Modulation of the baroreflex control of heart rate by angiotensin-(1-7) at the nucleus tractus solitarii of normotensive and spontaneously hypertensive rats.

Objectives In the present study, we evaluated the effect of angiotensin-(1-7) [Ang-(1-7)] and its selective antagonist, D-Ala 7 -Ang-(1-7) (A-779), at the nucleus tractus solitarii (nTS), in the modulation of the bradycardic component of the baroreceptor reflex. Methods Mean arterial pressure (MAP) and heart rate were continuously recorded. Reflex changes in heart rate elicited by bolus injection of graded doses of phenylephrine were evaluated before and after bilateral microinjection (glass micropipette) of Ang-(1-7) (10 pmol or 25 pmol), A-779 (50 pmol) or saline (vehicle) into the nTS of urethane anesthetized male Wistar rats or spontaneously hypertensive rats (SHR). The averaged ratio between reflex changes in heart rate and changes in MAP was used as index of baroreflex sensitivity. Results Microinjection of Ang-(1-7) into the nTS elicited significant decreases in MAP and heart rate in both Wistar and SHR. While the decrease in MAP was similar in both strains, the changes in heart rate were smaller in SHR. A-779 produced small changes in MAP and heart rate that were no different from those induced by saline. After microinjection of 10 pmol of Ang-(1-7) into the nTS of normotensive rats, there was a significant increase in baroreflex sensitivity. In SHR, only the microinjection of a higher dose (25 pmol) of Ang-(1-7) produced a significant increase in baroreflex sensitivity. A significant reduction inbaroreflex sensitivity was observed after microinjection of A-779 (50 pmol) in both strains. Conclusions These results indicate that Ang-(1-7) exerts a tonic modulatory effect on the baroreflex control of heart rate at the nTS, probably through a non-AT 1 non-AT2 receptor subtype. In addition, our data showed a reduced sensitivity to Ang-(1-7) at the nTS of SHR, that could be accounting, at least in part, for the decreased baroreflex sensitivity present in this model of hypertension.