A DYNAMIC-MODEL OF THE CARDIAC VENTRICULAR ACTION-POTENTIAL .2. AFTERDEPOLARIZATIONS, TRIGGERED ACTIVITY, AND POTENTIATION

The action potential model presented in our accompanying article in th is journal is used to investigate phenomena that involve dynamic chang es of [Ca2+](i), as described below. Delayed afterdepolarizations (DAD s) are induced by spontaneous Ca2+ release from the sarcoplasmic retic ulum (SR), which, in turn, activates both the Na+-Ca2+ exchanger (I-Na Ca) and a nonspecific Ca2+-activated current (I-ns(Ca)). The relative contributions of I-NaCa and of I-ns(Ca) to the generation of DADs are different under different degrees of Ca2+ overload. Early afterdepolar izations (EADs) can be categorized into two types: (1) plateau EADs, r esulting from a secondary activation of the L-type Ca2+ current during the plateau of an action potential, and (2) phase-3 EADs, resulting f rom activation of I-NaCa and I-ns(Ca) by increased [Ca2+](i) due to sp ontaneous Ca2+ release from the SR during the late repolarization phas e. Spontaneous rhythmic activity and triggered activity are caused by spontaneous Ca2+ release from the SR under conditions of Ca2+ overload . Postextrasystolic potentiation reflects the time delay associated wi th translocation of Ca2+ from network SR to junctional SR, The cell is paced at high frequencies to investigate the long-term effects on the intracellular ionic concentrations.