The binding sites of Ca
2+ entering the injured cardiac muscle cell was investigated by
45Ca electron microscope autoradiography in guinea pig papillary muscle and in frog ventricular muscle. The muscle was injured in Ca-free Tyrode's solution, transfered into
45Ca-Tyrode's solution and, after 30 min, fixed and cut into thin sections. An autoradiogram was taken after exposure (4-7 weeks) the thin film of the section was developed. Fine, or sometimes filamentous, silver grains produced by radiation with
45Ca were frequently observed on the intercalated discs of injured cells where most of the nexal membranes were separated. The number of
45Ca grains on the discs of injured cells waseight times more than that found on intact cells. The concentration of
45 Ca grains estimated by the number of grains per unit area was 3.2 times higher in the disc region than that in the other cytoplasmic regions in the injured cells, while in the intact cells it was only 1/3 of that found in thelatter. The localization of
45Ca grains along the disc of the injured cell was also examined and the fine grains were seen to be located on the separated nexal membrane or at the cytoplasmic side of the desmosome and fascia adherens. It is likely that Ca
2+ binds with the nexal membrane and the resulting structural change, such as nexal separation, is indispensable for intercellular uncoupling or healing-over of cardiac muscles.
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