Role of CuZn superoxide dismutase on carotid body function in heart failure rabbits

Peripheral chemoreflex modulation of renal hemodynamics and renal tissue PO2 in chronic heart failure with reduced ejection fraction

Aberrant carotid body chemoreceptor (CBC) function contributes to increased sympathetic nerve activity (SNA) and reduced renal blood flow (RBF) in chronic heart failure (CHF). Intermittent asphyxia (IA) mimicking sleep apnea is associated with additional increases in SNA and may worsen reductions in RBF and renal PO2 (RPO2) in CHF. The combined effects of decreased RBF and RPO2 may contribute to biochemical changes precipitating renal injury. This study sought to determine the role of CBC activity on glomerular filtration rate (GFR), RBF and RPO2 in CHF, and to assess the additive effects of IA. Furthermore, we sought to identify changes in gene expression that might contribute to renal injury. We hypothesized that GFR, RBF, and RPO2 would be reduced in CHF, that decreases in RBF and RPO2 would be worsened by IA, and that these changes would be ameliorated by CBC ablation (CBD). Finally, we hypothesized that CHF would be associated with pro-oxidative pro-fibrotic changes in renal gene expression that would be ameliorated by CBD. CHF was induced in adult male Sprague Dawley rats using coronary artery ligation (CAL). Carotid body denervation was performed by cryogenic ablation. GFR was assessed in conscious animals at the beginning and end of the experimental period. At 8-weeks post-CAL, cardiac function was assessed via echocardiography, and GFR, baseline and IA RBF and RPO2 were measured. Renal gene expression was measured using qRT-PCR. GFR was lower in CHF compared to sham (p < 0.05) but CBD had no salutary effect. RBF and RPO2 were decreased in CHF compared to sham (p < 0.05), and this effect was attenuated by CBD (p < 0.05). RBF and RPO2 were reduced to a greater extent in CHF vs. sham during exposure to IA (p < 0.05), and this effect was attenuated by CBD for RBF (p < 0.05). Downregulation of antioxidant defense and fibrosis-suppressing genes was observed in CHF vs. sham however CBD had no salutary effect. These results suggest that aberrant CBC function in CHF has a clear modulatory effect on RBF during normoxia and during IA simulating central sleep apnea.

Homocysteine, Vitamins B6 and Folic Acid in Experimental Models of Myocardial Infarction and Heart Failure—How Strong Is That Link?

Cardiovascular diseases are the leading cause of death and the main cause of disability. In the last decade, homocysteine has been found to be a risk factor or a marker for cardiovascular diseases, including myocardial infarction (MI) and heart failure (HF). There are indications that vitamin B6 plays a significant role in the process of transsulfuration in homocysteine metabolism, specifically, in a part of the reaction in which homocysteine transfers a sulfhydryl group to serine to form α-ketobutyrate and cysteine. Therefore, an elevated homocysteine concentration (hyperhomocysteinemia) could be a consequence of vitamin B6 and/or folate deficiency. Hyperhomocysteinemia in turn could damage the endothelium and the blood vessel wall and induce worsening of atherosclerotic process, having a negative impact on the mechanisms underlying MI and HF, such as oxidative stress, inflammation, and altered function of gasotransmitters. Given the importance of the vitamin B6 in homocysteine metabolism, in this paper, we review its role in reducing oxidative stress and inflammation, influencing the functions of gasotransmitters, and improving vasodilatation and coronary flow in animal models of MI and HF.

The protective role of SOD1 overexpression in central mediation of bradycardia following chronic intermittent hypoxia in mice

Obstructive sleep apnea (OSA) is a highly prevalent sleep disorder that is associated with many cardiovascular complications. Similar to OSA, chronic intermittent hypoxia (CIH) (a model for OSA) leads to oxidative stress and impairs baroreflex control of the heart rate (HR) in rodents. The baroreflex arc includes the aortic depressor nerve (ADN), vagal efferent, and central neurons. In this study, we used mice as a model to examine the effects of CIH on baroreflex sensitivity, aortic baroreceptor afferents, and central and vagal efferent components of the baroreflex circuitry. Furthermore, we tested whether human Cu/Zn Superoxide Dismutase (SOD1) overexpression in transgenic mice offers protection against CIH-induced deficit of the baroreflex arc. Wild-type C57BL/6J and SOD1 mice were exposed to room air (RA) or CIH and were then anesthetized, ventilated, and catheterized for measurement of mean arterial pressure (MAP) and HR. Compared with wild-type RA control, CIH impaired baroreflex sensitivity but increased maximum baroreceptor gain and bradycardic response to vagal efferent stimulation. Additionally, CIH reduced the bradycardic response to ADN stimulation, indicating a diminished central regulation of bradycardia. Interestingly, SOD1 overexpression prevented CIH-induced attenuation of HR responses to ADN stimulation and preserved HR responses to vagal efferent stimulation in transgenic mice. We suggest that CIH decreased central mediation of the baroreflex and SOD1 overexpression may prevent the CIH-induced central deficit.

Treating oxidative stress in heart failure: past, present and future

Advances in cardiovascular research have identified oxidative stress as an important pathophysiological pathway in the development and progression of heart failure. Oxidative stress is defined as the imbalance between the production of reactive oxygen species (ROS) and the endogenous antioxidant defence system. Under physiological conditions, small quantities of ROS are produced intracellularly, which function in cell signalling, and can be readily reduced by the antioxidant defence system. However, under pathophysiological conditions, the production of ROS exceeds the buffering capacity of the antioxidant defence system, resulting in cell damage and death. Over the last decades several studies have tried to target oxidative stress with the aim to improve outcome in patients with heart failure, with very limited success. The reasons as to why these studies failed to demonstrate any beneficial effects remain unclear. However, one plausible explanation might be that currently employed strategies, which target oxidative stress by exogenous inhibition of ROS production or supplementation of exogenous antioxidants, are not effective enough, while bolstering the endogenous antioxidant capacity might be a far more potent avenue for therapeutic intervention. In this review, we provide an overview of oxidative stress in the pathophysiology of heart failure and the strategies utilized to date to target this pathway. We provide novel insights into modulation of endogenous antioxidants, which may lead to novel therapeutic strategies to improve outcome in patients with heart failure.

