The relationship of the sympathetic nervous system and the renin-angiotensin system in congestive heart failure

Review of the Pathophysiologic and Clinical Aspects of Hypokalemia in Children and Young Adults: an Update

This article examines the regulatory function of the skeletal muscle, renal, and adrenergic systems in potassium homeostasis. The pathophysiologic bases of hypokalemia, systematic approach for an early diagnosis, and therapeutic strategy to avert life-threatening complications are highlighted. By promoting skeletal muscle uptake, intense physical exercise (post), severe trauma, and several toxins produce profound hypokalemia. Hypovolemia due to renal and extra-renal fluid losses and ineffective circulation activate secondary aldosteronism causing urinary potassium wasting. In addition to hypokalemic alkalosis, primary aldosteronism causes low-renin hypertension. Non-aldosterone mineralocorticoid activation leading to low-renin and low-aldosterone hypertension occurs in Liddle's syndrome and apparent mineralocorticoid excess. Although there is enzymatic inhibition of cortisol synthesis in congenital adrenal hyperplasia, precursors of aldosterone produce low-renin hypokalemic hypertension. In addition to the glucocorticoid effect, hypercortisolism activates mineralocorticoid receptors in Cushing's syndrome. Genetic mutations involving furosemide-sensitive Na+-K+-2Cl- co-transporters and thiazide-sensitive Na+-Cl- transporters result in (non-hypertensive) salt-wasting nephropathy. Proximal and distal renal tubular acidosis is associated with hypokalemia. Eating disorders causing hypokalemia include bulimia, laxative abuse, and diuretic misuse. Low urinary potassium (<15 mmol/day) and/or low urinary chloride (<20 mol/L) suggest a gastrointestinal pathology. Co-morbidity of hypokalemia with chronic pulmonary and cardiovascular diseases may increase the fatality rate.

Impaired cerebral oxygenation in heart failure patients at rest and during head-up tilt testing

Aims

Heart failure (HF) confers potentially negative effects on the brain and autonomic nervous system. The measurement cerebral tissue oxygen saturation (SctO₂) may aid in understanding such effects. We aimed to investigate if compensated HF affects SctO₂ at rest and during orthostatic challenge.

Methods and results

Non‐invasive haemodynamic monitoring and near‐infrared spectroscopy were applied during head‐up tilt (HUT) in 61 HF patients [mean (SD) 71 (11) years, 82% male, New York Heart Association (NYHA) class I–III] and 60 controls [60 (12) years, 42% male). Group differences in continuous variables were compared using Student's t‐test. Associations between HF and SctO₂ were studied using multivariable linear regression models adjusted for age, sex, diabetes, smoking, systolic blood pressure (SBP), and heart rate in supine position and after 10 min of HUT.

Mean SctO₂ was lower in HF patients compared with controls both in the supine position (67 vs. 71%; P < 0.001) and after 10 min of HUT (64 vs. 69%; P < 0.001). The HUT‐induced SctO₂ decrease was greater in HF patients compared with controls (P = 0.026). SBP did not change in neither HF patients nor controls during HUT, whereas diastolic blood pressure and heart rate increased in both groups. HF was associated with lower SctO₂ in supine (B = −2.5%, P = 0.023) and after 10 min of HUT (B = −2.6%, P = 0.007) after multivariable adjustments.

Conclusions

Cerebral tissue oxygenation is lower in HF patients both at rest and during orthostasis compared with subjects without HF. Future studies should test if the lower cerebral oxygenation associates with negative prognosis and with impaired cognitive function.

Relationship Between Serum Sodium Level Within the Low-Normal Range on Admission and Long-Term Clinical Outcomes in Patients with Acute Decompensated Heart Failure

Although hyponatremia during hospitalization for acute decompensated heart failure (ADHF) is reportedly related with poor prognosis, the available data regarding the impact of serum sodium level within the low-normal range at admission on clinical events in patients with ADHF is limited.We studied eligible patients admitted to our institution in 2007-2011. All the patients were categorized into 3 groups according to the admission serum sodium levels of < 135 mmol/L (hyponatremia), ≥ 135 and < 140 mmol/L (low-normal range), or ≥ 140 mmol/L (normal range). The association between admission serum sodium levels and long-term clinical events, a composite of all-cause deaths and re-hospitalizations for ADHF, was assessed by multivariable Cox proportional analysis.Of the 584 eligible patients, 208 (35.6%) were in the low-normal range and 99 (16.9%) had hyponatremia on admission. On multivariable analysis, compared with those with a sodium level ≥ 140 mmol/L, patients with hyponatremia were at increased risk for clinical events (hazard ratio [HR], 1.53; P = 0.041), whereas the HR of those in the low-normal range was attenuated and insignificant (HR, 1.08; P = 0.625). However, the HR of each category increased significantly as sodium level decreased (P value for HR trend, 0.024). In addition, when serum sodium level was treated as a continuous variable, the lower the serum sodium level, the greater the risk of clinical events (P = 0.012). The cut-off value of serum sodium level to predict mortality was < 138 mmol/L.In conclusion, a low serum sodium level on admission for ADHF, even if low-normal, can increase the risk of long-term mortality and/or re-hospitalization for ADHF.

The renal and cardiovascular effects of natriuretic peptides

The landmark report by de Bold et al. in 1981 signified the heart as one of the endocrine organs involved in fluid and salt balance (de Bold AJ, Borenstein HB, Veress AT, Sonnenberg H. Life Sci 28: 89-94, 1981). Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are secreted from cardiomyocytes in response to cardiac stretch as in the case of heart failure, whereas C-type natriuretic peptide (CNP) is secreted from endothelial and renal cells in response to cytokines and endothelium-dependent agonists, such as acetylcholine. Binding ANP or BNP to natriuretic peptide receptor A induces cyclic guanylyl monophosphate as second messenger in the target cells to mediate the following: natriuresis; water diuresis; increasing glomerular filtration rate; decreasing systemic sympathetic activities; plasma volume; cardiac output and blood pressure; and curbing mitoses of heart fibroblasts and hypertrophy of cardiovascular muscle cells. ANP, BNP, and CNP are cleared from the bloodstream by natriuretic peptide receptor C and degraded by an ectoenzyme called neprilysin (NEP). The plasma levels of BNP are typically >100 pg/ml in patients with congestive heart failure. Sacubitril/valsartan is an angiotensin receptor NEP inhibitor that prevents the clinical progression of surviving patients with heart failure more effectively than enalapril, an angiotensin-converting enzyme inhibitor. A thorough understanding of the renal and cardiovascular effects of natriuretic peptides is of major importance for first-year medical students to gain insight into the significance of plasma levels of BNP in patients with heart failure.

Baroreflex control of renal sympathetic nerve activity in early heart failure assessed by the sequence method

KEY POINTS

The integrity of the baroreflex control of sympathetic activity in heart failure (HF) remains under debate. We proposed the use of the sequence method to assess the baroreflex control of renal sympathetic nerve activity (RSNA). The sequence method assesses the spontaneous arterial pressure (AP) fluctuations and their related changes in heart rate (or other efferent responses), providing the sensitivity and the effectiveness of the baroreflex. Effectiveness refers to the fraction of spontaneous AP changes that elicits baroreflex-mediated variations in the efferent response. Using three different approaches, we showed that the baroreflex sensitivity between AP and RSNA is not altered in early HF rats. However, the sequence method provided evidence that the effectiveness of baroreflex in changing RSNA in response to AP changes is markedly decreased in HF. The results help us better understand the baroreflex control of the sympathetic nerve activity.

