We determined the effect of microinjection of ANG-(1-7) and ANG II into two key regions of the medulla that control the circulation rostral and caudal ventrolateral medulla (RVLM and CVLM, respectively) on baroreflex control of heart rate (HR) in anesthetized rats. Reflex bradycardia and tachycardia were induced by increases and decreases in mean arterial pressure produced by intravenous phenylephrine and sodium nitroprusside, respectively. The pressor effects of ANG-(1-7) and ANG II (25 pmol) after RVLM microinjection (11 +/- 0.8 and 10 +/- 2 mmHg, respectively) were not accompanied by consistent changes in HR. In addition, RVLM microinjection of these angiotensin peptides did not alter the bradycardic or tachycardic component of the baroreflex. CVLM microinjections of ANG-(1-7) and ANG II produced hypotension (-11 +/- 1.5 and -11 +/- 1.9 mmHg, respectively) that was similarly not accompanied by significant changes in HR. However, CVLM microinjections of angiotensins induced differential changes in the baroreflex control of HR. ANG-(1-7) attenuated the baroreflex bradycardia (0.26 +/- 0.06 ms/mmHg vs. 0.42 +/- 0.08 ms/mmHg before treatment) and facilitated the baroreflex tachycardia (0.86 +/- 0.19 ms/mmHg vs. 0.42 +/- 0.10 ms/mmHg before treatment); ANG II produced the opposite effect, attenuating baroreflex tachycardia (0.09 +/- 0.06 ms/mmHg vs. 0.31 +/- 0.07 ms/mmHg before treatment) and facilitating the baroreflex bradycardia (0.67 +/- 0.16 ms/mmHg vs. 0.41 +/- 0.05 ms/mmHg before treatment). The modulatory effect of ANG II and ANG-(1-7) on baroreflex sensitivity was completely abolished by peripheral administration of methylatropine. These results suggest that ANG II and ANG-(1-7) at the CVLM produce a differential modulation of the baroreflex control of HR, probably through distinct effects on the parasympathetic drive to the heart.
Alzamora et al. (2005) studied this question.