Binding of 45Ca to intercalated discs of cardiac muscles studied by electron microscope autoradiography

The binding sites of Ca2+ 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 was eight times more than that found on intact cells. The concentration of 45Ca 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 the latter. 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 Ca2+ 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.