ABSTRACT

In heart failure (HF), the reflex control of the heart rate is known to be markedly impaired; however, the baroreceptor control of the sympathetic drive remains under debate. Applying the sequence method to a series of arterial pressure (AP) and renal sympathetic nerve activity (RSNA), we demonstrated a clear dysfunction in the baroreflex control of sympathetic activity in rats with early HF. We analysed the baroreflex control of the sympathetic drive using three different approaches: AP vs. RSNA curve, cross-spectral analysis and sequence method between AP and RSNA. The sequence method also provides the baroreflex effectiveness index (BEI), which represents the percentage of AP ramps that actually produce a reflex response. The methods were applied to control rats and rats with HF induced by myocardial infarction. None of the methods employed to assess the sympathetic baroreflex gain were able to detect any differences between the control and the HF group. However, rats with HF exhibited a lower BEI compared to the controls. Moreover, an optimum delay of 1 beat was observed, i.e. 1 beat is required for the RSNA to respond after AP changing, which corroborates with the findings related to the timing between these two variables. For delay 1, the BEI of the controls was 0.45 ± 0.03, whereas the BEI of rats with HF was 0.29 ± 0.09 (P < 0.05). These data demonstrate that while the gain of the baroreflex is not affected in early HF, its effectiveness is markedly decreased. The analysis of the spontaneous changes in AP and RSNA using the sequence method provides novel insights into arterial baroreceptor reflex function.

A novel role for miR-133a in centrally mediated activation of the renin-angiotensin system in congestive heart failure

An activated renin-angiotensin system (RAS) within the central nervous system has been implicated in sympathoexcitation during various disease conditions including congestive heart failure (CHF). In particular, activation of the RAS in the paraventricular nucleus (PVN) of the hypothalamus has been recognized to augment sympathoexcitation in CHF. We observed a 2.6-fold increase in angiotensinogen (AGT) in the PVN of CHF. To elucidate the molecular mechanism for increased expression of AGT, we performed in silico analysis of the 3'-untranslated region (3'-UTR) of AGT and found a potential binding site for microRNA (miR)-133a. We hypothesized that decreased miR-133a might contribute to increased AGT in the PVN of CHF rats. Overexpression of miR-133a in NG108 cells resulted in 1.4- and 1.5-fold decreases in AGT and angiotensin type II (ANG II) type 1 receptor (AT₁R) mRNA levels, respectively. A luciferase reporter assay performed on NG108 cells confirmed miR-133a binding to the 3'-UTR of AGT. Consistent with these in vitro data, we observed a 1.9-fold decrease in miR-133a expression with a concomitant increase in AGT and AT₁R expression within the PVN of CHF rats. Furthermore, restoring the levels of miR-133a within the PVN of CHF rats with viral transduction resulted in a significant reduction of AGT (1.4-fold) and AT₁R (1.5-fold) levels with a concomitant decrease in basal renal sympathetic nerve activity (RSNA). Restoration of miR-133a also abrogated the enhanced RSNA responses to microinjected ANG II within the PVN of CHF rats. These results reveal a novel and potentially unique role for miR-133a in the regulation of ANG II within the PVN of CHF rats, which may potentially contribute to the commonly observed sympathoexcitation in CHF.NEW & NOTEWORTHY Angiotensinogen (AGT) expression is upregulated in the paraventricular nucleus of the hypothalamus through posttranscriptional mechanism interceded by microRNA-133a in heart failure. Understanding the mechanism of increased expression of AGT in pathological conditions leading to increased sympathoexcitation may provide the basis for the possible development of new therapeutic agents with enhanced specificity.

Carotid body chemoreceptors, sympathetic neural activation, and cardiometabolic disease

The carotid body (CB) is the main peripheral chemoreceptor that senses the arterial PO₂, PCO₂ and pH. In response to hypoxemia, hypercapnia and acidosis, carotid chemosensory discharge elicits reflex respiratory, autonomic and cardiovascular adjustments. The classical construct considers the CB as the main peripheral oxygen sensor, triggering reflex physiological responses to acute hypoxemia and facilitating the ventilatory acclimation to chronic hypoxemia at high altitude. However, a growing body of experimental evidence supports the novel concept that an abnormally enhanced CB chemosensory input to the brainstem contributes to overactivation of the sympathetic nervous system, and consequent pathology. Indeed, the CB has been implicated in several diseases associated with increases in central sympathetic outflow. These include hypertension, heart failure, sleep apnea, chronic obstructive pulmonary disease and metabolic syndrome. Indeed, ablation of the CB has been proposed for the treatment of severe and resistant hypertension in humans. In this review, we will analyze and discuss new evidence supporting an important role for the CB chemoreceptor in the progression of autonomic and cardiorespiratory alterations induced by heart failure, obstructive sleep apnea, chronic obstructive pulmonary disease and metabolic syndrome.

Relevance of the Carotid Body Chemoreflex in the Progression of Heart Failure

Chronic heart failure (CHF) is a global health problem affecting millions of people. Autonomic dysfunction and disordered breathing patterns are commonly observed in patients with CHF, and both are strongly related to poor prognosis and high mortality risk. Tonic activation of carotid body (CB) chemoreceptors contributes to sympathoexcitation and disordered breathing patterns in experimental models of CHF. Recent studies show that ablation of the CB chemoreceptors improves autonomic function and breathing control in CHF and improves survival. These exciting findings indicate that alterations in CB function are critical to the progression of CHF. Therefore, better understanding of the physiology of the CB chemoreflex in CHF could lead to improvements in current treatments and clinical management of patients with CHF characterized by high chemosensitivity. Accordingly, the main focus of this brief review is to summarize current knowledge of CB chemoreflex function in different experimental models of CHF and to comment on their potential translation to treatment of human CHF.

Cardiac Output and Cerebral Blood Flow

Cerebral blood flow (CBF) is rigorously regulated by various powerful mechanisms to safeguard the match between cerebral metabolic demand and supply. The question of how a change in cardiac output (CO) affects CBF is fundamental, because CBF is dependent on constantly receiving a significant proportion of CO. The authors reviewed the studies that investigated the association between CO and CBF in healthy volunteers and patients with chronic heart failure. The overall evidence shows that an alteration in CO, either acutely or chronically, leads to a change in CBF that is independent of other CBF-regulating parameters including blood pressure and carbon dioxide. However, studies on the association between CO and CBF in patients with varying neurologic, medical, and surgical conditions were confounded by methodologic limitations. Given that CBF regulation is multifactorial but the various processes must exert their effects on the cerebral circulation simultaneously, the authors propose a conceptual framework that integrates the various CBF-regulating processes at the level of cerebral arteries/arterioles while still maintaining autoregulation. The clinical implications pertinent to the effect of CO on CBF are discussed. Outcome research relating to the management of CO and CBF in high-risk patients or during high-risk surgeries is needed.

Modulation of angiotensin II signaling following exercise training in heart failure

Sympathetic activation is a consistent finding in the chronic heart failure (CHF) state. Current therapy for CHF targets the renin-angiotensin II (ANG II) and adrenergic systems. Angiotensin converting enzyme (ACE) inhibitors and ANG II receptor blockers are standard treatments along with β-adrenergic blockade. However, the mortality and morbidity of this disease is still extremely high, even with good medical management. Exercise training (ExT) is currently being used in many centers as an adjunctive therapy for CHF. Clinical studies have shown that ExT is a safe, effective, and inexpensive way to improve quality of life, work capacity, and longevity in patients with CHF. This review discusses the potential neural interactions between ANG II and sympatho-excitation in CHF and the modulation of this interaction by ExT. We briefly review the current understanding of the modulation of the angiotensin type 1 receptor in sympatho-excitatory areas of the brain and in the periphery (i.e., in the carotid body and skeletal muscle). We discuss possible cellular mechanisms by which ExT may impact the sympatho-excitatory process by reducing oxidative stress, increasing nitric oxide. and reducing ANG II. We also discuss the potential role of ACE2 and Ang 1-7 in the sympathetic response to ExT. Fruitful areas of further investigation are the role and mechanisms by which pre-sympathetic neuronal metabolic activity in response to individual bouts of exercise regulate redox mechanisms and discharge at rest in CHF and other sympatho-excitatory states.

The central renin-angiotensin system and sympathetic nerve activity in chronic heart failure

CHF (chronic heart failure) is a multifactorial disease process that is characterized by overactivation of the RAAS (renin-angiotensin-aldosterone system) and the sympathetic nervous system. Both of these systems are chronically activated in CHF. The RAAS consists of an excitatory arm involving AngII (angiotensin II), ACE (angiotensin-converting enzyme) and the AT1R (AngII type 1 receptor). The RAAS also consists of a protective arm consisting of Ang-(1-7) [angiotensin-(1-7)], the AT2R (AngII type 2 receptor), ACE2 and the Mas receptor. Sympatho-excitation in CHF is driven, in large part, by an imbalance of these two arms, with an increase in the AngII/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. The present review focuses on the current literature that describes the components of these two arms of the RAAS and their imbalance in the CHF state. Moreover, the present 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.

