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 s eparate hyperinsulinemic infusion protocols were performed: 1) protocol I w as a euglycemic (similar to 6 mM)hyperinsulinemic (10 mU.kg(-1).min(-1)) cl amp, 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 (simila r 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 thr ee protocols for a 3-h period, during which time [1-C-13]glucose label inco rporation into [1-C-13]glycogen, [3-C-13]lactate, and [3-C-13]alanine was d etected in the hindlimb of awake rats via C-13-NMR. Combined steady-state a nd kinetic data were used to calculate rates of glycogen synthesis and glyc olysis. Additionally, glucose 6-phosphate (G-6-P) was measured in the hindl imb muscles with the use of in vivo P-31-NMR during the three infusion prot ocols. 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, respectiv ely. 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 meta bolic control analysis was used to calculate the distributed control among glucose transport/phosphorylation [GLUT4/hexokinase (HK)], glycogen synthes is, and glycolysis from the metabolic flux and G-6-P data. The calculated v alues for the control coefficients (C) of these three metabolic steps (C-GL UT-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 dispos al and that glucose transport phosphorylation is responsible for the majori ty of control of insulin-stimulated glucose disposal in skeletal muscle.