Effect of urea concentration of pelvic fluid on renal concentrating ability

The role of the pelvic urine in the renal concentration process was examined in anesthetized antidiuretic rats. The left ureter was cannulated with perfusion and collection catheters so that the composition of the fluid bathing the papilla could be altered by superperfusing solution of known composition without opening the renal pelvis, exposing the papilla, or altering intrapelvic pressure. The effect on urinary osmotic pressure (Uosmol) of papillary superperfusion with solutions of varying urea concentrations and total osmolality was examined. Papillary superperfusion with solutions containing no urea or less urea than preperfusion pelvic urine caused marked reductions in Uosmol regardless of the osmotic pressure of the solution. Superperfusion solutions with urea concentrations similar to that of preperfusion pelvic urine but lower in osmotic pressure than preperfusion urine produced no change in Uosmol. Superperfusion solutions with urea concentrations higher than that of preperfusion urine and with osmotic pressure equal to that of the preperfusion urine increased Uosmol. Finally, superperfusion of the papilla with urea solutions of greater than 2,370 mosmol/kg H2O and hypertonic to preperfusion urine decreased Uosmol. We conclude that the urea concentration of the pelvic urine plays an essential role in the urinary concentration mechanism.