Role of ATP-sensitive K⁺ channels in CGRP-induced dilatation of basilar artery in vivo

Stimulation of adenylate cyclase appears to activate ATP-sensitive K⁺ channels in the basilar artery. We tested the hypothesis that calcitonin gene-related peptide (CGRP), which increases intracellular adenosine 3',5'- cyclic monophosphate (cAMP) levels, activates ATP-sensitive K⁺ channels and thereby causes vasodilatation. Using a cranial window in anesthetized rats, we examined responses of the basilar artery to CGRP in vivo. We also examined responses of the artery to another vasoactive peptide, vasoactive intestinal peptide (VIP). Topical application of CGRP (10^-11 to 10^-8 M) increased diameter of the basilar artery. Responses of the basilar artery to CGRP were almost abolished by a CGRP₁ receptor antagonist, CGRP-(8-37). Vasodilatation in response to VIP was much smaller than that produced by CGRP. Dilator responses of the basilar artery to 10^-9 and 10^-8 M CGRP were inhibited by glibenclamide (10^-6 M), a selective inhibitor of ATP-sensitive K⁺ channels, by 69 ± 19 and 41 ± 9%, respectively. N(G)-nitro-L-arginine methyl ester (10^-5 M), an inhibitor of nitric oxide synthase, did not attenuate dilator response to 10^-8 M CGRP but inhibited responses to 10^{- 9} M CGRP by 34 ± 12%. Indomethacin did not alter dilator responses to CGRP. These findings suggest that a minor component of CGRP-induced dilatation of the basilar artery is mediated by endothelium-derived relaxing factor. Vasodilatation in response to CGRP appears to be mediated primarily by direct activation of CGRP₁ receptors on vascular muscle. Activation of ATP- sensitive K⁺ channels appears to be the major mechanism of dilatation of the basilar artery in response to CGRP in vivo.