Toll-like receptors 2 and 4 modulate autonomic control of heart rate and energy metabolism

Toll-like receptors (TLR) are innate immune receptors typically activated by microbial-associated molecular patterns (MAMPs) during infection or damage-associated molecular patterns (DAMPs) as a result of tissue injury. Recent findings suggest that TLR2 and TLR4 signaling play important roles in developmental and adult neuroplasticity, and in learning and memory. In addition, activation of TLR2 and TLR4 worsens ischemic injury to the heart and brain in animal models of myocardial infarction and stroke. TLR activation is also implicated in thermoregulation and fever in response to infection. However, it is not known whether TLRs participate in the regulation of the sympathetic and/or parasympathetic components of the autonomic nervous system (ANS). Here we provide evidence that TLR2 and TLR4 influence autonomic regulation of heart rate (HR) body temperature and energy metabolism in mice. We show that mice lacking TLR2 or TLR4 exhibit reduced basal HR, which results from an increase of parasympathetic tone. In addition, thermoregulatory responses to stress are altered in TLR2-/- and TLR4-/- mice, and brown fat-dependent thermoregulation is altered in TLR4-/- mice. Moreover, TLR2-/- and TLR4-/- mice consume less food and exhibit a greater mass compared to wild type mice. Collectively, our findings suggest important roles for TLR2 and TLR4 in the ANS regulation of cardiovascular function, thermoregulation, and energy metabolism.

Additional use of an aldosterone antagonist in patients with mild to moderate chronic heart failure: a systematic review and meta-analysis

AIMS

Aldosterone antagonists (AldoAs) have been used to treat severe chronic heart failure (CHF). There is uncertainty regarding the efficacy of using AldoAs in mild to moderate CHF with New York Heart Association (NYHA) classifications of I to II. This study summarizes the evidence for the efficacy of spironolactone (SP), eplerenone (EP) and canrenone in mild to moderate CHF patients.

METHODS

PubMed, MEDLINE, EMBASE and OVID databases were searched before June 2012 for randomized and quasi-randomized controlled trials assessing AldoA treatment in CHF patients with NYHA classes I to II. Data concerning the study's design, patients' characteristics and outcomes were extracted. Risk ratio (RR) and weighted mean differences (WMD) or standardized mean difference were calculated using either fixed or random effects models.

RESULTS

Eight trials involving 3929 CHF patients were included. AldoAs were superior to the control in all cause mortality (RR 0.79, 95% CI 0.66, 0.95) and in re-hospitalization for cardiac causes (RR 0.62, 95% CI 0.52, 0.74), the left ventricular ejection fraction was improved by AldoA treatment (WMD 2.94%, P = 0.52). Moreover, AldoA therapy decreased the left ventricular end-diastolic volume (WMD -14.04 ml, P < 0.00001), the left ventricular end-systolic volume (WMD -14.09 ml, P < 0.00001). A stratified analysis showed a statistical superiority in the benefits of SP over EP in reducing LVEDV and LVESV. AldoAs reduced B-type natriuretic peptide concentrations (WMD -37.76 pg ml(-1), P < 0.00001), increased serum creatinine (WMD 8.69 μmol l(-1), P = 0.0003) and occurrence of hyperkalaemia (RR 1.78, 95% CI 1.43, 2.23).

CONCLUSIONS

Additional use of AldoAs in CHF patients may decrease mortality and re-hospitalization for cardiac reasons, improve cardiac function and simultaneously ameliorate LV reverse remodelling.

Hyponatremia is an independent predictor of adverse clinical outcomes in hospitalized patients due to worsening heart failure

BACKGROUND AND PURPOSE

Hyponatremia is common and is associated with poor in-hospital outcomes in patients hospitalized with heart failure (HF). However, it is unknown whether hyponatremia is associated with long-term adverse outcomes. The purpose of this study was to clarify the characteristics, clinical status on admission, and management during hospitalization according to the serum sodium concentration on admission, and determine whether hyponatremia was associated with in-hospital as well as long-term outcomes in 1677 patients hospitalized with worsening HF on index hospitalization registered in the database of the Japanese Cardiac Registry of Heart Failure in Cardiology (JCARE-CARD).

METHODS AND SUBJECTS

We studied the characteristics and in-hospital treatment in 1659 patients hospitalized with worsening HF by using the JCARE-CARD database. Patients were divided into 2 groups according to serum sodium concentration on admission <135mEq/mL (n=176; 10.6%) or ≥135mEq/mL (n=1483; 89.4%).

RESULTS

The mean age was 70.7 years and 59.2% were male. Etiology was ischemic in 33.9% and mean left ventricular ejection fraction was 42.4%. After adjustment for covariates, hyponatremia was independently associated with in-hospital death [adjusted odds ratio (OR) 2.453, 95% confidence interval (CI) 1.265-4.755, p=0.008]. It was significantly associated also with adverse long-term (mean 2.1±0.8 years) outcomes including all-cause death (OR 1.952, 95% CI 1.433-2.657), cardiac death (OR 2.053, 95% CI 1.413-2.983), and rehospitalization due to worsening HF (OR 1.488, 95% CI 1.134-1.953).

CONCLUSIONS

Hyponatremia was independently associated with not only in-hospital but also long-term adverse outcomes in patients hospitalized with worsening HF.

Partially silencing brain toll-like receptor 4 prevents in part left ventricular remodeling with sympathoinhibition in rats with myocardial infarction-induced heart failure

BACKGROUND

Left ventricular (LV) remodeling and activation of sympathetic nervous system (SNS) are cardinal features of heart failure. We previously demonstrated that enhanced central sympathetic outflow is associated with brain toll-like receptor 4 (TLR4) probably mediated by brain angiotensin II type 1 receptor in mice with myocardial infarction (MI)-induced heart failure. The purpose of the present study was to examine whether silencing brain TLR4 could prevent LV remodeling with sympathoinhibition in MI-induced heart failure.

METHODOLOGY/PRINCIPAL FINDINGS

MI-induced heart failure model rats were created by ligation of left coronary artery. The expression level of TLR4 in brainstem was significantly higher in MI-induced heart failure treated with intracerebroventricular (ICV) injection of hGAPDH-SiRNA than in sham. TLR4 in brainstem was significantly lower in MI-induced heart failure treated with ICV injection of TLR4-SiRNA than in that treated with ICV injection of hGAPDH-SiRNA. Lung weight, urinary norepinephrine excretion, and LV end-diastolic pressure were significantly lower and LV dimension was significantly smaller in MI-induced heart failure treated with TLR4-SiRNA than in that treated with hGAPDH-SiRNA for 2 weeks.

CONCLUSIONS

Partially silencing brain TLR4 by ICV injection of TLR4-SiRNA for 2 weeks could in part prevent LV remodeling with sympathoinhibition in rats with MI-induced heart failure. Brain TLR4 has a potential to be a target of the treatment for MI-induced heart failure.

Therapeutic role of toll-like receptor modification in cardiovascular dysfunction

Toll-like receptors (TLR) are key pattern recognition receptors in the innate immune system. The TLR-mediated immune response against pathogens is usually protective however inappropriate TLR activation may lead to excessive tissue damage. It is well recognised that TLRs respond to a variety of endogenous as well as exogenous ligands. By responding to endogenous ligands that are exposed during cellular damage, TLRs have been implicated in a range of pathological conditions associated with cardiovascular dysfunction. Increasing knowledge on the mechanisms involved in TLR signalling has encouraged the exploration of therapeutic pharmacological modulation of TLR activation in conditions such as atherosclerosis, ischaemic heart disease, heart failure and ischaemic reperfusion injury. The aim of this review is to explore the translational potentials of TLR modification in cardiovascular dysfunction, where these agents have been studied.

