Physiological and Ultrastructural Studies on the Longitudinal Retractor Muscle of a Sea Cucumber Stichopus Japonicus I. Factors Influencing the Mechanical Response

1. The physiological properties of contraction and the ultrastructure of the longitudinal retractor muscle (LRM) of a sea cucumber Stichopus japonicus were studied to give information about the sources of activator Ca in echinoderm somatic smooth muscles. 2. The magnitude of AChand K-induced contractures was dependent on [Ca]0, and both contractures were eliminated in Ca-free solution, while they were not markedly influenced by Mn ions (10 mM) and low pH (4o). 3. Procaine (5 mM) decreased ACh-contracture tension with a long lasting after-effect, which was removed by the application of high [K]o. 4. ACh-contractures were markedly potentiated shortly after the termination of mechanical response to the removal of external Na. 5. The LRM could also be made to contract by caffeine (10 mM), the removal of external divalent cations and hypertonic solutions, indicating the presence of intracellularly stored Ca available for the activation of the contractile mechanism. 6. The LRM fibres contain poorly developed intracellular membranous structures. The inner surface of the plasma membrane and small vesicles located underneath the plasma membrane seem to be two of the sources of activator Ca.