Use-dependent loss of acetylcholine- and bradykinin-mediated vasodilation after nitric oxide synthase inhibition - Evidence for preformed stores of nitric oxide-containing factors in vascular endothelial cells

In the present study, we examined the possibility that the endothelium-dependent vasodilators acetylcholine and bradykinin release preformed poets of nitric oxide-containing factors, Successive injections of selected doses of acetylcholine (1.18+/-0.3 mu g/kg IV) or bradykinin (5 mu g/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-omega-nitro-L-arginine methyl ester (L-NAME, 25 mu mol/kg IV), time 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 mu g/kg IV) and the S-nitrosothiol compound 5-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.