Effects of several peptides with Pro-Arg-Leu-NH2 C-terminal sequence on invertebrate muscles.

1. Effects of the following PRLamide peptides on some invertebrate muscles were examined: APNFLAYPRLamide (a peptide of the bivalve mollusc Mytilus), AAPLPRLamide (a peptide of the echiuroid Urechis), FTPRLamide (a fragment of a peptide of the insect Pseudaletia), YFSPRLamide (a fragment of a peptide of the insect Heliothis) and pQTSFTPRLamide (a peptide of the insect Leucophaea). 2. In the ABRM of Mytilus, the Mytilus and Urechis peptides potentiated phasic contraction by repetitive electrical stimulation. The former was more potent than the latter. The three insect-origin peptides did not show any potentiating effect but showed a weak antagonistic action on the potentiating effect of the Mytilus peptide. Except the Mytilus peptide, the PRLamide peptides showed little or no relaxing effect on catch tension, though they accelerated catch-relaxing response to repetitive electrical stimulation. 3. In the inner circular body-wall of muscle of Urechis, the PRLamide peptides showed a potentiating effect on twitch contraction. The Urechis peptide was most potent. In the esophagus of the annelid Perinereis, the PRLamide peptides showed a contractile effect. The Urechis peptide was also most potent. 4. In the crop of the cricket Gryllus, the insect-origin peptides markedly potentiated spontaneous rhythmic contractions. The Mytilus peptide showed a weak potentiating effect, but the Urechis peptide showed little or no effect.