Na⁺:Ca²⁺ Exchanger and EADs. Introduction: The ionic mechanisms for early afterdepolarizations (EADs) have not been fully clarified. It has been suggested that L‐type Ca²⁺ current (I_caL) is the primary current generating EADs that occur near the plateau level (E‐EADs) of the membrane potential (Vm) when I_caL is enhanced. The purpose of these studies was to determine accurately the range of Vm at which EADs occur in Purkinje fibers with K⁺ currents blocked by Cs⁺ and to investigate the importance of Na⁺:Ca²⁺ exchange current (I_Na:ca) as opposed to l_CaL and other currents in the generation of EADs occurring later during repolarization (L‐EADs).

Methods and Results: Shortened Purkinje strands from dogs and guinea pigs were superfused with physiologic solution containing Cs⁺ (3.6 mM) and a low [K⁺]_o (3.0 or 2.0 mM) to induce EADs. The Vm of origin of EADs and their evolution were measured with the aid of phase plane plots of the rate of repolarization against Vm. L‐EADs occurred over a wide range of Vm (−35 to −90 mV), generally more negative in guinea pig than in dog. Elevation of [Ca²⁺]_o, from 1.8 to 3.6 mM suppressed L‐EADs within a few cycles, and they returned with continued exposure. After repeated exposures to high [Ca^:2+]₀, L‐EADs migrated toward less negative Vm when |Ca²⁺]₀, was reestablished to 1.8 mM in the presence of Cs⁺. Reduction of [Na⁺]₀ from 147.5 to 112.5 mM by substitution with Li⁺ or sucrose also rapidly depressed L‐EADs.

Conclusions: The observation of Cs⁺‐induced L‐EADs over a wide range of Vm indicates that there is not a single inward gated current as a common ionic mechanism for L‐EADs but does not exclude an important role for I_Na:Ca, which can operate over a wide range of Vm. The rapid suppression of L‐EADs with elevated [Ca²⁺]_o, and reduced [Na⁺]_o, and the migration of EADs to more positive Vm after exposures to high |Ca²⁺]_o, are compatible with I_Nc:Ca as the major charge carrier for L‐EADs.