Abnormal baroreflex function is dissociated from central angiotensin II receptor expression in chronic heart failure

Neurohumoral disturbances characterize chronic heart failure (CHF) and are reflected, in part, as impairment of baroreflex sensitivity (BRS) and sympathetic function. However, the mechanisms that trigger these neurohumoral abnormalities in CHF are not clear. We hypothesized that the BRS is blunted early in CHF and that the humoral effects occur later and contribute to progressive loss of cardiovascular control in CHF. We assessed the BRS (beats/min per mmHg) and recorded renal sympathetic nerve activity (RSNA) in four groups of conscious rabbits at varying time intervals: control, 1-week CHF, 2-week CHF, and 3-week CHF. Chronic heart failure was induced by ventricular pacing at 360 beats/min and was assessed by echocardiography. Arterial blood pressure and heart rate were recorded by an implanted telemetric device and RSNA through an implanted electrode. A significant fall in the ejection fraction, fractional shortening, and an increase in left ventricular end-systolic diameter and left ventricular end-diastolic diameter were observed in all CHF groups. The BRS was significantly reduced in all the CHF groups with no significant change in the basal RSNA (% of maximum) after 1 week of pacing; a small but insignificant rise in RSNA was seen at 2 weeks, and a significant rise in RSNA was observed at 3 weeks. Angiotensin II type 1 (AT-1) receptor protein (Western Blot) and mRNA (reverse transcriptase-polymerase chain reaction) expression in the rostral ventrolateral medulla exhibited a progressive increase with the duration of CHF, reaching significance after 3 weeks, the same time point in which RSNA was significantly elevated. These data are the first to examine early changes in central AT-1 receptors in CHF and suggest that the fall in BRS and hemodynamic changes occur early in the development of CHF followed by sympathoexcitation and overexpression of AT-1 receptors with the progression of CHF, causing further impairment of cardiovascular control.

Dysfunctional nucleus tractus solitarius: its crucial role in promoting neuropathogenetic cascade of Alzheimer's dementia--a novel hypothesis

The pathophysiological mechanism(s) underlying Alzheimer's disease (AD) still remain unclear, and no disease-modifying or prophylactic therapies are currently available. Unraveling the fundamental neuropathogenesis of AD is an important challenge. Several studies on AD have suggested lesions in a number of CNS areas including the basal forebrain, hippocampus, entorhinal cortex, amygdale/insula, and the locus coeruleus. However, plausible unifying studies on the upstream factors that involve these heterogeneous regions and herald the onset of AD pathogenesis are not available. The current article presents a novel nucleus tractus solitarius (NTS) vector hypothesis that underpins several disparate biological mechanisms and neural circuits, and identifies relevant hallmarks of major presumptive causative factor(s) linked to the NTS, in older/aging individuals. Aging, obesity, infection, sleep apnea, smoking, neuropsychological states, and hypothermia-all activate inflammatory cytokines and oxidative stress. The synergistic impact of systemic proinflammatory mediators activates microglia and promotes neuroinflammation. Acutely, the innate immune response is protective defending against pathogens/toxins; however, when chronic, it causes neuroinflammation and neuronal dysfunction, particularly in brainstem and neocortex. The NTS in the brainstem is an essential multiple signaling hub, and an extremely important central integration site of baroreceptor, chemoreceptor, and a multitude of sensory afferents from gustatory, gastrointestinal, cardiac, pulmonary, and upper airway systems. Owing to persistent neuroinflammation, the dysfunctional NTS exerts deleterious impact on nucleus ambiguus, dorsal motor nucleus of vagus, hypoglossal, parabrachial, locus coeruleus and many key nuclei in the brainstem, and the hippocampus, entorhinal cortex, prefrontal cortex, amygdala, insula, and basal forebrain in the neocortex. The neuronal and synaptic dysfunction emanating from the inflamed NTS may affect its interconnected pathways impacting almost the entire CNS--which is already primed by neuroinflammation, thus promoting cognitive and neuropsychiatric symptoms. The upstream factors discussed here may underpin the neuropathopgenesis of AD. AD pathology is multifactorial; the current perspective underscores the value of attenuating disparate upstream factors--in conjunction with anticholinesterase, anti-inflammatory, immunosuppressive, and anti-oxidant pharmacotherapy. Amelioration of the NTS pathology may be of central importance in countering the neuropathological cascade of AD. The NTS, therefore, may be a potential target of novel therapeutic strategies.

Central angiotensin (1-7) enhances baroreflex gain in conscious rabbits with heart failure

In chronic heart failure (CHF), arterial baroreflex function is impaired, in part, by activation of the central renin-angiotensin system. A metabolite of angiotensin (Ang) II, Ang-(1-7), has been shown to exhibit cardiovascular effects that are in opposition to that of Ang II. However, the action of Ang-(1-7) on sympathetic outflow and baroreflex function is not well understood, especially in CHF. The aim of this study was to determine the effect of intracerebroventricular infusion of Ang-(1-7) on baroreflex control of heart rate and renal sympathetic nerve activity in conscious rabbits with CHF. We hypothesized that central Ang-(1-7) would improve baroreflex function in CHF. Ang-(1-7) (2 nmol/1 μL per hour) or artificial cerebrospinal fluid (1 μL per hour) was infused by an osmotic minipump for 4 days in sham and pacing-induced CHF rabbits (n=3 to 6 per group). Ang-(1-7) treatment had no effects in sham rabbits but reduced heart rate and increased baroreflex gain (7.4±1.5 versus 2.5±0.4 bpm/mm Hg; P<0.05) in CHF rabbits. The Ang-(1-7) antagonist A779 (8 nmol/1 μL per hour) blocked the improvement in baroreflex gain in CHF. Baroreflex gain increased in CHF+Ang-(1-7) animals when only the vagus was allowed to modulate baroreflex control by acute treatment with the β-1 antagonist metoprolol, indicating increased vagal tone. Baseline renal sympathetic nerve activity was significantly lower, and baroreflex control of renal sympathetic nerve activity was enhanced in CHF rabbits receiving Ang-(1-7). These data suggest that augmentation of central Ang-(1-7) inhibits sympathetic outflow and increases vagal outflow in CHF, thus contributing to enhanced baroreflex gain in this disease state.

Hypokalemia and sudden cardiac death

Worldwide, approximately three million people suffer sudden cardiac death annually. These deaths often emerge from a complex interplay of substrates and triggers. Disturbed potassium homeostasis among heart cells is an example of such a trigger. Thus, hypokalemia and, also, more transient reductions in plasma potassium concentration are of importance. Hypokalemia is present in 7% to 17% of patients with cardiovascular disease. Furthermore, up to 20% of hospitalized patients and up to 40% of patients on diuretics suffer from hypokalemia. Importantly, inadequate management of hypokalemia was found in 24% of hospitalized patients. Hypokalemia is associated with increased risk of arrhythmia in patients with cardiovascular disease, as well as increased all-cause mortality, cardiovascular mortality and heart failure mortality by up to 10-fold. Long-term potassium homeostasis depends on renal potassium excretion. However, skeletal muscles play an important role in short-term potassium homeostasis, primarily because skeletal muscles contain the largest single pool of potassium in the body. Moreover, due to the large number of Na(+)/K(+) pumps and K(+) channels, the skeletal muscles possess a huge capacity for potassium exchange. In cardiovascular patients, hypokalemia is often caused by nonpotassium-sparing diuretics, insufficient potassium intake and a shift of potassium into stores by increased potassium uptake stimulated by catecholamines, beta-adrenoceptor agonists and insulin. Interestingly, drugs with a proven significant positive effect on mortality and morbidity rates in heart failure patients all increase plasma potassium concentration. Thus, it may prove beneficial to pay more attention to hypokalemia and to maintain plasma potassium levels in the upper normal range. The more at risk of fatal arrhythmia and sudden cardiac death a patient is, the more attention should be given to the potassium homeostasis.

