Low pO2 selectively inhibits K channel activity in chemoreceptor cells of the mammalian carotid body

Carotid body interoception in health and disease

Interoception entails perceiving or being aware of the internal state of the body, playing a pivotal role in regulating processes such as heartbeat, digestion, glucose metabolism, and respiration. The carotid body (CB) serves as an interoceptive organ, transmitting information to the brain via its sensitive nerve, the carotid sinus nerve, to maintain homeostasis. While traditionally known for sensing oxygen, carbon dioxide, and pH levels, the CB is now recognized to possess additional interoceptive properties, detecting various mediators involved in blood pressure regulation, inflammation, and glucose homeostasis, among other physiological functions. Furthermore, in the last decades CB dysfunction has been linked to diseases like sleep apnea, essential hypertension, and diabetes. In this review manuscript, we make a concise overview of the traditional interoceptive functions of the CB, acting as a sensor for oxygen levels, carbon dioxide levels, and pH, and introduce the novel interoceptive properties of the CB related to vascular, glucose and energy regulation. Additionally, we revise the contribution of the CB to the onset and progression of metabolic diseases, delving into the potential dysfunction of its interoceptive metabolic functions as a contributing factor to pathophysiology. Finally, we postulate the use of therapeutic interventions targeting the metabolic interoceptive properties of the CB as a potential avenue for addressing metabolic diseases.

Mitochondrial Redox Signaling in O2-Sensing Chemoreceptor Cells

Significance: Acute responses to hypoxia are essential for the survival of mammals. The carotid body (CB), the main arterial chemoreceptor, contains glomus cells with oxygen (O₂)-sensitive K⁺ channels, which are inhibited during hypoxia to trigger adaptive cardiorespiratory reflexes. Recent Advances: In this review, recent advances in molecular mechanisms of acute O₂ sensing in CB glomus cells are discussed, with a special focus on the signaling role of mitochondria through regulating cellular redox status. These advances have been achieved thanks to the use of genetically engineered redox-sensitive green fluorescent protein (roGFP) probes, which allowed us to monitor rapid changes in ROS production in real time in different subcellular compartments during hypoxia. This methodology was used in combination with conditional knockout mice models, pharmacological approaches, and transcriptomic studies. We have proposed a mitochondria-to-membrane signaling model of acute O₂ sensing in which H₂O₂ released in the mitochondrial intermembrane space serves as a signaling molecule to inhibit K⁺ channels on the plasma membrane. Critical Issues: Changes in mitochondrial reactive oxygen species (ROS) production during acute hypoxia are highly compartmentalized in the submitochondrial regions. The use of redox-sensitive probes targeted to specific compartments is essential to fully understand the role of mitochondrial ROS in acute O₂ sensing. Future Directions: Further studies are needed to specify the ROS and to characterize the target(s) of ROS in chemoreceptor cells during acute hypoxia. These data may also contribute to a more complete understanding of the implication of ROS in acute responses to hypoxia in O₂-sensing cells in other organs. Antioxid. Redox Signal. 37, 274-289.

Are Multiple Mitochondrial Related Signalling Pathways Involved in Carotid Body Oxygen Sensing?

It is generally acknowledged that the carotid body (CB) type I cell mitochondria are unique, being inhibited by relatively small falls in PaO2 well above those known to inhibit electron transport in other cell types. This feature is suggested to allow for the CB to function as an acute O2 sensor, being stimulated and activating systemic protective reflexes before the metabolism of other cells becomes compromised. What is less clear is precisely how a fall in mitochondrial activity links to type I cell depolarisation, a process that is required for initiation of the chemotransduction cascade and post-synaptic action potential generation. Multiple mitochondrial/metabolic signalling mechanisms have been proposed including local generation of mitochondrial reactive oxygen species (mitoROS), a change in mitochondrial/cellular redox status, a fall in MgATP and an increase in lactate. Although each mechanism is based on compelling experimental evidence, they are all not without question. The current review aims to explore the importance of each of these signalling pathways in mediating the overall CB response to hypoxia. We suggest that there is unlikely to be a single mechanism, but instead multiple mitochondrial related signalling pathways are recruited at different PaO2s during hypoxia. Furthermore, it still remains to be determined if mitochondrial signalling acts independently or in partnership with extra-mitochondrial O2-sensors.

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.

Mitochondrial acute oxygen sensing and signaling

Oxygen (O₂) is essential for life and therefore the supply of sufficient O₂ to the tissues is a major physiological challenge. In mammals, a deficit of O₂ (hypoxia) triggers rapid cardiorespiratory reflexes (e.g. hyperventilation and increased heart output) that within a few seconds increase the uptake of O₂ by the lungs and its distribution throughout the body. The prototypical acute O₂-sensing organ is the carotid body (CB), which contains sensory glomus cells expressing O₂-regulated ion channels. In response to hypoxia, glomus cells depolarize and release transmitters which activate afferent fibers terminating at the brainstem respiratory and autonomic centers. In this review, we summarize the basic properties of CB chemoreceptor cells and the essential role played by their specialized mitochondria in acute O₂ sensing and signaling. We focus on recent data supporting a "mitochondria-to-membrane signaling" model of CB chemosensory transduction. The possibility that the differential expression of specific subunit isoforms and enzymes could allow mitochondria to play a generalized adaptive O₂-sensing and signaling role in a wide variety of cells is also 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.

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.

Transcriptomic profiles of Dunaliella salina in response to hypersaline stress

Background

Dunaliella salina is a good model organism for studying salt stress. In order to have a global understanding of the expression profiles of Dunaliella salina in response to hypersaline stress, we performed quantitative transcriptomic analysis of Dunaliella salina under hypersaline stress (2.5 M NaCl) of different time duration by the second and third generation sequencing method.

Results

Functional enrichment of the up-regulated genes was used to analyze the expression profiles. The enrichment of photosynthesis was observed, accompanied by enrichments of carbon fixation, pigment biosynthetic process and heme biosynthetic process, which also imply the enhancement of photosynthesis. Genes responsible for starch hydrolysis and glycerol synthesis were significantly up-regulated. The enrichment of biosynthesis of unsaturated fatty acids implies the plasma membrane undergoes changes in desaturation pattern. The enrichment of endocytosis implies the degradation of plasma membrane and might help the synthesis of new glycerophospholipid with unsaturated fatty acids. Co-enrichments of protein synthesis and degradation imply a higher protein turnover rate. The enrichments of spliceosome and protein processing in endoplasmic reticulum imply the enhancement of regulations at post-transcriptional and post-translational level. No up-regulation of any Na⁺ or Cl⁻ channels or transporters was detected, which implies that the extra exclusion of the ions by membrane transporters is possibly not needed. Voltage gated Na⁺ and Cl⁻ channels, mechanosensitive ion channel are possible signal receptors of salt stress, and Ca²⁺ and MAP kinase pathways might play a role in signal transduction.

