Long-QT syndrome-related sodium channel mutations probed by the dynamic action potential clamp technique

Long-QT3 syndrome (LQT3) is linked to cardiac sodium channel gene ( SCN5A ) mutations. In this study, we used the ‘dynamic action potential clamp’ (dAPC) technique to effectively replace the native sodium current ( I Na ) of the Priebe–Beuckelmann human ventricular cell model with wild-type (WT) or mutant I Na generated in a human embryonic kidney (HEK)-293 cell that is voltage clamped by the free-running action potential of the ventricular cell. We recorded I Na from HEK cells expressing either WT or LQT3-associated Y1795C or A1330P SCN5A at 35°C, and let this current generate and shape the action potential (AP) of subepicardial, mid-myocardial and subendocardial model cells. The HEK cell's endogenous background current was completely removed by a real-time digital subtraction procedure. With WT I Na , AP duration (APD) was longer than with the original Priebe–Beuckelmann model I Na , due to a late I Na component of ∼30 pA that could not be revealed with conventional voltage-clamp protocols. With mutant I Na , this late component was larger (∼100 pA), producing a marked increase in APD (∼70–80 ms at 1 Hz for the subepicardial model cell). The late I Na magnitude showed reverse frequency dependence, resulting in a significantly steeper APD–frequency relation in the mutant case. AP prolongation was more pronounced for the mid-myocardial cell type, resulting in increased APD dispersion for each of the mutants. For both mutants, a 2 s pause following rapid (2 Hz) pacing resulted in distorted AP morphology and beat-to-beat fluctuations of I Na . Our dAPC data directly demonstrate the arrhythmogenic nature of LQT3-associated SCN5A mutations.