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

We observed severe overhydration in an 18-month-old Japanese girl with prim ary 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 aquapo rin-2 to ADH was essentially preserved. These findings suggest that the wat er channel itself was intact and that overhydration resulting from polydips ia was responsible for the decreased ADH secretion and suppression of the w ater channel protein.