Functional, proteomic and bioinformatic analyses of Nrf2- and Keap1- null skeletal muscle.

KEY POINTS: Nrf2 is a master regulator of endogenous cellular defences, governing the expression of more than 200 cytoprotective proteins, including a panel of antioxidant enzymes. Nrf2 plays an important role in redox haemostasis of skeletal muscle in response to the increased generation of reactive oxygen species during contraction. Employing skeletal muscle-specific transgenic mouse models with unbiased-omic approaches, we uncovered new target proteins, downstream pathways and molecular networks of Nrf2 in skeletal muscle following Nrf2 or Keap1 deletion. Based on the findings, we proposed a two-way model to understand Nrf2 function: a tonic effect through a Keap1-independent mechanism under basal conditions and an induced effect through a Keap1-dependent mechanism in response to oxidative and other stresses. ABSTRACT: Although Nrf2 has been recognized as a master regulator of cytoprotection, its functional significance remains to be completely defined. We hypothesized that proteomic/bioinformatic analyses from Nrf2-deficient or overexpressed skeletal muscle tissues will provide a broader spectrum of Nrf2 targets and downstream pathways than are currently known. To this end, we created two transgenic mouse models; the iMS-Nrf2flox/flox and iMS-Keap1flox/flox , employing which we demonstrated that selective deletion of skeletal muscle Nrf2 or Keap1 separately impaired or improved skeletal muscle function. Mass spectrometry revealed that Nrf2-KO changed expression of 114 proteins while Keap1-KO changed expression of 117 proteins with 10 proteins in common between the groups. Gene ontology analysis suggested that Nrf2 KO-changed proteins are involved in metabolism of oxidoreduction coenzymes, purine ribonucleoside triphosphate, ATP and propanoate, which are considered as the basal function of Nrf2, while Keap1 KO-changed proteins are involved in cellular detoxification, NADP metabolism, glutathione metabolism and the electron transport chain, which belong to the induced effect of Nrf2. Canonical pathway analysis suggested that Keap1-KO activated four pathways, whereas Nrf2-KO did not. Ingenuity pathway analysis further revealed that Nrf2-KO and Keap1-KO impacted different signal proteins and functions. Finally, we validated the proteomic and bioinformatics data by analysing glutathione metabolism and mitochondrial function. In conclusion, we found that Nrf2-targeted proteins are assigned to two groups: one mediates the tonic effects evoked by a low level of Nrf2 at basal condition; the other is responsible for the inducible effects evoked by a surge of Nrf2 that is dependent on a Keap1 mechanism.

Episodic stimulation of central chemoreceptor neurons elicits disordered breathing and autonomic dysfunction in volume overload heart failure.

Enhanced central chemoreflex (CC) gain is observed in volume overload heart failure (HF) and is correlated with autonomic dysfunction and breathing disorders. The aim of this study was to determine the role of the CC in the development of respiratory and autonomic dysfunction in HF. Volume overload was surgically created to induce HF in male Sprague-Dawley rats. Radiotelemetry transmitters were implanted for continuous monitoring of blood pressure and heart rate. After recovering from surgery, conscious unrestrained rats were exposed to episodic hypercapnic stimulation [EHS; 10 cycles/5 min, inspiratory fraction of carbon dioxide (FICO2) 7%] in a whole body plethysmograph for recording of cardiorespiratory function. To determine the contribution of CC to cardiorespiratory variables, selective ablation of chemoreceptor neurons within the retrotrapezoid nucleus (RTN) was performed via injection of saporin toxin conjugated to substance P (SSP-SAP). Vehicle-treated rats (HF+Veh and Sham+Veh) were used as controls for SSP-SAP experiments. Sixty minutes post-EHS, minute ventilation was depressed in sham animals relative to HF animals (ΔV̇e: -5.55 ± 2.10 vs. 1.24 ± 1.35 mL/min 100 g, P < 0.05; Sham+Veh vs. HF+Veh). Furthermore, EHS resulted in autonomic imbalance, cardiorespiratory entrainment, and ventilatory disturbances in HF+Veh but not Sham+Veh rats, and these effects were significantly attenuated by SSP-SAP treatment. Also, the apnea-hypopnea index (AHI) was significantly lower in HF+SSP-SAP rats compared with HF+Veh rats (AHI: 5.5 ± 0.8 vs. 14.4 ± 1.3 events/h, HF+SSP-SAP vs. HF+Veh, respectively, P < 0.05). Finally, EHS-induced respiratory-cardiovascular coupling in HF rats depends on RTN chemoreceptor neurons because it was reduced by SSP-SAP treatment. Overall, EHS triggers ventilatory plasticity and elicits cardiorespiratory abnormalities in HF that are largely dependent on RTN chemoreceptor neurons.