Role of blood flow in carotid body chemoreflex function in heart failure

Peripheral chemoreflex sensitivity is potentiated in clinical and experimental chronic heart failure (CHF). Blood supply to tissues is inevitably reduced in CHF. However, it remains poorly understood whether the reduced blood flow is the cause of increased peripheral chemoreflex sensitivity in CHF. This work highlights the effect of chronically reduced blood flow to the carotid body (CB) on peripheral chemoreflex function in rabbits. In pacing-induced CHF rabbits, blood flow in the carotid artery was reduced by 36.4 ± 5.2% after 3 weeks of pacing. For comparison, a similar level of blood flow reduction was induced by carotid artery occlusion (CAO) over a similar 3 week time course without pacing. CB blood supply was reduced by similar levels in both CHF and CAO rabbits as measured with fluorescent microspheres. Compared with sham rabbits, CAO enhanced peripheral chemoreflex sensitivity in vivo, increased CB chemoreceptor activity in an isolated CB preparation and decreased outward potassium current (Ik) in CB glomus cells to levels similar to those that were observed in CHF rabbits. In CAO CB compared to sham, neural nitric oxide (NO) synthase (nNOS) expression and NO levels were suppressed, and angiotensin II (Ang II) type 1 receptor (AT1-R) protein expression and Ang II concentration were elevated; these changes were similar to those seen in the CB from CHF rabbits. A NO donor and AT1-R antagonist reversed CAO-enhanced chemoreflex sensitivity. These results suggest that a reduction of blood flow to the CB is involved in the augmentation of peripheral chemoreflex sensitivity in CHF.

Regulation of central angiotensin type 1 receptors and sympathetic outflow in heart failure

Angiotensin type 1 receptors (AT(1)Rs) play a critical role in a variety of physiological functions and pathophysiological states. They have been strongly implicated in the modulation of sympathetic outflow in the brain. An understanding of the mechanisms by which AT(1)Rs are regulated in a variety of disease states that are characterized by sympathoexcitation is pivotal in development of new strategies for the treatment of these disorders. This review concentrates on several aspects of AT(1)R regulation in the setting of chronic heart failure (CHF). There is now good evidence that AT(1)R expression in neurons is mediated by activation of the transcription factor activator protein 1 (AP-1). This transcription factor and its component proteins are upregulated in the rostral ventrolateral medulla of animals with CHF. Because the increase in AT(1)R expression and transcription factor activation can be blocked by the AT(1)R antagonist losartan, a positive feedback mechanism of AT(1)R expression in CHF is suggested. Oxidative stress has also been implicated in the regulation of receptor expression. Recent data suggest that the newly discovered catabolic enzyme angiotensin-converting enzyme 2 (ACE2) may play a role in the modulation of AT(1)R expression by altering the balance between the octapeptide ANG II and ANG- (1-7). Finally, exercise training reduces both central oxidative stress and AT(1)R expression in animals with CHF. These data strongly suggest that multiple central and peripheral influences dynamically alter AT(1)R expression in CHF.

Beneficial effects of torasemide on systolic wall stress and sympathetic nervous activity in asymptomatic or mildly symptomatic patients with heart failure: comparison with azosemide

Loop diuretics could adversely influence prognosis due to activation of neurohumoral mechanism in the long term. Previous study showed torasemide, a loop diuretic with anti-aldosteronergic properties, was associated with lower mortality in patients with chronic heart failure (CHF). We evaluated the effects of torasemide, in comparison with azosemide, in patients with CHF. Patients received oral diuretic therapy with torasemide (8 mg/d, n = 15) or azosemide (60 mg/d, n = 15) for 3 months. Torasemide and azosemide were then switched, and the patients were treated for another 3 months. Torasemide treatment induced significant decreases in left ventricular (LV) systolic wall stress (from 259 +/- 95 to 232 +/- 80 kdyn/cm2) and the plasma level of aldosterone (from 133 +/- 61 to 95 +/- 50 pg/mL) and was not associated with a change in the plasma level of norepinephrine. In contrast, the plasma level of norepinephrine was significantly increased (from 370 +/- 170 to 481 +/- 247 pg/mL), whereas LV systolic wall stress was unchanged after azosemide treatment. This study indicates that torasemide treatment reduced LV systolic wall stress without activation of the sympathetic nervous system in patients with CHF. The anti-aldosteronergic properties of torasemide may contribute to its favorable effects.

Endogenous angiotensin II has fewer effects but neuronal nitric oxide synthase has excitatory effects on renal sympathetic nerve activity in salt-sensitive hypertension-induced heart failure

The effects of endogenous angiotensin II (Ang II) and neuronal nitric oxide synthase (nNOS) on tonic sympathetic activity were studied in salt-sensitive hypertension-induced heart failure. Dahl salt-sensitive rats were fed 8% NaCl diet for 9 weeks to induce chronic heart failure (CHF-DSS). The effects of intravenous administration of a selective nNOS inhibitor, S-methyl-L: -thiocitrulline (SMTC), and an Ang II type 1-receptor blocker, losartan, on renal sympathetic nerve activity (RSNA) were examined in chronically instrumented conscious rats. Baroreceptor (baro)-unloaded RSNA was obtained by decreasing arterial pressure with caval occlusion to determine tonic RSNA. SMTC significantly decreased baro-unloaded RSNA, and subsequent losartan recovered baro-unloaded RSNA to the control level in CHF-DSS rats. To compare the effects of the inhibitors between low- and high-activity states of the renin-angiotensin system (RAS), Sprague-Dawley rats were fed low (0.04%)- or high (8%)-salt diets. A significant difference was found in the effects of SMTC and/or losartan on RSNA between the high- and low-RAS states, which suggested that there is a difference in the effect of endogenous Ang II on RSNA between salt-induced and other-type heart failure. To examine the effects of heart failure on brain-tissue nNOS activity, we measured the activities of the diencephalon in heart-failure rats. Heart failure significantly suppressed diencephalon nNOS activity, which was significantly different from the results in salt-sensitive hypertension without heart failure. These results suggest that endogenous Ang II has fewer effects, but nNOS has excitatory effects on tonic RSNA in salt-sensitive hypertension-induced heart failure.

Anti-remodelling effect of canrenone in patients with mild chronic heart failure (AREA IN-CHF study): final results

AIMS

To test whether canrenone, an aldosterone receptor antagonist, improves left ventricular (LV) remodelling in NYHA class II heart failure (HF). Aldosterone receptor antagonists improve outcome in severe HF, but no information is available in NYHA class II.

METHODS AND RESULTS

AREA IN-CHF is a randomized, double-blind, placebo-controlled study testing canrenone on top of optimal treatment in NYHA class II HF with low ejection fraction (EF) to assess 12-month changes in LV end-diastolic volume (LVEDV). Brain natriuretic peptide (BNP) was also measured. Information was available for 188 subjects on canrenone and 194 on placebo. Left ventricular end-diastolic volume was similarly reduced (-18%) in both arms, but EF increased more (P = 0.04) in the canrenone (from 40% to 45%) than in the placebo arm (from 40-43%). Brain natriuretic peptide (n = 331) decreased more in the canrenone (-37%) than in the placebo arm (-8%; P < 0.0001), paralleling a significant reduction in left atrial dimensions (-4% vs. 0.2%; P = 0.02). The composite endpoint of cardiac death and hospitalization was significantly lower in the canrenone arm (8% vs. 15%; P = 0.02).

CONCLUSION

Canrenone on top of optimal treatment for HF did not have additional effects on LVEDV, but it increased EF, and reduced left atrial size and circulating BNP, with potential beneficial effects on outcome. A large-scale randomized study should be implemented to confirm benefits on cardiovascular outcomes in patients with HF in NYHA class II.

Heart rate, lifespan, and mortality risk

An increasing body of scientific research and observational evidence indicates that resting heart rate (HR) is inversely related to the lifespan among homeothermic mammals and within individual species. In numerous human studies with patients stratified by resting HR, increased HR is universally associated with greater risk of death. The correlation between HR and maximum lifespan seems to be due to both basal metabolic rate and cardiovascular-related mortality risk. Both intrinsic and extrinsic factors are already postulated to determine how the biological clock works, through regulating and modulating the processes such as protein oxidation, free radical production, inflammation and telomere shortening. Given the remarkable correlation between HR and lifespan, resting HR should be seriously considered as another possible cap on maximum lifespan. Future research is needed to determine whether deliberate cardiac slowing, through methods like lifestyle modification, pharmacological intervention, or medical devices, can decelerate biological clock of aging, reduce cardiovascular mortality and increase maximum lifespan in humans in general.

