Effects of adrenomedullin and calcitonin gene‐related peptide on contractions of the rat aorta and porcine coronary artery

Effects of adrenomedullin and α‐calcitonin gene‐related peptide (CGRP) on the contractions and cytosolic Ca2+ concentrations ([Ca2+]i) of the rat aorta and porcine coronary artery were investigated. Characteristics of the receptors mediating the effects of adrenomedullin and α‐CGRP were also investigated. Adrenomedullin and α‐CGRP caused a concentration‐dependent relaxation in the rat aorta contracted with noradrenaline. The IC50 values for adrenomedullin and α‐CGRP were 2.4 nM and 4.0 nM, respectively. The relaxant effects of these peptides were abolished by removal of the endothelium and significantly attenuated by an inhibitor of nitric oxide synthase, NG‐monomethyl‐L‐arginine (L‐NMMA, 100 μM), but not by a cyclo‐oxygenase inhibitor, indomethacin (10 μM). Adrenomedullin and α‐CGRP increased the endothelial [Ca2+]i in the rat aorta with endothelium, whereas they did not change [Ca2+]i in the smooth muscle. An antagonist of the CGRP1 receptor, CGRP (8–37), antagonized the relaxant effects of α‐CGRP and the β‐isoform of CGRP (β‐CGRP) but not those of adrenomedullin in the rat aorta. In the porcine coronary artery contracted with U46619, adrenomedullin and α‐CGRP caused a concentration‐dependent relaxation with an IC50 of 27.6 and 4.1 nM, respectively. Removal of the endothelium altered neither the IC50 values nor the maximal relaxations induced by adrenomedullin or α‐CGRP. When the artery was contracted with high K+ solution (72.7 mM), these peptides caused a small relaxation. Adrenomedullin and α‐CGRP increased cyclic AMP content and decreased the smooth muscle [Ca2+]i in the porcine coronary artery. CGRP (8–37) significantly antagonized the relaxant effects of adrenomedullin and α‐CGRP in the porcine coronary artery. However, it had little effect on the relaxations induced by the β‐isoform of CGRP (β‐CGRP). These results suggest that in the rat aorta, adrenomedullin and α‐CGRP increase the endothelial [Ca2+]i, activate nitric oxide synthase and release nitric oxide, without a direct inhibitory action on smooth muscle. In the porcine coronary artery, in contrast, adrenomedullin and α‐CGRP directly act on smooth muscle, increase cyclic AMP content, decrease the smooth muscle [Ca2+]i and inhibit contraction. The rat aortic endothelium seems to express the CGRP receptor which is sensitive to α‐CGRP, β‐CGRP and CGRP (8–37) and the adrenomedullin specific receptor. The porcine coronary smooth muscle, in contrast, seems to express two types of CGRP receptor; on