The saying goes that the first impression is the last impression. Then, imagine what reputation nitric oxide got only a decade ago. It was labeled as a possible carcinogen, ozone destroyer, precipitator of acid rain, and a pollutant of the anesthetic nitrous oxide. Now listen to this: Today, hardly a quarter passes without a new biological role assigned to this once petty poison. According to the Nitric Oxide Journal, more than 18,000 research papers have been published on nitric oxide to date. NO was declared Molecule of the Year by the esteemed scientific journal Science. And what's more, a fortnight ago, the Nobel Prize committee awarded the coveted prize to three scientists who work involved the biology of nitric oxide.

Sure, sayings sometimes don't hold on as they might.

Just what is so cool about nitric oxide? Actually, it is the lightest, smallest, molecule in the biological systems to act as a messenger. Moreover, NO is the first gas known to act in the way it does. According to current data, NO is a neurotransmitter, is an important biological mediator, produces erection in males, regulates blood pressure, plays a part in learning and remembering, has important anti-inflammatory properties, fights invading pathogens, has a role in diabetic complications, helps in food digestion - you name the process and NO sneaks in.

Since NO brushes almost every aspect of biology and medicine, it is hopeless to try to cover it in one magazine article. However, an attempt has been made here to keep you abreast of the clinical relevance of this gem of a molecule. If you want to probe further, I would be glad to provide you references that cover various shades of NO biology in detail.

Historical perspective

Amyl nitrate, a NO donor, has been used since 1867 for the treatment of angina pectoris, although, of course, without knowing the exact pharmacodynamics. In 1980, Furchgott and Zawadzki reported in Nature that endothelium releases a substance in response to acetylcholine that relaxes the smooth muscle and consequently baptized endothelium-derived relaxing factor, or. Furchgott and Zawadzki postulated EDRF could be a prostanoid metabolite of the pro-inflammation enzyme lipooxygenase. However, in 1987, the true identity of EDRF was unmasked and it was established that it is indeed NO. This finding catapulted the hitherto modest molecule into the spotlight. Papers started trickled in on NO, then formed a torrent and now nitric oxide research is converted in the form a sea where scientific data continues to pour in from all parts of the world.

Chemistry and biosynthesis

NO is one of the simplest molecules. It is composed of just two atoms: nitrogen and oxygen. It is a free radical, meaning that it has an unpaired electron in a 2p-p orbital. This makes NO extremely unstable and remarkably reactive. In biological systems, the half-life of NO is not more than a few seconds.

In the physical world, lightening catalyzes the reaction between nitrogen and oxygen to produce NO. It is thought that NO was first synthesized during the primordial electric storms, way back in the early days of earth formation. In the biological systems, however, less thunderous processes are utilized to produce NO. NO is synthesized in almost every cell of the body with the help of the enzyme nitric oxide synthase or NOS. NOS is a flavoprotein of the family of cytochrome P-450 enzyme system. Three isoenzyme have been so far identified: an endothelial type (eNOS), that produces NO for normal physiological functions; nNOS, or neuronal type, which creats NO for neurotransmission; and finally, iNOS, or inducible type that manufactures NO for defensive purposes. The chemical reaction is the hydroxylation of the amino acid L-arginine. The end products of the reaction are nitric oxide and citrulline. What is fascinating about this reaction is that citrulline is recycled back into L-arginine, and the cycle repeats itself. (See figure)

PHYSIOLOGICAL ROLES OF NITRIC OXIDE

NO IN THE CARDIOVASCULAR SYSTEM

When scientists at the US NIH infused a NO antagonist NG-monomethyl-L-arginine into the brachial artery of healthy volunteers, they recorded a sharp decline in the forearm blood flow. See fig. This suggests that NO plays a pivotal role in maintaining vascular basal tone and is continuously synthesized by the endothelium to preserve vessel wall homeostasis.

