Urinary excretion of aquaporin-2 water channel differentiates psychogenic polydipsia from central diabetes insipidus

The present study was undertaken to determine whether urinary excretion of aquaporin-2 (AQP-2) water channel under ad libitum water intake is of value to differentiate polyuria caused by psychogenic polydipsia from central di abetes insipidus. A 30-min urine collection was made at 0900 h in 3 groups of: 11 patients with central diabetes insipidus (22-68 yr old), 10 patients with psychogenic polydipsia (28-60 yr old), and 15 normal subjects (21-38 yr old). In the patients with central diabetes insipidus, the plasma argini ne vasopressin level was low despite hyperosmolality, resulting in hypotoni c urine. Urinary excretion of AQP-2 was 37 +/- 15 fmol/mg creatinine, a val ue one-fifth less than that in the normal subjects. In the patients with ps ychogenic polydipsia, plasma arginine vasopressin and urinary osmolality we re as low as those in the patients with central diabetes insipidus. However , urinary excretion of AQP-2 of 187 +/- 45 fmol/mg creatinine was not decre ased, and its excretion was equal to that in the normal subjects. These res ults indicate that urinary excretion of AQP-2, under ad libitum water drink ing, participates in the differentiation of psychogenic polydipsia from cen tral diabetes insipidus.