Conclusion

At global transcriptomic level, the response of Dunaliella salina to hypersaline stress is a systematic work, possibly involving enhancements of photosynthesis, carbon fixation, and heme biosynthetic process, acceleration of protein turnover, spliceosome, protein processing in endoplasmic reticulum, and endocytosis, as well as degradation of starch, synthesis of glycerol, membrane lipid desaturation. Altogether, the changes of these biological processes occurred at trancriptomic level will help understand how a new intracellular balance achieved in Dunaliella salina to adapt to hypersaline environment, which are worth being confirmed at the physiological levels.

Insights into the evolution of polymodal chemoreceptors

Respiratory chemoreceptors in vertebrates are specialized cells that detect chemical changes in the environment or arterial blood supply and initiate autonomic responses, such as hyperventilation or changes in heart rate, to improve O₂ uptake and delivery to tissues. These chemoreceptors are sensitive to changes in O₂, CO₂ and/or H⁺. In fish and mammals, respiratory chemoreceptors may be additionally sensitive to ammonia, hypoglycemia, and numerous other stimuli. Thus, chemoreceptors that affect respiration respond to different types of stimuli (or modalities) and are considered to be "polymodal". This review discusses the polymodal nature of respiratory chemoreceptors in vertebrates with a particular emphasis on chemoreceptors of the carotid body and pulmonary epithelium in mammals, and on neuroepithelial cells in water- and air-breathing fish. A major goal will be to examine the evidence for putative polymodal chemoreceptors in fish within the context of studies on mammalian models, for which polymodal chemoreceptors are well described, in order to improve our understanding of the evolution of polymodal chemoreceptors in vertebrates, and to aid in future studies that aim to identify putative receptors in air- and water-breathing fish.

A1899, PK-THPP, ML365, and Doxapram inhibit endogenous TASK channels and excite calcium signaling in carotid body type-1 cells

Sensing of hypoxia and acidosis in arterial chemoreceptors is thought to be mediated through the inhibition of TASK and possibly other (e.g., BKCa ) potassium channels which leads to membrane depolarization, voltage-gated Ca-entry, and neurosecretion. Here, we investigate the effects of pharmacological inhibitors on TASK channel activity and [Ca2+ ]i -signaling in isolated neonatal rat type-1 cells. PK-THPP inhibited TASK channel activity in cell attached patches by up to 90% (at 400 nmol/L). A1899 inhibited TASK channel activity by 35% at 400 nmol/L. PK-THPP, A1899 and Ml 365 all evoked a rapid increase in type-1 cell [Ca2+ ]i . These [Ca2+ ]i responses were abolished in Ca2+ -free solution and greatly attenuated by Ni2+ (2 mM) suggesting that depolarization and voltage-gated Ca2+ -entry mediated the rise in [Ca2+ ]i. Doxapram (50 μmol/L), a respiratory stimulant, also inhibited type-1 cell TASK channel activity and increased [Ca2+ ]i. . We also tested the effects of combined inhibition of BKCa and TASK channels. TEA (5 mmol/L) slightly increased [Ca2+ ]i in the presence of PK-THPP and A1899. Paxilline (300 nM) and iberiotoxin (50 nmol/L) also slightly increased [Ca2+ ]i in the presence of A1899 but not in the presence of PK-THPP. In general [Ca2+ ]i responses to TASK inhibitors, alone or in combination with BKCa inhibitors, were smaller than the [Ca2+ ]i responses evoked by hypoxia. These data confirm that TASK channel inhibition is capable of evoking membrane depolarization and robust voltage-gated Ca2+ -entry but suggest that this, even with concomitant inhibition of BKCa channels, may be insufficient to account fully for the [Ca2+ ]i -response to hypoxia.

Monitoring Functional Responses to Hypoxia in Single Carotid Body Cells

The carotid body is the main arterial chemoreceptor in mammals that mediates the cardiorespiratory reflexes activated by acute hypoxia. Here we describe the protocols followed in our laboratory to study responsiveness to hypoxia of single, enzymatically dispersed, glomus cells monitored by microfluorimetry and the patch-clamp technique.

Activation of voltage-dependent K+ channels strongly limits hypoxia-induced elevation of [Ca2+ ]i in rat carotid body glomus cells

KEY POINTS

We studied the role of the large-conductance Ca²⁺ -activated K⁺ channel (BK) and voltage-dependent K⁺ channels (Kv) on [Ca²⁺ ]_i responses to a wide range of hypoxia at different resting cell membrane potential (E_m ). BK/Kv were mostly closed at rest in normoxia. BK/Kv became basally active when cells were depolarized by elevated [KCl]_o (>12 mm). Regardless of whether BK/Kv were closed or basally open, hypoxia-induced elevation of [Ca²⁺ ]_i was enhanced 2- to 3-fold by inhibitors of BK/Kv. Hypoxia-induced elevation of [Ca²⁺ ]_i was enhanced ∼2-fold by an inhibitor of Kv2, a major Kv in rat glomus cells. Hypoxia did not inhibit BK in inside-out patches. Our study supports a scheme in which activation of BK/Kv strongly limits the magnitude of hypoxia-induced [Ca²⁺ ]_i rise, with Kv having a much greater effect than BK.

ABSTRACT

Large-conductance K_Ca (BK) and other voltage-dependent K⁺ channels (Kv) are highly expressed in carotid body (CB) glomus cells, but their role in hypoxia-induced excitation is still not well defined and remains controversial. We addressed this issue by studying the effects of inhibitors of BK (IBTX) and BK/Kv (TEA/4-AP) on [Ca²⁺ ]_i responses to a wide range of hypoxia at different levels of resting cell membrane potential (E_m ). IBTX and TEA/4-AP did not affect the basal [Ca²⁺ ]_i in isolated glomus cells bathed in 5 mm KCl_o , but elicited transient increases in [Ca²⁺ ]_i in cells that were moderately depolarized (11-20 mV) by elevation of [KCl]_o (12-20 mm). Thus, BK and Kv were mostly closed at rest and activated by depolarization. Four different levels of hypoxia (mild, moderate, severe, anoxia) were used to produce a wide range of [Ca²⁺ ]_i elevation (0-700 nm). IBTX did not affect the rise in [Ca²⁺ ]_i , but TEA/4-AP strongly (∼3-fold) enhanced [Ca²⁺ ]_i rise by moderate and severe levels of hypoxia. Guangxitoxin, a Kv2 blocker, inhibited the whole-cell current by ∼50%, and enhanced 2-fold the [Ca²⁺ ]_i rise elicited by moderate and severe levels of hypoxia. Anoxia did not directly affect BK, but activated BK via depolarization. Our findings do not support the view that hypoxia inhibits BK/Kv to initiate or maintain the hypoxic response. Rather, our results show that BK/Kv are activated as glomus cells depolarize in response to hypoxia, which then limits the rise in [Ca²⁺ ]_i . Inhibition of Kv may provide a mechanism to enhance the chemosensory activity of the CB and ventilation.

