DIFFERENTIAL BARORECEPTOR REFLEX MODULATION BY CENTRALLY INFUSED ANGIOTENSIN PEPTIDES

The present study was designed to investigate the effect of intracerebroventricular (icv) and intravenous (iv) infusion of angiotensin (ANG)-(1-7), ANG III, and ANG II on the baroreceptor control of heart rate (BHR) in conscious rats. Reflex changes in HR were elicited by bolus iv injection of either phenylephrine or sodium nitroprusside before and within 1 and 3 h of icv infusion of ANG II (n = 10), ANG III (n = 9), ANG-(1-7) (n = 9), or saline (n = 9) at a rate of 3 nmol.7.5-mu-l-1.h-1. In another group of animals (n = 23), iv infusion of the same amount of ANG peptides was carried out at a rate of 0.7 ml/h. The average ratio of changes in HR in beats per minute and changes in mean arterial pressure (MAP, mmHg) was used as an index of BHR sensitivity. ANG II and ANG III produced a significant increase in the basal levels of MAP, but only during the first hour of infusion (iv or icv). No significant changes in baseline HR were observed. ANG-(1-7) and saline infusion did not change basal levels of HR or MAP (iv or icv). ANG II (iv and icv) and ANG III (icv) caused a significant decrease in the BHR sensitivity for reflex bradycardia. In contrast, icv infusion of ANG-(1-7) induced a significant increase in BHR sensitivity for reflex bradycardia (-3.0 +/- 0.3, 1 h, and -2.8 +/- 0.1 beats.min-1.mmHg-1, 3 h vs. -2.1 +/- 0.2 beats.min-1.mmHg-1, before infusion). Intravenous infusion of ANG-(1-7) had no effect on the BHR sensitivity for either increases or decreases of MAP. None of the ANG peptides produced changes in reflex tachycardia. Saline infusion (icv or iv) did not produce any change in BHR sensitivity. These results show a differential effect of the ANG peptides on the modulation of the BHR, indicating that these peptides can express selective biological actions in the central control of blood pressure.