Endothelium-dependent vasorelaxation independent of nitric oxide and K+ release in isolated renal arteries of rats

1 We investigated whether K+ can act as an endothelium-derived hyperpolarizing factor (EDHF) in isolated small renal arteries of Wistar-Kyoto rats. 2 Acetylcholine (0.001-3 muM) caused relaxations that were abolished by removal of the endothelium. However, acetylcholine-induced relaxations were not affected by the nitric oxide (NO) synthase inhibitor N-omega-nitro-L-arginine methyl ester (L-NAME, 100 muM), by L-NAME plus the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 1 muM) or by L-NAME plus the cyclo-oxygenase inhibitor indomethacin (10 muM). In rings precontracted with high-K+(60 mM) physiological salt solution in the presence of L-NAME, acetylcholine-induced relaxations were abolished. 3 L-NAME-resistant relaxations were abolished by the large-conductance Ca2+-activated K+ channel inhibitor charybdotoxin plus the small-conductance Ca2+-activated K+ channel inhibitor apamin, while the inward rectifier K+ channel inhibitor Ba2+ or the gap junction inhibitor 18 alpha -glycyrrhetinic acid had no effect. Acetylcholine-induced relaxation was unchanged by ouabain (10 muM) but was partially inhibited by a higher concentration (100 muM). 4 In half of the tissues tested, K+(10 mM) itself produced L-NAME-resistant relaxations that were blocked by ouabain (10 muM) and partially reduced by charybdotoxin plus apamin, but not affected by 18 alpha -glycyrrhetinic acid or Ba2+. However, K+ did not induce relaxations in endothelium-denuded tissues. 5 In conclusion, acetylcholine-induced relaxations in this tissue are largely dependent upon hyperpolarization mechanisms that are initiated in the endothelium but do not depend upon NO release. K+ release cannot account for endothelium-dependent relaxation and cannot be an EDHF in this artery. However, K+ itself can initiate endothelium-dependent relaxations via a different pathway from acetylcholine, but the mechanisms of K+-induced relaxations remain to be clarified.