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Posts Tagged ‘Thrombosis’


Biomarkers and risk factors for cardiovascular events, endothelial dysfunction, and thromboembolic complications

Curator: Larry H Bernstein, MD, FCAP

 

 

Acute Coronary Syndrome

Predictive Cardiovascular and Circulation Biomarkers

Biomarkers are chemistry analytes measured in plasma, serum or whole blood that potentially identify injury or risk for injury.  They may be measured in the laboratory or at the bedside (point of care technology).  They may be measured as an enzyme (CK isoenzyme MB), a protein (troponins I & T), or as a micro RNA (miRNA).  In the last decade the discovery and use of cardiac biomarkers has moved toward very small quantities, even 100 times below the picogram range using Quanterix Simoa, compared with an enzyme immunoassay.

The time of sampling was based on time to appearance from time of damage, and the release of the biomarker is a stochastic process. The earliest studies of CK-MB appearance, peak height, and disappearance was by Burton Sobel and associates related to measuring the extent of damage, and determined that reperfusion had an effect.

There has been a nonlinear introduction of new biomarkers in that period, with an explosion of methods discovery and large studies to validate them in concert with clinical trials. The improvement of interventional methods, imaging methods, and the unraveling of patient characteristics associated with emerging cardiovascular disease is both cause for alarm (technology costs) and for raised expectations for both prevention, risk reduction, and treatment. What is strikingly missing is the kind of data analyses on the population database that could alleviate the burden of physician overload. It is an urgent requirement for the EHR, and it needs to be put in place to facilitate patient care.

 

Biomarkers: Diagnosis and Management, Present and Future

Curator: Larry H Bernstein, MD, FCAP
Biomarkers of Cardiovascular Disease : Molecular Basis and Practical Considerations.
RS Vasan .
Circulation. 2006;113:2335-2362. http://dx.doi.org/10.1161/CIRCULATIONAHA.104.482570
https://pharmaceuticalintelligence.com/2013/11/10/biomarkers-diagnosis-and-management/

sCD40L indicates soluble CD40 ligand; Fbg, fibrinogen; FFA, free fatty acid; ICAM, intercellular adhesion molecule; IL, interleukin; IMA, ischemia modified albumin; MMP, matrix metalloproteinases; MPO, myeloperoxidase; Myg, myoglobin; NT-proBNP, N-terminal proBNP; Ox-LDL, oxidized low-density lipoprotein; PAI-1, plasminogen activator inhibitor; PAPP-A, pregnancy-associated plasma protein-A; PlGF, placental growth factor; TF, tissue factor; TNF, tumor necrosis factor; TNI, troponin I; TNT, troponin T; VCAM, vascular cell adhesion molecule; and VWF, von Willebrand factor.

 

Accurate Identification and Treatment of Emergent Cardiac Events  

Author: Larry H Bernstein, MD, FCAP
https://pharmaceuticalintelligence.com/2013/03/15/accurate-identification-and-treatment-of-emergent-cardiac-events/

The main issue that we have a consensus agreement that PLAQUE RUPTURE is not the only basis for a cardiac ischemic event. The introduction of  high sensitivity troponin tests has made it no less difficult after throwing out the receiver-operator characteristic curve (ROC) and assuming that any amount of cardiac troponin released from the heart is pathognomonic of an acute ischemic event.  This has resulted in a consensus agreement that

  • ctn measurement at a coefficient of variant (CV) measurement in excess of 2 Std dev of the upper limit of normal is a “red flag” signaling AMI? or other cardiomyopathic disorder

This is the catch.  The ROC curve established AMI in ctn(s) that were accurate for NSTEMI – (and probably not needed with STEMI or new Q-wave, not previously seen) –

  1. ST-depression
  2. T-wave inversion
  3. in the presence of other findings
  • suspicious for AMI

Wouldn’t it be nice if it was like seeing a robin on your lawn after a harsh winter?  Life isn’t like that.  When acute illness hits the patient may well present with ambiguous findings.   We are accustomed to relying on

  • clinical history
  • family history
  • co-morbidities, eg., diabetes, obesity, limited activity?, diet?
  • stroke and/or peripheral vascular disease
  • hypertension and/or renal vascular disease
  • aortic atherosclerosis or valvular heart disease

these are evidence, and they make up syndromic classes

  • Electrocardiogram – 12 lead EKG (as above)
  • Laboratory tests
  • isoenzyme MB of creatine kinase (CK)… which declines after 12-18 hours
  • isoenzyme-1 of LD if the time of appearance is > day-1 after initial symptoms (no longer used)
  1. cardiac troponin cTnI or cTnT
  • genome testing
  • advanced analysis of EKG

This may result in more consults for cardiologists, but it lays the ground for better evaluation of the patient, in the long run.

Perspectives on the Value of Biomarkers in Acute Cardiac Care and Implications for Strategic Management
Antoine Kossaify, … STAR-P Consortium
Biomarker Insights 2013:8 115–126.
http://dx.doi.org:/10.4137/BMI.S12703

In addition to the conventional use of natriuretic peptides, cardiac troponin, and C-reactive protein, other biomarkers are outlined in variable critical conditions that may be related to acute cardiac illness. These include ST2 and chromogranin A in acute dyspnea and acute heart failure, matrix metalloproteinase in acute chest pain, heart-type fatty acid binding protein in acute coronary syndrome, CD40 ligand and interleukin-6 in acute myocardial infarction, blood ammonia and lactate in cardiac arrest, as well as tumor necrosis factor-alpha in atrial fibrillation. Endothelial dysfunction, oxidative stress and inflammation are involved in the physiopathology of most cardiac diseases, whether acute or chronic. In summary, natriuretic peptides, cardiac troponin, C-reactive protein are currently the most relevant biomarkers in acute cardiac care.

 Inverse Association between Cardiac Troponin-I and Soluble Receptor for Advanced Glycation End Products in Patients with Non-ST-Segment Elevation Myocardial Infarction

ED. McNair, CR. Wells, A.M. Qureshi, C Pearce, G Caspar-Bell, and K Prasad
Int J Angiol 2011;20:49–54
http://dx.doi.org/10.1055/s-0031-1272552

Interaction of advanced glycation end products (AGEs) with the receptor for advanced AGEs (RAGE) results in activation of nuclear factor kappa-B, release of cytokines, expression of adhesion molecules, and induction of oxidative stress. Oxygen radicals are involved in plaque rupture contributing to thromboembolism, resulting in acute coronary syndrome (ACS). Thromboembolism and the direct effect of oxygen radicals on myocardial cells cause cardiac damage that results in the release of cardiac troponin-I (cTnI) and other biochemical markers. The soluble RAGE (sRAGE) compete with RAGE for binding with AGE, thus functioning as a decoy and exerting a cytoprotective effect. Low levels of serum sRAGE would allow unopposed serum AGE availability for binding with RAGE, resulting in the generation of oxygen radicals and proinflammatory molecules that have deleterious consequences and promote myocardial damage. sRAGE may stabilize atherosclerotic plaques. It is hypothesized that low levels of sRAGE are associated with high levels of serum cTnI in patients with ACS.
The levels of cTnI were higher in NSTEMI patients (2.180.33 mg/mL) as compared with control subjects (0.0120.001 mg/mL). Serum sRAGE levels were negatively correlated with the levels of cTnI. In conclusion, the data suggest that low levels of serum sRAGE are associated with high serum levels of cTnI and that there is a negative correlation between sRAGE and cTnI.

Correlation of soluble receptor for advanced glycation end products (sRAGE) with cardiac troponin-I

Correlation of soluble receptor for advanced glycation end products (sRAGE) with cardiac troponin-I

 

Figure 1 Serum levels of soluble receptor for advanced glycation end products (sRAGE) in control subjects and in patients with non-ST-elevation myocardial infarction (NSTEMI). Results are expressed as meanstandard error. *p<0.05, control versus NSTEMI.

 

Serum levels of soluble receptor for advanced glycation end products

Serum levels of soluble receptor for advanced glycation end products

Figure 3 Correlation of soluble receptor for advanced glycation end products (sRAGE) with cardiac troponin-I (cTnI) in patients with non-ST-segment elevation myocardial infarction.

 

Heart Failure Complicating Non–ST-Segment Elevation Acute Coronary Syndrome

MC Bahit, RD. Lopes, RM. Clare, et al.
JACC: HtFail 2013; 1(3):223–9 .
http://dx.doi.org/10.1016/j.jchf.2013.02.007

This study sought to describe the occurrence and timing of heart failure (HF), associated clinical factors, and 30-day outcomes in patients with non–ST-segment elevation acute coronary syndromes (NSTE-ACS). Of 46,519 NSTE-ACS patients, 4,910 (10.6%) had HF at presentation. Of the 41,609 with no HF at presentation, 1,194 (2.9%) developed HF during hospitalization. A total of 40,415 (86.9%) had no HF at any time. Patients presenting with or developing HF during hospitalization were older, more often female, and had a higher risk of death at 30 days than patients without HF (adjusted odds ratio [OR]: 1.74; 95% confidence interval: 1.35 to 2.26). Older age, higher presenting heart rate, diabetes, prior myocardial infarction (MI), and enrolling MI were significantly associated with HF during hospitalization.

Other risk factors

Additive influence of genetic predisposition and conventional risk factors in the incidence of coronary heart disease: a population-based study in Greece
N Yiannakouris, M Katsoulis, A Trichopoulou, JM Ordovas, DTrichopoulos
BMJ Open 2014;4:e004387.
http://dx.doi.org:/10.1136/bmjopen-2013-004387

Genetic predisposition to CHD, operationalised through a multilocus GRS, and ConvRFs have essentially additive effects on CHD risk.

PTX3, A Prototypical Long Pentraxin, Is an Early Indicator of Acute Myocardial Infarction

G Peri, M Introna, D Corradi, G Iacuitti, S Signorini, et al.
Circulation. 2000;102:636-641
http://circ.ahajournals.org/content/102/6/636
http://dx.doi.org:/10.1161/01.CIR.102.6.636

PTX3 is a long pentraxin whose expression is induced by cytokines in endothelial cells, mononuclear phagocytes, and myocardium. PTX3 is present in the intact myocardium, increases in the blood of patients with AMI, and disappears from damaged myocytes. We suggest that PTX3 is an early indicator of myocyte irreversible injury in ischemic cardiomyopathy.

Early release of glycogen phosphorylase inpatients with unstable angina and transient ST-T alterations

J Mair, B Puschendorf, J Smidt, P Lechleitner, F Dienstl, et al.
BrHeartJ 1994;72:125-127.
http://www.ncbi.nlm.nih.gov/pubmed/7917682

Glycogen phosphorylase BB (molecular weight 96000 kDa as a monomer) is the predominant isotype in human myocardium where it occurs alongside the MM subtype. The release of glycogen phosphorylase from injured myocardium may reflect the burst in glycogenolysis initiated during acute myocardial ischaemia. This is supported by a rapid increase in serum concentrations of glycogen phosphorylase BB in patients with acute myocardial infarction before concentrations of creatine kinase, creatine kinase MB, myoglobin, and cardiac troponin T increase. Unstable angina, however, ranges from no myocardial cell damage to non-Q wave myocardial infarction.
All variables except for creatine kinase and creatine kinase MB activities were significantly higher on admission in patients with unstable angina and transient ST-T alterations than in patients without. However, glycogen phosphorylase BB concentration was the only marker that was significantly (p = 0-0001) increased above its discriminator value in most patients.

Endothelium and Vascular

Endothelial Dysfunction: An Early Cardiovascular Risk Marker in Asymptomatic Obese Individuals with Prediabetes
AK. Gupta, E Ravussin, DL. Johannsen, AJ. Stull, WT. Cefalu and WD. Johnson
Br J Med Med Res 2012; 2(3): 413-423.
http://www.ncbi.nlm.nih.gov/pubmed/22905340

Adults with desirable weight [n=12] and overweight [n=8] state, had normal fasting plasma glucose [Mean(SD)]: FPG [91.1(4.5), 94.8(5.8) mg/dL], insulin [INS, 2.3(4.4), 3.1(4.8) μU/ml], insulin sensitivity by homeostasis model assessment [HOMA-IR, 0.62(1.2), 0.80(1.2)] and desirable resting clinic blood pressure [SBP/DBP, 118(12)/74(5), 118(13)/76(8) mmHg]. Obese adults [n=22] had prediabetes [FPG, 106.5(3.5) mg/dL], hyperinsulinemia [INS 18.0(5.2) μU/ml], insulin resistance [HOMA-IR 4.59(2.3)], prehypertension [PreHTN; SBP/DBP 127(13)/81(7) mmHg] and endothelial dysfunction [ED; reduced RHI 1.7(0.3) vs. 2.4(0.3); all p<0.05]. Age-adjusted RHI correlated with BMI [r=-0.53; p<0.001]; however, BMI-adjusted RHI was not correlated with age [r=-0.01; p=0.89].

