Clotrimazole, an antimycotic drug, inhibits the sarcoplasmic reticulum calcium pump and contractile function in heart muscle

Clotrimazole (CLT), an antimycotic drug, has been shown to inhibit proliferation of normal and cancer cell lines and its systemic use as a new tool in the treatment of proliferative disorders is presently under scrutiny (Benzaquen, L. R., Brugnara, C., Byers, H. R., Gattoni-Celli, S., and Halperin, J. A. (1995) Nature Med. 1, 534-540). The action of CLT is thought to involve depletion of intracellular Ca2+ stores but the underlying mechanism has not been defined. The present study utilized membrane vesicles of rabbit cardiac sarcoplasmic reticulum (SR) to determine the mechanism by which CLT depletes intracellular Ca2+ stores. The results revealed a strong, concentration-dependent inhibitory action of CLT on the ATP-energized Ca2+ uptake activity of SR (50% inhibition with approximately 35 microM CLT). The inhibition was of rapid onset (manifested in <15 s), and was accompanied by a 7-fold decrease in the apparent affinity of the SR Ca2+-ATPase for Ca2+ and a minor decrement in the enzyme's apparent affinity toward ATP. Exposure of SR to CLT in the absence or presence of Ca2+ resulted in irreversible inhibition of Ca2+ uptake demonstrating that the Ca2+-bound and Ca2+-free conformations of the Ca2+-ATPase are CLT-sensitive. Introduction of CLT to the reaction medium subsequent to induction of enzyme turnover with Ca2+ and ATP resulted in instantaneous cessation of Ca2+ transport indicating that an intermediate enzyme species generated during turnover undergoes rapid inactivation by CLT. The inhibition of Ca2+ uptake by CLT was accompanied by inhibition of Ca2+-stimulated ATP hydrolysis and Ca2+-induced phosphoenzyme intermediate formation from ATP in the ATPase catalytic cycle. Phosphorylation of the Ca2+-deprived enzyme with Pi in the reverse direction of catalytic cycle and Ca2+ release from Ca2+-preloaded SR vesicles were unaffected by CLT. It is concluded that CLT depletes intracellular Ca2+ stores by inhibiting Ca2+ sequestration by the Ca2+-ATPase. The mechanism of ATPase inhibition involves a drug-induced alteration in the Ca2+-binding site(s) resulting in paralysis of the enzyme's catalytic and ion transport cycle. CLT (50 microM) caused marked depression of contractile function in isolated perfused, electrically paced rabbit heart preparations. The contractile function recovered gradually following withdrawal of CLT from the perfusate indicating the existence of mechanisms in the intact cell to inactivate, metabolize, or clear CLT from its target site.