Rostral ventrolateral medullary catecholaminergic neurones mediate irregular breathing pattern in volume overload heart failure rats.

KEY POINTS: A strong association between disordered breathing patterns, elevated sympathetic activity, and enhanced central chemoreflex drive has been shown in experimental and human heart failure (HF). The aim of this study was to determine the contribution of catecholaminergic rostral ventrolateral medulla catecholaminergic neurones (RVLM-C1) to both haemodynamic and respiratory alterations in HF. Apnoea/hypopnoea incidence (AHI), breathing variability, respiratory-cardiovascular coupling, cardiac autonomic control and cardiac function were analysed in HF rats with or without selective ablation of RVLM-C1 neurones. Partial lesion (∼65%) of RVLM-C1 neurones reduces AHI, respiratory variability, and respiratory-cardiovascular coupling in HF rats. In addition, the deleterious effects of central chemoreflex activation on cardiac autonomic balance and cardiac function in HF rats was abolished by ablation of RVLM-C1 neurones. Our findings suggest that RVLM-C1 neurones play a pivotal role in breathing irregularities in volume overload HF, and mediate the sympathetic responses induced by acute central chemoreflex activation. ABSTRACT: Rostral ventrolateral medulla catecholaminergic neurones (RVLM-C1) modulate sympathetic outflow and breathing under normal conditions. Heart failure (HF) is characterized by chronic RVLM-C1 activation, increased sympathetic activity and irregular breathing patterns. Despite studies showing a relationship between RVLM-C1 and sympathetic activity in HF, no studies have addressed a potential contribution of RVLM-C1 neurones to irregular breathing in this context. Thus, the aim of this study was to determine the contribution of RVLM-C1 neurones to irregular breathing patterns in HF. Sprague-Dawley rats underwent surgery to induce volume overload HF. Anti-dopamine ß-hydroxylase-saporin toxin (DßH-SAP) was used to selectively lesion RVLM-C1 neurones. At 8 weeks post-HF induction, breathing pattern, blood pressures (BP), respiratory-cardiovascular coupling (RCC), central chemoreflex function, cardiac autonomic control and cardiac function were studied. Reduction (∼65%) of RVLM-C1 neurones resulted in attenuation of irregular breathing, decreased apnoea-hypopnoea incidence (11.1 ± 2.9 vs. 6.5 ± 2.5 events h-1 ; HF+Veh vs. HF+DßH-SAP; P < 0.05) and improved cardiac autonomic control in HF rats. Pathological RCC was observed in HF rats (peak coherence >0.5 between breathing and cardiovascular signals) and was attenuated by DßH-SAP treatment (coherence: 0.74 ± 0.12 vs. 0.54 ± 0.10, HF+Veh vs. HF+DßH-SAP rats; P < 0.05). Central chemoreflex activation had deleterious effects on cardiac function and cardiac autonomic control in HF rats that were abolished by lesion of RVLM-C1 neurones. Our findings reveal that RVLM-C1 neurones play a major role in irregular breathing patterns observed in volume overload HF and highlight their contribution to cardiac dysautonomia and deterioration of cardiac function during chemoreflex activation.

Sympathoexcitation in response to cardiac and pulmonary afferent stimulation of TRPA1 channels is attenuated in rats with chronic heart failure.

Excessive sympathoexcitation characterizes the chronic heart failure (CHF) state. An exaggerated cardiac sympathetic afferent reflex (CSAR) contributes to this sympathoexcitation. Prior studies have demonstrated that the CSAR to capsaicin [transient receptor potential (TRP) vanilloid 1 agonist] is exaggerated in CHF animal models. We recently discovered that capsaicin application to the lung visceral pleura in anesthetized, vagotomized, open-chested rats increases mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA). We named this response the pulmonary spinal afferent reflex (PSAR). Due to the similarities between TRP vanilloid 1 and TRP ankyrin 1 (TRPA1) channels as well as the excessive sympathoexcitation of CHF, we hypothesized that stimulation of the CSAR and PSAR with a specific TRPA1 agonist would result in an augmented response in CHF rats (coronary ligation model) compared with sham control rats. In response to a TRPA1 agonist, both CSAR and PSAR in sham rats resulted in biphasic changes in MAP and increases in HR and RSNA 10-12 wk postmyocardial infarction (post-MI). These effects were blunted in CHF rats. Assessment of TRPA1 expression levels in cardiopulmonary spinal afferents by immunofluorescence, quantitative RT-PCR, and Western blot analysis 10-12 wk post-MI all indicates reduced expression in CHF rats but no reduction at earlier time points. TRPA1 protein was reduced in a dorsal root ganglia cell culture model of inflammation and simulated tissue ischemia, raising the possibility that the in vivo reduction of TRPA1 expression was, in part, caused by CHF-related tissue ischemia and inflammation. These data provide evidence that reflex responses to cardiopulmonary spinal afferent TRPA1 stimulation may be attenuated in CHF rather than enhanced. NEW & NOTEWORTHY Excessive sympathoexcitation characterizes chronic heart failure (CHF). The contribution of transient receptor potential ankyrin 1 (TRPA1) channel-mediated reflexes to this sympathoexcitation is unknown. We found that application of TRPA1 agonist to the heart and lung surface resulted in increased heart rate and sympathetic output and a biphasic change in mean arterial pressure in control rats. These effects were attenuated in CHF rats, decreasing the likelihood that TRPA1 channels contribute to cardiopulmonary afferent sensitization in CHF.