Interaction between cardiac sympathetic afferent reflex and chemoreflex is mediated by the NTS AT1 receptors in heart failure

Several sympathoexcitatory reflexes, such as the cardiac sympathetic afferent reflex (CSAR) and arterial chemoreflex, are significantly augmented and contribute to elevated sympathetic outflow in chronic heart failure (CHF). This study was undertaken to investigate the interaction between the CSAR and the chemoreflex in CHF and to further identify the involvement of angiotensin II type 1 receptors (AT1Rs) in the nucleus of the tractus solitarius (NTS) in this interaction. CHF was induced in rats by coronary ligation. Acute experiments were performed in anesthetized rats. The chemoreflex-induced increase in cardiovascular responses was significantly greater in CHF than in sham-operated rats after either chemical or electrical activation of the CSAR. The inhibition of the CSAR by epicardial lidocaine reduced the chemoreflex-induced effects in CHF rats but not in sham-operated rats. Bilateral NTS injection of the AT1R antagonist losartan (10 and 100 pmol) dose-dependently decreased basal sympathetic nerve activity in CHF but not in sham-operated rats. This procedure also abolished the CSAR-induced enhancement of the chemoreflex. The discharge and chemosensitivity of NTS chemosensitive neurons were significantly increased following the stimulation of the CSAR in sham-operated and CHF rats, whereas CSAR inhibition by epicardial lidocaine significantly attenuated chemosensitivity of NTS neurons in CHF but not in sham-operated rats. Finally, the protein expression of AT1R in the NTS was significantly higher in CHF than in sham-operated rats. These results demonstrate that the enhanced cardiac sympathetic afferent input contributes to an excitatory effect of chemoreflex function in CHF, which is mediated by an NTS-AT1R-dependent mechanism.

Angiotensin II-triggered p44/42 mitogen-activated protein kinase mediates sympathetic excitation in heart failure rats

Angiotensin II (Ang II), acting via angiotensin type 1 receptors in the brain, activates the sympathetic nervous system in heart failure (HF). We reported recently that Ang II stimulates mitogen-activated protein kinase (MAPK) to upregulate brain angiotensin type 1 receptors in HF rats. In this study we tested the hypothesis that Ang II-activated MAPK signaling pathways contribute to sympathetic excitation in HF. Intracerebroventricular administration of PD98059 and UO126, 2 selective p44/42 MAPK inhibitors, induced significant decreases in mean arterial pressure, heart rate, and renal sympathetic nerve activity in HF rats, but had no effect on these variables in sham-operated rats. Pretreatment with losartan attenuated the effects of PD98059. Intracerebroventricular administration of the p38 MAPK inhibitor SB203580 and the c-Jun N-terminal kinase inhibitor SP600125 had no effect on mean arterial pressure, heart rate, or renal sympathetic nerve activity in HF. The phosphatidylinositol 3-kinase inhibitor LY294002 induced a small decrease in mean arterial pressure and heart rate but no change in renal sympathetic nerve activity. Immunofluorescent staining demonstrated increased p44/42 MAPK activity in neurons of the paraventricular nucleus of the hypothalamus of HF rats, colocalized with Fra-like activity (indicating chronic neuronal excitation). Intracerebroventricular PD98059 and UO126 reduced Fra-like activity in the paraventricular nucleus of the hypothalamus neurons in HF rats. In confirmatory acute studies, intracerebroventricular Ang II increased mean arterial pressure, heart rate, and renal sympathetic nerve activity in baroreceptor-denervated rats and Fra-like immunoreactivity in the paraventricular nucleus of the hypothalamus of neurally intact rats. Central administration of PD98059 markedly reduced these responses. These data demonstrate that intracellular p44/42 MAPK activity contributes to Ang II-induced neuronal excitation in the paraventricular nucleus of the hypothalamus and augmented sympathetic nerve activity in rats with HF.

Exercise training enhances baroreflex sensitivity by an angiotensin II-dependent mechanism in chronic heart failure

Exercise training (EX) has become an important modality capable of enhancing the quality of life and survival of patients with chronic heart failure (CHF). Although 4 wk of EX in animals with CHF evoked a reduction in renal sympathetic nerve activity and ANG II plasma levels and an enhancement in baroreflex sensitivity at rest (Liu JL, Irvine S, Reid IA, Patel KP, Zucker IH, Circulation 102: 1854-1862, 2000; Liu JL, Kulakofsky J, Zucker IH, J Appl Physiol 92: 2403-2408, 2002), it is unclear whether these phenomena are causally related. CHF was induced in rabbits by ventricular pacing (360-380 beats/min) for 3 wk. CHF rabbits were EX for 4 wk at 15-18 m/min, 6 days/wk, 30-40 min/day. Three groups of rabbits were studied: CHF (with no EX), CHF-EX, and CHF-EX + ANG II infusion [in which ANG II levels were kept at or near levels observed in CHF (non-EX) rabbits by subcutaneous osmotic minipump infusion]. EX prevented the increase in plasma ANG II levels shown in CHF rabbits. CHF and CHF-EX + ANG II infusion rabbits had significantly depressed baroreflex sensitivity slopes (P < 0.01 for sodium nitroprusside and P < 0.001 for phenylephrine) and higher baseline renal sympathetic nerve activities than CHF-EX animals. EX downregulated mRNA and protein expression of ANG II type 1 receptors in the rostral ventrolateral medulla in CHF rabbits. This was prevented by ANG II infusion. These data are consistent with the view that the reduction in sympathetic nerve activity and the improvement in baroreflex function in CHF after EX are due to the concomitant reduction in ANG II and angiotensin receptors in the central nervous system.

Influence of cardiopulmonary reflex on the sympathetic activity during myocardial infarction

The time-course of changes in renal sympathetic nerve activity (RSNA), arterial and cardiopulmonary baroreflexes sensitivities was evaluated in conscious rats eight hours (8 h) and ten days (10 day) after myocardial infarction (MI), induced by coronary artery ligation. RSNA was recorded by a platinum electrode implanted in left renal nerve. Arterial and cardiopulmonary baroreflexes sensitivities were evaluated by changes in blood pressure and serotonin administration, respectively. Both 8 h and 10 day groups presented hypotension (103+/-4 vs. 102+/-2 vs. 115+/-4 mm Hg), but only 8 h showed tachycardia (422+/-22 vs. 378+/-11 vs. 384+/-9 bpm) when compared to Control rats. RSNA was depressed 8 h after MI and increased in 10 day group (12+/-2 vs. 39+/-8 vs. 22+/-2 mV/cycle). Although arterial baroreflex control of heart rate was similar in all groups, the arterial baroreflex control of RSNA in 8 h group was impaired during reductions (-0.35+/-0.10 vs. -1.66+/-0.23 vs. -0.09+/-0.14 mV/cycle/mm Hg) or increases (-0.77+/-0.17 vs. -1.63+/-0.58 vs. -1.66+/-0.17 mV/cycle/mm Hg) in blood pressure when compared to Control animals. Moreover, cardiopulmonary baroreflex bradycardic response was increased in 8 h rats and normalized in 10 day group. The results suggest that the increased cardiopulmonary baroreflex sensitivity in 8 h may contribute to the reduction in the tonic level of RSNA as well as in the impairment of the baroreflex control of RSNA in the presence of hypotension.