Translating carotid body function into clinical medicine

The carotid body (CB) is considered the main O₂ chemoreceptor, which contributes to cardiorespiratory homeostasis and ventilatory acclimatization. In clinical medicine, the most common pathologies associated with the CB are tumours. However, a growing body of evidence supports the novel idea that an enhanced CB chemosensory discharge contributes to the autonomic dysfunction and pathological consequences in obstructive sleep apnoea (OSA), hypertension, systolic heart failure (HF) and cardiometabolic diseases. Heightened CB chemosensory reactivity elicited by oxidative stress has been involved in sympathetic hyperactivity, cardiorespiratory instability, hypertension and insulin resistance. CB ablation, which reduces sympathetic hyperactivity, decreases hypertension in animal models of OSA and hypertension, eliminates breathing instability and improves animal survival in HF, and restores insulin tolerance in cardiometabolic models. Thus, data obtained from preclinical studies highlight the importance of the CB in the progression of sympathetic-related diseases, supporting the idea that appeasing the enhanced CB chemosensory drive may be useful in improving cardiovascular, respiratory and endocrine alterations. Accordingly, CB ablation has been proposed and used as a treatment for moderating resistant hypertension and HF-induced sympathetic hyperactivity in humans. First-in-human studies have shown that CB ablation reduces sympathetic overactivity, transiently reduces severe hypertension and improves quality of life in HF patients. Thus, CB ablation would be a useful therapy to reverse sympathetic overactivation in HF and severe hypertension, but caution is required before it is widely used due to the crucial physiological function played by the CB. Further studies in preclinical models are required to assess side-effects of CB ablation.

KLF2 mediates enhanced chemoreflex sensitivity, disordered breathing and autonomic dysregulation in heart failure

KEY POINTS

Enhanced carotid body chemoreflex activity contributes to development of disordered breathing patterns, autonomic dysregulation and increases in incidence of arrhythmia in animal models of reduced ejection fraction heart failure. Chronic reductions in carotid artery blood flow are associated with increased carotid body chemoreceptor activity. Krüppel-like Factor 2 (KLF2) is a shear stress-sensitive transcription factor that regulates the expression of enzymes which have previously been shown to play a role in increased chemoreflex sensitivity. We investigated the impact of restoring carotid body KLF2 expression on chemoreflex control of ventilation, sympathetic nerve activity, cardiac sympatho-vagal balance and arrhythmia incidence in an animal model of heart failure. The results indicate that restoring carotid body KLF2 in chronic heart failure reduces sympathetic nerve activity and arrhythmia incidence, and improves cardiac sympatho-vagal balance and breathing stability. Therapeutic approaches that increase KLF2 in the carotid bodies may be efficacious in the treatment of respiratory and autonomic dysfunction in heart failure.

ABSTRACT

Oscillatory breathing and increased sympathetic nerve activity (SNA) are associated with increased arrhythmia incidence and contribute to mortality in chronic heart failure (CHF). Increased carotid body chemoreflex (CBC) sensitivity plays a role in this process and can be precipitated by chronic blood flow reduction. We hypothesized that downregulation of a shear stress-sensitive transcription factor, Krüppel-like Factor 2 (KLF2), mediates increased CBC sensitivity in CHF and contributes to associated autonomic, respiratory and cardiac sequelae. Ventilation (Ve), renal SNA (RSNA) and ECG were measured at rest and during CBC activation in sham and CHF rabbits. Oscillatory breathing was quantified as the apnoea-hypopnoea index (AHI) and respiratory rate variability index (RRVI). AHI (control 6 ± 1/h, CHF 25 ± 1/h), RRVI (control 9 ± 3/h, CHF 29 ± 3/h), RSNA (control 22 ± 2% max, CHF 43 ± 5% max) and arrhythmia incidence (control 50 ± 10/h, CHF 300 ± 100/h) were increased in CHF at rest ( FIO2 21%), as were CBC responses (Ve, RSNA) to 10% FIO2 (all P < 0.05 vs. control). In vivo adenoviral transfection of KLF2 to the carotid bodies in CHF rabbits restored KLF2 expression, and reduced AHI (7 ± 2/h), RSNA (18 ± 2% max) and arrhythmia incidence (46 ± 13/h) as well as CBC responses to hypoxia (all P < 0.05 vs. CHF empty virus). Conversely, lentiviral KLF2 siRNA in the carotid body decreased KLF2 expression, increased chemoreflex sensitivity, and increased AHI (6 ± 2/h vs. 14 ± 3/h), RRVI (5 ± 3/h vs. 20 ± 3/h) and RSNA (24 ± 4% max vs. 34 ± 5% max) relative to scrambled-siRNA rabbits. In conclusion, down-regulation of KLF2 in the carotid body increases CBC sensitivity, oscillatory breathing, RSNA and arrhythmia incidence during CHF.

Carotid Body-Mediated Chemoreflex Drive in The Setting of low and High Output Heart Failure

Enhanced carotid body (CB) chemoreflex function is strongly related to cardiorespiratory disorders and disease progression in heart failure (HF). The mechanisms underlying CB sensitization during HF are not fully understood, however previous work indicates blood flow per se can affect CB function. Then, we hypothesized that the CB-mediated chemoreflex drive will be enhanced only in low output HF but not in high output HF. Myocardial infarcted rats and aorto-caval fistulated rats were used as a low output HF model (MI-CHF) and as a high output HF model (AV-CHF), respectively. Blood flow supply to the CB region was decreased only in MI-CHF rats compared to Sham and AV-CHF rats. MI-CHF rats exhibited a significantly enhanced hypoxic ventilatory response compared to AV-CHF rats. However, apnea/hypopnea incidence was similarly increased in both MI-CHF and AV-CHF rats compared to control. Kruppel-like factor 2 expression, a flow sensitive transcription factor, was reduced in the CBs of MI-CHF rats but not in AV-CHF rats. Our results indicate that in the setting of HF, potentiation of the CB chemoreflex is strongly associated with a reduction in cardiac output and may not be related to other pathophysiological consequences of HF.

