Role of SK Ca and IK Ca 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 P 450 metabolite and whether they are linked to the activation of two distinct populations of endothelial K Ca channels, SK Ca and IK Ca.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 ω ‐ L ‐nitro arginine (100  μ M ) and indomethacin (5  μ M ). 3 Under control conditions (Ca 2+ : 2.5 m M ), acetylcholine (10 n M to 10  μ M ) induced a concentration‐ and endothelium‐dependent hyperpolarization of the vascular smooth muscle cells. Two structurally different specific blockers of SK Ca , apamin (0.5  μ M ) or UCL 1684 (10  μ M ), produced a partial but significant inhibition of the hyperpolarization evoked by acetylcholine whereas charybdotoxin (0.1  μ M ) and TRAM‐34 (10  μ M ), a nonpeptidic and specific blocker of IK Ca , were ineffective. In contrast, the combinations of apamin plus charybdotoxin, apamin plus TRAM‐34 (10  μ M ) or UCL 1684 (10  μ M ) plus TRAM‐34 (10  μ M ) virtually abolished the acetylcholine‐induced hyperpolarization. 4 In the presence of a combination of apamin and a subeffective dose of TRAM‐34 (5  μ M ), the residual hyperpolarization produced by acetylcholine was not inhibited further by the addition of either an epoxyeicosatrienoic acid antagonist, 14,15‐EEZE (10  μ M ) or the specific blocker of BK Ca , iberiotoxin (0.1  μ M ). 5 In presence of 0.5 m M Ca 2+ , the hyperpolarization in response to acetylcholine (1  μ M ) was significantly lower than in 2.5 m M Ca 2+ . 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 P 450 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.British Journal of Pharmacology (2005) 144 , 477–485. doi: 10.1038/sj.bjp.0706003