Urea metabolism in sheep given conventional feeds or nourished by intragastric infusion

Urea recycling in sheep: effects of intake

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 x 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 2 h intervals 48-54 h 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 15.0 g N/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.

Urease (EC 3.5.1.5) inhibition in the sheep rumen and its effect on urea and nitrogen metabolism

The urease (EC 3.5.1.5) inhibitor, phenylphosphoryldiamidate (PPDA), was given by continuous infusion into the rumen of two sheep nourished by intragastric infusion and into either the rumen or abomasum of four sheep given a pelleted diet containing 119 g crude protein (nitrogen x 6.25)/kg dry matter. PPDA was given at 1 g/d in infusion sheep and 1.5 g/d in the normally-fed sheep. Measurements of urea kinetics were made using single injections of [14C]urea. Urease inhibition was complete within 24 h of starting PPDA infusions to the rumen; in this time-period, urea concentration in rumen contents reached equilibrium with that in plasma and this situation persisted until infusions were terminated. Relative to the control periods, plasma urea and rumen ammonia concentrations were unchanged but urea irreversible loss rate decreased by 26% in infusion sheep and 33% in fed sheep when PPDA was given. Urinary urea excretion was not affected, hence urea degradation, measured by difference, decreased by 77 and 58% respectively in response to urease inhibition. Administration of PPDA to the abomasum resulted in a reduction in rumen urease activity to about 40% of control values but had no effect on urea metabolism. Differences between treatments in daily nitrogen retention were not significant, indicating that under the dietary conditions imposed in these experiments, even substantial changes in urea recycling had only minor effects on the overall N economy of the animal.