Impact of aging and exercise on skeletal muscle mitochondrial capacity, energy metabolism, and physical function.

The relationship between the age-associated decline in mitochondrial function and its effect on skeletal muscle physiology and function remain unclear. In the current study, we examined to what extent physical activity contributes to the decline in mitochondrial function and muscle health during aging and compared mitochondrial function in young and older adults, with similar habitual physical activity levels. We also studied exercise-trained older adults and physically impaired older adults. Aging was associated with a decline in mitochondrial capacity, exercise capacity and efficiency, gait stability, muscle function, and insulin sensitivity, even when maintaining an adequate daily physical activity level. Our data also suggest that a further increase in physical activity level, achieved through regular exercise training, can largely negate the effects of aging. Finally, mitochondrial capacity correlated with exercise efficiency and insulin sensitivity. Together, our data support a link between mitochondrial function and age-associated deterioration of skeletal muscle.

NAD+-Precursor Supplementation With L-Tryptophan, Nicotinic Acid, and Nicotinamide Does Not Affect Mitochondrial Function or Skeletal Muscle Function in Physically Compromised Older Adults.

BACKGROUND: Boosting NAD+ via supplementation with niacin equivalents has been proposed as a potential modality capable of promoting healthy aging and negating age-dependent declines of skeletal muscle mass and function. OBJECTIVES: We investigated the efficacy of NAD+-precursor supplementation (tryptophan, nicotinic acid, and nicotinamide) on skeletal muscle mitochondrial function in physically compromised older adults. METHODS: A randomized, double-blind, controlled trial was conducted in 14 (female/male: 4/10) community-dwelling, older adults with impaired physical function [age, 72.9 ± 4.0 years; BMI, 25.2 ± 2.3 kg/m2]. Participants were supplemented with 207.5 mg niacin equivalents/day [intervention (INT)] and a control product (CON) that did not contain niacin equivalents, each for 32 days. The primary outcomes tested were mitochondrial oxidative capacity and exercise efficiency, analyzed by means of paired Student's t-tests. Secondary outcomes, such as NAD+ concentrations, were analyzed accordingly. RESULTS: Following supplementation, skeletal muscle NAD+ concentrations [7.5 ± 1.9 compared with 7.9 ± 1.6 AU, respectively] in INT compared with CON conditions were not significantly different compared to the control condition, whereas skeletal muscle methyl-nicotinamide levels were significantly higher under NAD+-precursor supplementation [INT, 0.098 ± 0.063 compared with CON, 0.025 ± 0.014; P = 0.001], suggesting an increased NAD+ metabolism. Conversely, neither ADP-stimulated [INT, 82.1 ± 19.0 compared with CON, 84.0 ± 19.2; P = 0.716] nor maximally uncoupled mitochondrial respiration [INT, 103.4 ± 30.7 compared with CON, 108.7 ± 33.4; P = 0.495] improved under NAD+-precursor supplementation, nor did net exercise efficiency during the submaximal cycling test [INT, 20.2 ± 2.77 compared with CON, 20.8 ± 2.88; P = 0.342]. CONCLUSIONS: Our findings are consistent with previous findings on NAD+ efficacy in humans, and we show in community-dwelling, older adults with impaired physical function that NAD+-precursor supplementation through L-tryptophan, nicotinic acid, and nicotinamide does not improve mitochondrial or skeletal muscle function. This study was registered at clinicaltrials.gov as NCT03310034.

Exercise training increases mitochondrial content and ex vivo mitochondrial function similarly in patients with type 2 diabetes and in control individuals.

AIMS/HYPOTHESIS: We previously showed that type 2 diabetic patients are characterised by compromised intrinsic mitochondrial function. Here, we examined if exercise training could increase intrinsic mitochondrial function in diabetic patients compared with control individuals. METHODS: Fifteen male type 2 diabetic patients and 14 male control individuals matched for age, BMI and VO(2max) enrolled in a 12 week exercise intervention programme. Ex vivo mitochondrial function was assessed by high-resolution respirometry in permeabilised muscle fibres from vastus lateralis muscle. Before and after training, insulin-stimulated glucose disposal was examined during a hyperinsulinaemic-euglycaemic clamp. RESULTS: Diabetic patients had intrinsically lower ADP-stimulated state 3 respiration and lower carbonyl cyanide 4-(trifluoro-methoxy)phenylhydrazone (FCCP)-induced maximal oxidative respiration, both on glutamate and on glutamate and succinate, and in the presence of palmitoyl-carnitine (p < 0.05). After training, diabetic patients and control individuals showed increased state 3 respiration on the previously mentioned substrates (p < 0.05); however, an increase in FCCP-induced maximal oxidative respiration was observed only in diabetic patients (p < 0.05). The increase in mitochondrial respiration was accompanied by a 30% increase in mitochondrial content upon training (p < 0.01). After adjustment for mitochondrial density, state 3 and FCCP-induced maximal oxidative respiration were similar between groups after training. Improvements in mitochondrial respiration were paralleled by improvements in insulin-stimulated glucose disposal in diabetic patients, with a tendency for this in control individuals. CONCLUSIONS/INTERPRETATION: We confirmed lower intrinsic mitochondrial function in diabetic patients compared with control individuals. Diabetic patients increased their mitochondrial content to the same extent as control individuals and had similar intrinsic mitochondrial function, which occurred parallel with improved insulin sensitivity.