Association of digital vascular function with cardiovascular risk factors: a population study.
T Kuznetsova, E Van Vlierberghe, J Knez, G Szczesny, L Thijs, et al.
BMJ Open 2014; 4:e004399.
http://dx.doi.org:/10.1136/bmjopen-2013-004399

Our study is the first to implement the new photoplethysmography (PPG) technique to measure digital pulse amplitude hyperemic in a sample of a general population. The correlates of hyperaemic response were as expected and constitute an internal validation of the PPG technique in assessment of digital vascular function.

Thrombotic/Embolic Events

Risk marker associations with venous thrombotic events: a cross-sectional analysis 
BA Golomb, VT Chan, JO Denenberg, S Koperski,  & MH Criqui.
BMJ Open 2014;4:e003208.
http://dx.doi.org:/10.1136/bmjopen-2013-003208

To examine the interrelations among, and risk marker associations for, superficial and deep venous events—superficial venous thrombosis (SVT), deep venous thrombosis (DVT) and pulmonary embolism (PE). Significant correlates on multivariable analysis were, for SVT: female sex, ethnicity (African-American=protective), lower educational attainment, immobility and family history of varicose veins. For DVT and DVE, significant correlates included: heavy smoking, immobility and family history of DVEs (borderline for DVE). For PE, significant predictors included immobility and, in contrast to DVT, blood pressure (BP, systolic or diastolic). In women, estrogen use duration for hormone replacement therapy, in all and among estrogen users, predicted PE and DVE, respectively.

Endothelium and hemorheology
T Gori, S Dragoni, G Di Stolfo and S Forconi
Ann Ist Super Sanità 2007 | Vol. 43, No. 2: 124-129
http://www.ncbi.nlm.nih.gov/pubmed/22951621

The mechanisms underlying the regulation of its function are extremely complex, and are principally determined by physical forces imposed on the endothelium by the flowing blood. In the present paper, we describe the interactions between the rheological properties of blood and the vascular endothelium.The role of shear stress, viscosity, cell-cell interactions, as well as the molecular mechanisms that are important for the transduction of these signals are discussed both in physiology and in pathology, with a particular attention to the role of reactive oxygen species. In the final conclusions, we propose an hypothesis regarding the implications of changes in blood viscosity, and particularly on the significance of secondary hyperviscosity syndromes..

Fig. 1 | Endothelial “function” (i.e.,the production of protective autacoids by the vascular endothelium) and “dysfunction” (i.e., the involvement of the endothelium in vascular pathology). EDHF: En d o t h e l i um-De r i v e d Hyperpolarizing Factor; LDL:Low-Density Lipoprotein

Fig. 2 | Endothelial production of nitric oxide (NO) is stimulated by oscillatory shear stress, transmitted by the endothelial surface layer to the endothelial cells. NO: Nitric Oxide; NOS: Nitrous Oxide Systems; ESL: Endothelial Surface Layer

 

 

 

 

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Triggering of Plaque Disruption and Arterial Thrombosis

Curator and Reporter: Larry H Bernstein, MD, FCAP

 

This discussion is a very interesting experimental model for the elucidation of plaque rupture in acute coronary syndromes.  The knowledge exists that there is a series of steps in develoiping atheromatous plaque.  We also know that platelets and endothelium are the location of this pathological development.  We don’t know exactly the role or mechanism of the contribution of hyperlipidemia, and what triggers plaque rupture.  This work reported is an experimental rabbit model that sheds light on the triggering of plaque rupture.

Triggering of Plaque Disruption and Arterial Thrombosis in an Atherosclerotic Rabbit Model

George S. Abela, MD, MSc; Paulo D. Picon, MD, MSc; Stephan E. Friedl, MEE; Otavio C. Gebara, MD; Akira Miyamoto, MD; Micheline Federman, PhD; Geoffrey H. Tofler, MB; James E. Muller, MD
From the Institute for Prevention of Cardiovascular Disease, Cardiovascular Division (G.S.A., S.E.F., G.H.T., J.E.M.), and the Department of Pathology (C.S.A., M.F.), Deaconess Hospital, Harvard Medical School, Boston, Mass; the Department of Pharmacology, Federal University and University of Passo Fundo (P.D.P.), Rio Grande de Sul, Brazil; the Heart Institute, University of São Paulo (O.C.G.), São Paulo, Brazil; and the First Department of Internal Medicine, National Defense Medical College (A.K.), Saitama, Japan.

Abstract

Background

It is now recognized that plaque disruption and thrombosis, a process often triggered by activities of the patient, is generally the cause of the onset of acute coronary syndromes. Understanding of disease onset could be greatly enhanced by the availability of a suitable animal model of plaque disruption and thrombosis. The aim of this study was to replicate and further characterize an atherosclerotic rabbit model of triggering of arterial thrombosis that was introduced by Constantinides and Chakravarti more than 30 years ago but not subsequently used.

  • Aortic plaques were induced by a high-cholesterol diet, by mechanical balloon injury of the artery, or by a combination of the two.
  • Triggering was attempted by injection of Russell’s viper venom (RVV), which is a proteolytic procoagulant, and histamine.

 Methods and Results

A total of 53 New Zealand White rabbits were exposed to one of four preparatory regimens:

  1. rabbits in group I (n=9) were fed a regular diet for 8 months;
  2. rabbits in group II (n=13) were fed a diet of 1% cholesterol for 2 months alternated with 2 months of a regular diet for a total of 8 months;
  3. rabbits in group III (n=5) underwent balloon-induced arterial wall injury, then were given a regular diet for 8 months; and
  4. rabbits in group IV (n=14) underwent balloon-induced arterial wall injury, then were given a diet of 1% cholesterol for 2 months followed by a regular diet for 2 months for a total of 4 months. After completion of the preparatory regimen, triggering of plaque disruption and thrombosis was attempted by injection of RVV (0.15 mg/kg IP) and histamine (0.02 mg/kg IV).
  • In group I, normal control rabbits without atherosclerosis, only one small thrombus was noted in 1 of 9 rabbits.
  • In group II, cholesterol-fed rabbits, thrombosis occurred in 3 of 13 rabbits.
  • Thrombus occurred in all rabbits in group III (5 of 5) and in 10 of 14 rabbits in group IV.
Although the frequency of thrombosis was not significantly different between groups I and II, possibly due to a small sample size, it was significantly different among all four groups (P<.001). Also, the frequency and amount of thrombus formation were significantly different among all four groups (P<.001; P<.0001) but not between groups I and II. Rabbits with atherosclerosis (those in groups II and IV) demonstrated plaque disruption and overlying platelet-rich thrombus formation similar to that observed in patients with acute coronary syndromes. The surface area covered by thrombus was
  1. 2 mm^2 in group I, 1
  2. 5.3±19.2 mm^2 in group II,
  3. 223±119 mm^2 in group III, and
  4. 263±222 mm^2 in group IV.
Rabbits in groups III and IV had the greatest amount of thrombus, and this amount was significantly greater than in rabbits in groups I and II (P<.001 and P<.03, respectively).

Conclusions

A suitable animal model is available for the study of plaque disruption and arterial thrombosis.

  • Hypercholesterolemia and mechanical arterial wall injury seemed to produce plaques vulnerable to triggering of disruption and thrombosis, whereas
  • normal arteries were relatively resistant to triggering.
This model provides a method to evaluate agents that might decrease the occurrence of vulnerable plaques or the amount of thrombus formed after triggering. Most important, the model can be used to identify the features of vulnerable plaques and the pharmacological stressors that trigger plaque disruption and thrombus formation.

Key Words: thrombosis, atherosclerosis, balloon, histamine, RVV

Introduction

Plaque disruption and subsequent arterial thrombosis are now recognized as critical to the onset of acute coronary ischemic syndromes. It is hypothesized that occurrence of thrombotic coronary occlusion has three components.
  1. First, a plaque that is vulnerable to disruption must be present.
  2. Second, acute physiological events are required to induce plaque disruption and thrombosis.
  3. Third, a relatively hypercoagulable state and heightened vasomotor tone increase the likelihood that arterial thrombosis will produce complete lumen occlusion.
 Recent epidemiological studies of human patients with myocardial infarction have demonstrated that in many cases a triggering activity, such as physical exertion, precipitates the acute onset of the disorder. Although a better understanding of plaque vulnerability and triggering would be of great value, knowledge of this process is limited because human studies are difficult and a suitable animal model has not been used.
In human patients, the opportunity to study factors responsible for acute onset of myocardial infarction is limited because coronary angiography performed before the event cannot prospectively identify plaques vulnerable to disruption.(9) After the event, angiography cannot distinguish the features of the plaque responsible for the disruption from those resulting from the disruption.(10) Although findings at autopsy provide detailed information about plaque disruption, these observations may be biased toward more advanced disease, since plaque disruptions producing total vascular occlusion and death may be more severe than those occurring in asymptomatic individuals or in patients with unstable angina or nonfatal myocardial infarction.
These difficulties, inherent in the study of plaque disruption and thrombosis in human patients, create a great need for an animal model of the process. More than 30 years ago, Constantinides and Chakravarti(13) developed such a model in atherosclerotic rabbits. Atherosclerotic plaques were produced in New Zealand White rabbits by intermittent cholesterol feeding. Triggering of plaque disruption and thrombosis was then accomplished by intraperitoneal injection of Russell’s viper venom (RVV, a procoagulant and endothelial toxin) followed by the intravenous injection of histamine, a vasopressor in rabbits. The aortas of the rabbits were then found to have disrupted atherosclerotic plaques with overlying platelet-rich thrombi.
Despite the similarity of these lesions to those observed in human patients, the model has received little attention or use during the past 3 decades. A recent review of the animal models of thrombosis currently in use noted that “thus far, it has not been possible to duplicate in a model the most common clinical cause of thrombosis—an ulcerated atherosclerotic plaque.”(14)
The advantage of the Constantinides model over other animal models used to study thrombosis is that it uses a biological intervention to trigger localized atherosclerotic plaque disruption and formation of platelet-rich arterial thrombi. The model facilitates the study of the process because the investigator determines when disruption and thrombosis will occur.
Disadvantages of the Constantinides model are
  • (1) the low yield of triggering (only about one third of the rabbits developed thrombosis) and
  • (2) the long (8-month) preparatory period. In addition, there is a need to replicate the findings of Constantinides and Chakravarti(13) from 30 years ago because of the biological variability of rabbit strains and RVV.
It cannot be assumed that the rabbits and RVV currently available will produce the results obtained in the 1960s.
In this study, we attempted to reproduce the original model of Constantinides.13 In addition, we wanted to determine whether mechanical injury to the aorta early in the preparatory phase could enhance the development of vulnerable plaques, thereby increasing the yield of disrupted plaques and shortening the preparatory period.

Methods

Fifty-three New Zealand White rabbits weighing between 2 and 3 kg were started on the experimental protocol; 41 survived until the time of attempted triggering. In these 41 rabbits, four dietary and interventional regimens were used in preparation for attempted triggering (Fig 1⇓). The control group, group I, consisted of normal rabbits (n=9) that were fed a regular diet for 8 months. Group II rabbits (n=13) were fed a high-cholesterol diet (1% cholesterol, ICN) for 2 months alternated with 2 months of a regular diet for a total of 8 months.15 Rabbits in group III (n=5) underwent balloon-induced arterial injury and were maintained on a regular diet for 8 months. Rabbits in group IV (n=14) underwent balloon-induced arterial injury, were maintained on a 1% cholesterol diet for 2 months, then were given a regular diet for 2 months for a total of 4 months.
Balloon-induced arterial wall injury of the aorta was performed with a 4F Fogarty catheter introduced through a femoral artery cutdown. The catheter was advanced in a retrograde fashion to the aortic valve and then withdrawn 3 cm. The balloon was inflated with 1.5 cm3 of air, and the catheter was retracted down to the iliofemoral artery. This was repeated three times in each rabbit as cm3 described previously.16 Rabbits were anesthetized with ketamine (50 mg/kg IM) and xylazine (20 mg/kg IM).

Of the 12 rabbits that died during the preparatory period, 5 were in group II, 2 in group III, and 5 in group IV. Seven of the 12 rabbits that died prematurely underwent an autopsy, and none had evidence of plaque disruption or arterial thrombosis. The causes of death included respiratory infection and liver failure from lipid infiltration.

The triggering agents RVV (Sigma Chemical Co) and histamine (Eli Lilly) were administered according to the method of Constantinides and Chakravarti.(13) RVV (0.15 mg/kg) was given by intraperitoneal injection 48 and 24 hours before the rabbits were killed. Thirty minutes after each RVV injection, histamine (0.02 mg/kg) was administered intravenously through an ear vein. Rabbits were killed by an overdose of intravenous pentobarbital and potassium chloride. The aorta and iliofemoral arteries were dissected and excised, and the intimal surface was exposed by an anterior longitudinal incision of the vessel.

