Circulating glucose levels are maintained during fasting by endogenous production of glucose through: 1) the breakdown of glycogen and 2) from gluconeogenesis. Recently published work from our lab has utilized stable isotope tracing with cutting edge LC-MS methods to determine metabolic fluxes in vivo. During these studies we observed a surprising lack of labeling in glycolytic intermediates in the majority of tissues when infusing 13C-glucose. Based on these results we hypothesized that glycogen metabolism and gluconeogenesis were active in more tissues than was previously thought. To gain a holistic understanding of glycolysis, glycogenolysis, and gluconeogenesis in vivo, we performed a series of intravenous infusions to determine the flux through these pathways. To measure the contribution of glycogen breakdown to glycolytic intermediate we performed pulse chase infusions of glucose (16h pulse of glucose infusion followed by an 8-hour chase with no infusion) and measured labeling in key glycolytic intermediates. To determine contribution from gluconeogenic, we infused stable isotope labeled lactate, glycerol, alanine, and glutamine. Finally, by utilizing a linear algebra approach to account for substrate interconversion, we were able to determine the direct contribution of glucose, glycogen breakdown, and different gluconeogenic substrates to glycolytic intermediate in 11 tissues.
T. TeSlaa: None.
National Institutes of Health
