Calcitonin gene-related peptide: Exploring its vasodilating mechanism of action in humans

Objective: In vitro studies suggest that the vasodilator mechanism of action of calcitonin gene-related peptide (CGRP) involves various endothelium-dependent and endothelium-independent mechanisms. An in vivo analysis of the contribution of nitric oxide, prostaglandins, calcium-sensitive potassium channels (K-Ca(+) channels), and adenosine triphosphate (ATP)-sensitive potassium channels (K-ATP(+) channels) to CGRP-induced vasodilation in humans was performed. Methods: CGRP (3, 10, and 30 ng . min(-1) . dL(-1) forearm) was infused into the brachial artery of 40 healthy subjects. Forearm vascular responses were measured by venous occlusion plethysmography. First, dose-response curves were constructed during coinfusion of CGRP with placebo (sodium chloride, 0.9%). After washout, in 5 subgroups (n = 8 each), the infusions of CGRP were repeated with placebo (time-control experiments), N-G-monomethyl-L-arginine (L-NMMA, a nitric oxide-synthase inhibitor), indomethacin (a cyclooxygenase inhibitor), tetraethylammonium chloride (TEAC) (a K-Ca(+)-channel blocker), and glyburide (INN, glibenclamide) (a K-ATP(+)-channel blocker), respectively. Results. CGRP induced a dose-dependent and reproducible decrease in forearm vascular resistance (P < .001). Compared with placebo, L-NMMA reduced the decrease in forearm vascular resistance induced by CGRP (P < .001) (3 and 10 ng . min(-1) . dL(-1) forearm). The absence of an inhibitory effect of L-NMMA on CGRP-induced vasodilation at the highest dose of CGRP suggests that still other mechanisms are involved. The vasodilator response to CGRP was not affected by coinfusion of indomethacin, tetraethylammonium chloride, or glyburide. Conclusions. The intrabrachial infusion of CGRP results in a dose-dependent and reproducible forearm vasodilator response. CGRP-induced vasodilation is dependent at least in part on the release of nitric oxide and does not involve the release of prostaglandins or the activation of K-Ca(+) channels or K-ATP(+) channels in humans.