Extracellular pH affects phosphorylation and intracellular trafficking of AQP2 in inner medullary collecting duct cells

Kidney collecting duct cells are continuously exposed to the changes of extracellular pH (pH_e). We aimed to study the effects of altered pHe on desmopressin (dDAVP)-induced phosphorylation (Ser²⁵⁶, Ser²⁶¹, Ser²⁶⁴, and Ser²⁶⁹) and apical targeting of aquaporin-2 (AQP2) in rat kidney inner medullary collecting duct (IMCD) cells. When freshly prepared IMCD tubule suspensions exposed to HEPES buffer with pH 5.4, 6.4, 7.4, or 8.4 for 1 h were stimulated with dDAVP (10^-10 M, 3 min), AQP2 phosphorylation at Ser²⁵⁶, Ser²⁶⁴, and Ser²⁶⁹ was significantly attenuated under acidic conditions. Next, IMCD cells primary cultured in transwell chambers were exposed to a transepithelial pH gradient for 1 h (apical pH 6.4, 7.4, or 8.4 vs. basolateral pH 7.4 and vice versa). Immunocytochemistry and cell surface biotinylation assay revealed that exposure to either apical pH 6.4 or basolateral pH 6.4 for 1 h was associated with decreased dDAVP (10^-9 M, 15 min, basolateral)-induced apical targeting of AQP2 and surface expression of AQP2. Fluorescence resonance energy transfer analysis revealed that the dDAVP (10^-9 M)-induced increase of PKA activity was significantly attenuated when LLC-PK1 cells were exposed to pH_e 6.4 compared with pHe 7.4 and 8.4. In contrast, forskolin (10^-7 M)-induced PKA activation and dDAVP (10^-9 M)-induced increases of intracellular Ca²⁺ were not affected. Taken together, dDAVPinduced phosphorylation and apical targeting of AQP2 are attenuated in IMCD cells under acidic pH_e, likely via an inhibition of vasopressin V2 receptor-G protein-cAMP-PKA actions.