Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 16, 2003
Publication Date: June 1, 2004
Citation: Oba, M., Baldwin, R.L., Owens, S.L., Bequette, B.J. 2004. Urea synthesis by ruminal epithelial and duodenal mucosal cells from growing sheep. Journal of Dairy Science. 87(6):1803-1805.
Interpretive Summary: Synthesis of urea, a nitrogen containing compound excreted in urine, by the gastrointestinal tract of ruminants may play a role in the total excretion losses of nitrogen to the environment. Additionally, because these tissues make urea, there is a metabolic cost associated with the digestion of certain feeds, which results in a decrease in the efficiency of nutrient use by the animal. In order to determine whether ruminant gut tissues have the capability to synthesize urea ruminal epithelial and duodenal mucosal cells were isolated from growing lambs. Cells were incubated for 90 min with combinations of substrates used for urea syntheis. For both rumen epithelial cells and duodenal mucosal cells, arginase, an important enzyme in making urea from arginine, activity appeared to be sufficient for catabolism of arginine to urea. Other additions were used to illustrate that the enzyme carbamoyl phosphate synthetase is probably controlling the amount of urea made by the gut tissues. The genetic control of these enzymes will help to decide if they can positively affect the nitrogen economy of the animal.
The objective was to determine whether ruminant gut tissues have capability to synthesize urea in a short-term incubation. Ruminal epithelial (REC) and duodenal mucosal cells (DMC) were isolated from growing Polypay ram lambs (n=4) fed a mixed forage-concentrate diet. Cells were incubated (90 min) with either acetate (5 mM) or propionate (5 mM) plus combinations of substrate intermediates (5 mM) for urea synthesis: arginine (Arg), aspartate + citrulline (AspC), aspartate + ornithine + ammonia (AspON), and AspON + N-carbamoylglutamate (AspONG). Treatments were arranged in a 2 x 4 factorial design. Type of volatile fatty acid (VFA) did not influence net urea synthesis. For REC, net urea synthesis (nmoles×106 cells-1×90 min-1) was greatest with Arg (54.5 ± 6.3) followed by AspC (4.6 ± 1.1) and AspONG (3.6 ± 1.4), and net urea synthesis for AspON (1.6 ± 0.8) did not differ from zero (Control background). For DMC, net urea synthesis for Arg (2.1 ± 0.7) and AspONG (1.9 ± 0.7) treatments was greater than for AspC (0.3 ± 0.7) and AspON (-0.6 ± 0.7) treatments. Thus, for both REC and DMC, arginase activity appeared to be sufficient for catabolism of arginine to urea. Furthermore, greater urea synthesis from ammonia, ornithine and aspartate with presence of N-acetylglutamate analogue suggests that carbamoyl phosphate synthetase is probably rate-limiting for urea synthesis and ammonia detoxification by ruminant gut tissues.