UREA RECYCLING IN SHEEP - EFFECTS OF INTAKE

The effect of intake on urea production, entry into the digestive trac t and return of N to the ornithine cycle was studied in four sheep. Ea ch sheep received 0.6, 1.2 and 1.8 x estimated maintenance energy inta ke quantities of grass pellets for 9 d. After 4 d of adjustment, N bal ance measurements were conducted between days 5 and 8. From day 7 to d ay 9 animals were continuously infused, via the jugular vein, with [(N N)-N-15-N-15]urea and three urine samples were collected at approximat ely 2 h intervals 48-54 h after the start of infusion. Total urea and enrichments of [(NN)-N-15-N-15]- and [(NN)-N-14-N-15]urea in the urine samples were determined. Urea production was calculated from the isot opic dilution of [(NN)-N-15-N-15]urea and entry into the gastrointesti nal tract (GIT) obtained from the difference between this and urinary urea elimination. Urea which enters the GIT undergoes hydrolysis to li berate NH3 which may be reabsorbed and enter the ornithine cycle, in w hich case the product is [(NN)-N-14-N-15]urea, based on the probabilit ies of labelled and unlabelled N providing ureagenic precursors. The q uantity of urea-N which returns to the ornithine cycle from the GIT ca n thus be calculated. Existing models based on this approach yield ove restimates of the fate of individual urea molecules due to a failure t o allow for multiple recycling of [(NN)-N-14-N-15]urea species through the GIT. Refinements introduced to cover this resulted in a 33-48 % r eduction in calculated return of label for the current study. The pres ent model also predicted that 95 % of the label movements across the G IT 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 produc tion increased with intake (P < 0.001) and exceeded digestible N value s at all intakes. Urea which entered the digestive tract, both in abso lute 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 a mount increased (5.6, 9.2 and 15.0 g N/d; P < 0.001) with intake. If a llowance is included for losses of N-15 in faeces then the approach of fers 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 te chniques used hitherto.