Physiological and ultrastructural studies on the longitudinal retractor muscle of a sea cucumber Stichopus japonicus. II. Intracellular localization and translocation of activator calcium during mechanical activity

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Abstract
The intracellular localization and translocation of activator Ca in the longitudinal retractor muscle (LRM) of a sea cucumber Stichopus japonicus were studied by fixing the LRM in a 1 % OsO4 solution containing 2 % K pyroantimonate. In the resting LRM fibres, electron-opaque pyroantimonate precipitate was mostly localized along the inner surface of the plasma membrane and at the subsarcolemmal vesicles in close apposition to the plasma membrane. In the LRM fibres fixed during the mechanical response to ACh and high [K]o, the precipitate was diffusely distributed in the myoplasm in the form of numerous particles with corresponding decrease in the amount of the precipitate at the peripheral structures. Electron probe X-ray microanalysis showed the presence of Ca in the precipitate, indicating that the precipitate provides a valid measure of Ca localization. These results accord with the view that, in the LRM, the contractile mechanism is activated by the release of Ca from the intracellular structures as well as by the inward movement of extracellular Ca.