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Reporter: Aviva Lev-Ari, PhD, RN

Prostacyclin and Nitric Oxide: Adventures in vascular biology –  a tale of two mediators

The e-Readers are encouraged to review two additional Sources on this topic on this Open Access Online Scientific Journal

Perspectives on Nitric Oxide in Disease Mechanisms

 and

Interaction of Nitric Oxide and Prostacyclin in Vascular Endothelium

S Moncada*

The Wolfson Institute for Biomedical Research, University College London, Gower Street, London WC1E 6BT, UK
* (Email: s.moncada@ucl.ac.uk)

Prof. Moncada:

I would like to thank the Royal Society for inviting me to deliver the Croonian Lecture. In so doing, the Society is adding my name to a list of very distinguished scientists who, since 1738, have preceded me in this task. This is, indeed, a great honour.

For most of my research career my main interest has been the understanding of the normal functioning of the blood vessel wall and the way this is affected in pathology. During this time, our knowledge of these subjects has grown to such an extent that many people now believe that the conquering of vascular disease is a real possibility in the foreseeable future.

My lecture concerns the discovery of two substances, prostacyclin and nitric oxide. I would like to describe the moments of insight and some of the critical experiments that contributed significantly to the uncovering of their roles in vascular biology. The process was often adventurous, hence the title of this lecture. It is the excitement of the adventure that I would like to convey in the text that follows.

Keywords: prostacyclin, aspirin, nitric oxide, oxidative stress, free radicals, cardiovascular pathology
Full article 
Philos Trans R Soc Lond B Biol Sci. 2006 May 29; 361(1469): 735–759.
Published online 2006 February 8. doi:  10.1098/rstb.2005.1775
PMCID: PMC1609404

9. THE TWO STORIES CONVERGE

Although the research fields of prostacyclin/thromboxane and NO are now mature, they have developed mostly as parallel research activities with few points of contact between them. Thus, our understanding of how both might operate in relation to each other in physiology and pathophysiology remains to be developed. Table 2 shows some of the similarities between prostacyclin and NO. Both mediators, from very different biochemical pathways, play a variety of roles in the modulation and protection of the vascular wall. The release of both mediators is dependent on constitutive enzymes, the activity of which seems to be regulated locally, predominantly by the shear stress caused by the blood passing over the endothelial surface (Grabowski et al. 1985Frangos et al. 1985; for review see Boo & Jo 2003). However, while the constitutive eNOS—localized only in the vascular endothelium—is the enzyme that responds to shear stress, the generation of prostacyclin is dependent on the activity of two enzymes, COX-1 and COX-2, in relation to which several questions remain unanswered. These include whether COX-2 is a constitutive as well as an inducible enzyme, and whether COX-1 or COX-2, or both, respond to shear stress by increases in their mRNA, their activity, or both (Topper et al. 1996Okahara et al. 1998;McCormick et al. 2000Garcia-Cardena et al. 2001). Prostacyclin, unlike NO, is constitutively generated throughout the vessel wall (Moncada et al. 1977c) and at this stage we also do not know whether the ratio between COX-1 and COX-2 changes in the different layers. In addition, the similarities and differences between regulation of NO and prostacyclin by shear stress are only now being investigated (Osanai et al. 2000McAllister et al. 2000Walshe et al. 2005).

Table 2

Table 2

Comparison of the properties of nitric oxide and prostacyclin.

A clear synergism between NO and prostacyclin has been demonstrated in regard to inhibition of platelet aggregation; however, only one of them (NO) plays a role in inhibiting platelet adhesion. The significance of this difference remains to be understood. Many years ago a physiological role for platelets in repairing the vessel wall was investigated (for discussion see Higgs et al. 1978). This subject has not been re-evaluated in the light of all this new knowledge about the roles of NO and prostacyclin in platelet/vessel wall interactions. Both mediators also regulate vascular smooth muscle proliferation and white cell vessel wall interactions through similar mechanisms which include, at least in part, the activation of adenylate cyclase and the soluble guanylate cyclase. The interactions between NO and prostacyclin in the control of these functions are not fully understood.

Both mediators are further increased by inflammatory stimuli; however, while in the case of prostacyclin the same COX-2 which responds to shear stress responds to such stimuli by a further increase in its expression, NO is generated during inflammation by a specific ‘inducible’ NO synthase which is not normally present physiologically in the vessel wall. The induction of both is inhibited by anti-inflammatory glucocorticoids (Axelrod 1983Knowles et al. 1990). It is remarkable that both compounds possess antioxidant properties (Wink et al. 1995Egan et al. 2004) but are themselves affected by oxidative stress, which inhibits the synthesis of prostacyclin and decreases the bioavailability of NO. This mechanism might be relevant to the ‘malfunctioning’ of the constitutive generation of both mediators and therefore to the genesis of endothelial dysfunction. This, however, is an early phenomenon. In advanced disease the situation is far more complex, akin to chronic inflammation in other parts of the body and, as such, probably varies significantly in the different stages of the disease. A simple hypothesis would suggest that any amount of prostacyclin which is bioavailable, although pro-inflammatory, will provide anti-thrombotic protection, while in the case of NO the balance will vary between bioavailable NO which is protective and cytotoxic peroxynitrite formed from the interaction of NO with O2. Currently, however, the results are not clear and on the crucial question of the role of both mediators in the progression of atherosclerosis, the information in relation to prostacyclin is contradictory (Burleigh et al. 2002Olesen et al. 2002Rott et al. 2003). The evidence in relation to NO, on the other hand, seems to suggest that, while constitutive NO generated by eNOS is protective (e.g. Kawashima & Yokoyama 2004), NO generated by the inducible enzyme favours the development of atherosclerosis (Chyu et al. 1999). Studies of genetically manipulated animals are providing some important clues. For example, knockout of the prostacyclin receptor (IP) leads to mice with normal blood pressure but an increased tendency to thrombosis when the endothelium is damaged (Murata et al. 1997) These animals also exhibit an increased platelet activation and proliferative response to injury that can be prevented by deletion or antagonism of the TXA2 receptor (Cheng et al. 2002). Furthermore, deletion of the IP receptor in animals prone to spontaneous atherosclerosis accelerates the development of the disease (Egan et al. 2004;Kobayashi et al. 2004). On the other hand, knocking out the thromboxane receptor or the thromboxane synthase gives rise to a mild bleeding tendency and a resistance to platelet aggregation and sudden death induced by arachidonic acid infusion (Thomas et al. 1998Yu et al. 2004). Deletion of the thromboxane receptor also seems to retard atherogenesis in murine models of atherosclerosis (Cayatte et al. 2000;Egan et al. 2005).

Although the lack of either mediator has been shown to increase the risk of thrombosis and atherosclerosis, especially in animals with additional risk factors such as ApoE deficiencies (Kuhlencordtet al. 2001Belton et al. 2003), there seems to be a certain specialization in their actions, so that NO has a more significant role in the regulation of blood pressure and blood flow, while prostacyclin has a clearer role in regulating platelet/vessel wall interactions. For example, inhibition of NO generation has an immediate and dramatic effect on blood flow and blood pressure and the eNOS−/− animal exhibits a clear hypertensive phenotype. On the other hand, inhibition of prostacyclin synthesis by the coxibs leads to a slow effect on blood pressure and apparently to a more thrombotic situation (Muscara et al. 2000;FitzGerald 2003). Similarly, COX-1−/− and COX-2−/− animals show no change in blood pressure (Norwood et al. 2000Cheung et al. 2002) and manipulation of COX or IP results in a prothrombotic phenotype.

Protection against decreases in the generation of constitutive NO and prostacyclin in the vasculature may prevent the development of vascular disease. In relation to NO, the most often tried interventions relate to the use of antioxidants (see Carr & Frei 2000) and the manipulation of eNOS expression by genetic means (Von der Leyen & Dzau 2001). Each of these interventions has shown promise in both animal experiments and in humans. An unexpected and highly interesting development relates to the effects of statins which, in the last few years, have been shown to increase the production of endothelial NO in endothelial cell cultures and in animals (for review see Laufs 2003). Many mechanisms have been claimed for this action. However, of interest in the context of our discussion is the fact that statins have been claimed to reduce oxidative stress by increasing the synthesis of BH4 (Hattori et al. 2002), increasing the coupling of the eNOS (Brouet et al. 2001) or reducing the activation of NADPH oxidase (Wagner et al. 2000). Reduction of oxidative stress is likely to preserve the generation of prostacyclin, and to our knowledge there is at least one report suggesting that statins also increase prostacyclin in endothelial cell cultures of human coronary arteries (Mueck et al. 2001). Studies on the transfection of COX-1 or COX-2 into endothelial and other cells, on the other hand, are at an early stage and clear results are not conclusive (Murakami et al. 1999Shyue et al. 2001). The full consequences of overexpression of both NO and prostacyclin in the vasculature remain to be investigated.

