In spite of intense recent investigation of the physiological and pathophysiological roles of endogenously produced nitric oxide (-NO) in mammalian systems, little quantitative information exists concerning the diffusion of this small nonelectrolyte from its source (NO synthase) to its targets of action. I present here a conceptual framework for analyzing the intracellular and intercellular diffusion and reaction of free NO, using kinetic modeling and calculations of the diffusibility of -NO and its reactions in aqueous solution based on published data. If the half-life of -NO is greater than -25 msec and the rates of reaction of -NO with its targets are slower than its diffusion or reaction with 02 (for which there is experimental evidence in at least some systems), then (i) 'NO acts in vivo in a mostly paracrine fashion for a collection of 'NO-producing cells, (ii) NO diffuses to significant concentrations at distances relatively far removed from a single 'NO-producing cell, and (iU) localized sites of vascularization wil scavenge'-NO (and thus decrease its actions) at distances many cell diameters away from that site. These conclusions have important implications with regard to the mechanism of endothelium-dependent retaxation, the autocrine vs. paracrine actions of 'NO, and the role of the spatial relationship between specific sites of 'NO formation and neighboring blood vessels in 'NO-effected and -affected neuronal signal transmission.Although chemists, biochemists, and microbiologists have examined the unique properties ofnitric oxide ('NO) for >200 years, it has only been within the last 7 years that its multiple physiological functions in mammals have been appreciated (reviewed in ref. 1). 'NO is synthesized by specific enzymes in many cell types, in response to inflammatory, neural, or vascular stimuli. In inflammation, 'NO production can be induced by exposure to bacterial products and/or cytokines and functions as a cytostatic/cytotoxic effector for defense against transformed or infected host cells and pathogenic organisms. In neural and endothelial cells, 'NO is an intercellular messenger, effecting signal transduction by stimulation of heme-containing soluble guanylate cyclase.Intense research has been directed toward characterizing the production of -NO by the various isoforms of 'NO synthase (2, 3) and the reactions of 'NO with its molecular targets (4). -NO is a small, neutral, relatively hydrophobic (5, 6) nonelectrolyte in aqueous solutiont consistent with its role as a diffusible intercellular messenger or immune effector. Little attention has been directed toward understanding the quantitative characteristics of this diffusion process and it is frequently characterized as a short-lived, short-range mediator. However, I demonstrate here that kinetic analysis of the production, diffusion, and reaction of -NO under physiologically significant conditions contradicts these assumptions. The distance of diffusion of -NO is in fact surprisingly long, and this finding has important implicati...

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The role of reactive nitrogen intermediates (RNI) such as nitric oxide ( . NO) in host defense against pyogenic microorganisms is unclear, and the actual interactive effect of RNI and reactive oxidative intermediates (ROI) for microbial killing has not been determined. Since, in nature, ROI and RNI might be generated together within any local infection, we evaluated the separate and interactive effects of . NO and O 2 ؊ on staphylococcal survival by using a simplified system devoid of eukaryotic cells. These studies showed that prolonged exposure of staphylococci to . NO does not result in early loss of viability but instead is associated with a dose-related delayed loss of viability. This effect is abrogated by the presence of hemoglobin, providing further evidence that the effect is RNI associated. Superoxide-mediated killing also is dose related, but in contrast to RNI-mediated killing, it is rapid and occurs within 2 h of exposure. We further show that the interaction of . NO and O 2 ؊ results in decreased O 2 ؊ -mediated staphylococcal killing at early time points. . NO, however, appears to enhance or stabilize microbial killing over prolonged periods of incubation. This study did not produce evidence of early synergism of ROI and RNI, but it does suggest that . NO may contribute to host defense, especially when ROI-mediated killing is compromised.

We describe the hemodynamic effects and metabolic fate of inhaled NO gas in 12 anesthetized piglets. Pulmonary and systemic hemodynamic responses to incremental [NO] (5-80 ppm) were tested during ventilation with high- [0.30 inspired O2 fraction (FIO2)] and low-O2 (0.10 FIO2) mixtures. In six animals, inhalation of 40 ppm NO was maintained over 6 h to test effects of prolonged exposure (0.30 FIO2). In the other six animals, pulmonary hypertension was induced by hypoxic ventilation (0.10 FIO2) and responses to NO were tested. Inhaled low [NO] partially reversed pulmonary hypertension induced by alveolar hypoxia; mean pulmonary arterial pressure decreased from 31.4 +/- 2.3 mmHg during hypoxia to 18.2 +/- 1.2 mmHg during 5 ppm NO. Mean pulmonary arterial pressure at 0.10 FIO2 did not fall further at higher [NO] (10-40 ppm) and never reached control levels. Pulmonary vascular resistance increased with institution of hypoxic ventilation and fell with subsequent administration of NO, ultimately reaching control levels. Inhaled NO did not affect systemic vascular resistance. Plasma levels of NO2- + NO3- and methemoglobin (MetHb) levels increased with increasing [NO]. Over 6 h of NO administration during high-O2 ventilation, MetHb equilibrated at subtoxic levels while NO2- + NO3- increased. Nitrosylhemoglobin, analyzed by electron paramagnetic resonance spectrophotometry was not detected in blood at any time. At the relatively low concentrations (5-80 ppm) that are effective in relieving experimental pulmonary hypertension induced by alveolar hypoxia, inhaled NO gas causes accumulation of NO2- + NO3- in plasma and a small increase in MetHb but no detectable nitrosylhemoglobin.

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