Effects of ruminally degradable nitrogen intake and in vitro addition of ammonia and propionate on the metabolic fate of L-[1-14C]alanine and L-[15N]alanine in isolated sheep hepatocytes.

Abstract

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 the partitioning of C from 1.25 mM L-[1-14C]alanine between oxidation and gluconeogenesis, and the flux of 15N from 1.25 mM L-[15N]alanine to [14N15N]urea and [15N15N]urea. Hepatocyte suspensions were incubated with NH4Cl (0, .31, .63, and 1.25 mM) and (or) propionate (0, .31, .63, and 1.25 mM) in the presence of either 1.25 mM L-[15N]alanine or 1.25 mM L-alanine plus 18.5 kBq of L-[1-14C]alanine. Feeding dietary urea did not affect [1-14C]alanine oxidation to 14CO2 (P = .601), or its conversion to [14C]glucose (P = .576) by isolated hepatocytes. Increasing in vitro concentrations of NH4Cl and propionate between 0 and 1.25 mM reduced [1-14C]alanine oxidation to 14CO2 (P < .001). Increasing NH4Cl concentration between 0 and 1.25 mM reduced [1-14C]alanine conversion to [14C]glucose in isolated hepatocytes (P < .001), whereas addition of propionate between 0 and 1.25 mM stimulated production of [14C]glucose from [1-14C]alanine (P < .001). Feeding urea did not affect in vitro rates of total urea production (P = .655) but increased the production of [14N15N]urea and [15N15N]urea (P < .05). Addition of NH4Cl increased total urea, [14N15N]urea, and [15N15N]urea production (P < .001), but reduced 15N isotopic enrichments of [14N15N]urea and [15N15N]urea (P < .001). Increasing propionate concentration between 0 and 1.25 mM reduced total urea production (P < .001), but [14N15N]urea and [15N15N]urea production was reduced only at 1.25 mM propionate (P < .001). We conclude that NH3 detoxification by isolated 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.