Pharmacological characterization of CGRP receptors mediating relaxation of the rat pulmonary artery and inhibition of twitch responses of the rat vas deferens

1 CGRP receptors mediating vasorelaxation of the rat isolated pulmonary artery and inhibition of contractions of the rat isolated prostatic vas deferens were investigated using CGRP agonists, homologues and the antagonist CGRP(8-37). 2 In the pulmonary artery, human (h)alpha-CGRP-induced relaxation of phenylephrine-evoked tone was abolished either by removal of the endothelium or by N-G-nitro-L-arginine (10(-5) M). The inhibitory effect of N-G-nitro-L-arginine was stereoselectively reversed by L- but not by D-arginine (10(-4) M). Thus, CGRP acts via nitric oxide released from the endothelium. 3 In the endothelium-intact artery, h alpha-CGRP, h beta-CGRP and human adrenomedullin (10(-10)-3 x 10(-7) M), dose-dependently relaxed the phenylephrine-induced tone with similar potency. Compared with h alpha-CGRP, rat amylin was around 50 fold less potent, while [Cys(ACM(2,7))] h alpha-CGRP (10(-7)-10(-4) M) was at least 3000 fold less potent. Salmon calcitonin was inactive (up to 10(-4) M). 4 Human alpha-CGRP(8-37) (3 x 10(-7)-3 x 10(-6) M) antagonized h alpha-CGRP (pA(2) 6.9, Schild plot slope 1.2+/-0.1) and h beta-CGRP (apparent pK(B) of 7.1+/-0.1 for h alpha-CGRP(8-37) 10(-6) M) in the pulmonary artery. Human beta-CGRP(8-37) (10(-6) M) antagonized h alpha-CGRP responses with a similar affinity (apparent pK(B) 7.1+/-0.1). Human adrenomedullin responses were not inhibited by h alpha-CGRP(8-37) (10(-6) M). 5 In the prostatic vas deferens, h alpha-CGRP, h beta-CGRP and rat beta-CGRP (10-(10)-3 x 10(-7) M) concentration-dependently inhibited twitch responses with about equal potency, while rat amylin (10(-8)-10(-5) M) was around 10 fold less potent and the linear analogue [Cys(ACM(2,7))] h alpha-CGRP was at least 3000 fold weaker. Salmon calcitonin was inactive (up to 10(-4) M). 6 The antagonist effect of h alpha-CGRP(8-37) (10(-5)-3 x 10(-5)) in the vas deferens was independent of the agonist, with pA(2) values against h alpha-CGRP of 6.0 (slope 0.9+/-0.1), against h beta-CGRP of 5.8 (slope 1.1+/-0.1), and an apparent pK(B) value of 5.8+/-0.1 against both rat beta-CGRP and rat amylin. Human beta-CGRP(8-37) (3 x 10(-5)-10(-4) M) competitively antagonized h alpha-CGRP responses (pA(2) 5.6, slope 1.1+/-0.2). The inhibitory effect of h alpha-CGRP on noradrenaline-induced contractions in both the prostatic and epididymal vas deferens was antagonized by h alpha CGRP(8-37) (pA(2) 5.8 and 5.8, slope 1.0+/-0.2 and 1.0+/-0.3 respectively). 7 The effects of h alpha-CGRP and h alpha-CGRP(8-37) in both rat pulmonary artery and vas deferens were not significantly altered by pretreatment with peptidase inhibitors (amastatin, bestatin, captopril, phosphoramidon and thiorphan, all at 10(-6) M). The weak agonist activity of [Cys(ACM(2,7))] h alpha-CGRP in the vas deferens was not increased by peptidase inhibitors. 8 These data demonstrate that two different CGRP receptors may exist in the rat pulmonary artery and vas deferens, a CGRP(1) receptor subtype in the rat pulmonary artery (CGRP(8-37) pA(2) 6.9), while the lower affinity for CGRP(8-37) (pA(2) 6.0) in the vas deferens is consistent with a CGRP(2) receptor.