Filamin C: a novel component of the KCNE2 interactome during hypoxia

Aim

KCNE2 encodes for the potassium voltage-gated channel, KCNE2. Mutations in KCNE2 have been associated with long-QT syndrome (LQTS). While KCNE2 has been extensively studied, the functions of its C-terminal domain remain inadequately described. Here, we aimed to elucidate the functions of this domain by identifying its protein interactors using yeast two-hybrid analysis.

Methods

The C-terminal domain of KCNE2 was used as bait to screen a human cardiac cDNA library for putative interacting proteins. Co-localisation and co-immunoprecipitation analyses were used for verification.

Results

Filamin C (FLNC) was identified as a putative interactor with KCNE2. FLNC and KCNE2 co-localised within the cell, however, a physical interaction was only observed under hypoxic conditions.

Conclusion

The identification of FLNC as a novel KCNE2 ligand not only enhances current understanding of ion channel function and regulation, but also provides valuable information about possible pathways likely to be involved in LQTS pathogenesis.

TASK channels in arterial chemoreceptors and their role in oxygen and acid sensing

Arterial chemoreceptors play a vital role in cardiorespiratory control by providing the brain with information regarding blood oxygen, carbon dioxide, and pH. The main chemoreceptor, the carotid body, is composed of sensory (type 1) cells which respond to hypoxia or acidosis with a depolarising receptor potential which in turn activates voltage-gated calcium entry, neurosecretion and excitation of adjacent afferent nerves. The receptor potential is generated by inhibition of Twik-related acid-sensitive K(+) channel 1 and 3 (TASK1/TASK3) heterodimeric channels which normally maintain the cells' resting membrane potential. These channels are thought to be directly inhibited by acidosis. Oxygen sensitivity, however, probably derives from a metabolic signalling pathway. The carotid body, isolated type 1 cells, and all forms of TASK channel found in the type 1 cell, are highly sensitive to inhibitors of mitochondrial metabolism. Moreover, type1 cell TASK channels are activated by millimolar levels of MgATP. In addition to their role in the transduction of chemostimuli, type 1 cell TASK channels have also been implicated in the modulation of chemoreceptor function by a number of neurocrine/paracrine signalling molecules including adenosine, GABA, and serotonin. They may also be instrumental in mediating the depression of the acute hypoxic ventilatory response that occurs with some general anaesthetics. Modulation of TASK channel activity is therefore a key mechanism by which the excitability of chemoreceptors can be controlled. This is not only of physiological importance but may also offer a therapeutic strategy for the treatment of cardiorespiratory disorders that are associated with chemoreceptor dysfunction.

Cxs and Panx- hemichannels in peripheral and central chemosensing in mammals

Connexins (Cxs) and Pannexins (Panx) form hemichannels at the plasma membrane of animals. Despite their low open probability under physiological conditions, these hemichannels release signaling molecules (i.e., ATP, Glutamate, PGE₂) to the extracellular space, thus subserving several important physiological processes. Oxygen and CO₂ sensing are fundamental to the normal functioning of vertebrate organisms. Fluctuations in blood PO₂, PCO₂ and pH are sensed at the carotid bifurcations of adult mammals by glomus cells of the carotid bodies. Likewise, changes in pH and/or PCO₂ of cerebrospinal fluid are sensed by central chemoreceptors, a group of specialized neurones distributed in the ventrolateral medulla (VLM), raphe nuclei, and some other brainstem areas. After many years of research, the molecular mechanisms involved in chemosensing process are not completely understood. This manuscript will review data regarding relationships between chemosensitive cells and the expression of channels formed by Cxs and Panx, with special emphasis on hemichannels.

Acute Hypoxia Activates an ENaC-like Channel in Rat Pheochromocytoma (PC12) Cells

Cells can resist and even recover from stress induced by acute hypoxia, whereas chronic hypoxia often leads to irreversible damage and eventually death. Although little is known about the response(s) to acute hypoxia in neuronal cells, alterations in ion channel activity could be preferential. This study aimed to elucidate which channel type is involved in the response to acute hypoxia in rat pheochromocytomal (PC12) cells as a neuronal cell model. Using perfusing solution saturated with 95% N(2) and 5% CO(2), induction of cell hypoxia was confirmed based on increased intracellular Ca(2+) with diminished oxygen content in the perfusate. During acute hypoxia, one channel type with a conductance of about 30 pS (2.5 pA at -80 mV) was activated within the first 2~3 min following onset of hypoxia and was long-lived for more than 300 ms with high open probability (P(o), up to 0.8). This channel was permeable to Na(+) ions, but not to K(+), Ca(+), and Cl(-) ions, and was sensitively blocked by amiloride (200 nM). These characteristics and behaviors were quite similar to those of epithelial sodium channel (ENaC). RT-PCR and Western blot analyses confirmed that ENaC channel was endogenously expressed in PC12 cells. Taken together, a 30-pS ENaC-like channel was activated in response to acute hypoxia in PC12 cells. This is the first evidence of an acute hypoxia-activated Na(+) channel that can contribute to depolarization of the cell.

K(+) channels in O(2) sensing and postnatal development of carotid body glomus cell response to hypoxia

The sensitivity of carotid body chemoreceptors to hypoxia is low just after birth and increases over the first few weeks of the postnatal period. At present, it is believed that the hypoxia-induced excitation of carotid body glomus cells begins with the inhibition of the outward K(+) current via one or more O(2) sensors. Although the nature of the O(2) sensors and their signals that inhibit the K(+) current are not well defined, studies suggest that the postnatal maturation of the glomus cell response to hypoxia is largely due to the increased sensitivity of K(+) channels to hypoxia. As K(V), BK and TASK channels that are O(2)-sensitive contribute to the K(+) current, it is important to identify the O(2) sensor and the signaling molecule for each of these K(+) channels. Various O(2) sensors (mitochondrial hemeprotein, hemeoxygenase-2, NADPH oxidase) and associated signals have been proposed to mediate the inhibition of K(+) channels by hypoxia. Studies suggest that a mitochondrial hemeprotein is likely to serve as an O(2) sensor for K(+) channels, particularly for TASK, and that multiple signals may be involved. Thus, changes in the sensitivity of the mitochondrial O(2) sensor to hypoxia, the sensitivity of K(+) channels to signals generated by mitochondria, and/or the expression levels of K(+) channels are likely to account for the postnatal maturation of O(2) sensing by glomus cells.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Tetrodotoxin as a tool to elucidate sensory transduction mechanisms: the case for the arterial chemoreceptors of the carotid body

Carotid bodies (CBs) are secondary sensory receptors in which the sensing elements, chemoreceptor cells, are activated by decreases in arterial PO₂ (hypoxic hypoxia). Upon activation, chemoreceptor cells (also known as Type I and glomus cells) increase their rate of release of neurotransmitters that drive the sensory activity in the carotid sinus nerve (CSN) which ends in the brain stem where reflex responses are coordinated. When challenged with hypoxic hypoxia, the physiopathologically most relevant stimulus to the CBs, they are activated and initiate ventilatory and cardiocirculatory reflexes. Reflex increase in minute volume ventilation promotes CO₂ removal from alveoli and a decrease in alveolar PCO₂ ensues. Reduced alveolar PCO₂ makes possible alveolar and arterial PO₂ to increase minimizing the intensity of hypoxia. The ventilatory effect, in conjunction the cardiocirculatory components of the CB chemoreflex, tend to maintain an adequate supply of oxygen to the tissues. The CB has been the focus of attention since the discovery of its nature as a sensory organ by de Castro (1928) and the discovery of its function as the origin of ventilatory reflexes by Heymans group (1930). A great deal of effort has been focused on the study of the mechanisms involved in O₂ detection. This review is devoted to this topic, mechanisms of oxygen sensing. Starting from a summary of the main theories evolving through the years, we will emphasize the nature and significance of the findings obtained with veratridine and tetrodotoxin (TTX) in the genesis of current models of O₂-sensing.

