Pharmacology of relaxing response of mytilus smooth muscle to the catch-relaxing peptide

Abstract

1. Pharmacological properties of relaxing action of the catch-relaxing peptide (CARP) on the anterior byssus retractor muscle of Mytilus were examined and compared with those of the actions of other relaxing stimuli. 2. Butaclamol (10 −5 M), a dopamine blocker, showed little effect on relaxing responses to CARP, serotonin and repetitive electrical pulses of stimulation. 3. Mersalyl (5 × 10 −4 M), a serotonin blocker, depressed but did not abolish relaxing response to CARP. Pre-treatment of the muscle with KCl-EGTA solution also depressed but did not abolish the response to CARP. 4. Propranolol (10 −3 M) depressed or abolished relaxing responses to serotonin, dopamine and repetitive electrical stimulation, whereas it enhanced the response to CARP. 5. Phenoxybenzamine (10 −4 M) enhanced relaxing responses to CARP, serotonin and repetitive electrical stimulation, but it showed little effect on the response to dopamine. 6. Simple exposure of the muscle to La 3+ (5 mM) showed little effect on relaxing response to CARP. 7. When the muscle was continuously immersed (for 30 min or more) in CARP (5 × 10 −8 M), relaxing response to repetitive electrical stimulation was depressed or abolished. 8. It is suggested that CARP relaxes catch tension of the ABRM by acting on postsynaptic sites in the muscle.