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Urea recycling in ruminants has been studied extensively in the past, but the mechanisms regulating the amount of urea recycled or excreted remain obscure. To elucidate the role of urea transporters (UT) in N recycling, nine Dorset-Finn ewe lambs (20.8 [+ or -]0.8 kg) were fed diets containing 15.5, 28.4, and 41.3 g of N/kg of DM for 25 d. Nitrogen balance and urea N kinetics were measured during the last 3 d of the period. Animals were then slaughtered and mucosa samples from the rumen, duodenum, ileum, and cecum, as well as kidney medulla and liver, were collected. Increasing N intake tended to increase N balance quadratically (1.5, 5.1, and 4.4[+ or -]0.86 g of N/d, P < 0.09), and linearly increased urinary N excretion (2.4, 10, and 16.5[+ or -]0.86 g N/d, P < 0.001) and plasma urea N concentration (4.3, 20.3, and 28.4[+ or -]2.62 mg of urea N/dL, P < 0.001), but did not affect fecal N excretion (5.0[+ or -]0.5 g of N/d; P < 0.94). Urea N production (2.4, 11.8, and 19.2[+ or -]0.83 g of N/d; P < 0.001) and urinary urea N excretion (0.7, 7.0, and 13.4[+ or -]0.73 g N/d; P < 0.001) increased linearly with N intake, as well as with the urea N recycled to the gastrointestinal tract (1.8, 4.8, and 5.8[+ or -]0.40 g of N/d, P < 0.001). No changes due to N intake were observed for creatinine excretion (518[+ or -]82.4 mg/d; P < 0.69) and clearance (46[+ or -]10.7 mL/min; P < 0.56), but urea N clearance increased linearly with N intake (14.9, 24.4, and 34.9[+ or -]5.9 mL/min; P < 0.04). Urea N reabsorption by the kidney tended to decrease (66.3, 38.5, 29.1 [+ or -]12.6%; P < 0.06) with increasing N content of the diet. Increasing the level of N intake increased linearly the weight of the liver as a proportion of BW (1.73, 1.88, and 2.22[+ or -]0.15%, P < 0.03) but only tended to increase the weight of the kidneys (0.36, 0.37, and 0.50[+ or -]0.05%, P < 0.08). Urea transporter B was present in all the tissues analyzed, but UT-A was detected only in kidney medulla, liver, and duodenum. Among animals on the three diets, no differences (P > 0.10) in UT abundance, quantified by densitometry, were found. Ruminal-wall urease activity decreased linearly (P < 0.02) with increasing level of N intake. Urease activity in duodenal, ileal, and cecal mucosa did not differ from zero (P > 0.10) in lambs on the high-protein diet. In the present experiment, urea transporter abundance in the kidney medulla and the gastrointestinal tract did not reflect the increase in urea-N reabsorption by the kidney and transferred into the gut. Key Words: Biochemical Transporters, Kinetics, Nitrogen Metabolism, Sheep, Urea Recycling

To study the effect of dietary N level on urea kinetics and recycling, four Holstein heifers (267 [+ or -] 3.6 kg) were used in a Youden square design. Isocaloric diets with a N content of 1.44, 1.89, 2.50, 2.97, and 3.40% were fed at approximately 1.8 times maintenance intake. Increasing the N content of the diet increased urinary N excretion (P < 0.001) and N balance (P < 0.01), but did not affect the fecal N excretion (P = 0.21). Increasing the level of dietary N, increased urea production (P < 0.001) and excretion (P < 0.001), but no effect (P = 0.24) could be detected in the amount of N recycled to the gut. Urea recycled with the saliva, however, increased (P < 0.001) both in absolute and relative terms, with increasing dietary N. No difference could be detected on the amount of recycled N that was used for anabolism or returned to the ornithine cycle, but less (P = 0.001) N originating from urea was excreted in feces as dietary N increased. Ruminal ammonia concentration increased (P < 0.001) with increasing N intake, but total tract neutral detergent fiber digestibility was depressed only on the lowest N intake diet. No difference (P = 0.30) was detected in ruminal microbial yield among diets, but more (P < 0.003) N was derived from blood urea at low N intakes, and the efficiency of use of the recycled N decreased (P < 0.001) with increasing levels of dietary N. Adaptive changes to low-N diets were a decrease (P < 0.003) in the renal clearance of urea and an increase (P < 0.001) in the gastrointestinal clearance of urea. Urea transporters were present in the rumen wall of the heifers and differentially expressed depending on dietary N content, but their role in the transfer of urea into the rumen remains uncertain. Different mechanisms of N salvage and recycling were involved when animals were fed low-N diets that ensured a supply of endogenous N to the gastrointestinal tract and, due to the reduced contribution of dietary N, an increased efficiency of the N recycled was observed. Key Words: Biochemical Transporters, Heifers, Kinetics, Nitrogen Metabolism, Urea Recycling