Calmodulin kinase II regulates atrial myocyte late sodium current, calcium handling, and atrial arrhythmia

Background: Atrial fibrillation (AF) is the most common type of arrhythmia. Abnormal atrial myocyte Ca²⁺ handling promotes aberrant membrane excitability and remodeling that are important for atrial arrhythmogenesis. The sequence of molecular events leading to loss of normal atrial myocyte Ca²⁺ homeostasis is not established. Late Na⁺ current (I_Na,L) is increased in atrial myocytes from AF patients together with an increase in activity of Ca²⁺/calmodulin-dependent kinase II (CaMKII).

Objective: The purpose of this study was to determine whether CaMKII-dependent phosphorylation at Ser571 on Na_V1.5 increases atrial I_Na,L, leading to aberrant atrial Ca²⁺ cycling, altered electrophysiology, and increased AF risk.

Methods: Atrial myocyte electrophysiology, Ca²⁺ handling, and arrhythmia susceptibility were studied in wild-type and Scn5a knock-in mice expressing phosphomimetic (S571E) or phosphoresistant (S571A) Na_V1.5 at Ser571.

Results: Atrial myocytes from S571E but not S571A mice displayed an increase in I_Na,L and action potential duration, and with adrenergic stress have increased delayed afterdepolarizations. Frequency of Ca²⁺ sparks and waves was increased in S571E atrial myocytes compared to wild type. S571E mice showed an increase in atrial events induced by adrenergic stress and AF inducibility in vivo. Isolated S571E atria were more susceptible to spontaneous atrial events, which were abrogated by inhibiting sarcoplasmic reticulum Ca²⁺ release, CaMKII, or the Na⁺/Ca²⁺ exchanger. Expression of phospho-Na_V1.5 at Ser571 and autophosphorylated CaMKII were increased in atrial samples from human AF patients.

Conclusion: This study identified CaMKII-dependent regulation of Na_V1.5 as an important upstream event in Ca²⁺ handling defects and abnormal impulse generation in the setting of AF.