Ablation of brainstem C1 neurons improves cardiac function in volume overload heart failure.

Activation of the sympathetic nervous system is a hallmark of heart failure (HF) and is positively correlated with disease progression. Catecholaminergic (C1) neurons located in the rostral ventrolateral medulla (RVLM) are known to modulate sympathetic outflow and are hyperactivated in volume overload HF. However, there is no conclusive evidence showing a contribution of RVLM-C1 neurons to the development of cardiac dysfunction in the setting of HF. Therefore, the aim of this study was to determine the role of RVLM-C1 neurons in cardiac autonomic control and deterioration of cardiac function in HF rats. A surgical arteriovenous shunt was created in adult male Sprague-Dawley rats to induce HF. RVLM-C1 neurons were selectively ablated using cell-specific immunotoxin (dopamine-ß hydroxylase saporin [DßH-SAP]) and measures of cardiac autonomic tone, function, and arrhythmia incidence were evaluated. Cardiac autonomic imbalance, arrhythmogenesis and cardiac dysfunction were present in HF rats and improved after DßH-SAP toxin treatment. Most importantly, the progressive decline in fractional shortening observed in HF rats was reduced by DßH-SAP toxin. Our results unveil a pivotal role played by RVLM-C1 neurons in cardiac autonomic imbalance, arrhythmogenesis and cardiac dysfunction in volume overload-induced HF.

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.

Revisiting the physiological effects of exercise training on autonomic regulation and chemoreflex control in heart failure: does ejection fraction matter?

Heart failure (HF) is a global public health problem that, independent of its etiology [reduced (HFrEF) or preserved ejection fraction (HFpEF)], is characterized by functional impairments of cardiac function, chemoreflex hypersensitivity, baroreflex sensitivity (BRS) impairment, and abnormal autonomic regulation, all of which contribute to increased morbidity and mortality. Exercise training (ExT) has been identified as a nonpharmacological therapy capable of restoring normal autonomic function and improving survival in patients with HFrEF. Improvements in autonomic function after ExT are correlated with restoration of normal peripheral chemoreflex sensitivity and BRS in HFrEF. To date, few studies have addressed the effects of ExT on chemoreflex control, BRS, and cardiac autonomic control in HFpEF; however, there are some studies that have suggested that ExT has a beneficial effect on cardiac autonomic control. The beneficial effects of ExT on cardiac function and autonomic control in HF may have important implications for functional capacity in addition to their obvious importance to survival. Recent studies have suggested that the peripheral chemoreflex may also play an important role in attenuating exercise intolerance in HFrEF patients. The role of the central/peripheral chemoreflex, if any, in mediating exercise intolerance in HFpEF has not been investigated. The present review focuses on recent studies that address primary pathophysiological mechanisms of HF (HFrEF and HFpEF) and the potential avenues by which ExT exerts its beneficial effects.

Chronic Heart Failure Abolishes Circadian Rhythms in Resting and Chemoreflex Breathing.

Physiological systems often display 24 h rhythms that vary with the light/dark cycle. Disruption of circadian physiological rhythms have been linked to the progression of various cardiovascular diseases, and advances in the understanding of these rhythms have led to novel interventions and improved clinical outcomes. Although respiratory function has been known to vary between the light and dark periods, circadian rhythms in breathing have been understudied in clinical conditions. In the current study, we have begun to assess light/dark variations in respiration in chronic heart failure (CHF), a condition associated with abnormal resting and chemoreflex breathing as well as exercise intolerance. CHF was induced using coronary artery ligation and verified using echocardiography. Sham animals underwent a thoracotomy without coronary artery ligation. Tidal volume, respiratory frequency, and minute ventilation were all determined by whole body plethysmography under resting conditions and in response to chemoreflex challenges during the light and dark periods. Light/dark differences in voluntary exercise were assessed using a running wheel. The sham control group showed light/dark differences in resting and chemoreflex breathing, as well as arterial pressure, and these effects were eliminated in the CHF group. Both groups completed more rotations on the running wheel during the dark period compared to during the light period. The data suggest that CHF disrupts cardiovascular and respiratory circadian rhythms.