Also relevant to this discussion are studies of the role that NO and prostacyclin play in the protection of the cardiovascular system provided by oestrogens, and therefore in the difference between genders in susceptibility to cardiovascular disease. Oestrogens increase the expression and the activity of eNOS (Weiner et al. 1994Yang et al. 2000) and the activity of the COX-2 enzyme (Akarasereenont et al. 2000;Egan et al. 2004). They could therefore reduce oxidative stress by simply increasing both mediators. Alternatively, it has been claimed that oestrogens increase the efficiency of the NO synthase, thus reducing free radical formation (Barbacanne et al. 1999).

In summary, the concept of the balance between prostacyclin and TXA2 has to be expanded to include NO. Furthermore, although not discussed in this review, the way in which these compounds interact with many other systems known to be involved in vessel wall physiology and pathophysiology requires further investigation. Both prostacyclin and NO synergize in the protection of the vessel wall. TXA2, however, lies on the negative side of this balance being responsible for, among other things, platelet aggregation and vasoconstriction. The investigation into the interplay between these three molecules is just beginning. This is a sobering thought when one is contemplating probably close to 100 000 papers and over 30 years of research! However, it is clear that the discoveries of prostacyclin and NO have transformed our comprehension of vascular physiology and opened avenues for further understanding of pathophysiological processes. This knowledge has already benefited clinical medicine and no doubt will continue providing clues that will guide future therapy and prevention of vascular disease. I have had the good fortune to be intimately involved with both discoveries. More importantly, many of the colleagues that I have interacted with in the process of doing this work have become life-long personal friends. To those with whom I have managed to combine scientific excitement with friendship I owe a double debt of gratitude.

Philos Trans R Soc Lond B Biol Sci. 2006 May 29; 361(1469): 735–759.
Published online 2006 February 8. doi:  10.1098/rstb.2005.1775

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Curated/reported by : Aviral Vatsa PhD, MBBS

Based on : S Moncada et al

It was in 1980 that Furchgott & Zawadzki first described endothelium- dependent relaxation of the blood vessels by acetylcholine. Further studies in 1984 revealed that other factors such as bradykinin, histamine and 5-hydroxytryptamine release endothelium derived relaxing factor (EDRF), which can modulate vessel tone. EDRF was shown to stimulate soluble guanylate cyclase and was inhibited by haemoglobin. In 1986 it was demonstrated that superoxide (O2) anions mediated EDRF inactivation and that the inhibitors of EDRF generated superoxide (O2) anions in solution as a mean to inhibit EDRF. It was later established that all compounds that inhibit EDRF have one property in common, redox activity, which accounted for their inhibitory action on EDRF. One exception was haemoglobin, which inactivates EDRF by binding to it. In 1987 Furchgott proposed that EDRF might be nitric oxide (NO) based on a study of the transient relaxations of endothelium-denuded rings of rabbit aorta to ‘acidified’ inorganic nitrite (NO) solutions and the observations that superoxide dismutase (SOD, which removes O2) protected EDRF. Till then NO was not known to be produced in mammalian cells. In 1988 Palmer et al could detect NO production both biologically and chemically by chemiluminescence. The following year in 1989 the enzyme responsible for NO production, NO synthase, was discovered and L-arginine:NO pathway was proposed.

Roles of L-arginine:NO pathway

By 1987 it was proposed that NO is generated in tissues other than endothelium. Hibbs et al and Marletta et al proposed that NO was generated by macrophages. Moreover release of EDRF was demonstrated in cerebellar cells following activation with N-methyl-D- aspartate (NMDA ). Both noradrenergic and cholinergic responses are ‘controlled’ by the nitrergic system so that the release of NO (e.g., during electrical field stimulation) counteracts and dominates the response to the noradrenergic or cholinergic stimulus (Cellek & Moncada, 1997). Mechanism of penile erection was unveiled by the studies on nitrergic neurotransmission that led to therapeutic intervention. Selective damage of nitrergic nerves in disease states was proposed as a potent mechanism of pathophysiology. Broadly three areas of research based on three isoforms of NOS came into being;

  • cardiovascular
  • nervous
  • immunology

Identification of NG-monomethyl-L-arginine (L-NMMA) as an inhibitor of the synthesis of NO lay the basis of future research into investigating the role of NO in biological systems. In 1989 it was demonstrated that intravenous infusion of L-NMMA resulted in increase in blood pressure that was reversible by infusing L-arginine. NO was thus implicated in constantly maintaining blood vessel tone. eNOS knockout studies showed a hypertensive phenotypes in the animal models and over expression of eNOS led to lowering of the blood pressure. Furthermore, eNOS activation was attributed to phosphorylation of a specific tyrosine residue in the enzyme.

NO and Mitochondria 

https://pharmaceuticalintelligence.com/2012/09/16/nitric-oxide-has-a-ubiquitous-role-in-the-regulation-of-glycolysis-with-a-concomitant-influence-on-mitochondrial-function/

NO reacts with some of the complexes of the respiratory chain, and inhibits mitochondrial respiration – this is a well accepted notion. Initially it was believed that the target for NO was soluble guanylate cyclase, which in vasculature would lead to elevation of cGMP that eventually results in NO mediated vasodilatation and platelet aggregation inhibition. In 1994, another potential target, cytochrome c oxidase, for inhibitory effects of NO was discovered. This was a reversible effect, in competition with oxygen concentrations. Increases in NO production were also shown to inhibit cellular respiration irreversibly by selectively inhibiting complex I . Hence in 2002 it was proposed that this might be a mechanism through which cell pathology was initiated in certain conditions. Furthermore, NO was proposed to be implicated in the activation of the grp78-dependent stress response , via modulating calcium-related interaction between mitochondria and endoplasmic reticulum . This host defence mechanism might also have role in vasculature. Further evidence was provided in 2003 to link the role of NO in mitochondrogenesis and thus indicating that NO might be involved in the regulation of the balance between glycolysis and oxidative phosphorylation in cells.

NO and Pathophysiology

Lack of NO: By 2000, NO was established as a haemostatic regulator in the vasculature. Its absence was implicated in pathological states such as hypertension and vasospasm. These pathophysiological states share a common beginning of endothelial dysfunction, which has low NO production as one of its characterstic features. This dysfunction has been observed prior to the appearance of cardiovascular disease in predisposed subjects with family history of essential hypertension and atherosclerosis. The most likely mechanism for endothelial dysfunction is that of a reduced bioavailability of NO . The mechanism of this aspect is discussed elsewhere on this site. Protection against reduction of NO bio-availability in the vasculature is a vital therapeutic target and is extensively explored. This can be achieved by the use of antioxidants and/or augmentation of eNOS expression. In 2003 statins were shown to increase the production of endothelial NO in endothelial cell cultures and in animals by the reduction of oxidative stress or by increasing the coupling of the eNOS. It was way back in 1994 that oestrogen was shown to increase both the activity and expression of eNOS. In addition, more recently in 2003, oestrogen was shown to reduce the breakdown of available NO.

Excess of NO: In 2000 it was shown that NO produced from iNOS in vasculature is involved in extensive vasodilatation in septic shock. Later it was demonstrated that inhibition of mitochondrial respiration is an important component of the NO-induced tissue damage. This inhibition of respiration, which is initially NO-dependent and reversible, becomes persistent with time as a result of oxidative stress . Such metabolic hypoxic states where in tissues cannot utilise available oxygen due to NO, could also contribute to other inflammatory and degenerative conditions. An obvious therapeutic target for reducing NO production in such conditions would be L-NMMA. L-NMM was tested in a clinical trial for septic shock in 2004. The results were however disappointing probably due to the blanket reduction in NO production from other NOS enzymes there by having deleterious effects on the treatment group. More specific inhibitors for NOS forms are being investigated for in different disease states.

In conclusion, the L-arginine: NO pathway has had a major impact in many areas of research, specially vascular biology. A lot has been understood about this pathway and its interactions, therapeutic targets are being aggressively investigated, but further investigations are required to delineate further the role of NO in human health and disease.

