Frog vasoactive intestinal polypeptide and galanin: Primary structures and effects on pituitary adenylate cyclase

Vasoactive intestinal polypeptide (VIP) and galanin were isolated in pure form from the stomach of the European green frog, Rana ridibunda. Frog VIP is identical to the previously characterized VIP from chicken and alligator. The primary structure of frog galanin contains only two amino acid substitutions (asparagine for histidine at position 23 and histidine for tyrosine at position 26) compared with porcine galanin. The data indicate that evolutionary pressure to conserve the amino acid sequence of both peptides during the evolution of amphibia to mammals has been strong. Synthetic frog VIP produced a dose-dependent increase in cAMP concentration in frog anterior pituitary fragments. The potency of the peptide (ED₅₀ = 1.2 x 10^-6 M; mean ± SE; n = 8) was comparable to that of porcine VIP (EC₅₀ = 1.3 x 10^{- 6} M), but was approximately 10-fold less than that of frog pituitary adenylate cyclase-activating polypeptide [PACAP-(1-38); ED₅₀ = 1.1 x 10^{- 7} M] in the same system. The increases in cAMP concentrations produced by maximal doses of PACAP (10^-5 M) and VIP (10^-5 M) were not additive. The data suggest that the effects of both peptides are mediated through a common PACAP-preferring receptor that is pharmacologically different from the mammalian PACAP type I receptor. Synthetic frog galanin also produced a dose- dependent increase in the concentration of cAMP in isolated frog anterior pituitary fragments (ED₅₀ = 9.3 x 10^-8 M) consistent with a possible role for the peptide as a hypophysiotropic factor in amphibians.