Aquaporins, also known as water channels, form pores in cell membranes and selectively transport water in and out of the cell. Aquaporins are involved in regulation of water balance and blood pressure, and thirteen different isoforms have been found in mammals. Aquaporin 2 (AQP2) is located in the collecting duct of the kidney, and is regulated by the peptide hormone vasopressin. AQP2 expression is increased in conditions where there is water retention, such as pregnancy and congestive heart failure, and mutations of AQP2 are associated with nephrogenic diabetes insipidus. Also, lithium treatment, often administered for bipolar disorder, can cause acquired diabetes insipidus by decreasing AQP2 expression.

The inventors have developed rabbit polyclonal antibodies directed against a peptide sequence in the C-terminal region of AQP2 (LKGLEPDTDWEEREVRRRQ). The sequence is upstream of phosphorylation sites in this region, and consequently the antibodies recognize both unphosphorylated and phosphorylated AQP2. The sequence is identical in human, rat, mouse, cow, and sheep.