Role of endothelin and endothelin A-type receptor in adaptation of the carotid body to chronic hypoxia

Possible Molecular Mechanisms of Hypertension Induced by Sleep Apnea Syndrome/Intermittent Hypoxia

Intermittent hypoxia (IH) is a central characteristic of sleep apnea syndrome (SAS), and it subjects cells in the body to repetitive apnea, chronic hypoxia, oxygen desaturation, and hypercapnia. Since SAS is linked to various serious cardiovascular complications, especially hypertension, many studies have been conducted to elucidate the mechanism of hypertension induced by SAS/IH. Hypertension in SAS is associated with numerous cardiovascular disorders. As hypertension is the most common complication of SAS, cell and animal models to study SAS/IH have developed and provided lots of hints for elucidating the molecular mechanisms of hypertension induced by IH. However, the detailed mechanisms are obscure and under investigation. This review outlines the molecular mechanisms of hypertension in IH, which include the regulation systems of reactive oxygen species (ROS) that activate the renin-angiotensin system (RAS) and catecholamine biosynthesis in the sympathetic nervous system, resulting in hypertension. And hypoxia-inducible factors (HIFs), Endotheline 1 (ET-1), and inflammatory factors are also mentioned. In addition, we will discuss the influences of SAS/IH in cardiovascular dysfunction and the relationship of microRNA (miRNA)s to regulate the key molecules in each mechanism, which has become more apparent in recent years. These findings provide insight into the pathogenesis of SAS and help in the development of future treatments.

Pathophysiological mechanisms and therapeutic approaches in obstructive sleep apnea syndrome

Obstructive sleep apnea syndrome (OSAS) is a common breathing disorder in sleep in which the airways narrow or collapse during sleep, causing obstructive sleep apnea. The prevalence of OSAS continues to rise worldwide, particularly in middle-aged and elderly individuals. The mechanism of upper airway collapse is incompletely understood but is associated with several factors, including obesity, craniofacial changes, altered muscle function in the upper airway, pharyngeal neuropathy, and fluid shifts to the neck. The main characteristics of OSAS are recurrent pauses in respiration, which lead to intermittent hypoxia (IH) and hypercapnia, accompanied by blood oxygen desaturation and arousal during sleep, which sharply increases the risk of several diseases. This paper first briefly describes the epidemiology, incidence, and pathophysiological mechanisms of OSAS. Next, the alterations in relevant signaling pathways induced by IH are systematically reviewed and discussed. For example, IH can induce gut microbiota (GM) dysbiosis, impair the intestinal barrier, and alter intestinal metabolites. These mechanisms ultimately lead to secondary oxidative stress, systemic inflammation, and sympathetic activation. We then summarize the effects of IH on disease pathogenesis, including cardiocerebrovascular disorders, neurological disorders, metabolic diseases, cancer, reproductive disorders, and COVID-19. Finally, different therapeutic strategies for OSAS caused by different causes are proposed. Multidisciplinary approaches and shared decision-making are necessary for the successful treatment of OSAS in the future, but more randomized controlled trials are needed for further evaluation to define what treatments are best for specific OSAS patients.

Neurochemical Plasticity of the Carotid Body

A striking feature of the carotid body (CB) is its remarkable degree of plasticity in a variety of neurotransmitter/modulator systems in response to environmental stimuli, particularly following hypoxic exposure of animals and during ascent to high altitude. Current evidence suggests that acetylcholine and adenosine triphosphate are two major excitatory neurotransmitter candidates in the hypoxic CB, and they may also be involved as co-transmitters in hypoxic signaling. Conversely, dopamine, histamine and nitric oxide have recently been considered inhibitory transmitters/modulators of hypoxic chemosensitivity. It has also been revealed that interactions between excitatory and inhibitory messenger molecules occur during hypoxia. On the other hand, alterations in purinergic neurotransmitter mechanisms have been implicated in ventilatory acclimatization to hypoxia. Chronic hypoxia also induces profound changes in other neurochemical systems within the CB such as the catecholaminergic, peptidergic and nitrergic, which in turn may contribute to increased ventilatory and chemoreceptor responsiveness to hypoxia at high altitude. Taken together, current data suggest that complex interactions among transmitters markedly influence hypoxia-induced transmitter release from the CB. In addition, the expression of a wide variety of growth factors, proinflammatory cytokines and their receptors have been identified in CB parenchymal cells in response to hypoxia and their upregulated expression could mediate the local inflammation and functional alteration of the CB under hypoxic conditions.

Neurochemical Anatomy of the Mammalian Carotid Body

Carotid body (CB) glomus cells in most mammals, including humans, contain a broad diversity of classical neurotransmitters, neuropeptides and gaseous signaling molecules as well as their cognate receptors. Among them, acetylcholine, adenosine triphosphate and dopamine have been proposed to be the main excitatory transmitters in the mammalian CB, although subsequently dopamine has been considered an inhibitory neuromodulator in almost all mammalian species except the rabbit. In addition, co-existence of biogenic amines and neuropeptides has been reported in the glomus cells, thus suggesting that they store and release more than one transmitter in response to natural stimuli. Furthermore, certain metabolic and transmitter-degrading enzymes are involved in the chemotransduction and chemotransmission in various mammals. However, the presence of the corresponding biosynthetic enzyme for some transmitter candidates has not been confirmed, and neuroactive substances like serotonin, gamma-aminobutyric acid and adenosine, neuropeptides including opioids, substance P and endothelin, and gaseous molecules such as nitric oxide have been shown to modulate the chemosensory process through direct actions on glomus cells and/or by producing tonic effects on CB blood vessels. It is likely that the fine balance between excitatory and inhibitory transmitters and their complex interactions might play a more important than suggested role in CB plasticity.

The carotid body: A novel key player in neuroimmune interactions

The idea that the nervous system communicates with the immune system to regulate physiological and pathological processes is not new. However, there is still much to learn about how these interactions occur under different conditions. The carotid body (CB) is a sensory organ located in the neck, classically known as the primary sensor of the oxygen (O2) levels in the organism of mammals. When the partial pressure of O2 in the arterial blood falls, the CB alerts the brain which coordinates cardiorespiratory responses to ensure adequate O2 supply to all tissues and organs in the body. A growing body of evidence, however, has demonstrated that the CB is much more than an O2 sensor. Actually, the CB is a multimodal sensor with the extraordinary ability to detect a wide diversity of circulating molecules in the arterial blood, including inflammatory mediators. In this review, we introduce the literature supporting the role of the CB as a critical component of neuroimmune interactions. Based on ours and other studies, we propose a novel neuroimmune pathway in which the CB acts as a sensor of circulating inflammatory mediators and, in conditions of systemic inflammation, recruits a sympathetic-mediated counteracting mechanism that appears to be a protective response.

The Role of Pharmacological Treatment in the Chemoreflex Modulation

From a physiological point of view, peripheral chemoreceptors (PCh) are the main sensors of hypoxia in mammals and are responsible for adaptation to hypoxic conditions. Their stimulation causes hyperventilation—to increase oxygen uptake and increases sympathetic output in order to counteract hypoxia-induced vasodilatation and redistribute the oxygenated blood to critical organs. While this reaction promotes survival in acute settings it may be devastating when long-lasting. The permanent overfunctionality of PCh is one of the etiologic factors and is responsible for the progression of sympathetically-mediated diseases. Thus, the deactivation of PCh has been proposed as a treatment method for these disorders. We review here physiological background and current knowledge regarding the influence of widely prescribed medications on PCh acute and tonic activities.

