AQP2 and chronic kidney disease: Immunocytochemistry and immunoelectron microscopy revealed that the level of AQP2 in the principal cells was reduced, implying that the lower level of AQP2 could be a significant factor contributing to the changes in the water permeability of the collecting duct and the decrement in vasopressin response in patients with CKD [12], which appears to be strongly found in a manner similar to our immunohistochemistry.