Cardiac diastolic and autonomic dysfunction are aggravated by central chemoreflex activation in heart failure with preserved ejection fraction rats

KEY POINTS

Heart failure with preserved ejection fraction (HFpEF) is associated with disordered breathing patterns, and sympatho-vagal imbalance. Although it is well accepted that altered peripheral chemoreflex control plays a role in the progression of heart failure with reduced ejection fraction (HFrEF), the pathophysiological mechanisms underlying deterioration of cardiac function in HFpEF are poorly understood. We found that central chemoreflex is enhanced in HFpEF and neuronal activation is increased in pre-sympathetic regions of the brainstem. Our data showed that activation of the central chemoreflex pathway in HFpEF exacerbates diastolic dysfunction, worsens sympatho-vagal imbalance and markedly increases the incidence of cardiac arrhythmias in rats with HFpEF.

ABSTRACT

Heart failure (HF) patients with preserved ejection fraction (HFpEF) display irregular breathing, sympatho-vagal imbalance, arrhythmias and diastolic dysfunction. It has been shown that tonic activation of the central and peripheral chemoreflex pathway plays a pivotal role in the pathophysiology of HF with reduced ejection fraction. In contrast, no studies to date have addressed chemoreflex function or its effect on cardiac function in HFpEF. Therefore, we tested whether peripheral and central chemoreflexes are hyperactive in HFpEF and if chemoreflex activation exacerbates cardiac dysfunction and autonomic imbalance. Sprague-Dawley rats (n = 32) were subjected to sham or volume overload to induce HFpEF. Resting breathing variability, chemoreflex gain, cardiac function and sympatho-vagal balance, and arrhythmia incidence were studied. HFpEF rats displayed [mean ± SD; chronic heart failure (CHF) vs. Sham, respectively] a marked increase in the incidence of apnoeas/hypopnoeas (20.2 ± 4.0 vs. 9.7 ± 2.6 events h^-1 ), autonomic imbalance [0.6 ± 0.2 vs. 0.2 ± 0.1 low/high frequency heart rate variability (LF/HF_HRV )] and cardiac arrhythmias (196.0 ± 239.9 vs. 19.8 ± 21.7 events h^-1 ). Furthermore, HFpEF rats showed increase central chemoreflex sensitivity but not peripheral chemosensitivity. Accordingly, hypercapnic stimulation in HFpEF rats exacerbated increases in sympathetic outflow to the heart (229.6 ± 43.2% vs. 296.0 ± 43.9% LF/HF_HRV , normoxia vs. hypercapnia, respectively), incidence of cardiac arrhythmias (196.0 ± 239.9 vs. 576.7 ± 472.9 events h^-1 ) and diastolic dysfunction (0.008 ± 0.004 vs. 0.027 ± 0.027 mmHg μl^-1 ). Importantly, the cardiovascular consequences of central chemoreflex activation were related to sympathoexcitation since these effects were abolished by propranolol. The present results show that the central chemoreflex is enhanced in HFpEF and that acute activation of central chemoreceptors leads to increases of cardiac sympathetic outflow, cardiac arrhythmogenesis and impairment in cardiac function in rats with HFpEF.

Contribution of peripheral and central chemoreceptors to sympatho-excitation in heart failure

Chronic heart failure (CHF) is a major public health problem. Tonic hyper-activation of sympathetic neural outflow is commonly observed in patients with CHF. Importantly, sympatho-excitation in CHF exacerbates its progression and is strongly related to poor prognosis and high mortality risk. Increases in both peripheral and central chemoreflex drive are considered markers of the severity of CHF. The principal peripheral chemoreceptors are the carotid bodies (CBs) and alteration in their function has been described in CHF. Mainly, during CHF the CB chemosensitivity is enhanced leading to increases in ventilation and sympathetic outflow. In addition to peripheral control of breathing, central chemoreceptors (CCs) are considered a dominant mechanism in ventilatory regulation. Potentiation of the ventilatory and sympathetic drive in response to CC activation has been shown in patients with CHF as well as in animal models. Therefore, improving understanding of the contribution of the peripheral and central chemoreflexes to augmented sympathetic discharge in CHF could help in developing new therapeutic approaches intended to attenuate the progression of CHF. Accordingly, the main focus of this review is to discuss recent evidence that peripheral and central chemoreflex function are altered in CHF and that they contribute to autonomic imbalance and progression of CHF.

Scavenging of superoxide anions by lecithinized superoxide dismutase in HL-60 cells

Superoxide dismutase covalently bound to four lecithin molecules (PC-SOD) on plasma membrane has been found to have beneficial therapeutic effects.

SOD1 Overexpression Preserves Baroreflex Control of Heart Rate with an Increase of Aortic Depressor Nerve Function

Overproduction of reactive oxygen species (ROS), such as the superoxide radical (O₂^∙−), is associated with diseases which compromise cardiac autonomic function. Overexpression of SOD1 may offer protection against ROS damage to the cardiac autonomic nervous system, but reductions of O₂^∙− may interfere with normal cellular functions. We have selected the C57B6SJL-Tg (SOD1)2 Gur/J mouse as a model to determine whether SOD1 overexpression alters cardiac autonomic function, as measured by baroreflex sensitivity (BRS) and aortic depressor nerve (ADN) recordings, as well as evaluation of baseline heart rate (HR) and mean arterial pressure (MAP). Under isoflurane anesthesia, C57 wild-type and SOD1 mice were catheterized with an arterial pressure transducer and measurements of HR and MAP were taken. After establishing a baseline, hypotension and hypertension were induced by injection of sodium nitroprusside (SNP) and phenylephrine (PE), respectively, and ΔHR versus ΔMAP were recorded as a measure of baroreflex sensitivity (BRS). SNP and PE treatment were administered sequentially after a recovery period to measure arterial baroreceptor activation by recording aortic depressor nerve activity. Our findings show that overexpression of SOD1 in C57B6SJL-Tg (SOD1)2 Gur/J mouse preserved the normal HR, MAP, and BRS but enhanced aortic depressor nerve function.

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.

