Depressed transient outward current density in ventricular myocytes from cardiomyopathic Syrian hamsters of different ages.

We determined whether the dilated cardiomyopathy which develops between 30 and 140 days of age in the Syrian hamster strain MS200, before the onset of cardiac hypertrophy and failure, is associated with alterations in both the action potential (AP) and the Ca(2+)-independent transient outward current, Ito1. AP was recorded in perfused hearts using microelectrodes and Ito1 was recorded in single ventricular myocytes using the whole-cell patch-clamp. The MS200 strain was compared to the control CHF148 strain at different periods of age (60, 90, 120 and 180 days). APs were markedly lengthened in MS200 compared to CHF148 hearts at all ages studied. Cell membrane capacitance increased with age in the two strains, but was not significantly higher in MS200 than in CHF148 of a given age, indicating the absence of cell hypertrophy. At 60 days of age, Ito1 density was the same in the two strains. Later on, Ito1 density increased markedly at 90-120 days then decreased at 180 days in CHF148, whereas this increase was delayed and of reduced amplitude in MS200. The sustained component of outward current, Isus, was not sizeably different in the two strains. The conductance-voltage and steady-state inactivation relationships were shifted with age towards positive potentials by 15 mV in the two strains, but earlier in MS200 (90 days) than in CHF148 (180 days). Similarly, the recovery of Ito1 from inactivation exhibited a slow component which increased with age in the two strains but was larger in MS200 than in CHF148. In conclusion, alterations of Ito1 may contribute to changes in shape of AP, but cannot entirely explain dilation-induced AP lengthening.