Comparative affinities and antagonistic potencies of various human calcitonin gene-related peptide fragments on calcitonin gene-related peptide receptors in brain and periphery.

Recent data have suggested that N-truncated human calcitonin gene-related peptide (hCGRP) fragments such as hCGRP8-37 and hCGRP12-37 behave as competitive antagonists in certain bioassays. The present study was undertaken to determine which amino acid residues between positions 8 to 12 are directly involved in ensuring high affinity and antagonist properties at CGRP receptors. In brain, atrium and vas deferens membrane preparations, hCGRP8-37 and hCGRP9-37 demonstrated affinities similar to or much higher than that of the native peptide hCGRP alpha. Shorter fragments such as hCGRP 10-37 and hCGRP 11-37 possessed less than 20% of the affinity of hCGRP alpha in these various assays demonstrating the critical importance of the Thr residue in position 9 for maintenance of adequate receptor affinity. In the in vitro guinea pig left and right atria bioassays, both hCGRP8-37 and hCGRP9-37 behaved as potent and competitive antagonists (pA2:7.0-7.7) of positive chronotropic and inotropic effects induced by hCGRP alpha. hCGRP 10-37 and hCGRP 11-37 were at least 10 times less potent (pA2: 6.1-6.6). Interestingly, both hCGRP 8-37 and hCGRP9-37 were much less potent (pA2: 6.2-6.3) in blocking the effects of hCGRP alpha in the rat vas deferens whereas only slight inhibition was observed at 1.0 microM with hCGRP10-37 and no blocking activity was detected with hCGRP11-37 at a low micromolar concentration. These results are in accordance with binding data and demonstrate further the importance of residues in positions 9 (Thr) and 10 (His) to ensure potent antagonistic properties of N-truncated hCGRP fragments.(ABSTRACT TRUNCATED AT 250 WORDS)