Chronic heart failure (CHF) is a multi-factorial disease process that is characterized by over activation of the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system. Both of these systems are chronically activated in CHF. The RAAS consists of an excitatory arm involving Angiotensin II (Ang II), Angiotensin Converting Enzyme (ACE), and the Ang II type 1 Receptor (AT1R). The RAAS also consists of a protective arm consisting of Angiotensin-1-7 (Ang -1-7), the Ang II type 2 receptor (AT2R), ACE2 and the mas receptor. Sympathoexcitation in CHF is driven, in large part, by an imbalance of these two arms, with an increase in the Ang II-AT1R-ACE arm and a decrease in the AT2R-ACE2 arm. This imbalance is manifested in cardiovascular-control regions of the brain such as the rostral ventrolateral medulla and paraventricular nucleus in the hypothalamus. This review focuses on current literature that describes the components of these two arms of the RAAS, and their imbalance in the CHF state. Moreover, this review provides additional evidence for the relevance of ACE2 and Ang-1-7 as key players in the regulation of central sympathetic outflow in CHF. Finally we also examine the effects of exercise training as a therapeutic strategy and the molecular mechanisms at play in CHF, in part, because of the ability of exercise training to restore the balance of the RAAS axis and sympathetic outflow.

Excessive sympathetic drive is a hallmark of chronic heart failure (HF). Disease progression can be correlated with plasma norepinephrine concentration. Renal function is also correlated with disease progression and prognosis. Because both the renal nerves and renin-angiotensin II system are activated in chronic HF we hypothesized that excessive renal sympathetic nerve activity decreases renal blood flow in HF and is associated with changes in angiotensin II type 1 receptor (AT1R) and angiotensin II type 2 receptor (AT2R) expression. The present study was carried out in conscious, chronically instrumented rabbits with pacing-induced HF. We found that rabbits with HF showed a decrease in mean renal blood flow (19.8±1.6 in HF vs. 32.0±2.5 ml/min from prepace levels; P<0.05) and an increase in renal vascular resistance (3.26±0.29 in HF vs. 2.21±0.13 mmHg·ml(-1)·min in prepace normal rabbits; P<0.05) while the blood flow and resistance was not changed in HF rabbits with the surgical renal denervation. Renal AT1R expression was increased by ∼67% and AT2R expression was decreased by ∼87% in rabbits with HF; however, kidneys from denervated rabbits with HF showed a near normalization in the expression of these receptors. These results suggest renal sympathetic nerve activity elicits a detrimental effect on renal blood flow and may be associated with alterations in the expression of angiotensin II receptors.

Brain ANG II plays an important role in modulating sympathetic function and homeostasis. The generation and degradation of ANG II are carried out, to a large extent, through the angiotensin-converting enzyme (ACE) and ACE2, respectively. In disease states, such as hypertension and chronic heart failure, central expression of ACE is upregulated and ACE2 is decreased in central sympathoregulatory neurons. In this study, we determined the expression of ACE and ACE2 in response to ANG II in a neuronal cell culture and the subsequent signaling mechanism(s) involved. A mouse catecholaminergic neuronal cell line (CATH.a) was treated with ANG II (30, 100, and 300 nM) for 24 h, and protein expression was determined by Western blot analysis. ANG II induced a significant dose-dependent increase in ACE and decrease in ACE2 mRNA and protein expression in CATH.a neurons. This effect was abolished by pretreatment of the cells with the p38 MAPK inhibitor SB-203580 (10 M) 30 min before administration of ANG II or the ERK1/2 inhibitor U-0126 (10 M). These data suggest that ANG II increases ACE and attenuates ACE2 expression in neurons via the ANG II type 1 receptor, p38 MAPK, and ERK1/2 signaling pathways.angiotensin; converting enzyme; signaling THE RENIN-ANGIOTENSIN SYSTEM (RAS) in the brain plays an important role in the regulation of blood pressure, water balance, and endocrine secretion. Functional studies have clearly established that augmented ANG II and ANG II type 1 receptor (AT 1 R) signaling in the central nervous system plays an essential role in the sympathoexcitation in disease states such as hypertension and chronic heart failure in various animal models (43). Elevated plasma ANG II can directly activate neurons in the circumventricular organs, which lack a tight blood-brain barrier (6). On the other hand, other brain regions, such as the presympathetic neurons in the paraventricular nucleus (PVN) and rostral ventrolateral medulla, do not have direct access to systemic ANG II but have also been associated with an upregulation of local ANG II signaling in disease states (44).The conversion of bioactive angiotensin peptides is mainly carried out by two angiotensin-converting enzymes: angiotensin-converting enzyme (ACE) and angiotensin-converting enzyme 2 (ACE2). ACE catalyzes the conversion from ANG I to the major sympathoexcitatory peptide ANG II, while ACE2 primarily metabolizes ANG II to form ANG-(1-7). ANG-(1-7) exhibits effects that are counter to the effects of ANG II through activation of the Mas receptor (30,32). ACE also mediates the degradation of ANG-(1-7) to form the nonactive peptide ANG-(1-5) (7). Therefore, a balance between ACE and ACE2 expression and activity may contribute to the control of sympathetic nerve activity. Reciprocal changes in ACE and ACE2 expression in brain autonomic regions have been shown in studies using different models of heart failure (13, 34). Similarly, an increase in the ratio of ACE to ACE2 has been observed in the brain (1) and kidneys (36) in hypertensive patients and anim...

