Urea synthesis by ruminal epithelial and duodenal mucosal cells from growing sheep

The objective was to determine whether ruminant gut tissues have the capability to synthesize urea in a short-term incubation. Mixed primary cell cultures containing ruminal epithelial (REC) or duodenal mucosal cells (DMC) were isolated from growing sheep (n = 4) fed a mixed forage-concentrate diet. Cells were incubated (90 min) in a Krebs salts-based buffer with either acetate (5 mM) or propionate (5 mM) plus a combination of substrate intermediates (5 mM) for urea synthesis: arginine, aspartate + citrulline (AspC), aspartate + ornithine + ammonia (AspON), or AspON + N-carbamoylglutamate (AspONG) in a 2 x 4 factorial arrangement of treatments. Volatile fatty acid, propionate vs. acetate, did not influence net urea synthesis. For REC, net urea synthesis (nmoles x (10(6) cells)(-1) x 90 min(-1)) was greatest with Arg (54.5 +/- 6.3) followed by AspC (4.6 +/- 1.1) and AspONG (3.6 +/- 1.4). 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 in the presence of the N-acetylglutamate analogue suggests that carbamoyl phosphate synthetase is probably rate-limiting for urea synthesis and ammonia detoxification by ruminant gut tissues.