Nitric oxide and Drugs

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.