HYPERHYDRATING EFFECT OF ACUTE ADMINISTRATION OF ANGIOTENSIN-II IN RATS

Water intake, urine output, and fluid exchange (water intake less urine output) were measured in rats at hourly intervals for 7 hours and at 24 hours following acute administration of angiotensin II (AII, 200-mu-g/kg SC). AII induced the expected abrupt increase in water intake and a more gradual increase in urine output. The change in fluid exchange (fluid exchange of the AII-treated group less fluid exchange of controls) became positive within the first hour after treatment with AII, decreased linearly with time, and reached 0 at approximately 10 to 12 hours after treatment with AII. When AII was administered intracerebroventricularly (50 ng), similar results were observed. In this case, the change in fluid exchange (DELTA-F) reached 0 in about 6 hours. Imposition of a water load (1% of body weight, IP) on the group receiving AII SC failed to affect the time required for DELTA-F to reach 0 if the water load was disregarded. However, inclusion of the load as a part of intake extended the time the rats remained in positive fluid balance beyond that of the nonloaded, AII-treated control group. In the case of the larger water load (3% of body weight, IP), DELTA-F returned to that of controls in about 4 to 5 hours if the water load was disregarded. However, inclusion of the load as part of intake extended the period of hyperhydration well beyond that of both the nonloaded, AII-treated group and the AII-treated group given the 1% load. The results indicate that an acute injection of AII, either peripherally or centrally, can induce a state of hyperhydration in rats that lasts for approximately 6 to 12 hours, depending on the route of administration of AII. Imposition of a water load in combination with acute administration of AII can extend the duration of hyperhydration. The results are also of interest because they provide a simple method for assessment of the duration of hyperhydration following administration of AII and other dipsogenic agents.