Mechanisms of carotid body chemoreflex dysfunction during heart failure

NEW FINDINGS

What is the topic of this review? Carotid body chemoreceptor activity is tonically elevated in heart failure and contributes to morbidity due to the reflex activation of sympathetic nerve activity and destabilization of breathing. The potential causes for the enhanced chemoreceptor activation in heart failure are discussed. What advances does it highlight? The role of a chronic reduction in blood flow to the carotid body due to cardiac failure and its impact on signalling pathways in the carotid body is discussed. Recent advances have attracted interest in the potential for carotid body (CB) ablation or desensitization as an effective strategy for clinical treatment and management of cardiorespiratory diseases, including hypertension, heart failure, diabetes mellitus, metabolic syndrome and renal failure. These disease states have in common sympathetic overactivity, which plays an important role in the development and progression of the disease and is often associated with breathing dysregulation, which in turn is likely to mediate or aggravate the autonomic imbalance. Evidence from both chronic heart failure (CHF) patients and animal models indicates that the CB chemoreflex is enhanced in CHF and contributes to the tonic elevation in sympathetic activity and the development of periodic breathing associated with the disease. Although this maladaptive change is likely to derive from altered function at all levels of the reflex arc, a tonic increase in afferent activity from CB glomus cells is likely to be a main driving force. This report focuses on our understanding of mechanisms that alter CB function in CHF and their potential translational impact on treatment of CHF.

Role of the Carotid Body Chemoreflex in the Pathophysiology of Heart Failure: A Perspective from Animal Studies

The treatment and management of chronic heart failure (CHF) remains an important focus for new and more effective clinical strategies. This important goal, however, is dependent upon advancing our understanding of the underlying pathophysiology. In CHF, sympathetic overactivity plays an important role in the development and progression of the cardiac and renal dysfunction and is often associated with breathing dysregulation, which in turn likely mediates or aggravates the autonomic imbalance. In this review we will summarize evidence that in CHF, the elevation in sympathetic activity and breathing instability that ultimately lead to cardiac and renal failure are driven, at least in part, by maladaptive activation of the carotid body (CB) chemoreflex. This maladaptive change derives from a tonic increase in CB afferent activity. We will focus our discussion on an understanding of mechanisms that alter CB afferent activity in CHF and its consequence on reflex control of autonomic, respiratory, renal, and cardiac function in animal models of CHF. We will also discuss the potential translational impact of targeting the CB in the treatment of CHF in humans, with relevance to other cardio-respiratory diseases.

Neuronal uptake of nanoformulated superoxide dismutase and attenuation of angiotensin II-dependent hypertension after central administration

Excessive production of superoxide (O2(-)) in the central nervous system has been widely implicated in the pathogenesis of cardiovascular diseases, including chronic heart failure and hypertension. In an attempt to overcome the failed therapeutic impact of currently available antioxidants in cardiovascular disease, we developed a nanomedicine-based delivery system for the O2(-)-scavenging enzyme copper/zinc superoxide dismutase (CuZnSOD), in which CuZnSOD protein is electrostatically bound to a poly-l-lysine (PLL50)-polyethylene glycol (PEG) block copolymer to form a CuZnSOD nanozyme. Various formulations of CuZnSOD nanozyme are covalently stabilized by either reducible or nonreducible crosslinked bonds between the PLL50-PEG polymers. Herein, we tested the hypothesis that PLL50-PEG CuZnSOD nanozyme delivers active CuZnSOD protein to neurons and decreases blood pressure in a mouse model of angiotensin II (AngII)-dependent hypertension. As determined by electron paramagnetic resonance spectroscopy, nanozymes retain full SOD enzymatic activity compared to native CuZnSOD protein. Nonreducible CuZnSOD nanozyme delivers active CuZnSOD protein to central neurons in culture (CATH.a neurons) without inducing significant neuronal toxicity. Furthermore, in vivo studies conducted in adult male C57BL/6 mice demonstrate that hypertension established by chronic subcutaneous infusion of AngII is significantly attenuated for up to 7 days after a single intracerebroventricular injection of nonreducible nanozyme. These data indicate the efficacy of nonreducible PLL50-PEG CuZnSOD nanozyme in counteracting excessive O2(-) and decreasing blood pressure in AngII-dependent hypertensive mice after central administration. Additionally, this study supports the further development of PLL50-PEG CuZnSOD nanozyme as an antioxidant-based therapeutic option for hypertension.

Brain delivery of proteins via their fatty acid and block copolymer modifications

It is well known that hydrophobic small molecules penetrate cell membranes better than hydrophilic molecules. Amphiphilic molecules that dissolve both in lipid and aqueous phases are best suited for membrane transport. Transport of biomacromolecules across physiological barriers, e.g. the blood-brain barrier, is greatly complicated by the unique structure and function of such barriers. Two decades ago we adopted a simple philosophy that to increase protein delivery to the brain one needs to modify this protein with hydrophobic moieties. With this general idea we began modifying proteins (antibodies, enzymes, hormones, etc.) with either hydrophobic fatty acid residues or amphiphilic block copolymer moieties, such as poy(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (pluronics or poloxamers) and more recently, poly(2-oxasolines). This simple approach has resulted in impressive successes in CNS drug delivery. We present a retrospective overview of these works initiated in the Soviet Union in 1980s, and then continued in the United States and other countries. Notably some of the early findings were later corroborated by brain pharmacokinetic data. Industrial development of several drug candidates employing these strategies has followed. Overall modification by hydrophobic fatty acids residues or amphiphilic block copolymers represents a promising and relatively safe strategy to deliver proteins to the brain.

Role of the carotid body in the pathophysiology of heart failure

Important recent advances implicate a role of the carotid body (CB) chemoreflex in sympathetic and breathing dysregulation in several cardio-respiratory diseases, drawing renewed interest in its potential implications for clinical treatment. Evidence from both chronic heart failure (CHF) patients and animal models indicates that the CB chemoreflex is enhanced in CHF, and contributes to the tonic elevation in sympathetic nerve activity (SNA) and periodic breathing associated with the disease. Although this maladaptive change likely derives from altered function at all levels of the reflex arc, a change in afferent function of the CB is likely to be a main driving force. This review will focus on recent advances in our understanding of the pathophysiological mechanisms that alter CB function in CHF and their potential translational impact on treatment of chronic heart failure (CHF).

