Nonequilibrium reactivation of Na⁺ current drives early afterdepolarizations in mouse ventricle

Background: Early afterdepolarizations (EADs) are triggers of cardiac arrhythmia driven by L-type Ca²⁺ current (I_CaL) reactivation or sarcoplasmic reticulum Ca²⁺ release and Na⁺/Ca²⁺ exchange. In large mammals the positive action potential plateau promotes I_CaL reactivation, and the current paradigm holds that cardiac EAD dynamics are dominated by interaction between I_CaL and the repolarizing K⁺ currents. However, EADs are also frequent in the rapidly repolarizing mouse action potential, which should not readily permit I_CaL reactivation. This suggests that murine EADs exhibit unique dynamics, which are key for interpreting arrhythmia mechanisms in this ubiquitous model organism. We investigated these dynamics in myocytes from arrhythmia-susceptible calcium calmodulin-dependent protein kinase II delta C (CaMKIIδC)-overexpressing mice (Tg), and via computational simulations. Methods and Results: In Tg myocytes, β-adrenergic challenge slowed late repolarization, potentiated sarcoplasmic reticulum Ca²⁺ release, and initiated EADs below the I_CaL activation range (-47±0.7 mV). These EADs were abolished by caffeine and tetrodotoxin (but not ranolazine), suggesting that sarcoplasmic reticulum Ca²⁺ release and Na⁺ current (I_Na), but not late I_Na, are required for EAD initiation. Simulations suggest that potentiated sarcoplasmic reticulum Ca²⁺ release and Na⁺/Ca²⁺ exchange shape late action potential repolarization to favor nonequilibrium reactivation of I_Na and thereby drive the EAD upstroke. Action potential clamp experiments suggest that lidocaine eliminates virtually all inward current elicited by EADs, and that this effect occurs at concentrations (40-60 μmol/L) for which lidocaine remains specific for inactivated Na⁺ channels. This strongly suggests that previously inactive channels are recruited during the EAD upstroke, and that nonequilibrium I_Na dynamics underlie murine EADs. Conclusions: Nonequilibrium reactivation of I_Na drives murine EADs.