Further Reading

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1760731/?tool=pubmed

Nitric Oxide and Platelet Aggregation

Inhaled NO in Pulmonary Artery Hypertension and Right Sided Heart Failure

Cardiovascular Disease (CVD) and the Role of agent alternatives in endothelial Nitric Oxide Synthase (eNOS) Activation and Nitric Oxide Production

Nitric Oxide in bone metabolism

Nitric oxide and signalling pathways

Rationale of NO use in hypertension and heart failure

Interaction of Nitric Oxide and Prostacyclin in Vascular Endothelium

Nitric Oxide has a ubiquitous role in the regulation of glycolysis -with a concomitant influence on mitochondrial function

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Authour/Curator: Aviral Vatsa PhD MBBS

This post is in continuation with posts on basics of NO metabolism and its effect on physiology. Other topics are covered under the following posts.

  1. Nitric Oxide and Platelet Aggregation

  2. Inhaled NO in Pulmonary Artery Hypertension and Right Sided Heart Failure

  3. Cardiovascular Disease (CVD) and the Role of agent alternatives in endothelial Nitric Oxide Synthase (eNOS) Activation and Nitric Oxide Production

  4. Nitric Oxide in bone metabolism

  5. Nitric oxide and signalling pathways

  6. Rationale of NO use in hypertension and heart failure

Nitric oxide plays wide variety of roles in cardiovascular system and acts as a central point for signal transduction pathway in endothelium. NO modulates vascular tone, fibrinolysis, blood pressure and proliferation of vascular smooth muscles. In cardiovascular system disruption of NO pathways or alterations in NO production can result in preponderance to hypertension, hypercholesterolemia, diabetes mellitus, atherosclerosis and thrombosis. The three enzyme isoforms of NO synthase family are responsible for generating NO in different tissues under various circumstances. The endothelial NOS (eNOS) is expressed in endothelial cells, the inducible NOS (iNOS) is expressed in macrophages and neuronal NOS (nNOS) is expressed in certain neurons and skeletal muscle. Although the basic mechanism of action for NO production is the same for all three NOS isoforms, yet deficiencies of each one of them manifest differently or with varying severity in the body e.g. eNOS deficiency might lead to hypertension, more severe form of vascular injury to cerebral ischaemia and more severe form of atherosclerosis induced by hypercholesterolemic diet whereas nNOS deficiency might show less severe form of vascular injury to cerebral ischaemia and absence of iNOS might lead to reduced hypotension in septic shock.

Reduction in NO production is implicated as one of the initial factors in initiating endothelial dysfunction. This reduction could be due to

  • reduction in eNOS production

  • reduction in eNOS enzymatic activity

  • reduced bioavailability of NO

eNOS production is increased by physiological sheer stress on endothelial cells resulting from normal flow of blood along the arterial walls. Alterations in fluid sheer stress patterns e.g due to arterial constriction has been shown to have detrimental effect on eNOS production in endothelial cells. eNOS production is decreased by LDL, angiotensin II and TNF alpha. eNOS is tightly coupled enzyme and its activity can be significantly reduced by reduction in availability of cofactors and substrates, and by competitive inhibitors such as ADMA. Furthermore, uncoupling of eNOS can result in increased production of reactive species of both oxygen (superoxide) and nitrogen (peroxinitrite), which inturn can further reduce eNOS bioavailability. A range of therapeutic targets aim at increasing bioavailability of eNOS and they are summarised here.

Increased production of ROS and peroxinitrite is associated with endothelial dysfunction. Coronary heart disease risk factors may increase NOS mediated ROS formation and peroxinitrite formation. Such risk factors are associated with decreased NO production levels in the vasculature. However, recent data suggests that reduction in bioavailable NO levels in the arteries could be due to increased local oxidative stress rather than reduction in basal NO production.

Oxidation dependent mechanisms have been implicated in endothelial dysfunction. Oxidized low density lipo-proteins (oxLDL) play an important role in early endothelial dysfunction and hence early atherosclerosis (see figure below)

oxLDL can uncouple eNOS and reduced uptake of L-ariginine that can lead to production of superoxide radical oxygen. OxLDL can interfere with NO production and lead to altered NO signalling in the vascular endothelium. In addition, different arteries can be affected differently by these physiological changes e.g. oxLDL affects carotid artery and not the basilar artery thereby implying that intracranial arteries might be protected from endothelium-mediated oxidative injury and hence atherosclerosis. And finally NO can modulate oxidation mediated apoptotic signals in the vessel wall. Hence atherosclerosis can result from the derangement of fine imbalance between NO bioavailability and local oxidative stress.

Therapeutic targets:

There are various pathways being targeted to modulate the bioavailability of eNOS and NO such as

  1. Recoupling of eNOS to cofactors and substrates

  2. Modulation of eNOS activity by genomic and non genomic mechanisms e.g. by statins, ACE inhibitors, angiotensin II receptor blockers, calcium channel blockers , KLF2 modulators

  3. The suppression of inflammatory signalling pathways by PPAR-α activation

  4. Modulation of caeviolin mediated endocytosis and thus dissociation of eNOS from caevolin

The above list is not exhaustive, but this post here summarises recent developments in therapeutic targets in NO /eNOS regulation.

Based on:

Nitric Oxide and Pathogenic Mechanisms Involved in the Development of Vascular Diseases

Claudio Napoli and Louis J. Ignarro

Arch Pharm Res Vol 32, No 8, 1103-1108, 2009 , DOI 10.1007/s12272-009-1801-1 

 

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Curator: Aviral Vatsa, PhD, MBBS

Systemic sclerosis (SSc) is a type of autoimmune disease when the body’s immune system attacks and destroys body’s healthy tissue. It is characterised by lesions in the vessels and accumulation of collagen in the tissues. Although the pathogenesis of this disease is not clear, but one of the suggestions is that the endothelium fails to produce NO upon cold stimulation. Physiologically, NO acts as a vasodilator and its deficiency has been implicated in diseases such as hypertension and atherosclerosis.

In the body NO is generated when L-arginine is converted to L-citruline in the presence of NO synthase (NOS) enzyme, molecular oxygen, NADPH, and other cofactors. Principally, three isoenzymes of NOS are present in the body to catalyse the production of NO in various anatomic locations and under various physiological conditions. Three distinct genes encode for the three types of NOS i.e. endothelial (eNOS or NOS-3), neuronal (nNOS or NOS-1), and inducible (iNOS or NOS-2) NOS.

The inducible type 2 NOS (iNOS) may act as an immunoregulator. Several reports have provided evidence for the existence of a NO pathway in human mononuclear cells.

It is not well established if NO production increases or decreases in SSc patients. In one study by Allanore et al, NO production was shown to be reduced in plasma and PBMC supernatants, and iNOS synthesis in PBMCs.

The authors of this study investigated NO metabolites in plasma and PBMC supernatants, and iNOS synthesis in PBMC to see if the level of NO production by peripheral blood mononuclear cells (PBMC) was low in SSc, as this might contribute to the vasodilatory abnormalities observed in this disease.

Eighteen patients with SSc were compared with two control groups: 16 patients with rheumatoid arthritis (RA) and 23 patients with mechanical sciatica.The NO metabolites nitrite and nitrates were determined by flurometeric and spectrometeric assays respectively. iNOS expression was determined by using monoclonal anti‐NOS2 antibody and FACS analyses.

The data suggested a decrease in plasma NO concentration and iNOS production in PBMC in patients with systemic sclerosis as compared with patients with rheumatoid arthritis and sciatica. Subgroup analysis showed no difference between limited and diffuse SSc forms. Total plasma nitrite concentrations in five healthy volunteers were similar to those in patients with sciatica, which is consistent with this group being an appropriate control group.

Thus the authors suggest that low NO production in Ssc patients might be involved in the tendency towards vasospam.

However in other studies authors have shown an increase in NO production in SSc patients. Takagi et al set out to investigate this discrepancy in NO production in SSc patients. They sought to determine whether increased NO levels are associated with various clinical subsets of SSc patients, and to assess the contribution of fibroblasts in skin lesions to NO synthesis.

In this study Takagi et al measured the levels of serum NO metabolites in SSc patients and determined the contribution of the excessive production of NO synthase (NOS)-2 by skin fibroblasts to NO synthesis. Serum NO levels of 45 patients with SSc were significantly higher than those of 20 healthy volunteers. In addition, some clinical features of SSc (the extent of skin fibrosis, short disease duration, and the complication of active fibrosing alveolitis) were all correlated positively with the levels of NO metabolites in SSc patients. RT PCR was used to determine NOS-2 mRNA expression levels in cultured fibroblasts derived from SSc patients.

The authors showed that serum NO levels were significantly elevated in patients with SSc as compared to healthy normal controls. They also demonstrated that NOS-2 was produced spontaneously by cultured SSc fibroblasts, suggesting that increased serum NO levels might reflect in part the elevated expression of NOS-2 by fibroblasts derived from SSc patients.

