BACKGROUND Multifocal ectopic Purkinje-related premature contractions (MEPPCs) are associated with SCN5A variants. However, it is not well understood why Purkinje fibers, but not ventricular myocardium, play a predominant role in arrhythmogenesis. OBJECTIVES This study sought to explore the underlying mechanisms of MEPPC. METHODS Whole-cell patch-clamp and molecular biology techniques were used in the present study. RESULTS Clinical data from one patient with R814W variant showed MEPPC syndrome, which is well responsive to amiodarone. Compared with canine ventricular myocytes, Purkinje cells (PCs) had significantly larger sodium current (INa), leftward shift of INa activation and inactivation curves, suggesting higher sodium channel excitability in PCs. Real-time polymerase chain reaction and Western blot analysis showed that the mRNA and protein expression of NaV(31 and NaV(33 was higher in canine Purkinje fibers than in ventricular myocardium. INa in heterologous Chinese hamster ovary cell expression system co-expressing NaV1.5 and NaV(31/NaV(33 exhibited similar biophysical properties of INa in PCs. R814W variant shifted INa activation in a hyperdepolarized direction, caused a larger window current, and generated an outward gating pore current at depolarized voltages. Coexpression of NaV(31/NaV(33 with Nav1.5-R814W further left-shifted INa activation and caused an even larger window current and gating pore current, suggesting higher susceptibility of Purkinje fibers to R814W variant. Amiodarone inhibited INa, shifted its inactivation to more negative voltages, and significantly decreased the window current. CONCLUSIONS A higher expression of (31 and (33 subunits contributes to higher sodium channel excitability in cardiac Purkinje fibers, making them more susceptible to MEPPC. (J Am Coll Cardiol EP 2023;9:2477-2490) (c) 2023 by the American College of Cardiology Foundation.
