Characterization of endothelium-dependent relaxation independent of NO and prostaglandins in guinea pig coronary artery

In the presence of N omega-nitro-L-arginine and indomethacin, acetylcholine (ACh) induced endothelium-dependent relaxation in guinea pig coronary artery preconstricted with 9,11-dideoxy-9 alpha, 11 alpha-epoxymethano prostaglandin F2 alpha. 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 omega-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.