The total surface area of the aorta, from the aortic arch to the distal common iliac branches, was measured. The surface area covered with atherosclerotic plaque and the surface area covered with antemortem thrombus were then determined. Images of the arterial surface were collected with a color charge-coupled device camera (TM 54, Pulnix) and digitized by an IBM PC/AT computer with a color image processing subsystem. The digitized images were calibrated by use of a graticule, and surface areas were measured by use of a customized quantitative image analysis package.

Tissue samples (1 cm in length) were taken from the thoracic aorta, 3 and 6 cm distal to the aortic valve; from the abdominal aorta, 7 and 4 cm proximal to the iliac bifurcation; and from the iliofemoral arteries. and prepared for and examined by light microscopy and they were examined by quantitative colorimetric assay.  Electron microscopy was also carried out with a Hitachi 600 microscope.

Biochemical analysis was done for tissue cholesterol. Free cholesterol and cholesteryl esters in the aorta were determined by high-performance liquid chromatography (HPLC) on the basis of the method of Kim and Chung. Each sample of aorta was ground to a fine powder with anhydrous sodium sulfate and extracted twice with 5 mL chloroform: methanol (2:1). The extract was dried under nitrogen and redissolved in 5 mL isopropanol.   Serum cholesterol, plasma fibrinogen, and platelet counts were done.

Overall comparison among the four groups was conducted with Fisher’s exact test and the Kruskal-Wallis test for discrete and continuous data, respectively. Comparisons between any two groups of rabbits were made by an exact Wilcoxon midrank test.23 P<.05 was considered statistically significant, and measured data were reported as mean±SD.

Results

Extent of Thrombosis After Triggering

In the 41 rabbits that underwent attempted triggering, the frequency of plaque disruption and focal thrombosis varied markedly depending on the type of preparatory regimen. In group I, only 1 of 9 control rabbits developed a thrombus. This was a small white thrombus with a surface area of 2 mm^2. Three of the 13 rabbits in group II on a 1% cholesterol diet developed white thrombi, all of which were small but were larger than that observed in group I (mean surface area, 15.3±19.2 mm^2). In group III, each of the 5 rabbits that had balloon-induced arterial wall injury developed large white thrombi (mean surface area, 223.0±119 mm^2). Ten of 14 group IV rabbits, with combined arterial wall injury and a high-cholesterol diet, developed white thrombi, all of which were large (mean surface area, 263.0±222 mm^2).

Both the frequency of occurrence and the amount of thrombus formation were significantly different among all four groups (P<.001 and P<.0001, respectively). However, the frequency and the amount of thrombus formation tested individually between groups I and II were not statistically different. The average surface area covered by thrombi in rabbits from groups III and IV was significantly greater than that observed in group II (P=.03 and P=.02) or group I (P=.001 and P=.001) rabbits. The average surface area covered by thrombi did not significantly differ between rabbits in group III versus those in group IV.

No white thrombi were noted in the ascending aorta or the aortic arch. In the non–balloon-treated rabbits in groups I and II, only 1 of 5 thrombi was in the abdominal aorta. In the balloon-injured rabbits in groups III and IV, the thrombi were almost evenly distributed between the thoracic and abdominal aorta (48 versus 66). There were more thrombi in the balloon-injured rabbits than in the non–balloon-injured rabbits (P<.002).

Extent of Plaque Covering the Arterial Surface

 The plaque surface area was significantly different among the four groups (P<.0001). Plaque was present in all the rabbits that were maintained on a high-cholesterol diet or that had balloon-induced arterial injury. The plaque distribution for each group is shown in Fig 4⇓. (not shown) Individual comparisons showed a larger amount of plaque in rabbits from groups III and IV than in those from group II (P=.04 and P=.001, respectively). There was no significant difference in the amount of the plaque in group III versus group IV rabbits. The Table demonstrates the relations of the various groups regarding frequency of disruption with the amount of thrombus formation and plaque surface area.
 The intima in group I rabbits appeared normal by gross inspection. In group II rabbits, white-yellow plaque was widely distributed over the arterial surface, with focal punctate ulceration occasionally noted under a dissecting microscope. In group III rabbits, the intima was smooth and widely covered with white plaque. Group IV rabbits had extensive sheets of elevated white-yellow plaque. By gross visualization, ulceration of the surface was present without superimposed thrombus in two rabbits in group IV.

Histological Features of Plaque Disruption and Thrombosis

 Over 4500 tissue sections were prepared and evaluated. Light microscopy of arterial samples from group I showed normal vascular histology. Group II samples had a predominance of foam cell infiltration of the intima surrounded with connective tissue. Group III samples had fibromuscular plaque composed mostly of muscular cell elements and minimal fibroconnective tissue. This was confirmed by Masson’s trichrome stains showing mostly red muscle cells and minimal blue fibrous tissue. Group IV samples had extensive plaque with an infiltration predominantly composed of foam cells.

Light microscopic examination of adjacent serial sections from thrombosis sites revealed platelet-rich thrombi with interrupted but long adhesion sites to the arterial wall over most of their length. Early organization and inflammatory cell infiltration were present within the thrombi. In sections from groups II and IV, some areas of plaque directly adjacent to the thrombi had marked thinning of the connective tissue cap and areas of dehiscent foam cells,. These observations were rare and were noted in <0.5% of the examined lesions. In most cases, the arterial thrombus was not located at a site of obvious plaque rupture. Foam cell infiltration was also noted adjacent to sites of thrombosis.

Figure 6.

A, Light micrograph shows that degenerated foam cells are present in a large cavity below a cap separating the cavity from the intimal surface of thoracic aorta from a rabbit in group IV (Movat’s pentachrome, magnification ×40). B, Light micrograph of large thrombus attached to the luminal surface of the thoracic aorta in the same rabbit shown in A. The cavitation is seen below the thrombus, and the intimal surface is markedly thinned (Masson’s trichrome, magnification ×16). C, Light micrograph of thrombus overlying a region of plaque with a large accumulation of foam cells from a rabbit in group IV. The free edges of the thrombus correspond to the underlying contour of the plaque, which suggests that the thrombus became detached during fixation (Masson’s trichrome, magnification ×25). D, Light micrograph of thrombus from the abdominal aorta in a rabbit from group IV, 48 hours after triggering. The thrombus is firmly attached and becoming organized. The yellow stain represents red blood cells, and the fibrin and platelets appear pink (Carstair’s stain, magnification ×25).
The degree of blue staining indicative of fibrous tissue in Masson’s trichrome–prepared slides was greatest in group II samples, as represented by values closer to the pure blue region (0.0) on CIE coordinates. Group II samples (0.33±0.046, mean±SD) were more blue than group III (0.43±0.06, P<.001) or group IV samples (0.38±0.05, P<.001). The degree of blue staining was not statistically different between samples from groups III and IV.
Scanning electron microscopy demonstrated fissures of various lengths below areas from which overlying thrombi were removed. Endothelial cells could be seen lining the intimal surface of the aorta in the rabbits that had undergone balloon-induced arterial wall injury 8 months earlier. Surface blebs and focal endothelial breakdown with ulcer formation, without grossly visible thrombosis, were occasionally seen in samples from groups II and IV. The base of these ulcers was layered with platelets, fibrin, and red blood cells. Transmission electron microscopy of areas with thrombosis confirms that the thrombi were platelet rich.

Biochemical Findings

 Baseline serum cholesterol for all rabbits was 50±25 mg/dL and did not differ among the four groups. In rabbits in groups II and IV, which received cholesterol feeding, serum cholesterol rose to an average peak level of 2500± 1200 mg/dL.
In the two groups that received cholesterol feeding, the total cholesterol content in tissue samples pooled from the thoracic and abdominal aorta was significantly higher in group IV (16±7.2 mg/g) than in group II (2.8±1.6 mg/g) (P<.0001). Rabbits that were maintained on a regular diet (groups I and III) had equally low levels of tissue cholesterol (0.05±0.04 versus 0.06±0.02 mg/g, P=NS).

Hematological Changes Accompanying Triggering

The average fibrinogen level before triggering in the 27 rabbits in which fibrinogen was measured was 210±119 mg/dL; it rose to 403±168 mg/dL 48 hours after triggering (P<.001). Plasma fibrinolytic activity did not change after triggering (85.5±37.8 versus 94.8±33.5 arbitrary units). Platelet counts (measured in only 19 rabbits in groups II and IV) decreased from 350±84×103 to 215±116×103 per cubic millimeter after triggering (P<.001). White blood cell count did not decrease after triggering (12.8±13.0 versus 12.8±7.1×103 cells per cubic millimeter). However, the hematocrit dropped from 35.7±3.8% to 32.0±5.8% (P<.0002).

Discussion

The results demonstrate that vulnerable plaques can be produced and that plaque disruption and platelet-rich arterial thrombus formation may be triggered pharmacologically in an animal model of arterial plaque. This finding documents that the New Zealand White rabbit strains and the RVV currently available can be used to obtain the same results observed by Constantinides and Chakravarti(13) more than 30 years ago.
The frequency of successful triggering was dependent on the type of preparatory regimen used. In control rabbits maintained on a regular diet, only 1 of 9 developed a small thrombus after injection of the triggering agents. Although rabbits fed a high-cholesterol diet had more thrombosis after triggering, the values were not statistically different between rabbits in groups I and II. In other studies of triggering of cholesterol-fed rabbits, a total of 7 of 30 rabbits have developed thrombi, but this also does not achieve statistical significance (unpublished data, 1994). The number of rabbits studied may have been too low to demonstrate a moderate difference of thrombus occurrence. However, earlier work by Constantinides and Chakravarti(13 24) demonstrated a frequency of thrombi in 1 of 22 rabbits not fed cholesterol versus 22 of 77 rabbits fed cholesterol, which does achieve statistical significance (P<.02). This indicates that a larger sample may demonstrate a difference between groups I and II and that cholesterol feeding increases the likelihood of the disruption and thrombosis process in the rabbit model. Thus, our results in conjunction with those of Constantinides and Chakravarti suggest that thrombosis triggered by RVV and histamine may be facilitated in the presence of atherosclerosis. However, these observations do not preclude the possibility of thrombosis in a normal artery, which can be induced by injury from various triggers.
Rabbits subjected to arterial balloon injury developed extensive thrombosis only after triggering, as did rabbits subjected to both arterial injury and a high-cholesterol diet. Thus, a high-cholesterol diet especially in the presence of mechanical injury is capable of producing a plaque vulnerable to disruption and thrombosis by triggering with RVV and histamine.

Production of Vulnerable Plaque by Cholesterol Feeding

The technique of pulsed cholesterol feeding used in this study has been shown to be an effective method of producing experimental atherosclerosis, as have continuous cholesterol feeding regimens. Recently, it has been demonstrated that cholesterol feeding induces an upregulation of vascular cell adhesion molecule-1 in rabbit endothelium. This may predispose a site to monocyte adhesion and migration into the subendothelial space. Continued macrophage accumulation may make the site particularly vulnerable to disruption and thrombosis.
Autopsy studies in humans have led to the hypothesis that a lesion with a lipid pool beneath a thin cap is particularly vulnerable to disruption and thrombosis.4 5 This morphology has been shown to generate stress concentrations that would predispose a plaque to disrupt.  Although sites with lipid pools and thin caps were noted in the present study, their occurrence was too limited to permit studies to determine whether these were sites particularly prone to thrombosis. Cholesterol feeding for 2 years may be required to produce a sufficient number of such lesions to determine their vulnerability to disruption.

Production of Vulnerable Plaque by Balloon-Induced Injury

An important finding of this study is that vulnerability to disruption and thrombosis was present 8 months after deendothelialization with balloon-induced arterial wall injury in rabbits on a regular diet (group III). This occurred in the presence of a regenerated endothelium overlying a diffuse fibromuscular plaque. Previous reports have demonstrated that endothelium that regenerates after balloon deendothelialization is physiologically dysfunctional for a prolonged period. From our study, it appears that endothelial function is compromised in its role as a thrombosis-resistant surface over a long period as well. An important factor that may contribute to the altered function is the presence of underlying plaque.

Triggering Agents RVV and Histamine

Among its numerous constituents, RVV contains proteases that activate factors V and X. Such activation leads to thrombosis, which is most likely to occur at sites of cell injury. In addition to this procoagulant effect, RVV is a direct endothelial toxin.31 However, in the absence of arterial abnormalities produced by cholesterol feeding or other means, RVV alone or in combination with a vasoconstrictor agent rarely produces thrombosis.4 The increase in fibrinogen levels and the stability of hematological factors during triggering indicate that RVV does not act by producing a disseminated coagulopathy. The localization of thrombus at focal arterial sites is further evidence that this model does not merely produce a nonspecific thrombotic effect.
Histamine is an arterial vasoconstrictor in rabbits. This effect is mediated by an H1 receptor that regulates release of norepinephrine at the presynaptic norepinephrine sites. Histamine may contribute to plaque disruption by raising the arterial pressure and stress on the plaque and/or by the development of vasospasm. Other, similar agents, thromboxane A2 and serotonin, also have been shown to result in severe vasoconstriction of epicardial coronary arteries that is mediated by platelet deposition at stenosed sites.

