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Systemic insulin administration causes hypoaminoacidemia by inhibiting protein degradation, which may in turn inhibit muscle protein synthesis (PS). Insulin enhances muscle mitochondrial PS and ATP production when hypoaminoacidemia is prevented by exogenous amino acid (AA) replacement. We determined whether insulin would stimulate mitochondrial PS and ATP production in the absence of AA replacement. Using l-[1,2-13C]leucine as a tracer, we measured the fractional synthetic rate of mitochondrial as well as sarcoplasmic and mixed muscle proteins in 18 participants during sustained (7-h) insulin or saline infusion ( n = 9 each). We also measured muscle ATP production, mitochondrial enzyme activities, mRNA levels of mitochondrial genes, and phosphorylation of signaling proteins regulating protein synthesis. The concentration of circulating essential AA decreased during insulin infusion. Mitochondrial, sarcoplasmic, and mixed muscle PS rates were also lower during insulin (2–7 h) than during saline infusions despite increased mRNA levels of selected mitochondrial genes. Under these conditions, insulin did not alter mitochondrial enzyme activities and ATP production. These effects were associated with enhanced phosphorylation of Akt but not of protein synthesis activators mTOR, p70S6K, and 4EBP1. In conclusion, sustained physiological hyperinsulinemia without AA replacement did not stimulate PS of mixed muscle or protein subfractions and did not alter muscle mitochondrial ATP production in healthy humans. These results support that insulin and AA act in conjunction to stimulate muscle mitochondrial function and mitochondrial protein synthesis.

High-fat diets are reported to increase oxidative stress in a variety of tissues, whereas antioxidant supplementation prevents many diseases attributed to high-fat diet. Rodent skeletal muscle mitochondrial DNA has been shown to be a potential site of oxidative damage. We hypothesized that the effects of a high-fat diet on skeletal muscle DNA functions would be attenuated or partially reversed by antioxidant supplementation. Gene expression profiling and measurement of mitochondrial ATP production capacity were performed in skeletal muscle from male rats after feeding one of three diets (control, high-fat diet with or without antioxidants) for 36 wk. The high-fat diet altered transcript levels of 18 genes of 800 surveyed compared with the control-fed rats. Alterations included reduced expression of genes involved in free-radical scavenging and tissue development and increased expression of stress response and signal transduction genes. The magnitude of these alterations due to high-fat diet was reduced by antioxidant supplementation. Real-time PCR measurements confirmed the changes in transcript levels of cytochrome c oxidase subunit III and superoxide dismutase-1 and -2 noted by microarray approach. Mitochondrial ATP production was unaltered by dietary changes or antioxidant supplemention. It is concluded that the high-fat diet increases the transcription of genes involved in stress response but reduces those of free-radical scavenger enzymes, resulting in reduced DNA repair/metabolism (increased DNA damage). Antioxidants partially prevent these changes. Mitochondrial functions in skeletal muscle remain unaltered by the dietary intervention due to many adaptive changes in gene transcription.

Rodent skeletal muscle mitochondrial DNA has been shown to be a potential site of oxidative damage during aging. Caloric restriction (CR) is reported to reduce oxidative stress and prolong life expectancy in rodents. Gene expression profiling and measurement of mitochondrial ATP production capacity were performed in skeletal muscle of male rats after feeding them either a control diet or calorie-restricted diet (60% of control diet) for 36 wk to determine the potential mechanism of the beneficial effects of CR. CR enhanced the transcripts of genes involved in reactive oxygen free radical scavenging function, tissue development, and energy metabolism while decreasing expression of those genes involved in signal transduction, stress response, and structural and contractile proteins. Real-time PCR measurments confirmed the changes in transcript levels of cytochrome- c oxidase III, superoxide dismutase (SOD)1, and SOD2 that were noted by the microarray approach. Mitochondrial ATP production and citrate synthase were unaltered by the dietary changes. We conclude that CR alters transcript levels of several genes in skeletal muscle and that mitochondrial function in skeletal muscle remains unaltered by the dietary intervention. Alterations in transcripts of many genes involved in reactive oxygen scavenging function may contribute to the increase in longevity reported with CR.