Polyuria after release of bilateral ureteral obstruction (BUO) is frequently seen in patients with urological disorders. In this study, we examined the effect of BUO and release of BUO on the expression of the vasopressin-regulated water channel aquaporin-2 (AQP-2) in rat kidney. Ureters were obstructed for 24 h in all experiments, and BUO was either not released or released for 24 or 48 h or 7 days. Each group of experimental rats were matched with sham-operated controls. One kidney was used for membrane fractionation and immunoblotting, whereas the contralateral was fixed for immunocytochemistry. Immunoblotting demonstrated a significant reduction in AQP-2 expression in inner medullar during 24 h of BUO to 26 +/- 8% (P < 0.001). Release of BUO was associated with immediate onset of a predominant osmotic-dependent polyuria. Forty-eight hours after release of BUO, the reduction in AQP-2 expression persisted (19 +/- 8%, P < 0.001), concurrent with a marked nonosmotic postobstructive polyuria, as determined by a significant reduction in free-water clearance (-50 +/- 7 vs. -85 +/- 10 microliters.min-1.kg-1, P < 0.05). Immunofluorescence and immunoelectron microscopy confirmed the reduced levels of AQP-2 in collecting duct principal cells. Seven days after release, the renal excretion of water and electrolytes had almost normalized. However, the downregulation of AQP-2 was not partly reversed (49 +/- 14%, P < 0.001), and, consistent with this, the urinary concentrating capacity was significantly reduced 7 days after release to a 18-h period of thirst. This strongly suggests that the persistent downregulation of AQP-2 is the cause of the slow recovery in concentration capacity. In conclusion, BUO and release of BUO were associated with a marked reduction in expression of AQP-2, coincident with the development and maintenance of postobstructive polyuria. Thus reduced AQP-2 levels may represent an important factor in the slow recovery from postobstructive diuresis.