Simvastatin treatment attenuates increased respiratory variability and apnea/hypopnea index in rats with chronic heart failure

Cheyne-Stokes respiration and cardiac arrhythmias are associated with increased morbidity and mortality in patients with chronic heart failure (CHF). Enhanced carotid body chemoreflex (CBC) sensitivity is associated with these abnormalities in CHF. Reduced carotid body (CB) 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, in part, by increasing Krüppel-like Factor 2 expression. We hypothesized that statin treatment would ameliorate enhanced CBC sensitivity as well as increased respiratory variability, apnea/hypopnea index, and arrhythmia index, in a rodent model of CHF. Resting breathing pattern, cardiac rhythm, and the ventilatory and CB chemoreceptor afferent responses to hypoxia 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 respiratory variability, apnea/hypopnea index, and arrhythmia index. Statin treatment prevented the increases in CBC sensitivity and the concomitant increases in respiratory variability, apnea/hypopnea index, and arrhythmia index. Krüppel-like Factor 2 and endothelial NOS protein were decreased in the CB and nucleus tractus solitarii 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 Cheyne-Stokes respiration and arrhythmias in patient populations with high chemoreflex sensitivity.

Ventilatory responses to chemoreflex stimulation are not enhanced by angiotensin II in healthy humans

The chemoreflexes exert significant control over respiration and sympathetic outflow. Abnormalities in chemoreflex function may contribute to various disease processes. Based on prior animal studies, we developed the hypothesis that acutely elevating circulating angiotensin II levels into the pathophysiological range increases chemoreflex responsiveness in healthy humans. Eighteen adults were studied before (Pre) and during (Post) low (protocol 1; 2ng/kg/min; n=9) or high (protocol 2; 5ng/kg/min; n=9) dose angiotensin II infusion (study day 1). Chemoreflex responses were quantified by the pure nitrogen breathing method [slope of the minute ventilation vs. arterial oxygen saturation plot generated during a series (n=10) of 100% inspired nitrogen exposures (1-8 breaths)] and by measuring responses to hypercapnia (7% inspired carbon dioxide). Responses to a non-chemoreflex stimulus were also determined (cold pressor test). Measurements were repeated on a subsequent day (study day 2) before and during infusion of a control vasoconstrictor (phenylephrine) infused at a dose (0.6-1.2μg/kg/min) sufficient to increase blood pressure to the same degree as that achieved during angiotensin II infusion. We found that despite increasing plasma angiotensin II levels to pathophysiological levels responses to pure nitrogen breathing, hypercapnia, and the cold pressor test were unchanged by low (2ng/kg/min) and high dose (5ng/kg/min) angiotensin II infusion (protocols 1 and 2). Similarly, responses measured during phenylephrine infusion (Post) were unchanged (from Pre). These findings indicate that acutely increasing plasma angiotensin II levels to levels observed in disease states, such as human heart failure, do not increase chemoreflex responsiveness in healthy humans.

Carotid body denervation improves autonomic and cardiac function and attenuates disordered breathing in congestive heart failure

In congestive heart failure (CHF), carotid body (CB) chemoreceptor activity is enhanced and is associated with oscillatory (Cheyne-Stokes) breathing patterns, increased sympathetic nerve activity (SNA) and increased arrhythmia incidence. We hypothesized that denervation of the CB (CBD) chemoreceptors would reduce SNA, reduce apnoea and arrhythmia incidence and improve ventricular function in pacing-induced CHF rabbits. Resting breathing, renal SNA (RSNA) and arrhythmia incidence were measured in three groups of animals: (1) sham CHF/sham-CBD (sham-sham); (2) CHF/sham-CBD (CHF-sham); and (3) CHF/CBD (CHF-CBD). Chemoreflex sensitivity was measured as the RSNA and minute ventilatory (VE) responses to hypoxia and hypercapnia. Respiratory pattern was measured by plethysmography and quantified by an apnoea-hypopnoea index, respiratory rate variability index and the coefficient of variation of tidal volume. Sympatho-respiratory coupling (SRC) was assessed using power spectral analysis and the magnitude of the peak coherence function between tidal volume and RSNA frequency spectra. Arrhythmia incidence and low frequency/high frequency ratio of heart rate variability were assessed using ECG and blood pressure waveforms, respectively. RSNA and VE responses to hypoxia were augmented in CHF-sham and abolished in CHF-CBD animals. Resting RSNA was greater in CHF-sham compared to sham-sham animals (43 ± 5% max vs. 23 ± 2% max, P < 0.05), and this increase was not found in CHF-CBD animals (25 ± 1% max, P < 0.05 vs. CHF-sham). Low frequency/high frequency heart rate variability ratio was similarly increased in CHF and reduced by CBD (P < 0.05). Respiratory rate variability index, coefficient of variation of tidal volume and apnoea-hypopnoea index were increased in CHF-sham animals and reduced in CHF-CBD animals (P < 0.05). SRC (peak coherence) was increased in CHF-sham animals (sham-sham 0.49 ± 0.05; CHF-sham 0.79 ± 0.06), and was attenuated in CHF-CBD animals (0.59 ± 0.05) (P < 0.05 for all comparisons). Arrhythmia incidence was increased in CHF-sham and reduced in CHF-CBD animals (213 ± 58 events h(-1) CHF, 108 ± 48 events h(-1) CHF-CBD, P < 0.05). Furthermore, ventricular systolic (3.8 ± 0.7 vs. 6.3 ± 0.5 ml, P < 0.05) and diastolic (6.3 ± 1.0 vs. 9.1 ± 0.5 ml, P < 0.05) volumes were reduced, and ejection fraction preserved (41 ± 5% vs. 54 ± 2% reduction from pre-pace, P < 0.05) in CHF-CBD compared to CHF-sham rabbits. Similar patterns of changes were observed longitudinally within the CHF-CBD group before and after CBD. In conclusion, CBD is effective in reducing RSNA, SRC and arrhythmia incidence, while improving breathing stability and cardiac function in pacing-induced CHF rabbits.

Angiotensin peptides and nitric oxide in cardiovascular disease

SIGNIFICANCE

The renin-angiotensin system (RAS) plays an important role in the normal control of cardiovascular and renal function in the healthy state and is a contributing factor in the development and progression of various types of cardiovascular diseases (CVD), including hypertension, diabetes, and heart failure.

RECENT ADVANCES

Evidence suggests that a balance between activation of the ACE/Ang II/AT1 receptor axis and the ACE2/Ang-(1-7)/Mas receptor axis is important for the function of the heart, kidney, and autonomic nervous system control of the circulation in the normal healthy state. An imbalance in these opposing pathways toward the ACE/Ang II/AT1 receptor axis is associated with CVD. The key component of this imbalance with respect to neural control of the circulation is the negative interaction between oxidative and NO• mechanisms, which leads to enhanced sympathetic tone and activation in disease conditions such as hypertension and heart failure.