Neurobiology of the carotid body

The carotid body (CB) is a bilateral arterial chemoreceptor located in the carotid artery bifurcation with an essential role in cardiorespiratory homeostasis. It is composed of highly perfused cell clusters, or glomeruli, innervated by sensory fibers. Glomus cells, the most abundant in each glomerulus, are neuron-like multimodal sensory elements able to detect and integrate changes in several physical and chemical parameters of the blood, in particular O₂ tension, CO₂ and pH, as well as glucose, lactate, or blood flow. Activation of glomus cells (e.g., during hypoxia or hypercapnia) stimulates the afferent fibers which impinge on brainstem neurons to elicit rapid compensatory responses (hyperventilation and sympathetic activation). This chapter presents an updated view of the structural organization of the CB and the mechanisms underlying the chemosensory responses of glomus cells, with special emphasis on the molecular processes responsible for acute O₂ sensing. The properties of the glomus cell-sensory fiber synapse as well as the organization of CB output are discussed. The chapter includes the description of recently discovered CB stem cells and progenitor cells, and their role in CB growth during acclimatization to hypoxemia. Finally, the participation of the CB in the mechanisms of disease is briefly discussed.

Carotid body chemoreceptors: physiology, pathology, and implications for health and disease

The carotid body (CB) is the main peripheral chemoreceptor for arterial respiratory gases O2 and CO2 and pH, eliciting reflex ventilatory, cardiovascular, and humoral responses to maintain homeostasis. This review examines the fundamental biology underlying CB chemoreceptor function, its contribution to integrated physiological responses, and its role in maintaining health and potentiating disease. Emphasis is placed on 1) transduction mechanisms in chemoreceptor (type I) cells, highlighting the role played by the hypoxic inhibition of O2-dependent K+ channels and mitochondrial oxidative metabolism, and their modification by intracellular molecules and other ion channels; 2) synaptic mechanisms linking type I cells and petrosal nerve terminals, focusing on the role played by the main proposed transmitters and modulatory gases, and the participation of glial cells in regulation of the chemosensory process; 3) integrated reflex responses to CB activation, emphasizing that the responses differ dramatically depending on the nature of the physiological, pathological, or environmental challenges, and the interactions of the chemoreceptor reflex with other reflexes in optimizing oxygen delivery to the tissues; and 4) the contribution of enhanced CB chemosensory discharge to autonomic and cardiorespiratory pathophysiology in obstructive sleep apnea, congestive heart failure, resistant hypertension, and metabolic diseases and how modulation of enhanced CB reactivity in disease conditions may attenuate pathophysiology.

Oxygen-dependent regulation of ion channels: acute responses, post-translational modification, and response to chronic hypoxia

Oxygen is a vital element for the survival of cells in multicellular aerobic organisms such as mammals. Lack of O₂ availability caused by environmental or pathological conditions leads to hypoxia. Active oxygen distribution systems (pulmonary and circulatory) and their neural control mechanisms ensure that cells and tissues remain oxygenated. However, O₂-carrying blood cells as well as immune and various parenchymal cells experience wide variations in partial pressure of oxygen (P_O2) in vivo. Hence, the reactive modulation of the functions of the oxygen distribution systems and their ability to sense P_O2 are critical. Elucidating the physiological responses of cells to variations in P_O2 and determining the P_O2-sensing mechanisms at the biomolecular level have attracted considerable research interest in the field of physiology. Herein, we review the current knowledge regarding ion channel-dependent oxygen sensing and associated signalling pathways in mammals. First, we present the recent findings on O₂-sensing ion channels in representative chemoreceptor cells as well as in other types of cells such as immune cells. Furthermore, we highlight the transcriptional regulation of ion channels under chronic hypoxia and its physiological implications and summarize the findings of studies on the post-translational modification of ion channels under hypoxic or ischemic conditions.

Comparative morphological and molecular studies on the oxygen-chemoreceptive cells in the carotid body and fish gills

Oxygen-chemoreceptive cells play critical roles for the respiration control. This review summarizes the chemoreceptive cells in the carotid body and fish gills from a morphological and molecular perspective. The cells synthesize and secrete biogenic amines, neuropeptides, and neuroproteins and also express many signaling molecules and transcription factors. In mammals, birds, reptiles, and amphibians, the carotid body primordium is consistently formed in the wall of the third arch artery which gives rise to the common carotid artery and the basal portion of the internal carotid artery. Consequently, the carotid body is located in the carotid bifurcation region, except birds in which the organ is situated at the lateral side of the common carotid artery. The carotid body receives branches of the cranial nerves IX and/or X dependent on the location of the organ. The glomus cell progenitors in mammals and birds are derived from the neighboring ganglion, i.e., the superior cervical sympathetic ganglion and the nodose ganglion, respectively, and immigrate into the carotid body primordium, constituting a solid cell cluster. In other animal species, the glomus cells are dispersed singly or forming small cell groups in intervascular stroma of the carotid body. In fishes, the neuroepithelial cells, corresponding to the glomus cells, are distributed in the gill filaments and lamellae. All oxygen-chemoreceptive cells sensitively respond to acute or chronic hypoxia, exhibiting degranulation, hypertrophy, hyperplasia, and upregulated expression of many genes.

Olfactory receptor 78 regulates erythropoietin and cardiorespiratory responses to hypobaric hypoxia

Olfactory receptor (Olfr) 78 is expressed in the carotid bodies (CB) and participates in CB responses to acute hypoxia. Olfr78 is also expressed in the kidney, which is a major site of erythropoietin (Epo) production by hypoxia. The present study examined the role of Olfr78 in cardiorespiratory and renal Epo gene responses to hypobaric hypoxia (HH), simulating low O₂ condition experienced at high altitude. Studies were performed on adult, male wild-type (WT) and Olfr78 null mice treated with 18 h of HH (0.4 atmospheres). HH-treated WT mice exhibited increased baseline breathing, augmented hypoxic ventilatory response, elevated blood pressure, and plasma norepinephrine (NE) levels. These effects were associated with increased baseline CB sensory nerve activity and augmented CB sensory nerve response to subsequent acute hypoxia. In contrast, HH-treated Olfr78 null mice showed an absence of cardiorespiratory and CB sensory nerve responses, suggesting impaired CB-dependent cardiorespiratory adaptations. WT mice responded to HH with activation of the renal Epo gene expression and elevated plasma Epo levels, and these effects were attenuated or absent in Olfr78 null mice. The attenuated Epo activation by HH was accompanied with markedly reduced hypoxia-inducible factor (HIF)-2α protein and reduced activation of HIF-2 target gene Sod-1 in Olfr78 null mice, suggesting impaired transcriptional activation of HIF-2 contributes to attenuated Epo responses to HH. These results demonstrate a hitherto uncharacterized role for Olfr78 in cardiorespiratory adaptations and renal Epo gene activation by HH such as that experienced at high altitude.NEW & NOTEWORTHY In this study, we delineated a previously uncharacterized role for olfactory receptor 78 (Olfr78), a G-protein-coupled receptor in regulation of erythropoietin and cardiorespiratory responses to hypobaric hypoxia. Our results demonstrate a striking loss of cardiorespiratory adaptations accompanied by an equally striking absence of carotid body sensory nerve responses to hypobaric hypoxia in Olfr78 null mice. We further demonstrate a hitherto uncharacterized role for Olfr78 in erythropoietin activation by hypobaric hypoxia.

