Rationale:: Pyridine nucleotides regulate the cardiac Na ⁺ current ( I _Na ) through generation of reactive oxygen species (ROS). Objective:: We investigated the source of ROS induced by elevated NADH. Methods and Results:: In human embryonic kidney (HEK) cells stably expressing the cardiac Na ⁺ channel, the decrease of I _Na (52±9%; P <0.01) induced by cytosolic NADH application (100 μmol/L) was reversed by mitoTEMPO, rotenone, malonate, DIDS (4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid), PK11195, and 4′-chlorodiazepam, a specific scavenger of mitochondrial superoxide and inhibitors of the mitochondrial complex I, complex II, voltage-dependent anion channels, and benzodiazepine receptor, respectively. Anti–mycin A (20 μmol/L), a complex III inhibitor known to generate ROS, decreased I _Na (51±4%, P <0.01). This effect was blocked by NAD ⁺ , forskolin, or rotenone. Inhibitors of complex IV, nitric oxide synthase, the NAD(P)H oxidases, xanthine oxidases, the mitochondrial permeability transition pore, and the mitochondrial ATP-sensitive K ⁺ channel did not change the NADH effect on I _Na . Analogous results were observed in cardiomyocytes. Rotenone, mitoTEMPO, and 4′-chlorodiazepam also blocked the mutant A280V GPD1-L (glycerol-3-phosphate dehydrogenase 1-like) effect on reducing I _Na , indicating a role for mitochondria in the Brugada syndrome caused by this mutation. Fluorescent microscopy confirmed mitochondrial ROS generation with elevated NADH and ROS inhibition by NAD ⁺ . Conclusions:: Altering the oxidized to reduced NAD(H) balance can activate mitochondrial ROS production, leading to reduced I _Na . This signaling cascade may help explain the link between altered metabolism, conduction block, and arrhythmic risk.

Hypomagnesemia is commonly observed in heart failure, diabetes mellitus, hypertension, and cardiovascular diseases. Low serum magnesium (Mg) is a predictor for cardiovascular and all-cause mortality and treating Mg deficiency may help prevent cardiovascular disease. In this review, we discuss the possible mechanisms by which Mg deficiency plays detrimental roles in cardiovascular diseases and review the results of clinical trials of Mg supplementation for heart failure, arrhythmias and other cardiovascular diseases.

Rationale : Mutations in glycerol-3-phosphate dehydrogenase 1-like (GPD1-L) protein reduce cardiac Na ⁺ current ( I _Na ) and cause Brugada Syndrome (BrS). GPD1-L has >80% amino acid homology with glycerol-3-phosphate dehydrogenase, which is involved in NAD-dependent energy metabolism. Objective : Therefore, we tested whether NAD(H) could regulate human cardiac sodium channels (Na _v 1.5). Methods and Results : HEK293 cells stably expressing Na _v 1.5 and rat neonatal cardiomyocytes were used. The influence of NADH/NAD ⁺ on arrhythmic risk was evaluated in wild-type or SCN5A ^+/− mouse heart. A280V GPD1-L caused a 2.48±0.17-fold increase in intracellular NADH level ( P <0.001). NADH application or cotransfection with A280V GPD1-L resulted in decreased I _Na (0.48±0.09 or 0.19±0.04 of control group, respectively; P <0.01), which was reversed by NAD ⁺ , chelerythrine, or superoxide dismutase. NAD ⁺ antagonism of the Na ⁺ channel downregulation by A280V GPD1-L or NADH was prevented by a protein kinase (PK)A inhibitor, PKAI _6–22 . The effects of NADH and NAD ⁺ were mimicked by a phorbol ester and forskolin, respectively. Increasing intracellular NADH was associated with an increased risk of ventricular tachycardia in wild-type mouse hearts. Extracellular application of NAD ⁺ to SCN5A ^+/− mouse hearts ameliorated the risk of ventricular tachycardia. Conclusions : Our results show that Na _v 1.5 is regulated by pyridine nucleotides, suggesting a link between metabolism and I _Na . This effect required protein kinase C activation and was mediated by oxidative stress. NAD ⁺ could prevent this effect by activating PKA. Mutations of GPD1-L may downregulate Na _v 1.5 by altering the oxidized to reduced NAD(H) balance.

In heart failure and type 2 diabetes mellitus (DM), the majority of patients have hypomagnesemia, and magnesium (Mg) supplementation has improved cardiac function and insulin resistance. Recently, we have shown that DM can cause cardiac diastolic dysfunction (DD). Therefore, we hypothesized that Mg supplementation would improve diastolic function in DM. High-fat diet–induced diabetic mouse hearts showed increased cardiac DD and hypertrophy. Mice with DM showed a significantly increased E/e’ ratio (the ratio of transmitral Doppler early filling velocity [E] to tissue Doppler early diastolic mitral annular velocity [e’]) in the echocardiogram, left ventricular end diastolic volume (LVEDV), incidence of DD, left ventricular posterior wall thickness in diastole (PWTd), and ratio of heart weight to tibia length (HW/TL) when compared with controls. DM mice also had hypomagnesemia. Ventricular cardiomyocytes isolated from DM mice exhibited decreased mitochondrial ATP production, a 1.7- ± 0.2-fold increase of mitochondrial ROS, depolarization of the mitochondrial membrane potential, and mitochondrial Ca ²⁺ overload. Dietary Mg administration (50 mg/ml in the drinking water) for 6 weeks increased plasma Mg concentration and improved cardiac function. At the cellular level, Mg improved mitochondrial function with increased ATP, decreased mitochondrial ROS and Ca ²⁺ overload, and repolarized mitochondrial membrane potential. In conclusion, Mg supplementation improved mitochondrial function, reduced oxidative stress, and prevented DD in DM.