Physiological and Ultrastructural Studies on the Origin of Activator Calcium in Body Wall Muscles of Spoon Worms

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To examine the origin of activator Ca and its translocation during contraction in body wall muscles (BWM) of spoon worms, Urechis unicinctus , physiological and ultrastructural studies, including cytochemistry, were performed. The potassium (K-) contracture tension was significantly reduced by the removal of external Ca, and by the application of Mn, La and verapamil. On the other hand, caffeine induced a prolonged contraction. The removal of Ca and Mg from the external solution, and the rapid cooling caused an irregular or oscillatory contraction. These results suggested that, in BWM fibers, the activator Ca is supplied partially from both external solution and intracellular Ca-accumulating structures. Ultrastructural observations revealed that the muscle fibers contain a relatively large amount of sarcoplasmic reticulum (SR). The fractional volume of the SR relative to the fiber volume was 2~5% in all fibers of three muscle layers. To demonstrate the Ca localization, the muscle fibers were fixed by pyroantimonate (PA) methods at resting and contracting states. In the resting fibers, the PA precipitates were exclusively localized in the SR and the inner surface of plasma membrane. On the other hand, in the contracting fibers, they were diffusely distributed in the central regions of myoplasm, and had disappeared from the SR and plasma membrane. X-ray microanalysis revealed that the PA precipitates contain Ca. With the results of physiological experiments, these results indicate that the activator Ca originates not only from the external solution, but also from the intracellular Ca-accumulating structures, the SR and the inner surface of plasma membrane.

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