MODULATION OF CARDIAC AUTONOMIC CONTROL IN HUMANS BY ANGIOTENSIN-II

Angiotensin II (Ang II) exerts an inhibitory action on vagal activity in animals and may also facilitate sympathetic activity. The object of this study was to compare autonomic activity resulting from equivalen t steady-state baroreflex activation during intravenous Ang II infusio n with that resulting from a control infusion of phenylephrine. Eight healthy subjects aged 22 to 34 years were studied in a single-blind, r andomized, prospective crossover study. Autonomic activity was determi ned by computer analysis of RR interval variability in the time and fr equency domains. Despite equal experimental hypertension with Ang II a nd phenylephrine infusion, at peak infusion rates the mean RR interval was significantly shorter with Ang II (983+/-179 milliseconds; mean+/ -SD) than with phenylephrine (1265+/-187 milliseconds, P<.01). The var iability of RR intervals was not significantly different, but the vari ability (median interquartile difference) of RR interval successive di fferences was significantly lower with Ang II (66 milliseconds) than w ith phenylephrine (104 milliseconds, P<.02). Power spectral analysis r evealed the power of the 0.25-Hz component in normalized units to be s ignificantly smaller during Ang II infusion (20.5+/-12.7 U) than durin g phenylephrine (38.2+/-14.7 U, P<.05), whereas the power of the 0.1-H z component was significantly greater during Ang II infusion (67.8+/-1 7.1 U) than phenylephrine (38.8+/-20.3 U, P<.05). Measures of vagal mo dulation of heart rate were significantly attenuated, and sympathetic modulation appeared to be increased during Ang II infusion compared wi th control phenylephrine infusions. These observations may underlie re ports of increased vagal activity during angiotensin-converting enzyme inhibitor therapy.