O(2) modulates large-conductance Ca(2+)-dependent K(+) channels of rat chemoreceptor cells by a membrane-restricted and CO-sensitive mechanism

The Heme Oxygenase/Biliverdin Reductase System and Its Genetic Variants in Physiology and Diseases

Heme oxygenase (HO) metabolizes heme into ferrous iron, carbon monoxide (CO), and biliverdin-IXα (BV), the latter being reduced into bilirubin-IXα (BR) by the biliverdin reductase-A (BVR). Heme oxygenase exists as two isoforms, HO-1, inducible and involved in the cell stress response, and HO-2, constitutive and committed to the physiologic turnover of heme and in the intracellular oxygen sensing. Many studies have identified genetic variants of the HO/BVR system and suggested their connection in free radical-induced diseases. The most common genetic variants include (GT)n dinucleotide length polymorphisms and single nucleotide polymorphisms. Gain-of-function mutations in the HO-1 and HO-2 genes foster the ventilator response to hypoxia and reduce the risk of coronary heart disease and age-related macular degeneration but increase the risk of neonatal jaundice, sickle cell disease, and Parkinson's disease. Conversely, loss-of-function mutations in the HO-1 gene increase the risk of type 2 diabetes mellitus, chronic obstructive pulmonary disease, and some types of cancers. Regarding BVR, the reported loss-of-function mutations increase the risk of green jaundice. Unfortunately, the physiological role of the HO/BVR system does not allow for the hypothesis gene silencing/induction strategies, but knowledge of these mutations can certainly facilitate a medical approach that enables early diagnoses and tailored treatments.

Oxygen Sensor of the Heart

How oxygen is sensed by the heart and what mechanisms mediate its sensing remain poorly understood. Since recent reports show that low PO₂ levels are detected by the cardiomyocytes in a few seconds, the rapid and short applications of low levels of oxygen (acute hypoxia), that avoid multiple effects of chronic hypoxia may be used to probe the oxygen sensing pathway of the heart. Here we explore the oxygen sensing pathway, focusing primarily on cellular surface membrane proteins that are first exposed to low PO₂. Such studies suggest that acute hypoxia primarily targets the cardiac calcium channels, where either the channel itself or moieties closely associated with it, for instance, heme-oxygenase-2 (HO-2) interacting through kinase phosphorylation, signals the α-subunit of the channel as to the altered levels of PO₂. Amino acids 1572-1651, the CaMKII phosphorylation sites (S1487 and S1545), CaM-binding site (I1624, Q1625) and Ser1928 of the carboxyl tail of the α-subunit appear to be critical residues that sense oxygen. Future studies in HO-2 knockout mice or CRISPR/Cas9 gene-edited hiPSC-CMs that reduce CaM-binding affinity are likely to provide deeper insights in the O₂-sensinsing mechanisms.

Carbon monoxide releasing molecule-2 suppresses stretch-activated atrial natriuretic peptide secretion by activating large-conductance calcium-activated potassium channels

Carbon monoxide (CO) is a known gaseous bioactive substance found across a wide array of body systems. The administration of low concentrations of CO has been found to exert an anti-inflammatory, anti-apoptotic, anti-hypertensive, and vaso-dilatory effect. To date, however, it has remained unknown whether CO influences atrial natriuretic peptide (ANP) secretion. This study explores the effect of CO on ANP secretion and its associated signaling pathway using isolated beating rat atria. Atrial perfusate was collected for 10 min for use as a control, after which high atrial stretch was induced by increasing the height of the outflow catheter. Carbon monoxide releasing molecule-2 (CORM-2; 10, 50, 100 µM) and hemin (HO-1 inducer; 0.1, 1, 50 µM), but not CORM-3 (10, 50, 100 µM), decreased high stretch-induced ANP secretion. However, zinc porphyrin (HO-1 inhibitor) did not affect ANP secretion. The order of potency for the suppression of ANP secretion was found to be hemin > CORM-2 >> CORM-3. The suppression of ANP secretion by CORM-2 was attenuated by pretreatment with 5-hydroxydecanoic acid, paxilline, and 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one, but not by diltiazem, wortmannin, LY-294002, or NG-nitro-L-arginine methyl ester. Hypoxic conditions attenuated the suppressive effect of CORM-2 on ANP secretion. In sum, these results suggest that CORM-2 suppresses ANP secretion via mitochondrial K_ATP channels and large conductance Ca²⁺-activated K⁺ channels.