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.

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/HFHRV )] 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/HFHRV , 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.

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 ß-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.

Exercise training attenuates chemoreflex-mediated reductions of renal blood flow in heart failure.

In chronic heart failure (CHF), carotid body chemoreceptor (CBC) activity is increased and contributes to increased tonic and hypoxia-evoked elevation in renal sympathetic nerve activity (RSNA). Elevated RSNA and reduced renal perfusion may contribute to development of the cardio-renal syndrome in CHF. Exercise training (EXT) has been shown to abrogate CBC-mediated increases in RSNA in experimental heart failure; however, the effect of EXT on CBC control of renal blood flow (RBF) is undetermined. We hypothesized that CBCs contribute to tonic reductions in RBF in CHF, that stimulation of the CBC with hypoxia would result in exaggerated reductions in RBF, and that these responses would be attenuated with EXT. RBF was measured in CHF-sedentary (SED), CHF-EXT, CHF-carotid body denervation (CBD), and CHF-renal denervation (RDNX) groups. We measured RBF at rest and in response to hypoxia (FiO2 10%). All animals exhibited similar reductions in ejection fraction and fractional shortening as well as increases in ventricular systolic and diastolic volumes. Resting RBF was lower in CHF-SED (29 ± 2 ml/min) than in CHF-EXT animals (46 ± 2 ml/min, P < 0.05) or in CHF-CBD animals (42 ± 6 ml/min, P < 0.05). In CHF-SED, RBF decreased during hypoxia, and this was prevented in CHF-EXT animals. Both CBD and RDNX abolished the RBF response to hypoxia in CHF. Mean arterial pressure increased in response to hypoxia in CHF-SED, but was prevented by EXT, CBD, and RDNX. EXT is effective in attenuating chemoreflex-mediated tonic and hypoxia-evoked reductions in RBF in CHF.

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.

Selective carotid body ablation in experimental heart failure: a new therapeutic tool to improve cardiorespiratory control.

NEW FINDINGS: What is the topic of this review? This review summarizes the physiological role played by the carotid body in the autonomic dysregulation and breathing disturbances during the progression of chronic heart failure and the therapeutic potential of carotid body ablation to control cardiorespiratory imbalance and improve survival in heart failure. What advances does it highlight? Carotid body ablation markedly improves breathing stability and normalizes autonomic function in chronic heart failure. More importantly, if carotid body ablation is performed early during the progression of the disease it significantly improves animal survival. Chronic heart failure (CHF) is a leading medical problem worldwide. Common hallmarks of CHF include autonomic imbalance and breathing disorders, both of which are closely related to the progression of the disease and strongly predict mortality in CHF patients. The role played by the carotid body (CB) chemoreceptors in the progression of CHF has received attention because enhanced carotid chemoreflex drive is thought to contribute to autonomic dysfunction, abnormal breathing patterns and increased mortality in CHF. Therefore, therapeutic tools intended to normalize CB-mediated chemoreflex drive could have the potential to improve quality of life and decrease mortality of CHF patients. In experimental CHF, an enhancement of the CB chemoreflex drive, elevated sympathetic outflow, increased resting breathing variability, increased incidence of apnoea and desensitization of the baroreflex have been shown. Notably, selective elimination of the CB reduced central presympathetic neuronal activation, normalized sympathetic outflow and baroreflex sensitivity and stabilized breathing function in CHF. More remarkably, CB ablation has been shown to be a valuable therapeutic tool that significantly reduced aberrant cardiac remodelling, improved left ventriclular ejection fraction and reduced cardiac arrhythmogenesis. Most importantly, animals with CHF that underwent CB ablation showed a marked improvement in survival rate. Interestingly, a case report from a heart failure patient in whom unilateral CB ablation was performed showed promising results, with significant improvement in autonomic balance and breathing variability. Together, the CHF data from experimental animals as well as humans unveil a major role for the CB chemoreceptors in the progression of heart failure and support the notion that CB ablation could represent a novel therapeutic strategy to reduce cardiorespiratory dysfunction and improve survival during heart failure.

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.

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.