Potential Role of a Membrane-Bound NADH Oxidoreductase in Nitric Oxide Release and Arterial Relaxation to Nitroprusside

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Abstract
—The site of metabolism in vascular smooth muscle responsible for the release of nitric oxide (NO) from nitroprusside is not well established. In this study we observed that a membrane-bound NADH oxidoreductase in the pulmonary artery activates nitroprusside to release NO, and we examined whether this process could potentially participate in relaxation to nitroprusside. Relaxation to nitroprusside in bovine calf pulmonary artery is inhibited by a scavenger of NO and by an antagonist of NO stimulation of guanylate cyclase. A flavoprotein probe that inhibits pulmonary artery NADH oxidoreductase (1 μmol/L diphenyliodonium) and electron acceptors for NADH oxidoreductase (0.3 mmol/L nitroblue tetrazolium and 0.1 mmol/L ferricyanide) inhibited pulmonary artery relaxation to nitroprusside, but not to nitroglycerin. Pulmonary arteries were observed to promote the release of NO from nitroprusside in vitro, and NO release was inhibited by the presence of nitroblue tetrazolium, ferricyanide, and diphenyliodonium. In homogenates of pulmonary arteries, NADH (0.1 mmol/L) increased the release of NO from nitroprusside by ≈6-fold, whereas NADPH, mitochondrial substrates, and other redox cofactors had minimal effects on NO release, and the action of NADH on nitroprusside was inhibited by nitroblue tetrazolium, ferricyanide, and diphenyliodonium. A membrane fraction enriched in NADH oxidoreductase activity showed a NADH-dependent release of NO from nitroprusside; nitroprusside caused NADH consumption, and it also inhibited the NADH-dependent reduction of nitroblue tetrazolium. Thus, a membrane-bound NADH oxidoreductase appears to contribute to the release of NO from nitroprusside, but not nitroglycerin, in calf pulmonary artery.