nitric oxide

A colorless, radical-free gas that reacts rapidly with O₂ to form other nitrogen oxides (e.g., NO₂, N₂O₃, and N₂O₄) and ultimately is converted to nitrite (NO₂⁻) and nitrate (NO₃⁻); a gaseous mediator of cell-to-cell communication and potent vasodilator, formed from l-arginine in bone, brain, endothelium, granulocytes, pancreatic beta cells, and peripheral nerves by a constitutive n. synthase, and in hepatocytes, Kupffer cells, macrophages, and smooth muscle by an inducible n. synthase (e.g., induced by endotoxin). NO activates soluble guanylate cyclase, mediates penile erection, and may be the first known retrograde neurotransmitter.The short-lived NO molecule is a product of various tissues and plays a role in various processes. NO elaborated by endothelium, which is identical to endothelium-derived relaxing factor, dilates vessels by relaxing vascular smooth muscle; nitrites used in coronary and peripheral vascular disease induce or mimic this action. The 1998 Nobel Prize in Medicine or Physiology was awarded to 3 U.S. pharmacologists, Robert F. Furchgott, Ferid Murad, and Louis J. Ignarro, for their independent discoveries of the role of n. in cardiovascular physiology. In the immune system, macrophages use NO as a cytotoxic agent. Deficiency or inactivation of NO may contribute to the pathogenesis of both hypertension and atherosclerosis. An excess of NO , which is a free radical, is toxic to brain cells, and NO is also responsible for the precipitate, often fatal, drop in blood pressure accompanying septic shock. Free NO in the bloodstream is rapidly reduced by the iron of hemoglobin.

- n. reductase an enzyme oxidizing N₂ with some acceptor to 2NO , a first step in the fixing of atmospheric nitrogen by bacteria.

- n. synthase (NO synthase) an enzyme that catalyzes the reaction of l-arginine with 2O₂ and 1.5NADPH to form NO, l-citrulline, 1.5NADP⁺, and 2H₂O; there are an inducible and two constitutive forms of this enzyme : the constitutive forms play significant roles in regulating vascular tone, tissue blood flow, renal function, etc.; in bone, brain, endothelium, granulocytes, pancreatic Z-cells, and peripheral nerves, the constitutive forms are calcium-calmodulin dependent; in brain, the enzyme is cytosolic; in endothelium, it is membrane bound; the inducible form of the enzyme ( e.g., by endotoxin) in hepatocytes, Kupffer cells, macrophages, and smooth muscle is not calmodulin dependent.

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nitric oxide n a poisonous colorless gas NO that occurs as a common air pollutant formed by the oxidation of atmospheric nitrogen and that is also formed by the oxidation of arginine in the mammalian body where it acts as a mediator of intracellular and intercellular communication regulating numerous biological processes (as vasodilation and neurotransmission)

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an important member of the group of gaseous mediators, which - together with amine mediators (e.g. adrenaline, noradrenaline, histamine, acetylcholine) and lipid mediators (e.g. prostaglandins) - produce many physiological responses (e.g. smooth muscle relaxation). Nitric oxide is involved in the manifestations of sepsis and septic shock. Formula: NO.

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1. NO, a naturally occurring gas that in the body is a short-lived dilator substance released from vascular endothelial cells in response to the binding of vasodilators to endothelial cell receptors; it causes activation of guanylate cyclase in vascular smooth muscle, leading to an increase in cyclic GMP, which inhibits muscular contraction and produces relaxation. Excess nitric oxide is toxic to cells of the central nervous system and causes the hypotension seen in septic shock. Called also endothelial- or endothelium-derived relaxing factor. 2. a preparation of nitric oxide used in conjunction with ventilatory support or other agents as a pulmonary antihypertensive in the treatment of hypoxic respiratory failure due to persistent fetal circulation in term or near-term neonates; administered by inhalation.