Isotope labeling ratios: a tool for the exploration of metabolic compartments

Glucose and glutamine provide similar proportions of energy to mucosal cells of rat small intestine

The objectives of this study were to establish a reliable method for quantifying glycolytic flux in intestinal epithelial cells, to determine the proportion of energy provided to small intestine epithelial cells by glucose vs. glutamine, and to determine whether there was an energetic advantage to having both substrates present simultaneously. There was substantial retention of 3H in alanine and lactate when [2-(3)H]glucose was used as tracer for quantifying glycolysis, and the magnitude of the 3H retention was influenced by the presence of other substrates and metabolites. Detritiation was at least 99% complete, however, when [3-(3)H]glucose was used as tracer in this system and the tritium was recovered as 3H2O. Glycolytic flux was six- to sevenfold higher in cells of the proximal than distal small intestine but was not significantly different for young adult (4 mo) vs. aged adult (24 mo) rats. Net ATP production from exogenous substrates was higher when both glucose and glutamine were present simultaneously than when either substrate was present alone, and glucose was calculated to provide 50-60% of the net ATP produced from these two substrates. Most of the energy produced from glucose was produced via the anaerobic metabolic pathways (78% for glucose alone, 95% with glucose and glutamine). Net energy production was calculated to be 10% lower in cells from aged animals than in those from young animals, since CO2 production from these major substrates was lower in cells from aged animals.