Abstract
Calcitonin gene-related peptide (CGRP) receptors are classified into CGRP subtype 1 (CGRP1) and CGRP subtype 2 (CGRP2) based on the affinity of the antagonist, human α (hα)-CGRP8-37. hα-CGRP8-37 antagonizes CGRP1 receptor-mediated responses with high affinity (KB <100 nM) and antagonizes CGRP2 receptor-mediated responses with low affinity (KB > 1 μM). CGRP2 receptors have been previously reported to mediate relaxation of large porcine coronary arteries because this action is antagonized with low affinity by hα-CGRP8-37. In the present study, we used reverse transcription-polymerase chain reaction, radioligand binding, and values from our previously reported isolated tissue experiments to compare the CGRP receptor in porcine coronary arteries with the porcine CGRP1 receptor stably expressed in human embryonic kidney (HEK) 293 cells. We identified calcitonin receptor-like receptor and receptor activity modifying protein 1 mRNA in coronary arteries. We also found that the ligand binding characteristics of the CGRP receptor in coronary arteries and the cloned CGRP1 receptor were highly similar. K1 values for hα-CGRP8-37 were 6.6 and 5.7 nM in porcine coronary arteries and the cloned CGRP1 receptor, respectively. The affinities (KB) of hα-CGRP8-37 and five other antagonists were 22- to 707-fold lower in functional experiments measuring relaxation of coronary arteries than in radioligand binding experiments. Despite this difference in absolute affinity values, there was a high correlation of the rank order of affinity for the antagonists determined by the two methods. Thus hα-CGRP8-37 antagonizes CGRP-induced relaxation of porcine coronary arteries with low affinity at the CGRP1 receptor. Taken together, these data do not support the existence of the CGRP2 receptor.