Hypoxia. 4. Hypoxia and ion channel function

The ability to sense and respond to oxygen deprivation is required for survival; thus, understanding the mechanisms by which changes in oxygen are linked to cell viability and function is of great importance. Ion channels play a critical role in regulating cell function in a wide variety of biological processes, including neuronal transmission, control of ventilation, cardiac contractility, and control of vasomotor tone. Since the 1988 discovery of oxygen-sensitive potassium channels in chemoreceptors, the effect of hypoxia on an assortment of ion channels has been studied in an array of cell types. In this review, we describe the effects of both acute and sustained hypoxia (continuous and intermittent) on mammalian ion channels in several tissues, the mode of action, and their contribution to diverse cellular processes.

Role of MaxiK-type calcium dependent K+ channels in rat carotid body hypoxia transduction during postnatal development

Carotid body chemoreceptors transduce a decrease in arterial oxygen tension into increased sinus nerve action potential (AP) activity which undergoes a maturational increase in the post-natal period. MaxiK-channels channels are proposed to play a major role in organ function based on their maturation-dependent expression in glomus cells and inhibition by acute hypoxia. To better resolve the role of this channel, single-unit AP activity of rat chemoreceptor neurons was recorded, in vitro, during a progressive decrease in oxygen from normoxia (∼150 Torr) to moderate hypoxia (∼60 Torr). Blockade of MaxiK channels with charybdotoxin (100 nM) in both older (P16-P18) and younger (P2-P3) animals resulted in no significant change in AP activity, but increased nerve conduction speed in the older animals. In dissociated glomus cells, charybdotoxin slightly enhanced the intracellular calcium response to acute hypoxia at both ages. We conclude that MaxiK channels play little or no role in mediating the response to acute, moderate hypoxia, either in the newborn or older animal.

Carotid body chemosensory responses in mice deficient of TASK channels

Background K⁺ channels of the TASK family are believed to participate in sensory transduction by chemoreceptor (glomus) cells of the carotid body (CB). However, studies on the systemic CB-mediated ventilatory response to hypoxia and hypercapnia in TASK1- and/or TASK3-deficient mice have yielded conflicting results. We have characterized the glomus cell phenotype of TASK-null mice and studied the responses of individual cells to hypoxia and other chemical stimuli. CB morphology and glomus cell size were normal in wild-type as well as in TASK1^−/− or double TASK1/3^−/− mice. Patch-clamped TASK1/3-null glomus cells had significantly higher membrane resistance and less hyperpolarized resting potential than their wild-type counterpart. These electrical parameters were practically normal in TASK1^−/− cells. Sensitivity of background currents to changes of extracellular pH was drastically diminished in TASK1/3-null cells. In contrast with these observations, responsiveness to hypoxia or hypercapnia of either TASK1^−/− or double TASK1/3^−/− cells, as estimated by the amperometric measurement of catecholamine release, was apparently normal. TASK1/3 knockout cells showed an enhanced secretory rate in basal (normoxic) conditions compatible with their increased excitability. Responsiveness to hypoxia of TASK1/3-null cells was maintained after pharmacological blockade of maxi-K⁺ channels. These data in the TASK-null mouse model indicate that TASK3 channels contribute to the background K⁺ current in glomus cells and to their sensitivity to external pH. They also suggest that, although TASK1 channels might be dispensable for O₂/CO₂ sensing in mouse CB cells, TASK3 channels (or TASK1/3 heteromers) could mediate hypoxic depolarization of normal glomus cells. The ability of TASK1/3^−/− glomus cells to maintain a powerful response to hypoxia even after blockade of maxi-K⁺ channels, suggests the existence of multiple sensor and/or effector mechanisms, which could confer upon the cells a high adaptability to maintain their chemosensory function.

Molecular identification and functional role of voltage-gated sodium channels in rat carotid body chemoreceptor cells. Regulation of expression by chronic hypoxia in vivo

We have assessed the expression, molecular identification and functional role of Na+ channels (Na(v)) in carotid bodies (CB) obtained from normoxic and chronically hypoxic adult rats. Veratridine evoked release of catecholamines (CA) from an in vitro preparation of intact CBs obtained from normoxic animals, the response being Ca2+ and Na+-dependent and sensitive to tetrodotoxin (TTX). TTX inhibited by 25-50% the CA release response evoked by graded hypoxia. Immunoblot assays demonstrated the presence of Na(v)alpha-subunit (c. 220 kDa) in crude homogenates from rat CBs, being evident an up-regulation (60%) of this protein in the CBs obtained from chronically hypoxic rats (10% O2; 7 days). This up-regulation was accompanied by an enhanced TTX-sensitive release response to veratridine, and by an enhanced ventilatory response to acute hypoxic stimuli. RT-PCR studies demonstrated the expression of mRNA for Na(v)1.1, Na(v)1.2, Na(v)1.3, Na(v)1.6 and Na(v)1.7 isoforms. At least three isoforms, Na(v)1.1, Na(v)1.3 and Na(v)1.6 co-localized with tyrosine hydroxylase in all chemoreceptor cells. RT-PCR and immunocytochemistry indicated that Na(v)1.1 isoform was up-regulated by chronic hypoxia in chemoreceptor cells. We conclude that Na(v) up-regulation represents an adaptive mechanism to increase chemoreceptor sensitivity during acclimatization to sustained hypoxia as evidenced by enhanced ventilatory responses to acute hypoxic tests.

Oxygen sensitive Kv channels in the carotid body

Hypoxic inhibition of K(+) channels has been documented in many native chemoreceptor cells, and is crucial to initiate reflexes directed to improve tissue O(2) supply. In the carotid body (CB) chemoreceptors, there is a general consensus regarding the facts that a decrease in P(O2) leads to membrane depolarization, increase of Ca(2+) entry trough voltage-dependent Ca(2+) channels and Ca(2+)-dependent release of neurotransmitters. Central to this pathway is the modulation by hypoxia of K(+) channels that triggers depolarization. However, the details of this process are still controversial, and even the molecular nature of these oxygen-sensitive K(+) (K(O2)) channels in the CB is hotly debated. Clearly there are inter-species differences, and even in the same preparation more that one K(O2) may be present. Here we recapitulate our present knowledge of the role of voltage dependent K(+) channels as K(O2) in the CB from different species, and their functional contribution to cell excitability in response to acute and chronic exposure to hypoxia.

