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.
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.
Maltby
S A
SA
ARS, USDA, Beltsville, MD 20705, USA.
Reynolds
C K
CK
Lomax
M A
MA
Beever
D E
DE
D000596:Amino Acids / Q000097:blood; D000641:Ammonia / Q000378:metabolism / Q000652:urine; D000821:Animal Feed / Q000032:analysis; D000818:Animals; D001120:Arginine / Q000008:administration & dosage / Q000378:metabolism; D001835:Body Weight; D002417:Cattle / Q000378:metabolism*; D004032:Diet / Q000662:veterinary*; D006400:Hematocrit / Q000662:veterinary; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D000455:Medicago sativa / Q000378:metabolism; D009584:Nitrogen / Q000652:urine; D017672:Nitrogen Compounds / Q000097:blood / Q000378:metabolism*; D011169:Portal Vein / Q000502:physiology; D011897:Random Allocation; D014508:Urea / Q000008:administration & dosage / Q000494:pharmacology*; D014556:Urine / Q000737:chemistry / Q000502:physiology; D014781:Viscera / Q000187:drug effects / Q000378:metabolism*
D000596:Amino Acids; D017672:Nitrogen Compounds; D000641:Ammonia; D014508:Urea; D001120:Arginine; D009584:Nitrogen