The biological lifetime of nitric oxide: Implications for the perivascular dynamics of NO and O ₂

Abstract

Endothelial nitric oxide (nitrogen monoxide) is synthesized at the intravascular͞extravascular interface. We previously have reported the intravascular half-life of NO, as a result of consumption by erythrocytes, as approximately 2 ms. We report here studies designed to estimate the lifetime of NO in the parenchymal (extravascular) tissue and describe the implications of these results for the distribution of NO and oxygen concentration gradients away from the blood vessel. The rate of consumption of NO by parenchymal cells (hepatocytes) linearly depends on both NO and O2 concentration. We estimate that the extravascular half-life of NO will range from 0.09 to > 2 s, depending on O2 concentration and thus distance from the vessel. Computer modeling reveals that this phenomenon, coupled with reversible NO inhibition of cellular mitochondrial oxygen consumption, substantially extends the zone of adequate tissue cellular oxygenation away from the blood vessel, with an especially dramatic effect during conditions of increased tissue work (oxygen consumption). This represents a second action of NO, in addition to vasodilation, in enhancing tissue cellular respiration and provides a possible physiological function for the known reversible inhibition of mitochondrial respiration by low concentrations of NO. Q uite possibly, the two most important properties of nitric oxide (nitrogen monoxide) as a messenger and effector molecule in mammals are its ready diffusibility and its relatively short lifetime (1). With regard to the lifetime of NO, initial measurements using cascade perfusion experiments suggested a disappearance of NO with a lifetime on the order of seconds (2); however, measurements with intact tissue (the coronary circulation) yield a much faster rate, on the order of 0.1 s (3). Although reaction of NO with oxygen (4) is commonly cited as the explanation for this disappearance, this is highly unlikely because it will be too slow with physiologically relevant NO concentrations, even considering the 300-fold acceleration of this process in hydrophobic compartments such as the cell membrane (5). The characteristics of the disappearance of NO by parenchymal cells, which will determine its perivascular diffusion, have not been reported. We have previously characterized the consumption of NO by erythrocytes and estimate that the half-life of NO in the vascular lumen is approximately 2 ms (6). Here we characterize NO consumption by parenchymal cells (isolated rat hepatocytes) and present the implications of these results in terms of the concentration gradients of NO and of O 2 away from a blood vessel. Materials and MethodsChemicals. Chemicals and supplies were from standard sources and were of the highest purity available.Hepatocyte Isolation. Rat hepatocytes were isolated as described (7). Cells were finally resuspended in 10 mM phosphate buffer ϩ 20 mM dextrose, pH 7.4 and kept on ice until use. Viability was Ͼ90% for all experiments, and exposure to NO did not result in appreciable decreases in this number.Measu...

Nitrogen monoxide (NO)-mediated iron release from cells is linked to NO-induced glutathione efflux via multidrug resistance-associated protein 1

“…''Free NO'' has a short half-life (t 1/2 ) of 2 ms-2 s, but DNICs increase NO t 1/2 and are natural carriers of NO (30)(31)(32)(33)(34)(35)(36). In fact, glutatione S-transferase, an enzyme associated with MRP1, can increase the t 1/2 of DNICs to 4.…”

Nitric Oxide Storage and Transport in Cells Are Mediated by Glutathione S-Transferase P1-1 and Multidrug Resistance Protein 1 via Dinitrosyl Iron Complexes

“…In fact, DNICs are a natural storage form of NO that possesses a longer half-life than NO alone (15). In addition, DNICs associate with GST to stabilize NO for many hours (15), which markedly exceeds the half-life of "free NO" (2 ms-2 s) (33). Further, DNICs permeate cells to donate iron and trans-nitrosylate molecular targets, demonstrating their bioavailability (1,34).…”

Nitric oxide inhibition of respiration involves both competitive (heme) and noncompetitive (copper) binding to cytochromecoxidase

“…However, in 1994 it was additionally shown that mitochondrial oxygen consumption by cytochrome c oxidase (CCO) is reversibly inhibited by NO in a manner apparently competitive with the oxygen tension (4)(5)(6). Inhibition of mitochondrial respiration by NO at CCO has since been implicated in a wide range of physiological processes (7)(8)(9)(10)(11)(12), and several of these require a competitive (with respect to oxygen) element to the interaction, e.g., activating hypoxia-inducible factor (13); maintaining flow-metabolism coupling in the brain (14); and allowing cells distant from capillaries to maintain adequate oxygenation (15). Under pathological conditions such as sepsis, the elevated NO concentration, generated by inducible NO synthase, is associated with mitochondrial dysfunction and is implicated in mortality in the critically ill (16).…”