Doxorubicin (DXR) is an effective antitumor agent in a wide spectrum of neoplasms. Chronic treatment is associated with cardiomyopathy and characteristic myocardial ultrastructural changes, which include swelling of the t tubules. Accordingly, we investigated excitation-contraction coupling in cardiomyopathic rat heart resulting from chronic DXR treatment. Using the whole-cell patch clamp technique, we studied the L-type calcium channel in single cells enzymatically isolated from normal (CTRL) and DXR rat hearts. Despite similar cell dimensions, the total membrane capacitance was significantly smaller in the DXR cells (138±9 pF) than in the CTRL cells (169±11 pF) (mean±SEM, n = 9, P < 0.05). The mean current and the current density-voltage relationships of the CTRL and the DXR cells were significantly different (n = 9, P < 0.001) with the maximal peak L-type calcium current (Ic) density increased from 6A±0.9 in CIRL cells to 10.5±2.4 .A/cm2 in the DXR cells (P < 0.05). There was no shift either in the currentvoltage relationship or the steady-state inactivation curve in the two cell groups. However, the fast time constant of inactivation was increased at a membrane voltage of -10 to 10 mV. Calcium channel antagonist equilibrium binding assays using 13H1 PN200-110 revealed no difference in the maximal receptor binding capacity (CrRL, 194±27 and DXR 211±24 fmol/mg protein; P > 0.05, n = 6) and in receptor affinity (CTRL, 0.15±0.05 and DXR 0.13±0.03 nM; P < 0.05). These data suggest that a decrease in effective capacitance might be associated with t-tubular damage. Despite this decrease, IcJ was increased in the DXR cells. Such an increase may result from an alteration in the properties of the calcium channels and/or recruitment of "hibernating" channels in the remaining surface and t-tubular membranes. (J. Clin. Invest. 1991. 87:2108-2113