CGRP 2 receptor in the internal anal sphincter of the rat: implications for CGRP receptor classification

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. In isolated IAS strips, the spontaneously developed tone was concentration‐dependently relaxed by hα CGRP, hβ CGRP and rat β 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 hα CGRP (pEC 50 9.0±0.1; 100% maximum relaxation). [Cys(ACM 2.7 )] hα CGRP and salmon calcitonin were inactive (up to 10 −5   M ). Hα CGRP 8–37 (10 −5   M ) antagonized responses to hα CGRP (apparent p K B 5.7±0.3) and rat β CGRP (apparent p K B 5.8±0.2), but not to VIP. Hβ CGRP 8–37 (10 −5   M ) was an antagonist against hα CGRP (apparent p K B 6.1±0.1). Hα CGRP 8–37 analogues (10 −5   M ), with substitutions at the N‐terminus by either glycine 8 or des ‐NH 2 valine 8 or proline 8 , antagonized hα CGRP responses with similar affinities (apparent p K B 5.8±0.1, 5.8±0.1 and 5.5±0.1, respectively). Peptidase inhibitors (amastatin, bestatin, captopril, phosphoramidon and thiorphan, 10 −6   M each) did not increase the agonist potency of either hα CGRP or [Cys(ACM 2,7 )] hα CGRP, or the antagonist affinity of hα CGRP 8–37 against hα CGRP or rat β CGRP. These data demonstrate for the first time a CGRP receptor in the rat IAS for which hα 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 ).British Journal of Pharmacology (2000) 130 , 464–470; doi: 10.1038/sj.bjp.0703315