Role of NADH/NAD⁺ transport activity and glycogen store on skeletal muscle energy metabolism during exercise: in silico studies

Abstract

Skeletal muscle can maintain ATP concentration constant during the transition from rest to exercise, whereas metabolic reaction rates may increase substantially. Among the key regulatory factors of skeletal muscle energy metabolism during exercise, the dynamics of cytosolic and mitochondrial NADH and NAD+ have not been characterized. To quantify these regulatory factors, we have developed a physiologically based computational model of skeletal muscle energy metabolism. This model integrates transport and reaction fluxes in distinct capillary, cytosolic, and mitochondrial domains and investigates the roles of mitochondrial NADH/NAD+ transport (shuttling) activity and muscle glycogen concentration (stores) during moderate intensity exercise (60% maximal O2 consumption). The underlying hypothesis is that the cytosolic redox state (NADH/NAD+) is much more sensitive to a metabolic disturbance in contracting skeletal muscle than the mitochondrial redox state. This hypothesis was tested by simulating the dynamic metabolic responses of skeletal muscle to exercise while altering the transport rate of reducing equivalents (NADH and NAD+) between cytosol and mitochondria and muscle glycogen stores. Simulations with optimal parameter estimates showed good agreement with the available experimental data from muscle biopsies in human subjects. Compared with these simulations, a 20% increase (or approximately 20% decrease) in mitochondrial NADH/NAD+ shuttling activity led to an approximately 70% decrease (or approximately 3-fold increase) in cytosolic redox state and an approximately 35% decrease (or approximately 25% increase) in muscle lactate level. Doubling (or halving) muscle glycogen concentration resulted in an approximately 50% increase (or approximately 35% decrease) in cytosolic redox state and an approximately 30% increase (or approximately 25% decrease) in muscle lactate concentration. In both cases, changes in mitochondrial redox state were minimal. In conclusion, the model simulations of exercise response are consistent with the hypothesis that mitochondrial NADH/NAD+ shuttling activity and muscle glycogen stores affect primarily the cytosolic redox state. Furthermore, muscle lactate production is regulated primarily by the cytosolic redox state.

NAD+/NADH and skeletal muscle mitochondrial adaptations to exercise

“…Overall, the free cytosolic and nuclear NAD ϩ (H) compartments are traditionally thought to be in equilibrium, with NAD ϩ (H) being able to freely pass through pore complexes in the nuclear membrane (46, 98 -103, 187, 190). In Cos7 cells, the free nuclear NAD ϩ concentration is estimated to be ϳ10 -100 M (53, 188), which is comparable to the estimations for the cytosol (ϳ150 M) of muscle (42,119). Thus, in response to exercise, it would be expected that the pyridine redox state in the nucleus reflects changes that occur in the cytosol.…”

“…mmol/kg dry wt muscle, respectively (62,80,93,154,155,159,160). Based on the approximate volumes of distributions of mitochondria, the extramitochondrial space (i.e., cytosol) and their mass fractions [i.e., %cell volume: cytosol ϭ 90% and mitochondria ϭ 10% (50)], Cabrera and colleagues (42,119) estimate the total, mitochondrial, and cytosolic compartment concentrations in skeletal muscle for NAD ϩ and NADH, respectively, to be approximately as follows: total, 0.45 and 0.05 mmol/kg cell wet wt; cytosol, 0.15 and 0.00028 mmol/kg cytosolic wet wt; mitochondria, 3.15 and 0.5 mmol/kg mitochondrial wet wt [Note: to convert to dry weight muscle, multiply by ϳ4. 2 (145)].…”

“…In skeletal muscle, NAD ϩ levels are highest in mitochondria (42,119); thus, by extension one might infer that oxidative skeletal muscle (with a greater abundance of mitochondria) would have overall higher NAD ϩ levels compared with glycolytic muscle. Supporting this notion, in human resting muscle, NAD ϩ concentration is positively correlated with the percentage of slow-twitch fibers (62).…”

“…Considering that the cytosolic/nuclear content of NAD ϩ in muscle is estimated to be ϳ540-fold higher than NADH (42,119), conversion of NAD ϩ to NADH, or vice versa, would therefore result in a greater change in NADH levels. By extension, and as reasoned by others (53,188,189), changes in nuclear NADH, rather than NAD ϩ , could link perturbations in NAD ϩ /NADH ratio to gene transcription.…”

Hyperpolarized 13C NMR observation of lactate kinetics in skeletal muscle

“…The observations confirm that an activated PDH can compete immediately and effectively with the near-equilibrium LDH and ALT enzymes for the precursor pyruvate. mitochondrial wet wt (Li et al, 2009;White and Schenk, 2012). The NAD + /NADH ratio in the cytosol (∼540) stands at a higher oxidization level than the NAD + /NADH ratio in the mitochondria (∼6.3).…”