In the renal collecting duct (CD) the major physiological role of aldosterone is to promote Na+ reabsorption. In addition, aldosterone may also influence CD water permeability elicited by vasopressin (AVP). We have previously shown that endogenous expression of the aquaporin-2 (AQP2) water channel in immortalized mouse cortical CD principal cells (mpkCCDC14) grown on filters is dramatically increased by administration of physiological concentrations of AVP. In the present study, we investigated the influence of aldosterone on AQP2 expression in mpkCCDC14 cells by RNase protection assay and Western blot analysis. Aldosterone reduced AQP2 mRNA and protein expression when administered together with AVP for short periods of time (< or =24 h). For longer periods of time, however, aldosterone increased AQP2 protein expression despite sustained low expression levels of AQP2 mRNA. Both events were dependent on mineralocorticoid receptor occupancy because they were both induced by a low concentration of aldosterone (10-9 m) and were abolished by the mineralocorticoid receptor antagonist canrenoate. Inhibition of lysosomal AQP2 protein degradation increased AQP2 protein expression in AVP-treated cells, an effect that was potentiated by aldosterone. Finally, both aldosterone and actinomycin D delayed AQP2 protein decay following AVP washout, but in a non-cumulative manner. Taken together, our data suggest that aldosterone tightly modulates AQP2 protein expression in cultured mpkCCDC14 cells by increasing AQP2 protein turnover while maintaining low levels of AQP2 mRNA expression.