In mpkCCD cells, long-term regulation of aquaporin-2 by vasopressin occurs independent of protein kinase A and CREB but may involve Epac.

Urine concentration involves the hormone vasopressin (AVP), which stimulates cAMP production in renal principal cells, resulting in translocation and transcription of aquaporin-2 (AQP2) water channels, greatly increasing the water permeability, leading to a concentrated urine. As cAMP levels decrease shortly after AVP addition, whereas AQP2 levels still increase and are maintained for days, we investigated in the present study the mechanism responsible for the AQP2 increase after long-term 1-desamino-8-d-arginine vasopressin (dDAVP) application using mouse collecting duct (mpkCCD) cells. While 30 min of dDAVP incubation strongly increased cAMP, cAMP was lower with 1 day and was even further reduced with 4 days of dDAVP, although still significantly higher than in control cells. One day of dDAVP incubation increased AQP2 promoter-dependent transcription, which was blocked by the protein kinase A (PKA) inhibitor H89. Moreover, phosphorylation of the cAMP-responsive element binding protein (CREB) and CRE-dependent transcription was observed after short-term dDAVP stimulation. With 4 days of dDAVP, AQP2 transcription remained elevated, but this was not blocked by H89, and CRE-dependent transcription and CREB phosphorylation were not increased. Exchange factor directly activated by cAMP (Epac) 1 and 2 were found to be endogenously expressed in mpkCCD cells. Application of dDAVP increased the expression of Epac1, while Epac2 was reduced. Incubation with a specific Epac activator after dDAVP pretreatment increased both AQP2 abundance and transcription compared with cells left unstimulated the last day. In conclusion, the PKA-CRE pathway is involved in the initial rise in AQP2 levels after dDAVP stimulation but not in the long-term effect of dDAVP. Instead, long-term regulation of AQP2 may involve the activation of Epac.