Neurotransmitter Modulation of Carotid Body Germinal Niche

The carotid body (CB), a neural-crest-derived organ and the main arterial chemoreceptor in mammals, is composed of clusters of cells called glomeruli. Each glomerulus contains neuron-like, O₂-sensing glomus cells, which are innervated by sensory fibers of the petrosal ganglion and are located in close contact with a dense network of fenestrated capillaries. In response to hypoxia, glomus cells release transmitters to activate afferent fibers impinging on the respiratory and autonomic centers to induce hyperventilation and sympathetic activation. Glomus cells are embraced by interdigitating processes of sustentacular, glia-like, type II cells. The CB has an extraordinary structural plasticity, unusual for a neural tissue, as it can grow several folds its size in subjects exposed to sustained hypoxia (as for example in high altitude dwellers or in patients with cardiopulmonary diseases). CB growth in hypoxia is mainly due to the generation of new glomeruli and blood vessels. In recent years it has been shown that the adult CB contains a collection of quiescent multipotent stem cells, as well as immature progenitors committed to the neurogenic or the angiogenic lineages. Herein, we review the main properties of the different cell types in the CB germinal niche. We also summarize experimental data suggesting that O₂-sensitive glomus cells are the master regulators of CB plasticity. Upon exposure to hypoxia, neurotransmitters and neuromodulators released by glomus cells act as paracrine signals that induce proliferation and differentiation of multipotent stem cells and progenitors, thus causing CB hypertrophy and an increased sensory output. Pharmacological modulation of glomus cell activity might constitute a useful clinical tool to fight pathologies associated with exaggerated sympathetic outflow due to CB overactivation.

Growth Factors in the Carotid Body-An Update

The carotid body may undergo plasticity changes during development/ageing and in response to environmental (hypoxia and hyperoxia), metabolic, and inflammatory stimuli. The different cell types of the carotid body express a wide series of growth factors and corresponding receptors, which play a role in the modulation of carotid body function and plasticity. In particular, type I cells express nerve growth factor, brain-derived neurotrophic factor, neurotrophin 3, glial cell line-derived neurotrophic factor, ciliary neurotrophic factor, insulin-like-growth factor-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-α and -β, interleukin-1β and -6, tumor necrosis factor-α, vascular endothelial growth factor, and endothelin-1. Many specific growth factor receptors have been identified in type I cells, indicating autocrine/paracrine effects. Type II cells may also produce growth factors and express corresponding receptors. Future research will have to consider growth factors in further experimental models of cardiovascular, metabolic, and inflammatory diseases and in human (normal and pathologic) samples. From a methodological point of view, microarray and/or proteomic approaches would permit contemporary analyses of large groups of growth factors. The eventual identification of physical interactions between receptors of different growth factors and/or neuromodulators could also add insights regarding functional interactions between different trophic mechanisms.

G-Protein-Coupled Receptor (GPCR) Signaling in the Carotid Body: Roles in Hypoxia and Cardiovascular and Respiratory Disease

The carotid body (CB) is an important organ located at the carotid bifurcation that constantly monitors the blood supplying the brain. During hypoxia, the CB immediately triggers an alarm in the form of nerve impulses sent to the brain. This activates protective reflexes including hyperventilation, tachycardia and vasoconstriction, to ensure blood and oxygen delivery to the brain and vital organs. However, in certain conditions, including obstructive sleep apnea, heart failure and essential/spontaneous hypertension, the CB becomes hyperactive, promoting neurogenic hypertension and arrhythmia. G-protein-coupled receptors (GPCRs) are very highly expressed in the CB and have key roles in mediating baseline CB activity and hypoxic sensitivity. Here, we provide a brief overview of the numerous GPCRs that are expressed in the CB, their mechanism of action and downstream effects. Furthermore, we will address how these GPCRs and signaling pathways may contribute to CB hyperactivity and cardiovascular and respiratory disease. GPCRs are a major target for drug discovery development. This information highlights specific GPCRs that could be targeted by novel or existing drugs to enable more personalized treatment of CB-mediated cardiovascular and respiratory disease.

Sustained Hypoxia Alters nTS Glutamatergic Signaling and Expression and Function of Excitatory Amino Acid Transporters

Glutamate is the major excitatory neurotransmitter in the nucleus tractus solitarii (nTS) and mediates chemoreflex function during periods of low oxygen (i.e. hypoxia). We have previously shown that nTS excitatory amino acid transporters (EAATs), specifically EAAT-2, located on glia modulate neuronal activity, cardiorespiratory and chemoreflex function under normal conditions via its tonic uptake of extracellular glutamate. Chronic sustained hypoxia (SH) elevates nTS synaptic transmission and chemoreflex function. The goal of this study was to determine the extent to which glial EAAT-2 contributes to SH-induced nTS synaptic alterations. To do so, male Sprague-Dawley rats (4-7 weeks) were exposed to either 1, 3, or 7 days of SH (10% O2, 24 h/day) and compared to normoxic controls (21% O2, 24 h/day, i.e., 0 days SH). After which, the nTS was harvested for patch clamp electrophysiology, quantitative real-time PCR, immunohistochemistry and immunoblots. SH induced time- and parameter-dependent increases in excitatory postsynaptic currents (EPSCs). TS-evoked EPSC amplitude increased after 1D SH which returned at 3D and 7D SH. Spontaneous EPSC frequency increased only after 3D SH, which returned to normoxic levels at 7D SH. EPSC enhancement occurred primarily by presynaptic mechanisms. Inhibition of EAAT-2 with dihydrokainate (DHK, 300 µM) did not alter EPSCs following 1D SH but induced depolarizing inward currents (Ihold). After 3D SH, DHK decreased TS-EPSC amplitude yet its resulting Ihold was eliminated. EAAT-2 mRNA and protein increased after 3D and 7D SH, respectively. These data suggest that SH alters the expression and function of EAAT-2 which may have a neuroprotective effect.

Physiology of the Carotid Body: From Molecules to Disease

The carotid body (CB) is an arterial chemoreceptor organ located in the carotid bifurcation and has a well-recognized role in cardiorespiratory regulation. The CB contains neurosecretory sensory cells (glomus cells), which release transmitters in response to hypoxia, hypercapnia, and acidemia to activate afferent sensory fibers terminating in the respiratory and autonomic brainstem centers. Knowledge of the physiology of the CB has progressed enormously in recent years. Herein we review advances concerning the organization and function of the cellular elements of the CB, with emphasis on the molecular mechanisms of acute oxygen sensing by glomus cells. We introduce the modern view of the CB as a multimodal integrated metabolic sensor and describe the properties of the CB stem cell niche, which support CB growth during acclimatization to chronic hypoxia. Finally, we discuss the increasing medical relevance of CB dysfunction and its potential impact on the mechanisms of disease.

Chronic hypoxia changes gene expression profile of primary rat carotid body cells: consequences on the expression of NOS isoforms and ET-1 receptors

Sustained chronic hypoxia (CH) produces morphological and functional changes in the carotid body (CB). Nitric oxide (NO) and endothelin-1 (ET-1) play a major role as modulators of the CB oxygen chemosensory process. To characterize the effects of CH related to normoxia (Nx) on gene expression, particularly on ET-1 and NO pathways, primary cultures of rat CB cells were exposed to 7 days of CH. Total RNA was extracted, and cDNA-32P was synthesized and hybridized with 1,185 genes printed on a nylon membrane Atlas cDNA Expression Array. Out of 324 differentially expressed genes, 184 genes were upregulated, while 140 genes were downregulated. The cluster annotation and protein network analyses showed that both NO and ET-1 signaling pathways were significantly enriched and key elements of each pathway were differentially expressed. Thus, we assessed the effect of CH at the protein level of nitric oxide synthase (NOS) isoforms and ET-1 receptors. CH induced an increase in the expression of endothelial NOS, inducible NOS, and ETB. During CH, the administration of SNAP, a NO donor, upregulated ETB. Treatment with Tezosentan (ET-1 receptor blocker) during CH upregulated all three NOS isoforms, while the NOS blocker L-NAME induced upregulation of iNOS and ETB and downregulated the protein levels of ETA. These results show that CH for 7 days changed the cultured cell CB gene expression profile, the NO and ET-1 signaling pathways were highly enriched, and these two signaling pathways interfered with the protein expression of each other.

