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 (I(NaCa)) 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 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 I(NaCa) and I(ns(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.