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1 Howard Florey Institute of Physiology and Medicine, Parkville, Victoria, Australia
2 Department of Physiology, Selcuk University, Konya, Turkey
* To whom correspondence should be addressed. E-mail: c.may{at}hfi.unimelb.edu.au.
Central actions of angiotensin play an important role in cardiovascular control and have been implicated in the pathogenesis of hypertension and heart failure. One feature of centrally or peripherally administered angiotensin is that the bradycardia in response to an acute pressor effect is blunted. It is unknown whether following central angiotensin this is due partly to increased cardiac sympathetic nerve activity (CSNA). We recorded CSNA and arterial pressure in conscious sheep, at least 3 days after electrode implantation. The effects of intracerebroventricular (ICV) infusions of angiotensin II (3nmol/h for 30 min) and artificial cerebrospinal fluid (CSF)(1mL/h) were determined. The response to ICV hypertonic saline (0.6M NaCl in CSF at 1 mL/h) was examined as there is evidence that hypertonic saline acts via angiotensinergic pathways. ICV angiotensin increased CSNA by 23±7% (P<0.001) and mean arterial pressure (MAP) by 7.6±1.2 mmHg (P<0.001) but did not significantly change heart rate (n=5). During ICV Ang II the reflex relation between CSNA and diastolic blood pressure was significantly shifted to the right (P<0.01). ICV hypertonic saline increased CSNA (+9.4±6.6%, P<0.05) and MAP but did not alter heart rate. The responses to angiotensin and hypertonic saline were prevented by ICV losartan (1 mg/h). In conclusion, in conscious sheep angiotensin acts within the brain to increase CSNA, despite increased MAP. The increase in CSNA may account partly for the lack of bradycardia in response to the increased arterial pressure. The responses to angiotensin and hypertonic saline were losartan-sensitive, indicating they were mediated by angiotensin AT-1 receptors.
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