AENDO July 40/1

Previs, Stephen F., Gary W. Cline, and Gerald I. Shulman. A critical evaluation of mass isotopomer distribution analysis of gluconeogenesis in vivo. Am. J. Physiol. 277 (Endocrinol. Metab. 40): E154–E160, 1999.—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 [13C]glycerol infusion.1 We have evaluated substrate-induced effects on rate of appearance (Ra) of glycerol and glucose and f during [2-13C]glycerol infusion in vivo. Five groups of mice were fasted for 30 h and then infused with [2-13C]glycerol at variable rates and variable 13C enrichments (group I: 20 μmol ·kg21 ·min21, 99% 13C; group II: 60 μmol ·kg21 ·min21, 60% 13C; group III: 60 μmol ·kg21 ·min21, 99% 13C; group IV: 120 μmol · kg21 · min21, 40% 13C; or group V: 120 μmol ·kg21 ·min21, 99% 13C). The total glycerol Ra increased from ,104 to ,157 and to ,210 μmol ·kg21 ·min21 as the infusion of [2-13C]glycerol increased from 20 to 60 and to 120 μmol ·kg2 1 ·min21, respectively. As the amount of 99% enriched [2-13C]glycerol increased from 20 to 60 and to 120 μmol ·kg21 ·min21 (groups I, III, and V, respectively), plasma glycerol enrichment increased from ,21 to ,42 and to ,57% and the calculated f increased from ,27 to ,56 and to ,87%, respectively. Similar plasma glycerol enrichments were observed in groups I, II, and IV (i.e., ,21–24%), yet f increased from ,27 to ,57 and to ,86% in groups II and IV, respectively. Estimates of absolute gluconeogenesis increased from ,14 to ,33 and ,86 μmol ·kg21 ·min21 as the infusion of [2-13C]glycerol increased from 20 to 60 and 120 μmol·kg21 ·min21. Plausible estimates of f were obtained only under conditions that increased total glycerol Ra ,2-fold (P , 0.001) and increased glucose Ra ,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 [2-13C]glycerol yield erroneous results and 2) reasonable estimates of f are obtained at glycerol infusion rates that perturb glycerol and glucose metabolism.