Higher Sodium Channel Excitability in Cardiac Purkinje Fibers: Implications for Multifocal Ectopic Purkinje-Related Premature Contractions

Abstract

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 (I_Na), leftward shift of I_Na 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 Na_Vβ1 and Na_Vβ3 was higher in canine Purkinje fibers than in ventricular myocardium. I_Na in heterologous Chinese hamster ovary cell expression system co-expressing Na_V1.5 and Na_Vβ1/Na_Vβ3 exhibited similar biophysical properties of I_Na in PCs. R814W variant shifted I_Na activation in a hyperdepolarized direction, caused a larger window current, and generated an outward-gating pore current at depolarized voltages. Coexpression of Na_Vβ1/Na_Vβ3 with Nav1.5-R814W further left-shifted I_Na activation and caused an even larger window current and gating pore current, suggesting higher susceptibility of Purkinje fibers to R814W variant. Amiodarone inhibited I_Na, shifted its inactivation to more negative voltages, and significantly decreased the window current.

Conclusions: A higher expression of β1 and β3 subunits contributes to higher sodium channel excitability in cardiac Purkinje fibers, making them more susceptible to MEPPC.

Keywords: SCN5A mutation; cardiac Purkinje fibers; multifocal ectopic Purkinje-related premature contractions; sodium current; ventricular arrhythmias.