TASK-like potassium channels and oxygen sensing in the carotid body

Chemosensing by type-1 cells of the carotid body involves a series of events which culminate in the calcium-dependent secretion of neurotransmitter substances which then excite afferent nerves. This response is mediated via membrane depolarisation and voltage-gated calcium entry. Studies utilising isolated cells indicates that the membrane depolarisation in response to hypoxia, and acidosis, appears to be primarily mediated via the inhibition of a background K(+)-current. The pharmacological and biophysical characteristics of these channels suggest that they are probably closely related to the TASK subfamily of tandem-P-domain K(+)-channels. Indeed they show greatest similarity to TASK-1 and -3. In addition to being sensitive to hypoxia and acidosis, the background K(+)-channels of the type-1 cell are also remarkably sensitive to inhibition of mitochondrial energy metabolism. Metabolic poisons are known potent stimulants of the carotid body and cause membrane depolarisation of type-1 cells. In the presence of metabolic inhibitors hypoxic sensitivity is lost suggesting that oxygen sensing may itself be mediated via depression of mitochondrial energy production. Thus these TASK-like background channels play a central role in mediating the chemotransduction of several different stimuli within the type-1 cell. The mechanisms by which metabolic/oxygen sensitivity might be conferred upon these channels are briefly discussed.

A search for genes that may confer divergent morphology and function in the carotid body between two strains of mice

The carotid body (CB) is the primary hypoxic chemosensory organ. Its hypoxic response appears to be genetically controlled. We have hypothesized that: 1) genes related to CB function are expressed less in the A/J mice (low responder to hypoxia) compared with DBA/2J mice (high responder to hypoxia); and 2) gene expression levels of morphogenic and trophic factors of the CB are significantly lower in the A/J mice than DBA/2J mice. This study utilizes microarray analysis to test these hypotheses. Three sets of CBs were harvested from both strains. RNA was isolated and used for global gene expression profiling (Affymetrix Mouse 430 v2.0 array). Statistically significant gene expression was determined as a minimum six counts of nine pairwise comparisons, a minimum 1.5-fold change, and P ≤ 0.05. Our results demonstrated that 793 genes were expressed less and that 568 genes were expressed more in the A/J strain vs. the DBA/2J strain. Analysis of individual genes indicates that genes encoding ion channels are differentially expressed between the two strains. Genes related to neurotransmitter metabolism, synaptic vesicles, and the development of neural crest-derived cells are expressed less in the A/J CB vs. the DBA/2J CB. Through pathway analysis, we have constructed a model that shows gene interactions and offers a roadmap to investigate CB development and hypoxic chemosensing/chemotransduction processes. Particularly, Gdnf, Bmp2, Kcnmb2, Tph1, Hif1a, and Arnt2 may contribute to the functional differences in the CB between the two strains. Bmp2, Phox2b, Dlx2, and Msx2 may be important for the morphological differences.

The role of NADPH oxidase in carotid body arterial chemoreceptors

O(2)-sensing in the carotid body occurs in neuroectoderm-derived type I glomus cells where hypoxia elicits a complex chemotransduction cascade involving membrane depolarization, Ca(2+) entry and the release of excitatory neurotransmitters. Efforts to understand the exquisite O(2)-sensitivity of these cells currently focus on the coupling between local P(O2) and the open-closed state of K(+)-channels. Amongst multiple competing hypotheses is the notion that K(+)-channel activity is mediated by a phagocytic-like multisubunit enzyme, NADPH oxidase, which produces reactive oxygen species (ROS) in proportion to the prevailing P(O2). In O(2)-sensitive cells of lung neuroepithelial bodies (NEB), multiple studies confirm that ROS levels decrease in hypoxia, and that E(M) and K(+)-channel activity are indeed controlled by ROS produced by NADPH oxidase. However, recent studies in our laboratories suggest that ROS generated by a non-phagocyte isoform of the oxidase are important contributors to chemotransduction, but that their role in type I cells differs fundamentally from the mechanism utilized by NEB chemoreceptors. Data indicate that in response to hypoxia, NADPH oxidase activity is increased in type I cells, and further, that increased ROS levels generated in response to low-O(2) facilitate cell repolarization via specific subsets of K(+)-channels.

Postnatal development of carotid body glomus cell response to hypoxia

This study examines developmental changes in CB glomus cell depolarization, intracellular calcium ([Ca(2+)](i)) and the magnitude of an O(2)-sensitive background ionic conductance that may play roles in the postnatal increase in oxygen sensitivity of glomus cells isolated from rats of 1-3 days and 11-14 days postnatal age. Using fura-2 and perforated patch whole cell recordings, we simultaneously measured [Ca(2+)](i) and membrane potential (E(m)) during normoxia and hypoxia. Resting E(m) in normoxia was similar at both ages. Hypoxia caused a larger E(m) depolarization and correspondingly larger [Ca(2+)](i) response in glomus cells from 11- to 14-day-old rats compared to 1-3-day-old rats. E(m) and [Ca(2+)](i) responses to 40mM K(+) were identical between the two age groups. Under normoxic conditions both age groups had similar background conductances. Under anoxic conditions (at resting membrane potential) background K(+) conductance decreased significantly more in cells from 11- to 14-day-old rats compared to cells from 1- to 3-day-old rats. Glomus cells from newborns therefore have less O(2)-sensitive background K(+) conductance. These results support the hypothesis that postnatal maturation of glomus cell O(2) sensitivity involves developmental regulation of the expression and/or O(2)-sensitivity of background ionic conductances.

Postnatal development of carotid body glomus cell O2 sensitivity

In mammals, the main sensors of arterial oxygen level are the carotid chemoreceptors, which exhibit low sensitivity to hypoxia at birth and become more sensitive over the first few days or weeks of life. This postnatal increase in hypoxia sensitivity of the arterial chemoreceptors, termed "resetting", remains poorly understood. In the carotid body, hypoxia is transduced by glomus cells, which are secretory sensory neurons that respond to hypoxia at higher P(O2) levels than non-chemoreceptor cell types. Maturation or resetting of carotid body O2 sensitivity potentially involves numerous aspects of the O2 transduction cascade at the glomus cell level, including glomus cell neurotransmitter secretion, neuromodulator function, neurotransmitter receptor expression, glomus cell depolarization in response to hypoxia, [Ca2+]i responses to hypoxia, K+ and Ca2+ channel O2 sensitivity and K+ channel expression. However, although progress has been made in the understanding of carotid body development, the precise mechanisms underlying postnatal maturation of these numerous aspects of chemotransduction remain obscure.

