Renal Nerve Activity and Heart Rate

Angiotensin II (Ang II) exerts complex actions on sympathetic nerve activity and heart rate, but these actions are incompletely understood. We performed three series of experiments in conscious rabbits to analyze the actions of exogenous and endogenous Ang II on renal sympathetic nerve activity and heart rate. (1) Graded intravenous doses of phenylephrine and Ang II suppressed renal sympathetic nerve activity to the same degree, whereas Ang II decreased heart rate much less than phenylephrine. (2) Ang II infusion at 10 ng/kg per minute increased mean arterial pressure by 13±2 mm Hg (P<.01) and decreased renal sympathetic nerve activity by 67±13% (P<.01) but did not change heart rate. In the same rabbits, nitroprusside and phenylephrine infusions were used to generate baroreceptor reflex curves. Ang II shifted the heart rate-mean arterial pressure curve to the right but did not alter the renal nerve activity-mean arterial pressure curve. (3) The Ang II type 1 receptor antagonist losartan decreased mean arterial pressure by 8±3 mm Hg (P<.01) and increased renal I t is generally accepted that angiotensin II (Ang II) modulates the baroreceptor reflex control of heart rate (HR). This modulation results from a resetting of the baroreceptor reflex control of HR to a higher pressure with' or without' a decrease in baroreceptor reflex gain. The resetting is apparently due to an action of Ang II at the area postrema that is mediated by Ang II type 1 (ATt) receptors. 9 It can be elicited by endogenous and exogenous Ang II. Ang II may also modulate the baroreceptor reflex control of sympathetic nerve activity, although less information is available and some of it is conflicting. The receptors that mediate effects of Ang II on sympathetic nerve activity have not been characterized, and the role of endogenous Ang II in the baroreceptor reflex control of sympathetic nerve activity has not been