URINARY-EXCRETION OF AQUAPORIN-2 WATER CHANNEL PROTEIN IN HUMAN AND RAT

Previous studies by the authors demonstrated that the response of urin ary aquaporin-2 (AQP2) excretion to dDAVP (deamino-8-D-arginine vasopr essin) infusion is an index of vasopressin action on the kidney (N Eng l J Med 332: 1540-1545, 1995). In the study presented here, the charac teristics of urinary excretion of AQP2 were examined further. An RIA s uitable for AQP2 in the urine was established. Relatively high concent rations of detergent and bovine serum albumin in the RIA buffer allowe d analysis of urine samples with a wide range of concentrations and in creased the sensitivity of the assay. AQP2 in the urine existed as a h igh molecular weight form of approximately 190 kD by HPLC analysis. Th e mean urinary AQP2 concentration corrected for creatinine in spot uri ne samples of healthy subjects who voided in the morning was 1081 +/- 699 fmol/mg creatinine (mean +/- SD, n = 208). The amount of daily exc retion of AQP2 in the urine was the same in men and women. Urinary AQP 2 content was not affected by age of the subjects and showed a positiv e correlation with urine osmolality. Finally, the fraction of AQP2 exc reted in the urine compared with whole kidney content was determined i n the rat. Approximately 3% of AQP2 in the kidney was excreted daily, and this fraction did not change when rats were dehydrated for 3 d. Th ese data demonstrate the necessity of establishing well-designed proto cols to use urinary AQP2 as a marker of AVP action.