CRITICAL ISSUES

The key mechanisms that disrupt normal regulation of Ang II and Ang-(1-7) signaling and promote pathogenesis of CVD at all organ levels remain poorly understood. The reciprocal relation between ACE and ACE2 expression and function suggests they are controlled interdependently at pre- and post-translational levels. Insights from neural studies suggest that an interaction between oxidative and nitrosative pathways may be key.

FUTURE DIRECTIONS

The role of redox mechanisms in the control of expression and activity of RAS enzymes and Ang receptors may provide important insight into the function of local tissue RAS in health and disease.

Carotid chemoreceptor ablation improves survival in heart failure: rescuing autonomic control of cardiorespiratory function

OBJECTIVES

This study sought to investigate whether selective ablation of the carotid body (CB) chemoreceptors improves cardiorespiratory control and survival during heart failure.

BACKGROUND

Chronic heart failure (CHF) is a recognized health problem worldwide, and novel treatments are needed to better improve life quality and decrease mortality. Enhanced carotid chemoreflex drive from the CB is thought to contribute significantly to autonomic dysfunction, abnormal breathing patterns, and increased mortality in heart failure.

METHODS

Chronic heart failure was induced by coronary ligation in rats. Selective CB denervation was performed to remove carotid chemoreflex drive in the CHF state (16 weeks post-myocardial infarction). Indexes of autonomic and respiratory function were assessed in CB intact and CB denervated animals. CB denervation at 2 weeks post-myocardial infarction was performed to evaluate whether early targeted CB ablation decreases the progression of left ventricular dysfunction, cardiac remodeling, and arrhythmic episodes and improves survival.

RESULTS

The CHF rats developed increased CB chemoreflex drive and chronic central pre-sympathetic neuronal activation, increased indexes of elevated sympathetic outflow, increased breathing variability and apnea incidence, and desensitization of the baroreflex. Selective CB ablation reduced the central pre-sympathetic neuronal activation by 40%, normalized indexes of sympathetic outflow and baroreflex sensitivity, and reduced the incidence of apneas in CHF animals from 16.8 ± 1.8 events/h to 8.0 ± 1.4 events/h. Remarkably, when CB ablation was performed early, cardiac remodeling, deterioration of left ventricle ejection fraction, and cardiac arrhythmias were reduced. Most importantly, the rats that underwent early CB ablation exhibited an 85% survival rate compared with 45% survival in CHF rats without the intervention.

CONCLUSIONS

Carotid chemoreceptors play a seminal role in the pathogenesis of heart failure, and their targeted ablation might be of therapeutic value to reduce cardiorespiratory dysfunction and improve survival during CHF.

Antioxidant-based therapies for angiotensin II-associated cardiovascular diseases

Cardiovascular diseases, including hypertension and heart failure, are associated with activation of the renin-angiotensin system (RAS) and increased circulating and tissue levels of ANG II, a primary effector peptide of the RAS. Through its actions on various cell types and organ systems, ANG II contributes to the pathogenesis of cardiovascular diseases by inducing cardiac and vascular hypertrophy, vasoconstriction, sodium and water reabsorption in kidneys, sympathoexcitation, and activation of the immune system. Cardiovascular research over the past 15-20 years has clearly implicated an important role for elevated levels of reactive oxygen species (ROS) in mediating these pathophysiological actions of ANG II. As such, the use of antioxidants, to reduce the elevated levels of ROS, as potential therapies for various ANG II-associated cardiovascular diseases has been intensely investigated. Although some antioxidant-based therapies have shown therapeutic impact in animal models of cardiovascular disease and in human patients, others have failed. In this review, we discuss the benefits and limitations of recent strategies, including gene therapy, dietary sources, low-molecular-weight free radical scavengers, polyethylene glycol conjugation, and nanomedicine-based technologies, which are designed to deliver antioxidants for the improved treatment of cardiovascular diseases. Although much work has been completed, additional research focusing on developing specific antioxidant molecules or proteins and identifying the ideal in vivo delivery system for such antioxidants is necessary before the use of antioxidant-based therapies for cardiovascular diseases become a clinical reality.

SOD1 gene transfer into paraventricular nucleus attenuates hypertension and sympathetic activity in spontaneously hypertensive rats

Excessive sympathetic activity contributes to the initiation and progression of hypertension. Reactive oxygen species in the paraventricular nucleus (PVN) is involved in sympathetic overdrive and hypertension. The present study was designed to investigate whether superoxide dismutase 1 (SOD1) overexpression in the PVN attenuated sympathetic activation and hypertension. Adenoviral vectors containing human SOD1 or null adenoviral vectors were microinjected into the PVN of Wistar rats and spontaneously hypertensive rats (SHR). Significant depressor effects were observed from weeks 1 to 4 after SOD1 gene transfer in SHR. Acute experiments were carried out at the end of the 3rd week. In the PVN, superoxide anion and angiotensin II levels were increased while SOD1 activity and protein expression were decreased in SHR, which were attenuated by SOD1 overexpression in the PVN. However, SOD1 overexpression had no significant effect on the SOD2 activity in the PVN. The blood pressure response to ganglionic blockade, cardiac sympathetic nerve activity, and cardiac sympathetic afferent reflex (CSAR) were enhanced, and the plasma norepinephrine level was increased in SHR, which were prevented by SOD1 gene transfer in the PVN. Furthermore, SOD1 overexpression decreased the ratio of left ventricular weight to body weight, cross-sectional areas of myocardial cells, media thickness, and the media/lumen ratio of small arteries in the heart in SHR. These results indicate that SOD1 overexpression in the PVN reduces arterial blood pressure, attenuates excessive sympathetic activity and CSAR, and improves myocardial and vascular remodeling in SHR.

Neurohumoral stimulation

The temporal relationship between the development of heart failure and activation of the neurohumoral systems involved in chronic heart failure (CHF) has not been precisely defined. When a compensatory mechanism switches to a deleterious contributing factor in the progression of the disease is unclear. This article addresses these issues through evaluating the contribution of various cardiovascular reflexes and cellular mechanisms to the sympathoexcitation in CHF. It also sheds light on some of the important central mechanisms that contribute to the increase in sympathetic nerve activity in CHF.

