Metabolic control analysis of insulin-stimulated glucose disposal in rat skeletal muscle

Metabolic control analysis was used to calculate the distributed control of insulin-stimulated skeletal muscle glucose disposal in awake rats. Three separate hyperinsulinemic infusion protocols were performed: 1) protocol I was a euglycemic (similar to 6 mM)hyperinsulinemic (10 mU.kg(-1).min(-1)) clamp, 2) protocol II was a hyperglycemic (similar to 11 mM)-hyperinsulinemic (10 mU.kg(-1).min(-1)) clamp, and 3) protocol III was a euglycemic (similar to 6 mM)-hyperinsulinemic (10 mU.kg(-1).min(-1))-lipid/heparin (increased plasma free fatty acid) clamp. [1-C-13]glucose was administered in all three protocols for a 3-h period, during which time [1-C-13]glucose label incorporation into [1-C-13]glycogen, [3-C-13]lactate, and [3-C-13]alanine was detected in the hindlimb of awake rats via C-13-NMR. Combined steady-state and kinetic data were used to calculate rates of glycogen synthesis and glycolysis. Additionally, glucose 6-phosphate (G-6-P) was measured in the hindlimb muscles with the use of in vivo P-31-NMR during the three infusion protocols. The clamped glucose infusion rates were 31.6 +/- 2.9, 49.7 +/- 1.0, and 24.0 +/- 1.5 mg.kg(-1).min(-1) at 120 min in protocols I-III, respectively. Rates of glycolysis were 62.1 +/- 10.3, 71.6 +/- 11.8, and 19.5 +/- 3.6 nmol.g(-1).min(-1) and rates of glycogen synthesis were 125 +/- 15, 224 +/- 23, and 104 +/- 17 nmol.g(-1).min(-1) in protocols I-III, respectively. Insulin-stimulated G-6-P concentrations were 217 +/- 8, 265 +/- 12, and 251 +/- 9 nmol/g in protocols I-III, respectively. A top-down approach to metabolic control analysis was used to calculate the distributed control among glucose transport/phosphorylation [GLUT4/hexokinase (HK)], glycogen synthesis, and glycolysis from the metabolic flux and G-6-P data. The calculated values for the control coefficients (C) of these three metabolic steps (C-GLUT-4/HK(J) = 0.55 +/- 0.10, C-glycogen(J) = 0.30 +/- 0.06, and C-glycolysis(J) = 0.15 +/- 0.02; where J is glucose disposal flux, and glycogen syn is glycogen synthesis) indicate that there is shared control of glucose disposal and that glucose transport phosphorylation is responsible for the majority of control of insulin-stimulated glucose disposal in skeletal muscle.