Angiotensin: From the Kidney to Coronavirus

Angiotensin: From the Kidney to Coronavirus, a new volume in the Molecular Mediators in Health and Disease series, presents the communication role of the hormone in both health and disease states. Beyond the most common conditions, the book also explores the role of Angiotensin in infectious diseases, like COVID-19. Sections provide background to its discovery and role in homeostasis, focus on molecular biology aspects, including genetics and measurements of its associated proteins, describe the specific actions of angiotensin in normal physiology with different organ systems, survey different classes of drugs that act on the Renin-angiotensin-aldosterone system, cover non-angiotensin II peptides, and more.

The final part of the book is dedicated to angiotensin’s role in disease states, making this the ideal reference for researchers in life sciences interested in understanding the physiological role of Angiotensin in a complete fashion. Research physicians will also benefit from the book’s complete coverage of organ systems and diseases where Angiotensin plays a key role.

• Covers the traditional disorders related to Angiotensin and new findings like its role in infectious diseases like COVID-19
• Offers an overview of all important aspects of angiotensin, from discovery to clinical application, from physical and chemical properties to the physiological/cellular interaction
• Expands on the role of angiotensin beyond the individual system the researcher is focusing, providing a comprehensive coverage of the additional effects at cell, tissue, organ, and system levels

• Language: English
• Publisher: Academic Press
• Release date: Feb 20, 2023
• ISBN: 9780323996198

Book preview

Preface

Paul M. Pilowsky,     University of Sydney, Camperdown, NSW, Australia

This volume addresses the distribution and physiological functions of angiotensin peptides and related proteins, the potential for therapeutic agents to interact with the angiotensin system, and the importance of angiotensin system in relation to COVID19.

In late September 1939, IH Page and OM Helmer, working at the Indianapolis City Hospital, submitted a manuscript on the hypertensive effects of a substance produced by the action of renin on a substrate found in plasma: For this substance we suggest the name angiotonin [Greek αγγειον, blood vessel, + τονος (τεινω), strain] [1–3]. At that time, it was not known that angiotensin was a peptide, although the idea that its precursor was present in blood was well understood.

In the years that followed, the physiology of angiotensin and its relationship to the development of hypertension was clarified [4]. If the blood flow to the kidney is reduced, prorenin is released and converted to the active enzyme renin [5]. Renin acts on the liver protein angiotensinogen to produce angiotensin I, a key precursor peptide. Within the lungs, high concentrations of the enzyme ACE (angiotensin-converting enzyme) generate angiotensin II, which then passes into the circulation where it has many effects including vasoconstriction (by acting on smooth muscle type 1 angiotensin II receptors: AT1R) [6] and thirst (by acting at circumventricular organs in the brain) release of aldosterone from the adrenal cortex (leading to sodium and water retention) [7]. Angiotensin II also has profound effects on blood pressure regulation within the central nervous system [8] and on sympathetic neurons in the spinal cord [9].

In the decades that followed, it became clear that the angiotensin and aldosterone systems are critical in the control of fluid and electrolyte balance generally, and the long-term control of arterial blood pressure.

Upregulation of the AT1R is present in hypertension [10] and, with recognition of the importance of the RAAS (renin–angiotensin–aldosterone system), development of drugs that blocked formation of angiotensin or the AT1R were introduced to great effect as pharmacological tools to manage human hypertension likely including the hypertension that is associated with intermittent hypoxia and obstructive sleep apnea [6].

The finding that functional renin–angiotensin systems exist locally within tissues further highlighted the importance of this system.

A second critical breakthrough occurred with the discovery that angiotensin [1–7], another peptide product of the angiotensin precursor, could be produced by a second angiotensin-converting enzyme (ACE2) that is also distributed throughout the body [11]. The actions of angiotensin [1–7] are generally opposite to those of angiotensin II.

ACE2 is also a critical receptor for coronaviruses. ACE2 is commonly found as a membrane-bound protein and is the protein that coronaviruses bind to in order to gain access to cells. The ACE2-binding spike protein is present on all coronaviruses that cause disease in humans including the virus responsible for COVID19 [12].

References

1. Page I.H. On the nature of the pressor action of renin. J Exp Med. 1939;70(5):521–542.

2. Page I.H, Helmer O.M. A crystalline pressor substance (angiotonin) resulting from the reaction between renin and renin-activator. J Exp Med. 1940;71(1):29–42.

3. Page I.H, Helmer O.M, Plentl A.A, Kohlstaedt K.G, Corcoran A.C. Suggested change in designation of renin-activator (hypertensinogen) to renin-substrate (α globulin). Science. 1943;98(2537):153–154.

4. Johnston C.I. Biochemistry and pharmacology of the renin-angiotensin system. Drugs. 1990;39(1):21–31.

5. Johnston C.I, Matthews P.G, Davis J.M, Morgan T. Renin measurement in blood collected from the efferent arteriole of the kidney of the rat. Pflugers Arch. 1975;356(3):277–286.

6. Kim S.J, Fong A.Y, Pilowsky P.M, Abbott S.B.G. Sympathoexcitation following intermittent hypoxia in rat is mediated by circulating angiotensin II acting at the carotid body and subfornical organ. J Physiol. 2018;596(15):3217–3232.

7. Mazzocchi G, Gottardo G, Macchi V, Malendowicz L.K, Nussdorfer G.G. The AT2 receptor-mediated stimulation of adrenal catecholamine release may potentiate the AT1 receptor-mediated aldosterone secretagogue action of angiotensin-II in rats. Endocr Res. 1998;24(1):17–28.

8. McMullan S, Goodchild A.K, Pilowsky P.M. Circulating angiotensin II attenuates the sympathetic baroreflex by reducing the barosensitivity of medullary cardiovascular neurones in the rat. J Physiol. 2007;582(Pt 2):711–722.

9. Oshima N, Kumagai H, Onimaru H, Kawai A, Pilowsky P.M, Iigaya K, et al. Monosynaptic excitatory connection from the rostral ventrolateral medulla to sympathetic preganglionic neurons revealed by simultaneous recordings. Hypertens Res. 2008;31(7):1445–1454.

10. Reja V, Goodchild A.K, Phillips J.K, Pilowsky P.M. Upregulation of angiotensin AT1 receptor and intracellular kinase gene expression in hypertensive rats. Clin Exp Pharmacol Physiol. 2006;33(8):690–695.

11. Burrell L.M, Johnston C.I, Tikellis C, Cooper M.E. ACE2, a new regulator of the renin-angiotensin system. Trends Endocrinol Metab. 2004;15(4):166–169.

12. Li W, Moore M.J, Vasllieva N, Sui J, Wong S.K, Berne M.A, et al. Angiotensin-converting enzyme 2 is a functional receptor for the SARS coronavirus. Nature. 2003;426(6965):450–454.

Chapter 1: Regulation of sympathetic nerve activity by the central angiotensin system in heart failure

Julia Shanks, and Rohit Ramchandra     Manaaki Manawa - The Centre for Heart Research and the Department of Physiology, The University of Auckland, Faculty of Medical and Health Sciences, Auckland, New Zealand

Abstract

Heart failure (HF) is associated with activation of both the renin-angiotensin-aldosterone system and the sympathetic nervous system. The increase in the levels of sympathetic nerve activity to the heart and kidney is particularly damaging and is associated with increased mortality. This chapter will focus on the role of angiotensin II within the brain and spinal cord in mediating the increase in sympathetic nerve activity during HF. The finding that blockade of angiotensin receptors within the brain significantly reduces both cardiac and renal sympathetic drive toward normal levels indicates a critical role for central angiotensinergic mechanisms in HF. The literature indicates that the paraventricular nucleus of the hypothalamus and the rostral ventral lateral medulla play a critical role in setting the increased sympathetic drive to the kidney in HF. In contrast, a circumventricular organ, the area postrema plays an important role in modulating sympathetic drive to the heart in this disease. Emerging evidence also points to an important role for spinal angiotensin II in modulating sympathetic nerve activity in HF. These findings highlight the important role of central angiotensin II in modulating sympathetic nerve activity during HF. Further research into the site-specific actions of central and spinal angiotensin II in HF is needed to advance our understanding of this chronic disease.

Keywords

Angiotensin II; Brain; Heart; Heart failure; Kidney; Spinal cord; Sympathetic nerve activity

1. Introduction

Heart failure (HF) is a major public health epidemic that is increasingly prevalent in developed countries and is the leading cause of hospital admission for those over 65 years of age. An estimated 5.7 million Americans have HF, and it is predicted that a further 3 million will have HF by 2030 [1]. Despite advances in new treatments and therapies, HF patients continue to have a high mortality and morbidity burden. The Framingham Study reported that 59% of men and 45% of women with HF would die within 5years of diagnosis [2]. This 5-year mortality rate is worse than most cancers [3]. The increasing incidence of HF is due to the aging of the population, an increase in the prevalence of risk factors such as obesity and diabetes, and a decrease in fatality associated with improved treatment of acute coronary syndromes.