Hypoxic Regulation of the Large-Conductance, Calcium and Voltage-Activated Potassium Channel, BK

Hypoxia is a condition characterized by a reduction of cellular oxygen levels derived from alterations in oxygen balance. Hypoxic events trigger changes in cell-signaling cascades, oxidative stress, activation of pro-inflammatory molecules, and growth factors, influencing the activity of various ion channel families and leading to diverse cardiovascular diseases such as myocardial infarction, ischemic stroke, and hypertension. The large-conductance, calcium and voltage-activated potassium channel (BK) has a central role in the mechanism of oxygen (O₂) sensing and its activity has been related to the hypoxic response. BK channels are ubiquitously expressed, and they are composed by the pore-forming α subunit and the regulatory subunits β (β1–β4), γ (γ1–γ4), and LINGO1. The modification of biophysical properties of BK channels by β subunits underly a myriad of physiological function of these proteins. Hypoxia induces tissue-specific modifications of BK channel α and β subunits expression. Moreover, hypoxia modifies channel activation kinetics and voltage and/or calcium dependence. The reported effects on the BK channel properties are associated with events such as the increase of reactive oxygen species (ROS) production, increases of intracellular Calcium ([Ca²⁺]_i), the regulation by Hypoxia-inducible factor 1α (HIF-1α), and the interaction with hemeproteins. Bronchial asthma, chronic obstructive pulmonary diseases (COPD), and obstructive sleep apnea (OSA), among others, can provoke hypoxia. Untreated OSA patients showed a decrease in BK-β1 subunit mRNA levels and high arterial tension. Treatment with continuous positive airway pressure (CPAP) upregulated β1 subunit mRNA level, decreased arterial pressures, and improved endothelial function coupled with a reduction in morbidity and mortality associated with OSA. These reports suggest that the BK channel has a role in the response involved in hypoxia-associated hypertension derived from OSA. Thus, this review aims to describe the mechanisms involved in the BK channel activation after a hypoxic stimulus and their relationship with disorders like OSA. A deep understanding of the molecular mechanism involved in hypoxic response may help in the therapeutic approaches to treat the pathological processes associated with diseases involving cellular hypoxia.

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.

High Efficacy by GAL-021: A Known Intravenous Peripheral Chemoreceptor Modulator that Suppresses BKCa-Channel Activity and Inhibits IK(M) or Ih

: GAL-021 has recently been developed as a novel breathing control modulator. However, modifications of ionic currents produced by this agent remain uncertain, although its efficacy in suppressing the activity of big-conductance Ca2+-activated K+ (BKCa) channels has been reported. In pituitary tumor (GH3) cells, we found that the presence of GAL-021 decreased the amplitude of macroscopic Ca2+-activated K+ current (IK(Ca)) in a concentration-dependent manner with an effective IC50 of 2.33 μM. GAL-021-mediated reduction of IK(Ca) was reversed by subsequent application of verteporfin or ionomycin; however, it was not by that of diazoxide. In inside-out current recordings, the addition of GAL-021 to the bath markedly decreased the open-state probability of BKCa channels. This agent also resulted in a rightward shift in voltage dependence of the activation curve of BKCa channels; however, neither the gating charge of the curve nor single-channel conductance of the channel was changed. There was an evident lengthening of the mean closed time of BKCa channels in the presence of GAL-021, with no change in mean open time. The GAL-021 addition also suppressed M-type K+ current with an effective IC50 of 3.75 μM; however, its presence did not alter the amplitude of erg-mediated K+ current, or mildly suppressed delayed-rectifier K+ current. GAL-021 at a concentration of 30 μM could also suppress hyperpolarization-activated cationic current. In HEK293T cells expressing α-hSlo, the addition of GAL-021 was also able to suppress the BKCa-channel open probabilities, and GAL-021-mediated suppression of BKCa-channel activity was attenuated by further addition of BMS-191011. Collectively, the GAL-021 effects presented herein do not exclusively act on BKCa channels and these modifications on ionic currents exert significant influence on the functional activities of electrically excitable cells occurring in vivo.

Mechanisms and Consequences of Oxygen and Carbon Dioxide Sensing in Mammals

Molecular oxygen (O₂) and carbon dioxide (CO₂) are the primary gaseous substrate and product of oxidative phosphorylation in respiring organisms, respectively. Variance in the levels of either of these gasses outside of the physiological range presents a serious threat to cell, tissue, and organism survival. Therefore, it is essential that endogenous levels are monitored and kept at appropriate concentrations to maintain a state of homeostasis. Higher organisms such as mammals have evolved mechanisms to sense O₂ and CO₂ both in the circulation and in individual cells and elicit appropriate corrective responses to promote adaptation to commonly encountered conditions such as hypoxia and hypercapnia. These can be acute and transient nontranscriptional responses, which typically occur at the level of whole animal physiology or more sustained transcriptional responses, which promote chronic adaptation. In this review, we discuss the mechanisms by which mammals sense changes in O₂ and CO₂ and elicit adaptive responses to maintain homeostasis. We also discuss crosstalk between these pathways and how they may represent targets for therapeutic intervention in a range of pathological states.

