Aquaporin-2 (AQP-2) has been shown to be a vasopressin-sensitive water channel in collecting duct (CD) cells of the kidney. To prove the role of the vasopressin V2 receptor (V2R) in the regulation of intracellular AQP-2 shuttling, we examined the acute effects of vasopressin and V2R antagonist on the distribution of AQP-2 in the cells. Normal Wistar rats were given continuous infusions of vasopressin, vasopressin V2R antagonist (OPC31260), or both. The kidneys were then processed for immunofluorescent studies with an affinity-purified specific antibody to AQP-2. One hour after the infusion of the V2R antagonist, AQP-2 staining was diffusely distributed in the CD cells from the cortex to the inner medulla. This tendency was not changed by the concomitant infusion with vasopressin. Vasopressin infusion without antagonist, however, induced intensified AQP-2 staining of the apical membrane in the CD cells. The ratio of the fluorescence intensity of the apical to subapical region was determined by confocal laser microscopy. In the inner medulla, this ratio was significantly increased in the vasopressin treatment group (2.26 +/- 0.76) as compared to the V2R antagonist group (1.03 +/- 0.34) and the combined treatment group (0.84 +/- 0.43). The increase in the ratio was also demonstrated in the cortex and the outer medulla in the vasopressin-treated group. In addition, Northern blotting studies clearly revealed that mRNA of AQP-2 in the vasopressin-treated group was increased when compared to the combined treatment animals. Our present results reveal that localization and gene expressions of AQP-2 are acutely regulated via vasopressin V2R.