A dynamic model of the cardiac ventricular action potential. II. Afterdepolarizations, triggered activity, and potentiation.

The action potential model presented in our accompanying article in this journal is used to investigate phenomena that involve dynamic changes of [Ca2+]i, as described below. Delayed afterdepolarizations (DADs) are induced by spontaneous Ca2+ release from the sarcoplasmic reticulum (SR), which, in turn, activates both the Na(+)-Ca2+ exchanger (INaCa) and a nonspecific Ca(2+)-activated current (Ins(Ca)). The relative contributions of INaCa and of Ins(Ca) to the generation of DADs are different under different degrees of Ca2+ overload. Early afterdepolarizations (EADs) can be categorized into two types: (1) plateau EADs, resulting from a secondary activation of the L-type Ca2+ current during the plateau of an action potential, and (2) phase-3 EADs, resulting from activation of INaCa and Ins(Ca) by increased [Ca2+]i due to spontaneous Ca2+ release from the SR during the late repolarization phase. 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 with 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.