In vivo NMR investigation of intramuscular glucose metabolism in conscious rats

In vivo C-13 nuclear magnetic resonance (NMR) spectroscopy was used to determine quantitatively the flux of muscle glycolysis, glycogen synthesis, pyruvate dehydrogenase, and pyruvate carboxylation in the hindlimb of conscious rats. C-13 NMR spectroscopy was used to observe [1-C-13]glucose label precursor incorporation into intramuscular [1-C-13]glycogen, [3-C-13]lactate, and [3-C-13]alanine during a hyperglycemic (similar to 11 mM)-hyperinsulinemic (10 mu.kg(-1).min(-1)) clamp. The glycogen synthesis rate was calculated to be 224 +/- 23 nmol.g(-1).min(-1). The kinetic data obtained from the label turnover in the intramuscular C-3 lactate and C-3 alanine metabolite pools, as well as in plasma C-3 lactate and C-3 alanine, were combined with a steady-state rate analysis to determine the glycolytic flux (67.4 +/- 10.1 nmol.g(-1).min(-1)). Steady-state isotopomer analysis of glutamate and pyruvate in skeletal muscle tissue extracts was used to determine the anaplerotic contribution of substrate via pyruvate carboxylation (V-pc). The pyruvate dehydrogenase flux (V-pdh) was calculated after a steady-state flux correction for V-pc. Calculated values of V-pc and V-pdh were 24.8 +/- 4.3 and 110.0 +/- 18.7 nmol.g(-1).min(-1), respectively. In addition, [2-C-13]acetate was used in a separate study to determine that pyruvate carboxylation was the major pathway for anaplerosis in skeletal muscle under conditions of hyperglycemia-hyperinsulinemia.