ROLE OF NITRIC OXIDE IN PATHOLOGY

Student, Guilherme Lima PaschoaliniGroup 29 3rd year, 2nd semester

SUPERVISOR,MD. PhD. Ass Prof., Elena Vladislavovna Antopol

State Educational Institution of Higher Professional EducationKursk State Medical University

Ministry of Health of the Russian Federation Department of Pathophysiology

2015

INTRODUCTION

• Nitric oxide is a molecule that our body produces to help its 50 trillion cells communicate with each other by transmitting signals throughout the entire body.

• There have been over 60,000 studies done on nitric oxide in the last 20 years and in 1998, The Nobel Prize for Medicine was given to three scientists that discovered the signaling role of nitric oxide.

• In mammals including humans, NO is an important cellular signaling molecule involved in many physiological and pathological processes.

• Nitric oxide is seen as an important player in both normal cellular and whole body physiology and also in the pathology of many diseases:

1. Nitric oxide and heart disease 2. Nitric oxide and neurological disorders3. Nitric oxide in inflammation

THE BASIC ABOUT NITRIC OXIDE• A chemical compound with formula NO is a free radical gas.• It is first identified as endothelial derived releasing factor(E D R F ).• At high concentration , fight against infectious organism and cancer cell.• At lower concentration helps in regulating the circulatory and central nervous system.• Nitric oxide differs from other neurotransmitter and hormones in a way that it is not regulated by storage,

release , or targeted degradation.• No does not require receptor for its action when synthesized immediately utilized.• Ca++ clamudulin complex is necessary for nitric oxide synthesis.

Nitric oxide is a di atomic free radical consisting of one atom of nitrogen and one atom of oxygen.

Lipid soluble and very small for easy passage between cell membranes.

Short lived, usually degraded or reacted within a few seconds.

SYNTHESIS OF NITRIC OXIDE Nitric oxide is synthesized from L-arginine. This reaction is catalyzed by nitric oxide synthase, a 1,2,9,4 amino acid enzyme.

INTRACELLULAR MECHANISM• When nitric oxide forms in large parts because superoxide anion has a height affinity for No.• Superoxide anion reduces No bioavailability.• Nitric oxide also binds to the heme moiety of hemoglobin and heme moiety of enzyme gunayl cyclase ,

which is found in smooth muscle cell and most other cells of body.• When NO formed by vascular endothelium it rapidly diffuses into the blood where it binds to hemoglobin

& subsequently broken down. • It also diffuses into vascular smooth muscle cells adjacent to the endothelium where it binds to & activate

gunyl cyclase . This enzyme catalyse the dephosprylation of GTP to cGMP which serve as a second messenger for many important cellular function , particular for signaling smooth muscle contraction.

• cGMP induces smooth muscle relaxation by multiple mechanism including- increased intracellular cGMP which inhibit ca++ entry into the cell and decrease intracellular ca++

concentration. activates k+ channel which leads to hyper polarization & relaxation .• Stimulates a cGMP dependent protein kinase that activates myosin light chain phosphate (MLCK) the

enzyme that dephosphorylate myosin light chain leads to smooth muscle relaxation

The role of endothelium derived NO in the regulation of blood flow and platelet activation.

TYPES OF NITRIC OXIDE

Nitric Oxide in the human body has many uses which are best summarized under five categories.• NO in the nervous system• NO in the circulatory system• NO in the muscular system• NO in the immune system• NO in the digestive system

THE ROLE OF NITRIC OXIDE IN CARDIOVARCULAR DISEASES• NO plays an important role in the protection against the onset and progression of cardiovascular disease.• The cardioprotective roles of NO include regulation of blood pressure and vascular tone, inhibition of

platelet aggregation and leukocyte adhesion, and prevention smooth muscle cell proliferation.• Reduced bioavailability of NO is thought to be one of the central factors common to cardiovascular

disease, leads to a loss of the cardio protective actions and in some case may even increase disease progression.

Factors that may contribute to a reduction in nitric oxide bioavailability. One of the major contributing factors to the progression of cardiovascular diseases is a loss of NO-dependent actions. It is proposed that this is due to a reduction in the bioavailability of NO. There are a variety of factors which could reduce NO availability including (1) a reaction in the availability of the substrate L-arginine, (2) increased concentration of circulating inhibitors such as ADMA, (3) altered levels of eNOS expression, (4) perturbed signal transduction reducing agonist-induced eNOS activation, (5) reduced availability of terahydrobiopterin (BH4) an essential co-factor, or (6) the destruction of NO by other free radical species.

