Angiotensin II antagonists in dehydrated rabbits without baroreceptor reflexes

Blood pressure effects of angiotensin II antagonists were studied in sham-operated and baroreceptor-denervated rabbits in the normal water-replete state or after 6 days of water deprivation (dehydrated). Experiments were performed in awake rabbits. Dehydrated rabbits had significantly higher plasma sodium concentrations, hematocrits, and plasma renin activities, but lower plasma potassium concentrations and body weights than water-replete rabbits. Administration of angiotensin II antagonists caused a significant decrease in mean arterial pressure in dehydrated rabbits (-16 mmHg in sham-dehydrated and -19 mmHg in denervated-dehydrated) but not in water-replete ones, whether the baroreceptor reflexes were intact or not (-1 mmHg in sham replete and -4 mmHg in denervated replete). The open-loop feedback gain of the renin-angiotensin system in blood pressure control was calculated as -1.6. The results demonstrate an important role of angiotensin II in blood pressure regulation during the high-renin, dehydrated state, but not during the normal renin, water-replete state. Abolishment of baroreceptor reflexes did not unmask an important role of normal levels of angiotensin II in blood pressure regulation.