Cheyne-Stokes respiration (CSR) and cardiac arrhythmias are associated with increased morbidity and mortality in patients with congestive heart failure (CHF). Enhanced carotid body chemoreflex (CBC) sensitivity is associated with these abnormalities in CHF. Reduced carotid body nitric oxide and nitric oxide synthase (NOS) levels play an important role in the enhanced CBC. In other disease models, Simvastatin (statin) treatment increases endothelial NOS (eNOS) in part by increasing Kruppel like Factor 2 (KLF2) expression. We hypothesized that statin treatment would ameliorate enhanced CBC sensitivity as well as increased respiratory variability (RV), apnea/hypopnea index (AHI), and arrhythmia index (AI), in a rodent model of CHF. Resting breathing pattern, cardiac rhythm, and the ventilatory and carotid body (CB) chemoreceptor afferent responses to hypoxia (CBC) were assessed in rats with CHF induced by coronary ligation. CHF was associated with enhanced ventilatory and CB afferent responses to hypoxia as well as increased RV, AHI, and AI. Statin treatment prevented the increases in CBC sensitivity and the concomitant increases in RV, AHI, and AI. KLF2 and eNOS protein were decreased in the CB and nucleus tractus solitarii (NTS) of CHF animals and statin treatment increased the expression of these proteins. Our findings demonstrate that the increased CBC sensitivity, respiratory instability and cardiac arrhythmias observed in CHF are ameliorated by statin treatment and suggest that statins may be an effective treatment for CSR and arrhythmias in patient populations with high chemoreflex sensitivity.

Nuclear factor kappa B (NF-κB) and the Ets like gene-1 (Elk-1) are two transcription factors that have been previously established to contribute to the Angiotensin II mediated upregulation of Angiotensin II type 1 receptor (AT1R) in neurons. The cAMP response element binding protein (CREB) is another transcription factor that has also been implicated in AT1R gene transcription. The goal of the current study was to determine if NF-κB and CREB association was required for AT1R upregulation. We hypothesized that the transcription of the AT1R gene occurs via an orchestration of transcription factor interactions including NF-κB, CREB, and Elk-1. The synergistic role of CREB and NFκB in promoting AT1R gene expression was determined using siRNA-mediated silencing of CREB. Electrophorectic Mobility Shift Assay studies employing CREB and NF-κB demonstrated increased protein – DNA binding as a result of Ang II stimulation which was blunted by siRNA silencing of CREB. Upstream inhibition of p38 mitogen activated protein kinase (p38 MAPK) with SB203580 or inhibition of the calmodulin kinase (CAMK) pathway using KN-62 blunted changes in CREB and NF-κB expression. These findings suggest that Ang II may activate multiple signaling pathways involving p38 MAPK leading to the activation of NF-κB and CREB, which feed back to upregulate the AT1R gene. This study provides insight into the molecular mechanisms involving multiple transcription factor activation in a coordinated fashion which may be partially responsible for sympathoexcitation in clinical conditions associated with increased activation of the renin angiotensin system.