EFFECTS OF RUMINALLY DEGRADABLE NITROGEN INTAKE AND IN-VITRO ADDITIONOF AMMONIA AND PROPIONATE ON THE METABOLIC-FATE OF L-[1-C-14]ALANINE AND L-[N-15]ALANINE IN ISOLATED SHEEP HEPATOCYTES

Isolated hepatocytes prepared from sheep fed a basal diet (bromegrass hay-corn, 50:50 wt/wt, as-fed basis) with or without urea were used to determine the effects of added ammonia (as NH4Cl) and propionate on t he partitioning of C from 1.25 mM L-[1-C-14]alanine between oxidation and gluconeogenesis, and the flux of N-15 from 1.25 mM L-[N-15]alanine to [(NN)-N-14-N-15]urea and [(NN)-N-15-N-15]urea. Hepatocyte suspensi ons were incubated with NH4Cl (0, .31, .63, and 1.25 mM) and(or) propi onate (0, .31, .63, and 1.25 mM) in the presence of either 1.25 mM L-[ N-15]alanine or 1.25 mM L-alanine plus 18.5 kBq of L-[1-C-14]alanine. Feeding dietary urea did not affect [1-C-14]alanine oxidation to (CO2) -C-14 (P = .601), its conversion to [C-14]glucose (P = .576) by isolat ed hepatocytes. Increasing in vitro concentrations of NH4Cl and propio nate between 0 and 1.25 mM reduced [1-C-14]alanine oxidation to (CO2)- C-14 (P < .001). Increasing NH4Cl concentration between 0 and 1.25 mM reduced [1-C-14]alanine conversion to [C-14]glucose in isolated hepato cytes (P < .001), whereas addition of propionate between 0 and 1.25 mM stimulated production of [C-14]glucose from [1-C-14]alanine (P < .001 ). Feeding urea did not affect in vitro rates of total urea production (P = .655) but increased the production of [(NN)-N-14-N-15] and [(NN) -N-15-N-15]urea (P < .05). Addition of NH4Cl increased total urea, [(N N)-N-14-N-15]urea, and [(NN)-N-15-N-15]urea production (P < .001) but reduced N-15 isotopic enrichments of [(NN)-N-14-N-15] urea and [(NN)-N -15-N-15]urea (P < .001). Increasing propionate concentration between 0 and 1.25 mM reduced total urea production (P < .001), but [(NN)-N-14 -N-15]urea and [(NN)-N-15-N-15]urea production was reduced only at 1.2 5 mM propionate(P < .001). We conclude that NH3 detoxification by isol ated sheep hepatocytes increases amino acid deamination and this might have implications for nitrogen retention in ruminants consuming diets that promote considerable NH3 absorption from the digestive tract.