Abstract
The incubation of 10(5) single neonatal rat cardiomyocytes with 1 microM-cardiotoxin in a bath medium, Tyrode solution in the presence of 1 mM-Ca2+, at 37 degrees C evoked the following chain of events. Firstly, there appeared a latent period of about 10 min during which the cells behaved normally. Neither lactate dehydrogenase nor ATP leaked from the cells. Cytosolic free Ca2+ increased considerably, as measured by the fluorescence intensity of fura-2-Ca2+ complex. At the same time a large portion of endogenous ATP was depleted. Secondly, after the latent period, the cell beating became irregular and eventually stopped. Thirdly, blebs appeared on the cell surface, leading to cell degeneration. If, before the appearance of blebs, the cells were washed with the bath medium exhaustively or incubated in the presence of the toxin antibody, cytosolic free Ca2+ and endogenous ATP returned to normal levels and cells resumed regular beating. Preincubation of the cells with 3.75 microM-flunarizine or 3.75 microM-diltiazem (both are Ca2+ antagonists), or 1.5 microM-fura-2 acetoxymethyl ester (a chelate for Ca2+), or 200 microM-leupeptin or 50 microM-antipain (both are proteinase inhibitors) considerably suppressed the toxin's ability to degenerate the cells. On the other hand, lysosomal proteinase inhibitor, autophage inhibitor, serine proteinase inhibitor, phospholipase inhibitor and calmodulin antagonist did not inhibit the toxin's activity. The results suggest that the toxin may act on the extracellular surface of intact cardiomyocytes to increase cytosolic free Ca2+. The subsequent cell degeneration may result from the activation of a Ca2+-dependent non-lysosomal proteolytic system.