Mechanism of anthraquinone-induced calcium release from skeletal muscle sarcoplasmic reticulum

The anthraquinones, doxorubicin, mitoxantrone, daunorubicin and rubidazone are shown to be potent stimulators of Ca²⁺ release from skeletal muscle sarcoplasmic reticulum (SR) vesicles and to trigger transient contractions in chemically skinned psoas muscle fibers. These effects of anthraquinones are the direct consequence of their specific interaction with the [³H]ryanodine receptor complex, which constitutes the Ca²⁺ release channel from the triadic junction. In the presence of adenine nucleotides and physiological Mg²⁺ concentrations (~1.0 mM), channel activation by doxorubicin and daunorubicin exhibits a sharp dependence on submicromolar Ca²⁺ which is accompanied by a selective, dose-dependent increase in the apparent affinity of the ryanodine binding sites for Ca²⁺, in a manner similar to that previously reported with caffeine. Unlike caffeine, however, anthraquinones increase [³H]ryanodine receptor occupancy to the level observed in the presence of adenine nucleotides. A strong interaction between the anthraquinone and the caffeine binding sites on the Ca²⁺ release channel is also observed when monitoring Ca²⁺ fluxes across the SR. Millimolar caffeine both inhibits anthraquinone-stimulated Ca²⁺ release, and reduced anthraquinone-stimulated [³H]ryanodine receptor occupancy, without changing the effective binding constant of the anthraquinone for its binding site. The degree of cooperativity for daunorubicin activation of Ca²⁺ release from SR also increases in the presence of caffeine. These results demonstrate that the mechanism by which anthraquinones stimulate Ca²⁺ release is caused by a direct interaction with the [³H]ryanodine receptor complex, and by sensitization of the Ca²⁺ activator site for Ca²⁺.