Functional calcitonin gene-related peptide subtype 2 receptors in porcine coronary arteries are identified as calcitonin gene-related peptide subtype1 receptors by radioligand binding and reverse transcription-polymerase chain reaction

Calcitonin gene-related peptide (CGRP) receptors are classified into CGRP s ubtype 1 (CGRP(1)) and CGRP subtype 2 (CGRP(2)) based on the affinity of th e antagonist, human alpha (h alpha)-CGRP(8-37). h alpha -CGRP(8-37) antagon izes CGRP(1) receptor-mediated responses with high affinity (K-B < 100 nM) and antagonizes CGRP(2) receptor-mediated responses with low affinity (K-B > 1 muM). CGRP(2) receptors have been previously reported to mediate relaxa tion of large porcine coronary arteries because this action is antagonized with low affinity by h alpha -CGRP(8-37). In the present study, we used rev erse transcription-polymerase chain reaction, radioligand binding, and valu es from our previously reported isolated tissue experiments to compare the CGRP receptor in porcine coronary arteries with the porcine CGRP(1) recepto r 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 characte ristics of the CGRP receptor in coronary arteries and the cloned CGRP(1) re ceptor were highly similar. K-i values for h alpha CGRP(8-37) were 6.6 and 5.7 nM in porcine coronary arteries and the cloned CGRP, receptor, respecti vely. The affinities (K-B) of h alpha -CGRP(8-37) and five other antagonist s 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 th e rank order of affinity for the antagonists determined by the two methods. Thus h alpha -CGRP(8-37) antagonizes CGRP-induced relaxation of porcine co ronary arteries with low affinity at the CGRP(1) receptor. Taken together, these data do not support the existence of the CGRP(2) receptor.