A quantitative analysis of the metabolic pathways of hepatic glucose synthesis in vivo with 13C-labeled substrates.

A quantitative analysis of the major metabolic pathways of hepatic glucose synthesis in fasted rats was conducted. [2-13C]Acetate was administered intraintestinally into awake fasted rats. 13C NMR and GC-MS analysis were used to quantitate the isotopic enrichments of glutamate, glutamine, lactate, alanine and the newly synthesized liver glucose. By measuring the ratio of carbon atoms in glutamate molecules derived from acetyl-CoA to carbon atoms in the glucose molecule derived from oxaloacetate and gluconeogenic substrates, such as lactate and alanine, the relative activities of the Krebs cycle and gluconeogenesis were quantified. Our results indicate that the percentage of glucose carbons originating by 'metabolic exchange' with the oxaloacetate pool, via the Krebs cycle, is less than 7%.