Nicotinamide-rich diet improves physical endurance by up-regulating SUR2A in the heart

Abstract

SUR2A is an ATP-binding protein that serves as a regulatory subunit of cardioprotective ATP-sensitive K(+) (K(ATP) ) channels. Based on signalling pathway regulating SUR2A expression and SUR2A role in regulating numbers of fully assembled K(ATP) channels, we have suggested that nicotinamide-rich diet could improve physical endurance by stimulating SUR2A expression. We have found that mice on nicotinamide-rich diet significantly improved physical endurance, which was associated with significant increase in expression of SUR2A. Transgenic mice with solely overexpressed SUR2A on control diet had increased physical endurance in a similar manner as the wild-type mice on nicotinamide-rich diet. The experiments focused on action membrane potential and intracellular Ca(2+) concentration have demonstrated that increased SUR2A expression was associated with the activation of sarcolemmal K(ATP) channels and steady Ca(2+) levels in cardiomyocytes in response to β-adrenergic stimulation. In contrast, the same challenge in the wild-type was characterized by a lack of the channel activation and rise in intracellular Ca(2+) . Nicotinamide-rich diet was ineffective to increase physical endurance in mice lacking K(ATP) channels. This study has shown that nicotinamide-rich diet improves physical endurance by increasing expression of SUR2A and that this is a sole mechanism of the nicotinamide-rich diet effect. The obtained results suggest that oral nicotinamide is a regulator of SUR2A expression and has a potential as a drug that can improve physical endurance in conditions where this effect would be desirable.

Fig 1

Expression of K_ATP channel subunits in hearts of mice on control and nicotinamide-rich diet. Representative progress curves for the real-time PCR amplification of SUR2A (A and C) and GAPDH (B and D) cDNA from mice on control or nicotinamide-rich diet (as labelled in the figure) and a corresponding bar graphs (E and F) as well as bar graphs depicting cycle threshold values for Kir6.2 and Kir6.1. Each bar represents mean ± S.E. of the mean (n = 4–6). *P < 0.05.

Fig 2

Acute electrophysiological effect of nicotinamide on isolated cardiomyocytes and expression of different SUR types in cardiac and some extra-cardiac tissues. (A) Typical whole cell currents at 80 mV in a cardiomyocyte in the absence (control) and presence (nicotinamide) of nicotinamide (1 mM) and a current density graph corresponding to conditions (A1). Each bar represents mean ± S.E. of the mean (n = 4). (B) Bar graphs showing real time RT-PCR cycle threshold values for depicted tissues and genes. Each bar represents mean ± S.E. of the mean (n = 3–6).

Fig 3

Physical endurance in mice on control and nicotinamide-rich diet. Bar graphs showing energy expenditure (A) and time spent (B) on treadmill of mice on control and nicotinamide-rich diet. Each bar represents mean ± S.E. of the mean (n = 6). *P < 0.05.

Fig 4

Physical endurance in wild-type (WT) and SUR2A mice on control diet. Bar graphs showing energy expenditure (A) and time spent (B) on treadmill of WT and SUR2A mice. Each bar represents mean ± S.E. of the mean (n = 6). *P < 0.05.

Fig 5

Action membrane potential duration in wild-type and SUR2A cardiomyocytes. (A) Duration of action membrane potential in cardiomyocytes from wild-type (WT) and SUR2A (SUR2A) at depicted time-points. Original action membrane potentials from wild-type (B) and SUR2A cardiomyocyte (C) in the absence (control) and presence of isoprenaline (500 nM) and isoprenaline (500 nM)/glybenclamide (10 μM). (D–E) Graphs corresponding to (B) and (C). Each bar represents mean ± S.E. of the mean (n = 6–10). *P < 0.05.

Fig 6

Intracellular Ca²⁺ in cardiac cells in response to long lasting stimulation with isporenaline. Left panels: Laser confocal images of Fluo 3-loaded cardiac cells from wild-type (WT) and SUR2A (SUR2A) mice in the absence and presence of isoprenaline (500 nM). White bar corresponds to 40 μm. Graph: Time course of Fluo-3 fluorescence in cardiac cells from wild-type (WT) and SUR2A (SUR2A) mice in the presence of isoprenaline (500 nM). Each point represents mean ± S.E.M. (n = 8–12).

Fig 7

Physical endurance in Kir6.2 knockout (Kir6.2 KO) mice on control and nicotinamide-rich diet. Bar graphs showing energy expenditure (A) and time spent (B) on treadmill of Kir6.2 KO mice on control and nicotinamide-rich diet. Each bar represents mean ± S.E. of the mean (n = 6–7).