K+ channel blockers and cytochrome P450 inhibitors on acetylcholine-induced, endothelium-dependent relaxation in rabbit mesenteric artery

Acetylcholine caused an endothelium-dependent relaxation in isolated rabbit mesenteric small artery in the presence of nitro L-arginine and indomethac in. The acetylcholine-induced relaxation was attenuated by high K+ solution , suggesting that the response is mediated by a membrane potential-sensitiv e mechanism, presumably an endothelium-derived hyperpolarizing factor. The acetylcholine-induced relaxation was also inhibited with tetraethylammonium , 4-aminopyridine and charybdotoxin, but not with Ba2+, apamin, iberiotoxin nor glibenclamide. The relaxation was abolished by a combination of apamin and charybdotoxin, but iberiotoxin could not replace charybdotoxin in this combination. The responses to charybdotoxin and 4-aminopyridine were syner gistic but neither apamin nor iberiotoxin increased the effect of 4-aminopy ridine. Clotrimazole and proadifen inhibited the acetylcholine-induced rela xation, but these drugs also inhibited the cromakalim-induced relaxation, w hile protoporphyrin IX inhibited the acetylcholine- but not cromakalim-indu ced relaxation. 17-Octadecynoic acid and 1-aminobenzotriazole did not affec t the response to acetylcholine. Four regioisomers of epoxyeicosatrienoic a cids did not relax endothelium-denuded artery. A gap junction inhibitor 18 alpha-glycyrrhetinic acid attenuated the relaxation to acetylcholine. It is suggested that in rabbit mesenteric artery, the acetylcholine-induced, nit ric oxide- and prostacyclin-independent relaxation is mainly mediated by 4- aminopyridine- and charybdotoxin-sensitive K+ channels and that the relaxat ion is not mediated through cytochrome P450 enzyme metabolites. The contrib ution of heterocellular gap junctional communication to the relaxation is d iscussed. (C) 1999 Elsevier Science B.V. All rights reserved.