Role of SK _Ca and IK _Ca in endothelium-dependent hyperpolarizations of the guinea-pig isolated carotid artery

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, SK _Ca and IK _Ca. 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 ^ω-L-nitro arginine (100 μM) and indomethacin (5 μM). Under control conditions (Ca ²⁺: 2.5 mM), acetylcholine (10 nM to 10 μM) induced a concentration- and endothelium-dependent hyperpolarization of the vascular smooth muscle cells. Two structurally different specific blockers of SK _Ca, apamin (0.5 μM) or UCL 1684 (10 μM). produced a partial but significant inhibition of the hyperpolarization evoked by acetylcholine whereas charybdotoxin (0.1 μM) and TRAM-34 (10 μM), a nonpeptidic and specific blocker of IK _Ca. were ineffective. In contrast, the combinations of apamin plus charybdotoxin, apamin plus TRAM-34 (10 μ) or UCL 1684 (10 μM) plus TRAM-34 (10 μM) virtually abolished the acetylcholine-induced hyperpolarization. In the presence of a combination of apamin and a subeffective dose of TRAM-34 (5 μM), the residual hyperpolarization produced by acetylcholine was not inhibited further by the addition of either an epoxyeicosatrienoic acid antagonist, 14,15-EEZE (10 μM) or the specific blocker of BK _Ca, iberiotoxin (0.1 μM). In presence of 0.5 mM Ca ²⁺, the hyperpolarization in response to acetylcholine (1 μM) was significantly lower than in 2.5 mM Ca ²⁺. The EDHF-mediated responses became predominantly sensitive to charybdotoxin or TRAM-34 but resistant to apamin. This investigation shows that the production of a cytochrome P450 metabolite, and the subsequent activation of BK _Ca, 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 IK _Ca and SK _Ca channels, the activation of either one being able to produce a true hyperpolarization.