Pharmacology of FMRFamide in Mytilus catch muscle

In the anterior byssus retractor muscle (ABRM) of Mytilus, low concentrations of FMRFamide (10(-8)-10(-7) M) relax ACh-induced catch-tension, whereas high concentrations (greater than 10(-7) M) cause contraction. To study the structure-activity relations of these actions, a number of peptide analogs of FMRFamide were screened for their biological activities on the ABRM. The structure-activity relations for contraction were different from those for relaxation. Among the peptides tested, FMR-[D-Phe]-amide and gamma 1-MSH substantially antagonized FMRFamide contractions; but only gamma 1-MSH was even slightly antagonistic to FMRFamide-induced relaxation. Relaxations produced by 10(-7) M FMRFamide, or by 10(-5) M FMRFamide-relating relaxing peptides, were markedly depressed by treating the muscle first with 10(-5) M FMRFamide or with 10(-5) M FMRFamide-related contractile peptides. However, contractile agents that are structurally unrelated to FMRFamide, such as 3 X 10(-5) M SCPB and 2 X 10(-2) M caffeine, showed little or no such after-effect on the relaxation. Relaxations in response to submaximal serotonin, dopamine and repetitive electrical pulses of stimulation were not affected by a pretreatment with 10(-5) M FMRFamide. These results suggest that the ABRM of Mytilus has at least two pharmacologically distinct classes of receptors which are capable of being activated by FMRFamide.