Comparison With Other Models

This is a unique model that combines features of several other animal models that have been used to study atherosclerosis and thrombosis. With regard to thrombosis, the model provides the opportunity to extend observations previously made in other animal models of thrombosis to the special conditions surrounding triggering of acute cardiovascular syndromes. While the model of Folts et al has been invaluable in assessing enhanced platelet deposition in dog and pig coronary arteries, it requires both endothelial injury and the production of a 60% to 70% lumen stenosis. Moreover, it does not use an atherosclerotic artery with a vulnerable plaque.
Badimon et al used a flow chamber to evaluate platelet deposition on activated arterial surfaces. They demonstrated that deep arterial injury results in more thrombus formation than superficial injury. However, their model does not recreate the in vivo environment or provide an opportunity for evaluation of various thrombogenic sites, as does the model presented in this study.

Relation of the Model to Human Coronary Thrombosis

Certain features of the lesions seen in this model are similar to those of human lesions seen at autopsy of patients with fatal myocardial infarction, ie, a lesion with a fissured collagen cap overlying a lipid mass of amorphous and crystalline lipid. However, most of the lesions in the model did not have these features and were more consistent with a recent pathological study of fatal coronary thrombosis, which revealed that in approximately half the cases, the plaque was relatively intact but an inflammatory infiltrate was present.36 Perhaps the incidence of plaque rupture causing thrombus may be even lower in patients with nonfatal coronary thrombosis, as suggested from angioscopic studies of coronary arteries that have shown plaque ulceration of various severities.
Although the model we used produced lesions with many similarities to the nonruptured lesions described in patients, extension of this preparation for a 2-year period has been documented to produce lesions with deep fissures similar to those observed in many patients with fatal coronary thrombosis. Also, use of balloon injury in this model to enhance plaque development resulted in plaques that were morphologically similar to advanced plaques induced by the alternating high-cholesterol diet.
Analyses of human plaques have demonstrated that disrupted plaques have significantly less collagen, glycosaminoglycans, and smooth muscle cells and more extracellular lipid and macrophages than do nondisrupted plaques. This is consistent with findings in our study that rabbits in group II had more connective tissue and a lower rate of disruption and thrombosis than those in groups III and IV.
Perhaps the major limitation of this study is that it used a complex pharmacological mixture as the trigger, which makes speculation on the mechanism of action difficult. Further studies will be necessary to determine which components of RVV and histamine are responsible for the focal thrombosis.

Potential Utility of the Model to Study Plaque Disruption and Thrombosis

The observation that large, platelet-rich thrombi can be obtained by triggering in animals with underlying plaques produced by cholesterol feeding or by balloon injury broadens the types of plaque that can be studied for vulnerability. Various types of preparatory regimens could be studied for their ability to promote or retard the development of vulnerable plaque.
The model also can be used to test pharmacological agents that may reduce the development of vulnerable atherosclerotic plaques, such as lipid-lowering agents, antioxidants, calcium channel blocking agents, and angiotensin-converting enzyme inhibitors. Antiplatelet and other antithrombotic drug therapies can be tested for the ability to reduce the amount of thrombus complicating plaque disruption. Finally, the model can be used to characterize the biochemical and cellular bases for plaque vulnerability by comparing the features of sites that do and do not develop thrombi soon after triggering.

 References

3 Friedman M, van den Bovenkamp GJ. The pathogenesis of a coronary thrombus. Am J Pathol. 1966;80:19-44.
4 Constantinides P. Plaque fissures in human coronary thrombosis. J Atheroscler Res. 1966;6:1-17.
5 Davies MJ, Thomas AC. Plaque fissuring: the cause of acute myocardial infarction causing sudden ischaemic death, and crescendo angina. Br Heart J. 1985;53:363-373. FREE Full Text
8 Tofler GH, Stone PH, Maclure M, Edelman E, Davis VG, Robertson T, Antman EM, Muller JE, and the MILIS Study Group. Analysis of possible triggers of acute myocardial infarction (the MILIS Study). Am J Cardiol. 1990;66:22-27. CrossRefMedline
9  Little WC, Constantinescu M, Applegate RJ, Kutcher MA, Burrows MT, Kahl FR, Santamore WP. Can coronary angiography predict the site of a subsequent myocardial infarction in patients with mild-to-moderate coronary artery disease? Circulation. 1988;78:1157-1166. Abstract/FREE Full Text
10 Ambrose JA, Winters SL, Arora RR, Eng A, Riccio A, Gorlin R, Fuster V. Angiographic evolution of coronary artery morphology in unstable angina. J Am Coll Cardiol. 1986;7:472-478. Abstract
11 Davies MJ, Bland MJ, Hartgartner WR, Angelini A, Thomas AC. Factors influencing the presence or absence of acute coronary thrombi in sudden ischemic death. Eur Heart J. 1989;10:203-208. Abstract/FREE Full Text
12  JH, Fuster V, Badimon L, Taubman M, Badimon J, Cheseboro JH. Syndromes of accelerated atherosclerosis: role of vascular injury and smooth muscle cell proliferation. J Am Coll Cardiol. 1990;15:1667-1687. Abstract
13 Constantinides P, Chakravarti RN. Rabbit arterial thrombosis production by systemic procedures. Arch Pathol. 1961;72:197-208. Medline
14  Runge RS, Haber E. Animal models for the study of thrombolysis in vivo. Circulation. 1991;83(suppl IV): IV-1-IV-2. Abstract.
15 Constantinides P, Booth J, Carlson G. Production of advanced cholesterol atherosclerosis in the rabbit. Arch Pathol. 1960;70:80-92.

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Platelets in Translational Research – Part 1

 

Reviewer and Curator: Larry H Bernstein, MD, FCAP 

 

Introduction

This article is one of a 2 part presentation posted as an example of a central role of platelet biology in translational medicine investigations leading to the prevention and control of hemolytic and coagulopathic conditions, and to an understanding of atherosclerotic cardiovascular disease. The study of coagulation traces back to the early work on Warfarin in bleeding, and even earlier than that to the geneological evidence of inherited hemophilia in the Royal family of 18th Century Victoria.  The amount of work has been voluminous, and the conceptual framework has been difficult to put into practice over generations of postgraduate physicians.  No wonder, considering the clotting proteins and the amazing platelet.

Part I of Platelets in Translaional Research is a comprehensive coberage of the signaling and control involved in platelet-endothelial reactions, platelet-platelet reactions, and platelet transciptomics, all of which have a significant bearing on atherosclerotic plaque buildup, plaque rupture, and acute coronary syndrome as well as chronic ischemic heart disease.

Part II will cover a range of studies pointing to anti-platelet therapeutic targets.

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‎ Special Considerations in Blood Lipoproteins, Viscosity, Assessment and Treatment

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Platelet Role in Atheroscleosis

Platelets and Cardiovascular Disease

David Gregg, MD; Pascal J. Goldschmidt-Clermont, MD
Duke University Medical Center, Durham, NC.

Platelets are specialized disk-shaped cells in the blood stream that are involved in the formation of blood clots that play an important role in heart attacks, strokes, and peripheral vascular disease. In most people, the more than 200 million platelets in a milliliter of blood act as tiny building blocks to form the basis of a clot to stop bleeding from cuts or injuries. Platelets can detect a disruption in the lining of a blood vessel and react to build a wall to stop bleeding.

F1.large  platelr forms plug

Figure 1. Platelets form a platelet plug to stop bleeding from an injured blood vessel.

In cardiovascular disease, abnormal clotting occurs that can result in heart attacks or stroke. Blood vessels injured by smoking, cholesterol, or high blood pressure develop cholesterol-rich build-ups (plaques) that line the blood vessel; these plaques can rupture and cause the platelets to form a clot. Even though no bleeding is occurring, platelets sense the plaque rupture and are confused, thinking that an injury has taken place that will cause bleeding. Instead of sealing the vessel to prevent bleeding as would occur with a cut, a clot forms in an intact blood vessel, causing a blockage of blood flow (Figure 2). Without blood, a portion of the heart muscle can die, leading to a heart attack.

F2.large   clot formation blocks flow

Figure 2. Plaque rupture results in clot formation to block blood flow, which may result in a heart attack or stroke

Platelet Disorders

Platelets may be abnormal either quantitatively (too many or too few) or qualitatively (the right number but they do not work correctly). The number of platelets is routinely tested as part of the complete blood count (CBC). Normal counts range from 150 000 to 450 000. A decrease in the number of platelets indicates a condition known as thrombocytopenia and may result in increased bleeding, the first signs of which may include gum bleeding, nose bleeds, and increased bruising. In cardiology, the most frequent cause of a low platelet count is an abnormal immune response caused by drug therapy, particularly with the intravenous blood thinner heparin (heparin-induced thrombocytopenia), and rarely with other drugs to control high blood pressure or symptoms of congestive heart failure (diuretics), to control diabetes (antidiabetic medications), or to regulate your blood clotting (antiplatelet drugs). Elevated platelet counts can also occur, usually in association with diseases in the elderly, and can result in either excess clotting or even abnormal bleeding.

Because platelets are so important in stopping bleeding from everyday injuries such as cuts or bruises, severe inherited disorders of platelets are quite rare. Researchers, however, have discovered more subtle genetic variations in platelets called polymorphisms that may alter platelets in subtle ways to raise the risk of cardiovascular disease when combined with other risk factors, but which on their own do not result in overt disease. These polymorphisms may also be important in understanding who may gain the greatest benefit from anti-platelet drugs.

The most commonly used antiplatelet agent is aspirin, although you may also be prescribed other oral agents, such as ticlopidine, clopidogrel, or dipyridamole, or intravenous antiplatelet drugs such as abciximab or eptifibatide while you are in the hospital or undergoing angioplasty procedures. Each agent affects platelets in slightly different ways and may have unique side effects, but all either cause the platelets to stick together or induce them to clot less well. Your doctor will choose the drug that best suits your situation. Table 1 shows some of the unique features of commonly used oral antiplatelet drugs

TABLE 1. Commonly Used Oral Antiplatelet Drugs: Uses and Side Effects

Aspirin Clopidogrel Ticlopidine Dipyridamole + Aspirin
Uses Heart disease and stroke; inexpensive Heart disease and stroke, particularly after stenting Mostly in stroke; requires blood count monitoring Stroke; may not be suitable for patients with heart disease
Side effects Gastrointestinal (GI) intolerance; GI bleeding Diarrhea (much less common than with ticlopidine); rash and itching Diarrhea and GI upset (usually resolves in 2 weeks); may decrease blood counts, particularly white blood cells Headache; GI bleeding; GI intolerance

Antiplatelet drugs are different than blood thinners or anticoagulants such as warfarin (Coumadin, Bristol-Myers Squibb) or heparin. Anticoagulants block a second step in clotting known as coagulation but do not directly affect the platelets.

Eur J Cardiovasc Nurs. 2002 Dec;1(4):273-88.

Platelets and cardiovascular disease

Willoughby S, Holmes A, Loscalzo J.
Queen Elizabeth Hospital, Adelaide University, South Australia, Adelaide, Au

Platelets play an important, but often under-recognized role in cardiovascular disease. For example, the normal response of the platelet can be altered, either by increased pro-aggregatory stimuli or by diminished anti-aggregatory substances to produce conditions of increased platelet activation/aggregation and occur in active cardiovascular disease states both on a chronic (e.g. stable angina pectoris) and acute basis (e.g. acute myocardial infarction). In addition, platelet hyperaggregability is also associated with the risk factors for coronary artery disease (e.g. smoking, hypertension, and hypercholesterolaemia). Finally, the utility of an increasing range of anti-platelet therapies in the management of the above disease states further emphasizes the pivotal role platelets play in the pathogenesis of cardiovascular disease. This paper provides a comprehensive overview of the normal physiologic role of platelets in maintain homeostasis, the pathophysiologic processes that contribute to platelet dysfunction in cardiovascular disease and the associated role and benefits of anti-platelet therapies.   PMID: 14622657

Triggering of Plaque Disruption and Arterial Thrombosis in an Atherosclerotic Rabbit Model

GS Abela, PD Picon, SE Friedl, OC Gebara, A Miyamoto, et al.
Deaconess Hospital, Harvard Medical School, Boston, Mass; Federal Univ and Univ Passo Fundo (P.D.P.), Rio Grande de Sul, Brazil;  Heart Institute, Univ São Paulo (O.C.G.), São Paulo, Brazil; National Defense Medical College (A.K.), Saitama, Jp.

It is now recognized that plaque disruption and thrombosis, a process often triggered by activities of the patient, is generally the cause of the onset of acute coronary syndromes. Plaque disruption and subsequent arterial thrombosis are now recognized as critical to the onset of acute coronary ischemic syndromes. It is hypothesized that occurrence of thrombotic coronary occlusion has three components. First, a plaque that is vulnerable to disruption must be present. Second, acute physiological events are required to induce plaque disruption and thrombosis. Third, a relatively hypercoagulable state and heightened vasomotor tone increase the likelihood that arterial thrombosis will produce complete lumen occlusion.

