Multiple ionic mechanisms of early afterdepolarizations in isolated ventricular myocytes from guinea-pig hearts.

Ionic mechanisms of early afterdepolarization (EAD) induced by the K(+)-free solution or veratridine were studied with guinea-pig ventricular myocytes using the patch-clamp technique of whole-cell and cell-attached patch configurations. In the K(+)-free solution, myocytes exhibited prolonged action potential duration with humps on the final repolarization phase, which eventually turned into EAD starting around -70 mV and induced triggered activity. Application of 0.5 mM Cd2+ inhibited the development of EAD and caused depolarization of maximum diastolic potentials around -30 mV, although Cd2+ did not prevent prolongation of the action potential. Application of 50-100 microM Ni2+ or 30 microM tetrodotoxin had little effects on EAD and diastolic potentials. The background current-voltage relation examined by a ramp voltage clamp showed inhibition of the inward rectifier K+ current, induction of steady inward current between -40 and -10 mV, and increase in the outward tail current upon repolarization in the K(+)-free solution. Cd2+ completely blocked the steady inward current at the plateau level and partially depressed the delayed outward K+ current, while Ni2+ had no effects on the background I-V relation. Tetrodotoxin showed a mild inhibitory effect on the inward component of the background current negative to -50 mV, but left the steady inward current at the plateau level. Therefore, EAD in the K(+)-free condition is mainly formed by decreased inward rectifier K+ current, activation of the L-type Ca2+ current, and time-dependent decay of the delayed outward K+ current upon repolarization. Application of 25-100 microM veratridine caused marked prolongation of action potential with appearance of regenerative EADs. Action potential prolongation and EADs were partially abolished by Cd2+ and completely eliminated by tetrodotoxin. The single channel current recordings showed a decreased current amplitude, and prolonged and delayed openings of the Na+ channel currents by veratridine. Thus, an ensemble average current showed markedly prolonged decay time constant of 609 msec in veratridine from 3.6 msec in the control. These results indicate that veratridine-induced EAD is mainly formed by altered properties of the Na+ channel current and partly by the L-type Ca2+ current due to slowed repolarization. Thus, EAD can be induced by different ionic mechanisms depending on the basal conditions.