Pharmacological discrimination of calcitonin receptor: receptor activity-modifying protein complexes

Calcitonin (CT) receptors dimerize with receptor activity-modifying proteins (RAMPs) to create high-affinity amylin (AMY) receptors, but there is no reliable means of pharmacologically distinguishing these receptors. We used agonists and antagonists to define their pharmacology, expressing the CT(a) receptor alone or with RAMPs in COS-7 cells and measuring cAMP accumulation. Intermedin short, otherwise known as adrenomedullin 2, mirrored the action of alpha CGRP, being a weak agonist at CT(a), AMY(2a), and AMY(3a) receptors but considerably more potent at AMY(1a) receptors. Likewise, the linear calcitonin gene-related peptide (CGRP) analogs (Cys(ACM)(2,7))h alpha CGRP and (Cys(Et)(2,7))h alpha CGRP were only effective at AMY(1a) receptors, but they were partial agonists. As previously observed in COS-7 cells, there was little induction of the AMY(2a) receptor phenotype; thus, AMY(2a) was not examined further in this study. The antagonist peptide salmon calcitonin(8-32) (sCT(8-32)) did not discriminate strongly between CT and AMY receptors; however, AC187 was a more effective antagonist of AMY responses at AMY receptors, and AC413 additionally showed modest selectivity for AMY(1a) over AMY(3a) receptors. CGRP(8-37) also demonstrated receptor-dependent effects. CGRP(8-37) more effectively antagonized AMY at AMY(1a) than AMY(3a) receptors, although it was only a weak antagonist of both, but it did not inhibit responses at the CT(a) receptor. Low CGRP(8-37) affinity and agonism by linear CGRP analogs at AMY(1a) are the classic signature of a CGRP2 receptor. Our data indicate that careful use of combinations of agonists and antagonists may allow pharmacological discrimination of CT(a), AMY(1a), and AMY(3a) receptors, providing a means to delineate the physiological significance of these receptors.