Insulin fails to enhance mTOR phosphorylation, mitochondrial protein synthesis and ATP production in human skeletal muscle without amino acid replacement

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-(13)C]-leucine as a tracer we measured the fractional synthetic rate of mitochondrial as well as sarcoplasmic and mixed muscle proteins in eighteen participants during sustained (7-hour) 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 amino acids decreased during insulin infusion. Mitochondrial, sarcoplasmic and mixed muscle PS rates were also lower during insulin (2-7 hours) 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, p70S(6)K, and 4EBP(1). In conclusion, sustained physiological hyperinsulinemia without AA replacement did not stimulate PS of mixed muscle or protein sub-fractions, 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