In human patients, the opportunity to study factors responsible for acute onset of myocardial infarction is limited because coronary angiography performed before the event cannot prospectively identify plaques vulnerable to disruption. After the event, angiography cannot distinguish the features of the plaque responsible for the disruption from those resulting from the disruption. Moreover, plaque disruptions producing total vascular occlusion and death may be more severe than those occurring in asymptomatic individuals or in patients with unstable angina or nonfatal myocardial infarction. These difficulties, inherent in the study of plaque disruption and thrombosis in human patients, create a great need for an animal model of the process.  An atherosclerotic rabbit model of triggering of arterial thrombosis that was introduced by Constantinides and Chakravarti more than 30 years ago but not subsequently used. Aortic plaques were induced by a high-cholesterol diet, by mechanical balloon injury of the artery, or by a combination of the two. Triggering was attempted by injection of Russell’s viper venom (RVV), which is a proteolytic procoagulant, and histamine. A recent review of the animal models of thrombosis currently in use noted that “thus far, it has not been possible to duplicate in a model the most common clinical cause of thrombosis—an ulcerated atherosclerotic plaque.” The advantage of the Constantinides model over other animal models used to study thrombosis is that it uses a biological intervention to trigger localized atherosclerotic plaque disruption and formation of platelet-rich arterial thrombi.  Disadvantages of the Constantinides model are (1) the low yield of triggering (only about one third of the rabbits developed thrombosis) and (2) the long (8-month) preparatory period. In addition, there is a need to replicate the findings of Constantinides and Chakravarti13 from 30 years ago because of the biological variability of rabbit strains and RVV. It cannot be assumed that the rabbits and RVV currently available will produce the results obtained in the 1960s.

A total of 53 New Zealand White rabbits were exposed to one of four preparatory regimens: rabbits in group I (n=9) were fed a regular diet for 8 months; rabbits in group II (n=13) were fed a diet of 1% cholesterol for 2 months alternated with 2 months of a regular diet for a total of 8 months; rabbits in group III (n=5) underwent balloon-induced arterial wall injury, then were given a regular diet for 8 months; and rabbits in group IV (n=14) underwent balloon-induced arterial wall injury, then were given a diet of 1% cholesterol for 2 months followed by a regular diet for 2 months for a total of 4 months. After completion of the preparatory regimen, triggering of plaque disruption and thrombosis was attempted by injection of RVV (0.15 mg/kg IP) and histamine (0.02 mg/kg IV). In group I, normal control rabbits without atherosclerosis, only one small thrombus was noted in 1 of 9 rabbits. In group II, cholesterol-fed rabbits, thrombosis occurred in 3 of 13 rabbits. Thrombus occurred in all rabbits in group III (5 of 5) and in 10 of 14 rabbits in group IV. Although the frequency of thrombosis was not significantly different between groups I and II, possibly due to a small sample size, it was significantly different among all four groups (P<.001). Also, the frequency and amount of thrombus formation were significantly different among all four groups (P<.001; P<.0001) but not between groups I and II. Rabbits with atherosclerosis (those in groups II and IV) demonstrated plaque disruption and overlying platelet-rich thrombus formation similar to that observed in patients with acute coronary syndromes. The surface area covered by thrombus was 2 mm2 in group I, 15.3±19.2 mm2 in group II, 223±119 mm2 in group III, and 263±222 mm2 in group IV. Rabbits in groups III and IV had the greatest amount of thrombus, and this amount was significantly greater than in rabbits in groups I and II (P<.001 and P<.03, respectively).

The intima in group I rabbits appeared normal by gross inspection. In group II rabbits, white-yellow plaque was widely distributed over the arterial surface, with focal punctate ulceration occasionally noted under a dissecting microscope. In group III rabbits, the intima was smooth and widely covered with white plaque. Group IV rabbits had extensive sheets of elevated white-yellow plaque. By gross visualization, ulceration of the surface was present without superimposed thrombus in two rabbits in group IV.  In sections from groups II and IV, some areas of plaque directly adjacent to the thrombi had marked thinning of the connective tissue cap and areas of dehiscent foam cells. These observations were rare and were noted in <0.5% of the examined lesions. In most cases, the arterial thrombus was not located at a site of obvious plaque rupture. Foam cell infiltration was also noted adjacent to sites of thrombosis. Scanning electron microscopy demonstrated fissures of various lengths below areas from which overlying thrombi were removed. Endothelial cells could be seen lining the intimal surface of the aorta in the rabbits that had undergone balloon-induced arterial wall injury 8 months earlier. Surface blebs and focal endothelial breakdown with ulcer formation, without grossly visible thrombosis, were occasionally seen in samples from groups II and IV. The base of these ulcers was layered with platelets, fibrin, and red blood cells. Transmission electron microscopy of areas with thrombosis confirms that the thrombi were platelet rich.

In the two groups that received cholesterol feeding, the total cholesterol content in tissue samples pooled from the thoracic and abdominal aorta was significantly higher in group IV (16±7.2 mg/g) than in group II (2.8±1.6 mg/g) (P<.0001). Rabbits that were maintained on a regular diet (groups I and III) had equally low levels of tissue cholesterol (0.05±0.04 versus 0.06±0.02 mg/g, P=NS). The average fibrinogen level before triggering in the 27 rabbits in which fibrinogen was measured was 210±119 mg/dL; it rose to 403±168 mg/dL 48 hours after triggering (P<.001). Plasma fibrinolytic activity did not change after triggering (85.5±37.8 versus 94.8±33.5 arbitrary units). Platelet counts (measured in only 19 rabbits in groups II and IV) decreased from 350±84×103 to 215±116×103 per cubic millimeter after triggering (P<.001).

Conclusions

The results demonstrate that vulnerable plaques can be produced and that plaque disruption and platelet-rich arterial thrombus formation may be triggered pharmacologically in an animal model of arterial plaque. This finding documents that the New Zealand White rabbit strains and the RVV currently available can be used to obtain the same results observed by Constantinides and Chakravarti13 more than 30 years ago. This animal model is suitable for the study of plaque disruption and arterial thrombosis. Hypercholesterolemia and mechanical arterial wall injury seemed to produce plaques vulnerable to triggering of disruption and thrombosis, whereas normal arteries were relatively resistant to triggering. The model provides a method to evaluate agents that might decrease the occurrence of vulnerable plaques or the amount of thrombus formed after triggering. Most important, the model can be used to identify the features of vulnerable plaques and the pharmacological stressors that trigger plaque disruption and thrombus formation.

Certain features of the lesions seen in this model are similar to those of human lesions seen at autopsy of patients with fatal myocardial infarction, ie, a lesion with a fissured collagen cap overlying a lipid mass of amorphous and crystalline lipid. However, most of the lesions in the model did not have these features and were more consistent with a recent pathological study of fatal coronary thrombosis, which revealed that in approximately half the cases, the plaque was relatively intact but an inflammatory infiltrate was present. Perhaps the incidence of plaque rupture causing thrombus may be even lower in patients with nonfatal coronary thrombosis, as suggested from angioscopic studies of coronary arteries that have shown plaque ulceration of various severities.  Analyses of human plaques have demonstrated that disrupted plaques have significantly less collagen, glycosaminoglycans, and smooth muscle cells and more extracellular lipid and macrophages  than do nondisrupted plaques. This is consistent with findings in our study that rabbits in group II had more connective tissue and a lower rate of disruption and thrombosis than those in groups III and IV.

references

Herrick JB. Clinical features of sudden obstruction of the coronary arteries. JAMA. 1912;59:2015-2020.
Chapman I. Morphogenesis of occluding coronary artery thrombosis. Arch Pathol. 1965;80:256-261.
Friedman M, van den Bovenkamp GJ. The pathogenesis of a coronary thrombus. Am J Pathol. 1966;80:19-44.

This reader sees a validation in this study of the noted cardiologist, Alan Jaffe, at Mayo Clinic, in referring to Type I and Type II myocardial infarcts, which accounts for differences in troponin elevations in patients.

The Platelet in Cardiovascular Disease

1. microthrombi adhering to foam cells
2. Platelets secrete

–  Platelet-derived growth factor (PDGF) that promotes smooth muscle cell migration and collagen production

– Plasminogen activator inhibitor-1 (PAI-1) that suppresses fibrinolysis

Davies MJ, Woolf N, Rowles PM, et al. Morphology of the endothelium over atherosclerotic plaques in human coronary arteries. Br Heart J 1988;60:459-64.

– Microhemorrhages attract and activate neighboring platelets, support fibrin generation

Inoue M, Itoh H, Ueda M, et al. Vascular endothelial growth factor (VEGF) expression in human coronary atherosclerotic lesions: Possible pathophysiological significance in progression of atherosclerosis. Circulation 1998;98:2108-16.

Systems biology of platelet-vessel wall interactions

Scott L. Diamond*, Jeremy Purvis, Manash Chatterjee and Matthew H. Flamm
Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, PA
Front Phys 26 August 2013    http://dx.doi.org/10.3389/fphys.2013.00229

Blood systems biology seeks to quantify outside-in signaling as platelets respond to numerous external stimuli, typically under flow conditions. Platelets can activate via GPVI collagen receptor and numerous G-protein coupled receptors (GPCRs) responsive to ADP, thromboxane, thrombin, and prostacyclin. A bottom-up ODE approach allowed prediction of platelet calcium and phosphoinositides following P2Y1 activation with ADP, either for a population average or single cell stochastic behavior. The homeostasis assumption (i.e., a resting platelet stays resting until activated) was particularly useful in finding global steady states for these large metabolic networks. Alternatively, a top-down approach involving Pairwise Agonist Scanning (PAS) allowed large data sets of measured calcium mobilization to predict an individual’s platelet responses. The data was used to train neural network (NN) models of signaling to predict patient-specific responses to combinatorial stimulation. A kinetic description of platelet signaling then allows prediction of inside-out activation of platelets as they experience the complex biochemical milieu at the site of thrombosis. Multiscale lattice kinetic Monte Carlo (LKMC) utilizes these detailed descriptions of platelet signaling under flow conditions where released soluble species are solved by finite element method and the flow field around the growing thrombus is updated using computational fluid dynamics or lattice Boltzmann method. Since hemodynamic effects are included in a multiscale approach, thrombosis can then be predicted under arterial and venous thrombotic conditions for various anatomical geometries. Such systems biology approaches accommodate the effect of anti-platelet pharmacological intervention where COX1 pathways or ADP signaling are modulated in a patient-specific manner.

CLOTTING UNDER FLOW CONDITIONS

Collagen is sufficient to capture and activate platelets under venous wall shear rates (ãw  100–200s_1). In the arterial circulation (ãw 1000–2000 s_1), collagen adsorbed von Willebrand factor (vWF) facilitates platelet capture, allowing col­lagen induced GPVI signaling and subsequent á2â1 and á2bâ3 activation. Under flow conditions, red blood cells help enrich the platelet concentration by 3–8x in the plasma layer near the wall. At pathological high shear exposures (>5000 s_1) encountered in severe stenosis, mechanical heart valves, and continuous LVAD pumps, the plasma vWF may undergo structural changes, such as a transition from a globular to an extended state (Schneider et al., 2007), likely increasing the availability of A1 domains in the vWF polymer for multivalent contacting with platelet GPIb receptors.

GROWTH OF THE PLATELET AGGREGATE VIA AUTOCATALYTIC SIGNALING

Collagen triggers GPVI clustering, leading to rapid phosphorylation of the GPVI-associated Fc receptor by Src family tyrosine kinases. Such phosphotyrosine residues are recognized by Syk, and the binding and activation of Syk activates PLCã2. PLCã2 converts phosphatidylinositol (PI)-4,5-P2 (PIP2) to inositol 1,4,5-trisphosphate (1,4,5-IP3 or IP3) and diacyclglycerol (DAG). IP3 opens Ca2+ channels in the platelet dense tubular system (DTS). Depletion of DTS Ca2+ results in STIM1 activation and bind­ing to Orai1, leading to store operated calcium entry (SOCE). DAG/Ca2+ activates protein kinase C (PKC) in platelets, which in turn governs several serine/threonine phosphorylation events.