Atherosclerosis and hypercholesterolaemia:• Atherosclerosis is a progressive disease that begins with fatty streaks and through a process of lipoprotein

deposition and cellular dysfunction progresses to complicated plaques• NO in this setting can have both pro- and anti-atherosclerotic effects.• The anti-atherosclerotic effects rely upon sufficient amounts of NO to regulate platelet function, leukocyte

adhesion and extravasation, inhibit LDL oxidation and prevent SMC proliferation.• The reduced bioavailability of NO is thought to occur early in the development of atherosclerosis.• Patients with hypercholesterolaemia show impaired acetylcholine-induced vasorelaxation prior to the

development of atherosclerosis as visualised by angiography.• The loss of NO has considerable effect on the development of the disease. In the early stages of the disease

reduced NO would leave the endothelium vulnerable to increased leukocyte diapedesis and increase the possibility of LDL oxidation in the subendothelial space. Furthermore, the smooth muscle cell (SMC) proliferation associated with neointimal thickening would proceed unchecked. In the final stages, loss of NO could aggravate platelet activation, leading to thrombosis and myocardial infarction.

NO aids in gas exchange between hemoglobin and cells• Hemoglobin is a vasoconstrictor, Fe scavenges

NO• NO is protected by cysteine group when O2

binds to hemoglobin.• During O2 delivery, NO locally dilates blood

vessels to aid in gas exchange

Diabetes and nitric oxide:

• Diabetes mellitus is associated with increased rates of morbidity and mortality caused primarily by the accelerated development of atherosclerotic disease.

• Similar to other atherosclerotic pathologies diabetic vascular disease is characterised by endothelial dysfunction.

Nitric oxide and hypertension:• NO is crucial to the maintenance of normal blood pressure and therefore its

relationship to essential hypertension has been the subject of intense investigation.• As with coronary artery disease and diabetes initial evidence suggesting a NO-

dependent component of the disease came from studies assessing endothelium-dependent vasodilatation

• A number of studies have demonstrated the impairment of NO-mediated vasodilatation in brachial, coronary and renal arteries in patients with essential hypertension compared to controls.

• Evidence suggests that the impaired NO responses are genetically determined, since basal NO production is impaired in offspring of patients with essential hypertension.

Nitric oxide and septic shock• Septic shock represents a disorder in which overproduction of NO is critical.• Is characterised by hypotension, compromised vascular function and multi-

organ failure.• The hypotension associated with septic shock is brought about by a massive

increase in NO production, emanating primarily from NOSII present in VSMC.

• Lipopolysaccharide (LPS) found in the cell wall of gram-positive bacteria is the endotoxin responsible for the induction of septic shock, It has been postulated that LPS leads to a widespread increased expression of iNOS, resulting in a large systemic increase in NO production. LPS induces the production of several cytokines such as tissue necrosis factor (TNF-α), interferon-γ (INF-γ), interleukins (IL)-1 and 6: these cytokines have the capacity to increase NOSII expression.

• The activation of NOSII by endotoxin results in far larger quantities of NO produced than under normal conditions. This leads to an increase in vasodilation and an increased resistance to the actions of vasoconstrictors

NITRIC OXIDE AND NEUROLOGICAL DISORDERS

NO serves in the body as a neurotransmitter, but there are definite differences.– NO is a signaling molecule, but not necessarily a neurotransmitter.– NO signals inhibition of smooth muscle contraction, adaptive

relaxation, and localized vasodilation.– n nos action in C N S have been associated with pain perception in

spinal cord level.– NO diffuses out of the cells making it vescular storage in vesicles and

release by exocytosis– NO does not bind to surface receptors, but instead exits cytoplasm,

enters the target cell, and binds with intracellular guanyl cyclase– Present in presynaptic terminal– Natural removal from synaptic junction

Implicated in :- Alzheimer disease

Parkinson disease

Huntington disease

Amyotrophic leteral sclerosis

All are related to the excessive release of NO & glutamate

both.

But in Parkinson's disease Glial cells produce excessive levels of nitric oxide, which may be neurotoxic for a sub population of dopaminergic neurons, especially those not expressing NADPH- diaphorase activity.

The presence of glial cells expressing nitric oxide synthase in the substantia nigra of patients with Parkinson's disease represents a consequence of dopaminergic neuronal loss.

