Structural determinants for binding to CGRP receptors expressed by human SK-N-MC and Col 29 cells: Studies with chimeric and other peptides

1. Structure-activity relationships for the binding of human α-calcitonin gene-related peptide 8-37 (hαCGRP_8-37) have been investigated at the CGRP receptors expressed by human SK-N-MC (neuroblastoma) and Col 29 (colonic epithelia) cells by radioligand binding assays and functional assays (hαCGRP stimulation of adenylate cyclase). 2. On SK-N-MC cells the potency order was hαCGRP_8-37 > hαCGRP_19-37 = AC187 > rat amylin_8-37 > hα[Tyr⁰]-CGRP_28-37 (apparent pKBS of 7.49 ± 0.25, 5.89 ± 0.20, 6.18 ± 0.19, 5.85 ± 0.19 and 5.25 ± 0.07). The SK-N-MC receptor appeared CGRP₁-like. 3. On Col 29 cells, only hαCGRP_8-37 of the above compounds was able to antagonize the actions of hαCGRP (apparent pKB = 6.48 ± 0.28). Its receptor appeared CGRP₂-like. 4. hα[Ala^11,18]-CGRP_8-37, where the amphipathic nature of the N-terminal α-helix has been reduced, bound to SK-N-MC cells a 100 fold less strongly than hαCGRP_8-37. 5. On SK-N-MC cells, hαCGRP(8-18, 28-37) (M433) and mastoparan-hαCGRP_28-37 (M432) had apparent pKBS of 6.64 ± 0.16 and 6.42 ± 0.26, suggesting that residues 19-27 play a minor role in binding. The physico-chemical properties of residues 8-18 may be more important than any specific side-chain interactions. 6. M433 was almost as potent as hαCGRP_8-37 on Col 29 cells (apparent pKB = 6.17 ± 0.20). Other antagonists were inactive.