The effects of increased plasma
free fatty acids (FFA) on
insulin-dependent whole body
glucose disposal, skeletal muscle glycolysis,
glycogen synthesis,
pyruvate versus FFA/
ketone oxidation, and
glucose 6-phosphate (Glu-6-P) were investigated in the awake rat. A control group (
glycerol-infused) and high plasma FFA group (
Liposyn-infused) were clamped at euglycemia (approximately 6 mM)-
hyperinsulinemia (10 milliunits/kg/min) throughout the experiment (180-240 min). In the initial experiment, 13C NMR was used to observe [1-13C]
glucose incorporation into [1-13C]
glycogen in the rat hindlimb for
glycogen synthesis calculations and into [3-13C]
lactate and [3-13C]
alanine for glycolytic flux calculations. These experiments were followed by 31P 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 the high plasma FFA and control groups respectively (p < 0.0005).
Glucose infusion rates (Ginf) decreased significantly in the
Liposyn-infused 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 entering the tricarboxylic acid cycle was greater in the control (57 +/- 9%) versus high plasma FFA group (25 +/- 4%) (p < 0.005) as assessed by [4-13C]
glutamate/[3-13C]
lactate steady state isotopic enrichment measurements. 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 conclusion, we have demonstrated the ability to use in vivo NMR to elucidate the metabolic fate of
glucose within skeletal muscle of an awake rat during a euglycemic-hyperinsulinemic clamp and increased levels of plasma FFA. These data suggest that increased concentrations of plasma FFA inhibit
insulin-stimulated muscle
glucose metabolism in the rat through inhibition of glycolysis.