Effect of hyperinsulinemia on whole body and skeletal muscle leucine carbon kinetics in humans.

Data documenting the isolated effect of systemic hyperinsulinemia on whole body and skeletal muscle leucine carbon kinetics in humans are limited. Using steady-state [14C]leucine kinetics, 10 normal volunteers were studied in the baseline postabsorptive state and then under euglycemic, hyperinsulinemic (71 +/- 5 microU/ml), and euleucinemic conditions. Systemic hyperinsulinemia resulted in a significant decrease in whole body and forearm leucine rate of appearance (Ra) by 17 and 37%, respectively, (P less than 0.0003, 0.03), without a significant change in the nonoxidized rate of disappearance for either (P = 0.23, 0.66). The baseline contribution of total body skeletal muscle (TBSM) leucine Ra and rate of disappearance (Rd) to whole body leucine Ra and Rd was 27 +/- 6 and 24 +/- 5%, respectively. During hyperinsulinemia TBSM Ra decreased by 34%, whereas whole body Ra decreased by 16%. We conclude that the primary effect of insulin in the whole body and skeletal muscle is to decrease leucine release from protein without a significant effect on leucine incorporation into protein. This antiproteolytic effect of insulin is more pronounced in skeletal muscle than in other tissues in the body.