Surface ultrastructural features of isolated perfused rat hearts during calcium paradox

Cellular damage caused by calcium (Ca⁺⁺) paradox is of significant importance under both experimental and clinical conditions. Electron microscopy was used to study the surface changes of the isolated rat hearts perfused with normal Krebs-Henseleit (KH) medium, Ca⁺⁺ -free KH medium, Ca⁺⁺- free KH medium followed by normal KH medium. The hearts perfused with Ca⁺⁺ -free medium and normal KH medium showed elongated parallel myofibrils with transverse ridges spaced at regular intervals. The major change in Ca⁺⁺ -free hearts consisted of wide cellular separation. For the Ca⁺⁺ -paradox hearts, the myofibrils were twisted and narrowed at several locations. The myocardial fiber surfaces were bulged out, frequently with sarcolemmal ruptures and holes. Supercontraction of cells caused breaks in the sarcolemma and aggregation of mitochondria at the cell periphery. These results suggest that Ca⁺⁺ -depletion primarily affects the myocardial cell junctions which, following reintroduction of Ca⁺⁺, accelerate the entry of Ca⁺⁺ into the cells, supercontraction of cells and stretching of sarcolemma.