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Anthracycline therapy is associated with a life-threatening but poorly understood cardiotoxicity. Effects of treatment are consistent with drug-induced disruption of cardiac sarcoplasmic reticulum (SR) calcium homeostasis, including inhibition of calcium release by anthracyclines. This effect, which depends on luminal SR calcium concentration, is hypothesized to involve interactions of anthracyclines with the calcium binding protein calsequestrin (CSQ). This study was designed to test the hypothesis that an interaction between CSQ and anthracyclines could be related to alterations in SR calcium release and cardiac function. The effects of the anthracycline, daunorubicin, and its metabolite daunorubicinol were compared with those of a known CSQ inhibitor, trifluoperazine (TFP). Protein fluorescence quenching studies demonstrated that TFP, daunorubicin, and daunorubicinol bind to CSQ with apparent binding affinities in the low micromolar range. The presence of calcium decreases the drug-dependent fluorescence quenching, probably because of calcium-induced CSQ conformational changes. TFP also inhibited SR calcium release. Although the TFP IC50 value is somewhat larger than for anthracyclines, the TFP effect is also dependent on luminal SR calcium concentration. In a muscle preparation, micromolar TFP decreased cardiac contractility in a manner that implicates the involvement of SR calcium and resembles the effects of anthracyclines. These data are consistent with a mechanism in which TFP or anthracyclines impair SR calcium release and cardiac function through a mechanism involving disruption of CSQ function. Such a mechanism may contribute to anthracycline cardiotoxicity.