Age- and chamber-specific differences in oxidative stress after ischemic injury

Each year, tens of thousands of children undergo cardiopulmonary bypass (CPB) to correct congenital heart defects. Although necessary for surgery, CPB involves stopping the heart and exposing it to ischemic conditions. On reoxygenation, the heart can experience effects similar to that of acute myocardial infarction. Although much is known about adult injury, little is known about the effects of global ischemia on newborn ventricles. We studied newborn (2 to 4 days old) and adult (>8 weeks old) rabbit hearts subjected to ischemia-reperfusion (30 min of ischemia and 60 min of reperfusion). Our data demonstrated chamber- and age-specific changes in oxidative stress. During ischemia, hydrogen peroxide (H(2)O(2)) increased in both right-ventricular (RV) and left-ventricular (LV) myocytes of the newborn, although only the RV change was significant. In contrast, there was no significant increase in H(2)O(2) in either RV or LV myocytes of adults. There was a fivefold increase in H(2)O(2) formation in newborn RV myocytes compared with adults (P = 0.006). In whole-heart tissue, superoxide dismutase activity increased from sham versus ischemia in the left ventricle of both adult and newborn hearts, but it was increased only in the right ventricle of the newborn heart. Catalase activity was significantly increased after ischemia in both adult ventricles, whereas no increase was seen in newborn compared with sham hearts. In addition, catalase levels in newborns were significantly lower, indicating less scavenging potential. Nanoparticle-encapsulated ebselen, given as an intracardiac injection into the right or left ventricle of newborn hearts, significantly increased functional recovery of developed pressure only in the right ventricle, indicating the potential for localized antioxidant therapy during and after pediatric surgical procedures.

SOD1 overexpression in paraventricular nucleus improves post-infarct myocardial remodeling and ventricular function

Excessive sympathetic activation contributes to the progression of chronic heart failure. Reactive oxygen species in paraventricular nucleus (PVN) play an important role in the enhanced sympathetic outflow. This study was designed to determine whether superoxide dismutase 1 (SOD1) overexpression in the PVN attenuated the sympathetic activation and cardiac dysfunction in rats after an episode of myocardial infarction (MI). Adenoviral vectors containing human SOD1 (Ad-SOD) or null adenoviral vectors (Ad-null) were immediately microinjected into the PVN of rats with coronary artery ligation or sham operation. At the eighth week, the SOD1 protein level and activity in the PVN increased while the superoxide anions in the PVN decreased in Ad-SOD rats. The SOD1 overexpression in the PVN prevented the increases in left ventricular end-diastolic pressure and volume, and the decreases in ejection fraction and peak velocities of contraction in MI rats. In addition, there was an attenuation of renal sympathetic nerve activity, cardiac sympathetic afferent reflex and plasma norepinephrine level in MI rats. Furthermore, the SOD1 overexpression in the PVN reduced cardiomyocyte size, collagen deposition and the TUNEL-positive cardiomyocytes in MI rats. These results indicate that the SOD1 overexpression in the PVN attenuates the excessive sympathetic activation, myocardial remodeling, cardiomyocyte apoptosis and ventricular dysfunction in MI rats.

ROS and RNS signaling in heart disorders: could antioxidant treatment be successful?

There is not too much success in the antioxidant treatment of heart deceases in humans. However a new approach is now developed that suggests that depending on their structures and concentrations antioxidants can exhibit much more complicated functions in many pathological disorders. It is now well established that physiological free radicals superoxide and nitric oxide together with their derivatives hydrogen peroxide and peroxynitrite (all are named reactive oxygen species (ROS) and reactive nitrogen species (RNS)) play a more important role in heart diseases through their signaling functions. Correspondingly this work is dedicated to the consideration of damaging signaling by ROS and RNS in various heart and vascular disorders: heart failure (congestive heart failure or CHF), left ventricular hypertrophy (LVH), coronary heart disease, cardiac arrhythmias, and so forth. It will be demonstrated that ROS overproduction (oxidative stress) is a main origin of the transformation of normal physiological signaling processes into the damaging ones. Furthermore the favorable effects of low/moderate oxidative stress through preconditioning mechanisms in ischemia/reperfusion will be considered. And in the last part we will discuss the possibility of efficient application of antioxidants and enzyme/gene inhibitors for the regulation of damaging ROS signaling in heart disorders.

Angiotensin and carotid body chemoreception in heart failure

The carotid body (CB) plays an important role in the control of breathing and in autonomic control of cardiovascular function. CB chemoreceptor activity is enhanced in chronic heat failure (CHF) and contributes to the sympathetic hyperactivity that exacerbates the progression of the disease. Studies in the past few years have revealed that a local angiotensin (Ang) system exists in the CB and plays an important role in altering CB function in CHF as well as other conditions, such as chronic hypoxia. This brief review highlights recent revelations that Ang I metabolites exert effects within the CB, and focuses on the influence of Ang II and Ang-(1-7) on CB function in CHF.

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.

The role of redox changes in oxygen sensing

The specialized oxygen-sensing tissues include the carotid body and arterial smooth muscle cells in the pulmonary artery (PA) and ductus arteriosus (DA). We discuss the evidence that changes in oxygen tension are sensed through changes in redox status. "Redox" changes imply the giving or accepting of electrons. This might occur through the direct tunneling of electrons from mitochondria or redox couples to an effector protein (e.g. ion channel). Alternatively, the electron might be transferred through reactive oxygen species from mitochondria or an NADPH oxidase isoform. The PA's response to hypoxia and DA's response to normoxia result from reduction or oxidation, respectively. These opposing redox stimuli lead to K+ channel inhibition, membrane depolarization and an increase in cytosolic calcium and/or calcium sensitization that causes contraction. In the neuroendocrine cells (the type 1 cell of the carotid body, neuroepithelial body and adrenomedullary cells), the response is secretion. We examine the roles played by superoxide anion, hydrogen peroxide and the anti-oxidant enzymes in the signaling of oxygen tensions.

