A new active peptide xenopsin was obtained from the skin of Xenopus laevis, possessing a potent contractile activity on rat stomach strip in vitro. The amino acid sequence of xenopsin has been found to be pyrGlu-Gly-Lys-Arg-Pro-Trp-Ile-Leu-OH. The contractile responses on a rat stomach strip were proportional to the peptide doses. This peptide also had a moderate contractile activity on guinea-pig ileum but the response was not reproducible. Intravenous injection of this peptide lowered arterial blood pressure of the anesthetized rat but a prompt and intense tachyphylaxis was observed. Xenopsin and several of its analogous peptides were synthesized and their biological activities were determined. Essential sequence that displayed the activity was Arg-Pro-Trp-Ile-Leu-OH. In addition to xenopsin and caerulein, it was confirmed that the skin of Xenopus laevis contained biological active amines, serotonin, buffotenidine, and N-methylserotonin.
