Reduction of myofibrillar ATPase activity and isomyosin shift in delayed doxorubicin cardiotoxicity

The aim of this study was to determine whether variations of isomyosin expression occurred during doxorubicin-induced cardiomyopathy. A suitable experimental model in which pure delayed cardiotoxic effects could be easily studied was adopted. Young adult female Sprague Dawley rats received 9 mg/kg of doxorubicin (DXR) i.v. divided into three subdoses of 3 mg/kg every third day. Control animals received equal volumes of saline. The animals were examined 9 weeks after treatment. At this time the animals treated with DXR showed ECG alterations, reduction of body weight and a marked decrease of both atrial and ventricular mass, but were still fully hemodynamically compensated. Loss of myofibrillar material could be documented by the reduced recovery of myofibril and myosin. The contractile response of papillary muscles isolated from the right ventricle of treated animals was markedly impaired. Ca-Mg-activated and Mg-activated myofibrillar ATPase activity and Ca-activated myosin ATPase activity were determined on ventricular myocardium of control and treated animals. Both myofibrillar and myosin ATPase activities were found to be significantly reduced. Pyrophosphate gel electrophoresis of purified myosin was carried out. The isomyosin pattern of DXR-treated animals showed a pronounced shift towards V3, the percent of alpha heavy chains being 54.6% in treated rats (80.5% in control rats). This isomyosin shift can explain the reduced myofibrillar and myosin ATPase activity found in treated animals