Acute hypoxia selectively inhibits KCNA5 channels in pulmonary artery smooth muscle cells

Acute hypoxia causes pulmonary vasoconstriction in part by inhibiting voltage-gated K(+) (Kv) channel activity in pulmonary artery smooth muscle cells (PASMC). The hypoxia-mediated decrease in Kv currents [I(K(V))] is selective to PASMC; hypoxia has little effect on I(K(V)) in mesenteric artery smooth muscle cells (MASMC). Functional Kv channels are homo- and/or heterotetramers of pore-forming alpha-subunits and regulatory beta-subunits. KCNA5 is a Kv channel alpha-subunit that forms functional Kv channels in PASMC and regulates resting membrane potential. We have shown that acute hypoxia selectively inhibits I(K(V)) through KCNA5 channels in PASMC. Overexpression of the human KCNA5 gene increased I(K(V)) and caused membrane hyperpolarization in HEK-293, COS-7, and rat MASMC and PASMC. Acute hypoxia did not affect I(K(V)) in KCNA5-transfected HEK-293 and COS-7 cells. However, overexpression of KCNA5 in PASMC conferred its sensitivity to hypoxia. Reduction of Po(2) from 145 to 35 mmHg reduced I(K(V)) by approximately 40% in rat PASMC transfected with human KCNA5 but had no effect on I(K(V)) in KCNA5-transfected rat MASMC (or HEK and COS cells). These results indicate that KCNA5 is an important Kv channel that regulates resting membrane potential and that acute hypoxia selectively reduces KCNA5 channel activity in PASMC relative to MASMC and other cell types. Because Kv channels (including KCNA5) are ubiquitously expressed in PASMC and MASMC, the observation from this study indicates that a hypoxia-sensitive mechanism essential for inhibiting KCNA5 channel activity is exclusively present in PASMC. The divergent effect of hypoxia on I(K(V)) in PASMC and MASMC also may be due to different expression levels of KCNA5 channels.

Down regulation of Kv3.4 channels by chronic hypoxia increases acute oxygen sensitivity in rabbit carotid body

The carotid body (CB) chemoreceptors participate in the ventilatory responses to acute and chronic hypoxia (CH). Arterial hypoxaemia increases breathing within seconds, and CB chemoreceptors are the principal contributors to this reflex hyperventilatory response. Acute hypoxia induces depolarization of CB chemoreceptors by inhibiting certain K+ channels, but the role of these channels in CH, as in high-altitude acclimatization, is less known. Here we explored the effects of prolonged (24-48 h) hypoxic exposure of rabbit CB chemoreceptor cells in primary cultures on the voltage-dependent K+ currents and on their response to acute hypoxia. We found that CH induces a decrease in the amplitude of outward K+ currents due to a reduction in a fast-inactivating BDS- and highly TEA-sensitive component of the current. In spite of this effect, acute hypoxic inhibition of K+ currents is increased in CH cultures, as well as hypoxia-induced depolarization. These data suggest that downregulation of this component (that does not contribute to the oxygen-sensitive K+ current (IKO2) participates in the hypoxic sensitization. Pharmacological, immunocytochemical and quantitative PCR (qPCR) experiments demonstrate that CH-induced decrease in outward K+ currents is due to a downregulation of the expression of Kv3.4 channels. Taken together, our results suggest that CH sensitization in rabbit CB could be achieved by an increase in the relative contribution of IKO2 to the outward K+ current as a consequence of the decreased expression of the oxygen-insensitive component of the current. We conclude that acute and chronic hypoxia can exert their effects acting on different molecular targets.

Acute hypoxia differentially regulates K+ channels. Implications with respect to cardiac arrhythmia

The first ion channels demonstrated to be sensitive to changes in oxygen tension were K(+) channels in glomus cells of the carotid body. Since then a number of hypoxia-sensitive ion channels have been identified. However, not all K(+) channels respond to hypoxia alike. This has raised some debate about how cells detect changes in oxygen tension. Because ion channels respond rapidly to hypoxia it has been proposed that the channel is itself an oxygen sensor. However, channel function can also be modified by thiol reducing and oxidizing agents, implicating reactive oxygen species as signals in hypoxic events. Cardiac ion channels can also be modified by hypoxia and redox agents. The rapid and slow components of the delayed rectifier K(+) channel are differentially regulated by hypoxia and beta-adrenergic receptor stimulation. Mutations in the genes that encode the subunits for the channel are associated with Long QT syndrome and sudden cardiac death. The implications with respect to effects of hypoxia on the channel and triggering of cardiac arrhythmia will be discussed.

Postnatal development of E-4031-sensitive potassium current in rat carotid chemoreceptor cells

The O2 sensitivity of dissociated type I cells from rat carotid body increases with age until approximately 14-16 days. Hypoxia-induced depolarization appears to be mediated by an O2-sensitive K+ current, but other K+ currents may modulate depolarization. We hypothesized that membrane potential may be stabilized in newborn type I cells by human ether-a-go-go-related gene (HERG)-like K+ currents that inhibit hypoxia-induced depolarization and that a decrease in this current with age could underlie, in part, the developmental increase in type I cell depolarization response to hypoxia. In dissociated type I cells from 0- to 1- and 11- to 16-day-old rats, using perforated patch-clamp and 70 mM K+ extracellular solution, we measured repolarization-induced inward K+ tail currents in the absence and presence of E-4031, a specific HERG channel blocker. This allowed isolation of the E-4031-sensitive HERG-like current. E-4031-sensitive peak currents in type I cells from 0- to- 1-day-old rats were 2.5-fold larger than in cells from 11- to 16-day-old rats. E-4031-sensitive current density in newborn type I cells was twofold greater than in cells from 11- to 16-day-old rats. Under current clamp conditions, E-4031 enhanced hypoxia-induced depolarization in type I cells from 0- to- 1-day-old but not 11- to 16-day-old rats. With use of fura 2 to measure intracellular Ca2+, E-4031 increased the cytosolic Ca2+ concentration response to anoxia in cells from 0- to- 1-day-old but not cells from 11- to 16-day-old rats. E-4031-sensitive K+ currents are present in newborn carotid body type I cells and decline with age. These findings are consistent with a role for E-4031-sensitive K+ current, and possibly HERG-like K+ currents, in the type I cell hypoxia response maturation.

