Ultrastructural and physiological studies on the longitudinal body wall muscle of Dolabella auricularia. II. Localization of intracellular calcium and its translocation during mechanical activity.

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Abstract
The localization of Ca-accumulating structures in the longitudinal body wall muscle (LBWM) of the opisthobranch mollusk D. auricularia and their role in the contraction-relaxation cycle were studied by fixing the LBWM fibers at rest and during mechanical response to 400 mM K or to 10-4-10-3 M acetylcholine in a 1% OsO4 solution containing 2% potassium pyroantimonate. In the resting fibers, electron-opaque pyroantimonate precipitate was mostly localized at the peripheral structures, i.e., along the inner surface of the plasma membrane, at the membrane of the surface tubules, and at the sarcoplasmic reticulum. In fibers fixed during mechanical activity, 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 may serve as a measure of Ca localization. These results support the view that in the LBWM, Ca stored in the peripheral structures is released into the myoplasm to activate the contractile mechanism.