Bioactive beta-bend structures for the antagonist h alpha CGRP(8-37) at the CGRP(1) receptor of the rat pulmonary artery

1 The aim of this study was to determine beta-bend structures and the role of the N- and C-terminus in the antagonist h alpha CGRP(8-37) at the rat pu lmonary artery CGRP receptor mediating h alpha CGRP relaxation. 2 H alpha CGRP(8-37) Pro(16) (10(-6) M), with a bend-biasing residue (proli ne) at position 16, did not antagonize h alpha CGRP responses, while a stru cture-conserving amino acid (alanine(16)) at the same position retained ant agonist activity (apparent pK(B) 6.6+/-0.1; 10(-6) M). H alpha CGRP(8-37) P ro(19) (10(-6) M), with proline at position 19 was an antagonist (apparent pK(B) 6.9+/-0.1). 3 Incorporation of a beta-bend forcing residue, BTD (beta-turn dipeptide), at positions 19 and 20 in h alpha CGRP(8-37) (10(-6) M) antagonized h alpha CGRP responses (apparent pK(B) 7.2+/-0.2); and BTD at positions 19,20 and 33,34 within h alpha CGRP(8-37) was a competitive antagonist (pA(2) 7.2; Sc hild plot slope 1.0+/-0.1). 4 H alpha CGRP(8-37) analogues, substituted at the N-terminus by either gly cine(8) or des-NH2 valine(8) or proline(8) were all antagonists (apparent p K(B) 6.9+/-0.1; (10(-6) M), 7.0+/-0.1 (10(-6) M), and pA(2) 7.0 (slope 1.0/-0.2), respectively); while replacements by prolines together with glutami c acid(10,14) in h alpha CGRP(8-37) (10(-6) M) or alanine amide(37) at the C-terminus of h alpha CGRP(8-37) (10(-5) M) were both inactive compounds. 5 In conclusion, possible bioactive structures of h alpha CGRP8-37 include two beta-bends (at 18-21 and 32-35), which were mimicked by BTD incorporati on. Within h alpha CGRP(8-37), the N-terminus is not essential for antagoni sm while the C-terminus may interact directly with CGRP(1) receptors in the rat pulmonary artery.