Water movement across cell membranes through aquaporin water channels creates osmotic equilibrium between extracellular and intracellular fluid compartments. Plasma osmolality is tightly regulated by the kidneys and brain through the process of osmoregulation. The antidiuretic hormone, arginine vasopressin (AVP), is normally released from the posterior pituitary in response to increased osmolality or decreased intravascular volume. Defects in the synthesis or release of AVP result in AVP deficie