Time course of hepatic gluconeogenesis during hindlimb suspension unloading.

The goal of this work was to determine the time-dependent changes in fractional hepatic gluconeogenesis (GNG) during conditions of hindlimb suspension unloading (HSU), a 'ground-based' method for inducing muscular atrophy to simulate space flight. We hypothesized that GNG would increase in HSU conditions as a result of metabolic shifts in the liver and skeletal muscle. A significant and progressive atrophy was observed in the soleus (30, 47 and 55%) and gastrocnemius muscles (0, 15 and 17%) after 3, 7 and 14 days of HSU, respectively. Fractional hepatic GNG was determined following the incorporation of deuterium from deuterated water ((H2O)-H-2) into C-H bonds of newly synthesized glucose after an 8 h fast. Enrichment of plasma glucose with H-2 was measured using the classic method of Landau et al. (the 'hexamethylenetetramine (HMT) method'), based on specific H-2 labelling of glucose carbons, and the novel method of Chacko et al. ('average method'), based on the assumption of equal H-2 enrichment on all glucose carbons (except C2). After 3 days of HSU, fractional GNG was significantly elevated in the HSU group, as determined by either method (similar to 13%, P < 0.05). After 7 and 14 days of HSU, gluconeogenesis was not significantly different. Both analytical methods yielded similar time-dependent trends in gluconeogenic rates, but GNG values determined using the average method were consistently lower (similar to 30%) than those found by the HMT method. To compare and validate the average method against the HMT method further, we starved animals for 13 h to allow for hepatic GNG to contribute 100% to endogenous glucose production. The HMT method yielded 100% GNG, while the average method yielded GNG of similar to 70%. As both methods used the same values of precursor enrichment, we postulated that the underestimation of gluconeogenic rate was as a result of differences in the measurements of product enrichment (H-2 labelling of plasma glucose). This could be explained by the following factors: (i) loss of deuterium via exchange between acetate and glucose; (ii) interference caused by fragment m/z 169, representing multiple isobaric species; and (iii) interference from other sugars at m/z 169. In conclusion, HSU caused a time-dependent increase in hepatic gluconeogenesis, irrespective of the analytical methods used.