Previous studies have suggested that mineralocorticoids are needed for a normal action of vasopressin on collecting duct osmotic water permeability. However, the mechanisms behind this are unknown. To investigate if aldosterone-receptor blockade influences vasopressin type 2 receptor (V(2))-mediated renal water reabsorption and the renal expression of the vasopressin-regulated water channel aquaporin-2 (AQP2), rats were treated with the aldosterone-receptor antagonist canrenoate (20 mg/day iv) for 4 wk. Daily urine flow was increased significantly by 44%, and urine osmolality was decreased by 27% in canrenoate-treated rats. Acute V(2)-receptor blockade (OPC-31260, 800 microgram. kg(-1). h(-1)) was performed under conditions in which volume depletion was prevented. In control rats, OPC-31260 induced a significant increase in urine flow rate (V, +25%) and free water clearance (C(H(2)O), -29%). In canrenoate-treated rats, the effect of OPC-31260 was significantly reduced, and semiquantiative immunoblotting demonstrated a significant reduction (45%) in AQP2 expression. Because rats with common bile duct ligation (CBL) have a reduced vasopressin-mediated water reabsorption compared with normal rats (V: -24%; C(H(2)O): -28%, and 86% downregulation of AQP2), the effect of canrenoate combined with OPC-31260 was tested. Canrenoate treatment of CBL rats significantly increased daily urine flow, decreased urine osmolality, and impaired the aquaretic response to OPC-31260 (V: -23%; C(H(2)O): -31%) with maintained suppression of the renal AQP2 expression. Thus canrenoate treatment of normal and CBL rats showed 1) increased urine production, 2) reduced aquaretic effect of acute V(2)-receptor blockade, and 3) a marked reduction in AQP2 expression. This strongly supports the view that aldosterone plays a significant role for vasopressin-mediated water reabsorption.