Use-Dependent Loss of Acetylcholine- and Bradykinin-Mediated Vasodilation After Nitric Oxide Synthase Inhibition

Abstract

In the present study, we examined the possibility that the endothelium-dependent vasodilators acetylcholine and bradykinin release preformed pools of nitric oxide–containing factors. Successive injections of selected doses of acetylcholine (1.18±0.3 μg/kg IV) or bradykinin (5 μg/kg IV) caused reproducible hypotensive and vasodilator responses within sympathetically intact and sympathetically denervated hindlimbs of conscious rats. After administration of the nitric oxide synthesis inhibitor N ω -nitro- l -arginine methyl ester (L-NAME, 25 μmol/kg IV), the first injection of acetylcholine or bradykinin produced pronounced depressor and vasodilator responses that, in the case of bradykinin, were greater than those observed before L-NAME administration. However, each successive injection of acetylcholine and bradykinin produced progressively smaller responses, such that the later injections elicited a markedly diminished hypotension and vasodilation. This “use-dependent” loss of endothelium-dependent vasodilation was not due to the diminished vasorelaxant potency of nitric oxide–containing factors because the vasodilator effects of the nitric oxide donor sodium nitroprusside (32 μg/kg IV) and the S -nitrosothiol compound S -nitrosocysteine (200 nmol/kg IV) were augmented in the presence of L-NAME. These results suggest that the use-dependent loss of the hemodynamic effects of acetylcholine and bradykinin in L-NAME–treated rats may be due to the release and subsequent depletion of a factor whose synthesis depends on the bioavailability of nitric oxide. Taken together, these results suggest that preformed pools of nitric oxide–containing factors exist within the endothelium of resistance vessels and that endothelium-dependent agonists exert their vasorelaxant effects at least in part by the mobilization of these preformed pools.