Beyond the first monolayer of platelets adherent to colla-gen/VWF, the addition of subsequent layers of platelets to the growing thrombus is strongly potentiated by locally released ADP and thromboxane (TXA2) as well as locally generated thrombin. ADP activates P2Y1 and P2Y12 while TXA2 activates the TP receptor and thrombin cleaves PAR1 and PAR4. Activation of a GPCR causes an exchange of GTP for GDP on the α subunit of the G protein and dissociation of the α and γ subunits. Both these units in turn interact with secondary effectors such as PLC and adenylate cyclase. Human platelets express at least 10 forms of Gα (including members of the Gq, Gi, G12, and Gs fami­lies) (Brass et al., 2006; Offermanns, 2006). Thrombin, ADP, and TXA2 activate PLC via Gq. PLC generates IP3 from membrane PIP2. Rising Ca2+ levels activate the Ras family member, Rap1B via Cal-DAG GEF. Rap1B activation is a precursor to αIIb 3 acti­vation and allows the platelets to form aggregates with other platelets through fibrinogen cross-bridging. Ca2+-dependent signaling drives myosin light chain kinase and activation of GTP binding proteins of the Rho family. Rho acti­vation in turn activates kinases like p160ROCK and 5 LIM-kinase that can phosphorylate myosin light chain kinase and cofilin to regulate actin-dependent cytoskeletal shape changes. Endothelial derived prostacyclin (PGI2) binds the IP recep­tor and causes Gs mediated increase in adenyl cyclase activity. Also, NO from the endothelium and platelets can activate guany-late cyclase resulting in elevated cGMP levels that subsequently inhibit the hydrolysis of cAMP by intracellular phosphodi-esterases. Taken together these mechanisms elevate intracellular cAMP levels, which strongly downregulate platelet signaling. Agonists coupled to Gi family members inhibit cAMP production in platelets, thus allowing activation to proceed unhindered. Additionally the âã subunits of these receptors can activate PLCâ and the ã isoform of PI3K. The effectors for PI3K include Rap1b and Akt.

Fig 1 reaction schemes for platelet signaling

FIGURE 1 | Detailed reaction schemes for platelet signaling modules. Four interconnected models were defined: (A) Ca2+ module: cytosolic and DTS compartments are separated by the DTS membrane, which contains the IP3R and SERCA. (B) Phosphoinositide (PI) module: Membrane-bound PIs are cleaved by PLC-â to form diffusible inositol phosphates and DAG, which are substrates for resynthesis of PIs. (C) PKC module: Ca2+i and DAG activate PKC, which migrates to the plasmamembrane where it phosphorylates PLC-â. (D) P2Y1 module: extracellular ADP binds to and activates P2Y1. Active P2Y1 accelerates guanine nucleotide exchange on bound Gq. The Gq·GTP binds and activates PLC-â, which increases the GTPase activity of Gq·GTP.

ADP is stored in platelet dense granules and is released upon activation. P2Y1 and P2Y12 are the primary receptors for this agonist. P2Y1 is Gq coupled and signaling through this receptor causes Ca2+ mobilization, shape change, and thromboxane generation. P2Y12 is the target of the commonly used anti-platelet drug Plavix, and is a Gi2 coupled receptor that inhibits cAMP production in platelets. Thrombin is a potent platelet agonist that causes fast mobi­lization of intracellular Ca2+, and activation of phospholipase A2 and subsequent thromboxane generation (Offermanns et al., 1997). Also, thrombin can trigger Rho dependent signaling pathways in platelets (Moers et al., 2003), that contribute to actin modeling and shape change. Thrombin signals through the protease-activated receptor (PAR) family of GPCRs. PAR1 and PAR4 are expressed on human platelets, while PAR3 and PAR4 are expressed on mouse platelets. Thrombin cleaves the N-terminus of these receptors, exposing a new N-terminus that serves as a tethered ligand for these receptors. Synthetic pep­tides are able to selectively activate these receptors and mimic the actions of thrombin (for example, SFLLRN for PAR1, and AYPGKF for PAR4). Kinetic studies have shown that the human platelet response to thrombin is biphasic and involves first signal­ing through PAR1 and subsequent signaling through PAR4 (Covic et al., 2000). In mouse platelets signaling occurs primarily via PAR4, and is facilitated by PAR3. In addition to the PAR recep­tors, GP1bá has high affinity for thrombin. Absence of GP1bá reduces responses to low doses of thrombin and diminishes PAR1 signaling, suggesting that this receptor facilitates signaling through the PARs (Dormann et al., 2000). Ca2+ mobilization also activates phospholipase A2 (PLA2), which in turn converts mem­brane phospholipids to arachidonic Acid. TXA2 is produced from membrane arachidonate by the aspirin sensitive cyclooxygenase (COX-1) enzyme. TXA2 causes Ca2+ mobilization, aggregation, secretion, phosphoinositide hydrolysis, and protein phosphoryla-tion. TXA2 can diffuse across the membrane and activate nearby platelets, but its activity is limited by the molecule’s short half life (∼30 s).

These modules use previously validated or data-consistent kinetic networks for SERCA, IP3-Receptor, PKC translocation, and GPCR signaling (Figures 1E–H). Assembling the four modules together results in a global ODE model that has 77 reactions, 132 fixed kinetic rate constants, and 70 species. Since the reaction network (Figure 1) and the kinetic parameters are fixed, the reaction topology of the model is also fixed. Such a model takes the general form: dc/dt = F(c) and c(t = 0) = co where c is a vector of all species concentrations and co is a specified initial condition vector at t = 0. To determine appropriate sets of co that are suitable for use in modeling platelets, a challenge exists that the copy number of each species in a resting platelet is not known. Imposing a homeostasis assumption results in powerful tool to define a set of acceptable co vectors. The homeostasis assumption states that a resting platelet remains resting until activated. This means that an acceptable ini­tial condition co also represents a steady state for the system and will satisfy the equation dc/dt = 0. Finding a global co involves assembling the steady state solutions of each module (Figure 2).

Fig 2 Assembly of full model from steady-state modules

FIGURE 2 | Homeostasis requirement: Assembly of full model from steady-state modules using principle component analysis (PCA). The full model is assembled by combining PCA-reduced, steady-state solution spaces from each module into a combined steady state solution space. This global space is searched for full-length, steady-state solution vectors that satisfy both the steady state requirements of each module and the desired time-dependent properties when the steady-state is perturbed. A simple linear constraint is imposed for every pair of modules that share a common molecule ci to ensure that steady state solutions are Keywords: platelet, thrombosis, hemodynamic, ADP, thromboxane consistent. To assemble the platelet signaling model, a set of 16 PC vectors representing all 72 unknown variables in the model were used as search directions in a global optimization routine. The global solution space was searched for models with accurate dynamic behavior using experimental time-series data for ADP-stimulated Ca2+ release. Species are grouped according to compartment. Color values correspond to molar concentrations (mol/L or mol/m2) or as indicated: DTS species (mol L1). †Extracellular species (mol L1). DTS volume (L). §PM leak conductance/area (S m−2).

The first phase of the method involves generating a com­pact representation of the steady-state solutions for each module. First, conservative bounds are chosen for c based on physiological and practical considerations. Also, because molecular concentra­tions can span several orders of magnitude, it is most efficient to delineate this range of values on a logarithmic scale rather than a linear scale. Once the sampling distribution for c has been defined, steady-state solutions (co = c55) for each module are cal­culated using fixed kinetic parameters for each reaction in the module. For non-oscillating systems, steady-state solutions may be obtained by simulating the system until equilibrium is reached (i.e., until dc/dt = 0). In the third step, a large collection of steady-state solutions for each module is subjected to principal component analysis (PCA) (Purvis et al., 2009). PCA is then used to transform these points to a new coordinate set that optimally covers the space of steady-state solutions using the fewest num­ber of dimensions. For example, if two molecule concentrations in the steady-state space are highly correlated due to participation in the same reaction, PCA will locate a single dimension to rep­resent each pair of points in the transformed space. Ultimately, these new dimensions will be combined across all modules to search for global solutions that lie in the steady-state space for the fully combined network. Since PCA is a linear method, a steady-state solution space that is highly nonlinear may require more principal component vectors to accurately estimate the solutions. The reduction procedure is shown for the human platelet model comprising 4 interlinked signaling modules (Figure 2). For this step, we generated more than 109 sets of initial guesses (co) for each module, computed the initial value problem for each co until a steady state was reached (dc/dt ≈ 0), and selected only those steady states (c55) that were consistent with known con­centrations (i.e., [Ca2+]o ∼100 nM).  Interestingly, only a small fraction of initial guesses produce steady-state solutions that are also consistent with known concentration values. For example, it was shown that only 50,000 of 109 initial guesses (0.005%) in the Ca2+ balance module (Figure 1A) met both requirements and were suitable for further analysis. This observation shows that the kinetic topology of these molecular networks places very strong constraints on the range of concentrations that can exist at steady state. In biological terms, this suggests that fixed kinetic proper­ties at the molecular level (e.g., IP3R and SERCA kinetics) can affect not only the dynamical features of a biochemical system but can also determine the abundance of chemical species and the compartmental structures that contain them. A fully assem­bled initial condition vector results (bottom, Figure 2) results in new hypotheses about allowable concentrations and ratios of con­centrations (i.e., IP3/SERCA ratio is very small). The allowed co = css is consistent with the known resting levels of Ca2+, IP3, P2Y1, DAG, PA, PI, PIP2, and PIP (bottom, Figure 2) as well as the stimulated response of platelets to increasing amounts of ADP (right, Figure 2). With a global simulation of P2Y1 signaling, it is possible to simulate the ADP dose-response of calcium mobiliza­tion and IP3 generation in platelets as well as the mobilization of intracellular calcium in a single platelet due to stochastic fluctuations (Figure 3).

Fig 3. P2Y1 signalink model

FIGURE 3 | Tests of P2Y1 signaling model. ADP dose response for the full platelet model from 100 nM to 10 ìM ADP for calcium mobilization (A) or IP3 generation (B). Stochastic simulation of a single platelet (C). A single, fura-2-loaded platelet was immobilized on a fibrinogen-coated coverslip and activated with 40 ìM ADP at t = 90 [Ca2+ trace from Heemskerk et al. (2001)]. After 90 s of simulated rest, the platelet model was activated by setting extracellular [ADP] to 40ìM. Simulated interval times were binned in 2s increments for direct comparison with experiment (inset).

Since many initial condition vectors can be found to allow a resting platelet to remain resting and then respond appropriately to stimulation, investigation of these multiple steady states and associated cell responses can allow an ad-hoc sensitivity analysis. Some species (flexible nodes) may vary widely in the allowed ini­tial condition vectors but have little effect on system response. In contrast, other species (rigid nodes) may be forced to take on val­ues in a very narrow range due to the kinetic constraints of the problem.

To examine the changes in steady-state properties caused by kinetic perturbations in the P2Y1 model, we altered the rates of important regulatory reactions and observed the system response to each perturbation. Each perturbation cause a brief adjustment phase lasting ∼200 s followed by a more gradual phase char­acterized by a new steady-state profile. After 1 h of simulated time, steady-state concentrations and reaction fluxes were quan­tified relative to their original steady-state levels (Figure 4). In a computational perturbation, the inhibition of phospholipase C-β (PLC-β) activity by PKC was reduced 10-fold. Since PKC has a negative-feedback role in suppressing the platelet-stimulating activity of PLC-β, this perturbation caused a 2-fold increase

in steady-state PIP2 hydrolysis, elevated IP3 concentration, and accelerated Ca2+ release. This was a compensatory effect caused by the negative feedback loop involving Ca2+-regulated activity of PKC, a resulting new hypothesis that can be probed experi­mentally. In another example, increasing the hydrolytic activity of PLC-â for the substrate PIP2 by 10-fold caused an expected stimulatory effect, raising intracellular calcium and steady-state levels of cytosolic inositol phosphates (IP3, IP2, and IP) between 2- and 3-fold. Interestingly, reaction fluxes for phosphoinositide hydrolysis were diminished, possibly due to substrate depletion. Taken together, these examples illustrate the system-wide effects of perturbations in the kinetic rate processes. The procedure could easily be extended to examine multiple simultaneous per­turbations in both reaction rates and steady-state concentrations. In future applications of this approach, genomic or proteomic information of multiple perturbations could be used to help predict platelet signaling phenotypes.

Fig 4 Shifts in steady-state profiles caused by kinetic perturbations

FIGURE 4 | Shifts in steady-state profiles caused by kinetic perturbations. The steady-state platelet model was perturbed by changing selected kinetic parameters (±10-fold) and simulating for 1 h. After approaching a new steady state, the model concentrations and fluxes were determined relative to their original steady-state values and colored according to fold-change. Green indicates no change (NC) relative to initial flux/concentration. Red indicates a relative increase and blue indicates a relative decrease. Note that the color scale in each panel is normalized separately to maximize distinctions in fold change. New steady states were achieved after (top) 10-fold decrease in PKC-mediated inhibition of PLC-β, and (bottom) 10-fold increase in PIP2 hydrolysis (10-fold increase in kcat of hydrolysis). ∗, active state.

Fig 5. predicting global calcium response

FIGURE 5 | Pairwise agonist scanning to predict global calcium response in human platelets. (A) Simplified schematic of signaling pathways examined in this study that converge on intracellular calcium release in human platelets. (B) Dynamic NN model used to train platelet response to combinatorial agonist activation. A sequence of input signals representing agonist concentrations is introduced to the network at each time point. Processing layers integrate input values with feedback signals to predict the next time point. (C) A total of 154 calcium traces were measured for single and pairwise activation using 6 different agonists (“Experiment”) and used for neural network training. The NN training accurately predicted (“NN Prediction”) the training data.

