MECHANISM OF CONCENTRATING UREA IN KIDNEY .2. MICROPUNCTURE STUDIES ON RATS

Anesthetized, nondiuretic rats were infused with hypertonic urea solution. Micropuncture and clearance techniques were employed in order to investigate urea concentration in the proximal and distal convolution, the collecting duct and in final urine. 1. PU increased from 5 to approximately 20 mMol/l. 2. GFR remained unchanged. 3. Up to PU of 15 mMol/l UU increased proportionally with rising PU. Above 15 mMol/l PU, PU remained constant. 4. Neither an increase of fractional water reabsorption nor of non-urea-solute concentration in the final urine was observed. These results were in agreement with findings obtained in unanesthetized dogs. 5. About 54% of the filtered urea were reabsorbed in the convoluted portion of the proximal tubule. Late proximal TF/PIn remained constant (2,6±0,4), indicating a linearly increasing proximal urea was approximately 50% of filtered urea load. 6. The amount of recirculating urea was approximately 50% of filtered urea at PU=5 mMol/l, and decreased continuously with increasing PU. At a PU of 20 mMol/l the addition of urea to the loop of Henle approximated zero. Thus, urea concentration in the early distal tubule did not increase proportionally to PU. 7. Under the condition of low PU levels, and unchanged water reabsorption the concentration of urea rose 2.7 fold in the course of the distal convolution, at high plasma urea levels 2.2 fold. This decrease in concentrating ability was caused by an increasing urea reabsorption in the distal tubule with increasing PU. 8. Urea reabsorption in the distal tubule was caused mainly by diffusion (80%) and the rest by solvent drag. The data suggest that distal urea permeability increases with increasing intratubular urea concentration. 9. The recirculating urea is reabsorbed from the collecting ducts. A precise analysis, however, cannot be obtained, since both the urea concentration of the interstitial fluid and the action of the deep nephrons are unknown. Probably urea concentration in the collecting ducts and in the interstitial fluid approximated each other with increasing PU. This could be accounted for the declining urea reabsorption from the collecting ducts. 10. The data suggest that the cortical distal tubules were of great importance for the selective urea concentration in the final urine, which we have demonstrated. The distal tubule is able to maintain and to build up a high urea concentration gradient across the tubular wall. Therefore an increasing urea concentration reaches the collecting duct as a consequence of an augmented urea load. At high urea load, however, the concentrating ability" of the distal tubule is limited by the increasing urea reabsorption in the course of the distal tubule. © 1969 Springer-Verlag."