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 t wo ways to account for overall control of collecting duct water permea bility. First, vasopressin has a short-term effect in triggering trans location of aquaporin-2-containing intracytoplasmic vesicles to the ap ical plasma membrane, thus increasing principal cell water permeabilit y. Second, vasopressin has a long-term effect in increasing the abunda nce 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 severa l disorders of water balance, including central diabetes insipidus, co ngenital nephrogenic diabetes insipidus, acquired diabetes insipidus, syndrome of inappropriate antidiuretic hormone secretion, and several extracellular fluid volume expanded states.