Fig 6. Multiscale modeling with 4 components

FIGURE 6 | Multiscale modeling. The multiscale model has four main components (A) fluid flow, transport of soluble species, motion and binding of platelets, and the activation state of each platelet. The fluid flow is perturbed by the growing clot and is determined using the lattice Boltzmann method. The released soluble agonists form a boundary layer in the flow, and this process is determined using the finite element method. Platelet motion and bonding are simulated with lattice kinetic Monte Carlo. Platelet activation state is estimated from the history of intracellular calcium concentration, which is determined by a neural network model. (B) Multiscale simulation of patient-specific platelet deposition under flow for a specific donor and PAS-trained neural network of calcium signaling. Platelet activation (black, unactivated; white, activated) and deposition at 500 s (inlet wall shear rate, 200 s−1) showing released ADP (top) and TXA2 (middle) and perturbation of the flow field (bottom). Flow: left to right (streamlines, black lines); surface collagen (250 ìm long): red bar.  

PLATELET INTERACTIONS WITH THE VESSEL WALL

The multiscale systems biology model accommodates platelet sig­naling, platelet adhesion to collagen and other activated platelets, release of soluble agonists, thrombus growth, and distortion of the prevailing flow field (Figure 6A). The lattice Boltzmann (LB) method is used to solve for the velocity field of the fluid. Platelets in the growing aggregate release ADP and TXA2 into the fluid, and a boundary layer is formed with the flow. The dynamics of this process are determined with a finite element method solution of the convection-diffusion-reaction equation for each of the soluble species, ADP and TXA2. Platelets move in the fluid by convection and RBC-augmented dispersion. They also bind to the collagen surface as well as previously bound platelets. The motion and binding of platelets is simulated using the convective lattice kinetic Monte Carlo (LKMC) algorithm validated for stochastic convective-diffusive particle transport (Flamm et al., 2009, 2011, 2012). The level of integrin activation and associated adhesiveness for each platelet is related to the cumulative intracellular calcium concentration. The intracellu­lar calcium concentration is determined using a NN trained on a specifc patient’s platelet PAS phenotyping experiment. Using this multiscale approach, Multiscale simulations predicted the density of platelets adherent to the surface, platelet activation states, as well as the spatiotemporal dynamics of ADP and TXA2 release, morphology of the growing aggregate, and the distribu­tion of shear along the solid-fluid boundary (Figure 6B). Platelets stick to the collagen surface and release ADP and TXA2 which forms a boundary layer extending up to 10 pm from the throm­bus. Boundary layer concentrations of up to 10 pM ADP and 0.1 pM TXA2 were found by simulation. TXA2 concentrations were found to be sub-physiological (<0.0067 pM or <0.1 xEC50) until a sufficient platelet mass accumulated at the surface after ∼250 s. Boundary layer ADP concentrations were within the effective dynamic range (0.1–10 pM) throughout the simulation. The strong temporal and spatial fluctuations in the concentration of ADP were predominately driven by the short release time (5 s), whereas the longer release time of TXA2 (100 s) smoothed fluc­tuations. The shear rate along the solid-fluid boundary became nonuniform during the simulation (5–10-fold increase above 200 s−1) due to surface roughness. At 500 s, the platelet deposit was characterized by platelet clusters 20–30 pm in length, fully consistent with microfluidic measurements of platelet cluster size on collagen at this shear rate.

Developing tools to define platelet variations between patients and the relationship of platelet phenotype to prothrombotic or bleeding traits will have significant impact in stratifying patients according to risk. This multiscale approach also makes feasible patient-specific prediction of platelet deposi­tion and drug response in more complex in vivo geometries such as stenosis, aneurysms, stented vessels, valves, bifurcations, or ves­sel rupture (for prediction of bleeding risks) or in geometries encountered in mechanical biomedical devices.

 Platelet–Leukocyte–Endothelial Cell Interactions After Middle Cerebral Artery Occlusion

Mami Ishikawa, *Dianne Cooper, *Thiruma V. Arumugam, †John H. Zhang, †Anil Nanda, and *D. Neil Granger
Departments of *Molecular and Cellular Physiology, and †Neurosurgery, Louisiana State University Health Sciences Center, Shreveport, LA
Journal of Cerebral Blood Flow & Metabolism 24:907–915 © 2004 

Summary: The adhesion of both leukocytes and platelets to microvascular endothelial cells has been implicated in the pathogenesis of ischemia/reperfusion (I/R) injury in several vascular beds. The objectives of this study were to (1) assess the platelet–leukocyte–endothelial cell interactions induced in the cerebral microvasculature by middle cerebral artery occlu­sion (MCAO)/reperfusion, and (2) define the molecular deter­minants of the prothrombogenic and inflammatory responses in this model of focal I/R. MCAO was induced for 1 hour in wild-type (WT) mice, WT mice treated with a monoclonal antibody (mAb) to either P-selectin or GPIIb/IIIa, and in P-selectin−/−(P-sel−/−) chimeras. Isolated platelets labeled with carboxyfluorescein diacetate succinimidyl ester (CFDASE) were administered intravenously and observed with intravital fluorescence microscopy. Leukocytes were observed after in­travenous injection of rhodamine 6G. One hour of MCAO fol­lowed by 1 hour of reperfusion resulted in the rolling and adhesion of leukocytes in venules, and after 4 hours of reperfusion, the adhesion of both leukocytes and platelets was de­tected. Although both the P-selectin and GPIIb/IIIa mAbs sig­nificantly reduced the adhesion of leukocytes and platelets at 4 hours of reperfusion, the antiadhesive effects of the P-selectin mAb were much greater. The leukocyte and platelet adhesion responses were significantly attenuated in both P-sel−/−-WT and WT-P-sel−/− bone marrow chimeras, compared with WT-WT chimeras. Neutropenia, induced by antineutrophil serum treatment, also reduced the recruitment of leukocytes and platelets after cerebral I/R. These findings implicate a ma­jor role for both platelet-associated and endothelial cell– associated P-selectin, as well as neutrophils in the inflamma­tory and prothrombogenic responses in the microcirculation after focal cerebral I/R.
Key Words: Platelet—Leukocyte—P-selectin—GPIIb/IIIa—Cerebral ischemia—Reperfusion.

Adhesion of leukocytes and platelets after treatment with mAb against P-selectin or GPIIIb/IIIa

The I/R-induced recruitment of rolling and adherent leuko­cytes was significantly attenuated in P-selectin mAb-treated mice, compared with the responses noted in untreated mice exposed to 1-hour MCAO and 4-hour reperfusion (Figs. 3A and 3B). However, the number of adherent leukocytes after P-selectin mAb treatment remained elevated above the level de­tected in sham experiments. Both the rolling and firm adhesion of platelets was reduced to sham levels in the P-selectin mAb-treated mice. Although treatment with a GPIIb/IIIa mAb sig­nificantly reduced the adhesion of both platelets and leukocytes after I/R, the reductions noted were relatively small compared with the responses seen with the P-selectin mAb.

Leukocyte and platelet adhesion in P-selectin–deficient bone marrow chimeras

Our findings related to the role of platelet-associated and endothelial cell–associated P-selectin in mediating the I/R-induced rolling and adhesion of leukocytes and platelets are summarized in Fig. 4.  In P-sel / -WT chimeras, the number of rolling and adherent leukocytes were significantly but not completely reduced compared with WT—WT chimeras. However, compared with WT—WT chimeras, the rolling and firm adhesion of platelets was virtually abolished after I/R. In WT—P-sel−/− chimeras, the number of rolling and adherent leu­kocytes and platelets also decreased significantly com­pare with WT—WT chimeras; however, some adhesion of leukocytes and platelets was still detected after I/R, similar to the responses noted in the group treated with the P-selectin blocking mAb.

Plateletleukocyte interaction

Platelets were noted to adhere directly onto adherent leukocytes and platelet-bearing leukocytes were occa­sionally observed rolling in postischemic venules. Some free-flowing platelets were seen to suddenly bind (with-out rolling) on adherent leukocytes. Some of these plate­lets detached from the adherent leukocyte whereas others adhered firmly on the leukocyte. Other platelets were seen to roll and adhere directly on venular endothelium. To quantify the contribution of leukocytes to I/R-induced platelet recruitment, some mice were rendered neutropenic with antineutrophil serum. Although leuko­cyte rolling and adherence were still observed in cerebral venules of serum-treated mice after I/R, the responses were dramatically reduced. The cerebral venules of neutropenic mice also exhibited large and significant reduc­tions in rolling and adherent platelets after I/R (Fig. 5).

Fig  platelet and endothelial cell–associated P-selectin in mediating rolling and adhesion of leukocytes

FIG. 4. Role of platelet-associated and endothelial cell–associated P-selectin in mediating I/R-induced rolling and adhesion of leuko­cytes (A) and platelets (B). Four or five animals were studied in each group. Mice in all groups were exposed to 1 hour of MCAO followed by 4 hours of reperfusion. WT—*WT and WT—*P-sel−/− chi­meras received CFDASE-labeled platelets from WT mice. P-sel−/−—*WT chimeras received CFDASE-labeled platelets from P-sel−/− mice. *P < 0.05 relative to the WT*WT (control) chimeras.

Signal-Dependent Protein Synthesis by Activated Platelets: New Pathways to Altered Phenotype and Function

Guy A. Zimmerman and Andrew S. Weyrich
Arterioscler Thromb Vasc Biol. 2008;28:s17-s24       http://dx.do.org/10.1161/ATVBAHA.107.160218      http://atvb.ahajournals.org/content/28/3/s17         Online ISSN: 1524-4636

New biologic activities of platelets continue to be discovered, indicating that concepts of platelet function in hemostasis, thrombosis, and inflammation require reconsideration as new paradigms evolve. Studies done over 3 decades ago demonstrated that mature circulating platelets have protein synthetic capacity, but it was thought to be low level and inconsequential. In contrast, recent discoveries demonstrate that platelets synthesize protein products with important biologic activities in a rapid and sustained fashion in response to cellular activation. This process, termed signal-dependent translation, uses a constitutive transcriptome and specialized pathways, and can alter platelet phenotype and functions in a fashion that can have clinical relevance. Signal-dependent translation and consequent protein synthesis are examples of a diverse group of posttranscriptural mechanisms in activated platelets that are now being revealed. (Arterioscler Thromb Vasc Biol. 2008;28:s17-s24)
Key Words: platelets . translation . protein synthesis . transcriptome . proteome . thrombosis

This article is part of a multi-part CME-certified activity titled Translational Therapeutics at the Platelet Vascular Interface. 

New Paradigms at the Vascular Interface

The acute hemostatic functions of platelets are well known, have dominated the attention of the field for decades, and have been the founda­tion for discoveries that generated new molecular therapies. Rapid, immediate activation responses mediate platelet-dependent thrombosis in a variety of pathologic conditions, and pharmacological antiplatelet strategies are largely aimed at these events. Nevertheless, the focus on adhesion, aggre­gation, and secretion, and the view that platelets have a repertoire of activities primarily restricted to these acute processes, have also generated a central dogma that may inappropriately limit our view of their actions at the vascular interface and in other settings in health and disease. Clearly, our understanding of the molecular mechanisms by which platelets influence hemostasis, thrombosis, regulated and dysregulated inflammation, and neoplasia remains incom­plete and continues to evolve. New paradigms are emerging as previously unrecognized pathways in platelets are identi­fied, and unanticipated activities are characterized. In this regard, the current state of the field of platelet biology may be akin to that of endothelial cells several decades ago, when endothelium was thought by most investigators and physi­cians to have a limited range of responses; on the contrary, however, when this dogma was reexamined using new approaches that included primary culture of human endothe-lium, active participation of these cells in interactions with leukocytes and a variety of other previously unrecognized functions were discovered. If the comparison is accurate, new paradigms relevant to activities of platelets at the vascular interface are likely to be reported with some frequency.

Alternative and traditional views of selected features of platelet biology are listed in the Table. There is already considerable evidence for some of the alternative themes, such as inflammatory and immune activities of platelets,10–16 whereas others are less well explored and more speculative. The remainder of this review summarizes evidence for one such functional capability not generally recognized in plate­lets until recent discoveries revealed it: synthesis of new protein products in response to cellular activation (reviewed in references5,17).

Table. New Biology of Platelets: Traditional Paradigms May Be Insufficient to Understand Platelet Activities at the Vascular Interface

Traditional View                                                                                                                                              Alternate View

Platelets are biologically simple because they are anucleate                   Platelets have specializations and biologic activities that are novel and complex. Some

and have a limited repertoire of responses.                                                                                     activities are yet to be discovered.

Platelets do not express new gene products.                                             Platelets have diverse posttranscriptional mechanisms and use a transcriptome and

specialized pathways to modify their proteome, phenotype, and functions.

Platelets are short-acting cells in clots and damaged tissue.                      Platelets can be relatively long-lived and can mediate cell-cell interactions for many

hours after initial adhesion, aggregation, and secretion.

Platelets operate exclusively in the intravascular                                               Platelets can influence critical events in the extravascular milieu in direct

compartment.                                                                                                                                            and indirect fashions.

