Cellular mechanisms of nitrate action.

It is now generally accepted that organic nitrates generate their vasodilator action via production of nitric oxide. However, the cellular location of the metabolic enzyme(s) responsible for such conversion has not been defined. We examined the production of nitric oxide, via chemiluminescence detection, by various cellular fractions of the bovine coronary artery. We were able to show that the highest activity resides in the plasma membrane. Future isolation and characterization of such metabolic systems will greatly assist our understanding of nitrate action and tolerance. Several cellular mechanisms for nitrate tolerance have been proposed. Among the most popular theories is the "intracellular sulfhydryl depletion hypothesis" originally proposed by Needleman et al. The primary supportive data for this mechanism are that exogeneously added thiols (such as N-acetylcysteine) can potentiate the in vivo activity of nitroglycerin and can partially reverse nitrate tolerance. We showed that a cellular-impermeant thiol, viz: glutathione, can also potentiate the hemodynamic effect of nitroglycerin in rats. We subsequently showed that exogenously administered thiols can promote the formation of vasoactive S-nitrosothiols in blood. Thus, the beneficial effects of thiols on nitrate action might be mediated through an extracellular pathway. Another cellular mechanism for nitrate tolerance suggested that tolerance is caused by an alteration of the enzyme, guanylate cyclase. We showed, however, that blood vessels made tolerant to nitroglycerin remain fully responsive (in terms of in vitro relaxation) toward nitric oxide and S-nitrosothiols. These data showed that, as far as relaxation is concerned, nitrate tolerance did not cause a significant alteration of guanylate cyclase activity toward nitric oxide and S-nitrosothiols.