Urea flux in the ureter.

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 microliter/min (a moderate antidiuresis), the ratio of proximal-to-distal urea concentration, corrected for water movement, was 0.93 +/- 0.03 (P less than 0.01 compared with unity), indicating that approximately 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 microliter/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 less than 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 less than 0.01) and 3.5 +/- 1.2% (P less than 0.05), respectively], whereas sodium was secreted [9.2 +/- 2.3% (P less than 0.01)].(ABSTRACT TRUNCATED AT 250 WORDS)