Role of voltage-dependent calcium channels in stimulus-secretion coupling in rabbit carotid body chemoreceptor cells

We have defined Ca2+ channel subtypes expressed in rabbit carotid body (CB) chemoreceptor cells and their participation in the stimulus-evoked catecholamine (CA) release. Ca2+ currents (I(Ca)) activated at -30 mV, peaked at +10 mV and were fully blocked by 200 microm Cd2+. L-type channels (sensitive to 2 microm nisoldipine) activated at -30 mV and carried 21 +/- 2% of total I(Ca). Non-L-type channels activated at potentials positive to -10 mV and carried: N channels (sensitive to 1 microM omega-conotoxin-GVIA) 16 +/- 1% of total I(Ca), P/Q channels (sensitive to 3 microM omega-conotoxin-MVIIC after nisoldipine plus GVIA) 23 +/- 3% of total I(Ca) and R channels (resistant to all blockers combined) 40 +/- 3% of total I(Ca). CA release induced by hypoxia, hypercapnic acidosis, dinitrophenol (DNP) and high K(+)(o) in the intact CB was inhibited by 79-98% by 200 microm Cd2+. Hypoxia, hypercapnic acidosis and DNP, depolarized chemoreceptor cells and eventually generated repetitive action potential discharge. Nisoldipine plus MVIIC nearly abolished the release of CAs induced by hypoxia and hypercapnic acidosis and reduced by 74% that induced by DNP. All these secretory responses were insensitive to GVIA. 30 and 100 mm K(+)(o) brought resting membrane potential (E(m)) of chemoreceptor cells (-48.1 +/- 1.2 mV) to -22.5 and +7.2 mV, respectively. Thirty millimolar K(+)(o)-evoked release was abolished by nisoldipine but that induced by 100 mm K(+)(o) was mediated by activation of L, N, and P/Q channels. Data show that tested stimuli depolarize rabbit CB chemoreceptor cells and elicit CA release through Ca2+ entry via voltage-activated channels. Only L and P/Q channels are tightly coupled to the secretion of CA.

Regulation of oxygen sensing by ion channels

O(2) sensing is of critical importance for cell survival and adaptation of living organisms to changing environments or physiological conditions. O(2)-sensitive ion channels are major effectors of the cellular responses to hypoxia. These channels are preferentially found in excitable neurosecretory cells (glomus cells of the carotid body, cells in the neuroepithelial bodies of the lung, and neonatal adrenal chromaffin cells), which mediate fast cardiorespiratory adjustments to hypoxia. O(2)-sensitive channels are also expressed in the pulmonary and systemic arterial smooth muscle cells where they participate in the vasomotor responses to low O(2) tension (particularly in hypoxic pulmonary vasoconstriction). The mechanisms underlying O(2) sensing and how the O(2) sensors interact with the ion channels remain unknown. Recent advances in the field give different support to the various current hypotheses. Besides the participation of ion channels in acute O(2) sensing, they also contribute to the gene program developed under chronic hypoxia. Gene expression of T-type calcium channels is upregulated by hypoxia through the same hypoxia-inducible factor-dependent signaling pathway utilized by the classical O(2)-regulated genes. Alteration of acute or chronic O(2) sensing by ion channels could participate in the pathophysiology of human diseases, such as sudden infant death syndrome or primary pulmonary hypertension.

CO2/pH chemosensory signaling in co-cultures of rat carotid body receptors and petrosal neurons: role of ATP and ACh

The neurotransmitter mechanisms that process acid hypercapnia in the mammalian carotid body (CB) are poorly understood. Using a co-culture model containing rat CB chemoreceptor (type 1 cell) clusters and petrosal neurons (PN), we tested the hypothesis that co-released ACh and ATP was an important mechanism. Sensory transmission from type I clusters to PN in co-culture occurred at chemical synapses via co-release of ATP and ACh because isohydric hypercapnia depolarized and/or increased firing in co-cultured PN, but not in PN cultured alone; PN chemoexcitatory responses were inhibited by decreasing the extracellular Ca(2+):Mg2+ ratio; partial inhibition of these responses occurred during separate perfusion of cholinergic (hexamethonium or mecamylamine) and P2X (suramin) receptor blockers, although inhibition was often complete with both blockers present; and rapid chemoexcitatory responses to hypercapnia were inhibited by acetazolamide (10 microM), an inhibitor of carbonic anhydrase, localized in type I cells. Acidosis (pH = 7.0, 7.2) enhanced the ATP-induced whole cell current in functional PN relative to that at physiologic pH (7.4), suggesting that increased sensitivity of postsynaptic P2X receptors may contribute to acid chemotransmission. Type I cells in CB tissue sections expressed vesicular acetylcholine transporter (VAChT), a cholinergic marker, as revealed by confocal immunofluorescence. Thus co-release of ACh and ATP is an important neurotransmitter mechanism for processing isohydric and acidic hypercapnia in the rat carotid body.

Patch clamp study of mouse glomus cells using a whole carotid body

Some electrophysiological characteristics of mouse glomus cells (DBA/2J strain) were investigated using an undissociated carotid body. The carotid body with major carotid arteries was placed in a recording chamber, and glomus cells were visualized with a water immersion lens combined with an infrared differential interference video camera. Patch clamp experiments revealed that voltage-gated outward current, but not inward current, was easily observed in glomus cells. Pharmacological experiments and the kinetics of the current suggest that outward current is via delayed rectifier, A type, and large conductance calcium-activated K channels. Furthermore, K current was reversibly attenuated by mild hypoxia. The results suggest electrophysiological similarities of glomus cells among the cat, the rat, and the DBA/2J mouse. The method appears useful for physiological experiments.

Effects of reducing agents on glutathione metabolism and the function of carotid body chemoreceptor cells

Two current hypotheses of O2 sensing in the carotid body (CB) chemoreceptors suggest participation of oxygen reactive (ROS) species, but they are mechanistically opposed. One postulates that hypoxia decreases ROS levels; the other that hypoxia increases them. Yet, both propose that the ensuing alteration in the cellular redox environment is the key signal triggering hypoxic chemoreception. Since the glutathione redox pair is the main cellular buffer for ROS and the main determinant of the general redox environment of the cells, a way to test whether ROS participate in chemoreception is to determine glutathione levels and to correlate them with the activity of CB chemoreceptor cells. We found that hypoxia does not alter the glutathione reduction potential but that it activates chemoreceptor cell neurosecretion. Incubation of tissues with reduced glutathione increases the glutathione-reducing potential but does not activate chemoreceptor cells in normoxia nor does it modify hypoxic activation. Like reduced glutathione, N-acetylcysteine promoted a general reducing environment in the cells without alteration of chemoreceptor cell activity. N-(mercaptopropionyl)-glycine, like the two previous agents, increases the reduction potential of glutathione. In contrast, the compound activated chemoreceptor cells in normoxia, promoting a dose- and Ca(2+)-dependent neurosecretion and a potentiation of the hypoxic responses. The existence of multiple relationships between glutathione reduction potential in the cells and their activity indicates that the general cellular redox environment is not a factor determining chemoreceptor cell activation. It cannot be excluded that the local redox environments of restricted microdomain(s) in the cells with specific regulating mechanisms are important signals for chemoreceptor cell activity.

Mechanisms for Hypoxia Detection in O2-Sensitive Cells

Since O(2) is the bare necessity for multicellular organisms, they develop multiple protective mechanisms against hypoxia. Mammals will adapt to hypoxia in short and long terms. The short-term responses include enhancement of the respiratory and cardiac functions, adrenaline secretion from adrenal medullary cells, and pulmonary vasoconstriction, whereas the long-term response is the increase in erythropoietin production with the consequent increase in red blood cells. Although much work has been done to elucidate molecular mechanisms for O(2)-sensing for the last ten years, the majority of the mechanisms remain unclear. We will review mechanisms proposed for hypoxia detection in carotid body type I cells, pulmonary artery smooth muscle, adrenal medullary cells, and liver cells, with the special focus on adrenal medullary cells.

