ANGIOTENSIN-II MODULATES CARDIOVASCULAR AUTONOMIC CONTROL IN THE ABSENCE OF BAROREFLEX LOADING

Objective-To investigate the effects of angiotensin II in the absence of baroreflex activation. Design-Ten healthy male volunteers were stud ied in a single blind, randomised, crossover study of heart rate varia bility during intravenous angiotensin II infusion (5-20 ng/kg/min) com pared with a control presser infusion of phenylephrine (10.7- 2.8 mu g /kg/min). Each infusion was titrated to increase mean blood pressure b y 20mmHg; sodium nitroprusside was then infused simultaneously to rest ore blood pressure to baseline values. Results-During concomitant angi otensin II (AII) and sodium nitroprusside (SNP) infusion, the mean (SD ) RR interval (864 (117) ms) was significantly shorter than during phe nylephrine (PE) and sodium nitroprusside infusion (1057 (163) ms), and was significantly shorter than at baseline (999 (164) ms), despite co mparable levels of blood pressure. Values of high frequency heart rate variability measured in the time and frequency domains were: signific antly lower during AII/SNP infusion than during PE/SNP: percentage of successive RR interval differences exceeding 50 ms, 30(16)% upsilon 57 (21)%; root mean square of successive RR interval differences, 63 (39) upsilon 90 (40) ms; high frequency power 0.48 (0.19) upsilon 0.66 (0. 26) nu. Conclusions-When the presser response is controlled by sodium nitroprusside, angiotensin II infusion is associated with tachycardia. Analysis of heart rate variability suggests that this reflects inhibi tion of cardiac vagal activity.