Suppressed urinary excretion of aquaporin-2 in an infant with primary polydipsia.

We observed severe overhydration in an 18-month-old Japanese girl with primary polydipsia. The secretion of antidiuretic hormone (ADH) was decreased, and urinary excretion of aquaporin-2, a vasopressin-sensitive water channel protein, was suppressed under basal conditions, but the response of aquaporin-2 to ADH was essentially preserved. These findings suggest that the water channel itself was intact and that overhydration resulting from polydipsia was responsible for the decreased ADH secretion and suppression of the water channel protein.