The discrepancy in NO production could be explained by disease stage, severity of tissue fibrosis and various circumstances of endothelial damage. Takagi et al found increased NO production in early stages of SSc with tissue fibrosis and not in later stages of the disease. Hence they suggest that NO levels may be a sensitive marker of the early stages of the development of severe tissue fibrosis in SSc patients, although a longitudinal and prospective study is needed to confirm this.

NO is known to have dual functions in the body, both beneficial and cyototoxic. Generally it depends upon the concentration and the duration of NO production. Similarily in SSC, the dual functions of NO seem to be both beneficial (as a vasodilator) and harmful (as a cytotoxic effector) in regard to the clinical manifestations of SSc. One of the limitations of these studies is that they did not investigate the absolute concentrations of NO production but only its metaoblites were measured. This might be due the fact that NO is a short-lived molecule (half-life < 5 s) and it is challenging to quantify NO production at single cell level. Such techniques (e.g.DAR 4M AM flurophore) have been developed but are challenging to apply to various experimental set ups. DAR 4M AM has been used to quantify NO production online in single cells. In SSc determination of absolute NO concentrations at cellular or tissue level at various stages of the disease process will go a long way in solving the discrepancy of NO production in SSc patients.

Sources

Low levels of nitric oxide (NO) in systemic sclerosis: inducible NO synthase production is decreased in cultured peripheral blood monocyte/macrophage cells. Y. Allanore, D. Borderie, P. Hilliquin, A. Hernvann, M. Levacher, H. Lemaréchal, O. G. Ekindjian, and A. Kahan. (2001) 40(10): 1089-1096 doi:10.1093/rheumatology/40.10.1089.

Serum nitric oxide (NO) levels in systemic sclerosis patients: correlation between NO levels and clinical features. K. Takagi, Y. Kawaguchi, M. Hara, T. Sugiura, M. Harigai, N. Kamatani . Article first published online: 24 NOV 2003. DOI: 10.1111/j.1365-2249.2003.02320.x Clinical & Experimental Immunology: Volume 134, Issue 3, pages 538–544, December 2003

Extracellular NO signalling from a mechanically stimulated osteocyte. Aviral Vatsa, Theo H. Smit, Jenneke Klein-Nulend. Journal of biomechanics 1 January 2007 (volume 40 issue Pages S89-S95 DOI: 10.1016/j.jbiomech.2007.02.015)

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Bystolic’s generic Nebivolol – Positive Effect on circulating Endothelial  Progenitor Cells Endogenous Augmentation

Curator: Aviva Lev-Ari, PhD, RN

 

Bystolic’s generic Nebivolol – FDA approved for Treatment of Hypertension since 2008 – Pharmacological agent hypothesized to have positive effect on circulating Endothelial  Progenitor Cells (cEPCs) endogenous augmentation: Low number of cEPCs found to be associated with high Macrovascular Risk Events

Induction of NO Production and Stimulation of eNOS

Mechanism of Action (MOA) for Nitric Oxide (NO) and endothelial Nitric Oxide Syntase (eNOS) are described in George T. and P. Ramwell, (2004). Nitric Oxide, Donors, & Inhibitors. Chapter 19 in Katzung, BG., Basic & Clinical Pharmacology. McGraw-Hill, 9th Edition, pp. 313 – 318

http://books.google.com/books/about/Basic_and_Clinical_Pharmacology.html?id=4O7ghcthkt4C

Nitric oxide (NO) is a relative newcomer to pharmacology, as the paper which initiated the field was published only 25 years ago. In 2006, it is known that Arginine-vasopressin (AVP) is a hormone that is essential for both osmotic and cardiovascular homeostasis and exerts physiological regulation through three receptors, It causes a decrease in BP which occurs through mediated release of NO from the vascular endothelium (Koshimizu et al., 2006).

Dr. S. H. Snyder of Johns Hopkins University has established gases as a new class of neurotransmitters, beginning with his demonstrating the role of nitric oxide in mediating glutamate synaptic transmission and neurotoxicity. His isolation and molecular cloning of nitric oxide synthase led to major insights into the neurotransmitter functions of nitric oxide throughout the body. http://nrc88.nas.edu/pnas_search/memberDetails.aspx?ctID=50282

http://www.pnas.org/content/108/46/E1137.abstract

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3219156/

http://www.drproctor.com/O2NOpnas.htm

Predominant role of endothelial nitric oxide synthase in vascular endothelial growth factor-induced angiogenesis and vascular permeability

http://www.pnas.org/content/98/5/2604.short

Intracellular processing of endothelial nitric oxide synthase isoforms associated with differences in severity of cardiopulmonary diseases: Cleavage of proteins with aspartate vs. glutamate at position 298

http://www.pnas.org/content/97/6/2832.short

Stroke protection by 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitors mediated by endothelial nitric oxide synthase

http://www.pnas.org/content/95/15/8880.short

Superoxide generation by endothelial nitric oxide synthase: The influence of cofactors

NO impact is such that to date more than 31,000 papers have been published with NO in the title and more than 65,000 refer to it in some way. The identification of NO with endothelium-derived relaxing factor and the discovery of its synthesis from L-arginine led to the realization that the L-arginine: NO pathway is widespread and plays a variety of physiological roles. These include the maintenance of vascular tone, neurotransmitter function in both the central and peripheral nervous systems, and mediation of cellular defense. In addition, NO interacts with mitochondrial systems to regulate cell respiration and to augment the generation of reactive oxygen species, thus triggering mechanisms of cell survival or death.

Review of the role of NO in the cardiovascular system found, that in addition to maintaining a vasodilator tone, it inhibits platelet aggregation and adhesion and modulates smooth muscle cell proliferation. NO has been implicated in a number of cardiovascular diseases and virtually every risk factor for these appears to be associated with a reduction in endothelial generation of NO. Reduced basal NO synthesis or action leads to vasoconstriction, elevated blood pressure and thrombus formation. By contrast, overproduction of NO leads to vasodilatation, hypotension, vascular leakage, and disruption of cell metabolism. Appropriate pharmacological or molecular biological manipulation of the generation of NO will doubtless prove beneficial in such conditions (Moncada and Higgs, 2006).

http://onlinelibrary.wiley.com/doi/10.1038/sj.bjp.0706458/full

Evidence of HDL Modulation of eNOS in Humans

 Whereas the functional link between HDL and eNOS has been appreciated only recently, the relationship between HDL and endothelium-dependent vasodilation has been known for some time. In studies of coronary vasomotor responses to acetylcholine, it was noted in 1994 that patients with elevated HDL have greater vasodilator and attenuated vasoconstrictor responses (Zeiher et al., 1994).

Circulation, 89:2525–2532.

Studies of flow-mediated vasodilation of the brachial artery have also shown that HDL cholesterol is an independent predictor of endothelial function (Li et al., 2000).

Int. J. Cardiol., 73:231–236

The direct, short-term impact of HDL on endothelial function also has recently been investigated in humans. One particularly elegant study recently evaluated forearm blood flow responses in individuals who are heterozygous for a loss-of-function mutation in the ATP-binding cassette transporter 1 (ABCA1) gene. Compared with controls, ABCA1 heterozygotes (six men and three women) had HDL levels that were decreased by 60%, their blood flow responses to endothelium-dependent vasodilators were blunted, and endothelium-independent responses were unaltered. After a 4-hour infusion of apoAI/phosphatidylcholine disks, their HDL level increased threefold and endothelium-dependent vasomotor responses were fully restored (Bisoendial et al., 2003). It has also been observed that endothelial function is normalized in hypercholesterolemic men with normal HDL levels shortly following the administration of apoAI/phosphatidylcholine particles (Spieker et al., 2002).

Circulation, 105:1399–1402.

Thus, evidence is now accumulating that HDL is a robust positive modulator of endothelial NO production in humans (Shaul & Mineo, 2004).

J Clin Invest., 15; 113(4): 509–513.

HDL is more than an eNOS Agonist

 In addition to the modulation of NO production by signaling events that rapidly dictate the level of enzymatic activity, important control of eNOS involves changes in the abundance of the enzyme. In a clinical trial by the Karas laboratory of niacin therapy in patients with low HDL levels (nine males and two females), flow-mediated dilation of the brachial artery was improved in association with a rise in HDL of 33% over 3 months (Kuvin et al., 2002).

Am. Heart J., 144:165–172.

They also demonstrated that eNOS expression in cultured human endothelial cells is increased by HDL exposure for 24 hours. They further showed that the increase in eNOS is related to an increase in the half-life of the protein, and that this is mediated by PI3K–Akt kinase and MAPK (Ramet et al., 2003).

