ANGIOTENSIN-II EXERTS DIFFERENTIAL ACTIONS ON 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 sympathetic nerve activity by 63+/-15% (P<.05) but did not change heart rate. Losartan shifted the heart rate-mean arterial pressure curve to the left but did not alter the renal nerve activity-mean arterial pressure curve. These results demonstrate that whereas exogenous Ang II resets the baroreceptor reflex control of heart rate to a higher pressure, it does not increase resting renal sympathetic nerve activity or alter the baroreceptor reflex control of renal nerve activity. The results also confirm that endogenous Ang II resets the reflex control of heart rate by stimulating Ang II type 1 receptors but fail to provide evidence for an action of endogenous Ang II on resting renal sympathetic nerve activity or on the baroreceptor reflex control of renal nerve activity.