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 halpha CGRP, hbeta 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 halpha CGRP (pEC(50) 9.0+/-0.1; 100% maximum relaxation). [Cys(ACM(2.7))] halpha CGRP and salmon calcitonin were inactive (up to 10(-5) M). Halpha CGRP(8 - 37) (10(-5) M) antagonized responses to halpha CGRP (apparent pK(B) 5.7+/-0.3) and rat beta CGRP (apparent pK(B) 5.8+/-0.2), but not to VIP. Hbeta CGRP(8 - 37) (10(-5) M) was an antagonist against halpha CGRP (apparent pK(B) 6.1+/-0.1). Halpha 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 halpha CGRP responses with similar affinities (apparent pK(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 halpha CGRP or [Cys(ACM(2,7))] halpha CGRP, or the antagonist affinity of halpha CGRP(8 - 37) against halpha CGRP or rat beta CGRP. These data demonstrate for the first time a CGRP receptor in the rat IAS for which halpha 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).