Contribution of urea to urinary concentrating ability in the dog.

SCHMIDT-NIELSEN, BODIL, AND ROSCOE R. ROBINSON. Contribution of urea to urinary concentrating ability in the dog. Am. J. Physiol. 218(5): 1363-1369. 1970.-Dogs were maintained on balanced highand low-protein diets for 3 weeks prior to the experiments. Urea clearance, inulin clearance, and solute concentrations in the various zones of the kidney were determined. The glomerular filtration rates and osmolar clearances were approximately equal in the two dietary groups. Maximum osmolar urine-to-plasma ratios were far higher in the high(8.3) than in the low-protein dogs (3.0). This difference was primarily due to urea accumulation in the inner zone of the renal medulla. Blood urea concentration was 2.5 times higher in highthan in low-protein dogs, whereas the amount of urea per gram urea-free dry tissue at the medullary crest was 6 times higher in the highthan in the low-protein dogs. Uphill transport out of the collecting duct was indicated by a higher urea concentration in the tissue of the inner medulla than in the urine in the low-protein dogs. Compared to other mammals that conserve urea during low-protein feeding, the mechanism in the dog appears to be less pronounced.