Metabolic pathways leading to liver glycogen repletion in vivo, studied by GC-MS and NMR.

A quantitative analysis of the pathways leading to glycogen repletion in rats was conducted. [U-13C]Glucose was administered intra-intestinally into awake fasted animals. The distribution of glucose isotopomers derived from liver glycogen, liver extracts and plasma was performed by GC-MS and 13C NMR. The potential gluconeogenic precursors for liver glycogen, lactate, alanine, glutamate and glutamine, were also analyzed. The amount of glycogen that is synthesized by the direct pathway was found to be 35%. The 13C enrichment of liver lactate, alanine and glucose is similar, indicating that they are the major precursors for liver glycogen synthesis via the indirect pathway. Our results demonstrate that after 24 h fasting, when glucose is supplied, gluconeogenesis from endogenous sources is not shut off.