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We tested the hypothesis that pressor infusions of angiotensin II (AII) could stimulate the sympathetic nervous system as reflected by norepinephrine (NE) spillover in humans. AII was infused at 5 ng/kg/min in six healthy volunteers, with vehicle and phenylephrine infusions as controls, on 3 separate days. Heart rate, mean arterial pressure, plasma NE, NE clearance, and NE spillover were assessed before and after 30-minute infusions of AII, vehicle, or phenylephrine in the supine position and then after 15 minutes of head-up and 15 minutes of head-down tilt. Both AII and phenylephrine raised mean arterial pressure (88 +/- 9.6 to 103 +/- 14 mm Hg, p less than 0.001, and 91 +/- 7.6 to 104 +/- 9.2 mm Hg, p less than 0.001, respectively), whereas heart rate fell only with phenylephrine (60 +/- 6 to 51 +/- 6.3 beats/min, p less than 0.001). Neither plasma NE nor NE spillover was affected by either infusion, and NE clearance declined slightly with both. No changes occurred in any variable during vehicle infusions in the supine position. During upright tilt, NE spillover increases were attenuated by both AII and phenylephrine while NE clearance changes were slightly greater, leaving plasma NE increases similar on each day. During head-down tilt, NE and NE spillover declined comparably on each study day.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a pressor infusion of angiotensin II on sympathetic activity and heart rate in normal humans.