Urea Metabolism in Sheep

The rate at which urea enters the body pool of urea in sheep was investigated using isotope dilution techniques. Both single injection and constant infusion of carbon-14 urea were used. Urea excretion rates were calculated by measuring the urea in urine collected from a catheter in the bladder. Urea entry and excretion rates were measured in sheep fed rations containing 3.5, 9, 17 and 27% crude protein. The entry and excretion rates on each diet in mg/min were 2.5 t 0.23 and 0.2 2 0.09 (3 experiments), 8.5 2 0.77 and 4.6 * 1.13 (8), 26.4 2 1.3 1 and 17.7 t 0.52 (8), 39.4 2 2.45 and 28.3 t2.37 (8), respectively, showing that of the urea entering the body pool of urea in sheep 8, 56, 67 and 70% respectively was excreted on each ration. The entry rate and the excretion rate of urea both appeared to be linearly correlated with the concentration of urea in the blood of sheep.