Effect of chronic intracerebroventricular angiotensin II infusion on vasopressin release in rats

Angiotensin II (AII) was infused into the lateral cerebral ventricle of rats given water, isotonic saline, or hypertonic saline ad libitum, or 40 ml water/day. Fluid intake, change in body weight, plasma [Na+], and plasma and pituitary arginine vasopressin (AVP) levels were measure. Isotonic saline or AII (1 micrograms/microliter saline) was infused at 1 microliter/h for 5 days using osmotic minipumps. AII increased fluid intake of rats given isotonic saline to drink; they consumed an average of 269 +/- 25 ml/day on day 5. AII infusions in rat given water or isotonic saline to drink decreased plasma [Na+] with no changes in plasma or pituitary AVP. However, in rats given hypertonic saline, plasma [Na+] remained at control levels while plasma AVP increased. In water-restricted rats, the effects of AII were intermediate: a small decrease in plasma [Na+] and a small increase in plasma AVP. From these results, it is suggested that although acute AII administration elicits AVP release, this effect diminishes during chronic AII infusion, coincident with reduced plasma [Na+].