Abstract

In the presence of N_ω-nitro-l-arginine and indomethacin, acetylcholine (ACh) induced endothelium-dependent relaxation in guinea pig coronary artery preconstricted with 9,11-dideoxy-9α,11α-epoxymethano prostaglandin F_2α. Dexamethasone and arachidonyltrifluoromethyl ketone, inhibitors of phospholipase A₂, and 17-octadecynoic acid, an inhibitor of cytochrome P450 epoxygenase, had no effect on the response to ACh. Although proadifen, which is used widely as an inhibitor of cytochrome P450-dependent enzymes, suppressed the ACh-induced relaxation, the drug also inhibited the relaxation induced by cromakalim, a K⁺channel opener. In isolated smooth muscle cells of guinea pig coronary artery, proadifen, but not 17-octadecynoic acid, almost abolished delayed rectifier K⁺ current. Epoxyeicosatrienoic acids failed to relax the artery. Apamin and iberiotoxin, inhibitors of small- and large-conductance Ca⁺⁺-activated K⁺channels, respectively, did not affect the relaxation induced by ACh. A combination of charybdotoxin plus apamin, but not iberiotoxin plus apamin, abolished the response. However, the combination of charybdotoxin plus apamin had no effect on ACh-induced increase in intracellular free Ca⁺⁺ concentration in endothelial cells. These results suggest that epoxyeicosatrienoic acids do not contribute to N_ω-nitro-l-arginine/indomethacin-resistant relaxation induced by ACh in the guinea pig coronary artery. The present study also proposes that K⁺ channels on vascular smooth muscle cells, which both charybdotoxin and apamin must affect for inhibition to occur, are the target for endothelium-derived hyperpolarizing factor.