The carotid body: a physiologically relevant germinal niche in the adult peripheral nervous system

Oxygen constitutes a vital element for the survival of every single cell in multicellular aerobic organisms like mammals. A complex homeostatic oxygen-sensing system has evolved in these organisms, including detectors and effectors, to guarantee a proper supply of the element to every cell. The carotid body represents the most important peripheral arterial chemoreceptor organ in mammals and informs about hypoxemic situations to the effectors at the brainstem cardiorespiratory centers. To optimize organismal adaptation to maintained hypoxemic situations, the carotid body has evolved containing a niche of adult tissue-specific stem cells with the capacity to differentiate into both neuronal and vascular cell types in response to hypoxia. These neurogenic and angiogenic processes are finely regulated by the niche and by hypoxia itself. Our recent data on the cellular and molecular mechanisms underlying the functioning of this niche might help to comprehend a variety of different diseases coursing with carotid body failure, and might also improve our capacity to use these stem cells for the treatment of neurological disease. Herein, we review those data about the recent characterization of the carotid body niche, focusing on the study of the phenotype and behavior of multipotent stem cells within the organ, comparing them with other well-documented neural stem cells within the adult nervous system.

Receptor-Receptor Interactions of G Protein-Coupled Receptors in the Carotid Body: A Working Hypothesis

In the carotid body (CB), a wide series of neurotransmitters and neuromodulators have been identified. They are mainly produced and released by type I cells and act on many different ionotropic and metabotropic receptors located in afferent nerve fibers, type I and II cells. Most metabotropic receptors are G protein-coupled receptors (GPCRs). In other transfected or native cells, GPCRs have been demonstrated to establish physical receptor–receptor interactions (RRIs) with formation of homo/hetero-complexes (dimers or receptor mosaics) in a dynamic monomer/oligomer equilibrium. RRIs modulate ligand binding, signaling, and internalization of GPCR protomers and they are considered of relevance for physiology, pharmacology, and pathology of the nervous system. We hypothesize that RRI may also occur in the different structural elements of the CB (type I cells, type II cells, and afferent fibers), with potential implications in chemoreception, neuromodulation, and tissue plasticity. This ‘working hypothesis’ is supported by literature data reporting the contemporary expression, in type I cells, type II cells, or afferent terminals, of GPCRs which are able to physically interact with each other to form homo/hetero-complexes. Functional data about cross-talks in the CB between different neurotransmitters/neuromodulators also support the hypothesis. On the basis of the above findings, the most significant homo/hetero-complexes which could be postulated in the CB include receptors for dopamine, adenosine, ATP, opioids, histamine, serotonin, endothelin, galanin, GABA, cannabinoids, angiotensin, neurotensin, and melatonin. From a methodological point of view, future studies should demonstrate the colocalization in close proximity (less than 10 nm) of the above receptors, through biophysical (i.e., bioluminescence/fluorescence resonance energy transfer, protein-fragment complementation assay, total internal reflection fluorescence microscopy, fluorescence correlation spectroscopy and photoactivated localization microscopy, X-ray crystallography) or biochemical (co-immunoprecipitation, in situ proximity ligation assay) methods. Moreover, functional approaches will be able to show if ligand binding to one receptor produces changes in the biochemical characteristics (ligand recognition, decoding, and trafficking processes) of the other(s). Plasticity aspects would be also of interest, as development and environmental stimuli (chronic continuous or intermittent hypoxia) produce changes in the expression of certain receptors which could potentially invest the dynamic monomer/oligomer equilibrium of homo/hetero-complexes and the correlated functional implications.

AMPA Receptor-Dependent Glutamatergic Signaling is Present in the Carotid Chemoreceptor

Exposure to both sustained and intermittent hypoxia for as little as a day produces sustained augmentation of carotid chemoreceptor sensitivity; however, the molecular basis for this chemoreflex plasticity remains uncertain. We previously reported that NMDA receptor-dependent glutamatergic signaling in rat carotid body played a role in altered hypoxic sensitivity after exposure to cyclic intermittent hypoxia (CIH). Here we found that mRNAs of multiple AMPA and Kainate glutamate receptors were expressed in rat carotid body. The AMPA receptor subunit GluR1 showed intense immunoreactivity in the carotid body, co-localizing with tyrosine hydroxylase in type I cells. Treatment of rat carotid body-derived primary cells with AMPA activated ERK1/2 in a time-dependent manner. Exposing Sprague-Dawley rats to CIH for 8 h/day for 3 weeks significantly enhanced the expression level of GluA1 mRNA as well as GluR1 protein in the carotid body. In addition, our results showed that multiple of vesicular glutamate transporters (VGLUTs) and excitatory amino acid transporters (EAATs) were expressed in the rat carotid body, indicating that glutamate might be as a neurotransmitter stored, released and uptake in the carotid body. Finally, we found that mRNAs of GluA1, GluA2 and GluA3 as well as PSD-95-like membrane-associated granulate kinase family members, PSD-95, PSD-93, and SAP97, were expressed in human carotid body. Our data suggest AMPA receptor-dependent glutamatergic signaling is present in the carotid body and might be involved in the carotid chemoreceptor response to hypoxia.

Sensory Processing and Integration at the Carotid Body Tripartite Synapse: Neurotransmitter Functions and Effects of Chronic Hypoxia

Maintenance of homeostasis in the respiratory and cardiovascular systems depends on reflexes that are initiated at specialized peripheral chemoreceptors that sense changes in the chemical composition of arterial blood. In mammals, the bilaterally-paired carotid bodies (CBs) are the main peripheral chemoreceptor organs that are richly vascularized and are strategically located at the carotid bifurcation. The CBs contribute to the maintenance of O₂, CO₂/H⁺, and glucose homeostasis and have attracted much clinical interest because hyperactivity in these organs is associated with several pathophysiological conditions including sleep apnea, obstructive lung disease, heart failure, hypertension, and diabetes. In response to a decrease in O₂ availability (hypoxia) and elevated CO₂/H⁺ (acid hypercapnia), CB receptor type I (glomus) cells depolarize and release neurotransmitters that stimulate apposed chemoafferent nerve fibers. The central projections of those fibers in turn activate cardiorespiratory centers in the brainstem, leading to an increase in ventilation and sympathetic drive that helps restore blood PO₂ and protect vital organs, e.g., the brain. Significant progress has been made in understanding how neurochemicals released from type I cells such as ATP, adenosine, dopamine, 5-HT, ACh, and angiotensin II help shape the CB afferent discharge during both normal and pathophysiological conditions. However, type I cells typically occur in clusters and in addition to their sensory innervation are ensheathed by the processes of neighboring glial-like, sustentacular type II cells. This morphological arrangement is reminiscent of a "tripartite synapse" and emerging evidence suggests that paracrine stimulation of type II cells by a variety of CB neurochemicals may trigger the release of "gliotransmitters" such as ATP via pannexin-1 channels. Further, recent data suggest novel mechanisms by which dopamine, acting via D2 receptors (D2R), may inhibit action potential firing at petrosal nerve endings. This review will update current ideas concerning the presynaptic and postsynaptic mechanisms that underlie chemosensory processing in the CB. Paracrine signaling pathways will be highlighted, and particularly those that allow the glial-like type II cells to participate in the integrated sensory response during exposures to chemostimuli, including acute and chronic hypoxia.

