Quantitative aspect of the endogenous urea cycle in goats.

The turnover of plasma urea was studied in goats fed rations with differentprotein contents by using "C- and "N-urea as tracers. The biological half-life time ofurea determined with "N-urea was always longer than that determined with "C-urea, reflecting the difference in the metabolic pathway between the urea hydrolysates, ammoniaand carbon dioxide. It was roughly estimated from the turnover studies that more thana 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 proteincontent of the ration, the percentage of transfer and dissociation increased. The "C-ureadischarged into the urine was less than 57% of the administered dose. It decreased to3.9% in goats fed rations with a low protein content.The rate of urea turnover determined with "N-urea was nearly consistent with the rateof urea discharge into the urine in goats fed higher protein rations. In goats fed lowerprotein rations, on the other hand, the rate of urea discharge decreased and its consistencywith the rate of turnover determined with "N-urea was upset. This was caused byassimilation of urea-nitrogen in rumen microorganisms, as was convinced by studying theincorporation of urea-"N into the fraction of rumen microorganisms. It was assumedthat in goats fed lower protein rations the bulk of urea synthesized might have enteredthe endogenous recycling system within the body and assimilated by rumen micro-organusms.