K V 4.3 Expression Modulates Na V 1.5 Sodium Current.

In cardiomyocytes, the voltage-gated transient outward potassium current (I _to ) is responsible for the phase-1 repolarization of the action potential (AP). Gain-of-function mutations in KCND3 , the gene encoding the I _to carrying K _V 4.3 channel, have been associated with Brugada syndrome (BrS). While the role of I _to in the pro-arrhythmic mechanism of BrS has been debated, recent studies have suggested that an increased I _to may directly affect cardiac conduction. However, the effects of an increased I _to on AP upstroke velocity or sodium current at the cellular level remain unknown. We here investigated the consequences of K _V 4.3 overexpression on Na _V 1.5 current and consequent sodium channel availability. We found that overexpression of K _V 4.3 protein in HEK293 cells stably expressing Na _V 1.5 (HEK293-Na _V 1.5 cells) significantly reduced Na _V 1.5 current density without affecting its kinetic properties. In addition, K _V 4.3 overexpression decreased AP upstroke velocity in HEK293-Na _V 1.5 cells, as measured with the alternating voltage/current clamp technique. These effects of K _V 4.3 could not be explained by alterations in total Na _V 1.5 protein expression. Using computer simulations employing a multicellular in silico model, we furthermore demonstrate that the experimentally observed increase in K _V 4.3 current and concurrent decrease in Na _V 1.5 current may result in a loss of conduction, underlining the potential functional relevance of our findings. This study gives the first proof of concept that K _V 4.3 directly impacts on Na _V 1.5 current. Future studies employing appropriate disease models should explore the potential electrophysiological implications in (patho)physiological conditions, including BrS associated with KCND3 gain-of-function mutations.