Interactions between ruminal degradable nitrogen intake and in vitro addition of substrates on patterns of amino acid metabolism in isolated ovine hepatocytes.

The effects of ammonia (as NH4Cl) and propionate on the partitioning of amino acids between oxidation and gluconeogenesis were determined in isolated hepatocytes obtained from sheep fed a basal diet (50:50 bromegrass hay:corn; asfed basis) with or without urea. Hepatocyte suspensions were incubated with NH4Cl (0, 1.25, 2.5 and 5.0 mmol/L) and/or propionate (0, 2.5 and 5.0 mmol/L), in the presence of either 2.5 mmol/L L-alanine and 18.5 kBq L-[1-14C]alanine or 2.5 mmol/L L-glutamate and 18.5 kBq L-[1-14C]glutamate. Increasing the level of ruminal degradable nitrogen with urea increased in vitro rates of oxidation to 14CO2 of [1-14C]alanine, but not [1-14C]glutamate. Increasing in vitro concentrations of NH4Cl and propionate between 0 and 5 mmol/L reduced the rates of oxidation to 14CO2 of both [1-14C]alanine and [1-14C]glutamate. Synthesis of [14C]glucose with [1-14C]alanine, but not [1-14C]glutamate as the substrate, was increased 100% by feeding urea. Increasing in vitro levels of NH4Cl between 0 and 5 mmol/L reduced the rates of conversion of [1-14C]alanine and [1-14C]glutamate to [14C]glucose in hepatocytes isolated from sheep fed both diets. Increasing in vitro levels of propionate between 0 and 2.5 mmol/L elevated production rates of [14C]glucose from both radiotracers, but from 2.5 to 5.0 mmol/L propionate no further increase was evident. Feeding urea increased in vitro rates of urea nitrogen production. Increasing propionate levels between 0 and 5 mmol/L reduced ureagenic rates in liver cells isolated from sheep fed both diets. Oxygen (O2) uptake was unaffected by diet and NH4Cl; however, increasing propionate between 0 and 5 mmol/L increased rates of O2 uptake. It is concluded that in isolated sheep hepatocytes, detoxification of excessive ammonia may cause a repartitioning of alanine and glutamate metabolism towards oxidation and gluconeogenesis.