We have investigated the mechanisms of the anabolic effect of insulin on muscle protein metabolism in healthy volunteers, using stable isotopic tracers of amino acids. Calculations of muscle protein synthesis, breakdown, and amino acid transport were based on data obtained with the leg arteriovenous catheterization and muscle biopsy. Insulin was infused (0.15 mU/min per 100 ml leg) into the femoral artery to increase femoral venous insulin concentration (from 10 +/- 2 to 77 +/- 9 microU/ml) with minimal systemic perturbations. Tissue concentrations of free essential amino acids decreased (P < 0.05) after insulin. The fractional synthesis rate of muscle protein (precursor-product approach) increased (P < 0.01) after insulin from 0.0401 +/- 0.0072 to 0.0677 +/- 0.0101%/h. Consistent with this observation, rates of utilization for protein synthesis of intracellular phenylalanine and lysine (arteriovenous balance approach) also increased from 40 +/- 8 to 59 +/- 8 (P < 0.05) and from 219 +/- 21 to 298 +/- 37 (P < 0.08) nmol/min per 100 ml leg, respectively. Release from protein breakdown of phenylalanine, leucine, and lysine was not significantly modified by insulin. Local hyperinsulinemia increased (P < 0.05) the rates of inward transport of leucine, lysine, and alanine, from 164 +/- 22 to 200 +/- 25, from 126 +/- 11 to 221 +/- 30, and from 403 +/- 64 to 595 +/- 106 nmol/min per 100 ml leg, respectively. Transport of phenylalanine did not change significantly. We conclude that insulin promoted muscle anabolism, primarily by stimulating protein synthesis independently of any effect on transmembrane transport.
G Biolo, R Y Declan Fleming, R R Wolfe