Vasopressin responses to hypoxia in conscious rats: interaction with water restriction

The purpose of this study was to determine the effect of water restriction on the vasopressin response to hypoxia in conscious Long-Evans rats. Rats were prepared with chronic indwelling femoral artery and vein catheters 1 week before experimentation. At 24 h before the first blood sample, the supply of drinking water was maintained ad libitum (water replete) or removed (water deplete). At 24 h, a control blood sample was taken and then normoxia (21% O2) was maintained or hypoxia (10% O2) induced. Additional blood samples were taken at 1, 18 and 24 h. All blood samples (2.5 ml) were simultaneously replaced with donor blood to maintain isovolaemia. Hypoxia led to a very small and transient increase in vasopressin in the water-replete rats. The combination of hypoxia and water restriction led to a greatly augmented vasopressin response at 1 h (60 +/- 16 pmol/l); this response was also not sustained. Additional non-cannulated rats were exposed to 24 h of normoxia or hypoxia with or without water available ad libitum and posterior pituitaries were collected after decapitation for measurement of vasopressin content. Water restriction, hypoxia and water restriction plus hypoxia all led to decreased pituitary vasopressin content. We conclude that the vasopressin response to hypoxia in conscious rats is small and transient, and that concomitant water restriction augments the vasopressin response to acute but not chronic hypoxia.