Utilization of endogenous urea secreted into the gastrointestinal tract of the sheep

The possibility of the utilization of endogenous urea secreted into the gastrointestinal tract was studied in sheep to which a single dose of 3 g 15N urea (50.4% enrichment) was administered by slow infusion via a permanent surgical cannula into the caecum. During a 7 days' experiment, 31.15 +/- 1.08% of the dose was excreted in the urine and 19.65 +/- 1.08% in the faeces, while 49.20 +/- 1.49% was retained in the organism. The maximum 15N values in blood from the jugular vein were found 10 h after starting the infusion and a high concentration was maintained for a further 24 h; sixty hours after the infusion, no more 15N was found in the blood. The maximum 15N values in the total nitrogen of the rumen contents were observed 24 h after the infusion and low levels could still be detected on the 6th day of the experiment. Evidence that 15N participates in enterohepatic and ruminohepatic circulation, and that it is also utilized, is based on the finding of the total, but mainly the protein nitrogen concentration in the duodenum. The maximum 15N values in the total nitrogen (0.247 +/- 0.041 At % 15N) and protein nitrogen (0.262 +/- 0.023 At % 15N) of the duodenal contents were found 24 h after the infusion and 15N could still be demonstrated there in the nitrogen fractions on the 5th day of the experiment.