Inhibition of endogenous nitric oxide synthase potentiates nitrovasodilators in experimental pulmonary hypertension

Abstract

Background: The role of endogenous nitric oxide (NO) in the regulation of pulmonary vascular tone is complex. Inhibition of endogenous NO synthase, potentially through upregulation of guanylyl cyclase, results in an increase in potency of nitrovasodilators in the systemic circulation. This study considered whether inhibition of endogenous NO synthase would increase the potency of nitrovasolidators, but not of cyclic adenosine monophosphate- dependent vasodilators, in the pulmonary vasculature. Methods: We used the isolated buffer-perfused rabbit lung. Preparations were randomized to received either pretreatment with N(G)-nitro-L-arginine methyl ester (or L- NAME, an inhibitor of endogenous NO synthase) or no pretreatment. Stable pulmonary hypertension was then produced by infusing the thromboxane A 2 analog U46619. The dose-response characteristics of two nitrovasodilators, sodium nitroprusside and nitroglycerin, and two nonnitrovasodilators, prostaglandin E 1 and 5'-N-ethylcarboxamidoadenosine, were studied. Results: Inhibition of endogenous NO synthase caused no significant changes in baseline pulmonary artery pressure but did significantly reduce the U46619 infusion rate required to produce pulmonary hypertension. Pretreatment with L-NAME (vs. no L-NAME) resulted in significantly lower values of the log median effective dose with sodium nitroprusside and nitroglycerin. In contrast, pretreatment with L-NAME resulted in no changes in the dose- response characteristics of the cyclic adenosine monophosphate-mediated, NO- independent vasodilators prostaglandin E 1 and 5'-N- ethylcarboxamidoadenosine. Conclusions: These data suggest that endogenous NO synthase is not an important regulator of basal pulmonary tone in this model but is an important of pulmonary vascular responses to vasoconstriction and to nitrovasodilators. The pulmonary vasodilator effects of nitrovasodilators, but not of nonnitrovasodilators, may depend on the level of activity of NO synthase.