A hallmark of HF is the activation of many neurohumoral systems, such as the renin–angiotensin–aldosterone system and the sympathetic nervous system [4,5], in response to decreased cardiac output and subsequent underperfusion of tissues. Although these compensatory mechanisms are beneficial in the short term, chronic activation of these systems leads to further deterioration and contributes to a worse prognosis. Thus, the sympathetic nervous system and the renin–angiotensin system remain current therapeutic targets [4,5]. It is important to note that the sympathetic nervous system appears to be activated in both HF with reduced ejection fraction and HF with preserved ejection fraction [6]. This is relevant since HF with preserved ejection fraction is estimated to include around 50% of the patients with clinical features of HF. This chapter will focus specifically on the actions of angiotensin II within the brain and spinal cord in modulation of sympathetic nerve activity (SNA) in HF.

2. Heart failure results in an increase in resting levels of sympathetic nerve activity

While activation of the sympathetic nervous system is a hallmark feature of HF, it is important to note that there are differential changes to individual organs, in both the extent of the increase and the timeline of when SNA increases. In a seminal study in 1986, cardiac norepinephrine spillover in patients with severe HF was elevated up to 50-fold, whereas that from the kidney was only increased 3-fold, and that from the lungs, gut, and liver was unchanged [7]. These findings reiterate that SNA is differentially regulated and implicates distinct mechanisms in mediating the increased SNA to individual organs during HF. A further important finding was that cardiac spillover of norepinephrine occurred before that to the kidney and other organs [8]. These results indicate that the heart is not only exposed to higher levels of norepinephrine release than other organs but for longer. These findings led to the first trial demonstrating the lifesaving properties of a β-adrenergic blocker in HF [9], leading to the current widespread use of β-blockers as a primary therapy in HF.

In addition to norepinephrine spillover studies in patients with HF, there is also good evidence in preclinical animal models of HF. Direct recordings of SNA have predominantly focused on the heart and the kidney. In a large animal model of ventricular pacing-induced HF, a significant increase in directly recorded cardiac SNA was observed [10,11], indicating the increase in cardiac spillover of norepinephrine in patients with HF is mediated by both an increase in SNA and a decrease in reuptake [8]. In normal healthy animals, the resting levels of cardiac SNA were set much lower than that of renal SNA, indicating differential central control of these sympathetic outflows under normal physiological conditions [12]. In terms of the timeline of activation, norepinephrine spillover from the heart increases approximately threefold in patients with mild to moderate HF (NYHA II, IIIA, ejection fraction=29±7%). However, there is no change in spillover from the kidney at this stage. This result is in agreement with findings from the preclinical large animal model. Renal norepinephrine spillover increases in patients with severe HF (NYHA IIIB, IV, ejection fraction=18±5%) and is accompanied by a further increase of spillover of norepinephrine from the heart [8]. In both rat and rabbit models of HF, where either ligation or pacing induces HF, there is a consistent, reproducible increase in renal SNA [13–17]. These results establish that the increase in renal norepinephrine spillover observed in patients with HF is due to an increase in directly recorded renal SNA.

2.1. Consequence of increased sympathetic nerve activity during heart failure

The sustained high levels of SNA have numerous deleterious consequences that play an important contributing role to the poor prognosis of patients with HF. The increase in spillover of norepinephrine to the heart and kidney accounts for 62% of the increase in total plasma norepinephrine spillover in patients with HF [7]. The importance of these increases in SNA to the heart and kidney in HF is emphasized by the finding that increases in the sympathetic drive to both of these organs are associated with reduced survival [18,19].

Increased sympathetic drive to the heart increases the levels of norepinephrine released, which causes downregulation of cardiac β-adrenoceptors, and the released norepinephrine has toxic effects on sympathetic nerve terminals [20]. Excess norepinephrine release also induces left ventricular fibrosis and hypertrophy [21] and promotes the development of arrhythmias and sudden death [18]. While there is downregulation of cardiac β-adrenoceptors, there is no change in the density of vascular α receptors [22]. Consequently, in HF, coronary vasoconstriction is mediated by the excess norepinephrine and increased release of the coreleased peptide neuropeptide Y [23]. In addition to the direct effects of norepinephrine, the oxidative metabolites of norepinephrine also exert deleterious effects on the heart [24]. High levels of cardiac norepinephrine spillover remain the strongest prognostic marker in HF patients [18].

3. Role of angiotensin II

3.1. Circulating levels of angiotensin II in heart failure

HF is associated with increased circulating hormones such as angiotensin II and endothelin. Previous studies have suggested an important role for both angiotensin II and endothelin in modulating sympathetic drive [25]. Several clinical trials have shown that blockade of the RAS, using angiotensin AT1 receptor blockers (ARBs) and angiotensin-converting enzyme (ACE) inhibitors, reduces mortality and morbidity in patients with HF [5,26]. These drugs decrease the deleterious effects of increased circulating angiotensin II, which are thought to result from its actions to cause vasoconstriction, renal sodium retention, cardiac fibrosis, and increased SNA [27,28]. However, the exact mechanisms of the beneficial effects of ARBs in HF patients remain unclear. Chronic treatment with ACE inhibitors and ARBs has been shown to reduce measures of SNA in HF patients [29,30], but it is uncertain whether this is a direct effect of RAS inhibition or secondary to the hemodynamic improvement that occurs with these drugs. In addition, there is evidence that ARBs can cross the blood–brain barrier and inhibit the effect of angiotensin II in the brain [31]. We have previously investigated whether circulating angiotensin II was responsible for the increased cardiac SNA during HF. Inhibition of peripheral AT1 receptors using an i.v. infusion of irbesartan did not decrease the elevated cardiac SNA in the HF animals. It is important to note that despite a significant reduction in mean arterial pressure, there was no baroreflex-mediated increase in CSNA [32]. These data suggest that circulating angiotensin II may have a minor contribution to the increased cardiac SNA in HF.

3.2. Role of central angiotensinergic mechanisms

There is extensive evidence that an alteration in central angiotensinergic mechanisms plays an important role in mediating both the increased SNA and the altered reflex regulation of SNA in HF [33,34]. Infusion of angiotensin II into the intracerebroventricular space increases SNA to both the spleen [35] and the kidney [36,37] in rats. This effect of angiotensin II is mediated by the angiotensin II type 1 receptor (AT1R), as infusion of the AT1R antagonist losartan into the intracerebroventricular space blocks the effect of angiotensin II on renal SNA [37]. A previous study has shown that intracerebroventricular (ICV) infusion of angiotensin II increases cardiac SNA in normal conscious sheep [38], suggesting that increased activity of this system could account for the cardiac sympathoexcitation in HF as well. It is also known that AT1R blockers can cross the blood–brain barrier to varying degrees [31,39]. Therefore, any reduction in SNA may be due to putative inhibition of central angiotensin AT1 receptors.

Regarding what might lead to activation of AT1Rs in HF, increased cerebrospinal fluid levels of angiotensin II have been measured in dogs with pacing-induced HF [40]. In addition, increased levels of AT1Rs have been observed in brain nuclei associated with central cardiovascular control in multiple models of HF [41,42]. This evidence for central angiotensin II signaling in HF includes but is not limited to increased mRNA expression of the AT1R in the rostral ventral lateral medulla (RVLM) of rabbits with pacing-induced HF [40,41], and increased AT1R density measured by autoradiography in the subfornical organ, organum vasculosum laminae terminalis, paraventricular nucleus of the hypothalamus (PVN), and the median preoptic nucleus in rats with HF produced by an aortocaval shunt [42]. We have shown that pacing-induced HF in sheep resulted in differential changes in AT1R density; there was a decrease in the area postrema and the NTS at the level of the area postrema but an increase in the paraventricular nucleus of the hypothalamus (Fig. 1.1). The reasons for the discrepancies regarding changes in the AT1R levels in the NTS in the sheep and rodent models of HF are not clear. Irrespective of small differences in species, the data indicate an increase in AT1R within specific brain regions during HF. The following section will focus on the effects of inhibition of these receptors on SNA.

3.3. Blockade of central angiotensin type 1 receptor decreases SNA

Administration of the AT1R antagonist losartan into the intracerebroventricular space decreases renal SNA to a greater extent in rats with HF compared with control rats. This is observed both in HF induced by myocardial infarction [43,44] and in rabbits with rapid ventricular pacing-induced HF [41]. In sheep with HF, infusion of losartan significantly reduced cardiac SNA back to almost normal levels (Fig. 1.2) [45]. There was no change in the resting levels of renal SNA in this study, which may reflect the moderate level of HF in these sheep. These results indicate a critical role for central angiotensinergic mechanisms in setting the high levels of cardiac and renal SNA in HF. Endogenous angiotensin II also contributes to the suppressed baroreflex in HF, as AT1R blockade normalizes the depressed baroreflex control of renal SNA in a rat model of chronic HF [43,46]. The finding that central AT1R blockade in HF restored baseline levels of cardiac SNA and renal SNA in multiple animal models points toward the important role of central angiotensin II in the modulation of SNA. The next sections will focus on the specific sites within the brain that may be responsible for the sympathoexcitation observed in HF.