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.

Acute oxygen sensing-Role of metabolic specifications in peripheral chemoreceptor cells

Acute oxygen sensing is essential for humans under hypoxic environments or pathologic conditions. This is achieved by the carotid body (CB), the key arterial chemoreceptor, along with other peripheral chemoreceptor organs, such as the adrenal medulla (AM). Although it is widely accepted that inhibition of K⁺ channels in the plasma membrane of CB cells during acute hypoxia results in the activation of cardiorespiratory reflexes, the molecular mechanisms by which the hypoxic signal is detected to modulate ion channel activity are not fully understood. Using conditional knockout mice lacking mitochondrial complex I (MCI) subunit NDUFS2, we have found that MCI generates reactive oxygen species and pyridine nucleotides, which signal K⁺ channels during acute hypoxia. Comparing the transcriptomes from CB and AM, which are O₂-sensitive, with superior cervical ganglion, which is practically O₂-insensitive, we have found that CB and AM contain unique metabolic gene expression profiles. The "signature metabolic profile" and their biophysical characteristics could be essential for acute O₂ sensing by chemoreceptor cells.

Acute oxygen sensing by the carotid body: a rattlebag of molecular mechanisms

The molecular underpinnings of the oxygen sensitivity of the carotid body Type I cells are becoming better defined as research begins to identify potential interactions between previously separate theories. Nevertheless, the field of oxygen chemoreception still presents the general observer with a bewildering array of potential signalling pathways by which a fall in oxygen levels might initiate Type I cell activation. The purpose of this brief review is to address five of the current oxygen sensing hypotheses: the lactate-Olfr 78 hypothesis of oxygen chemotransduction; the role mitochondrial ATP and metabolism may have in chemotransduction; the AMP-activated protein kinase hypothesis and its current role in oxygen sensing by the carotid body; reactive oxygen species as key transducers in the oxygen sensing cascade; and the mechanisms by which H2 S, reactive oxygen species and haem oxygenase may integrate to provide a rapid oxygen sensing transduction system. Over the previous 15 years several lines of research into acute hypoxic chemotransduction mechanisms have focused on the integration of mitochondrial and membrane signalling. This review places an emphasis on the subplasmalemmal-mitochondrial microenvironment in Type I cells and how theories of acute oxygen sensing are increasingly dependent on functional interaction within this microenvironment.

Central and peripheral chemoreceptors in sudden infant death syndrome

The pathogenesis of sudden infant death syndrome (SIDS) has been ascribed to an underlying biological vulnerability to stressors during a critical period of development. This paper reviews the main data in the literature supporting the role of central (e.g. retrotrapezoid nucleus, serotoninergic raphe nuclei, locus coeruleus, orexinergic neurons, ventral medullary surface, solitary tract nucleus) and peripheral (e.g. carotid body) chemoreceptors in the pathogenesis of SIDS. Clinical and experimental studies indicate that central and peripheral chemoreceptors undergo critical development during the initial postnatal period, consistent with the age range of SIDS (<1 year). Most of the risk factors for SIDS (gender, genetic factors, prematurity, hypoxic/hyperoxic stimuli, inflammation, perinatal exposure to cigarette smoke and/or substance abuse) may structurally and functionally affect the developmental plasticity of central and peripheral chemoreceptors, strongly suggesting the involvement of these structures in the pathogenesis of SIDS. Morphometric and neurochemical changes have been found in the carotid body and brainstem respiratory chemoreceptors of SIDS victims, together with functional signs of chemoreception impairment in some clinical studies. However, the methodological problems of SIDS research will have to be addressed in the future, requiring large and highly standardized case series. Up-to-date autopsy protocols should be produced, involving substantial, and exhaustive sampling of all potentially involved structures (including peripheral arterial chemoreceptors). Morphometric approaches should include unbiased stereological methods with three-dimensional probes. Prospective clinical studies addressing functional tests and risk factors (including genetic traits) would probably be the gold standard, allowing markers of intrinsic or acquired vulnerability to be properly identified.

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.

