A critical evaluation of mass isotopomer distribution analysis of gluconeogenesis in vivo

There are conflicting reports concerning the reliability of mass isotopomer distribution analysis (MIDA) for estimating the contribution of gluconeogenesis to total glucose production (f) during [(13)C]glycerol infusion. We have evaluated substrate-induced effects on rate of appearance (R(a)) of glycerol and glucose and f during [2-(13)C]glycerol infusion in vivo. Five groups of mice were fasted for 30 h and then infused with [2-(13)C]glycerol at variable rates and variable (13)C enrichments (group I: 20 micromol. kg(-1). min(-1), 99% (13)C; group II: 60 micromol. kg(-1). min(-1), 60% (13)C; group III: 60 micromol. kg(-1). min(-1), 99% (13)C; group IV: 120 micromol. kg(-1). min(-1), 40% (13)C; or group V: 120 micromol. kg(-1). min(-1), 99% (13)C). The total glycerol R(a) increased from approximately 104 to approximately 157 and to approximately 210 micromol. kg(-1). min(-1) as the infusion of [2-(13)C]glycerol increased from 20 to 60 and to 120 micromol. kg(- 1). min(-1), respectively. As the amount of 99% enriched [2-(13)C]glycerol increased from 20 to 60 and to 120 micromol. kg(-1). min(-1) (groups I, III, and V, respectively), plasma glycerol enrichment increased from approximately 21 to approximately 42 and to approximately 57% and the calculated f increased from approximately 27 to approximately 56 and to approximately 87%, respectively. Similar plasma glycerol enrichments were observed in groups I, II, and IV (i. e., approximately 21-24%), yet f increased from approximately 27 to approximately 57 and to approximately 86% in groups II and IV, respectively. Estimates of absolute gluconeogenesis increased from approximately 14 to approximately 33 and approximately 86 micromol. kg(-1). min(-1) as the infusion of [2-(13)C]glycerol increased from 20 to 60 and 120 micromol. kg(-1). min(-1). Plausible estimates of f were obtained only under conditions that increased total glycerol R(a) approximately 2-fold (P < 0.001) and increased glucose R(a) approximately 1.5-fold (P < 0.01) above basal. We conclude that in 30-h fasted mice, 1) estimates of f by MIDA with low infusion rates of [2-(13)C]glycerol yield erroneous results and 2) reasonable estimates of f are obtained at glycerol infusion rates that perturb glycerol and glucose metabolism.