Figure 1.1  Original X-ray images of Ang II binding in the hypothalamus and brainstem of one normal (Panel A, C) and one HF animal (Panel B, D). The binding was totally displaced after coincubation with the AT1R antagonist losartan. The histogram shows the density of AT1R binding in major central cardiovascular control nuclei in normal (open bars, n=5) and HF animals (filled bars, n=5). Receptor binding densities, determined by in vitro autoradiography, are normalized to control values. HF, heart failure; PVN, paraventricular nucleus of the hypothalamus; SON, Supraoptic nucleus; AP, area postrema; NTS, nucleus of the solitary tract. Results are expressed as means±SE. ∗P <0.05 compared with normals using Student's t-test. Reproduced from Ramchandra R, Hood SG, Watson AM, Allen AM, May CN. Central angiotensin type 1 receptor blockade decreases cardiac but not renal sympathetic nerve activity in heart failure. Hypertension. 2012;59(3):634–641.

Figure 1.2  Changes in MAP, HR, CSNA, and RSNA over 5h ICV infusion of losartan in conscious normal (solid line) and heart failure (dashed line) sheep. Time 0 shows resting levels before the start of the losartan infusion. Results are mean±SEM. † indicates significant decrease in HF animals after 5h Losartan. # indicates significant difference between normal and HF animals at the control time point. Reproduced from Ramchandra R, Hood SG, Watson AM, Allen AM, May CN. Central angiotensin type 1 receptor blockade decreases cardiac but not renal sympathetic nerve activity in heart failure. Hypertension. 2012;59(3):634–641.

3.4. Central regions that respond to angiotensin II

The finding that central infusion of AT1R blockers can reduce SNA has led to numerous investigations examining the site putatively responsible for the elevated sympathetic drive in HF. We will focus on the data examining the actions of two sympathetic premotor sites: the PVN and the rostral ventral lateral medulla (RVLM) as well as a circumventricular organ, the area postrema.

3.4.1. The role of the paraventricular nucleus of the hypothalamus in heart failure

Earlier studies using transsynaptic neuronal transport of pseudorabies virus from the kidney and stellate ganglion showed neurons in the PVN anatomically innervate both the kidney and the heart [47,48]. In particular, there is good evidence that the PVN mediates the SNA response to changes in volume status [49–51]. Volume expansion and right atrial stretch increase the expression of the early gene marker c-fos in parvocellular neurons of the PVN [52], and inhibition of PVN neurons with muscimol attenuated the volume expansion-induced inhibition of renal SNA [53]. Microinjection of angiotensin II into the PVN in normal rats increases both mean arterial pressure and renal SNA [54,55]; this excitation is inhibited by the AT1R antagonist losartan. Endogenous angiotensin II may not regulate baseline levels of renal SNA under normal conditions, as microinjection of losartan into the PVN reduced mean arterial pressure but did not change renal SNA in anesthetized rats in one study [56], although this finding is not universal [57].

Given that HF is associated with volume expansion and significant increases in sympathetic drive to the heart and the kidney, there has been a major interest in determining whether the PVN mediates the increased SNA in HF. Activation of the PVN in HF is indicated by the increased expression of c-fos- as well as Fos-related antigens in neurons in the PVN [58,59]. In addition, studies in a rat model of HF indicate that the PVN contributes to the increased renal SNA [54,57,60] and that PVN neurons have a higher firing rate. A major factor leading to the increased sympathetic outflow to the kidneys from the PVN in HF appears to be a reduced inhibitory GABAergic input [54,61]. In particular, AT1R mRNA and protein levels are higher in the PVN of rats with HF than controls [57]. This is in line with functional data for renal SNA where microinjection of the AT1R antagonist losartan into the PVN decreases renal SNA in HF rats [57], while the renal SNA response to losartan is greater in rats with HF.

We have shown that AT1R within the PVN is upregulated in our sheep model of HF, as assessed by an autoradiography receptor binding assay [62]. In contrast to renal SNA, however, the PVN does not appear to play an important role in modulating cardiac SNA during HF. Both, inhibition of neurons in the PVN and ablation of the PVN do not alter cardiac SNA in the ovine HF model [62]. Taken together, these studies indicate a crucial role for the PVN in modulating SNA to the kidney but not to the heart during HF.

3.4.2. Actions of angiotensin II within the rostral ventral lateral medulla

There is good evidence that overactivity of neurons within the RVLM may also play an essential role in the high renal SNA in HF. Microinjection of angiotensin II into the RVLM increases renal SNA in an infarction model of HF in rats [63]. The expression of the AT1R within the RVLM is increased in HF in this model [63,64]. The increase in renal SNA during the development of HF in the pacing-induced model also reflects the upregulation of the AT1R within the RVLM [65]. These studies suggest that an upregulation of angiotensin II within the RVLM may drive the increase in renal SNA during HF.

3.4.3. Actions of Ang II within the area postrema

In addition to sympathetic premotor areas, there is also evidence that the sensory circumventricular organs, areas of the brain without a blood–brain barrier, play an important role in determining the increased SNA in HF [66–68]. These organs include the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema, which have receptors for numerous circulating hormones, including angiotensin II [69,70], many of which are increased in HF. Therefore, these circumventricular organs may detect humoral signals from the periphery and relay this information to autonomic centers in the brain, which may contribute to the increased SNA in HF [71,72].

A number of studies have implicated the OVLT and SFO in mediating the increase in renal SNA in rodent models of HF [71,72]. In rats with HF induced by myocardial infarction, lesion of the anteroventral third ventricle (AV3V) wall, which includes the OVLT, prevented the increase in renal SNA and improved baroreflex function [67], indicating an important role for this circumventricular organ in mediating the increase in renal SNA. Lesion of the AV3V did increase mortality in rats with HF, but it is unclear if this was due to lesion of the OVLT or other forebrain areas damaged by this lesion. In rabbits with pacing-induced HF, lesion of the area postrema did not reduce resting levels or improve the baroreflex control of renal SNA. However, the ability of an AT1R antagonist to increase the sensitivity of the arterial baroreflex control of renal SNA was prevented, suggesting that, in HF, an action of angiotensin on the area postrema contributes to the reduction in baroreflex sensitivity [68].

In contrast to the results with renal SNA, lesion of the area postrema caused a large decrease in the high levels of cardiac SNA in ovine HF [66]. The finding that there is a decrease in AT1R in the area postrema in this model makes the role of angiotensin II at the area postrema unclear. The pathways from the area postrema to the cardiac sympathetic nerves remain to be elucidated. It is possible that direct efferent projections from the area postrema to the RVLM, which have been described in rabbits [73], play a role in maintaining the increased cardiac SNA in HF. The different responses of cardiac SNA and renal SNA to lesion of the area postrema, both in pacing models of HF, may result from a species effect and/or highlight the differential central control of sympathetic outflow to the heart and kidneys.

4. Actions of angiotensin II within the spinal cord

4.1. AT1R within the spinal cord

In addition to central brain regions, recent studies have examined the importance of angiotensin II within the spinal cord in modulating SNA. In this context, sympathetic preganglionic neurons of the intermediolateral cell column of the spinal cord (IML) are an important site, relaying sympathetic outflow to the heart and the kidney from the RVLM and the PVN [74,75]. Previous studies have shown that sympathetic preganglionic neurons express AT1R in the IML in multiple species, including humans [76], rats [77,78], and sheep [79]. Consistent with previous studies, we found expression of the AT1R in discrete regions of the spinal cord, including the IML, dorsal horn, and around the central canal at spinal segmental levels T1-2 and T11-12 (Fig. 1.3). Interestingly, normal sheep show higher AT1R binding density in the T11-12 sections than the corresponding T1-2 sections. There were no significant differences in AT1R expression of the dorsal horn or around the central canal. The higher levels of AT1R at the level of T11-12 compared with T1-2 mirror the higher baseline levels of sympathetic drive to the kidney compared with the heart.

4.2. Regulation of sympathetic nerve activity by AT1R within the spinal cord

As expected, based on the distribution of AT1R in the IML, when angiotensin II is given intrathecally, there is an increase in SNA. In anesthetized rats, intrathecal administration of angiotensin II at the level of T9-10 [80,81] increased splanchnic and renal SNA as well as mean arterial pressure. Additional evidence from conscious normal sheep has shown that intrathecal administration of angiotensin II at T1-2 significantly increased cardiac SNA [82]. Simultaneously recorded cardiac SNA and renal SNA showed that renal SNA did not change when either angiotensin II or losartan was injected at the level of T1-2. These findings indicate that angiotensin II has selective effects on individual neurons at specific locations in the spinal cord; stimulation of sympathetic outflow to the heart at T1-2 and to the kidney at T11-12. It is possible that due to size, the regional selectivity of the intrathecal responses is more pronounced in sheep, although very few studies have recorded from sympathetic nerves to two organs at the same time. In normal conditions, endogenous angiotensin via the AT1R does not play a role in IML sympathetic outflow since infusion of losartan intrathecally does not alter baseline levels of blood pressure [83] but can abolish the dose-dependent increases in mean arterial pressure and SNA in both anesthetized [84] and conscious preparations.

