|Baldwin, Ransom - Randy|
Submitted to: Journal of Dairy Science
Publication Type: Abstract Only
Publication Acceptance Date: 9/8/2003
Publication Date: 12/16/2003
Citation: Baldwin, R.L., Bequette, B.J., Oba, M., Owens, S.L., 2003. Urea synthesis by ruminal epithelial and duodenal mucosal cells isolated from growing sheep [abstract]. Journal of Dairy Science 86(Suppl. 1):59.
Technical Abstract: To determine the capability of ruminant gut tissues to synthesize urea, ruminal epithelial (REC) and duodenal mucosal cells (DMC) were isolated from growing Polypay ram lambs (n=4) fed a mixed forage-concentrate diet. Isolated cells were incubated for 90 min with either acetate (5mM) or propionate (5mM) plus four combinations of substrate to support urea synthesis (arginine, Arg; aspartate + citrulline, AspC; aspartate + ornithine + ammonia, AspON; aspartate + ornithine + ammonia + N-carbamoylglutamate, AspONG; 5 mM each) in a 2 x 4 factorial arrangement of treatments. Urea production was determined by stable-isotope dilution with gas chromatography-mass spectrometry and correction for background urea concentration present in 0 time controls (Control). For both cell types, effects of VFA and interactions between VFA and substrate combinations were not observed. For REC, total urea release was 56.6, 6.7, 3.7, 5.8, and 2.2 nmol per 106 cells respectively for Arg, AspC, AspON, AspONG, and Control. Arg addition resulted in 10-fold greater (P < 0.001) urea production than other treatments which were not different from control. For DMC, total urea release was 4.2, 2.4, 1.5, 4.0, and 2.1 nmol per 106 cells respectively for Arg, AspC, AspON, AspONG, and Control. Arg and AspONG treatments resulted in greater (P < 0.001) urea production than other treatments and control, indicating that ammonia N can be utilized for urea synthesis by DMC if carbamoyl phosphate synthetase is activated. These results demonstrate that ruminant gut tissues are capable of synthesizing urea, particularly by REC when degrading Arg. However, further research is necessary to determine the extent to which arginine degradation by the REC contributes to N recycling in vivo and to assess the impact on the arginine requirement of ruminants.