[Urinary excretion of aquaporin-2 in water metabolism disorders]

Water metabolism plays an essential role in the homeostasis of body fluids in animals and humans. It is regulated by arginine vasopressin (AVP), renal function and water drinking. Disorders of water metabolism result in an increase or decrease in a body water or fluid, which manifest as hyponatremia, hypernatremia, polyuria, dehydration or edema. In the pathogenesis of such pathological conditions AVP is either directly or indirectly involved. Aquaporin-2 (AQP-2) is an AVP-dependent water channel in renal collecting duct cells. Approximately 3% of AQP-2 is excreted into the urine, which is measurable by RIA or Western blot using a specific antibody against AQP-2. There was positive relationship between urinary excretion of AQP-2 (UAQP-2) and plasma AVP levels in normal subjects. UAQP-2 varied in a wide range under ad libitum water drinking. The level of UAQP-2 was one eighth less in patients with central diabetes insipidus than in normal subjects, and it was 2.8-fold greater in patients with water retention. A hypertonic saline infusion test manifested the difference in the UAQP-2 response to an increase in plasma osmolality between the patients with central diabetes insipidus and the normal subjects. Acute oral water load clarified the impaired water excretion and the persistent elevation of UAQP-2 in patients with water retention. Such increased UAQP-2 was linked to nonsuppressible levels of plasma AVP despite hypoosmolality. These results indicate that UAQP-2 is a useful marker to diagnose disorders of water metabolism.