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Splanchnic metabolism of nitrogenous compounds and urinary nitrogen excretion in steers fed alfalfa under conditions of increased absorption of ammonia and L-arginine supply across the portal-drained viscera^1,2,3

S. A. Maltby, C. K. Reynolds^*,4, M. A. Lomax^,5 and D. E. Beever^,6

^* ARS, USDA, Beltsville, MD 20705; and Department of Biochemistry and Physiology, The University of Reading, Whiteknights, Reading, RG6 2AJ U.K.; and and School of Agriculture, Policy, and Development, The University of Reading, Earley Gate, Reading, RG6 6AR U.K.

⁴ Correspondence and current address: Dept. of Anim. Sci., The Ohio State Univ., OARDC, 1680 Madison Ave., Wooster 44691-4096 (phone: 330-263-3793; fax: 330-263-3949; e-mail: Reynolds.345 @osu.edu).

Effects of increased ammonia and/or arginine absorption across the portal-drained viscera (PDV) on net splanchnic (PDV and liver) metabolism of nitrogenous compounds and urinary N excretion were investigated in six catheterized Hereford x Angus steers (501 ± 1 kg BW) fed a 75% alfalfa:25% (as-fed basis) corn-soybean meal diet (0.523 MJ of ME/[kg BW^0.75•d]) every 2 h without (27.0 g of N/kg of dietary DM) and with 20 g of urea/kg of dietary DM (35.7 g of N/kg of dietary DM) in a split-plot design. Net splanchnic flux measurements were obtained immediately before beginning and ending a 72-h mesenteric vein infusion of L-arginine (15 mmol/h). For 3 d before and during arginine infusion, daily urine voided was measured and analyzed for N composition. Feeding urea increased PDV absorption (P < 0.01) and hepatic removal (P < 0.01) of ammonia N, accounting for 80% of increased hepatic urea N output (P < 0.01). Numerical increases in net hepatic removal of AA N could account for the remaining portion of increased hepatic urea N output. Arginine infusion increased hepatic arginine removal (P < 0.01) and hepatic urea N output (P < 0.03) and switched hepatic ornithine flux from net uptake to net output (P < 0.01), but numerical changes in net hepatic removal of ammonia and AA N could not account fully for the increase in hepatic urea N output. Increases in urine N excretion equaled quantities of N fed as urea or infused as arginine. Estimated salivary urea N excretion was not changed by either treatment. Urea cycle regulation occurs via a complex interaction of mechanisms and requires N sources other than ammonia, but the effect of increased ammonia absorption on hepatic catabolism of individual AA in the present study was not significant.

Key Words: Amino Acids • Arginine • Bovidae • Liver • Urea

This article has been cited by other articles:

				C. K. Reynolds and N. B. Kristensen Nitrogen recycling through the gut and the nitrogen economy of ruminants: An asynchronous symbiosis J Anim Sci, April 1, 2008; 86(14_suppl): E293 - E305. [Abstract] [Full Text] [PDF]

				S. A. Maltby, C. K. Reynolds, M. A. Lomax, and D. E. Beever Splanchnic metabolism of nutrients and hormones in steers fed alfalfa under conditions of increased absorption of ammonia and L-arginine supply across the portal-drained viscera J Anim Sci, May 1, 2005; 83(5): 1088 - 1096. [Abstract] [Full Text] [PDF]