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Articles in PresS, published online ahead of print April 18, 2002
Am J Physiol Regu Physiol, 10.1152/ajpregu.00648.2001
Submitted on October 31, 2001
Accepted on January 22, 2002
1 Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA, USA
* To whom correspondence should be addressed. E-mail: Sved{at}bns.pitt.edu.
Angiotensin II (ANGII) has complex actions on the cardiovascular system. ANGII may act to increase sympathetic vasomotor outflow, but acutely the sympathoexcitatory actions of exogenous ANGII may be opposed by ANGII-induced increases in arterial pressure evoking baroreceptor-mediated decreases in sympathetic nerve activity. To examine this hypothesis, the effect of ANGII infusion on lumbar sympathetic nerve activity was measured in unanaesthetized chronic sino-aortic denervated rats. Chronic sino-aortic denervated rats had no reflex heart rate responses to pharmacologically evoked increases or decreases in arterial pressure. Similarly, in these denervated rats, nitroprusside-induced hypotension had no effect on lumbar sympathetic nerve activity; however, phenylephrine-induced increases in arterial pressure were still associated with transient decreases in sympathetic nerve activity. In control rats, intravenous infusion of 100 ng/kg/min ANGII increased arterial pressure and decreased heart rate and sympathetic nerve activity. In contrast, ANGII infusion caused an increase in both lumbar sympathetic nerve activity and heart rate in sino-aortic denervated rats. In rats that underwent sino-aortic denervation surgery but still had residual baroreceptor reflex-evoked changes in heart rate, the effect of ANGII on heart rate and sympathetic nerve activity were variable and correlated to the extent of baroreceptor reflex impairment. The present data suggest that pressor concentrations of ANGII in rats act rapidly to increase lumbar sympathetic nerve activity and heart rate, though baroreceptor reflexes normally mask these effects of ANGII. Furthermore, these studies highlight the importance of fully characterizing sino-aortic denervated rats used in experiments examining the role of baroreceptor reflexes.
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