Calcium-related changes in the ultrastructure of mammalian myocardium.

Isolated rat hearts, perfused with Ca2+-free medium for 5 min, were perfusion-fixed for ultrastructural studies. Increased pinocytotoic activity and characteristic changes in the staining of the intercalated disc coupled with conspicuous staining of the middle lamina in the tight junctions in these hearts indicated an alteration in the sarcolemmal activity as well as cell--cell relationship. The intracellular effects of Ca2+-free perfusion were indicated by the presence of an active Golgi body as well as the loss of heterogenic staining of the nucleoplasm. The latter appears to be a direct consequence of Ca2+ depletion of the cell, as heterogeneity reappeared upon reintroduction of the calcium in the perfusion medium. The extent of structural damage upon reintroduction of calcium was dependent upon the extracellular calcium concentration. These observations suggest that perfusion with Ca2+-free medium causes some membrane changes in the heart which probably make it more vulnerable to the reintroduction of calcium. Furthermore, the structural damage observed upon reintroduction of calcium appeared to be related to the amount of calcium overload.