Vasopressin V₂-receptor-dependent regulation of AQP2 expression in Brattleboro rats

The role of AVP-V₂ receptor (AVP-V₂R)-dependent regulation of aquaporin-2 (AQP2) expression was evaluated in vasopressin-deficient Brattleboro (BB) rats. AQP2 levels were relatively high in BB rats (52 ± 8% of levels in Wistar rats), and treatment with the AVP-V₂R antagonist SR-121463A (0.8 mg/day) for 48 h was associated with 1) increased urine output (170 ± 9%), 2), reduced AQP2 protein levels (42 ± 10% in whole kidney and 53 ± 8% in inner medulla), and 3) reduced AQP2 mRNA levels (36 ± 7%). In addition, the levels of AQP2 phosphorylated in the protein kinase A (PKA) consensus site (Ser²⁵⁶ of AQP2) was reduced to 3 ± 1% of control levels. Lithium (Li) treatment of BB rats for 1 mo, known to reduce adenylyl cyclase (AC) activity, downregulated AQP2 protein levels (15 ± 6%) and increased urine output (220%). Downregulation of AQP2 expression in response to SR-121463A or Li treatment indicates that AQP2 expression in BB rats depends in part on activation of AVP-V₂Rs and that the signaling cascade(s) involves AC and hence cAMP. Complete water restriction of BB rats produced only a small increase in AQP2 mRNA (235 ± 33%) and AQP2 protein (156 ± 22%) levels. Immunoelectron microscopy confirmed the increase in AQP2 abundance but revealed no change in AQP2 apical plasma membrane labeling in response to thirsting. In conclusion, the expression and phosphorylation of AQP2 in BB rats are in part dependent on AVP-V₂R signaling, and AVP-V₂-mediated regulation of AQP2 trafficking and expression is effectively decoupled in BB rats, indicating differences in AVP-V₂R-mediated regulation of AQP2 trafficking and expression.