CGRP2 receptor in the internal anal sphincter of the rat:: implications for CGRP receptor classification

1 The CGRP receptor mediating relaxation of the rat internal anal sphincter (IAS) has been characterized using CGRP analogues, homologues, the antagonist CGRP(8-37) and its analogues. 2 In isolated IAS strips, the spontaneously developed tone was concentration-dependently relaxed by her CGRP, h beta CGRP and rat beta CGRP (pEC(50) 8.1+/-0.2, 8.3+/-0.1 and 8.4+/-0.2, respectively; 100% maximum response). Vasoactive intestinal polypeptide (VIP) was around 7 fold more potent than ha CGRP (pEC(50) 9.0+0.1; 100% maximum relaxation). [Cys(ACM(2.7))1 ha CGRP and salmon calcitonin were inactive (up to 10(-5) M). 3 H alpha CGRP(8-37) (10(-5) M) antagonized responses to h alpha CGRP (apparent pK(B) 5.7+/-0.3) and rat beta CGRP (apparent pK(B) 5.8+/-0.2), but not to VIP. H beta CGRP(8-37) (10(-5) M) was an antagonist against ha CGRP (apparent pK(B) 6.1+/-0.1). H alpha CGRP(8-37) analogues (10-5 M), with substitutions at the N-terminus by either glycines or des-NH2 valine(8) or prolines, antagonized her CGRP responses with similar affinities (apparent pK(B) 5.8+/-0.1, 5.8+/-0.1 and 5.5+/-0.1, respectively). 4 Peptidase inhibitors (amastatin, bestatin, captopril, phosphoramidon and thiorphan, 10(-6) M each) did not increase the agonist potency of either ha CGRP or [Cys(ACM(2,7))] h alpha CGRP, or the antagonist affinity of ha CGRP(8-37) against ha CGRP or rat beta CGRP. 5 These data demonstrate for the first time a CGRP receptor in the rat IAS for which ha CGRP(8-37) and its analogues have an affinity that is consistent with a CGRP(2) receptor. However, there is a marked species difference as the antagonist has a 100 fold lower affinity in the rat than in the same tissue of the opossum (Chakder & Rattan, 1991).