Play a role in long term memory– As a retrograde messenger that facilitates long term potentiation of neurons

(memory)– Synthesis mechanism involve Ca2+/Calmodulin

NO-mediated effects mediated by astrocytes and neurons. NO released from astrocytes as a result of NOS II activation by proinflammatory mediators, can have toxic effects on both astrocytes and, by diffusion to neighbouring cells, also neurons. Additionally, NO can mediate neurotoxicity by conversion to its more toxic metabolite peroxynitrite (ONOO−). Glutamate release, causing Ca2+ entry can also activate the Ca2+-dependant NOS I. It has also been proposed that NO can potentiate glutamate release. Given the effects on nearby cells, it is possible that a vicious circle chain of events could ensue.

• Stroke can be either occlusive (ischemic) or haemorrhagic, and NO appears to play a role in both types.

• It is proposed that ischemia causes release of NO from the vascular endothelium in an attempt to limit the degree of damage by increasing local blood flow. As ischemia develops and the infarct evolves, NO may have a more deleterious effect.

• Glutamate excitotoxicity has been implicated in stroke: ischemia markedly increases release due to depolarisation of the pre-synaptic neuron. In addition to NMDAR-mediated Ca2+ entry and stimulation of NOS I, NOS II expression also occurs in astrocytes

• The release of other inflammatory mediators including NF-κβ, TNFα and IRF-1 also potentiates NOS II-mediated generation of NO, which peaks between 12 and 24 h after ischemia develops.

• Haemorrhagic stroke results in blood spilling into the brain tissue. In addition to focal ischemia “downstream” of the bleed, as haemoglobin is a powerful scavenger of NO, when blood comes into contact with the outside of a blood vessel, this can lead to acute vasospasm.

• The presence of blood out with the vasculature will mediate inflammation, again resulting in activation of the inflammatory cascade including NOS II.

Stroke:

IMMUNE SYSTEM AND NITRIC OXIDE NOS II catalyzes synthesis of NO used in host defense reactions

– Activation of NOS II is independent of Ca2+ in the cell– Synthesis of NO happens in most nucleated cells,

particularly macrophages.– NO is a potent inhibitor of viral replication.

NO is a bactericidal agent– NO is created from the nitrates extracted from food

near the gums.– This kills bacteria in the mouth that may be harmful

to the body.

Nitric oxide in inflammation and asthma:• NO is generated at high levels during human inflammatory reactions such as

asthma and, as in the immune response, the principal NOS isotype involved is NOS-2 .

• Although the majority of mediators generated during inflammation, such as IFN-γ, TNF-α, leukotrienes and most prostaglandins, are pro-inflammatory, others such as the cyclopentanone prostaglandins are anti-inflammatory.

• The first event in IgE-mediated inflammatory diseases such as asthma is the activation of mast cells by antigen. Mast cells release histamine that, in synergy with other mediators, dilates and permeabilises the blood vessels, leading to increased blood supply to the tissues, raised temperature, edema and infiltration of white blood cells. Mast cells also release several cytokines, such as TNF-α, that may promote the later phases of inflammation by recruiting other inflammatory cell types. It has been known for some time that NO inhibits mast cell activation and more recently NO has been reported to mediate the inhibitory effects of IFN-γ on mast cells in mixed cell populations.

• In human asthma NO may be beneficial by virtue of its capacity to dilate airway smooth muscle, probably reflecting the natural function of NOS-1 in the lung, and as mentioned above, may inhibit mast cell activation. On the other hand, NO causes dilation and permeation of blood vessels and this may contribute to airway edema.

• Although glucocorticoids are an effective treatment for asthma and inhibit NO synthesis in asthmatic lungs, this does not necessarily imply a causative link between NO and the disease.

DIGESTIVE SYSTEM AND NITRIC OXIDE

NO is used in adaptive relaxation– NO promotes the stretching of the stomach in response

to filling.– When the stomach gets full, stretch receptors trigger

smooth muscle relaxation through NO releasing neurons.Role In the Reproductive system: Nos localized in pelvic nerve neuron innervating the

corpora cavrinosa and the neuronal plexuses of the adventitial layer of the penile arteries – proven most effective for erectyl dysfunction.

CONCLUSION

• Having read this article, we hope that readers who are new to the field will recognise the breadth and depth of nitric oxide research as it has evolved and how is it is currently evolving. Far from being a simple molecule the only action of which is to modulate cyclic GMP concentrations, nitric oxide clearly has a wide range of actions on other molecules large and small and therefore on diverse cellular processes. The likely growth area is most likely to be the effects of NO and its products on protein structure and function. The analyses of these changes in different disease states with the help of modern genomics, proteomics and metabolomics will eventually reveal the complete actions of this rather special molecule.

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