CHRONIC DOXORUBICIN INDUCED CARDIOMYOPATHY IN RABBITS - MECHANICAL, INTRACELLULAR ACTION-POTENTIAL, AND BETA-ADRENERGIC CHARACTERISTICS OF THE FAILING MYOCARDIUM

Study objective - The aim was to assess myocardial, electrophysiological, and adrenergic changes caused by chronic administration of doxorubicin.Design - Doxorubicin induced cardiotoxicity was produced in three groups of rabbits by injecting doxorubicin 0.75 mg·kg-1 three times a week for 7, 9 and 11 weeks. There were 36 controls. All studies were conducted within 16 to 36 h after the last injection. Histological, mechanical, and action potential changes produced by doxorubicin were examined in vitro. The effects of doxorubicin on β adrenergic receptors and cyclic adenosine monophosphate (AMP) generation in myocardial membrane preparations were also evaluated.Experimental material - 145 New Zealand white rabbits, 2.4-2.7 kg, were used. After excision of the heart, a papillary muscle was used for mechanical studies, a portion of the septum for intracellular action potential studies, and the rest of the heart for histological or biochemical studies.Measurements and results - Histological studies showed widespread myocardial damage that became more severe as the cumulative doses increased. Right ventricular papillary muscles of doxorubicin treated rabbits had lower total tension (1.5 v 3.3 g·mm-2 for controls, p<0.05) and dT/dt, shorter contraction duration, and lower velocity of shortening than the control muscles under all loading conditions. The changes progressed as the cumulative doxorubicin dose increased. Action potential duration was shorter in the doxorubicin treated groups (ADD50 = 76 v 62 ms for controls, p<0.01), although resting action potential amplitude was normal. Tension-frequency response (6-36 stimuli·min-1) and response to increasing calcium concentrations (2.54-6.32 mM) were attenuated in the doxorubicin group. Percent change in tension and dT/dt in response to noradrenaline (50 μM), isoprenaline (20 μM), or dibutyryl cyclic AMP (40 mM), was increased in the doxorubicin group v controls (300-600% v 100-200% respectively), despite chronic increase in circulating catecholamines, depletion of myocardial catecholamines, and no change in β adrenergic receptor number or affinity. The apparent increase in β adrenergic responsiveness in the doxorubicin group may have been partly due to decreased basal cyclic AMP production (13 v 31 pMol·mg-1 protein·min-1, p<0.01), although maximum catecholamine stimulated cyclic AMP production was only mildly decreased (251 v 315 pMol·mg-1 protein·min-1, p<0.05).Conclusions - The subacute effects of chronic doxorubicin become progressively more marked as the cumulative dose increases, and there are significant differences in the myocardial characteristics between this chronic model and other models of heart failure. These differences may be related to the cytotoxic effects of doxorubicin on membranes and membrane bound enzymes.