caa a22 4500 38806353X CHVBK 20180307125127.0 cr unu---uuuuu 161130e199801 xx s 000 0 eng 10.1079/BJN19980011 doi S0007114598000129 pii (NATIONALLICENCE)cambridge-10.1079/BJN19980011 Urea recycling in sheep: effects of intake [Elektronische Daten] The effect of intake on urea production, entry into the digestive tract and return of N to the ornithine cycle was studied in four sheep. Each sheep received 0-6, 1-2 and 1.8 × estimated maintenance energy intake quantities of grass pellets for 9 d. After 4 d of adjustment, N balance measurements were conducted between days 5 and 8. From day 7 to day 9 animals were continuously infused, via the jugular vein, with [15N15N]urea and three urine samples were collected at approximately 2h intervals 48-54h after the start of infusion. Total urea and enrichments of [15N15N]- and [14N15N]urea in the urine samples were determined. Urea production was calculated from the isotopic dilution of [15N15N]urea and entry into the gastrointestinal tract (GIT) obtained from the difference between this and urinary urea elimination. Urea which enters the GIT undergoes hydrolysis to liberate NH3 which may be reabsorbed and enter the ornithine cycle, in which case the product is [14N15N]urea, based on the probabilities of labelled and unlabelled N providing ureagenic precursors. The quantity of urea-N which returns to the ornithine cycle from the GIT can thus be calculated. Existing models based on this approach yield overestimates of the fate of individual urea molecules due to a failure to allow for multiple recycling of [14N15N]urea species through the GIT. Refinements introduced to cover this resulted in a 33-48 % reduction in calculated return of label for the current study. The present model also predicted that 95 % of the label movements across the GIT could be accommodated by three or fewer entries and returns of urea-N and 99 % by recycling for a maximum of six occasions. Urea-N production increased with intake (P < 0.001) and exceeded digestible N values at all intakes. Urea which entered the digestive tract, both in absolute terms (P < 0.001) and as a proportion of production (0.62, 0.69, 0.73; P = 0.027), increased with intake. The proportion of entry into the digestive tract which was returned to the ornithine cycle remained reasonably constant (0.37-0.41) across all intakes but the absolute amount increased (5.6, 9.2 and 15.0gN/d; P < 0.001) with intake. If allowance is included for losses of 15N in faeces then the approach offers a relatively simple means of estimating anabolic reuse of urea by digestive tract micro-organisms and can complement data obtained from the technically more demanding arterio-venous and multiple-isotope techniques used hitherto. Urea: Gastrointestinal tract: [15N]kinetics: Sheep Copyright © The Nutrition Society 1998 Urea nationallicence Gastrointestinal tract nationallicence [15N]kinetics nationallicence Sheep nationallicence Sarraseca A. 1Department of Agriculture, University of Aberdeen, Aberdeen, AB24 5UA, UK Milne E. Rowett Research Institute, Bucksburn, Aberdeen, AB21 9SB, UK Metcalf M. J. BioSS, Rowett Research Institute, Bucksburn, Aberdeen, AB21 9SB, UK Lobley G. E. Rowett Research Institute, Bucksburn, Aberdeen, AB21 9SB, UK British Journal of Nutrition Cambridge University Press 79/1(1998-01), 79-88 0007-1145 79:1<79 1998 79 BJN https://doi.org/10.1079/BJN19980011 text/html Onlinezugriff via DOI 1 research-article jats NATIONALLICENCE 856 40 https://doi.org/10.1079/BJN19980011 text/html Onlinezugriff via DOI NATIONALLICENCE 700 1- Sarraseca A. 1Department of Agriculture, University of Aberdeen, Aberdeen, AB24 5UA, UK NATIONALLICENCE 700 1- Milne E. Rowett Research Institute, Bucksburn, Aberdeen, AB21 9SB, UK NATIONALLICENCE 700 1- Metcalf M. J. BioSS, Rowett Research Institute, Bucksburn, Aberdeen, AB21 9SB, UK NATIONALLICENCE 700 1- Lobley G. E. Rowett Research Institute, Bucksburn, Aberdeen, AB21 9SB, UK NATIONALLICENCE 773 0- British Journal of Nutrition Cambridge University Press 79/1(1998-01), 79-88 0007-1145 79:1<79 1998 79 BJN CC0 http://creativecommons.org/publicdomain/zero/1.0 nationallicence BK010053 XK010053 XK010000 NATIONALLICENCE NATIONALLICENCE NL-cambridge