C-13 AND P-31 NMR-STUDIES ON THE EFFECTS OF INCREASED PLASMA-FREE FATTY-ACIDS ON INTRAMUSCULAR GLUCOSE-METABOLISM IN THE AWAKE RAT

The effects of increased plasma free fatty acids (FFA) on insulin-depe ndent whole body glucose disposal, skeletal muscle glycolysis, glycoge n synthesis, pyruvate versus FFA/ketone oxidation, and glucose 6-phosp hate (Glu-6-P) were investigated in the awake rat, A control group (gl ycerol-infused) and high plasma FFA group (Liposyn-infused) were clamp ed at euglycemia (similar to 6 mM). hyperinsulinemia (10 milliunits/kg /min) throughout the experiment (180-240 min), In the initial experime nt, C-13 NMR was used to observe [1-C-13]glucose incorporation into [1 -C-13]glycogen in the rat hindlimb for glycogen synthesis calculations and into [3-C-13]lactate and [3-C-13]alanine for glycolytic flux calc ulations, These experiments were followed by P-31 NMR measurements of Glu-6-P changes under identical conditions of the initial experiment, Plasma FFA concentrations were 2.25 +/- 0.36 and 0.20 +/- 0.03 mM in t he high plasma FFA and control groups respectively (p < 0.0005), Gluco se infusion rates (G(inf)) decreased significantly in the Liposyn-infu sed rats (29.5 +/- 0.7 and 27.2 +/- 1.2 mg/kg/min for control and high plasma FFA group, respectively, at 15 min to 30.7 +/- 2.3 and 17.7 +/ - 1.3 mg/kg/min, respectively, at the end of the experiment, p < 0.002 ), Glycogen synthesis rates were 163 +/- 32 and 104 +/- 17 nmol/g/min, and glycolytic rates were 57.9 +/- 8.0 and 19.5 +/- 3.6 nmol/g/min (p < 0.002) in the control and high plasma FFA groups, respectively, The relative flux of pyruvate versus free fatty acids and ketones enterin g the tricarboxylic acid cycle was greater in the control (57 +/- 9%) versus high plasma FFA group (25 +/- 4%) (p < 0.005) as assessed by [4 -C-13]glutamate/[3-C-13]lactate steady state isotopic enrichment measu rements, Finally, Glu-6-P concentrations increased by 29.8 +/- 7.0 and 52.8 +/- 12.3% (p < 0.05) in the control and high plasma FFA groups, respectively, above their basal concentrations by 180 min. In conclusi on, we have demonstrated the ability to use in vivo MMR to elucidate t he metabolic fate of glucose within skeletal muscle of an awake rat du ring a euglycemic-hyperinsulinemic clamp and increased levels of plasm a FFA. These data suggest that increased concentrations of plasma FFA inhibit insulin-stimulated muscle glucose metabolism in the rat throug h inhibition of glycolysis.