Pluronic-modified superoxide dismutase 1 attenuates angiotensin II-induced increase in intracellular superoxide in neurons

Overexpressing superoxide dismutase 1 (SOD1; also called Cu/ZnSOD), an intracellular superoxide (O(2)(*-))-scavenging enzyme, in central neurons inhibits angiotensin II (AngII) intraneuronal signaling and normalizes cardiovascular dysfunction in diseases associated with enhanced AngII signaling in the brain, including hypertension and heart failure. However, the blood-brain barrier and neuronal cell membranes impose a tremendous impediment for the delivery of SOD1 to central neurons, which hinders the potential therapeutic impact of SOD1 treatment on these diseases. To address this, we developed conjugates of SOD1 with poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer (Pluronic) (SOD1-P85 and SOD1-L81), which retained significant SOD1 enzymatic activity. The modified SOD1 effectively scavenged xanthine oxidase/hypoxanthine-derived O(2)(*-), as determined by HPLC and the measurement of 2-hydroxyethidium. Using catecholaminergic neurons, we observed an increase in neuronal uptake of SOD1-Pluronic after 1, 6, or 24h, compared to neurons treated with pure SOD1 or PEG-SOD1. Importantly, without inducing neuronal toxicity, SOD1-Pluronic conjugates significantly inhibited AngII-induced increases in intraneuronal O(2)(*-) levels. These data indicate that SOD1-Pluronic conjugates penetrate neuronal cell membranes, which results in elevated intracellular levels of functional SOD1. Pluronic conjugation may be a new delivery system for SOD1 into central neurons and therapeutically beneficial for AngII-related cardiovascular diseases.

Mitochondrial complex III: an essential component of universal oxygen sensing machinery?

Oxygen is necessary for the survival of mammalian cells. In order to maintain adequate cellular oxygenation, mammals have evolved multiple acute and long-term adaptive responses to hypoxia. These include hypoxic increases in erythropoiesis, pulmonary vasoconstriction and carotid body neurosecretion. Collectively, these responses help maintain oxygen homeostasis as oxygen levels remain scarce. There are multiple effectors proposed to underlie these diverse responses to hypoxia including PHD2, AMPK, NADPH oxidases, and mitochondrial complex III. Here I propose a model wherein complex III is integral to oxygen sensing in regulating diverse response to hypoxia.

The Walter B. Cannon Memorial Award Lecture, 2009. Physiology in perspective: The wisdom of the body. In search of autonomic balance: the good, the bad, and the ugly

Walter B. Cannon's research on the sympathetic nervous system and neurochemical transmission was pioneering. Wisdom has endowed our body with a powerful autonomic neural regulation of the circulation that provides optimal perfusion of every organ in accordance to its metabolic needs. Exquisite sensors tuned to an optimal internal environment trigger central and peripheral sympathetic and parasympathetic motor neurons and allow desirable and beneficial adjustments to physiologic needs as well as to acute cardiovascular stresses. This short review, presented as The Walter B. Cannon Memorial Award Lecture for 2009, addresses the mechanisms that disrupt sensory signaling and result in a chronic maladjustment of the autonomic neural output that in many cardiovascular diseases results in excessive increases in the risks of dying. The hopes for any reduction of those risks resides in an understanding of the molecular determinants of neuronal signaling.

Chemoreceptor hypersensitivity, sympathetic excitation, and overexpression of ASIC and TASK channels before the onset of hypertension in SHR

RATIONALE

Increased sympathetic nerve activity has been linked to the pathogenesis of hypertension in humans and animal models. Enhanced peripheral chemoreceptor sensitivity which increases sympathetic nerve activity has been observed in established hypertension but has not been identified as a possible mechanism for initiating an increase in sympathetic nerve activity before the onset of hypertension.

OBJECTIVE

We tested this hypothesis by measuring the pH sensitivity of isolated carotid body glomus cells from young spontaneously hypertensive rats (SHR) before the onset of hypertension and their control normotensive Wistar-Kyoto (WKY) rats.

METHODS AND RESULTS

We found a significant increase in the depolarizing effect of low pH in SHR versus WKY glomus cells which was caused by overexpression of 2 acid-sensing non-voltage-gated channels. One is the amiloride-sensitive acid-sensing sodium channel (ASIC3), which is activated by low pH and the other is the 2-pore domain acid-sensing K(+) channel (TASK1), which is inhibited by low pH and blocked by quinidine. Moreover, we found that the increase in sympathetic nerve activity in response to stimulation of chemoreceptors with sodium cyanide was markedly enhanced in the still normotensive young SHR compared to control WKY rats.

CONCLUSIONS

Our results establish a novel molecular basis for increased chemotransduction that contributes to excessive sympathetic activity before the onset of hypertension.

Elevated mitochondrial superoxide contributes to enhanced chemoreflex in heart failure rabbits

Peripheral chemoreflex sensitivity is enhanced in both clinical and experimental chronic heart failure (CHF). Here we investigated the role of manganese superoxide dismutase (MnSOD), the SOD isoform specially targeted to mitochondria, and mitochondrial superoxide levels in the enhanced chemoreceptor activity and function of the carotid body (CB) in CHF rabbits. CHF suppressed MnSOD protein expression and elevated mitochondrial superoxide levels in CB compared with that in sham CB. Adenovirus (Ad) MnSOD (1 x 10(8) plaque-forming units/ml) gene transfer selectively to the CBs normalized mitochondrial superoxide levels in glomus cells from CHF CB. In addition, Ad MnSOD reduced the elevation of superoxide level in CB tissue from CHF rabbits. Ad MnSOD significantly increased MnSOD expression in CHF CBs and normalized the baseline renal sympathetic nerve activity and the response of renal sympathetic nerve activity to hypoxia in CHF rabbits. Ad MnSOD decreased baseline single-fiber discharge from CB chemoreceptors compared with Ad Empty (6.3 + or - 1.5 vs. 12.7 + or - 1.4 imp/s at approximately 100-Torr Po(2), P < 0.05) and in response to hypoxia (20.5 + or - 1.8 vs. 32.6 + or - 1.4 imp/s at approximately 40-Torr Po(2), P < 0.05) in CHF rabbits. Compared with Ad Empty, Ad MnSOD reversed the blunted K(+) currents in CB glomus cells from CHF rabbits (385 + or - 11 vs. 551 + or - 20 pA/pF at +70 mV, P < 0.05). The results suggest that decreased MnSOD in the CB and elevated mitochondrial superoxide levels contribute to the enhanced CB chemoreceptor activity and peripheral chemoreflex function in CHF rabbits.