MECHANISM OF CONCENTRATING UREA IN KIDNEY.I. CLEARANCE STUDIES IN UNANESTHETIZED DOGS

The role of urea in the concentrating process of the kidney was studied in conscious dogs. Food and water were withdrawn 24 hours before the experiments. An increase of plasma urea concentration (PU) was obtained by constant intravenous infusion of a hypertonic urea solution over a period of 10-11 hours. In order to exclude the influence of a variable sodium intake the dogs were kept on a low sodium diet for a period of 4 weeks preceeding the experiment. 1. Infusion of urea results in an increase of PU from 5 to 20 mmol/l. During the same period U/Posm rises from 3.8 to 5.1. This effect is entirely due to a higher urine urea concentration. (UU rises from 403 to 1107 mmol/l.) 2. A linear correlation between PU and UU up to an PU of 10 mmol/l is observed. 3. There is no change in GFR during the infusion of urea. 4. 72 hours without water and food intake results in an U/Posm and UU which is similar to the values obtained during increased PU. 5. From the data it is concluded that there is a selective urea concentrating process which is linearly correlated to the urea load. Urea does not stimulate water reabsorption, and does not increase the concentrating ability of the kidney for non-urea solutes. © 1969 Springer-Verlag.