J. Am. Coll. Cardiol., 41:2288–2297.

Thus, the same mechanisms that underlie the acute activation of eNOS by HDL appear to be operative in upregulating the expression of the enzyme.

The current understanding of the mechanism by which HDL enhances endothelial NO production is summarized in Shaul & Mineo (2004), Figure 1.

J Clin Invest., 15; 113(4): 509–513.

It describes the mechanism of action for HDL enhancement of NO production by eNOS in vascular endothelium.

(a)   HDL causes membrane-initiated signaling, which stimulates eNOS activity. The eNOS protein is localized in cholesterol-enriched (orange circles) plasma membrane caveolae as a result of the myristoylation and palmitoylation of the protein. Binding of HDL to SR-BI via apoAI causes rapid activation of the nonreceptor tyrosine kinase src, leading to PI3K activation and downstream activation of Akt kinase and MAPK. Akt enhances eNOS activity by phosphorylation, and independent MAPK-mediated processes are additionally required (Duarte, et al., 1997). .Eur J Pharmacol, 338:25–33. HDL also causes an increase in intracellular Ca2+ concentration (intracellular Ca2+ store shown in blue; Ca2+ channel shown in pink), which enhances binding of calmodulin (CM) to eNOS. HDL-induced signaling is mediated at least partially by the HDL-associated lysophospholipids SPC, S1P, and LSF acting through the G protein–coupled lysophospholipid receptor S1P3. HDL-associated estradiol (E2) may also activate signaling by binding to plasma membrane–associated estrogen receptors (ERs), which are also G protein coupled. It remains to be determined if signaling events are also directly mediated by SR-BI (Yuhanna et al., 2001), (Nofer et al., 2004), (Gong et al., 2003), (Mineo et al., 2003).

Nat. Med., 7:853–857.

J. Clin. Invest.,113:569–581.

J. Clin. Invest., 111:1579–1587.

J. Biol. Chem., 278:9142–9149.

(b)   HDL regulates eNOS abundance and subcellular distribution. In addition to modulating the acute response, the activation of the PI3K–Akt kinase pathway and MAPK by HDL upregulates eNOS expression (open arrows). HDL also regulates the lipid environment in caveolae (dashed arrows). Oxidized LDL (OxLDL) can serve as a cholesterol acceptor (orange circles), thereby disrupting caveolae and eNOS function. However, in the presence of OxLDL, HDL maintains the total cholesterol content of caveolae by the provision of cholesterol ester (blue circles), resulting in preservation of the eNOS signaling module (Ramet et al., 2003), (Blair et al., 1999), (Uittenbogaard et al., 2000).

J. Am. Coll. Cardiol., 41:2288–2297.

J. Biol. Chem., 274:32512–32519.

J. Biol. Chem., 275:11278–11283.

Source for HDL-eNOS Figure: Shaul & Mineo (2004).

 

HDL enhances NO production by eNOS in vascular endothelium.

Nebivolol:  DRUG RESEARCH & CLINICAL TRIALS

Agent selection: Nebivolol

Rationale:            Patient’s pharmacological beneficial effects derived from usage of Nebivolol include the following but are not limited to this list

  •       Vasodilatory actions (Mukherjee et al., 2004).
  •      Inhibition of NADPH oxidase activity in inflammatory cells (Mollnau et al., 2003),
  •       Increase in arterial distensibility (McEniery et al., 2004)
  •       Reduction in nitroxidative stress and restores nitric oxide bioavailability in endothelium (Mason et al., 2005)
  •       Stimulation of nitric oxide release from endothelial cells through ATP efflux: a novel mechanism for antihypertensive   action (Kalinowski et al., 2003)
  •       {beta}-Adrenergic Receptor Stimulation and Nitric Oxide Release on Tissue Perfusion and Metabolism (Jordan et al., 2001)
  •       Correction of impaired adrenergic vasorelaxation in hypertension in use in conjunction to gene therapy implantation in the endothelium (Iaccarino, et al., 2002)
  •       Vasorelaxation of Coronary Microvessels (Dessy et al., 2005)
  •       Exploratory treatment for the Brugada syndrome, a disease caused by increased electrical heterogeneity between the right ventricular endo- and epicardium. The degree of electrical heterogeneity may be greater in the free wall in some patients, the outflow tract in others, or even in the inferior wall. The ST-segment elevation may then be recorded at the normal precordial position of V1–V3 in the first situation, at one or two intercostal spaces higher in the second, and in the inferior leads II, III and a VF in the third situation, representing a variant of the Brugada syndrome (Brugada et al., 2001).
  •       Endothelial ß2-Adrenergic Receptor–Mediated Nitric Oxide Production, two actions in one therapeutic agent for populations with prevalent polypharmacy due to multiple co-morbidities (Broeders et al., 2000).

The rationale for Agent selection supports the hypothesis that Nebovolol would have positive effect on cEPCs endogenous augmentation. It was a solution sought for the observations made be Werner in 2003 and in 2005 that Low number of cEPCs found in patient blood is statistically associated with high incidence of Macrovascular Risk Events.

 Nebivolol is a long-acting, cardioselective beta-blocker currently licensed for the treatment of hypertension. It has mild vasodilating properties attributed to its interaction with the L-arginine/nitric oxide pathway, a property not shared by other beta-blockers. To date this has been demonstrated in volunteers and small numbers of patients. If this mechanism is shown to result in improved clinical outcomes, nebivolol could be of value in managing hypertensive patients with endothelial dysfunction e.g., those with diabetes mellitus or hypercholesterolaemia and in patients with ischemic heart disease. It is an effective antihypertensive agent. Short-term (up to 12 weeks), published clinical studies in patients with mild-to-moderate essential hypertension have shown that it lowers sitting systolic and diastolic blood pressure to a similar extent as standard therapies – atenolol, metoprolol, enalapril, lisinopril, nifedipine and hydrochlorothiazide. One open non-comparative study showed that a significant reduction in BP is maintained over 1 year. It is well-tolerated; the frequency and severity of adverse events is similar to that reported for placebo, atenolol or enalapril in published studies. In the largest comparative study the numbers of patients complaining of fatigue was smaller for nebivolol compared with atenolol, although the numbers in both groups were too small for any meaningful comparisons to be made. In addition, in single comparative studies with nifedipine or metoprolol, the overall incidence of adverse events was smaller in the nebivolol groups. Although uncontrolled heart failure is listed as a contra-indication in the SPC, preliminary studies have shown that nebivolol has beneficial effects on left ventricular function in patients with hypertension and heart failure.

Nebivolol is considerably more expensive than atenolol, but costs less than carvedilol or celiprolol

How does it work?

Nebivolol belongs to a group of medicines called beta-blockers, which block beta receptors in the heart, lungs and other organs of the body. Blocking these receptors prevents the action of two chemicals called noradrenaline and adrenaline that occur naturally in the body. These are often referred to as the ‘fight or flight’ chemicals as they are responsible for the body’s reaction to stressful situations.

Blocking the beta receptors in the heart causes the heart to beat more slowly and with less force. This means that the pressure at which blood is pumped out of the heart to the rest of the body is reduced. This medicine also widens the blood vessels. These are two of the ways in which nebivolol helps to reduce blood pressure, however the whole mechanism is not fully understood.

What is it used for?

  •       High blood pressure (hypertension)

In vivo metabolized nebivolol increases vascular NO production. This phenomenon involves endothelial ß2-adrenergic receptor ligation, with a subsequent rise in endothelial free [Ca2+]i and endothelial NO synthase–dependent NO production. This may be an important mechanism underlying the nebivolol-induced, NO-mediated arterial dilation in humans. Nebivolol is a ß1-selective adrenergic receptor antagonist with proposed nitric oxide (NO)–mediated vasodilating properties in humans. In this study, they explored whether nebivolol indeed induces NO production and, if so, by what mechanism. They hypothesized that not nebivolol itself but rather its metabolites augment NO production (Broeders et al., 2000).

Circulation, 102:677.

http://www.dailymedplus.com/monograph/view/setid/673f5ad2-c09b-4a89-9407-efdadd007917

Relation between Beta-adrenoceptor Stimulation and Nitric Oxide Synthesis in Vascular Control.