Characterization of ectonucleotidase expression in the rat carotid body: regulation by chronic hypoxia

The carotid body (CB) chemoreflex maintains blood Po₂ and Pco₂/H⁺ homeostasis and displays sensory plasticity during exposure to chronic hypoxia. Purinergic signaling via P1 and P2 receptors plays a pivotal role in shaping the afferent discharge at the sensory synapse containing catecholaminergic chemoreceptor (type I) cells, glial-like type II cells, and sensory (petrosal) nerve endings. However, little is known about the family of ectonucleotidases that control synaptic nucleotide levels. Using quantitative PCR (qPCR), we first compared expression levels of ectonucleoside triphosphate diphosphohydrolases (NTPDases1,2,3,5,6) and ecto-5'-nucleotidase (E5'Nt/CD73) mRNAs in juvenile rat CB vs. brain, petrosal ganglia, sympathetic (superior cervical) ganglia, and a sympathoadrenal chromaffin (MAH) cell line. In whole CB extracts, qPCR revealed a high relative expression of surface-located members NTPDase1,2 and E5'Nt/CD73, compared with low NTPDase3 expression. Immunofluorescence staining of CB sections or dissociated CB cultures localized NTPDase2,3 and E5'Nt/CD73 protein to the periphery of type I clusters, and in association with sensory nerve fibers and/or isolated type II cells. Interestingly, in CBs obtained from rats reared under chronic hypobaric hypoxia (~60 kPa, equivalent to 4,300 m) for 5-7 days, in addition to the expected upregulation of tyrosine hydroxylase and VEGF mRNAs, there was a significant upregulation of NTPDase3 and E5'Nt/CD73 mRNA, but a downregulation of NTPDase1 and NTPDase2 relative to normoxic controls. We conclude that NTPDase1,2,3 and E5'Nt/CD73 are the predominant surface-located ectonucleotidases in the rat CB and suggest that their differential regulation during chronic hypoxia may contribute to CB plasticity via control of synaptic ATP, ADP, and adenosine pools.

Chronic hypoxia alters fetal cerebrovascular responses to endothelin-1

In utero hypoxia influences the structure and function of most fetal arteries, including those of the developing cerebral circulation. Whereas the signals that initiate this hypoxic remodeling remain uncertain, these appear to be distinct from the mechanisms that maintain the remodeled vascular state. The present study explores the hypothesis that chronic hypoxia elicits sustained changes in fetal cerebrovascular reactivity to endothelin-1 (ET-1), a potent vascular contractant and mitogen. In fetal lambs, chronic hypoxia (3,820-m altitude for the last 110 days of gestation) had no significant effect on plasma ET-1 levels or ETA receptor density in cerebral arteries but enhanced contractile responses to ET-1 in an ETA-dependent manner. In organ culture (24 h), 10 nM ET-1 increased medial thicknesses less in hypoxic than in normoxic arteries, and these increases were ablated by inhibition of PKC (chelerythrine) in both normoxic and hypoxic arteries but were attenuated by inhibition of CaMKII (KN93) and p38 (SB203580) in normoxic but not hypoxic arteries. As indicated by Ki-67 immunostaining, ET-1 increased medial thicknesses via hypertrophy. Measurements of colocalization between MLCK and SMαA revealed that organ culture with ET-1 also promoted contractile dedifferentiation in normoxic, but not hypoxic, arteries through mechanisms attenuated by inhibitors of PKC, CaMKII, and p38. These results support the hypothesis that chronic hypoxia elicits sustained changes in fetal cerebrovascular reactivity to ET-1 through pathways dependent upon PKC, CaMKII, and p38 that cause increased ET-1-mediated contractility, decreased ET-1-mediated smooth muscle hypertrophy, and a depressed ability of ET-1 to promote contractile dedifferentiation.

Chronic use of PAH-specific therapy in World Health Organization Group III Pulmonary Hypertension: a systematic review and meta-analysis

Pulmonary hypertension (PH) complicating chronic obstructive pulmonary disease (COPD-PH) and interstitial lung disease (ILD-PH) (World Health Organization [WHO] Group III PH) increases medical costs and reduces survival. Despite limited data, many clinicians are using pulmonary arterial hypertension (PAH)-specific therapy to treat WHO Group III PH patients. To further investigate the utility of PAH-specific therapy in WHO Group III PH, we performed a systematic review and meta-analysis. Relevant studies from January 2000 through May 2016 were identified in the MEDLINE, EMBASE, and COCHRANE electronic databases and www.clinicaltrials.gov. Change in six-minute walk distance (6MWD) was estimated using random effects meta-analysis techniques. Five randomized controlled trials (RCTs) in COPD-PH (128 placebo or standard treatment and 129 PAH-medication treated patients), two RCTs in ILD-PH (23 placebo and 46 treated patients), and four single-arm clinical trials (50 patients) in ILD-PH were identified. Treatment in both COPD-PH and ILD-PH did not worsen hypoxemia. Symptomatic burden was not consistently reduced but there were trends for reduced pulmonary artery pressures and pulmonary vascular resistance with PAH-specific therapy. As compared to placebo, 6MWD was not significantly improved with PAH-specific therapy in the five COPD-PH RCTs (42.7 m; 95% confidence interval [CI], –1.0 – 86.3). In the four single-arm studies in ILD-PH patients, there was a significant improvement in 6MWD after PAH-specific treatment (46.2 m; 95% CI, 27.9–64.4), but in the two ILD-PH RCTs there was not an improvement (21.6 m; 95% CI, –17.8 – 61.0) in exercise capacity when compared to placebo. Due to the small numbers of patients evaluated and inconsistent beneficial effects, the utility of PAH-specific therapy in WHO Group III PH remains unproven. A future clinical trial that is appropriately powered is needed to definitively determine the efficacy of this widely implemented treatment approach.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

Single cell transcriptome analysis of mouse carotid body glomus cells

KEY POINTS

Carotid body (CB) glomus cells mediate acute oxygen sensing and the initiation of the hypoxic ventilatory response, yet the gene expression profile of these cells is not available. We demonstrate that the single cell RNA-Seq method is a powerful tool for identifying highly expressed genes in CB glomus cells. Our single cell RNA-Seq results characterized novel CB glomus cell genes, including members of the G protein-coupled receptor signalling pathway, ion channels and atypical mitochondrial electron transport chain subunits. A heterologous cell-based screening identified acetate (which is known to affect CB glomus cell activity) as an agonist for the most highly abundant G protein-coupled receptor (Olfr78) in CB glomus cells. These data established the first transcriptome profile of CB glomus cells, highlighting genes with potential implications in CB chemosensory function.

ABSTRACT

The carotid body (CB) is a major arterial chemoreceptor containing glomus cells whose activities are regulated by changes in arterial blood content, including oxygen. Despite significant advancements in the characterization of their physiological properties, our understanding of the underlying molecular machinery and signalling pathway in CB glomus cells is still limited. To overcome this, we employed the single cell RNA-Seq method by performing next-generation sequencing on single glomus cell-derived cDNAs to eliminate contamination of genes derived from other cell types present in the CB. Using this method, we identified a set of genes abundantly expressed in glomus cells, which contained novel glomus cell-specific genes. Transcriptome and subsequent in situ hybridization and immunohistochemistry analyses identified abundant G protein-coupled receptor signalling pathway components and various types of ion channels, as well as members of the hypoxia-inducible factors pathway. A short-chain fatty acid olfactory receptor Olfr78, recently implicated in CB function, was the most abundant G protein-coupled receptor. Two atypical mitochondrial electron transport chain subunits (Ndufa4l2 and Cox4i2) were among the most specifically expressed genes in CB glomus cells, highlighting their potential roles in mitochondria-mediated oxygen sensing. The wealth of information provided by the present study offers a valuable foundation for identifying molecules functioning in the CB.

