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Interactions between Ruminal Degradable Nitrogen Intake and In Vitro^1,2,

Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario, N1G 2W1, Canada

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.

KEY WORDS: • hepatocytes • sheep • amino acid oxidation • gluconeogenesis • ammonia

¹ Supported by the Ontario Ministry of Agriculture, Food and Rural Affairs, Toronto, ON and the Natural Sciences and Engineering Research Council of Canada, T. Mutsvangwa is supported by a Canadian Commonwealth Scholarship.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence should be addressed.

Manuscript received 20 March 1995. Revision accepted 25 August 1995.

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