The antidiuretic hormone arginine vasopressin (AVP) regulates water reabsorption in renal principal cells by inducing a cAMP/protein kinase A-dependent translocation of water channels [aquaporin-2 (AQP2)] from intracellular vesicles into the apical cell membranes. Using primary cultured rat inner medullary collecting duct (IMCD) cells, it has been shown that AQP2 translocation in response to AVP stimulation occurs only if protein kinase A (PKA) is anchored to PKA anchoring proteins (AKAPs), which are present in various subcellular compartments. The identity of the AKAPs involved has not yet been elucidated. One potential candidate is a new splice variant of AKAP18, namely AKAP18 delta.