The present study was carried out to determine the effects of chronic central infusion of Ang-(1–7) on baroreflex control of sympathetic outflow and cardiac sympathovagal balance in CHF. The most important findings of this study are that, 1) central Ang-(1–7) enhances arterial baroreflex control of RSNA in CHF, 2) Ang-(1–7) enhances baroreflex control of HR by a profound effect on vagal outflow, and 3) these effects were blocked by central administration of a mas receptor antagonist.
Chronic ICV infusion of Ang-(1–7) resulted in a decrease in resting HR and RSNA and an increase in baroreflex gain in animals with CHF. HRV was significantly increased in CHF rabbits treated with Ang-(1–7). The HR response to cigarette smoke (a vagally mediated bradycardia)24
was blunted in CHF animals and restored to control following central Ang-(1–7) infusion. The change in baseline HR following administration of atropine was augmented by central Ang-(1–7) infusion. An enhancement of vagal outflow in CHF rabbits receiving ICV Ang-(1–7) is also supported by the increased baroreflex slope following metoprolol. The change in HR following administration of the β-1 blocker metoprolol was also significantly reduced in CHF animals receiving central infusion of Ang-(1–7) indicating decreased cardiac sympathetic outflow. The fact that both cardiac sympathetic outflow and RSNA were decreased following ICV Ang-(1–7) in CHF suggests a global reduction in sympathetic outflow by Ang-(1–7). Thus, Ang-(1–7) exerts both a cardiac vagotonic effect as well as a sympathoinhibitory effect in the setting of CHF thus restoring sympathovagal balance.
The central effects of Ang-(1–7) are not uniformly agreed upon. Gomes da Silva et al.
injected the mas
receptor antagonist, A-779 in the paraventricular nucleus of anesthetized rats and observed a decrease in RSNA suggesting that Ang-(1–7) is sympatho-excitatory.18
However, this effect was transient compared to Ang II. On the other hand, other studies suggest that Ang-(1–7) may have a sympathoinhibitory role. Gironacci et al.
showed that Ang-(1–7) decreased norepinephrine release from the hypothalamus of SHR.27
The current study supports the view that Ang-(1–7) opposes the actions of Ang II with a sympathoinhibitory effect in CHF.
The mechanism(s) by which Ang-(1–7) enhances arterial baroreflex function is not completely understood. There is substantial data showing that Ang II reduces baroreflex sensitivity by actions at several sites in the medulla.28
For instance, Ang II acting through the AT1R in the NTS inhibits baroreflex function.29
Sakima et al.
showed that in rats with low levels of brain angiotensinogen, inhibition of Ang-(1–7) reduced baroreflex sensitivity.30
Data from this same laboratory have convincingly shown that blockade of Ang-(1–7) receptors reduced baroreflex function in Sprague-Dawley rats when the Ang-(1–7) antagonist, (D-Ala7
)-Ang-(1–7) was injected into the NTS.23
This maneuver most likely shifted the balance between Ang II and Ang-(1–7) towards the baroreflex inhibitory effects of Ang II.
A unique finding in the present study is that chronic Ang-(1–7) infusion into the brain activates the cardiac vagus in animals with CHF, that have low resting vagal tone. While these studies cannot determine the site of Ang-(1–7) activation of central nuclei that may contribute to this vagal effect it is likely that at least one area is the NTS. A study by Campagnole-Santos et al.
showed that microinjection of Ang-(1–7) into the NTS in anesthetized rats resulted in bradycardia.31
Importantly, Barnes et al.
showed that Ang-(1–7) stimulated neurons in the dorsal motor nucleus of the vagus in a canine brain slice preparation.32
Furthermore, Becker et al
. have shown intense binding of Ang-(1–7) to mas
receptors in neurons in the NTS.33
Thus, it is likely that the bradycardic effect of central Ang-(1–7) along with the enhancement in baroreflex function is mediated by activation of the NTS. This notion would also explain the sympatho-inhibition and improvement in baroreflex function following Ang-(1–7) infusion. Since mas
receptors have also been identified in other nuclei known to be rich in Ang II receptors such as the paraventricular nucleus and rostral ventrolateral medulla,33
it is also possible that other structures are responsible for the central effects of Ang-(1–7).
The cellular actions of Ang-(1–7) are still being defined. However it is clear that in neurons, working through the G-protein coupled mas
receptor, Ang-(1–7) activates a nNOS dependent mechanism that regulates neuronal excitability. In catecholaminergic neurons in culture (CATH.a) Yang et al.
showed that Ang-(1–7) increased nitric oxide (NO) formation.34
This effect was inhibited by both the mas
receptor antagonist, A779 and by selective nNOS inhibition but not by eNOS or iNOS inhibition. More importantly, patch clamp recordings of outward potassium current showed that Ang-(1–7) significantly increased this current. The increase in current was also blocked by A779 or specific nNOS inhibition. These data would suggest that Ang-(1–7) contributes to a reduction in neuronal excitability. This is pertinent to the CHF state and sympathoexcitation. We have recently shown a reduction in the potassium channel protein Kv4.3 in the medulla of rats with CHF.35
Furthermore, Ang II working through the AT1R reduces potassium channel current thus contributing to an increase in neuronal excitability.35
Therefore, based on the current study and those cited above we propose that one function of Ang-(1–7) is to limit neuronal discharge in opposition to the actions of Ang II.
This effect may be mediated by enhanced NO formation. The effects of Ang-(1–7) on baroreflex function in our study were blocked by the mas
receptor antagonist A779. This suggests that the effects of Ang-(1–7) are mediated by the mas
receptor. However, some studies suggest that Ang-(1–7) also has affinity for the Ang II receptors. 36, 37
It is also possible that the mas
receptor itself may functionally interact with AT1
It is also unknown whether endogenous mas
receptor expression is altered in CHF. Thus, studies investigating the physiological changes and interactions of the mas
receptor are necessary to determine its role in the two axes of the RAS.
While previous studies show central administration of Ang-(1–7) evokes hypotensive effects in both normal and disease states,12, 17
the current study only shows significant effects of Ang-(1–7) in the CHF state and no change in MAP in either state. We did however observe a trend for a decrease in MAP and HR in the Sham+Ang-(1–7) group. It is also possible that higher doses of Ang-(1–7) may show significant effects in Sham animals. In addition, previous studies have been performed only in the anesthetized state or in rodent models. The lack of an effect in Sham animals in the present study may also be due to the use of the conscious rabbit model. Finally, it is possible that the physiological effects of Ang-(1–7) are primarily seen in disease states following an imbalance of Ang II and Ang-(1–7).
The balance between Ang II and Ang-(1–7) is critically dependent on the production and activity of ACE and ACE2. In a previous study we showed differential expression of ACE and ACE2 in the medulla and hypothalamus from CHF rabbits.7
In CHF animals the balance between these enzymes is tipped towards ACE and the generation of Ang II thereby promoting sympathoexcitation. These data therefore suggest that overexpression of ACE2 and concurrent increased levels of Ang-(1–7) would provide protection from excessive sympathetic outflow.