Expression of the AT2R within the spinal cord has been shown to have opposing actions to the AT1R, and inhibit SNA [84]. Selective activation of AT2R in the IML evokes hypotension and inhibition of renal SNA, which were abolished completely by AT2R inhibitors in anesthetized rats. Interestingly, blockade of endogenous AT2R in the IML significantly increased baseline mean arterial pressure and renal SNA. These data indicate that AT2R may have an endogenous role in modulation of SNA in the spinal cord, but more studies are needed in this area.

Regarding the mechanisms whereby intrathecal angiotensin II alters SNA, a previous study has examined the actions of angiotensin II on sympathetic preganglionic neurons using the whole-cell patch-clamp technique in spinal cord slice preparations. Minoura et al. demonstrated that both silent and firing sympathetic preganglionic neurons in the IML were significantly depolarized by angiotensin II [85]. In addition, after application of tetrodotoxin, angiotensin II continued to depolarize firing sympathetic preganglionic neurons suggesting both presynaptic and postsynaptic actions of angiotensin II. Application of AT1R antagonist candesartan abolished this depolarization effect indicating a role for the AT1R. It is important to note that when intrathecal injections are used, angiotensin II can also bind to the AT1R in the dorsal horn; these are likely presynaptically located on dorsal root ganglion sensory neurons [86]. While these dorsal root ganglion cells do express AT1R, the sensory modality of these neurons is not known. It is possible that angiotensin II may activate these terminals to induce a spinal reflex activation of sympathetic efferent activity.

Figure 1.3  Original X-ray images of angiotensin II binding in the spinal cord of a normal (A and C) and a heart failure (HF) sheep (B and D). Panels A and B represent spinal cord sections at thoracic level T1-2, and panels C and D represent spinal cord sections at thoracic level T11-12. The binding was totally displaced after coincubation with the angiotensin type 1 receptor (AT1R) antagonist losartan. Scale bar=5mm. The histogram shows the density of AT1R binding in the spinal cord in the normal (open bars n=5) and heart failure groups (filled bars; n=5). ∗ denotes a significant difference between the groups using an ANOVA; # denotes a significant difference between the levels at T1-2 and T11-12. P <0.05. IML, intermediolateral cell column; DH, dorsal horn; CC, central canal. Reproduced from Leversha S., Allen A.M., May C.N., Ramchandra R. Intrathecal administration of losartan reduces directly recorded cardiac sympathetic nerve activity in ovine heart failure. Hypertension 2019;74(4):896–902.

4.3. Changes in spinal cord AT1R during heart failure

Angiotensin II given within the spinal cord can increase SNA. In sheep with HF, there was increased binding of the AT1R at the level of T1-2 in the IML and a tendency to increased binding in the dorsal horn, but no differences around the central canal (Fig. 1.3). Interestingly, there were no differences in the binding of 125I-[Sar1, Ile8] angiotensin II in the IML, dorsal horn, or central canal between normal and HF sheep at the level of T11-12 (Fig. 1.3).

Consistent with the higher binding of AT1R in the spinal cord at the level of T1-2, intrathecal infusion of losartan in the sheep with pacing-induced HF did not change mean arterial pressure but caused a significant reduction in cardiac SNA and heart rate (Fig. 1.4). This inhibition appears to be site-specific since when renal SNA was simultaneously recorded in these animals, there was no change in renal SNA when losartan was injected at the level of T1-2. Our data are consistent with the idea that angiotensin II within the spinal cord may mediate changes in SNA. Importantly, this suggests that endogenous angiotensin II within the spinal cord contributes to the increased cardiac SNA in HF. This finding in a model of HF is similar to what has been observed in renovascular hypertensive rats where the RVLM-mediated increase in renal SNA is inhibited by intrathecal administration of an AT1R blocker [87–89].

When the AT1R blocker losartan was infused into the lateral or fourth cerebral ventricles, there was a long time delay of around 3h before cardiac SNA levels were reduced to levels observed in normal animals [45,66] (Fig. 1.2). This result contrasts with a shorter delay of 1h before inhibition of cardiac SNA when losartan is infused via the intrathecal infusion route (Fig. 1.4). This indicates that part of the inhibition of cardiac SNA with losartan infusion in the cerebral ventricles may be mediated by its actions on the spinal cord. Interestingly, infusion of losartan into the cisterna magna for 5h does not reduce cardiac SNA in HF sheep. However, this infusion would bathe the outside of the spinal cord and would not access the central canal. Therefore, intracisternal infusion of losartan would be unlikely to achieve a sufficient concentration at the IML to alter sympathetic preganglionic neuron activity. Such access issues may contribute to the reason high doses of losartan are required to attenuate sympathetic outflow in patients with HF [90].

Figure 1.4  Changes in mean arterial pressure, heart rate, CSNA, and RSNA in the normal (n=6) and heart failure (n=5) groups in response to intrathecal administration of losartan (1mg/ml/h) at T1-2 in normal (solid lines) and HF animals (dotted lines). N=4 for the RSNA data. # denotes a significant interaction effect of time X group. If a significant effect of time was found, post hoc analysis was done on each time point versus the baseline level. ∗ denotes a significant post hoc effect of each time point against baseline; P <0.05 using a two-way ANOVA. Reproduced from Leversha S., Allen A.M., May C.N., Ramchandra R. Intrathecal administration of losartan reduces directly recorded cardiac sympathetic nerve activity in ovine heart failure. Hypertension 2019;74(4):896–902.

5. Conclusions

In HF, the increase in the levels of SNA to the heart and the kidney are particularly damaging and are associated with increased mortality. There is a differential effect in the extent of the increase in cardiac versus renal SNA and the time course of the increase in SNA. The finding that infusion of losartan into the brain results in a significant reduction in both cardiac and renal SNA toward normal levels indicates a critical role for central angiotensinergic mechanisms. Previous studies have indicated a critical role for the PVN in setting the increased renal SNA in HF, but surprisingly this does not appear to be the case for cardiac SNA. In contrast, the area postrema plays an important role in modulating cardiac SNA in HF, although the role of angiotensin II in this circumventricular organ is not as clear. Finally, emerging evidence points toward an important role for spinal angiotensin II in modulating SNA to the heart in HF. These findings highlight the important role of central angiotensin II in differential control of SNA, and this extends to both baseline conditions and HF.

6. Future directions

It is now clear that angiotensin II within the brain and the spinal cord plays an essential role in mediating the increase in SNA during HF (Fig. 1.5). While the role of the area postrema in mediating the high levels of cardiac SNA in HF is established, the projections from the area postrema to the sympathetic premotor region, which drives this increase in cardiac SNA, are not known and require further investigation. The important role of the PVN in the control of renal SNA but not cardiac SNA in HF further highlights the need to record SNA to different vascular beds and study differential control of SNA in HF. Currently, investigations that have explored the role of the AT1R within the spinal cord in modulating sympathetic drive during HF are limited, and further research into the role of the spinal cord is needed. Increased research into the site-specific actions of central and spinal angiotensin II in HF may open new avenues for therapeutics while further advancing our understanding of this chronic disease.

Figure 1.5  Schematic showing the contribution of angiotensin II type 1 receptors in the central and spinal regions that contribute to the elevated sympathetic nerve activity in heart failure. The plus symbol denotes a contributory action of angiotensin II to the elevated sympathetic nerve activity. CVO, circumventricular organ; PVN, paraventricular nucleus of the hypothalamus, RVLM, rostral ventral lateral medulla; AP, area postrema.

Acknowledgments

Work in the author's laboratory was supported by grants from the National Health and Medical Research Council of Australia, the Heart Foundation of Australia, the Health Research Council of New Zealand, and the National Heart Foundation of New Zealand.

References

1. Roger V.L, Go A.S, Lloyd-Jones D.M, Benjamin E.J, Berry J.D, Borden W.B, et al. Heart disease and stroke statistics--2012 update: a report from the American Heart Association. Circulation. 2012;125(1):e2–e220.

2. Levy D, Kenchaiah S, Larson M.G, Benjamin E.J, Kupka M.J, Ho K.K, et al. Long-term trends in the incidence of and survival with heart failure. N Engl J Med. 2002;347(18):1397–1402.

3. Stewart S, MacIntyre K, Hole D.J, Capewell S, McMurray J.J. More ‘malignant' than cancer? Five-year survival following a first admission for heart failure. Eur J Heart Fail. 2001;3(3):315–322.

4. A randomized trial of beta-blockade in heart failure. The cardiac insufficiency bisoprolol study (CIBIS). CIBIS investigators and committees. Circulation. 1994;90(4):1765–1773.

