Plasma angiotensin II levels in hypoxic and hypovolemic stress in unanesthetized rabbits.

Plasma levels of angiotensin II were determined by radioimmunoassay in unanesthetized white rabbits exposed to acute hypoxia (FIO2 10% for 10 min), chronic hypoxia (0.5 atm up to 16 days), or hypovolemic stress (bleeding 20 ml/kg). Angiotensin II levels significantly decreased after 10 min of acute hypoxia in normal rabbits and significantly increased when the same procedure was applied to animals previously exposed to hypoxia by 6-8 days of permanence in the hypobaric chamber or sodium deprivation. Chronic hypoxia resulted in a temporary increase of angiotensin II already evident on the 3rd day, but maximal at the 9th day with return to normal values within 16 days. Hypovolemic stress resulted in the expected rise of angiotensin II levels 10 min postbleeding both in normal and acclimatized rabbits. The response of the renin-angiotensin-aldosterone system to hypoxic and hypovolemic stress is different. The direction and magnitude of the response to hypoxia depends on the underlying state of activation of the system and the cardiovascular condition of the animal at the time of hypoxic stress.