ABSTRACT. We reported previously that doxorubicin, an anticancer agent that has an anthracycline structure, alters Ca 2+ releasing and uptake mechanisms in the sarcoplasmic reticulum of myocardial cells. These effects of doxorubicin are apparently related to its cardiotoxicity. Mitoxantrone is a similar anticancer agent with an anthracenedion structure that has been shown to be significantly less cardiotoxic. In the present study, the effects of mitoxantrone on the functions of the sarcoplasmic reticulum were examined in isolated muscle preparations obtained from the guinea-pig heart. In electrically-stimulated left atrial muscle preparations, incubation in vitro for 4 hr with 30 or 100 µM mitoxantrone significantly prolonged the time to the peak of twitch tension, markedly increased the developed tension observed at lower stimulation frequencies, thereby attenuating the slope of positive force-frequency relationships, and increased the postrest contraction observed after a 60-sec quiescent period. In myocytes isolated from ventricular muscles, 30 µM mitoxantrone increased the peak and the size of intracellular Ca 2+ concentrations ([Ca 2+ ]i), and prolonged the time to peak [Ca 2+ ]i. In skinned muscle fiber preparations obtained from the left ventricular muscle, 30 µM mitoxantrone significantly increased the caffeine-induced contraction without affecting the Ca 2+ sensitivity of contractile proteins. These results suggest that mitoxantrone enhances Ca 2+ release from the sarcoplasmic reticulum in isolated atrial muscle preparations obtained from the guinea-pig heart. Apparent enhancement of the sarcoplasmic reticulum functions, in contrast to anthracyclines that has been shown to suppress these functions, seems to explain the relative lack of marked cardiotoxicity of mitoxantrone. KEY WORDS: caffeine-induced contraction, Ca 2+ release and uptake by sarcoplasmic reticulum, inotropic effects, intracellular Ca 2+ concentration, mitoxantrone.J. Vet. Med. Sci. 70(3): 255-264, 2008 Anthracyclines are efficacious anti-tumor agents; however, their clinical usefulness is limited by serious cardiotoxic effects. Mitoxantrone, an anthracenedion derivative, has been synthesized in an attempt to develop an agent that has a reliable anticancer activity without the compromising cardiotoxic effects [9,35].Human as well as animal studies demonstrated that the cardiotoxic effects of mitoxantrone are significantly less severe at clinically equivalent anticancer doses compared to those observed with anthracyclines [2,9,11,13,20,22,33].We reported previously [7] that in vivo treatment of rats with doxorubicin results in a prolongation of the time to the peak of twitch tension, a reduction of the developed tension observed at lower stimulation frequencies and an attenuation of the postrest contraction (contraction that is evoked by the first stimulus after a brief quiescent period) observed in electrically-stimulated heart muscle preparations obtained from the drug-treated animals. These changes developed slowly with time aft...