Peripheral chemoreception and arterial pressure responses to intermittent hypoxia

Carotid bodies are the principal peripheral chemoreceptors for detecting changes in arterial blood oxygen levels, and the resulting chemoreflex is a potent regulator of blood pressure. Recurrent apnea with intermittent hypoxia (IH) is a major clinical problem in adult humans and infants born preterm. Adult patients with recurrent apnea exhibit heightened sympathetic nerve activity and hypertension. Adults born preterm are predisposed to early onset of hypertension. Available evidence suggests that carotid body chemoreflex contributes to hypertension caused by IH in both adults and neonates. Experimental models of IH provided important insights into cellular and molecular mechanisms underlying carotid body chemoreflex-mediated hypertension. This article provides a comprehensive appraisal of how IH affects carotid body function, underlying cellular, molecular, and epigenetic mechanisms, and the contribution of chemoreflex to the hypertension.

Morphological changes in the rat carotid body following acute sodium nitrite treatment

The carotid body (CB) is a small neural crest-derived chemosensory organ that detects the chemical composition of the arterial blood and responds to its changes by regulating breathing. The effects of acute nitrite treatment on the CB morphology in rats were examined by morphometry. We found that 1h after administrating a single dose of sodium nitrite, the CB underwent structural changes characterized by a prominent increase in its size with a marked, several-fold dilation of the blood vessels. The obvious CB enlargement mostly due to apparent vasodilation and glomus cell hypertrophy was at its highest one day later and persisted until the fifth day. 20 days after the treatment, the CB regained its size to the normoxic control state. Morphometric analysis revealed that the CB size increase in treated animals is statistically significant when compared to that of untreated controls. It can be inferred that the nitrite-exposed CB displays remarkable structural plasticity and enlarges its size mostly through vascular expansion.

Sympathoexcitation and arterial hypertension associated with obstructive sleep apnea and cyclic intermittent hypoxia

Obstructive sleep apnea (OSA) is characterized by repetitive episodes of upper airway obstruction during sleep. These obstructive episodes are characterized by cyclic intermittent hypoxia (CIH), by sleep fragmentation, and by hemodynamic instability, and they result in sustained sympathoexcitation and elevated arterial pressure that persist during waking, after restoration of normoxia. Early studies established that 1) CIH, rather than sleep disruption, accounts for the increase in arterial pressure; 2) the increase in arterial pressure is a consequence of the sympathoactivation; and 3) arterial hypertension after CIH exposure requires an intact peripheral chemoreflex. More recently, however, evidence has accumulated that sympathoactivation and hypertension after CIH are also dependent on altered central sympathoregulation. Furthermore, although many molecular pathways are activated in both the carotid chemoreceptor and in the central nervous system by CIH exposure, two specific neuromodulators-endothelin-1 and angiotensin II-appear to play crucial roles in mediating the sympathetic and hemodynamic response to intermittent hypoxia.

Hydrogen sulfide and hypoxia-induced changes in TASK (K2P3/9) activity and intracellular Ca(2+) concentration in rat carotid body glomus cells

Acute hypoxia depolarizes carotid body chemoreceptor (glomus) cells and elevates intracellular Ca(2+) concentration ([Ca(2+)]i). Recent studies suggest that hydrogen sulfide (H2S) may serve as an oxygen sensor/signal in the carotid body during acute hypoxia. To further test such a role for H2S, we studied the effects of H2S on the activity of TASK channel and [Ca(2+)]i, which are considered important for mediating the glomus cell response to hypoxia. Like hypoxia, NaHS (a H2S donor) inhibited TASK activity and elevated [Ca(2+)]i. To inhibit the production of H2S, glomus cells were incubated (3h) with inhibitors of cystathionine-β-synthase and cystathionine-γ-lyase (DL-propargylglycine, aminooxyacetic acid, β-cyano-L-alanine; 0.3 mM). SF7 fluorescence was used to assess the level of H2S production. The inhibitors blocked L-cysteine- and hypoxia-induced elevation of SF7 fluorescence intensity. In cells treated with the inhibitors, hypoxia produced an inhibition of TASK activity and a rise in [Ca(2+)]i, similar in magnitude to those observed in control cells. L-cysteine produced no effect on TASK activity or [Ca(2+)]i and did not affect hypoxia-induced inhibition of TASK and elevation of [Ca(2+)]i. These findings suggest that under normal conditions, H2S is not a major signal in hypoxia-induced modulation of TASK channels and [Ca(2+)]i in isolated glomus cells.

Neurotrophic Properties, Chemosensory Responses and Neurogenic Niche of the Human Carotid Body

The carotid body (CB) is a polymodal chemoreceptor that triggers the hyperventilatory response to hypoxia necessary for the maintenance of O(2) homeostasis essential for the survival of organs such as the brain or heart. Glomus cells, the sensory elements in the CB, are also sensitive to hypercapnia, acidosis and, although less generally accepted, hypoglycemia. Current knowledge on CB function is mainly based on studies performed on lower mammals, but the information on the human CB is scant. Here we describe the structure, neurotrophic properties, and cellular responses to hypoxia and hypoglycemia of CBs dissected from human cadavers. The adult CB parenchyma contains clusters of chemosensitive glomus (type I) and sustentacular (type II) cells as well as nestin-positive progenitor cells. This organ also expresses high levels of the dopaminotrophic glial cell line-derived neurotrophic factor (GDNF). GDNF production and the number of progenitor and glomus cells were preserved in the CBs of human subjects of advanced age. As reported for other mammalian species, glomus cells responded to hypoxia by external Ca(2+)-dependent increase of cytosolic [Ca(2+)] and quantal catecholamine release. Human glomus cells are also responsive to hypoglycemia and together the two stimuli, hypoxia and hypoglycemia, can potentiate each other's effects. The chemo-sensory responses of glomus cells are also preserved at an advanced age. Interestingly, a neurogenic niche similar to that recently described in rodents is also preserved in the adult human CB. These new data on the cellular and molecular physiology of the CB pave the way for future pathophysiological studies involving this organ in humans.

Pathogenic roles of the carotid body inflammation in sleep apnea

Breathing difficulties in sleep are a hallmark of sleep-disordered breathing commonly observed in patients with sleep disorders. The pathophysiology of sleep apnea is in part due to an augmented activity of the carotid body chemoreflex. Arterial chemoreceptors in the carotid body are sensitive to inflammatory cytokines and immunogenic molecules in the circulation, because cytokine receptors are expressed in the carotid body in experimental animals and human. Intriguingly, proinflammatory cytokines are also locally produced and released in the carotid body. Also, there are significant increases in the expression of proinflammatory cytokines, cytokine receptors, and inflammatory mediators in the carotid body under hypoxic conditions, suggesting an inflammatory response of the carotid body. These upregulated cytokine signaling pathways could enhance the carotid chemoreceptor activity, leading to an overactivity of the chemoreflex adversely effecting breathing instability and autonomic imbalance. This review aims to summarize findings of the literature relevant to inflammation in the carotid body, with highlights on the pathophysiological impact in sleep apnea. It is concluded that local inflammation in the carotid body plays a pathogenic role in sleep apnea, which could potentially be a therapeutic target for the treatment of the pathophysiological consequence of sleep apnea.

