To determine whether activation by insulin of glycogen synthase (GS), phosphofructokinase (PFK), or pyruvate dehydrogenase (PDH) in skeletal muscle regulates intracellular glucose metabolism, subjects were studied basally and during euglycemic insulin infusions of 12, 30, and 240 mU/m2·min. Glucose disposal, oxidative and nonoxidative glucose metabolism were determined. GS, PFK, and PDH were assayed in skeletal muscle under each condition. Glucose disposal rates were 2.37 ± 0.11, 3.15 ± 0.19, 6.71 ± 0.44, and 11.7 ± 1.73 mg/kg·min; glucose oxidation rates were 1.96 ± 0.18, 2.81 ± 0.28, 4.43 ± 0.32, and 5.22 ± 0.52. Nonoxidative glucose metabolism was 0.39 ± 0.13, 0.34 ± 0.26, 2.28 ± 0.40, and 6.52 ± 1.21 mg/kg·min. Both the proportion of active GS and the proportion of active PDH were increased by hyperinsulinemia. PFK activity was unaffected. Activation of GS was correlated with nonoxidative glucose metabolism, while activation of PDH was correlated with glucose oxidation. Sensitivity to insulin of GS was similar to that of nonoxidative glucose metabolism, while the sensitivity to insulin of PDH was similar to that of glucose oxidation. Therefore, the activation of these enzymes in muscle may regulate nonoxidative and oxidative glucose metabolism.
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