Nitric oxide
- It is one of the oxides of nitrogen with molecular formula NO indicating that it is a free radical with an unpaired electron.
- smallest endogenous signal molecule and the only one which is a gas at normal temperature.
- Furchgott and Zwadzki ( 1980) demonstrated that ACh caused vascular smooth muscle relaxation only when the endothelial lining was intact, but not when it was denuded by rubbing.
- They proposed that on activation by ACh, the endothelial cells released a soluble ‘endothelium derived relaxing factor’ (EDRF) which diffused to the underlying smooth muscle cell causing relaxation.
- A little earlier Farid Murad ( 1977) had already shown that GTN caused vasodilatation by releasing N0.
- lgnarro in 1987 confirmed that EDRF infact was NO. All three, Furchgott, Murad and lgnarro received Nobel Prize in 1998 for these discoveries.
Generation and fate of NO:
- Nitric oxide is produced in the cells from the amino acid L-arginine by the enzyme nitric oxide synthase (NOS) using molecular oxygen and NADPH, FAD, haeme act as coenzyme to NOS.
- Three isoforms of NOS, viz.
- neuronal NOS (nNOS or NOS- I ),
- inducible NOS (iNOS or NOS-2) and (found in macrophages, monocytes, neutrophils, hepatocytes, etc. is induced by bacterial endotoxins, cytokines and TNFa)
- endothelial NOS (eNOS or NOS-3).
- Activation of eNOS and nNOS is Ca2+-calmodulin dependent.
- Agents and mediators activate eNOS/nNOS by raising cytosolic Ca2+.
- In contrast, iNOS is active as such, and not Ca2+ dependent.
Actions and roles of NO
Vascular system:
- Potent vasodilator and serves to regulate vascular tone.
- Produced in the endothelium
- freely diffuses to the nearby smooth muscle cell and
- activates a soluble guanylyl cyclase (sGC) raising cytosolic cGMP concentration
- This in turn activates a cGMP-dependent protein kinase (PKG) which dephosphorylates the myosin light chain kinase (MLCK) à reduced availability of phosphorylated (active) MLCK
- actin-myosin interaction is interfered
- resulting in muscle relaxation.
- Bronchial, gastrointestinal and other visceral smooth muscles are also relaxed by NO, but the effect is less marked than in blood vessels
Platelets:
- Just as in smooth muscle, NO activates the cGMP pathway in platelets to inhibit aggregation and thrombus formation.
- Defective endothelial and platelet NOS-NO function increases the risk of thrombosis.
- NO also has antiatherogenic property.
- Adhesion of leucocytes to the endothelium and cellular proIiferation/migration are suppressed by NO, conferring it a protective effect on plaque formation.
Infection, inflammation and immune function:
- Normally the iNOS activity in macrophages, etc. is very low.
- Bacterial endotoxins, TNFa, interferon and interleukins induce iNOS in macrophages and other inflammatory cells à generate large quantities of NO.
- Activated macrophages also produce superoxides that combine with NO à decomposes to release free radicals à broad antimicrobiaI action on bacteria, viruses, fungi and protozoa.
- Thus, N0 helps in combating infection.
- N0 has many proinflammatory actions, viz. vasodilatation, increased vascular permeability and enhanced PG production.
- excess and prolonged NO production à detrimental consequences à tissue injury.
- Excessive production of NO has been implicated in causing severe hypotension attending septic shock
Nervous system:
- NO serves as neurotransmitter/ neuromodulator in central as well as in peripheral nervous system.
- Activation of NMDA glutamate receptors causes long-lasting Ca2+ influx triggering NO release
- Function à nociception, increased cerebral blood flow, learning and memory.
- NO is believed to mediate ‘synaptic plasticity ‘ a process by which junctional transmission is strengthened in a use dependent manner.
- Excessive production of NO has been implicated in promoting ‘excitotoxicity’ due to over- stimulation of glutamate neurones.
- Aberrant NO generation à causative role in degenerative neurological diseases like Huntington ‘s disease, Parkinson’s disease, amyotrophic lateral sclerosis, etc.
- pelvic nerve stimulation causes penile erection by releasing NO from neurones in the corpora cavernosa.
- NO relaxes cavernosal smooth muscle, filling the sinusoids with blood and causing penile tumescence
- Accordingly, sildenafil and other PDE-5 inhibitors are effective in the treatment of erectile dysfunction, because they inhibit degradation of cGMP which mediates NO action.
- Defective NO signaling is believed to be involved in pyloric stenosis and gastric dilatation.
Lungs:
- NO appears to play a prominent role in the regulation of pulmonary vascular resistance and the development of pulmonary arterial hypertension (PAH)
- Sildenafil à indirectly potentiates the vasodilator action of NO by preventing inactivation of cGMP

Nitric oxide donors
These are compounds which contain a NO or similar moiety in their structure, that is released in the body, mostly enzymatically, to elicit smooth muscle relaxation.
- Organic nitrates, nitrites and sodium nitroprusside are the clinically useful NO donors. They are also called ‘nitro-dilators’.
Glyceryl trinitrate (GTN):
- rapidly acting à antianginal drug à mainly dilates veins and coronary arteries.
- Mitochondrial aldehyde dehydrogenase is the most important enzyme which releases NO from GTN.
- This enzyme is abundantly expressed in veins, offering an explanation for the preferential venodilator action of GTN.
Isosorbide dinitrate and lsosorbide mononitrate
- slower acting, but the pattern of action is similar to GTN .
- When administered continuouslyà developing tolerance (within few hours), which also weans off rapidly.
à reactive oxygen species generated during metabolism of organic nitrates inhibit mitochondrial aldehyde dehydrogenase and other enzymes which produce the NO species, limiting further action.
Sodium nitroprusside:
- Administered by i.v. infusion, nitroprusside equally dilates arterioles and veins by releasing NO as well as by directly activating guanylyl cyclase.
- Rapidly lowers BP and the action is titratable.
Nitic oxide as a therapeutic agent:
- Apart from being an important signal molecule. exogenously administered N0 gas has limited therapeutic application as well.
- Inhaled NO à indicated in severe respiratory distress in neonates with elevated pulmonary arterial pressure.
- It dilates pulmonary vessels, particularly in areas of the lung that are better ventilated, reducing the ventilation-perfusion mismatch and improving gaseous exchange.
- The elevated pulmonary artery pressure is lowered and cardiopulmonary function is improved.
- Inhaled NO improves cardiopulmonary function in adult patients as well with severe PAH, but this can only be a short-term measure.