The Molecular Mechanism of Cardiac Sodium Channel Current Reduction with Metabolic Stress

Abstract

Background Human cardiomyopathy is associated with decreased cardiac Na + channel current (I Na ), and Na + channel changes have been implicated in the increased risk of sudden death in heart failure. We have reported that in cardiomyopathy, elevated intracellular NADH downregulates I Na acutely to a magnitude seen in Brugada syndrome through protein kinase C (PKC) activation without changing the channel membrane expression. The decrease in I Na can be ameliorated by NAD + and protein kinase A (PKA) activators. Here, we investigate the molecular mechanism by which I Na is changed during metabolic stress. Methods HEK293 cells stably expressing human cardiac channel (Na v 1.5) were used to measure single channel currents at –40, –60, and –80 mV using the cell-attached voltage-clamp configuration. Results The single channel currents and conductance of Na v 1.5 were significantly decreased by NADH (100 µM) and phorbol 12-myristate 13-acetate (PMA, 30 nM) ( P + (500 µM) or forskolin (1 µM) restored the single channel currents and conductance to the untreated level ( P >.05 vs untreated group, P Conclusions Because we have already shown no reduction in cell surface channel number with metabolic stress, the decrease of macroscopic current of cardiac Na v 1.5 induced by NADH and PKC results from a decrease of single channel conductance, whereas NAD + and PKA elevate the single channel conductance. These results are consistent with the time course of the current changes seen with NADH/NAD + .