5. Pfeffer M.A, Braunwald E, Moye L.A, Basta L, Brown Jr. E.J, Cuddy T.E, et al. Effect of captopril on mortality and morbidity in patients with left ventricular dysfunction after myocardial infarction. Results of the survival and ventricular enlargement trial. The SAVE Investigators. N Engl J Med. 1992;327(10):669–677.

6. Kitzman D.W, Little W.C, Brubaker P.H, Anderson R.T, Hundley W.G, Marburger C.T, et al. Pathophysiological characterization of isolated diastolic heart failure in comparison to systolic heart failure. JAMA. 2002;288(17):2144–2150.

7. Hasking G.J, Esler M.D, Jennings G.L, Burton D, Johns J.A, Korner P.I. Norepinephrine spillover to plasma in patients with congestive heart failure: evidence of increased overall and cardiorenal sympathetic nervous activity. Circulation. 1986;73(4):615–621.

8. Rundqvist B, Elam M, Bergmann-Sverrisdottir Y, Eisenhofer G, Friberg P. Increased cardiac adrenergic drive precedes generalized sympathetic activation in human heart failure. Circulation. 1997;95(1):169–175.

9. Packer M, Bristow M.R, Cohn J.N, Colucci W.S, Fowler M.B, Gilbert E.M, et al. The effect of carvedilol on morbidity and mortality in patients with chronic heart failure. U.S. Carvedilol Heart Failure Study Group. N Engl J Med. 1996;334(21):1349–1355.

10. Ramchandra R, Hood S.G, Frithiof R, May C.N. Discharge properties of cardiac and renal sympathetic nerves and their impaired responses to changes in blood volume in heart failure. Am J Physiol Regul Integr Comp Physiol. 2009;297(3):R665–R674.

11. Watson A.M, Hood S.G, Ramchandra R, McAllen R.M, May C.N. Increased cardiac sympathetic nerve activity in heart failure is not due to desensitization of the arterial baroreflex. Am J Physiol Heart Circ Physiol. 2007;293(1):H798–H804.

12. Ramchandra R, Hood S.G, Denton D.A, Woods R.L, McKinley M.J, McAllen R.M, et al. Basis for the preferential activation of cardiac sympathetic nerve activity in heart failure. Proc Natl Acad Sci U S A. 2009;106(3):924–928.

13. Dibona G.F, Jones S.Y, Sawin L.L. Reflex influences on renal nerve activity characteristics in nephrosis and heart failure. J Am Soc Nephrol. 1997;8(8):1232–1239.

14. DiBona G.F, Sawin L.L. Role of renal nerves in sodium retention of cirrhosis and congestive heart failure. Am J Physiol. 1991;260(2 Pt 2):R298–R305.

15. Liu J.L, Zucker I.H. Regulation of sympathetic nerve activity in heart failure: a role for nitric oxide and angiotensin II. Circ Res. 1999;84(4):417–423.

16. Pinkham M.I, Whalley G, Guild S.J, Malpas S.C, Barrett C.J. Arterial baroreceptor reflex control of renal sympathetic nerve activity following chronic myocardial infarction in male, female and ovariectomized female rats. Am J Physiol Regul Integr Comp Physiol. 2015;309(2):R169–178 doi:10.1152/ajpregu.00026.2015..

17. Zhang K, Li Y.F, Patel K.P. Reduced endogenous GABA-mediated inhibition in the PVN on renal nerve discharge in rats with heart failure. Am J Physiol Regul Integr Comp Physiol. 2002;282(4):R1006–R1015.

18. Kaye D.M, Lefkovits J, Jennings G.L, Bergin P, Broughton A, Esler M.D. Adverse consequences of high sympathetic nervous activity in the failing human heart. J Am Coll Cardiol. 1995;26(5):1257–1263.

19. Petersson M, Friberg P, Eisenhofer G, Lambert G, Rundqvist B. Long-term outcome in relation to renal sympathetic activity in patients with chronic heart failure. Eur Heart J. 2005;26(9):906–913.

20. Liang C, Rounds N.K, Dong E, Stevens S.Y, Shite J, Qin F. Alterations by norepinephrine of cardiac sympathetic nerve terminal function and myocardial beta-adrenergic receptor sensitivity in the ferret: normalization by antioxidant vitamins. Circulation. 2000;102(1):96–103.

21. Briest W, Holzl A, Rassler B, Deten A, Leicht M, Baba H.A, et al. Cardiac remodeling after long term norepinephrine treatment in rats. Cardiovasc Res. 2001;52(2):265–273.

22. Jensen B.C, O'Connell T.D, Simpson P.C. Alpha-1-adrenergic receptors in heart failure: the adaptive arm of the cardiac response to chronic catecholamine stimulation. J Cardiovasc Pharmacol. 2014;63(4):291–301.

23. Kaye D.M, Lambert G.W, Lefkovits J, Morris M, Jennings G, Esler M.D. Neurochemical evidence of cardiac sympathetic activation and increased central nervous system norepinephrine turnover in severe congestive heart failure. J Am Coll Cardiol. 1994;23(3):570–578.

24. Dhalla N.S, Adameova A, Kaur M. Role of catecholamine oxidation in sudden cardiac death. Fundam Clin Pharmacol. 2010;24(5):539–546.

25. Abukar Y, May C.N, Ramchandra R. Role of endothelin-1 in mediating changes in cardiac sympathetic nerve activity in heart failure. Am J Physiol Regul Integr Comp Physiol. 2016;310(1):R94–R99.

26. Garg R, Yusuf S. Overview of randomized trials of angiotensin-converting enzyme inhibitors on mortality and morbidity in patients with heart failure. Collaborative Group on ACE Inhibitor Trials. JAMA. 1995;273(18):1450–1456.

27. Brunner-La Rocca H.P, Vaddadi G, Esler M.D. Recent insight into therapy of congestive heart failure: focus on ACE inhibition and angiotensin-II antagonism. J Am Coll Cardiol. 1999;33(5):1163–1173.

28. Esler M. Differentiation in the effects of the angiotensin II receptor blocker class on autonomic function. J Hypertens Suppl. 2002;20(5):S13–S19.

29. Kasama S, Toyama T, Kumakura H, Takayama Y, Ichikawa S, Suzuki T, et al. Effects of candesartan on cardiac sympathetic nerve activity in patients with congestive heart failure and preserved left ventricular ejection fraction. J Am Coll Cardiol. 2005;45(5):661–667.

30. Takeishi Y, Atsumi H, Fujiwara S, Takahashi K, Tomoike H. ACE inhibition reduces cardiac iodine-123-MIBG release in heart failure. J Nucl Med. 1997;38(7):1085–1089.

31. Gohlke P, Kox T, Jurgensen T, von Kugelgen S, Rascher W, Unger T, et al. Peripherally applied candesartan inhibits central responses to angiotensin II in conscious rats. Naunyn-Schmiedeberg’s Arch Pharmacol. 2002;365(6):477–483. .

32. Ramchandra R, Watson A.M, Hood S.G, May C.N. Response of cardiac sympathetic nerve activity to intravenous irbesartan in heart failure. Am J Physiol Regul Integr Comp Physiol. 2010;298(4):R1056–R1060.

33. May C.N, Yao S.T, Booth L.C, Ramchandra R. Cardiac sympathoexcitation in heart failure. Auton Neurosci: Basic Clin. 2013;175(1–2):76–84.

34. Zucker I.H, Patel K.P, Schultz H.D. Neurohumoral stimulation. Heart Fail Clin. 2012;8(1):87–99.

35. Ganta C.K, Lu N, Helwig B.G, Blecha F, Ganta R.R, Zheng L, et al. Central angiotensin II-enhanced splenic cytokine gene expression is mediated by the sympathetic nervous system. Am J Physiol Heart Circ Physiol. 2005;289(4):H1683–H1691.

36. Lu N, Helwig B.G, Fels R.J, Parimi S, Kenney M.J. Central Tempol alters basal sympathetic nerve discharge and attenuates sympathetic excitation to central ANG II. Am J Physiol Heart Circ Physiol. 2004;287(6):H2626–H2633.

37. Wei S.G, Yu Y, Zhang Z.H, Weiss R.M, Felder R.B. Angiotensin II-triggered p44/42 mitogen-activated protein kinase mediates sympathetic excitation in heart failure rats. Hypertension. 2008;52(2):342–350.

38. Watson A.M, Mogulkoc R, McAllen R.M, May C.N. Stimulation of cardiac sympathetic nerve activity by central angiotensinergic mechanisms in conscious sheep. Am J Physiol Regul Integr Comp Physiol. 2004;286(6):R1051–R1056.

39. Polidori C, Ciccocioppo R, Nisato D, Cazaubon C, Massi M. Evaluation of the ability of irbesartan to cross the blood-brain barrier following acute intragastric treatment. Eur J Pharmacol. 1998;352(1):15–21.

40. Wang W, Ma R. Cardiac sympathetic afferent reflexes in heart failure. Heart Fail Rev. 2000;5(1):57–71.

41. Gao L, Wang W, Li Y.L, Schultz H.D, Liu D, Cornish K.G, et al. Sympathoexcitation by central ANG II: roles for AT1 receptor upregulation and NAD(P)H oxidase in RVLM. Am J Physiol Heart Circ Physiol. 2005;288(5):H2271–H2279.