This commentary reviews recent evidence that implicates nitric oxide (NO) as a mediator of beta(2)-adrenoceptor (beta(2)-AR)-initiated vasodilatation. Emphasis is placed on the following: 1) in vivo studies that demonstrate potential physiological importance, 2) mechanistic studies performed in vitro in human umbilical vein endothelial cells (HUVEC), 3) effects of beta(2) agonists on arterial pulse wave reflection, and 4) therapeutic opportunities offered by the combination of beta(2) agonist action with selective beta(1) antagonism. Vascular beta(2)-AR-initiated mechanisms provide a physiologically important control mechanism during exercise. Activation of beta(2)-AR in HUVEC leads to vasodilatation that is partly NO-mediated via activation of protein kinase A (PKA) and of phosphatidylinositol-3 kinase (PI3K)/Akt pathways, leading to serine phosphorylation of the endothelial NO synthase (eNOS). In vivo, beta(2)-AR activation limits the rise in blood pressure during exercise and reduces arterial pulse wave reflection. Nebivolol is a selective beta(1)-AR antagonist with vasodilator actions operating through these pathways, offering novel therapeutic opportunities.

Ritter JM, Ferro A, Chowienczyk PJ., (2006). Relation between beta-adrenoceptor stimulation and nitric oxide synthesis in vascular control.

Eur J Clin Pharmacol., 62 (Supplement 13):109-113. 

eNOS is not Activated by Nebivolol in Human Failing Myocardium.

Nebivolol is a highly selective beta(1)-adrenoceptor blocker with additional vasodilatory properties, which may be due to an endothelial-dependent beta(3)-adrenergic activation of the endothelial nitric oxide synthase (eNOS). beta(3)-adrenergic eNOS activation has been described in human myocardium and is increased in human heart failure. Therefore, this study investigated whether nebivolol may induce an eNOS activation in cardiac tissue. Immunohistochemical stainings were performed using specific antibodies against eNOS translocation and eNOS serine(1177) phosphorylation in rat isolated cardiomyocytes, human right atrial tissue (coronary bypass-operation), left ventricular non-failing (donor hearts) and failing myocardium after application of the beta-adrenoceptor blockers nebivolol, metoprolol and carvedilol, as well as after application of BRL 37344, a specific beta(3)-adrenoceptor agonist. BRL 37344 (10 muM) significantly increased eNOS activity in all investigated tissues (either via translocation or phosphorylation or both). None of the beta-blockers (each 10 muM), including nebivolol, increased either translocation or phosphorylation in any of the investigated tissues. In human failing myocardium, nebivolol (10 muM) decreased eNOS activity. In conclusion, nebivolol shows a tissue-specific eNOS activation. Nebivolol does not activate the endothelial eNOS in end-stage human heart failure and may thus reduce inhibitory effects of NO on myocardial contractility and on oxidative stress formation. This mode of action may be of advantage when treating heart failure patients.

Brixius K, Song Q, Malick A, Boelck B, Addicks K, Bloch W, Mehlhorn U, Schwinger R, (2006). eNOS is not activated by nebivolol in human failing myocardium.

Life Sci. 2006 Apr 25

A Dose-response Trial of Nebivolol in Essential Hypertension.

Report by International Clinical R&D, Janssen Research Foundation, Beerse, Belgium.

A double-blind placebo-controlled dose-response trial of nebivolol, a cardioselective beta-blocking drug which also induces endothelium-dependent dilatation via nitric oxide, has been performed. Nebivolol reduced blood pressure (BP) in a dose dependent way, and was shown to be effective given once daily, without appreciable differences between peak and trough drug levels. There was no postural component to the BP fall. There was no clear inferiority of efficacy in black patients. A single daily dose of 5 mg was appropriate, with no evident advantage at 10 mg. The drug was well tolerated, even at 10 mg daily. BP control was achieved largely in the absence of typical side effects of beta-blockade. The combination of properties of nebivolol renders it an attractive addition to the antihypertensive repertoire.

Van Nueten L, Dupont AG, Vertommen C, Goyvaerts H, Robertson JI., (1997). A dose-response trial of nebivolol in essential hypertension.

J Hum Hypertens., 11(2):139-44.

Other eNOS Agonists – Exploration of Different Aspects related to eNOS Mechanism of Action

ACEI and NO stimulation

Carboxypeptidase cleavage of the C-terminal Arg of kinins generates specific agonists of the B1 receptor. Activation of B1 receptors produces nitric oxide via eNOS in bovine endothelial cells and iNOS in cytokine-stimulated human endothelial cells. Angiotensin-converting enzyme (ACE) inhibitors are direct agonists of B1 receptors in endothelial cells, although they release NO via a different signaling pathway than peptide ligands in bovine cells. This brief review discusses carboxypeptidase M as a required processing enzyme for generating B1 agonists, how ACE inhibitors and peptide ligands stimulate NO production and the evidence for, as well as some consequences of, the direct activation of B1 receptors by ACE inhibitors (Skidgel et al., 2006).

Biol Chem., 387(2):159-65.

Fenofibrate

 Fenofibrate improves endothelial function by lipid-lowering and anti-inflammatory effects. Additionally, fenofibrate has been demonstrated to upregulate endothelial nitric oxide synthase (eNOS). AMP-activated protein kinase (AMPK) has been reported to phosphorylate eNOS at Ser-1177 and stimulate vascular endothelium-derived nitric oxide (NO) production. We report here that fenofibrate activates AMPK and increases eNOS phosphorylation and NO production in human umbilical vein endothelial cells (HUVEC). Incubation of HUVEC with fenofibrate increased the phosphorylation of AMPK and acetyl-CoA carboxylase. Fenofibrate simultaneously increased eNOS phosphorylation and NO production. Inhibitors of protein kinase A and phosphatidylinositol 3-kinase failed to suppress the fenofibrate-induced eNOS phosphorylation. Neither bezafibrate nor WY-14643 activated AMPK in HUVEC. Furthermore, fenofibrate activated AMPK without requiring any transcriptional activities. These results indicate that fenofibrate stimulates eNOS phosphorylation and NO production through AMPK activation, which is suggested to be a novel characteristic of this agonist and unrelated to its effects on peroxisome proliferator-activated receptor alpha (Murakami et al., 2006). Biochem Biophys Res Commun., 341(4):973-8. Epub 2006 Jan 24.

Function of Ca2+ on NO response

Nitric oxide (NO) produced in the endothelium via the enzyme endothelial nitric-oxide synthase (eNOS) is an important vasoactive compound. Wild-type (WT) eNOS is localized to the plasma membrane and perinuclear/Golgi region by virtue of N-terminal myristoylation and palmitoylation. Acylation-deficient mutants (G2AeNOS) remain cytosolic and release less NO in response to Ca2+-elevating agonists; a disparity that we hypothesized was attributed to the greater distance between G2AeNOS and plasma membrane Ca2+ influx channels. The reduced activity of G2AeNOS versus WT was reversed upon disruption of cellular integrity with detergents or sonication. NO production from both constructs relied almost exclusively on the influx of extracellular Ca2+, and elevating intracellular Ca2+ to saturating levels with 10 microM ionomycin in the presence of 10 mM extracellular Ca2+ equalized NO production. To identify the contribution of calcium to the differences in activity between these enzymes, we created Ca2+/CaM-independent eNOS mutants by deleting the two putative autoinhibitory domains of eNOS. There was no difference in NO production between WT and G2A-targeted Ca2+-independent eNOS, suggesting that Ca2+ was the factor responsible. When eNOS constructs were fused in-frame to the bioluminescent probe aequorin, membrane-bound probes were exposed to higher [Ca2+] in unstimulated cells but upon ionomycin stimulation, the probes experienced equal amounts of Ca2+. The WT and G2A enzymes displayed significant differences in the phosphorylation state of Ser617, Ser635, and Ser1179, and mutating all three sites to alanine or restoring phosphorylation with the phosphatase inhibitor calyculin abolished the differences in activity. We therefore conclude that the disparity in NO production between WTeNOS and G2AeNOS is not caused by different localized [Ca2+] upon stimulation with ionomycin, but rather differences in phosphorylation state between the two constructs (Church & Fulton, 2006).

 J Biol Chem., 2006 Jan 20;281(3):1477-88. Epub 2005 Oct 28.

Muscarinic ACh and Purinergic (ADP) – mediated eNOS activation

Nitric oxide (NO) regulates flow and permeability. Acetylcholine (ACh) and platelet-activating factor (PAF) lead to eNOS phosphorylation and NO release. While ACh causes only vasodilation, PAF induces vasoconstriction and hyperpermeability. The key differential signaling mechanisms for discriminating between vasodilation and hyperpermeability are unknown. We tested the hypothesis that differential translocation may serve as a regulatory mechanism of eNOS to determine specific vascular responses. We used ECV-304 cells permanently transfected with eNOS-green fluorescent protein (ECVeNOS-GFP) and demonstrated that the agonists activate eNOS and reproduce their characteristic endothelial permeability effects in these cells. We evaluated eNOS localization by lipid raft analysis and immunofluorescence microscopy. After PAF and ACh, eNOS moves away from caveolae. eNOS distributes both in the plasma membrane and Golgi in control cells. ACh (10(-5) M, 10(-4) M) translocated eNOS preferentially to the Trans Golgi network (TGN) and PAF (10(-7) M) preferentially to the cytosol. We suggest that PAF-induced eNOS translocation preferentially to cytosol reflects a differential signaling mechanism related to changes in permeability, whereas ACh-induced eNOS translocation to the TGN is related to vasodilation (Sanchez et al., 2006).