Quantifying hypoxia-induced chemoreceptor sensitivity in the awake rodent

We evaluated several methods for characterizing hypoxic chemosensitivity in the conscious rat. Adult Sprague-Dawley rats (n = 30) were exposed to normobaric hypoxia [inspired oxygen fraction (Fio2) 0.15, 0.12, and 0.09]. We measured ventilation (V̇e; barometric plethysmography), arterial oxygen saturation (SpO2; pulse oximeter), and oxygen consumption and carbon dioxide production (V̇o2 and V̇co2; analysis of expired air). Linear regression analysis was used to define stimulus-response relationships. Testing was performed on 2 days to assess day-to-day reproducibility. Exposure to graded, steady-state hypoxia caused progressive reductions in SpO2 that were, for any given Fio2, quite variable (SpO2 range, 20-30%) among individuals. Hypoxia produced progressive increases in V̇e caused by increases in both tidal volume (VT) and breathing frequency. Hypoxia also increased the VT:inspiratory time (Ti) ratio, an indicator of central respiratory "drive." Hypoxia caused consistent, progressive declines in V̇o2, V̇co2, and core temperature (>20% at the lowest SpO2). We propose that optimal quantification of carotid chemoreceptor hypoxic sensitivity in the unanesthetized rodent should employ SpO2 [a surrogate for arterial Po2 (PaO2 )] as the stimulus variable and the ventilatory equivalent for V̇co2 (V̇e/V̇co2) and/or mean inspiratory flow rate (VT/Ti) normalized for V̇co2 as the response variables. Both metrics take into account not only the important influence of a falling metabolic rate, but also SpO2, which represents the hypoxic stimulus at the carotid body. Because of the somewhat curvilinear nature of these responses, exposure to multiple levels of graded hypoxia provides the most complete characterization of hypoxic chemosensitivity.

An O2-sensitive glomus cell-stem cell synapse induces carotid body growth in chronic hypoxia

Neural stem cells (NSCs) exist in germinal centers of the adult brain and in the carotid body (CB), an oxygen-sensing organ that grows under chronic hypoxemia. How stem cell lineage differentiation into mature glomus cells is coupled with changes in physiological demand is poorly understood. Here, we show that hypoxia does not affect CB NSC proliferation directly. Rather, mature glomus cells expressing endothelin-1, the O2-sensing elements in the CB that secrete neurotransmitters in response to hypoxia, establish abundant synaptic-like contacts with stem cells, which express endothelin receptors, and instruct their growth. Inhibition of glomus cell transmitter release or their selective destruction markedly diminishes CB cell growth during hypoxia, showing that CB NSCs are under the direct "synaptic" control of the mature O2-sensitive cells. Thus, glomus cells not only acutely activate the respiratory center but also induce NSC-dependent CB hypertrophy necessary for acclimatization to chronic hypoxemia.

Molecular genetics of paragangliomas of the skull base and head and neck region: implications for medical and surgical management

Paragangliomas are rare, slow-growing tumors that frequently arise in the head and neck, with the carotid bodies and temporal bone of the skull base being the most common sites. These neoplasms are histologically similar to pheochromocytomas that form in the adrenal medulla and are divided into sympathetic and parasympathetic subtypes based on functionality. Skull base and head and neck region paragangliomas (SHN-PGs) are almost always derived from parasympathetic tissue and rarely secrete catecholamines. However, they can cause significant morbidity by mass effect on various cranial nerves and major blood vessels. While surgery for SHN-PG can be curative, postoperative deficits and recurrences make these lesions challenging to manage. Multiple familial syndromes predisposing individuals to development of paragangliomas have been identified, all involving mutations in the succinate dehydrogenase complex of mitochondria. Mutations in this enzyme lead to a state of “pseudohypoxia” that upregulates various angiogenic, survival, and proliferation factors. Moreover, familial paraganglioma syndromes are among the rare inherited diseases in which genomic imprinting occurs. Recent advances in gene arrays and transcriptome/exome sequencing have identified an alternate mutation in sporadic SHN-PG, which regulates proto-oncogenic pathways independent of pseudohypoxia-induced factors. Collectively these findings demonstrate that paragangliomas of the skull base and head and neck region have a distinct genetic signature from sympathetic-based paragangliomas occurring below the neck, such as pheochromocytomas. Paragangliomas serve as a unique model of primarily surgically treated neoplasms whose future will be altered by the elucidation of their genomic complexities. In this review, the authors present an analysis of the molecular genetics of SHN-PG and provide future directions in patient care and the development of novel therapies.

Role of chemoreception in cardiorespiratory acclimatization to, and deacclimatization from, hypoxia

During sojourn to high altitudes, progressive time-dependent increases occur in ventilation and in sympathetic nerve activity over several days, and these increases persist upon acute restoration of normoxia. We discuss evidence concerning potential mediators of these changes, including the following: 1) correction of alkalinity in cerebrospinal fluid; 2) increased sensitivity of carotid chemoreceptors; and 3) augmented translation of carotid chemoreceptor input (at the level of the central nervous system) into increased respiratory motor output via sensitization of hypoxic sensitive neurons in the central nervous system and/or an interdependence of central chemoreceptor responsiveness on peripheral chemoreceptor sensory input. The pros and cons of chemoreceptor sensitization and cardiorespiratory acclimatization to hypoxia and intermittent hypoxemia are also discussed in terms of their influences on arterial oxygenation, the work of breathing, sympathoexcitation, systemic blood pressure, and exercise performance. We propose that these adaptive processes may have negative implications for the cardiovascular health of patients with sleep apnea and perhaps even for athletes undergoing regimens of "sleep high-train low"!

Petrosal ganglion responses to acetylcholine and ATP are enhanced by chronic normobaric hypoxia in the rabbit

In mammals, adaptation to chronic hypoxia requires the integrity of the arterial chemoreceptors, specially the carotid body (CB). Chronic hypoxia increases the sensibility of the CB by acting on the receptor cells, but there is limited information on the effects of chronic hypoxia on the sensory neurons that innervate the CB. Therefore, we studied the responses evoked by ACh and ATP, the main transmitters that generate the chemoafferent activity, on the petrosal ganglion (PG) of rabbits exposed to chronic normobaric hypoxia (CNH) during fourteen days. ATP and ACh increased the activity of PG neurons in a dose-dependent manner, in a similar way than in rabbits not exposed to hypoxia (naïve). However, the duration of the responses were significantly increased by CNH, with the mean maximal responses to ACh and ATP increased by a factor of two and four, respectively. Our results suggest that CNH increases duration of the responses by modifying the expression and/or content of ACh and ATP receptors.

Effect of endothelin receptor antagonist bosentan on chronic hypoxia-induced inflammation and chemoafferent neuron adaptation in rat carotid body

Chronic hypoxia (CH) induces an inflammatory response in rat carotid body that is characterized by immune cell invasion and the expression of pro-inflammatory cytokines. In the present study, we have investigated the role of type-A endothelin (ET-A) receptors in the development of CH-induced inflammation. After 7 days of CH (380 Torr), double-label immunofluorescence studies demonstrated elevated levels of ET-A receptor and tyrosine hydroxylase (TH) in O(2)-sensitive type I cells. Following CH, ET-A receptors were also expressed on resident and invasive CD45+ immune cells distributed in tissue surrounding chemosensory cell lobules. Immnofluorescence and quantitative PCR studies showed that concurrent treatment with the ET-A/B receptor antagonist, bosentan (200 mg/kg/day), blocked CH-induced ED-1+ macrophage invasion and the upregulation of cytokines, including interleukin-1β (IL-1β), interleukin-6 (IL-6), tumor necrosis factor α (TNFα), and monocyte chemoattractant protein-1 (MCP-1). Moreover, bosentan treatment blocked the CH-induced increases in expression of acid-sensitive ion channels (ASICs) in chemoafferent neurons in the petrosal ganglion (PG). Our findings are consistent with the hypothesis that CH-induced inflammation involves the upregulation and release of ET-1 from type I cells. ET-1 may act in an autocrine/paracrine mechanism via ET-A receptors on chemosensory type I cells and immune cells to promote an inflammatory response.

