Role of aquaporins in water balance disorders.

The aquaporins are a recently recognized family of water channels that mediate water transport in kidney and in other organs. Aquaporin-2, 'vasopressin-regulated water channel', is regulated by vasopressin in two ways to account for overall control of collecting duct water permeability. First, vasopressin has a short-term effect in triggering translocation of aquaporin-2-containing intracytoplasmic vesicles to the apical plasma membrane, thus increasing principal cell water permeability. Second, vasopressin has a long-term effect in increasing the abundance of aquaporin-2 in collecting duct principal cells, increasing the maximal attainable water permeability. Using animal models, defects in these control mechanisms have been shown to be associated with several disorders of water balance, including central diabetes insipidus, congenital nephrogenic diabetes insipidus, acquired diabetes insipidus, syndrome of inappropriate antidiuretic hormone secretion, and several extracellular fluid volume expanded states.