Gastrin-releasing peptides from Xenopus laevis: purification, characterization, and myotropic activity

Two molecular forms of gastrin-releasing peptide (GRP) were isolated from a n extract of the intestine of the tetraploid frog Xenopus laevis. The prima ry structure of GRP-1 (APTSQQHTEQ(10)LSRSNINTRG(20) SHWAVGHLM.NH2) differs from that of GRP-2 by a single amino acid substitution (Asn(15)--> Thr(15)) . GRP-(20-29) peptide (neuromedin C) was also isolated from the extract. Sy nthetic GRP-1 produced concentration-dependent contractions of longitudinal smooth muscle strips from Xenopus cardiac stomach (PD2 = 8.93 +/-0.32; n=6 ). The responses were unaffected by tetrodotoxin, atropine, and methysergid e, indicating a direct action of the peptide on smooth muscle cells. GRP-1 elicited concentration-dependent relaxations of precontracted (5 muM carbac hol) circular smooth muscle strips from the same region (pD(2) = 8.96 +/-0. 21; n=8). The responses were significantly (P<0.05) attenuated (71<plus/min us>24% decrease in maximum response; n=6) by indomethacin, indicating media tion, at least in part, by prostanoids. Despite the fact that Xenopus GRP-1 differs from pig GRP at 15 amino acid sites, both peptides are equipotent and equally effective for both contractile and relaxant responses, demonstr ating that selective evolutionary pressure has acted to conserve the functi onal COOH-terminal domain in the peptide. The data suggest a physiologicall y important role for GRP in the regulation of gastric motility in X. laevis .