PURPOSE: Impaired concentrating capacity has been demonstrated in human refluxing kidneys and in kidneys of growing animals with experimental congenital vesicoureteral reflux. Aquaporin water channel proteins are the principal mediators of water homeostasis in the kidney. We examined the impact of experimental congenital vesicoureteral reflux on renal aquaporin protein expression. MATERIALS AND METHODS: Vesicoureteral reflux was surgically induced bilaterally in male fetal sheep at 95 days of gestation (term 140 days). Following birth animals received antibiotic prophylaxis, and reflux grades were assessed by fluoroscopic voiding cystogram. After sacrifice at age 6 months 1 kidney of each animal (5 refluxing kidneys, and 3 from normal age and sex matched controls) was retrieved, frozen in liquid nitrogen and used for immunoblotting. The second kidney was perfusion fixed and processed for immunohistochemical analysis. RESULTS: Semiquantitative immunoblotting of inner medulla showed that vesicoureteral reflux was associated with a marked down-regulation in expression of aquaporin 1 (arbitrary units 7.0 +/- 4.3 vs 22.5 +/- 2.8, p <0.05) and aquaporin 2 (5.7 +/- 5.1 vs 24.8 +/- 3.8, p <0.05), which was confirmed by immunocytochemical analysis. Dot blot analysis revealed a homogeneous down-regulation of aquaporin 2 expression throughout the refluxing kidney to 0.029 vs 0.1 in normal kidneys (p = 0.026). CONCLUSIONS: Progressively impaired renal concentrating capacity induced by experimental fetal reflux is associated with decreased levels of renal aquaporin 1 and 2 expression. This long-term down-regulation of aquaporin 1 and 2 provides important molecular evidence for a defect in resorptive capacity of water induced by vesicoureteral reflux.