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 gluconeoge nesis to total glucose production (f) during [C-13]glycerol infusion.(1) We have evaluated substrate-induced effects on rate of appearance (R-a) of gl ycerol and glucose and f during [2-C-13]glycerol infusion in vivo. Five gro ups of mice were fasted for 30 h and then infused with [2-C-13]glycerol at variable rates and variable C-13 enrichments (group I: 20 mu mol . kg(-1) . min(-1), 99% C-13; group II: 60 mu mol . kg(-1) . min(-1), 60% C-13; group III: 60 mu mol . kg(-1) . min(-1), 99% C-13; group IV: 120 mu mol . kg(-1) . min(-1), 40% C-13; or group V: 120 mu mol . kg(-1) . min(-1), 99% C-13). The total glycerol R-a increased from similar to 104 to similar to 157 and to similar to 210 mu mol . kg(-1) . min(-1) as the infusion of [2-C-13]gly cerol increased from 20 to 60 and to 120 mu mol . kg(-1) . min(-1), respect ively. As the amount of 99% enriched [2-C-13]glycerol increased from 20 to 60 and to 120 mu mol kg(-1) . min(-1) (groups I, III, and V, respectively), plasma glycerol enrichment increased from similar to 21 to similar to 42 a nd to similar to 57% and the calculated f increased from similar to 27 to s imilar to 56 and to similar to 87%, respectively. Similar plasma glycerol e nrichments were observed in groups I, II, and IV (i.e., similar to 21-24%), yet f increased from similar to 27 to similar to 57 and to similar to 86% in groups II and IV, respectively. Estimates of absolute gluconeogenesis in creased from similar to 14 to similar to 33 and similar to 86 mu mol . kg(- 1) . min(-1) as the infusion of [2-C-13]glycerol increased from 20 to 60 an d 120 mu mol . kg(-1) . min(-1). Plausible estimates of f were obtained onl y under conditions that increased total glycerol R-a similar to 2-fold (P < 0.001) and increased glucose R-a similar to 1.5-fold (P < 0.01) above basa l. We conclude that in 30-h fasted mice, 1) estimates of f by MIDA with low infusion rates of [2-C-13]glycerol yield erroneous results and 2) reasonab le estimates of f are obtained at glycerol infusion rates that perturb glyc erol and glucose metabolism.