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 [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 ⋅ kg-1 ⋅ min-1, 99% 13C; group II: 60 μmol ⋅ kg-1 ⋅ min-1, 60% 13C; group III: 60 μmol ⋅ kg-1 ⋅ min-1, 99% 13C; group IV: 120 μmol ⋅ kg-1 ⋅ min-1, 40% 13C; or group V: 120 μmol ⋅ kg-1 ⋅ min-1, 99% 13C). The total glycerol Ra increased from ∼104 to ∼157 and to ∼210 μmol ⋅ kg-1 ⋅ min-1as the infusion of [2-13C]glycerol increased from 20 to 60 and to 120 μmol ⋅ kg- 1 ⋅ min-1, respectively. As the amount of 99% enriched [2-13C]glycerol increased from 20 to 60 and to 120 μmol ⋅ kg-1 ⋅ min-1( 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 ⋅ kg-1 ⋅ min-1as the infusion of [2-13C]glycerol increased from 20 to 60 and 120 μmol ⋅ kg-1 ⋅ min-1. 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.