Body water balance is regulated in the kidney via the vasopressin (AVP) regulated water channel aquaporin-2 (AQP2) expressed in the connecting tubule (CNT) and the collecting duct (CD). Although crucial for the urinary concentrating mechanism, the relative role of AQP2 in the CNT and CD are not fully understood. To study the role of AQP2 in the CNT we generated a mouse model with a CNT-specific knock-out of AQP2. Confocal immunofluorescence microscopy of kidney sections demonstrated an absence of AQP2 immunolabeling in the CNT of the knock-out animals. 24 hour urine output was significantly increased (KO: 3.0 ± 0.3 mL/20 g BW versus WT: 1.9 ± 0.3 mL/20 g BW) and urine osmolality decreased (KO: 1179 ±107 mOsm/kg versus WT: 1790 ±146 mOsm/kg) in knock-out mice compared to controls. After a 24 hour water restriction, urine volume was decreased in both groups, while urine osmolality was increased. Urine osmolality was still significantly lower in knock-out mice compared to controls (KO 2087 ±169 mOsm/kg versus WT: 2678 ±144 mOsm/kg), although 24 hour urine output between groups was not significantly different. A significant difference in urine osmolality between groups before intraperitoneal injection of dDAVP (KO: 873 ± 129 versus WT 1387 ± 163 mOsm/kg) was no longer observed 2 hours after injection, where urine osmolality had increased significantly from baseline in both groups (KO: 2944 ±41; WT 3133 ±66 mOsm/kg). Immunoblotting demonstrated a significant decrease in AQP2 abundance in cortical kidney fractions with no compensatory changes in the AVP-regulated transporters NKCC2, AQP3 and AQP4. In conclusion, we have demonstrated that deletion of AQP2 from the CNT results in a mild urinary concentrating defect, without diminished maximal urinary concentrating ability. Our studies suggest that the CNT plays a minor role in regulation of whole body water balance, or that dysfunction of the CNT can be compensated for by increased water reabsorption in the collecting duct.
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American Society of Nephrology 44th Annual Meeting & Scientific Exposition, 2011