Prolonged blood pressure elevation following continuous infusion of angiotensin II-a baroreflex study in healthy humans

Abstract

ANG II interacts with the sympathetic nervous system at central nervous blood pressure-regulating structures, including the baroreflex. It is unknown whether prolonged BP elevation mediated by high ANG II plasma levels could induce a persistent shift of the central nervous baroreflex setpoint, lasting beyond the short ANG II plasmatic half time of a few seconds, thereby consolidating elevated BP and/or increased SNA in healthy humans. In a blinded crossover design, ANG II or placebo (saline) was infused for a 6-h period in 12 resting normotensive students (6 males, 6 females) raising BP to borderline hypertensive levels. Between 60 and 120 min after the infusion period, muscle sympathetic nerve activity (MSNA) was assessed microneurographically and correlated with oscillometric BP measurements and heart rate at supine rest (baseline) and during pharmacologic baroreceptor challenge. Infusion of ANG II increased BP to borderline-hypertensive levels, as intended, whereas heart rate remained unaltered. At baroreflex assessment (i.e., 60-120 min after end of infusion period), systolic BP was significantly higher compared with placebo (Δ8.4 ± 3.1 mmHg; P < 0.05), whereas diastolic values were nearly equal between conditions. Baseline MSNA was neither decreased nor increased, and baroreflex sensitivity to vasoactive drug challenge was not altered. Our results show that elevation of ANG II plasma levels over 6 h was able to increase systolic, but not diastolic, BP far beyond blood-mediated ANG II effects. MSNA or heart rate did not counter-regulate this BP elevation, indicating that ANG II had sustainably reset the central nervous BP threshold of sympathetic baroreflex function to accept elevated BP input signals without counter-regulatory response.