Role of SKCa and IKCa in endothelium-dependent hyperpolarizations of the guinea-pig isolated carotid artery

1 This study was designed to determine whether the endothelium-dependent hyperpolarizations evoked by acetylcholine in guinea-pig carotid artery involve a cytochrome P450 metabolite and whether they are linked to the activation of two distinct populations of endothelial K-Ca channels, SKCa and IKCa. 2 The membrane potential was recorded in the vascular smooth muscle cells of the guinea-pig isolated carotid artery. All the experiments were performed in the presence of N(o)omegaL-nitro arginine (100 muM) and indomethacin (5 muM). 3 Under control conditions (Ca2+: 2.5 mM), acetylcholine (10 nM to 10 muM) induced a concentration- and endothelium-dependent hyperpolarization of the vascular smooth muscle cells. Two structurally different specific blockers of SKCa, apamin (0.5 muM) or UCL 1684 (10 muM), produced a partial but significant inhibition of the hyperpolarization evoked by acetylcholine whereas charybdotoxin (0.1 muM) and TRAM-34 (10 muM), a nonpeptidic and specific blocker of IKCa, were ineffective. In contrast, the combinations of apamin plus charybdotoxin, apamin plus TRAM-34 (10 muM) or U CL 1684 (10 muM) plus TRAM-34 (10 muM) virtually abolished the acetylcholine-induced hyperpolarization. 4 In the presence of a combination of apamin and a subeffective dose of TRAM-34 (5 muM), the residual hyperpolarization produced by acetylcholine was not inhibited further by the addition of either an epoxyeicosatrienoic acid antagonist, 14,15-EEZE (10 muM) or the specific blocker of BKCa, iberiotoxin (0.1 muM). 5 In presence of 0.5 mM Ca2+, the hyperpolarization in response to acetylcholine (1 muM) was significantly lower than in 2.5 mM Ca2+. The EDHF-mediated responses became predominantly sensitive to charybdotoxin or TRAM-34 but resistant to apamin. 6 This investigation shows that the production of a cytochrome P450 metabolite, and the subsequent activation of BKCa, is unlikely to contribute to the EDHF-mediated responses in the guinea-pig carotid artery. Furthermore, the EDHF-mediated response involves the activation of both endothelial IKCa and SKCa channels, the activation of either one being able to produce a true hyperpolarization.