Observations from a number of laboratories now demonstrate that physiologically relevant activation signals induce translation of proteins with impor­tant functions from constitutive or posttranscriptionally pro­cessed messenger RNAs (mRNAs) in human and murine platelets, a process that we have termed signal-dependent translation. These and other studies indicate that the platelet has intricate posttranscriptional mechanisms that allow it to alter its proteome, phenotype, and functions by accomplish­ing new protein synthesis in response to cellular activation. This capacity may allow platelets to modify the complex milieu of the vascular interface in ways that were previously unrecognized.

Essentially, all of the platelets isolated from normal subjects incorporated radiolabeled amino acids into new protein, demonstrating that this function is not a property of a subset of immature cells. Platelets from splenectomized subjects with idiopathic throm-bocytopenic purpura had increased levels of amino acid incorporation into protein, indicating that the physiological state of the subject or the age and maturity of the platelets influence protein synthesis. Extracellular factors were re­ported to alter protein synthesis by human platelets under some conditions. This provided evidence suggesting that the synthetic mechanisms involved are regulated.

The Platelet Transcriptome

Circulating human platelets have a substantial and diverse transcriptome, in addition to protein synthetic machinery. RNA-selective fluorescent dyes stain the entire population of platelets isolated from normal subjects, indicating the presence of RNA species transcribed by parent megakaryocytes. Messenger RNAs with 5′-methylguanosyl (m7G) caps and 3′ untranslated region polyadenylated tails are present, as are 18S and 28S ribosomal proteins, which are integral to the structure of ribo-somes. Early experiments with intact platelets from nor­mal subjects indicated that some of the mRNA transcripts are competent to serve as templates for proteins and have relatively long functional half lives that correlate with the lifespan of platelets in the circulation. This observation then lay fallow, for the most part, until the advent of reverse transcriptase polymer-ase chain reaction (RT-PCR) analysis and cDNA cloning meth-odologies. This infusion of new technology resulted in construction of cDNA libraries from platelet transcripts. Most recently, transcript profiling by microarray analysis and serial analysis of gene expression (SAGE) have been applied to platelets, identifying 1500 to 3000 unique transcripts in platelets from normal subjects, depending on the approach. Both cytoplasmic and mitochondrial transcripts are represented.35 There is substantial consistency between data generated by microarray analysis and SAGE, and in platelets isolated from different normal donors.

Multiple Proteins Are Synthesized by Activated Human Platelets

Although early studies indicated that platelets have protein synthetic capacity, the general concept in the field has been that it is low level, vestigial, and likely inconsequential. Several texts of hemostasis and platelet biology do not mention this function, and some commentaries conclude that platelets are simply incapable of any new protein synthesis. Consistent with the notion that platelets have low basal protein synthesis, little incorporation of the radiolabeled amino acid is detectable when freshly isolated human plate­lets are incubated with [35S] methionine under resting condi­tions in the absence of activation. However, when an activat­ing signal is delivered to platelets incubated in parallel, multiple labeled proteins are synthesized when lysates and soluble fractions are analyzed by 1-dimensional or 2-dimensional gel electrophoresis (Lindemann S, Weyrich AS, Zimmerman GA, 2001). Some of these newly synthe­sized proteins have been identified and mechanisms of their signal-dependent translation determined.

Recent findings provided clear evidence for signal-dependent (that is, induced by activating signals) translation of Bcl-3 from mRNA that is transcribed in parent megakaryocytes but is repressed, or “silenced,” in circulating platelets under resting, basal conditions. Immunocytochemical de­tection of Bcl-3 in platelets in inflamed and thrombosed human vessels in surgical specimens (Figure 1D) provided in situ evidence that the experimental observations have physi­ological and clinical relevance. We subsequently found that collagen, platelet-activating factor, ADP, and epinephrine are also agonists for signal-dependent translation in plate-lets. Collagen was recently reported to induce Bcl-3 synthesis by platelets in experiments by other investigators. The time course of Bcl-3 synthesis in response to thrombin yielded additional important insights: newly synthesized Bcl-3 could be detected in activated platelets within 15 to 30 minutes in some experiments, consistent with translation of constitutively present mRNA without a requirement for new transcription. This feature is also consistent with the biology of platelets as rapid response cells. Nevertheless, synthesis of Bcl-3 is also prolonged over many hours, indicating that platelets may have important functions in thrombi and injured vessels well beyond the first few minutes of acute activation.

We examined the effect of rapamycin and found that it completely and selectively inhibited Bcl-3 synthesis in thrombin-stimulated platelets, and also inhibited phosphorylation of 4E-BP1 assayed as a marker of mTOR activation in parallel. Pharmacological inhibition of phosphatidylinositol-3-kinase, which lies upstream from

mTOR in signaling cascades linking surface receptors to mTOR activation,49 also blocked both 4E-BP1 phosphoryla-tion and Bcl-3 synthesis.52 Together, these studies demon­strated that synthesis of Bcl-3 is controlled by mTOR and provided evidence for a new and previously unrecognized activity of mTOR as a regulator of expression of specific protein products and phenotypic changes in terminally differ­entiated cells in response to signals delivered via G protein– coupled receptors and integrins.52 This observation in platelets contributed to a parallel set of discoveries demonstrating that mTOR has similar roles in myeloid leukocytes.69–71 The find­ings also suggest that inhibition of mTOR by rapamycin may have novel therapeutic effects on gene expression by platelets and leukocytes independent of inhibition of proliferation of other cell types when this agent is applied in antiangiogenic strategies and in “drug-eluting” vascular stents in the clinic.72

Although Bcl-3 provided an index example of specialized, signal-dependent translation of a protein product in activated platelets the functional relevance of this event was not immediately obvious and was initially perplexing because the activity assigned to Bcl-3 at that time was as a transcriptional regulator. A clue lay in the domain structure of Bcl-3, which includes ankyrin repeats and proline-rich N and C termini, suggesting the possibility of multiple protein-protein interactions. Based on this information, we designed experi­ments to determine whether newly synthesized Bcl-3 interacts with other intracellular proteins. We found that Bcl-3 specifi­cally binds to the tyrosine kinase Fyn via the Fyn SH2 domain in activated platelets and transfected COS cells. Bcl-3 also associates with the actin cytoskeleton in platelets.53

Because Fyn and related intracellular tyrosine kinases influence contractile responses of activated platelets, we examined the contributions of Bcl-3 and mTOR to fibrin clot retraction. Clot retraction is proposed to stabilize thrombi and to modify thrombus remodeling and resolution. It can be modeled in vitro, where activated platelets retract and condense fibrin strands in a fashion that can be examined macroscopically and microscopically (Figure 1E). In paral­lel loss-of-function and gain-of-function strategies, inhibition of mTOR activity in human platelets using rapamycin under conditions that block Bcl-3 synthesis inhibited clot retraction,

Common and Specialized Elements in Platelet Translational Pathways and Transcripts

Biologic Advantages of Signal-Dependent Translation, and Potential Roles in Disease

Novel Pathways to Signal-Dependent Translation in Activated Human Platelets

Activated Platelets Synthesize Additional Proteins Under Signal-Dependent Control

Translation in Activated Human Platelets

Discovery of synthesis of Bcl-3 by activated platelets sparked a search for the identities of other protein products, yielding IL-1J3 and TF. It also led to the unexpected discovery that their synthesis is preceded by signal-dependent cytoplasmic splicing of IL-1J3 and TF pre-mRNAs, yielding mature transcripts that are translated into precursor (IL-1J3) and active (TF) proteins.24,43,44 This identified a novel mechanism not previously recognized in activated mammalian cells. The splicing capacities of activated platelets are intricate and will be reviewed separately. Signal-dependent splicing, to­gether with the mTOR-dependent translational control mech­anism and other regulatory pathways discussed here, indicate that platelets have unexpected diversity in posttranscriptional control. Previous and ongoing studies add to this conclusion and suggest that platelets may also use ribosomal “stalling” or polypeptide termination, participation of micro RNAs (Denis MM, Trask B, Schwertz H, Weyrich AS, Zimmerman GA, 2004) and, potentially, other modes of control.

References

  1. Lindemann S, McIntyre TM, Prescott SM, Zimmerman GA, Weyrich AS. Platelet signal-dependent protein synthesis. In: Quinn M, Fitzgerald D, eds. Platelet Function: Assessment, Diagnosis, and Treatment. Totowa, NJ: Humana Press Inc.;2005:149-74.
  2. Weyrich AS, Lindemann S, Tolley ND, Kraiss LW, Dixon DA, Mahoney TM, Prescott SP, McIntyre TM, Zimmerman GA. Change in protein phenotype without a nucleus: translational control in platelets. Semin Thromb Hemost. 2004;30:491–498.

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nihms-292073-f0002  platelet and vessel

Protein_Slide_2  proteome

nihms-292073-f0001  platelets support integrity and barrier function

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Reporter: Aviva Lev-Ari, PhD, RN

 

Stroke and Bleeding in Atrial Fibrillation with Chronic Kidney Disease

Original Article

Stroke and Bleeding in Atrial Fibrillation with Chronic Kidney Disease

J.B. Olesen and Others

The prevalence of both atrial fibrillation and chronic kidney disease increases with age. The prevalence of atrial fibrillation is 2.3% among persons 40 years of age or older and 5.9% among those 65 years of age or older, and the prevalence of end-stage renal disease increases from approximately 3.5% among persons 45 to 64 years of age to nearly 6% among those 75 years of age or older. Many patients have both disorders, and the number of such patients is increasing, owing in part to the aging population and the improved survival in both diseases.

Clinical Pearls

  What is the effect of chronic kidney disease on the risk of stroke?

The U.S.-based Renal Data System has reported that chronic kidney disease increases the risk of stroke by a factor of 3.7, and end-stage renal disease (i.e., disease requiring renal-replacement therapy) increases the risk by a factor of 5.8. Atrial fibrillation increases the risk of stroke by a factor of 5, and chronic kidney disease increases the risk of stroke among patients without atrial fibrillation.

  What is a CHA2DS2-VASc score?

This study evaluated the risk of stroke or systemic thromboembolism, with adjustment for CHA2DS2-VASc risk factors. The CHA2DS2-VASc score extends the CHADS2 algorithm to include additional nonmajor risk factors for stroke, including vascular disease (V), age between 65 to 74 years (A), and female gender (sex category or Sc).

Morning Report Questions

Q. What were the results of this study of patients with atrial fibrillation and chronic kidney disease with respect to risk of stroke or systemic embolism? 

A. This study demonstrated that warfarin treatment was associated with a significantly decreased risk of stroke or systemic thromboembolism overall and among patients requiring renal-replacement therapy, and with a nonsignificantly decreased risk among patients with non–end-stage chronic kidney disease. In an analysis that compared all patients who had any renal disease with those who had no renal disease, warfarin decreased the risk of stroke or systemic thromboembolism (hazard ratio, 0.76; 95% CI, 0.64 to 0.91; P=0.003), as did warfarin plus aspirin (hazard ratio, 0.74; 95% CI, 0.56 to 0.98; P=0.04). Aspirin alone was associated with an increased risk of stroke or systemic thromboembolism overall and among patients who had any renal disease, as compared with those who had no renal disease (hazard ratio, 1.17; 95% CI, 1.01 to 1.35; P=0.04).

Table 3. Hazard Ratios for Stroke or Systemic Thromboembolism.

Q. How did the risk of bleeding differ among patients with and without kidney disease? 

A. The risk of bleeding was higher among patients with non–end-stage chronic kidney disease and among patients requiring renal-replacement therapy as compared to patients without renal disease, and treatment with warfarin, aspirin, or both incrementally increased this risk. Among all patients who had any renal disease, as compared with those who had no renal disease, there was an increased risk of bleeding with warfarin (hazard ratio, 1.33; 95% CI, 1.16 to 1.53; P<0.001), aspirin (hazard ratio, 1.17; 95% CI, 1.02 to 1.34; P=0.03), and warfarin plus aspirin (hazard ratio, 1.61; 95% CI, 1.32 to 1.96; P<0.001). Among patients with non–end-stage chronic kidney disease, the risk of bleeding increased with a higher dose of loop diuretics. The risk of bleeding was highest among patients with chronic glomerulonephritis and lowest among those with chronic tubulointerstitial nephropathy.

Table 4. Hazard Ratios for Bleeding.

 Original article

Stroke and Bleeding in Atrial Fibrillation with Chronic Kidney Disease

Jonas Bjerring Olesen, M.D., Gregory Y.H. Lip, M.D., Anne-Lise Kamper, M.D., D.M.Sc., Kristine Hommel, M.D., Lars Køber, M.D., D.M.Sc., Deirdre A. Lane, Ph.D., Jesper Lindhardsen, M.D., Gunnar Hilmar Gislason, M.D., Ph.D., and Christian Torp-Pedersen, M.D., D.M.Sc.

N Engl J Med 2012; 367:625-635  August 16, 2012

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