Urinary excretion of the aquaporin-2 water channel exaggerated in pathological states of impaired water excretion.
OBJECTIVE
The present study was undertaken to determine whether the hydro-osmotic action of arginine vasopressin (AVP) is exaggerated in pathological states of impaired water excretion by measuring urinary excretion of the aquaporin-2 (AQP-2) water channel.


METHODS
Eighteen hyponatraemic patients with impaired water excretion and 12 control subjects were studied during an acute oral water load (20 ml/kg body weight).


RESULTS
In the patient group plasma AVP levels were 1.6 pmol/l, relatively high compared to plasma osmolality of 279.8 mmol/kg. Urinary excretion of AQP-2 under ad libitum water drinking was 41.1 fmol/micromol creatinine in the patient group, a value significantly greater than that of 21.7 fmol/micromol creatinine in the control subjects. The acute water load verified the impairment in water excretion in the patient group, as the excretion of the water load was only 28.2% (control, 77.3%, P < 0.001) and the minimum urinary osmolality was as high as 437.3 mmol/kg (control, 122.9 mmol/kg, P < 0.001). Also, the minimum urinary excretion of AQP-2 was significantly greater in the patient group than that in the control. There was a positive correlation between plasma AVP levels and urinary excretion of AQP-2 in the control subjects (r = 0.56, P < 0.01). In contrast, the urinary excretion of AQP-2 was exaggerated compared to the respective plasma AVP levels in the patient group, and thus the positive correlation disappeared.


CONCLUSIONS
These results indicate that hydroosmotic action of AVP is exaggerated more than that expected from plasma AVP levels in pathological states of impaired water excretion, with non-suppressible, but normal, arginine vasopressin levels in spite of the hypo-osmotic condition.
Saito
T
T
Division of Endocrinology and Metabolism, Department of Medicine, Jichi Medical School, Tochigi, Japan.
Higashiyama
M
M
Nakamura
T
T
Kusaka
I
I
Nagasaka
S
S
Saito
T
T
Ishikawa
S
S
D000368:Aged; D051399:Aquaporin 2; D051403:Aquaporin 6; D020346:Aquaporins / Q000652:urine*; D001127:Arginine Vasopressin / Q000502:physiology; D001834:Body Water / Q000378:metabolism*; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D007010:Hyponatremia / Q000378:metabolism* / Q000652:urine; D016014:Linear Models; D008297:Male; D008875:Middle Aged; D009994:Osmolar Concentration
C496023:AQP2 protein, human; D051399:Aquaporin 2; D051403:Aquaporin 6; D020346:Aquaporins; D001127:Arginine Vasopressin