Early Afterdepolarizations: Mechanism of Induction and Block

Early afterdepolarizations (EADs) are a type of triggered activity found in heart muscle. We used voltage-damped sheep cardiac Purkinje fibers to examine the mechanism underlying EADs induced near action potential plateau voltages with the Ca current agonist Bay K 8644 and the effect of several interventions known to suppress or enhance these EADs. Bay K 8644 produced an inward shift of the steady-state current-voltage relation near plateau voltages. Tetrodotoxin, lidocaine, verapamil, nitrendipine, and raising [K]o abolish EADs and shift the steady-state current-voltage relations outwardly. Using a two-pulse voltage-damp protocol, an inward current transient was present at voltages where EADs were induced. The voltage-dependence of availability of the inward current transient and of EAD induction were similar. The timedependence of recovery from inactivation of the inward current transient and of EAD amplitude were nearly identical. Without recovery of the inward current transient, EADs could not be elicited. The inward current transient was enhanced with Bay K 8644 and blocked by nitrendipine, but was not abolished by tetrodotoxin or replacement of [Nab with an impermeant cation. These results support a hypothesis that the induction of EADs near action potential plateau voltages requires 1) a conditioning phase controlled by the sum of membrane currents present near the action potential plateau and characterized by lengthening and flattening of the plateau within a voltage range where, 2) recovery from inactivation and reactivation of L-type Ca channels to carry the depolarizing charge can occur. Our results suggest an essential role for the L-type Ca "window" current and provide a framework for understanding the role of several membrane currents in the induction and block of EADs. (Circulation Research 1989;64:977-990)