Adaptive and maladaptive cardiorespiratory responses to continuous and intermittent hypoxia mediated by hypoxia-inducible factors 1 and 2

Hypoxia is a fundamental stimulus that impacts cells, tissues, organs, and physiological systems. The discovery of hypoxia-inducible factor-1 (HIF-1) and subsequent identification of other members of the HIF family of transcriptional activators has provided insight into the molecular underpinnings of oxygen homeostasis. This review focuses on the mechanisms of HIF activation and their roles in physiological and pathophysiological responses to hypoxia, with an emphasis on the cardiorespiratory systems. HIFs are heterodimers comprised of an O(2)-regulated HIF-1α or HIF-2α subunit and a constitutively expressed HIF-1β subunit. Induction of HIF activity under conditions of reduced O(2) availability requires stabilization of HIF-1α and HIF-2α due to reduced prolyl hydroxylation, dimerization with HIF-1β, and interaction with coactivators due to decreased asparaginyl hydroxylation. Stimuli other than hypoxia, such as nitric oxide and reactive oxygen species, can also activate HIFs. HIF-1 and HIF-2 are essential for acute O(2) sensing by the carotid body, and their coordinated transcriptional activation is critical for physiological adaptations to chronic hypoxia including erythropoiesis, vascularization, metabolic reprogramming, and ventilatory acclimatization. In contrast, intermittent hypoxia, which occurs in association with sleep-disordered breathing, results in an imbalance between HIF-1α and HIF-2α that causes oxidative stress, leading to cardiorespiratory pathology.

Chronic caffeine intake in adult rat inhibits carotid body sensitization produced by chronic sustained hypoxia but maintains intact chemoreflex output

Sustained hypoxia produces a carotid body (CB) sensitization, known as acclimatization, which leads to an increase in carotid sinus nerve (CSN) activity and ensuing hyperventilation greater than expected from the prevailing partial pressure of oxygen. Whether sustained hypoxia is physiological (high altitude) or pathological (lung disease), acclimatization has a homeostatic implication because it tends to minimize hypoxia. Caffeine, the most commonly ingested psychoactive drug and a nonselective adenosine receptor antagonist, alters CB function and ventilatory responses when administered acutely. Our aim was to investigate the effect of chronic caffeine intake on CB function and acclimatization using four groups of rats: normoxic, caffeine-treated normoxic, chronically hypoxic (12% O₂, 15 days), and caffeine-treated chronically hypoxic rats. Caffeine was administered in drinking water (1 mg/ml). Caffeine ameliorated ventilatory responses to acute hypoxia in normoxic animals without altering the output of the CB (CSN neural activity). Caffeine-treated chronically hypoxic rats exhibited a decrease in the CSN response to acute hypoxia tests but maintained ventilation compared with chronically hypoxic animals. The findings related to CSN neural activity combined with the ventilatory responses indicate that caffeine alters central integration of the CB input to increase the gain of the chemoreflex and that caffeine abolishes CB acclimatization. The putative mechanisms involved in sensitization and its loss were investigated: expression of adenosine receptors in CB (A(2B)) was down-regulated and that in petrosal ganglion (A(2A)) was up-regulated in caffeine-treated chronically hypoxic rats; both adenosine and dopamine release from CB chemoreceptor cells was increased in chronic hypoxia and in caffeine-treated chronic hypoxia groups.

Chronic hypoxia-induced acid-sensitive ion channel expression in chemoafferent neurons contributes to chemoreceptor hypersensitivity

Previously we demonstrated that chronic hypoxia (CH) induces an inflammatory condition characterized by immune cell invasion and increased expression of inflammatory cytokines in rat carotid body. It is well established that chronic inflammatory pain induces the expression of acid-sensitive ion channels (ASIC) in primary sensory neurons, where they contribute to hyperalgesia and allodynia. The present study examines the effect of CH on ASIC expression in petrosal ganglion (PG), which contains chemoafferent neurons that innervate oxygen-sensitive type I cells in the carotid body. Five isoforms of ASIC transcript were increased ∼1.5-2.5-fold in PG following exposure of rats to 1, 3, or 7 days of hypobaric hypoxia (380 Torr). ASIC transcript was not increased in the sympathetic superior cervical ganglion (SCG). In the PG, CH also increased the expression of channel-interacting PDZ domain protein, a scaffolding protein known to enhance the surface expression and the low pH-induced current density mediated by ASIC3. Western immunoblot analysis showed that CH elevated ASIC3 protein in PG, but not in SCG or the (sensory) nodose ganglion. ASIC3 transcript was likewise elevated in PG neurons cultured in the presence of inflammatory cytokines. Increased ASIC expression was blocked in CH rats concurrently treated with the nonsteroidal anti-inflammatory drug ibuprofen (4 mg·kg(-1)·day(-1)). Electrophysiological recording of carotid sinus nerve (CSN) activity in vitro showed that the specific ASIC antagonist A-317567 (100 μM) did not significantly alter hypoxia-evoked activity in normal preparations but blocked ∼50% of the hypoxic response following CH. Likewise, a high concentration of ibuprofen, which is known to block ASIC1a, reduced hypoxia-evoked CSN activity by ∼50% in CH preparations. Our findings indicate that CH induces inflammation-dependent phenotypic adjustments in chemoafferent neurons. Following CH, ASIC are important participants in chemotransmission between type I cells and chemoafferent nerve terminals, and these proton-gated channels appear to enhance chemoreceptor sensitivity.

Constitutive HIF-1α expression blunts the beneficial effects of cardiosphere-derived cell therapy in the heart by altering paracrine factor balance

Hypoxia-inducible factor-1alpha (HIF-1α) expression promotes angiogenesis and can influence stem cell engraftment. We investigated the effect of stable over-expression of constitutively active HIF-1α on cardiosphere-derived cell (CDC) engraftment and left ventricular function. CDCs were transduced with a lentivirus expressing a constitutively active mutant of human HIF-1α (LVHIF-1α). Two million male rat CDCs were injected into the infarct following ligation of the mid-LAD in female syngeneic rats. Left ventricular ejection fraction (EF) and circumferential strain were measured by echocardiography at 1 and 4 weeks post-MI in the following groups: PBS group (n = 7), CELL group (n = 7), and CELL-HIF group (n = 7). HIF-1α, VEGF, endothelin-1 expression, and CDC engraftment were measured by quantitative PCR. At 30 days, EF was unchanged in the CELL-HIF group (p = NS), increased in the CELL group (p = 0.025), and decreased in the PBS group (p = 0.021), but engraftment was similar (2.4% ± 3.3% vs 1.7% ± 0.8%, p = NS). Mean circumferential strain of the infarcted region was unchanged in the CELL-HIF group, but improved in the CELL group (p = 0.02). Endothelin-1 and VEGF expression were higher in HIF-CDCs exposed to hypoxia, compared with non-transduced CDCs. HIF-1α expression in CDCs blunted the beneficial functional effects of CDC transplantation, suggesting that paracrine factor balance may play an important role in cardiac regeneration.