Abstract
Calcitonin gene-related peptide (CGRP) receptors are divided into CGRP1 and CGRP2 receptor subtypes based on their affinity for CGRP8-37 in functional assays. CGRP1 receptors are blocked with high affinity (Kb < 100 nM) by CGRP8-37, and CGRP2 receptors are blocked with low affinity (Kb > 100 nM) by this antagonist. The CGRP1 receptor has been cloned and well characterized. In contrast, the CGRP2 receptor has not been well characterized and has only been identified by its low affinity for CGRPs.37 in functional assays. Furthermore, previous investigators reported that radioligand binding assays using [125I]CGRP do not support the existence of the CGRP2 receptor. The goals of my work were to further characterize the CGRP2 receptor and to determine why this receptor cannot be identified by radioligand binding.|Porcine coronary arteries were used to study CGRPi receptors because CGRPgg? blocks CGRP-induced relaxation of this tissue with low affinity. I used RT-PCR, radioligand binding, and affinity values from previously reported isolated tissue studies to compare the CGRP receptor in coronary arteries to the cloned CGRP1 receptor I identified mRNA encoding calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein (RAMP) 1 in porcine coronary arteries. In addition, the ligand binding characteristics of CGRP receptors in coronary arteries and in CGRP1 receptor-transfected HER 293 cells were similar. There was also a high correlation between antagonist affinities determined by radioligand binding and functional relaxation assays. My data support the conclusion that CGRP8-37 blocks CGRP1 receptors with low affinity in functional relaxation assays using porcine coronary arteries.|SV40LT-SMC cells were also used to study CGRP2 receptors. In contrast to coronary arteries, the putative CGRP2 receptor in this cell line was identified as an adrenomedullin receptor that has low affinity for CGRP8-37. The inability of [125I]CGRP to label adrenomedullin receptors is consistent with the fact that the CGRP2 receptor has not been previously identified by radioligand binding.|In summary, my data do not support the widely accepted view that CGRP- induced responses are mediated by two different CGRP receptor subtypes. Rather these data support the conclusions that the putative CGRP2 receptor is an adrenomedullin receptor and that CGRP8-37 blocks CGRP1 receptor mediated responses with low affinity in some isolated tissues. I anticipate that this work will have a significant impact on the currently accepted classification of CGRP receptors.