Binding Sites of Calcitonin Gene-Related Peptide (CGRP): Abundant Occurrence in Visceral Organs

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Abstract
Calcitonin gene-related peptide (CGRP) is a novel peptide amide of 37 amino acid residues, which was first identified as the product of alternative processing of RNA transcripts of the calcitonin gene in humans and rats. Using 125I-human CGRP (hCGRP) as the binding ligand and hCGRP or salmon calcitonin (sCT) as the specific inhibitor of binding, it was examined how the receptor-like binding sites distribute among rat tissues including the nervous system, which is already known to contain binding sites in discrete regions. Some visceral organs (liver, spleen and lung) and possibly the bone marrow of Wistar male rats (8-10 weeks old) were found to be relatively rich in these binding sites. The following parameters were calculated by Scatchard analysis of binding data for the cerebellum, spleen and liver; KD (nM) and Bmax (fmol/mg protein) were 0.61 and 408, 1.08 and 858, and 0.89 and 356, respectively. In these three tissues, both hCGRP and sCT were able to completely suppress the specific binding; the IC50s (nM) of hCGRP for the cerebellum, spleen and liver were 2.57, 2.29 and 3.02, respectively, and the IC50s (.mu.M) of sCT 2.69, 0.41 and 1.78, respectively. The results obtained herein strongly suggest the physiological function of CGRP in these visceral organs including bone marrow.

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