Ultrastructural Studies of Calcium Location during the “Catch” Contraction of Clam Smooth Adductor Muscle Cells

Contraction of molluscan adductors has been classified into three states; 1) resting state, 2) contracted state, 3) prolonged “catch” state. Among these, the “catch” state is considered a peculiar state, which requires little expenditure of energy, but in which contraction can be maintained for long periods. It is not yet known whether “catch” muscle contraction is regulated by Ca, or where Ca translocates during resting state through “catch” state, if the muscle contraction is regulated by Ca. We attempted to observe Ca translocation in muscle cells during contraction by the K-pyroantimonate method. We fixed “catch” muscle cells for electron microscopy with fixative including K-pyroantimonate, and observed where electron-dense precipitates, in which Ca is concentrated, were located in the muscle cells in the three states of contraction. At the resting state, precipitate was located at cell peripheries, in positions such as at the inner surface of cell membranes and in sarcoplasmic reticular systems (SRs). In the contracted state, they were located within the cytoplasm. At the “catch” state, they were found in both the cytoplasm and at peripheries, although the number of precipitates in peripheral areas was small. Thus, we show that calcium translocates in the cells during resting-contraction-catch cycles of “catch” muscle contraction.