42. Yoshimura R, Sato T, Kawada T, Shishido T, Inagaki M, Miyano H, et al. Increased brain angiotensin receptor in rats with chronic high-output heart failure. J Card Fail. 2000;6(1):66–72.

43. DiBona G.F, Jones S.Y, Brooks V.L. ANG II receptor blockade and arterial baroreflex regulation of renal nerve activity in cardiac failure. Am J Physiol. 1995;269(5 Pt 2):R1189–R1196.

44. Francis J, Wei S.G, Weiss R.M, Felder R.B. Brain angiotensin-converting enzyme activity and autonomic regulation in heart failure. Am J Physiol Heart Circ Physiol. 2004;287(5):H2138–H2146.

45. Ramchandra R, Hood S.G, Watson A.M, Allen A.M, May C.N. Central angiotensin type 1 receptor blockade decreases cardiac but not renal sympathetic nerve activity in heart failure. Hypertension. 2012;59(3):634–641.

46. Gao L, Schultz H.D, Patel K.P, Zucker I.H, Wang W. Augmented input from cardiac sympathetic afferents inhibits baroreflex in rats with heart failure. Hypertension. 2005;45(6):1173–1181.

47. Cano G, Card J.P, Sved A.F. Dual viral transneuronal tracing of central autonomic circuits involved in the innervation of the two kidneys in rat. J Comp Neurol. 2004;471(4):462–481.

48. Jansen A.S, Wessendorf M.W, Loewy A.D. Transneuronal labeling of CNS neuropeptide and monoamine neurons after pseudorabies virus injections into the stellate ganglion. Brain Res. 1995;683(1):1–24. .

49. Coote J.H. A role for the paraventricular nucleus of the hypothalamus in the autonomic control of heart and kidney. Exp Physiol. 2005;90(2):169–173.

50. Deering J, Coote J.H. Paraventricular neurones elicit a volume expansion-like change of activity in sympathetic nerves to the heart and kidney in the rabbit. Exp Physiol. 2000;85(2):177–186.

51. Pyner S, Deering J, Coote J.H. Right atrial stretch induces renal nerve inhibition and c-fos expression in parvocellular neurones of the paraventricular nucleus in rats. Exp Physiol. 2002;87(1):25–32.

52. Akama H, McGrath B.P, Badoer E. Volume expansion fails to normally activate neural pathways in the brain of conscious rabbits with heart failure. J Auton Nerv Syst. 1998;73(1):54–62.

53. Ng C.W, De Matteo R, Badoer E. Effect of muscimol and L-NAME in the PVN on the RSNA response to volume expansion in conscious rabbits. Am J Physiol Ren Physiol. 2004;287(4):F739–F746.

54. Li Y.F, Patel K.P. Paraventricular nucleus of the hypothalamus and elevated sympathetic activity in heart failure: the altered inhibitory mechanisms. Acta Physiol Scand. 2003;177(1):17–26.

55. Li Y.F, Wang W, Mayhan W.G, Patel K.P. Angiotensin-mediated increase in renal sympathetic nerve discharge within the PVN: role of nitric oxide. Am J Physiol Regul Integr Comp Physiol. 2006;290(4):R1035–R1043.

56. Silva A.Q, Santos R.A, Fontes M.A. Blockade of endogenous angiotensin-(1-7) in the hypothalamic paraventricular nucleus reduces renal sympathetic tone. Hypertension. 2005;46(2):341–348.

57. Zheng H, Li Y.F, Wang W, Patel K.P. Enhanced angiotensin-mediated excitation of renal sympathetic nerve activity within the paraventricular nucleus of anesthetized rats with heart failure. Am J Physiol Regul Integr Comp Physiol. 2009;297(5):R1364–R1374.

58. Patel K.P, Zhang K, Kenney M.J, Weiss M, Mayhan W.G. Neuronal expression of Fos protein in the hypothalamus of rats with heart failure. Brain Res. 2000;865(1):27–34.

59. Vahid-Ansari F, Leenen F.H. Pattern of neuronal activation in rats with CHF after myocardial infarction. Am J Physiol. 1998;275(6):H2140–H2146.

60. Patel K.P. Role of paraventricular nucleus in mediating sympathetic outflow in heart failure. Heart Fail Rev. 2000;5(1):73–86.

61. Zhang Z.H, Francis J, Weiss R.M, Felder R.B. The renin-angiotensin-aldosterone system excites hypothalamic paraventricular nucleus neurons in heart failure. Am J Physiol Heart Circ Physiol. 2002;283(1):H423–H433.

62. Ramchandra R, Hood S.G, Frithiof R, McKinley M.J, May C.N. The role of the paraventricular nucleus of the hypothalamus in the regulation of cardiac and renal sympathetic nerve activity in conscious normal and heart failure sheep. J Physiol. 2013;591(Pt 1):93–107.

63. Gao L, Wang W.Z, Wang W, Zucker I.H. Imbalance of angiotensin type 1 receptor and angiotensin II type 2 receptor in the rostral ventrolateral medulla: potential mechanism for sympathetic overactivity in heart failure. Hypertension. 2008;52(4):708–714.

64. Gao L, Wang W, Li Y.L, Schultz H.D, Liu D, Cornish K.G, et al. Superoxide mediates sympathoexcitation in heart failure: roles of angiotensin II and NAD(P)H oxidase. Circ Res. 2004;95(9):937–944.

65. Fahim M, Gao L, Mousa T.M, Liu D, Cornish K.G, Zucker I.H. Abnormal baroreflex function is dissociated from central angiotensin II receptor expression in chronic heart failure. Shock. 2012;37(3):319–324. .

66. Abukar Y, Ramchandra R, Hood S.G, McKinley M.J, Booth L.C, Yao S.T, et al. Increased cardiac sympathetic nerve activity in ovine heart failure is reduced by lesion of the area postrema, but not lamina terminalis. Basic Res Cardiol. 2018;113(5):35.

67. Francis J, Wei S.G, Weiss R.M, Beltz T, Johnson A.K, Felder R.B. Forebrain-mediated adaptations to myocardial infarction in the rat. Am J Physiol Heart Circ Physiol. 2002;282(5):H1898–H1906.

68. Liu J.L, Murakami H, Sanderford M, Bishop V.S, Zucker I.H. ANG II and baroreflex function in rabbits with CHF and lesions of the area postrema. Am J Physiol. 1999;277(1):H342–H350.

69. McKinley M.J, Albiston A.L, Allen A.M, Mathai M.L, May C.N, McAllen R.M, et al. The brain renin-angiotensin system: location and physiological roles. Int J Biochem Cell Biol. 2003;35(6):901–918.

70. McKinley M.J, McAllen R.M, Davern P, Giles M.E, Penschow J, Sunn N, et al. The sensory circumventricular organs of the mammalian brain. Adv Anat Embryol Cell Biol. 2003;172:III–XII 1-122, back cover.

71. Wei S.G, Yu Y, Felder R.B. Blood-borne interleukin-1beta acts on the subfornical organ to upregulate the sympathoexcitatory milieu of the hypothalamic paraventricular nucleus. Am J Physiol Regul Integr Comp Physiol. 2018;314(3):R447–R458.

72. Yu Y, Wei S.G, Weiss R.M, Felder R.B. TNF-alpha receptor 1 knockdown in the subfornical organ ameliorates sympathetic excitation and cardiac hemodynamics in heart failure rats. Am J Physiol Heart Circ Physiol. 2017;313(4):H744–H756.

73. Blessing W.W, Hedger S.C, Joh T.H, Willoughby J.O. Neurons in the area postrema are the only catecholamine-synthesizing cells in the medulla or pons with projections to the rostral ventrolateral medulla (C1-area) in the rabbit. Brain Res. 1987;419(1–2):336–340.

74. Sevigny C.P, Bassi J, Teschemacher A.G, Kim K.S, Williams D.A, Anderson C.R, et al. C1 neurons in the rat rostral ventrolateral medulla differentially express vesicular monoamine transporter 2 in soma and axonal compartments. Eur J Neurosci. 2008;28(8):1536–1544.

75. Sevigny C.P, Bassi J, Williams D.A, Anderson C.R, Thomas W.G, Allen A.M. Efferent projections of C3 adrenergic neurons in the rat central nervous system. J Comp Neurol. 2012;520(11):2352–2368.

76. MacGregor D.P, Murone C, Song K, Allen A.M, Paxinos G, Mendelsohn F.A. Angiotensin II receptor subtypes in the human central nervous system. Brain Res. 1995;675(1–2):231–240.

77. Ahmad Z, Milligan C.J, Paton J.F, Deuchars J. Angiotensin type 1 receptor immunoreactivity in the thoracic spinal cord. Brain Res. 2003;985(1):21–31.