Neuronal angiotensin II type 1 receptor upregulation in heart failure: activation of activator protein 1 and Jun N-terminal kinase

Chronic heart failure (CHF) is a leading cause of mortality in developed countries. Angiotensin II (Ang II) plays an important role in the development and progression of CHF. Many of the important functions of Ang II are mediated by the Ang II type 1 receptor (AT(1)R), including the increase in sympathetic nerve activity in CHF. However, the central regulation of the AT(1)R in the setting of CHF is not well understood. This study investigated the AT(1)R in the rostral ventrolateral medulla (RVLM) of rabbits with CHF, its downstream pathway, and its gene regulation by the transcription factor activator protein 1 (AP-1). Studies were performed in 5 groups of rabbits: sham (n=5), pacing-induced (3 to 4 weeks) CHF (n=5), CHF with intracerebroventricular (ICV) losartan treatment (n=5), normal with ICV Ang II treatment (n=5), and normal with ICV Ang II plus losartan treatment (n=5). AT(1)R mRNA and protein expressions, plasma Ang II, and AP-1-DNA binding activity were significantly higher in RVLM of CHF compared with Sham rabbits (240.4+/-30.2%, P<0.01; 206.6+/-25.8%, P<0.01; 280+/-36.5%, P<0.05; 207+/-16.4%, P<0.01, respectively). Analysis of the stress-activated protein kinase/Jun N-terminal kinase (SAPK/JNK) pathway showed that phosphorylated c-Jun proteins, phosphorylated JNK proteins, and JNK activity increased significantly in RVLM of CHF compared with sham (262.9+/-48.1%, 213.8+/-27.7%, 148.2+/-10.1% of control, respectively). Importantly, ICV losartan in CHF rabbits attenuated these increases. ICV Ang II in normal rabbits simulated the molecular changes seen in CHF. This effect was blocked by concomitant ICV losartan. In addition, Ang II-induced AT(1)R expression was blocked by losartan and a JNK inhibitor, but not by extracellular signal-regulated kinase or p38 MAP kinase inhibitors in a neuronal cell culture. These data suggest that central Ang II activates the AT(1)R, SAPK/JNK pathway. AP-1 may further regulate gene expression in RVLM in the CHF state.

Neurohormonal inhibition in heart failure: insights from recent clinical trials

Heart failure (HF) is a clinical syndrome characterized by chronic, persistent activation of the neuroendocrine system, which has been assumed to be linked to disease progression and adverse outcomes. Clinical trials have shown that adrenergic modulators, such as angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, aldosterone blockers, and beta-blockers, improve long-term outcomes in patients with HF. These findings have led to the hypothesis that inhibition of a single neurohormonal or cytokine pathway may continue to provide incremental benefits. However, the results of recent clinical trials, using centrally acting agents--endopeptidase inhibitors or endothelin and cytokine antagonists--suggest that selective inhibition of neurohormonal systems may not be advantageous and actually may have serious adverse effects. The reasons for this lack of benefit may be ascribed to the fact that long-term mortality benefits in patients with chronic HF are primarily the result of treatment of the diseases that have caused HF in the first place rather than treatment of neurohormonal abnormalities.

Angiotensin II--nitric oxide interactions in the control of sympathetic outflow in heart failure

Activation of the sympathetic nervous system is a compensatory mechanism which initially provides support for the circulation in the face of a falling cardiac output. It has been recognized for some time that chronic elevation of sympathetic outflow with the consequent increase in plasma norepinephrine, is counterproductive to improving cardiac function. Indeed, therapeutic targeting to block excessive sympathetic activation in heart failure is becoming a more accepted modality. The mechanism(s) by which sympathetic excitation occurs in the heart failure state are not completely understood. Components of abnormal cardiovascular reflex regulation most likely contribute to this sympatho-excitation. However, central mechanisms which relate to the elaboration of angiotensin II (Ang II) and nitric oxide (NO) may also play an important role. Ang II has been shown to be a sympatho-excitatory peptide in the central nervous system while NO is sympatho-inhibitory. Recent studies have demonstrated that blockade of Ang II receptors of the AT(1) subtype augments arterial baroreflex control of sympathetic nerve activity in the heart failure state, thereby predisposing to a reduction in sympathetic tone. Ang II and NO interact to regulate sympathetic outflow. Blockade of NO production in normal conscious rabbits was only capable of increasing sympathetic outflow when accompanied by a background infusion of Ang II. Conversely, providing a source of NO to rabbits with heart failure reduced sympathetic nerve activity when accompanied by blockade of AT(1) receptors. Chronic heart failure is also associated with a decrease in NO synthesis in the brain as indicated by a reduction in the mRNA for the neuronal isoform (nNOS). Chronic blockade of Ang II receptors can up regulate nNOS expression. In addition, exercise training of rabbits with developing heart failure has been shown to reduce sympathetic tone, decrease plasma Ang II, improve arterial baroreflex function and increase nNOS expression in the central nervous system. This review summarizes a large number of studies which have concentrated on the mechanisms of sympatho-excitation in heart failure. It now seems clear that one mechanism which is important in regulating sympathetic outflow in this disease state depends upon a central interaction between Ang II and NO at the cellular and nuclear levels.

Cardiac sympathetic afferent reflexes in heart failure

Heart failure is characterized by an elevation in sympathetic tone. The mechanisms responsible for this sympatho-excitation of heart failure are not completely understood. Several studies from this laboratory have compared differences in the cardiac "sympathetic afferent" reflex between sham dogs and dogs with pacing-induced heart failure. We found 1) that the cardiac sympathetic afferent reflex is augmented in heart failure, 2) tonic cardiac sympathetic afferent inputs play an important role in the elevated sympathetic tone in heart failure, 3) cardiac sympathetic afferents are sensitized in heart failure and 4) the central gain of the cardiac sympathetic afferent reflex in heart failure is sensitized and that this sensitization may be related to augmented central Ang II and blunted NO mechanisms. These studies integrate into the regulation of sympathetic outflow in heart failure which is likely to be mediated by a variety of peripheral inputs modulated by central substances. If the cardiac sympathetic afferent reflex is one of the excitatory reflexes which contribute to sympathetic activation in heart failure, a comprehensive understanding of neuro-humoral regulation of this reflex may result in more definitives and rational therapy targeted to the sympathetic nervous system in this disease state.

Chemoreflex function in heart failure

Peripheral and central chemoreflexes are the dominant autonomic mechanisms regulating ventilatory patterns in response to changes in partial pressures of oxygen and carbon dioxide in arterial blood and exert powerful effects on neural circulatory control. Both reflex pathways are capable of eliciting increases in sympathetic nerve traffic and consequent increases in blood pressure. Chronic heart failure is accompanied by a sustained elevation in sympathetic nerve traffic, which is thought to be an important component in the pathophysiology and progression of the disease. The role of chemoreflex mechanisms in the control of sympathetic function during heart failure is an important topic for which there are many questions and few answers. This review summarizes available evidence documenting peripheral and central chemoreflex function in heart failure, possible mechanisms for their alteration, and their possible contribution to ventilatory, and circulatory abnormalities that occur in heart failure.

Exercise training improves endogenous nitric oxide mechanisms within the paraventricular nucleus in rats with heart failure

Previously, we have demonstrated that an altered endogenous nitric oxide (NO) mechanism within the paraventricular nucleus (PVN) contributes to increased renal sympathetic nerve activity (RSNA) in heart failure (HF) rats. The goal of this study was to examine the effect of exercise training (ExT) in improving the endogenous NO mechanism within the PVN involved in the regulation of RSNA in rats with HF. ExT significantly restored the decreased number of neuronal NO synthase (nNOS)-positive neurons in the PVN (129 +/- 17 vs. 99 +/- 6). nNOS mRNA expression and protein levels in the PVN were also significantly increased in HF-ExT rats compared with HF-sedentary rats. To examine the functional role of NO within the PVN, an inhibitor of NOS, N(G)-monomethyl-L-arginine, was microinjected into the PVN. Dose-dependent increases in RSNA, arterial blood pressure (BP), and heart rate (HR) were produced in all rats. There was a blunted increase in these parameters in HF rats compared with the sham-operated rats. ExT significantly augmented RSNA responses in rats with HF (33% vs. 20% at the highest dose), thus normalizing the responses. The NO donor sodium nitroprusside, microinjected into the PVN, produced dose-dependent decreases in RSNA, BP, and HR in both sham and HF rats. ExT significantly improved the blunted decrease in RSNA in HF rats (36% vs. 17% at the highest dose). In conclusion, our data indicate that ExT improves the altered NO mechanism within the PVN and restores NO-mediated changes in RSNA in rats with HF.