Rotenone selectively occludes sensitivity to hypoxia in rat carotid body glomus cells

Carotid body glomus cells release transmitters in response to hypoxia due to the increase of excitability resulting from inhibition of O2 -regulated K+ channels. However, the mechanisms involved in the detection of changes of O2 tension are unknown. We have studied the interaction between glomus cell O2 sensitivity and inhibition of the mitochondrial electron transport chain (ETC) in a carotid body thin slice preparation in which catecholamine release from intact single glomus cells can be monitored by amperometry. Inhibition of the mitochondrial ETC at proximal and distal complexes induces external Ca2+-dependent catecholamine secretion. At saturating concentration of the ETC inhibitors, the cellular response to hypoxia is maintained. However, rotenone, a complex I blocker, selectively occludes the responsiveness to hypoxia of glomus cells in a dose-dependent manner. The effect of rotenone is mimicked by 1-methyl-4-phenylpyridinium ion (MPP+), an agent that binds to the same site as rotenone, but not by complex I inhibitors acting on different sites. In addition, the effect of rotenone is not prevented by incubation of the cells with succinate, a substrate of complex II. These data strongly suggest that sensitivity to hypoxia of carotid body glomus cells is not linked in a simple way to mitochondrial electron flow and that a rotenone (and MPP+)-sensitive molecule critically participates in acute oxygen sensing in the carotid body.

Graded response of K+ current, membrane potential, and [Ca2+]i to hypoxia in pulmonary arterial smooth muscle

Many studies indicate that hypoxic inhibition of some K+ channels in the membrane of the pulmonary arterial smooth muscle cells (PASMCs) plays a part in initiating hypoxic pulmonary vasoconstriction. The sensitivity of the K+ current (I(k)), resting membrane potential (E(m)), and intracellular Ca2+ concentration ([Ca2+]i) of PASMCs to different levels of hypoxia in these cells has not been explored fully. Reducing PO2 levels gradually inhibited steady-state I(k) of rat resistance PASMCs and depolarized the cell membrane. The block of I(k) by hypoxia was voltage dependent in that low O2 tensions (3 and 0% O2) inhibited I(k) more at 0 and -20 mV than at 50 mV. As expected, the hypoxia-sensitive I(k) was also 4-aminopyridine sensitive. Fura 2-loaded PASMCs showed a graded increase in [Ca2+]i as PO2 levels declined. This increase was reduced markedly by nifedipine and removal of extracellular Ca2+. We conclude that, as in the carotid body type I cells, PC-12 pheochromocytoma cells, and cortical neurons, increasing severity of hypoxia causes a proportional decrease in I(k) and E(m) and an increase of [Ca2+]i.

Cellular distribution of oxygen sensor candidates-oxidases, cytochromes, K+-channels--in the carotid body

The specific tissue of the carotid body is built up of groups of glomus cells, enveloped by glial-type sustentacular cells, and innervated by sensory nerve fibers. These units sense arterial pO(2) and respond to hypoxia by a variety of reactions that include initiation of the arterial chemoreflex, i.e., increasing firing activity in the carotid sinus nerve. Until now, neither the cellular localization of the initial events that lead to stimulation of chemoreceptor afferents nor the molecular mechanism of oxygen sensing in the carotid body have been unequivocally identified. Proposed molecular candidates for the mechanism of oxygen sensing include: 1). components of the mitochondrial respiratory chain, 2). NADPH oxidases generating reactive oxygen species in an oxygen-dependent manner, 3). oxygen-regulated plasmalemmal K(+)-channels, and 4). nonoxidase iron-proteins. Our still limited knowledge on their cellular distribution within the carotid body is reviewed here. It is evident that: 1). the distribution of at least some oxygen sensor candidates is not ubiquitous but cell-type-specific, and 2). each specific parenchymal cell type of the carotid body contains at least one of the proposed oxygen sensor candidates. This applies also for the glial-type sustentacular cells that exhibit immunoreactivity to the two-pore domain K(+)-channel, TASK-1. These observations fit best with the assumption that each cell type within the carotid body is principally responsive to hypoxia. The differential equipping of glomus cells, nerve endings, and sustentacular cells with sensor proteins might serve to determine different thresholds of sensitivity and/or to connect the process of oxygen sensing to different signaling pathways. It also favors the assumption that several mechanisms of oxygen sensing may act simultaneously. The cellular identification of the cell type initiating the chemoreceptor reflex, however, has to await the molecular identification of the particular oxygen sensor molecule that initiates increased carotid sinus nerve activity.

Ion channels in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a hemodynamic abnormality that ultimately results in mortality due to right heart failure. Although the clinical manifestations of primary and secondary PAH are diverse, medial hypertrophy and arterial vasoconstriction are key components in the vascular remodeling leading to PAH. Abnormalities in the homeostasis of intracellular Ca(2+), transmembrane flux of ions, and membrane potential may play significant roles in the processes leading to pulmonary vascular remodeling. Decreased activity of K(+) channels causes membrane depolarization, leading to Ca(2+) influx. The elevated cytoplasmic Ca(2+) is a major trigger for pulmonary vasoconstriction and an important stimulus for vascular smooth muscle proliferation. Dysfunctional K(+) channels have also been linked to inhibition of apoptosis and contribute further to the medial hypertrophy. This review focuses on the relative role of K(+) and Ca(2+) ions and channels in human pulmonary artery smooth muscle cells in the development of PAH.

Significance of ROS in oxygen sensing in cell systems with sensitivity to physiological hypoxia

Reactive oxygen species (ROS) are oxygen-containing molecular entities which are more potent and effective oxidizing agents than is molecular oxygen itself. With the exception of phagocytic cells, where ROS play an important physiological role in defense reactions, ROS have classically been considered undesirable byproducts of cell metabolism, existing several cellular mechanisms aimed to dispose them. Recently, however, ROS have been considered important intracellular signaling molecules, which may act as mediators or second messengers in many cell functions. This is the proposed role for ROS in oxygen sensing in systems, such as carotid body chemoreceptor cells, pulmonary artery smooth muscle cells, and erythropoietin-producing cells. These unique cells comprise essential parts of homeostatic loops directed to maintain oxygen levels in multicellular organisms in situations of hypoxia. The present article examines the possible significance of ROS in these three cell systems, and proposes a set of criteria that ROS should satisfy for their consideration as mediators in hypoxic transduction cascades. In none of the three cell types do ROS satisfy these criteria, and thus it appears that alternative mechanisms are responsible for the transduction cascades linking hypoxia to the release of neurotransmitters in chemoreceptor cells, contraction in pulmonary artery smooth muscle cells and erythropoietin secretion in erythropoietin producing cells.