78. Allen A.M, McKinley M.J, Oldfield B.J, Dampney R.A, Mendelsohn F.A. Angiotensin II receptor binding and the baroreflex pathway. Clin Exp Hypertens Theor Pract. 1988;10(Suppl. 1):63–78.

79. Oldfield B.J, Allen A.M, Hards D.K, McKinley M.J, Schlawe I, Mendelsohn F.A.Distribution of angiotensin II receptor binding in the spinal cord of the sheep. Brain Res. 1994;650(1):40–48.

80. Suter C, Coote J.H. Intrathecally administered angiotensin II increases sympathetic activity in the rat. J Auton Nerv Syst. 1987;19(1):31–37.

81. Yashpal K, Gauthier S, Henry J.L. Angiotensin II stimulates sympathetic output by a direct spinal action. Neuropeptides. 1989;14(1):21–29. .

82. Leversha S, Allen A.M, May C.N, Ramchandra R. Intrathecal administration of losartan reduces directly recorded cardiac sympathetic nerve activity in ovine heart failure. Hypertension. 2019;74(4):896–902.

83. Lewis D.I, Coote J.H. Angiotensin II in the spinal cord of the rat and its sympatho-excitatory effects. Brain Res. 1993;614(1–2):1–9.

84. Chao J, Gao J, Parbhu K.J, Gao L. Angiotensin type 2 receptors in the intermediolateral cell column of the spinal cord: negative regulation of sympathetic nerve activity and blood pressure. Int J Cardiol. 2013;168(4):4046–4055.

85. Minoura Y, Onimaru H, Iigaya K, Homma I, Kobayashi Y. Electrophysiological responses of sympathetic preganglionic neurons to ANG II and aldosterone. Am J Physiol Regul Integr Comp Physiol. 2009;297(3):R699–R706.

86. Pavel J, Tang H, Brimijoin S, Moughamian A, Nishioku T, Benicky J, et al. Expression and transport of Angiotensin II AT1 receptors in spinal cord, dorsal root ganglia and sciatic nerve of the rat. Brain Res. 2008;1246:111–122.

87. Milanez M.I.O, Nishi E.E, Bergamaschi C.T, Campos R.R. Role of spinal neurons in the maintenance of elevated sympathetic activity: a novel therapeutic target?Am J Physiol Regul Integr Comp Physiol. 2020;319(3):R282–R287.

88. Milanez M.I.O, Nishi E.E, Mendes R, Rocha A.A, Bergamaschi C.T, Campos R.R.Renal sympathetic activation triggered by the rostral ventrolateral medulla is dependent of spinal cord AT1 receptors in Goldblatt hypertensive rats. Peptides. 2021;146:170660.

89. Milanez M.I.O, Nishi E.E, Rocha A.A, Bergamaschi C.T, Campos R.R. Interaction between angiotensin II and GABA in the spinal cord regulates sympathetic vasomotor activity in Goldblatt hypertension. Neurosci Lett. 2020;728:134976.

90. Ruzicka M, Floras J.S, McReynolds A.J, Coletta E, Haddad H, Davies R, et al. Do high doses of AT(1)-receptor blockers attenuate central sympathetic outflow in humans with chronic heart failure?Clin Sci. 2013;124(9):589–595.

Chapter 2: The contribution of angiotensin peptides to cardiovascular neuroregulation in health and disease

Ewa Szczepanska-Sadowska, Tymoteusz Zera, Michal Kowara, and Agnieszka Cudnoch-Jedrzejewska     Chair and Department of Experimental and Clinical Physiology, Laboratory of Centre for Preclinical Research, Medical University of Warsaw, Warsaw, Poland

Abstract

The brain renin–angiotensin system (RAS) is critically involved in the cardiovascular regulation and energy homeostasis. The overwhelming evidence indicates that the central RAS closely cooperates with the systemic RAS by means of interactions with the autonomic nervous system and hormonal and humoral factors linking RAS with regulation of blood pressure, water–electrolyte equilibrium, and energy balance. Particularly significant roles in these interactions are played by aldosterone and vasopressin; however, other factors such as proinflammatory cytokines, growth factors, nitric oxide, prostaglandins, and ROS appear to be also essential players, especially during challenges (stress, hypoxia), and under pathological conditions (hypertension, heart failure, metabolic syndrome, atherosclerosis, COVID-19). Current pharmacotherapies of cardiovascular diseases are focused on the angiotensin-converting enzyme inhibitors and angiotensin type 1 receptor blockers and their effects on systemic RAS. A better understanding of neuroanatomical and neurochemical connections between central RAS and other regulatory systems should advance pharmacological interventions aimed at central RAS.

Keywords

Autonomic nervous system; Brain RAS; COVID-19; Depression; Heart failure; Hypertension; Metabolic disorders; RAS inhibitors; Stress

1. Introduction

Biologically active angiotensin peptides form a family of highly active compounds, playing significant role in the regulation of a variety of vital processes. Together with renin, they form local renin–angiotensin systems (RASs) acting in several organs and tissues. At the same time, they cooperate together as the systemic RAS. The harmonized cooperation of the local RASs largely depends on the coordinating function of the central nervous system RAS.

Angiotensin peptides are produced locally by cells of several organs, including the brain, heart, kidney, liver, and gastrointestinal system. Regulatory potentialities of particular peptides of this group significantly differ among various cells and organs. The potency and specificity of their action in the target cells largely depends on presence of specific receptors and efficacy of intracellular pathways mediating their action. A growing number of studies provide evidence that the complexity of processes involved in activation of various components of RAS and determining their regulatory potency significantly increases under pathological conditions, especially those affecting the cardiovascular system.

2. The overview of renin–angiotensin system organization: cooperation of central and systemic renin–angiotensin system

Details of organization of the RAS have been described in several previous studies [1–5], and they are briefly summarized in the following. The angiotensin peptides are generated from the angiotensinogen as a result of a proteolytic cleavage by renin. In the kidney, renin is synthesized as prorenin in the juxtaglomerular cells of terminal afferent arteriole and initiates generation of angiotensins from locally synthesized or circulating angiotensinogen. Synthesis of renin increases upon activation of the renal sympathetic nerves, and subsequent release of catecholamines and stimulation of β1 receptors. The release of renin in the kidney is also under control of several local factors such as hypoxia, reduced sodium content in the macula densa cells, and numerous hormonal and humoral factors (see Section 5).

The overview of the cleavage cascade engaged in formation of specific components of the RAS is presented in Fig. 2.1. Briefly, preprorenin and prorenin are enzymatically converted to renin, which is a highly potent enzyme. Renin acts on the N-terminus of its own substrate—angiotensinogen (Agt) cleaving a decapeptide angiotensin I [(Ang I, Ang-(1–10)], which is converted either to highly active angiotensin II [(Ang II, Ang-(1–8)] by angiotensin-converting enzyme 1 (ACE1) or to Ang-(1–9) by angiotensin-converting enzyme 2 (ACE2). Ang-(1–7), which is another biologically potent peptide of the RAS system, can be cleaved either from Ang-(1–9) by ACE1 or from Ang-(1–8) by ACE2. Other components of the RAS system [Ang III, Ang-(2–8); Ang IV, Ang-(3–8); Ang-(1–5), Ang-(5–8); Ang-(1–12)] are produced by enzymatic cleavage performed by other enzymes, such as aminopeptidases, which make a scission at the N-terminus of peptides and carboxypeptidases that act at the carboxy terminus or by endopeptidases.

Figure 2.1  The main components of the renin–angiotensin system engaged in the regulation of the cardiovascular system and water and electrolyte balance.ACE—angiotensin-converting enzyme inhibitor; ACE1, ACE2—, angiotensin-converting enzymes; APA, APN—aminopeptidases; AT1R—Ang II and Ang III receptors; AT2R—ANG II receptors; ATR4—Ang IV receptors; MasR—Ang-(1–7) receptors.

The main source of Agt is liver. Angiotensinogen is also locally synthetized in the brain [6,7], and astrocytes appear to be the main source of the protein in the central nervous system [8,9]. However, neurons of discrete regions of mouse brains, specifically the subfornical organ (SFO), the mesencephalic trigeminal nucleus, and the external lateral parabrachial nucleus, also express Agt [10]. Also neurons located in the paraventricular nucleus (PVN), the supraoptic nucleus (SON), and the accessory magnocellular nucleus were found to have positive immunostaining for Agt and Ang II [11]. Expression of Agt in pre/neonatal period seems to be more generalized and was found in astroglial cells of the hypothalamus and the brainstem, the subcortical, and cortical regions and in neurons present in limbic and sensorimotor regions of the brain [12].

