Substrate utilization for hepatic gluconeogenesis is altered by increased glucose demand in ruminants

Hepatocytes isolated from 10 Dorset wethers that were treated with excipien t or 1.0 g/d of phlorizin for 72 h were used to determine the effects of in creased glucose demand on utilization of [1-C-14]propionate and [1-C-14] al anine for oxidative metabolism and gluconeogenesis. Control and phlorizin-t reated wethers excreted 0 and 62.8 g/d of glucose into the urine, respectiv ely, Phlorizin treatment tended to increase conversion of propionate and al anine to CO2. A phlorizin x substrate interaction for conversion to glucose indicated that conversion of alanine to glucose was increased more by phlo rizin treatment than was conversion of propionate (285 vs 166% of controls) . Phlorizin treatment did not affect estimated K-s for conversion of substr ates to either CO2 or glucose; however, phlorizin increased estimated V-max for conversion of substrates to CO2 and tended to increase estimated V-max for conversion of substrates to glucose. Phlorizin treatment slightly incr eased the ratio of conversion of propionate to glucose compared with CO2 an d slightly decreased the ratio of conversion of alanine to glucose compared with CO2. In vitro addition of 2.5 mM NH4Cl decreased conversion of propio nate to CO2 and glucose but had little effect on conversion of alanine to C O2 and glucose. Estimated K-s and V-max for conversion of substrates to CO2 , K-s for conversion of substrates to glucose, and V-max for conversion of alanine to glucose were not affected by NH4Cl; however, V-max for conversio n of propionate to glucose was decreased by NH4Cl. These data indicate that although utilization of propionate for gluconeogenesis is extensive, amino acids have the potential to increase in importance as gluconeogenic substr ates when glucose demand is increased substantially. Furthermore, excess am monia decreases the capacity of hepatocytes to utilize propionate for oxida tion and gluconeogenesis.