Low aquaporin-2 levels in polyuric DI +/+ severe mice with constitutively high cAMP-phosphodiesterase activity

In the renal collecting duct, vasopressin acutely activates cAMP production , resulting in trafficking of aquaporin-2 water channels (AQP2) to the apic al plasma membrane, thereby increasing water permeability. This acute respo nse is modulated by long-term changes in AQP2 expression. Recently, a cAMP- responsive element has been identified in the AQP2 gene, raising the possib ility that changes in cAMP levels may control AQP2 expression. To investiga te this possibility, we determined AQP2 protein levels in a strain of mice, DI +/+ severe (DI), which have genetically high levels of cAMP-phosphodies terase activity, and hence low cellular cAMP levels, and severe polyuria. S emiquantitative immunoblotting of membrane fractions prepared from whole ki dneys revealed that AQP2 levels in DI mice were only 26 +/- 7% (+/-SE) of t hose in control mice (n = 10, P < 0.01). In addition, semiquantitative Nort hern blotting revealed a significantly lower AQP2 mRNA expression in kidney s from DI mice compared with control mice (43 +/- 6% vs. 100 +/- 10%; n = 6 in each group, P < 0.05). AQP3 levels were also reduced. The mice were pol yuric and urine osmolalities were accordingly substantially lower in the DI mice than in controls (496 +/- 53 vs. 1,696 +/- 105 mosmol/kgH(2)O, respec tively). Moreover, there was a linear correlation between urine osmolalitie s and AQP2 levels (P < 0.05). Immunoelectron microscopy confirmed the marke dly lower expression of AQP2 in collecting duct principal cells in kidneys of DI mice and, furthermore, demonstrated that AQP2 was almost completely a bsent from the apical plasma membrane. Thus expression of AQP2 and AQP2 tra fficking were severely impaired in DI mice. These results are consistent wi th the view that in vivo regulation of AQP2 expression by vasopressin is me diated by cAMP.