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Nitric Oxide

Weiqun Shen, Thomas H. Hintze, Michael S. Wolin

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Background Nitric oxide (NO) is known to be an inhibitor of mitochondrial function. However, the physiological significance of endothelium-derived NO in the control of tissue respiration is not established. Methods and Results Tissue O 2 consumption by skeletal muscle slices of the triceps brachii of normal dogs was measured with a Clark-type O 2 electrode/tissue bath system at 37°C. S -Nitroso- N -acetylpenicillamine (SNAP), carbachol (CCh), or bradykinin (BK) decreased tissue O 2 consumption by 12±3% to 55±8%, 15±6% to 36±11%, or 21±5% to 42±4% at doses of 10 −7 to 10 −4 mol/L, respectively. The effects of both CCh and BK but not SNAP were eliminated by nitro- l -arginine (NLA, 10 −4 mol/L), consistent with SNAP decomposing to release NO and both CCh and BK stimulating endogenous NO production from l -arginine. Oxygen consumption was also decreased by 8-bromo-cGMP. The mitochondrial uncoupler dinitrophenol blocked the effects of 8-bromo-cGMP but only slightly altered those of SNAP, indicating that the major site of action of NO is the mitochondria. In normal, chronically instrumented, resting conscious dogs, blockade of NO synthase by NLA increased mean arterial pressure by 28±2.5 mm Hg and hind limb vascular resistance by 114±12% and decreased blood flow by 39±3%. Most important, NLA also increased O 2 uptake by 55±9% in hind limb skeletal muscle ( P <.05), associated with decreases in P o 2 and O 2 saturation and an increase in reduced hemoglobin in hind limb venous blood. Conclusions Our results indicate that NO release from vascular endothelial cells appears to play an important physiological role in the regulation of tissue mitochondrial respiration in skeletal muscle and perhaps other organ systems.