USE-DEPENDENT LOSS OF ACETYLCHOLINE-MEDIATED 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 se lected 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 hindlimb s of conscious rats. After administration of the nitric oxide synthesi s inhibitor N-omega-nitro-L-arginine methyl ester (L-NAME, 25 mu mol/k g IV), time first injection of acetylcholine or bradykinin produced pr onounced depressor and vasodilator responses that, in the case of brad ykinin, were greater than those observed before L-NAME administration. However, each successive injection of acetylcholine and bradykinin pr oduced progressively smaller responses, such that the later injections elicited a markedly diminished hypotension and vasodilation. This ''u se-dependent'' loss of endothelium-dependent vasodilation was not due to the diminished vasorelaxant potency of nitric oxide-containing fact ors because the vasodilator effects of the nitric oxide donor sodium n itroprusside (32 mu g/kg IV) and the S-nitrosothiol compound 5-nitroso cysteine (200 nmol/kg IV) were augmented in the presence of L-NAME. Th ese results suggest that the use-dependent loss of the hemodynamic eff ects of acetylcholine and bradykinin in L-NAME-treated rats may be due to the release and subsequent depletion of a factor whose synthesis d epends on the bioavailability of nitric oxide. Taken together, these r esults suggest that preformed pools of nitric oxide-containing factors exist within the endothelium of resistance vessels and that endotheli um-dependent agonists exert their vasorelaxant effects at least in par t by the mobilization of these preformed pools.