Quantitative aspect of the endogenous urea cycle in goats

The turnover of plasma urea was studied in goats fed rations with different protein contents by using 14C- and 15N-urea as tracers. The biological half-life time of urea determined with 15N-urea was always longer than that determined with 14C-urea, reflecting the difference in the metabolic pathway between the urea hydrolysates, ammonia and carbon dioxide. It was roughly estimated from the turnover studies that more than a half the quantity of urea synthesized might be transferred to the alimentary canal, including the rumen, where it underwent dissociation, with a decrease in the protein content of the ration, the percentage of transfer and dissociation increased. The 14C-urea discharged into the urine was less than 57% of the administered dose. It decreased to 3.9% in goats fed rations with a low protein content. The rate of urea turnover determined with 15N-urea was nearly consistent with the rate of urea discharge into the urine in goats fed higher protein rations. In goats fed lower protein rations, on the other hand, the rate of urea discharge decreased and its consistency with the rate of turnover determined with 15N-urea was upset. This was caused by assimilation of urea-nitrogen in rumen microorganisms, as was convinced by studying the incorporation of urea-15N into the fraction of rumen microorganisms. It was assumed that in goats fed lower protein rations the bulk of urea synthesized might have entered the endogenous recycling system within the body and assimilated by rumen microorganisms. (auth.)