NITRIC OXIDE

Dr. KANIMOZHI.S1ST YEAR- MD PHYSIOLOGY

Prior to 1990: An air pollutant

Named “Molecule of the Year” by Science magazine in 1992

Robert Furchgott, Louis J Ignore, Ferid Murad got Nobel Prize in 1998

Furchgott IgnarroMurad

Nitric oxide (NO) is an endogenous messenger molecule that is extensively involved in the physiologic regulation of different tissues in the human body

First described in 1979 as a potent relaxant of peripheral vascular smooth muscle

Serves different functions depending on body system. i.e. neurotransmitter, vasodilator, bactericide etc

The structure and nature of Nitric The structure and nature of Nitric OxideOxide

Nitric oxide is a diatomic 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

The natural form is a gas

Synthesis of Nitric oxide

Common chemical reactions of nitric oxide

Interaction of NO with free radicals

Free radical formed by gaining an additional electron NO radical can react with peroxyl radical (RO2),hydroxyl radical (OH.) and NO- to produce peroxinitrite, nitrous acid and nitrous oxide respectively.

NO

NO bind to Fe 2+ haem group of Guanylyl Cyclase

Active Guanylate Cyclase

Increased cGMP

decreased intracellular Ca 2+

Relaxes muscle

Dilating the vessel &lowering B.P.

Production of NO

There are many organic nitrates and they can release NO through enzymatic or non-enzymatic reactions and have been used for a long time to relieve angina pectoris

The organic nitrites produce NO which can react with thiols to give RSNO that can transport it where it is needed

Other important classes of NO donors are the diazetinedioxides, Furoxans, C-nitoso compound, Oxatriazole-5-imines

One other possibility is to connect a NO-donor to known drugs. This can reduce the toxicity of the drug and add other effects connected to the NO-releasing capacity. eg NO aspirin

Inhibitors of NO

L-NAMEThiocitrullineNG –Nitro-L-arginineNG –Monomethyl-L-arginineacetateBenzamideAminoguanidine

Detection of NO and its derivatives

classical method is colorimetric and fluorimetric griess system

other methods are ESR and chemiluminescence

Nitric oxide synthase (NOS)

NOS I (nNos) Central and peripheral neuronal cells Ca+2 dependent, used for neuronal communication

NOS II (iNos) Most nucleated cells, particularly macrophagesIndependent of intracellular Ca+2

Inducible in presence of inflammatory cytokinesNOS III (eNos)

Vascular endothelial cellsCa+2 dependentVascular regulation

Types of NOS

Nitric Oxide in the human body has many uses which are best summarized under following categories

NO in the cardiovascular system NO in the respiratory system NO in the nervous system NO in reproductive system NO in the immune system NO in the muscular system NO in the digestive system NO in the kidneys

Cardiovascular system

Endothelial cells express eNOS, that is sensitive to Ca++, as well as iNOS , that is Ca++ insensitive

The largest bulk of NO within the cardiovascular system is produced by NOS III

Main stimuli that activate the biosynthesis of NO by endothelium is blood flow

Arg NO

GTP cGMP

5) NO binds to Guanylyl cyclase

Relaxation of smooth muscle

NO

Smooth muscle cell blood vessel wall

4) NO diffuses across membranes

2) ACh binds to receptors on endothelial cells

3) Activate NO synthase

1) Stimulated nerve releases Acetylcholine(ACh) at Nerve terminal

Nitric Oxide Signaling

Cells in cardiovascular tree synthesizing NO

platelets (eNOS, iNOS)smooth muscle cells (iNOS) macrophages (iNOS) neutrophils (iNOS) cardiomyocytes (eNOS)

NO and plateletsPlatelets and their progenitors contain a

constitutive NOS identical to endothelial NOS and produce and release NO upon activation

Platelet-derived NO inhibits P-selectin and GPIIb/IIIa expression in stimulated platelets

Antiplatelet effects of NO include the inhibition of 12-lipoxygenase and cyclooxygenase-2. platelet recruitment and aggregation is inhibited by platelet-derived NO,thus preventing thrombosis

NO as a therapeutic agent in cvs diseases

Direct NO donorsIndirect NO donorsCompounds that stimulate endogenous NO

biosynthesisNO-ASA

NO-ASA (Nitroaspirin)

Nitroaspirin has been shown to exert a wider antiplatelet action as compared with aspirin

In addition to the effects of aspirin other actions are:

Antiproliferative action on SMC

Prevents platelets induced pulmonary thromboembolism

Exerts tissue protective effects in focal spontaneous ischemia in hypertensives

Prevents restenosis and atherogenesis in aged and hypercholesterolemic individuals

Prevents gastric toxicity

Respiratory systemThe gas is primarily produced in the lungs

by cells in the airways (eg, epithelial cells inthe lungs, endothelial cells in vessels, andneurons)

NO promotes bronchodilation by directly relaxing the smooth muscles in the airway. Its produced continuously by the overlying airway epithelium

NO may also play a critical role in ventilation-perfusion coupling in the lung

Role of NO in lung pathology

Bronchial asthmaPrimary pulmonary hypertensionSmokingAcute respiratory distress syndromePersistent pulmonary hypertension in

newbornHigh altitude pulmonary oedema

Inhalational nitric oxide

NO is a potent vasodilator of vascular smooth muscle and when delivered by the inhalational route is a selective pulmonary vasodilator.

