Urea reabsorption in the medullary collecting duct of protein-depleted young rats before and after urea infusion.

The aims of the present study were to examine urea handling along the length of the medullary collecting duct (MCD) in protein-depleted young rats and to determine the effect of urea infusion on MCD function and urine concentrating ability. In 10 young rats on a low protein diet, urea reabsorption equivalent to 18.3% of the filtered load was observed along the MCD (4.5 mm) using the microcatheterization technique. Collecting duct urea reabsorption occurred almost entirely (16.6%) in the distal portion of the MCD (mid-zone to papillary tip, 2.8 mm). These results are in contrast to the lack of net urea reabsorption along the MCD in protein-replete adult rats [21]. After urea infusion which raised plasma urea level from 3.5 to 10.5 mmol/l in protein-depleted rats, urine non-urea solute concentration increased in the non-exposed right kidney from 827 to 1,199 mosm kg ( P<0.001) but the increase was not significant in the experimental left kidney (590 to 619 mosm kg). Thus exposure of the papilla interfered with urea-induced enhancement of urine concentrating ability. After urea infusion, fractional urea reabsorption in the distal portion of the MCD was similar to that before infusion (21.1%) of filtered load) but the absolute load of urea delivered to the MCD and reabsorbed along the duct was markedly increased (2.7-fold). In 6 rats with an increase in urine non-urea solute concentration in the experimental kidney after urea infusion, fluid reabsorption along the duct was significantly increased. The results indicate that in protein-depleted young rats (1) there is significant urea reabsorption in the distal portion of the medullary collecting duct, (2) urea infusion contributes to enhanced urine concentrating ability, in part, by increasing absolute urea reabsorption and also water reabsorption in the collecting duct. [ABSTRACT FROM AUTHOR]