Although some controversy exists how effective the local expression of renin is in the brain and whether the key source of renin activity in the brain is derived from the enzyme present in blood in the cerebral circulation [13,14], it is generally accepted that renin and prorenin can be effectively expressed in the central nervous system [15–18]. Experiments in double-transgenic mice expressing enhanced green fluorescent protein (eGFP) driven by the renin promoter and beta-galactosidase (beta-Gal) driven by the human Agt promoter revealed that both Agt and renin are expressed in adjacent cells in the SFO, the rostral ventrolateral medulla (RVLM), and hippocampal CA 1–3 regions [19], indicating that effective formation of Ang II is possible in the brain, including the cardiovascular centers. Accumulating evidence indicates that in addition to classic isoform of renin, a shorter form of the enzyme termed renin-b with expression restricted to the brain has been found in the neurons. It has also been suggested that this intracellular form of renin participates in the synthesis of Ang II that is subsequently released as a neurotransmitter [17,20–22].

The angiotensin-converting enzyme of type 1 (ACE, ACE1), which is the main convertor of angiotensin I (Ang I), acts predominantly in the lungs. ACE2, which is the membrane-bound metalloproteinase, cleaves Ang-(1–8) to Ang-(1–7). It also hydrolyzes other peptides, such as apelin, kinins, (des-Arg9)-bradykinin, neurotensin, and dynorphin A-(1–13) [23].

In many respects, and especially with regard to the regulation of the cardiovascular system, Ang-(1–7) exerts opposite effects to Ang II. Regulatory properties of Ang-(1–7) have been intensely investigated in context of its protective capability in cardiovascular diseases [24–28]. Besides, growing evidence indicates involvement of ACE2—Ang-(1–7) axis in the regulation of glycemia through actions exerted in the pancreatic, hepatic, and gastrointestinal cells [29] (see also Section 9). It was shown that Ang-(1–7) plays an essential role in the regulation of intestinal absorption of neutral amino acids, particularly of tryptophan, and it has been suggested that activation of ACE2 in the gastrointestinal tract may regulate substrate availability for neurotransmitters synthesis [30,31] (see also Section 9). Recent studies revealed that ACE2 plays a key role in cellular SARS-CoV2 invasion [32,33] (see also Section 6).

Studies on spatial distribution of ACE2 in the brain revealed that ACE2 is well expressed in the regions engaged in the regulation of cardiovascular and cardiorespiratory functions. Namely, they have been found in the olfactory bulb, the posterior cingulate, and temporal cortex; in the PVN and SON; in the substantia nigra, ventral tegmental area, the dorsal raphe nucleus, and the parabrachial nucleus (PBN); in the reticular nuclei, gigantocellular reticular, and retrotrapezoid nuclei; and in the area postrema (AP), the nucleus of the solitary tract (NTS), the RVLM, the dorsal motor nucleus of the vagus (DMNV), and the nucleus ambiguous [28,30,33].

Using pharmacological and molecular methods, it was possible to show that angiotensin peptides interact with AT1R, AT2R, AT4, and MasR receptors, which belong to G protein–coupled receptors. In rodents, AT1Rs receptors are subdivided into AT1a and AT1b receptors.

Stimulation of AT1R activates multiple intracellular enzymatic pathways, such as phospholipase C (PLC), phospholipase D, phospholipase A2 (PLA2), nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, and metalloproteinases (MMP). Mobilization of these pathways results in consecutive stimulation of other enzymes and highly active proteins, such as cyclooxygenases, lipooxygenases, cytochrome P450 enzymes, mitogen-activated protein kinases (MAPK), c-Jun N-terminal kinases (JNK), extracellular signal–regulated protein kinases 1 and 2 (ERK1/2), and some transcription factors (NF-κB, AP-1, HIF-1α). There is also evidence for functional cross-talk between the intracellular signaling pathways of AT1R and other angiotensin receptors [4,34–36]. Stimulation of AT2R causes activation phosphotyrosine phosphatases and inactivation of mitogen-activated protein kinases, activation of potassium channels, activation of phospholipase A2, and generation of arachidonic acid derivatives [5,35,37,38].

Effects of Ang II depend on proportion of types of receptors, which are stimulated. Binding to AT1 receptor (AT1R) results in activation of the sympathetic system, vasoconstriction, oxidative stress, inflammation, and fibrosis, whereas activation of AT2 receptor (AT2R) and Ang-(1–7) receptor promotes opposing effects. Prolonged activation of RAS is associated with Ang II—AT1R deleterious effects [4,35,39–42].

AT1 receptors are present in the brain, heart, and vessels, and in the renal glomeruli, tubules, vessels, and medullary interstitial cells. In the brain, high density of AT1Rs has been found in the circumventricular organs (CVOs), which lack the blood–brain barrier (BBB), and in several cardiovascular regions. Specifically they are present in the SFO, the organum vasculosum of the lamina terminalis (OVLT), the AP, the median preoptic area (MnPO), the PVN, the nucleus of the tractus solitarius (NTS), and the ventrolateral medulla (VLM) [39,41,43–47].

It has been shown that angiotensin IV (Ang IV) protects from harmful activity of Ang II in the cardiovascular system and exerts procognitive effects [48–52]. Autoradiography and functional studies provide evidence for presence of AT4 receptors in the brain, heart, vessels, and kidney [49,53–56]. At the cellular level, Ang IV stimulates AT4 receptor (AT4R), which has been identified as insulin-regulated aminopeptidase (IRAP). In the brain, it interacts also with hepatocyte growth factor/c-Met receptor pathway [57,58].

Ang-(1–7) activates the Mas receptor (MasR) and subsequently the ACE2—Ang-(1–7)—MasR axis [59–61]. Similarly, as other angiotensin receptors, MasRs are present in the brain, cardiovascular system, and the kidney [59,62,63].

3. Cooperation of the brain renin–angiotensin system with the autonomic nervous system: interactions with the sympathetic, parasympathetic, and enteric systems

The RAS affects activity of the sympathetic and parasympathetic divisions of the autonomic nervous system at the level of cardiovascular centers of the brainstem and the hypothalamus, and by acting on afferent and efferent pathways involved in the cardiovascular control. These interactions involve both the local brain RAS and Ang II present in the blood stream [14,64,65]. All components of the RAS are expressed in the key cardiovascular centers of the central nervous system [16,46].

The effects of RAS on the sympathetic branch of the autonomic nervous system have been studied since 1960, when early experiments in dogs revealed that the pressor effect of intravenously administered Ang II partially depends on central mechanism involving sympathetic response [66]. Furthermore, experiments in humans showed that intravenously administered Ang II at pressor doses inhibits sympathetic nerve activity by activating arterial baroreflex. However, this inhibition is significantly weaker in comparison with phenylephrine-induced activation of baroreflex, and when blood pressure increase is prevented by nitroprusside infusion to inactivate the arterial baroreflex, the peripherally administered Ang II causes increase in the muscle sympathetic nerve activity (MSNA) [67].

A large body of evidence shows that brain Ang II and AT1Rs are critically involved in winding up sympathetic activity to the heart, vasculature, and kidney with the key neural pathway involving connections between the SFO, PVN, RVLM, and the intermediolateral nucleus (IML), and the AP with the NTS and CVLM [14,16,64]. Angiotensinergic projections from the PVN stimulate AT1Rs in the RVLM and excite pressor neurons of the RVLM [68]. In addition, direct pathways from the PVN to the sympathetic neurons of the IML in the spinal cord are activated by Ang II acting on AT1Rs in the SFO [69]. Furthermore, AT1Rs are located in the presynaptic terminals on the sympathetic neurons of the IML at the thoracic spinal cord [70]. Thus, Ang II may bind to AT1Rs at all organizational levels of key structures involved in the sympathetic outflow. Experiments in rodents show that targeted genetic upregulation of specific components of RAS, such as Agt and/or renin in the central nervous system, leads to sympathoexcitation and increase in arterial blood pressure [71–73]. In this light, mice lacking brain-specific renin-b showed increased expression of AT1Rs in the PVN and increased expression of classic isoform of renin in the RVLM accompanied by enhanced responsiveness of the renal sympathetic nerves and suggesting that intracellular renin-b inhibits expression of renin [22]. In contrast to classic RAS, upregulation of ACE2—Ang-(1–7)—MasR axis or silencing (pro)renin receptor exerts sympathoinhibitory and pressure-lowering effects, especially under condition of hypertension or heart failure [74–78]. However, studies investigating the direct effects of Ang-(1–7) on sympathetic activity have provided varied results, with both excitatory and inhibitory effects on the sympathetic activity. Specifically, in rats, blockade of MasR for Ang-(1–7) in the PVN decreases sympathetic activity [79,80], which is suggestive of sympathoexcitatory effect of Ang-(1–7) in the PVN. However, Ang-(1–7) administered into the NTS exerts sympathoinhibitory effect in rats [81]. In anesthetized rabbits, microinjections of Ang-(1–7) into the RVLM increase sympathetic activity, whereas Ang-(1–7) administered into the CVLM attenuates sympathetic activity; however, the responses were observed for high doses of the peptide, suggestive of insignificant role of Ang-(1–7) under physiological conditions [82]. Nonetheless, Ang-(1–7) injected into the RVLM had also pressor effect in awake rats [83]. Furthermore, experiments in brainstem slices indicate that the plausible target of Ang-(1–7) in the RVLM are astrocytes