Rate-dependent changes in action potential duration and membrane currents in hamster ventricular myocytes

Hamsters are frequently studied as a model of cardiomyopathy, but the electrophysiological properties of a hamster heart are not well defined. We examined rate-dependent changes in action potentials and underlying ionic mechanisms in isolated ventricular myocytes from hamster hearts using the whole-cell configuration of the patch clamp technique. At 0.1 Hz stimulation, the mean action potential duration at 90% (APD90) and 20% (APD20) repolarization were 63+/-7 ms and 9+/-1 ms, respectively ( n=17). With increasing frequency of stimulation, APD progressively prolonged to 119+/-16 ms (APD90) and 36+/-7 ms (APD20) at 6.0 Hz. A further increase in the rate of stimulation to 8.0 Hz did not change APD significantly. Application of 4 mM 4-aminopyridine (4-AP) lengthened APD markedly and completely prevented the rate-dependent prolongation. Cd2+ (0.2 mM) shortened APD and generally attenuated the rate-dependent lengthening of APD up to 5.0 Hz, but unaffected the lengthening of APD with the further increase in the rate. At plateau voltages, there were two time-dependent currents, Ito1 and I(Ca,L). Recovery from inactivation for Ito1 had two components: t(slow)=980+/-129 ms accounting for 58% of the total fraction, and t(fast)=39+/-13 ms ( n=7). Recovery from inactivation for I(Ca,L) was rapid with t=20+/-4 ms ( n=6). Results suggest that the slow recovery from inactivation in Ito1 is the main reason for the rate-dependent prolongation of APD in hamster ventricular myocytes.