Skeletal muscle uncoupling protein-3 restores upon intervention in the prediabetic and diabetic state: implications for diabetes pathogenesis?

AIM: Skeletal muscle uncoupling protein-3 (UCP3) is reduced in type 2 diabetes, and in the pre-diabetic condition of impaired glucose tolerance (IGT). Here we examined whether intervention programs known to improve insulin sensitivity are paralleled by an increase in skeletal muscle UCP3 protein levels. METHODS: Skeletal muscle UCP3 protein content was measured before and after one year of an exercise intervention in muscle biopsies of eight diabetic subjects. In addition, UCP3 was measured in IGT subjects before and after 1 year of following a lifestyle-intervention program or serving as control. RESULTS: In the diabetic patients a significant increase of approximately 75% in UCP3 protein was found after 1 year of exercise training (P < 0.05). In IGT subjects UCP3 protein increased in the intervention group (P = 0.02), while UCP3 remained unaltered in the control group (P = 0.64). CONCLUSION: Both, exercise training and a lifestyle-intervention program increase UCP3 protein content in skeletal muscle of subjects with reduced glycaemic control, indicating a restoration towards normal UCP3 levels. These data support the idea that UCP3 has a role in the aetiology of type 2 diabetes mellitus.

Short-term training is accompanied by a down regulation of ACC2 mRNA in skeletal muscle.

Recently, we showed that short-term training induced a rapid increase in IMCL whilst insulin sensitivity tended to improve. Here we investigate molecular adaptations accompanying this physiological training-induced accumulation of IMCL. Nine untrained men (age: 23.3 +/- 3.2 y; maximal power output: 3.8 +/- 0.6 W/kg body weight) trained for two weeks. Before and after training, subjects cycled for three hours and biopsies were taken before and after exercise. mRNA concentrations of ACC2, HSL, LPL, Glut4 and HKII were quantified by RT-PCR and association of Glut4 with the membrane was quantified by immunohistochemical method. Endurance training resulted in a decrease of 29.1 % in ACC2 mRNA (p = 0.02). After training, ACC2 mRNA tended to decrease with acute exercise (- 24.4 % [p = 0.06]). HSL mRNA decreased with acute exercise after training (- 37.3 % [p = 0.002]). LPL mRNA concentrations increased with acute exercise before training (+ 42.4 % [p = 0.05]) and HKII mRNA increased with acute exercise before (+ 72.5 % [p = 0.025]) and after training (+ 99.3 % [p = 0.05]). After acute exercise, more Glut4 was associated with the membrane than before exercise, but it was not affected by training. We conclude that the training-induced increase in IMCL was accompanied by molecular adaptations in muscle to improve fat oxidative capacity, while markers of glucose metabolism were not yet changed. The present data are in line with the hypothesis that the fat oxidative capacity might be more important than the IMCL content in determining insulin sensitivity.

Effect of 2 weeks of endurance training on uncoupling protein 3 content in untrained human subjects.

AIM: The mitochondrial uncoupling protein-3 (UCP3) is able to lower the proton gradient across the inner mitochondrial membrane, thereby uncoupling substrate oxidation from ATP production and dissipating energy as heat. What the effect of endurance training on UCP3 is, is still controversial. Endurance-trained athletes are characterized by lower levels of UCP3, but longitudinal studies in rodents reported no effect of endurance training on muscular UCP3 levels. Here, we examined the effect of a 2-week training programme on skeletal muscle UCP3 protein content in untrained human subjects, and hypothesized that UCP3 will be reduced after the training programme. METHODS: Nine untrained men [age: 23.3 +/- 3.2 years; BMI: 22.6 +/- 2.6 kg m(-2); maximal power output (W(max)): 3.8 +/- 0.6 W kg(-1) body weight] trained for 2 weeks. Before and at least 72 h after the training period, muscle biopsies were taken for determination of UCP3 protein content. RESULTS: UCP3 protein content tended to be lower after the training programme [95 +/- 10 vs. 109 +/- 12 arbitrary units (AU), P = 0.08]. Cytochrome c content tended to increase with 33% in response to endurance training (52 +/- 6 vs. 39 +/- 6 AU, P = 0.08). The ratio UCP3 relative to cytochrome c tended to decrease significantly upon endurance training (2.0 +/- 0.4 vs. 3.2 +/- 0.6 AU, P = 0.01). CONCLUSION: A short-term (2-week) endurance training programme decreased UCP3 protein levels and significantly reduced the ratio of UCP3 to cytochrome c.