Modulation of the cardiac sodium channel Na1.5 peak and late currents by NAD precursors.

Rationale: The cardiac sodium channel Na1.5, encoded by SCN5A, produces the rapidly inactivating depolarizing current I that is responsible for the initiation and propagation of the cardiac action potential. Acquired and inherited dysfunction of Na1.5 results in either decreased peak I or increased residual late I (I), leading to tachy/bradyarrhythmias and sudden cardiac death. Previous studies have shown that increased cellular NAD and NAD/NADH ratio increase I through suppression of mitochondrial reactive oxygen species and PKC-mediated Na1.5 phosphorylation. In addition, NAD-dependent deacetylation of Na1.5 at K1479 by Sirtuin 1 increases Na1.5 membrane trafficking and I. The role of NAD precursors in modulating I remains unknown.

Objective: To determine whether and by which mechanisms the NAD precursors nicotinamide riboside (NR) and nicotinamide (NAM) affect peak I and Iin vitro and cardiac electrophysiology in vivo.

Methods And Results: The effects of NAD precursors on the NAD metabolome and electrophysiology were studied using HEK293 cells expressing wild-type and mutant Na1.5, rat neonatal cardiomyocytes (RNCMs), and mice. NR increased I in HEK293 cells expressing Na1.5 (500 μM: 51 ± 18%, p = .02, 5 mM: 59 ± 22%, p = .03) and RNCMs (500 μM: 60 ± 26%, p = .02, 5 mM: 74 ± 39%, p = .03) while reducing I at the higher concentration (RNCMs, 5 mM: -45 ± 11%, p = .04). NR (5 mM) decreased Na1.5 K1479 acetylation but increased I in HEK293 cells expressing a mutant form of Na1.5 with disruption of the acetylation site (Na1.5-K1479A). Disruption of the PKC phosphorylation site abolished the effect of NR on I. Furthermore, NAM (5 mM) had no effect on I in RNCMs or in HEK293 cells expressing wild-type Na1.5, but increased I in HEK293 cells expressing Na1.5-K1479A. Dietary supplementation with NR for 10-12 weeks decreased QTc in C57BL/6 J mice (0.35% NR: -4.9 ± 2.0%, p = .14; 1.0% NR: -9.5 ± 2.8%, p = .01).

Conclusions: NAD precursors differentially regulate Na1.5 via multiple mechanisms. NR increases I, decreases I, and warrants further investigation as a potential therapy for arrhythmic disorders caused by Na1.5 deficiency and/or dysfunction.