Modulation of the Cardiac Sodium Channel Na V 1.5 Peak and Late Currents by NAD ⁺ Precursors

Rationale The cardiac sodium channel Na V 1.5, encoded by SCN5A , produces the rapidly inactivating depolarizing current I Na that is responsible for the initiation and propagation of the cardiac action potential. Acquired and inherited dysfunction of Na V 1.5 results in either decreased peak I Na or increased residual late I Na (I Na,L ), leading to tachy/bradyarrhythmias and sudden cardiac death. Previous studies have shown that increased cellular NAD + and NAD + /NADH ratio increase I Na through suppression of mitochondrial reactive oxygen species and PKC-mediated Na V 1.5 phosphorylation. In addition, NAD + -dependent deacetylation of Na V 1.5 at K1479 by Sirtuin 1 increases Na V 1.5 membrane trafficking and I Na . The role of NAD + precursors in modulating I Na remains unknown. Objective To determine whether and by which mechanisms the NAD + precursors nicotinamide riboside (NR) and nicotinamide (NAM) affect peak I Na and I Na,L in 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 Na V 1.5, rat neonatal cardiomyocytes (RNCMs), and mice. NR increased I Na in HEK293 cells expressing Na V 1.5 (500 μM: 51 ± 18%, p=0.02, 5 mM: 59 ± 22%, p=0.03) and RNCMs (500 µM: 60 ± 26%, p=0.02, 5 mM: 75 ± 39%, p=0.03) while reducing I Na,L at the higher concentration (RNCMs, 5 mM: −45 ± 11%, p=0.04). NR (5 mM) decreased Na V 1.5 K1479 acetylation but increased I Na in HEK293 cells expressing a mutant form of Na V 1.5 with disruption of the acetylation site (Na V 1.5-K1479A). Disruption of the PKC phosphorylation site abolished the effect of NR on I Na . Furthermore, NAM (5 mM) had no effect on I Na in RNCMs or in HEK293 cells expressing wild-type Na V 1.5, but increased I Na in HEK293 cells expressing Na V 1.5-K1479A. Dietary supplementation with NR for 10-12 weeks decreased QTc in C57BL/6J mice (0.35% NR: −4.9 ± 2.0%, p=0.26; 1.0% NR: −9.5 ± 2.8%, p=0.01). Conclusions NAD + precursors differentially regulate Na V 1.5 via multiple mechanisms. NR increases I Na , decreases I Na,L , and warrants further investigation as a potential therapy for arrhythmic disorders caused by Na V 1.5 deficiency and/or dysfunction.