Baroreceptor reflex modulation by angiotensin II at the nucleus tractus solitarii.

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Abstract
This study characterized the effect of nucleus tractus solitarii (NTS) microinjection of the angiotensin II (Ang II) antagonist [Sar1, Thr8]Ang II on the baroreceptor control of heart rate in anesthetized rats. Reflex changes in heart rate were elicited by bolus intravenous injections of either phenylephrine or sodium nitroprusside before and after bilateral microinjection of [Sar1, Thr8]Ang II (100 pmol) or vehicle into the NTS. The slope of the relationship between the change in pulse interval and the change in mean arterial pressure was used as an index of baroreceptor reflex sensitivity. Bradycardia elicited by phenylephrine-induced increases in pressure was significantly greater after NTS injection of [Sar1, Thr8]Ang II. The slope of the pulse interval-arterial pressure relationship was 0.60 +/- 0.09 ms/mm Hg after injection, as compared with 0.42 +/- 0.07 ms/mm Hg before. In contrast, the baroreceptor reflex sensitivity index generated by decreases in pressure with nitroprusside was similar before and after injection. Vehicle injections did not alter the baroreceptor reflex index. Collectively, the data suggest that inhibition of endogenous Ang II in the NTS facilitates the baroreceptor reflex sensitivity to increases, but not decreases, in pressure. This new finding reveals the NTS as one site of action for the tonic effects of endogenous Ang II.