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Posts Tagged ‘basic’


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/Author: Aviral Vatsa PhD, MBBS

Nitric oxide is one of the smallest molecules involved in physiological functions in the body. It is a diatom and thus seeks formation of chemical bonds with its targets rather than structure-function configuration of say protein receptors. Nitric oxide can exert its effects principally by two ways:

  • Direct
  • Indirect

Direct actions, as the name suggests, result from direct chemical interaction of NO with its targets e.g. with metal complexes, radical species. These actions occur at relatively low NO concentrations (<200 nM)

Indirect actions result from the effects of reactive nitrogen species (RNS) such as NO2 and N2O3. These reactive species are formed by the interaction of NO with superoxide or molecular oxygen. RNS are generally formed at relatively high NO concentrations (>400 nM)

Credits: Nitric Oxide: Biology and Pathobiology By Louis J. Ignarro

Credits: Nitric Oxide: Biology and Pathobiology By Louis J. Ignarro

Although it can be tempting for scientists to believe that RNS will always have deleterious effects and NO will have anabolic effects, this is not entirely true as certain RNS mediated actions mediate important signalling steps e.g. thiol oxidation and nitrosation of proteins mediate cell proliferation and survival, and apoptosis respectively. As depicted in the figure above, NO concentration determines the action it exerts on different proteins. This is highlighted in the following examples from different studies:

  • Cells subjected to NO concentration between 10-30 nM were associated with cGMP dependent phosphorylation of ERK
  • Cells subjected to NO concentration between 30-60 nM were associated with Akt phosphorylation
  • Concentration nearing 100 nM resulted in stabilisation of hypoxia inducible factor-1
  • At nearly 400 nM NO, p53 can be modulated
  • >1μM NO, it nhibits mitochondrial respiration

Besides the concentration, duration of NO exposure also determines how proteins respond to NO. Hence proteins can be ‘immediate’ responders or ‘delayed’ responders. The response can be either ‘transient’ (short lived) or ‘sustained’ (prolonged). Different proteins fall into these different categories. These are not rigid categories rather a functional ‘classification’.

Endogenously generated NO concentration ranges from 2 nM as in endothelial cell to >1 μM in a fully activated macrophage. This wide range, along with the unique chemical reactivity of NO offers immense versatility to the physiological effects that it can exert in different cellular milieu in the body.

In addition to the concentration-dependent effects, other factors that determine the local cellular/tissue milieu add to the complexities involved with signal transduction undertaken by NO. These factors are

  • rate of NO production
  • diffusion distance
  • rates of consumption
  • reactivity of RNS with molecular targets.

These kinetic determinants play vital role in physiological functions and disease states.

Although it is not possible to detail the modes of modulation of biological functions by NO in a short post, but I hope the post gives a taste of the intricacies involved with NO functions and that there are various parameters that determine the exact role of NO in a biological milieu.

Sources

http://www.pnas.org/content/101/24/8894.short

http://onlinelibrary.wiley.com/doi/10.1002/ijc.22336/full

http://cancerres.aacrjournals.org/content/67/1/289.short

http://www.sciencedirect.com/science/article/pii/S0005272806000417

http://goo.gl/eVXFh

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