RESUMO
RATIONALE: The cardiac sodium channel NaV1.5, encoded by SCN5A, produces the rapidly inactivating depolarizing current INa that is responsible for the initiation and propagation of the cardiac action potential. Acquired and inherited dysfunction of NaV1.5 results in either decreased peak INa or increased residual late INa (INa,L), leading to tachy/bradyarrhythmias and sudden cardiac death. Previous studies have shown that increased cellular NAD+ and NAD+/NADH ratio increase INa through suppression of mitochondrial reactive oxygen species and PKC-mediated NaV1.5 phosphorylation. In addition, NAD+-dependent deacetylation of NaV1.5 at K1479 by Sirtuin 1 increases NaV1.5 membrane trafficking and INa. The role of NAD+ precursors in modulating INa remains unknown. OBJECTIVE: To determine whether and by which mechanisms the NAD+ precursors nicotinamide riboside (NR) and nicotinamide (NAM) affect peak INa and INa,Lin 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 NaV1.5, rat neonatal cardiomyocytes (RNCMs), and mice. NR increased INa in HEK293 cells expressing NaV1.5 (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 INa,L at the higher concentration (RNCMs, 5 mM: -45 ± 11%, p = .04). NR (5 mM) decreased NaV1.5 K1479 acetylation but increased INa in HEK293 cells expressing a mutant form of NaV1.5 with disruption of the acetylation site (NaV1.5-K1479A). Disruption of the PKC phosphorylation site abolished the effect of NR on INa. Furthermore, NAM (5 mM) had no effect on INa in RNCMs or in HEK293 cells expressing wild-type NaV1.5, but increased INa in HEK293 cells expressing NaV1.5-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 NaV1.5 via multiple mechanisms. NR increases INa, decreases INa,L, and warrants further investigation as a potential therapy for arrhythmic disorders caused by NaV1.5 deficiency and/or dysfunction.