Am J Physiol Heart Circ Physiol., May 5; [Epub ahead of print]

Nitric oxide (NO), derived from the endothelial isoform of NO synthase (eNOS), is a vital mediator of cerebral vasodilation. In the present study, we addressed the issue of whether the mechanisms responsible for agonist-induced eNOS activation differ according to the specific receptor being stimulated. Thus we examined whether heat shock protein 90 (HSP90), phosphatidylinositol-3-kinase (PI3K), and tyrosine kinase participate in ACh- versus ADP-induced eNOS activation in cerebral arterioles in vivo. Pial arteriolar diameter changes in anesthetized male rats were measured during sequential applications of ACh and ADP in the absence and presence of the nonselective NOS inhibitor N-nitro-L-arginine methyl ester (L-NAME), the neuronal NOS (nNOS)-selective inhibitor ARR-17477, the HSP90 blocker 17-(allylamino)-17-demethoxygeldanamycin (AAG), the PI3K inhibitor wortmannin (Wort), or the tyrosine kinase blocker tyrphostin 47 (T-47). Only NOS inhibition with L-NAME (not ARR-17477) reduced ACh and ADP responses (by 65-75%), which suggests that all of the NO dependence in the vasodilating actions of those agonists derived from eNOS. Suffusions of AAG, Wort, and T-47 were accompanied by substantial reductions in ACh-induced dilations but no changes in the responses to ADP. These findings suggest that muscarinic (ACh) and purinergic (ADP) receptor-mediated eNOS activation in cerebral arterioles involve distinctly different signal transduction pathways. (Xu et al., 2002).

Am J Physiol Heart Circ Physiol., 282:H237-H243

S-Nitrosylation of eNOS

Endothelial nitric-oxide synthase (eNOS) undergoes a complex pattern of post-translational modifications that regulate its activity. We have recently reported that eNOS is constitutively S-nitrosylated in endothelial cells and that agonists promote eNOS denitrosylation concomitant with enzyme activation (Erwin, P. A., Lin, A. J., Golan, D. E., and Michel, T. (2005),

J. Biol. Chem. 280, 19888–19894).

In the present studies, we use mass spectrometry to confirm that the zinc-tetrathiolate cysteines of eNOS are S-nitrosylated. eNOS targeting to the plasma membrane is necessary for enzyme S-nitrosylation, and we report that translocation between cellular compartments is necessary for dynamic eNOS S-nitrosylation. We transfected cells with cDNA encoding wild-type eNOS, which is membrane-targeted, or with acylation-deficient mutant eNOS (Myr–), which is expressed solely in the cytosol. While wild-type eNOS is robustly S-nitrosylated, we found that S-nitrosylation of the Myr– eNOS mutant is nearly abolished. When we transfected cells with a fusion protein in which Myr– eNOS is ligated to the CD8-transmembrane domain (CD8-Myr–), we found that CD8-Myr– eNOS, which does not undergo dynamic subcellular translocation, is hypernitrosylated relative to wild-type eNOS. Furthermore, we found that when endothelial cells transfected with wild-type or CD8-Myr– eNOS are stimulated with eNOS agonist, only wild-type eNOS is denitrosylated; CD8-Myr– eNOS S-nitrosylation is unchanged. These findings indicate that subcellular targeting is a critical determinant of eNOS S-nitrosylation. Finally, we show that eNOS S-nitrosylation can be detected in intact arterial preparations from mouse and that eNOS S-nitrosylation is a dynamic agonist-modulated process in intact blood vessels. These studies suggest that receptor-regulated eNOS S-nitrosylation may represent an important determinant of NO-dependent signaling in the vascular wall (Erwin et al., 2006).

 J. Biol. Chem., 281:1, 151-157.

Phosphorylation of eNOS

 The endothelial isoform of nitric-oxide synthase (eNOS) undergoes a complex pattern of covalent modifications, including acylation with the fatty acids myristate and palmitate as well as phosphorylation on multiple sites. eNOS acylation is a key determinant for the reversible subcellular targeting of the enzyme to plasmalemmal caveolae. We transfected a series of hemagglutinin epitope-tagged eNOS mutant cDNAs deficient in palmitoylation (palm) and/or myristoylation (myr) into bovine aortic endothelial cells; after treatment with the eNOS agonists sphingosine 1-phosphate or vascular endothelial growth factor, the recombinant eNOS was immunoprecipitated using an antibody directed against the epitope tag, and patterns of eNOS phosphorylation were analyzed in immunoblots probed with phosphorylation state-specific eNOS antibodies. The wild-type eNOS underwent agonist-induced phosphorylation at serine 1179 (a putative site for phosphorylation by kinase Akt), but phosphorylation of the myr eNOS at this residue was nearly abrogated; the palm eNOS exhibited an intermediate phenotype. The addition of the CD8 transmembrane domain to the amino terminus of eNOS acylation-deficient mutants rescued the wild-type phenotype of robust agonist-induced serine 1179 phosphorylation. Thus, membrane targeting, but not necessarily acylation, is the critical determinant for agonist-promoted eNOS phosphorylation at serine 1179. In striking contrast to serine 1179, phosphorylation of eNOS at serine 116 was enhanced in the myr eNOS mutant and was markedly attenuated in the CD8-eNOS membrane-targeted fusion protein. We conclude that eNOS targeting differentially affects eNOS phosphorylation at distinct sites in the protein and suggest that the inter-relationships of eNOS acylation and phosphorylation may modulate eNOS localization and activity and thereby influence NO signaling pathways in the vessel wall (Gonzalez et al., 2002).

J. Biol. Chem., 277;42:39554-39560.

eNOS translocation and Ca2+

In endothelial cells, two ways of endothelial nitric oxide (NO) synthase (eNOS) activation are known: 1) translocation and 2) Akt-dependent phosphorylation of the enzyme at Ser1177 (Ser1177 eNOS). We have recently shown that agonist-induced Ser1177 eNOS phosphorylation also occurs in human myocardium (10). In this study, we investigated the Ca2+ dependency of these two mechanisms in human atrium. Therefore, atrial tissue was obtained from patients who underwent coronary artery bypass operations. In immunohistochemical experiments, the translocated form of eNOS and phosphorylated Ser1177 eNOS were labeled using specific antibodies. eNOS translocation was measured in the absence and presence of the Ca2+ chelator BAPTA before and after application of BRL 37344 (BRL), a 3-adrenoceptor agonist that increases eNOS activity (34). In the absence of BAPTA, BRL time dependently increased the staining intensity of translocated eNOS, whereas in the presence of BAPTA, this effect was blunted. In contrast, BRL clearly increased the staining of phosphorylated Ser1177 eNOS even in the presence of BAPTA. This observation was confirmed using Western blot analysis. Using the NO-sensitive dye diaminofluorescein, we have demonstrated that BRL induced a strong NO release. This effect was completely abolished in the presence of BAPTA but was unaffected by LY-292004, an inhibitor of phosphatidylinositol 3-kinase activity and eNOS phosphorylation. Although Ca2+ dependent, neither the translocation of eNOS nor NO release was changed by the adenylate cyclase activator forskolin. In conclusion, 1) in human atrial myocardium, BRL-induced eNOS translocation but not Ser1177 eNOS phosphorylation is dependent on intracellular Ca2+. 2) In atrial myocardium, eNOS-translocation and not Ser1177 eNOS phosphorylation is responsible for generating the main amount of NO. 3) Although Ca2+ dependent, eNOS translocation and NO release could not be mimicked by adenylate cyclase activation as a mediator of -adrenergic stimulation (Pott et al., 2006).

Am J Physiol Cell Physiol 290: C1437-C1445.

Nebivolol  DRUG INFORMATION

http://www.intekom.com/pharm/adcock/nebilet.html – retrieved on 6/20/2006

 PHARMACOLOGICAL ACTION

 Pharmacodynamics

 Nebivolol is a racemate of two enantiomers, SRRR-nebivolol (or d-nebivolol) and RSSS-nebivolol (or l-nebivolol). It combines two pharmacological activities: –

• It is a competitive & selective B1-receptor antagonist which is attributable to the d-enantiomer

• It has mild vasodilating properties, possible due to an interaction with the L-arginine/nitric oxide pathway Nebivolol reduces heart rate & blood pressure at rest & during exercise. In healthy volunteers it has no significant effect on maximal exercise or endurance.

