CHRONIC ANG-II INFUSION AND REFLEX CONTROL OF NOREPINEPHRINE AND CORTICOSTERONE IN CONSCIOUS RABBITS

The hypothesis that long-term increases in angiotensin II (ANG II) pro duce pressure-independent resetting of baroreflex control of the sympa thetic nervous system and the hypothalamic-pituitary-adrenal axis was tested in rabbits by determining the effect of chronic ANG II infusion on reflex relationships between mean arterial pressure (MAP) and plas ma concentrations of norepinephrine (NE) and corticosterone (CS). Afte r 2 wk, ANG II increased MAP from 61 +/- 1 to 99 +/- 2 mmHg (P < 0.05) without altering heart rate or plasma NE concentration, but increased CS from 9.8 +/- 1.3 to 29.5 +/- 13.7 ng/ml (P < 0.05). Heart rate, NE , and CS baroreflex curves were all reset to a higher pressure level ( P < 0.05) after 24 h, 1 wk, and 2 wk of ANG II. Forty minutes after st opping ANG II on the same days, MAP decreased, and curves were shifted back toward control (P < 0.05), indicating that ANG II was required f or the resetting. Two findings suggest that the resetting action of AN G II is distinct from the presser effect. First, although stopping ANG II reversed the hypertension as it reversed the resetting, reversal o f the hypertension instead by prolonged infusion of nitroprusside alon g with ANG II did not have the same effect. Second, NE and heart rate baroreflex curves returned toward preinfusion positions after stopping ANG II (P < 0.05), even when the hypertension was nearly maintained b y phenylephrine infusion. In conclusion, chronic increases in ANG II m ay have a global baroreflex resetting effect by a mechanism that is in part independent of the hypertension.