The endothelium acts as a signal detector, sniffing changes in the stress that intraluminal blood exerts on its surface. If the blood pressure is on the rise, the endothelium is prompted to synthesize and release NO in the lumen. NO then stimulates soluble gc. See diagram for the mechanism

NO has a thrombo-protectant role as well. It inhibits platelet white cells' adhesion to the surface of endothelium. Other cardiovascular functions include inhibition of a potent vasoconstrictor endothelin-1and suppression of release of the sympathetic neurotransmitter, norepinephrine.

NO IN THE NERVOUS SYSTEM

In 1988, various investigators succeeded in staining NOS in different parts of the nervous system. This finding led to the iconoclastic theory that NO might serve as a neurotransmitter in the nervous system. Iconoclastic in the sense that conventional neurotransmitters are complex molecules that are stockpiled in storage vesicles at nerve endings and are released into the synaptic cleft in response to a nerve impulse. The neurotransmitters then bind to stereo-specific receptors and effect a response. NO, on the other hand, knows no rules. It's beguilingly simple, doesn't have any storage arrangements and binds to target molecules rather than receptors.

NO MEMORY

Understanding how memories are stored in the brain has remained a Holy Grail of biological research. One of the most promising theories is LTP, or long term potentiation. According to LTP, repeated signals trigger particular receptors called NMDA receptors. These receptors produce an efflux of calcium ions into the cell. Literature is amassing that the post-synaptic neuron releases NO, which diffuses back to the pre-synaptic receptor and "potentiates" or "strengthens" the pre-synaptic neuron, so that a successive signal causes greater firing from the neuron so that the receiving cell produces a bigger response. Thus NO serves the role of "retrograde" neurotransmitter.

NO also monitors the formation and functioning of ion channels and receptors. There is evidence that NO may be calling the shots in neurotransmission by regulating the vital potassium channels.

There is an extensive innervation of various body tissues via non-adrenergic non-cholinergic (NANC) transmission. One of the mediators in NANC system is NO in gastrointestinal tract and genitourinary tract. The major role of NO here is vasoregulation through its smooth muscle relaxing prowess. It is, however, undecided as yet that whether NO is released by the NANC neuron or by the central nervous system.

NO DEFENSE

Any pathologic alteration in the activity of NOS isoforms may lead to tissue damage and contribute to various inflammatory and autoimmune diseases. For example, an interruption in cNOS is shown to cause hypertension, whereas overproduction of iNOS activity may precipitate potentially fatal hypotension of septic shock.

Evidence is accumulating that endothelium-derived NO is responsible for the vasodilatory actions of both insulin and insulin-like growth factor in human skeletal muscle. Baron et al have demonstrated that NO blockade chokes the effects of insulin on leg blood flow. This translated into a one-fourth reduction of insulin-mediated glucose uptake.

Diabetes

Evidence indicates that the vascular complication like atherosclerosis, nephropathy and retinopathy annexed with type 1 diabetes mellitus result due to problems with EDNO. The primary mechanism seems to be that the initial damage is caused to the endothelium by hyperglycemia - for example, delay in endothelial cell replication and inhibition of growth due to free radicals generated by auto-oxidation of glucose, inhibition of growth - rendering the endothelium increasingly ineffective in releasing NO. Lack of NO further greases the wheels of vascular destruction due to continued vasoconstriction and platelet aggregation.

Atherosclerosis

Since NO is endogenous atherogenic agent due to its effects on monocyte adherence, platelet aggregation and smooth muscle cell proliferation, complications in its biosynthesis can adversely influence these processes and pave the way for athersclerosis. It has been shown that any therapeutic agents that might enhance NO dependent vasodilatation (e.g., dietary arginine) can arrest atherosclerosis.

Impotence

There is an extensive innervation of various body tissues via non-adrenergic non-cholinergic (NANC) transmission. One of the mediators in NANC system is NO in gastrointestinal tract and genitourinary tract. Muscles in carpora cavernosa of the penis relax in response to NANC nerve transmission to effect erection. Previously, it was thought that a vasodilator peptide and substance P are responsible for this process. Now it has been established that NO mediates an important step in the physiology of erection.