Sympathoexcitation by central ANG II: roles for AT1 receptor upregulation and NAD(P)H oxidase in RVLM

Chronic heart failure is often associated with sympathoexcitation and blunted arterial baroreflex function. These phenomena have been causally linked to elevated central ANG II mechanisms. Recent studies have shown that NAD(P)H oxidase-derived reactive oxygen species (ROS) are important mediators of ANG II signaling and therefore might play an essential role in these interactions. The aims of this study were to determine whether central subchronic infusion of ANG II in normal animals has effects on O2- production and expression of NAD(P)H oxidase subunits as well as ANG II type 1 (AT1) receptors in the rostral ventrolateral medulla (RVLM). Twenty-four male New Zealand White rabbits were divided into four groups and separately received a subchronic intracerebroventricular infusion of saline alone, ANG II alone, ANG II with losartan, and losartan alone for 1 wk. On day 7 of intracerebroventricular infusion, mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) values were recorded, and arterial baroreflex sensitivity was evaluated while animals were in the conscious state. We found that ANG II significantly increased baseline RSNA (161.9%; P < 0.05), mRNA and protein expression of AT1 receptors (mRNA, 66.7%; P < 0.05; protein, 85.1%; P < 0.05), NAD(P)H oxidase subunits (mRNA, 120.0-200.0%; P < 0.05; protein, 90.9-197.0%; P < 0.05), and O2- production (83.2%; P < 0.05) in the RVLM. In addition, impaired baroreflex control of HR (Gain(max) reduced by 48.2%; P < 0.05) and RSNA (Gain(max) reduced by 53.6%; P < 0.05) by ANG II was completely abolished by losartan. Losartan significantly decreased baseline RSNA (-49.5%; P < 0.05) and increased baroreflex control of HR (Gain(max) increased by 64.8%; P < 0.05) and RSNA (Gain(max) increased by 67.9%; P < 0.05), but had no significant effects on mRNA and protein expression of AT1 receptor and NAD(P)H oxidase subunits and O2- production in the RVLM. These data suggest that in normal rabbits, NAD(P)H oxidase-derived ROS play an important role in the modulation of sympathetic activity and arterial baroreflex function by subchronic central treatment of exogenous ANG II via AT1 receptors.

Superoxide mediates sympathoexcitation in heart failure: roles of angiotensin II and NAD(P)H oxidase

Chronic heart failure (CHF) is often associated with excitation of the sympathetic nervous system. This event is thought to be a negative predictor of survival in CHF. Sympathoexcitation and central angiotensin II (Ang II) have been causally linked. Recent studies have shown that NAD(P)H oxidase-derived reactive oxidant species (ROS) are important mediators of Ang II signaling. In the present study, we tested the hypothesis that central Ang II activates sympathetic outflow by stimulation of NAD(P)H oxidase and ROS in the CHF state. CHF was induced in male New Zealand White rabbits by chronic ventricular tachycardia. Using radio telemetry of arterial pressure and intracerebroventricular infusions, experiments were performed in the conscious state. Renal sympathetic nerve activity (RSNA) was recorded as a direct measure of sympathetic outflow. Intracerebroventricular Ang II significantly increased RSNA in sham (131.5+/-13.3% of control) and CHF (193.6+/-11.9% of control) rabbits. The increase in CHF rabbits was significantly greater than in sham rabbits (P<0.01). These responses were abolished by intracerebroventricular losartan, tempol, or apocynin. Resting RSNA was significantly reduced by intracerebroventricular losartan, tempol, or apocynin in CHF rabbits but not in sham rabbits. Intracerebroventricular administration of the superoxide dismutase inhibitor diethyldithio-carbamic acid increased RSNA significantly more in sham compared with CHF rabbits. NADPH-dependent superoxide anion production in the rostral ventrolateral medulla (RVLM) was increased by 2.9-fold in CHF rabbits compared with sham rabbits. Finally, increases in the RVLM mRNA and protein expression of Ang II type 1 (AT1) receptor and subunits of NAD(P)H oxidase (p40phox, p47phox, and gp91phox) were demonstrated in CHF rabbits. These data demonstrate intense radical stress in autonomic areas of the brain in experimental CHF and provide evidence for a tight relationship between Ang II and ROS as contributors to sympathoexcitation in CHF.

Redox signaling in central neural regulation of cardiovascular function

One of the most prominent concepts to emerge in cardiovascular research over the past decade, especially in areas focused on angiotensin II (AngII), is that reactive oxygen species (ROS) are critical signaling molecules in a wide range of cellular processes. Many of the physiological effects of AngII are mediated by ROS, and alterations in AngII-mediated redox mechanisms are implicated in cardiovascular diseases such as hypertension and atherosclerosis. Although most investigations to date have focused on the vasculature as a key player, the nervous system has recently begun to gain attention in this field. Accumulating evidence suggests that ROS have important effects on central neural mechanisms involved in blood pressure regulation, volume homeostasis, and autonomic function, particularly those that involve AngII signaling. Furthermore, oxidant stress in the central nervous system is implicated in the neuro-dysregulation associated with some forms of hypertension and heart failure. The main objective of this review is to discuss the recent progress and prospects for this new field of central redox signaling in cardiovascular regulation, while also addressing the molecular tools that have spurred it forward.

Pharmacologic therapy for patients with chronic heart failure and reduced systolic function: review of trials and practical considerations

Heart failure (HF) is a complex clinical syndrome resulting from any structural or functional cardiac disorder impairing the ability of the ventricles to fill with or eject blood. The approach to pharmacologic treatment has become a combined preventive and symptomatic management strategy. Ideally, treatment should be initiated in patients at risk, preventing disease progression. In patients who have progressed to symptomatic left ventricular dysfunction, certain therapies have been demonstrated to improve survival, decrease hospitalizations, and reduce symptoms. The mainstay therapies are angiotensin-converting enzyme (ACE) inhibitors and beta-blockers (bisoprolol, carvedilol, and metoprolol XL/CR), with diuretics to control fluid balance. In patients who cannot tolerate ACE inhibitors because of angioedema or severe cough, valsartan can be substituted. Valsartan should not be added in patients already taking an ACE inhibitor and a beta-blocker. Spironolactone is recommended in patients who have New York Heart Association (NYHA) class III to IV symptoms despite maximal therapies with ACE inhibitors, beta-blockers, diuretics, and digoxin. Low-dose digoxin, yielding a serum concentration <1 ng/mL can be added to improve symptoms and, possibly, mortality. The combination of hydralazine and isosorbide dinitrate might be useful in patients (especially in African Americans) who cannot tolerate ACE inhibitors or valsartan because of hypotension or renal dysfunction. Calcium antagonists, with the exception of amlodipine, oral or intravenous inotropes, and vasodilators, should be avoided in HF with reduced systolic function. Amiodarone should be used only if patients have a history of sudden death, or a history of ventricular fibrillation or sustained ventricular tachycardia, and should be used in conjunction with an implantable defibrillator [corrected]. Finally, anticoagulation is recommended only in patients who have concomitant atrial fibrillation or a previous history of cerebral or systemic emboli.

Effect of water immersion on renal sympathetic nerve activity and arterial baroreflex in conscious rabbits with heart failure

Several studies have indicated an interaction between cardiopulmonary mechanoreflex and arterial baroreflex. However, the contribution of cardiopulmonary mechanoreflex to an abnormal arterial baroreflex in chronic heart failure (CHF) has not been fully investigated. We examined the effect of the activation of cardiopulmonary mechanoreceptors induced by head-out water immersion (WI) on the arterial baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) in conscious rabbits with CHF induced by myocardial infarction. The arterial baroreflex sensitivity (BRS) of both HR and RSNA were decreased in CHF. WI induced a small decrease in RSNA in CHF compared to a sham-operated group (Sham), despite a similar increase in central venous pressure. WI did not affect BRS of HR or RSNA in either Sham or CHF. By averaging rectified RSNA recordings, we found that miniature RSNA in the control in CHF was higher than that in Sham. WI decreased the synchronized RSNA without changing miniature RSNA in either group. These results suggest that cardiopulmonary mechanoreflex control of RSNA is reduced in CHF, and that cardiopulmonary mechanoreflex has little effect on arterial baroreflex. An increase in miniature RSNA may reflect sympatho-excitation in CHF.