TABLE 2

Effects of TFP and anthracyclines on SR Ca²⁺ release

In control experiments, canine cardiac sarcoplasmic reticulum vesicles (0.34 mg of protein/ml) in 0.30 mM antipyrylazo III, 20 mM MOPS, pH 7.0, 50 mM KH₂PO₄, 5 mM KCl, 2 mM MgCl₂, and 2 mM ATP were loaded with Ca²⁺ (7 nmol/load). In drug experiments, TFP or anthracycline was added to the microsomes before Ca²⁺ loading or after the sixth or seventh load. In both drug and control experiments, Ca²⁺ efflux rates (nanomoles of Ca²⁺ per minute per milligram) as a result of the addition of 6 mM caffeine were determined spectrophotometrically. A total of 10 Ca²⁺ loads were used in the TFP experiments; 11 loads were used in the anthracycline experiments. Concentrations of drug used in Ca²⁺ load dependence studies were 50 μM TFP and 1 μM daunorubicin and daunorubicinol.