It has been hypothesized that urea from the final urine is recycled into the renal papilla through the pelvic epithelium. To test this hypothesis, samples of urine were collected by micropuncture proximally and distally through the intact, contracting ureter of the anesthetized rat. In 12 rats, in which urine flow was 5.89 ± 0.67 μl/min (a moderate antidiuresis), the ratio of proximal-to-distal urea concentration, corrected for water movement, was 0.93 ± 0.03 (P < 0.01 compared with unity), indicating that ~7% of urea in the urine emerging from the terminal collecting duct was reabsorbed by the time it reached the distal ureter. To assess the possible contribution of urea reabsorption by the ureter, the ureter was cannulated proximally and distally and perfused with urine of known composition at 6.26 ± 0.10 μl/min. In nine rats, the ratio of urea concentration in the perfusate collected from the distal end of the ureter to that in the perfusate entering the proximal end was 0.93 ± 0.02 (P < 0.01 compared with unity), indicating 7% reabsorption. Movement of solute across the ureteral epithelium was not restricted to urea. Potassium and creatinine were also reabsorbed [3.4 ± 0.9 (P < 0.01) and 3.5 ± 1.2% (P < 0.05), respectively], whereas sodium was secreted [9.2 ± 2.3% (P < 0.01)]. These data suggest that under the experimental conditions of a moderate antidiuresis, the difference in urea concentration of urine between the terminal collecting duct at the tip of the papilla and the distal end of the ureter can be accounted for by reabsorption through the ureteral epithelium without also necessitating urea reflux through the pelvic epithelium.
|Journal||American Journal of Physiology - Renal Fluid and Electrolyte Physiology|
|Issue number||2 (24/2)|
|State||Published - 1 Jan 1988|