Abstract
Highly purified sarcolemmal (SL) membrane vesicles were prepared from bovine cardiac tissue and used to evaluate the effects of cardiotoxin (CTX) on Ca 2+ transport systems of the SL membrane. The áddition of CTX, at 1.0 μM and 10 μM, stimulated the ATP-dependent transport of 45Ca 2+ by the Ca 2+-Mg 2+-ATPase to 138% and 193% of control levels, respectively. The increase in Ca 2+ accumulation by the SL vesicles in the presence of CTX was not affected by 100 μM ouabain but was reduced to 50% of control uptake levels by the addition of 0.5 mM dicyclohexylcarbodiimide. Additionally, no effect of CTX (1.0 μM to 10 μM) was seen on 45Ca 2+ transport by the Na-Ca exchange system. These effects of CTX on the SL membrane do not appear to be due to non-specific membrane disruption since SL vesicles preloaded with 45Ca 2+ did not release (efflux) the accumulated Ca 2+ more rapidly in the presence of CTX (1.0 μM to 250 μM). The rate of hydrolysis of ATP by the SL Ca 2+-Mg 2+-ATPase was observed to increase (29% to 52%) as CTX concentration increased (1.0 μM to 10 μM). The same concentrations of CTX had no effect on ATP hydrolysis by Na +-K +-ATPase. We conclude that 10 μM CTX stimulates the rate of ATP hydrolysis and Ca 2+ transport by the SL Ca 2+-Mg 2+-ATPase.
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