Pharmacology of fmrfamide in Mytilus catch muscle

Abstract

1. 1. 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 (>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. 2. 2. The structure-activity relations for contraction were different from those for relaxation. 3. 3. Among the peptides tested, FMR-[ d-Phe]-amide and γ 1-MSH substantially antagonized FMRF-amide contractions; but only γ 1-MSH was even slightly antagonistic to FMRFamide-induced relaxation. 4. 4. 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 × 10 −5 M SCP B and 2 × 10 −2 M caffeine, showed little or no such after-effect on the relaxation. 5. 5. Relaxations in response to submaximal serotonin, dopamine and repetitive electrical pulses of stimulation were not affected by a pretreatment with 10 −5 M FMRFamide. 6. 6. 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.