Mechanism of Action Diffuses rapidly from alveolus to pulmonary

vascular smooth muscle

Stimulates guanylate cyclase activity which increases the concentration of cyclic GMP which causes vasodilation

Selectively reverses acute pulmonary vasoconstriction caused by hypoxia or thromboxane

Rapidly inactivated by forming methemoglobin therefore does not cause systemic hypotension

Dosing of NO

Continuous inhalational agent given through inspiratory limb of the breathing circuit

Serum half-life is 3-4 seconds Theoretical effective range: 6-80 ppm Verify inhaled concentration of NO by using

inline chemiluminescence

Side effects of NO

Treatment

100% O2 methylene blue exchange transfusion hyperbaric oxygen

Nervous system

NO is a signaling molecule, also acts as a retrograde neurotransmitter

NO signals inhibition of smooth muscle contraction, adaptive relaxation, and localized vasodilation

Nitric oxide believed to play a role in long term potentiation in hippocampus

Difference between NO and other NT

NO is synthesized on demandNot stored in vesiclesRather than exocytosis it simply diffuses

between nerve terminalsNO does not bind to surface receptors, but

instead exits cytoplasm, enters the target cell, and binds with intracellular guanylyl cyclase

In the adrenal gland, nNOS occurs in discrete ganglion cells and fibers in the medulla, Splanchnic nerve stimulation augments both blood flow and catecholamine secretion from the adrenal medulla

In blood vessels, besides localizations in the endothelium, nNOS occurs in autonomic nerves in the outer, adventitial layers of various large blood vessels

nNOS neurons occur in the myenteric plexus throughout the gastrointestinal pathway. Depolarization of myenteric plexus neurons is associated with relaxation of the smooth muscle associated with peristalsis

Reproductive system

Nitric oxide (NO) plays a crucial role in reproduction at every level in the organism

In the brain, it activates the release of luteinizing hormone-releasing hormone (LHRH)

The axons of the LHRH neurons project to the mating centers in the brain stem and by afferent pathways evoke the lordosis reflex in females

In males, there is activation of NOergic terminals that release NO in the corpora cavernosa penis to induce erection by generation of cyclic guanosine monophosphate (cGMP)

NO also activates the release of LHRH which reaches the pituitary and activates the release of gonadotropins by activating neural NO synthase (nNOS) in the pituitary gland

In the gonad, NO plays an important role in inducing ovulation and in the reproductive tract, it relaxes uterine muscle via cGMP

Immune system

NOS II catalyzes synthesis of NO used in host defense reactionsActivation of NOS II is independent of Ca+2

in the cellSynthesis of NO happens in most nucleated

cells, particularly macrophagesNO is a potent inhibitor of viral replication

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

from food near the gumsThis kills bacteria in the mouth that may be

harmful to the body

Digestive system

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

in response to fillingWhen the stomach gets full, stretch

receptors trigger smooth muscle relaxation through NO releasing neurons

Muscular system

NO was orginally called EDRF (endothelium derived relaxation factor)

NO signals inhibition of smooth muscle contraction

Ca+2 is released from the vascular lumen activating NOS

NO is synthesized from NOS III in vascular endothelial cells

This causes guanylyl cyclase to produce cGMP

A rise in cGMP causes Ca2+pumps to be activated, thus reducing Ca2+ concentration in the cell

This causes muscle relaxation

Role of NO in kidneys

plays a major role in renal vascular biology interms of

anti-thrombotic anti-inflammatoryantiproliferative antioxidative effect

Functions of NO in kidneys

Renal macrovascular and microvascular dilatation(afferent > efferent)

Regulation of mitochondrial respiration Modulation renal medullary blood flow Stimulation of fluid, sodium and HCO3

reabsorption in the proximal tubule

Inhibition of Na+, Cl- and HCO3 reabsorption in the mTALH

Inhibition of Na+ conductance in the CCD Inhibition of H+-ATPase in CCD

Excessive release of NO causes

Neurotoxicity (stroke and neurodegenerative diseases)

Hypotension in septic shockReperfusion injuryCarcinogenesis

Conclusion NO is a universal messenger moleculeIt is involved in a wide variety of

pathophysiogical and biochemical reactions.In summary NO is involved in regulation of

B.P., prevention of aggregation and adhesion of platelets, promotion of penile erection.

Other way to increase active concentration of endogenous NO such as by prolonging its half life of duration of its actions

NO donating compounds can be used as replacement therapy to treat its impaired production

NO also as therapeutic potential for Ischemic CVS diseases, pulmonary hypertension associated with cardiac and respiratory diseases

They are far from ideal because of the associated side effect mainly due to the catabolism of NO into NO2

Therefore a technology to regulate in vivo synthesis of NO by genetic manipulation would be a welcome move