Further identification of bioactive peptides in the anterior byssus retractor muscle of Mytilus: two contractile and three inhibitory peptides.

1. Two contractile and three inhibitory peptides were newly isolated from the anterior byssus retractor muscles (ABRMs) of the bivalve mollusc Mytilus edulis by using the muscle as the bioassay system. 2. The structures of the two contractile peptides were GPFGTHIKamide (GPFG-8) and GPFGLNKHGamide (GPFG-9). The contractile response of the ABRM to the first-time application of GPFG-8 or GPFG-9 was of considerable size. The threshold concentrations of the peptides were around 10(-9) M. However, the contractile response to the second-time application was far smaller than that to the first-time application in both cases. Namely, the muscle showed tachyphylaxis to the peptides. 3. Two of the three inhibitory peptides were members of the Mytilus-inhibitory-peptide (MIP) family. Their structures were RAPLFIamide (MIP6) and RSPMFVamide (MIP7). The peptides, as well as the other MIPs previously identified, showed a potent inhibitory effect on phasic contraction of the ABRM in response to repetitive electrical stimulation. The remaining one was an MIP-related peptide (MIP-RP) having the sequence of MRYFVamide. The MIP-RP was less potent than the two MIPs in inhibiting the contraction of the ABRM.