In vivo NMR investigation of intramuscular glucose metabolism in conscious rats

Abstract

In vivo 13C 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. 13C NMR spectroscopy was used to observe [1-13C]glucose label precursor incorporation into intramuscular [1-13C]glycogen, [3-13C]lactate, and [3-13C]alanine during a hyperglycemic (approximately 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 (Vpc). The pyruvate dehydrogenase flux (Vpdh) was calculated after a steady-state flux correction for Vpc. Calculated values of Vpc and Vpdh were 24.8 +/- 4.3 and 110.0 +/- 18.7 nmol.g-1.min-1, respectively. In addition, [2-13C]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.