[Ca2+ localization in muscles with double calcium regulation]

Localization of Ca-accumulating structures was studied in scutal depressor muscles of barnacle (Balanus rostratus) by electron microscopy combined with osmium--pyroantimonate technique. In resting fibers the pyroantimonate precipitate was localized mainly in plasma membranes of the cleft system which can be considered as main calcium store. The precipitate was also located in membranes of sarcoplasmic reticulum and mitochondria and I--bonds diffusely. In contracted fibers fixed at the peak of mechanical activity the number of precipitate granules increased in the sarcoplasm and diminished at cleft plasma membranes. In contracted fibres the fine precipitate also appeared along thick filaments at the sites of myosin heads. The latter seems to be the proof of the presence of myosin control in crayfish muscles.