An in-vitro and in-vivo experiment in animals showed that nebivolol has no intrinsic sympathicomimetic activity and at pharmacological doses has no membrane stabilizing effect. It is also devoid of alpha-adrenergic antagonism at therapeutic doses.

Pharmacokinetics

Nebivolol can be given with or without meals with peak plasma concentrations occurring within 2 – 6 hours after dosing. It is extensively metabolized partly to active hydroxy metabolites. The bioavailability of nebivolol averages 12% in extensive metabolizers (EM’s) & is virtually complete in poor metabolizers (PM’s), but the mean bioavailability of the separate enantiomers and hydroxylated metabolites was fairly similar between EM’s & PM’s and no differences were found in the pharmacodynamic effects.

Steady-state plasma levels for nebivolol are reached within 24 hours in most subjects (EM’s). The elimination half-lives of the hydroxy-metabolites of both enantiomers average 24 hours in EM’s and are twice as long in PM’s. Plasma concentrations are dose proportional and the pharmacokinetics of nebivolol are unaffected by age. Nebivolol is highly protein bound; d-nebivolol being 98.1% and l-nebivolol 97,9% bound to albumin. About 52% of the dose is excreted in urine and about 15% in the faeces in PM’s one week after administration.

INDICATIONS: Treatment of mild to moderate essential hypertension.

 CONTRA-INDICATIONS

  • Hypersensitivity to Nebilet
  • Liver insufficiency or liver function impairment.
  • Pregnancy and lactation
  • Nebilet is contra-indicated in:

– Cardiogenic shock            – Untreated phaeochromocytoma

– Uncontrolled heart failure            – Metabolic acidosis

– Sick sinus syndrome, including            – Bradycardia (heart rate < 50 bpm)

– sino-atrial block            – Bronchial asthma

– 2nd & 3rd degree heart block            – Hypotension

– History of bronchospasm &             – Severe peripheral circulatory disorders

– bronchial asthma             – Verapamil therapy – Children, as safety and efficacy has not been demonstrated

 WARNINGS

Beta-adrenergic antagonists may increase the sensitivity to allergens and the severity of anaphylactic reactions

 SIDE-EFFECTS AND SPECIAL PRECAUTIONS:

 Side-Effects:

The most common side-effects (incidence between – 1-10%) are headache, dizziness, tiredness & paraesthesia. Other side-effects reported in 1% of patients are: diarrhea, constipation, nausea, dyspnea & edema. Typical beta-adrenergic antagonist side-effects reported in less than 1% of patients are: bradycardia, slowed AV conduction/AV-block, hypotension, heart failure, increase of intermittent claudication, impaired vision, impotence, depression, nightmare, dyspepsia, flatulence, vomiting, bronchospasm and rash.

The following side-effects have also been reported with some beta-adrenergic antagonists: hallucinations, psychoses, confusion, cold/cyanotic extremities, Raynaud phenomenon, dry eyes and mucocutaneous toxicity of the practolol-type, sleep disturbances and abdominal cramping.

Congestive heart failure or heart block may be precipitated in patients with underlying cardiac disorders. Pneumonitis, pleurisy, paraesthesia, peripheral neuropathy, overt psychosis, myopathies, skin rash, pruritis, and reversible alopecia have been reported. Ocular symptoms include decreased tear production, blurred vision and soreness.

Hematological reactions include nonthrombocytopenic purpura, thrombocytopenia, and less frequently agranulocytosis. Transient eosinophilia can occur.

Metabolic changes affect glucose control and cholesterol concentrations. Other side effects include a lupus like syndrome, male impotence, hypoglycemia, sclerosing peritonitis and retroperitoneal fibrosis. Severe peripheral vascular disease and even peripheral gangrene may be precipitated.

Special Precautions:

Cardiovascular:

Beta-adrenergic antagonists should not be used in patients with untreated congestive heart failure, unless their condition has been stabilized. One of the pharmacological actions of beta-blockers is to reduce the heart rate.

Abrupt discontinuation of therapy may cause exacerbation of angina pectoris in patients suffering from ischemic heart disease. Discontinuation of therapy should be gradual (over a period of 1-2 weeks) and patients should be advised to limit the extent of their physical activity during the period that their medicine may be discontinued. If the pulse rate drops below 50-55 bpm at rest and/or the patient experiences symptoms suggestive of bradychardia, the dosage should be reduced. Beta-adrenergic antagonists should be used with caution in:

• Peripheral circulatory disorders (Raynaud’s disease or syndrome, intermittent claudication) as the disorders may be aggravated

• 1st degree heart block because of the negative effect of beta-blockers on conduction time

• Prinzmetal’s angina due to unopposed alpha receptor mediated coronary artery vasoconstriction. Beta-blockers may increase the number and duration of anginal attacks

Metabolic/Endocrinological:

Symptoms of hypoglycemia (tachycardia, palpitations) may be masked in diabetic patients. Tachycardic symptoms may be masked in hyperthyroidism. Abrupt withdrawal may intensify symptoms.

Respiratory:

Bronchospasm may occur in patients suffering from asthma, bronchitis and other chronic pulmonary diseases.

Other:

Psoriasis may be aggravated. Patients with phaeochromocytoma should not receive beta-blockers without concomitant alpha-adrenoreceptor blocking therapy.

Beta-blockers may unmask myasthenia gravis.

Adverse reactions are more common in patients with renal decompensation, and in patients who receive beta-blockers intravenously.

INTERACTIONS

Calcium Antagonists:

Caution should be exercised when administering beta-blockers with calcium antagonists of the verapamil or diltiazem type because of their negative effect on contractility and atrio-ventricular conduction. Exaggeration of these effects can occur particularly in patients with impaired ventricular function and/or SA or AV conduction abnormalities. Neither medicine should therefore be administered intravenously within 48 hours of discontinuing the other.

Anti-arrhythmics:

Caution should be exercised when administering beta-blockers with Class I anti-arrhythmic drugs and amiodarone as their effect on atrial conduction time and their negative inotropic effect may be potentiated. Such interactions can have life threatening consequences.

Clonidine:

Beta-blockers increase the risk of rebound hypertension after sudden withdrawal of chronic clonidine treatment.

Digitalis:

Digitalis glycosides associated with beta-blockers may increase atrio-ventricular conduction times. Nebivolol does not influence the kinetics of digoxin & clinical trials have not shown any evidence of an interaction.

Special note: Digitalisation of patients receiving long term beta-blocker therapy may be necessary if congestive cardiac failure is likely to develop. The combination can be considered despite the potentiation of the negative chronotropic effect of the two medicines. Careful control of dosages and of individual patient’s response (notably pulse rate) is essential in this situation.

Insulin & Oral Antidiabetic drugs:

Glucose levels are unaffected, however symptoms of hypoglycemia may be masked.

Anaesthetics:

Concomitant use of beta-blockers & anaesthetics e.g. ether, cyclopropane & trichloroethylene may attenuate reflex tachycardia & increase the risk of hypotension

Other:

Provided Nebilet is taken with a meal & an antacid between meals, the two treatments can be co-prescribed.

Sympathicomimetic agents may counteract the effect of beta-blockers.

Concomitant administration of tricyclic antidepressants, barbiturates & phenothiazines may increase the blood pressure lowering effect.

Concomitant administration of serotonin re-uptake inhibitors or other compounds predominantly metabolized by the CYPZD6 pathway may delay oxidative metabolism of beta-blockers

 KNOWN SYMPTOMS OF OVERDOSAGE AND PARTICULARS OF ITS TREATMENT:

Symptoms:

Bradycardia, hypotension, bronchospasm and acute cardiac insufficiency

Treatment:

Blood glucose levels should be checked and symptomatic and supportive therapy given.

CONCLUSIONS

Nebvolol – one of the most interesting antihypertensive drugs on the market in 2012. Worldwide Sales of Nebivolol 2009-2011 in US $ (millions)

2009 – 179

2010 – 264  %increase 48

2011 – 348  %increase 32

http://www.evaluatepharma.com/Universal/View.aspx?type=Entity&entityType=Product&lType=modData&id=9552&componentID=1003

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Nebivolol is a long-acting, cardioselective beta-blocker currently licensed for the treatment of hypertension.

http://www.saha.org.ar/noticias/nebivolol2.htm  – retrieved on 6/20/2006

Nebivolol

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