Carbon monoxide in lung cell physiology and disease

Carbon monoxide (CO) is an endogenously produced gas that has gained recognition as a biological signal transduction effector with properties similar, but not identical, to that of nitric oxide (NO). CO, which binds primarily to heme iron, may activate the hemoprotein guanylate cyclase, although with lower potency than NO. Furthermore, CO can modulate the activities of several cellular signaling molecules such as p38 MAPK, ERK1/2, JNK, Akt, NF-κB, and others. Emerging studies suggest that mitochondria, the energy-generating organelle of cells, represent a key target of CO action in eukaryotes. Dose-dependent modulation of mitochondrial function by CO can result in alteration of mitochondrial membrane potential, mitochondrial reactive oxygen species production, release of proapoptotic and proinflammatory mediators, as well as the inhibition of respiration at high concentration. CO, through modulation of signaling pathways, can impact key biological processes including autophagy, mitochondrial biogenesis, programmed cell death (apoptosis), cellular proliferation, inflammation, and innate immune responses. Inhaled CO is widely known as an inhalation hazard due to its rapid complexation with hemoglobin, resulting in impaired oxygen delivery to tissues and hypoxemia. Despite systemic and cellular toxicity at high concentrations, CO has demonstrated cyto- and tissue-protective effects at low concentration in animal models of organ injury and disease. These include models of acute lung injury (e.g., hyperoxia, hypoxia, ischemia-reperfusion, mechanical ventilation, bleomycin) and sepsis. The success of CO as a candidate therapeutic in preclinical models suggests potential clinical application in inflammatory and proliferative disorders, which is currently under evaluation in clinical trials.

Gaseous Mediators and Mitochondrial Function: The Future of Pharmacologically Induced Suspended Animation?

The role of nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H₂S) as poisonous gases is well-established. However, they are not only endogenously produced but also, at low concentrations, exert beneficial effects, such as anti-inflammation, and cytoprotection. This knowledge initiated the ongoing debate, as to whether these molecules, also referred to as “gaseous mediators” or “gasotransmitters,” could serve as novel therapeutic agents. In this context, it is noteworthy, that all gasotransmitters specifically target the mitochondria, and that this interaction may modulate mitochondrial bioenergetics, thereby subsequently affecting metabolic function. This feature is of crucial interest for the possible induction of “suspended animation.” Suspended animation, similar to mammalian hibernation (and/or estivation), refers to an externally induced hypometabolic state, with the intention to preserve organ function in order to survive otherwise life-threatening conditions. This hypometabolic state is usually linked to therapeutic hypothermia, which, however, comes along with adverse effects (e.g., coagulopathy, impaired host defense). Therefore, inducing an on-demand hypometabolic state by directly lowering the energy metabolism would be an attractive alternative. Theoretically, gasotransmitters should reversibly interact and inhibit the mitochondrial respiratory chain during pharmacologically induced suspended animation. However, it has to be kept in mind that this effect also bears the risk of cytotoxicity resulting from the blockade of the mitochondrial respiratory chain. Therefore, this review summarizes the current knowledge of the impact of gasotransmitters on modulating mitochondrial function. Further, we will discuss their role as potential candidates in inducing a suspended animation.

Guinea Pig Oxygen-Sensing and Carotid Body Functional Properties

Mammals have developed different mechanisms to maintain oxygen supply to cells in response to hypoxia. One of those mechanisms, the carotid body (CB) chemoreceptors, is able to detect physiological hypoxia and generate homeostatic reflex responses, mainly ventilatory and cardiovascular. It has been reported that guinea pigs, originally from the Andes, have a reduced ventilatory response to hypoxia compared to other mammals, implying that CB are not completely functional, which has been related to genetically/epigenetically determined poor hypoxia-driven CB reflex. This study was performed to check the guinea pig CB response to hypoxia compared to the well-known rat hypoxic response. These experiments have explored ventilatory parameters breathing different gases mixtures, cardiovascular responses to acute hypoxia, in vitro CB response to hypoxia and other stimuli and isolated guinea pig chemoreceptor cells properties. Our findings show that guinea pigs are hypotensive and have lower arterial pO₂ than rats, probably related to a low sympathetic tone and high hemoglobin affinity. Those characteristics could represent a higher tolerance to hypoxic environment than other rodents. We also find that although CB are hypo-functional not showing chronic hypoxia sensitization, a small percentage of isolated carotid body chemoreceptor cells contain tyrosine hydroxylase enzyme and voltage-dependent K⁺ currents and therefore can be depolarized. However hypoxia does not modify intracellular Ca²⁺ levels or catecholamine secretion. Guinea pigs are able to hyperventilate only in response to intense acute hypoxic stimulus, but hypercapnic response is similar to rats. Whether other brain areas are also activated by hypoxia in guinea pigs remains to be studied.

BK channels in rat and human pulmonary smooth muscle cells are BKα-β1 functional complexes lacking the oxygen-sensitive stress axis regulated exon insert

A loss of K⁺ efflux in pulmonary arterial smooth muscle cells (PASMCs) contributes to abnormal vasoconstriction and PASMC proliferation during pulmonary hypertension (PH). Activation of high-conductance Ca²⁺-activated (BK) channels represents a therapeutic strategy to restore K⁺ efflux to the affected PASMCs. However, the properties of BK channels in PASMCs-including sensitivity to BK channel openers (BKCOs)-are poorly defined. The goal of this study was to compare the properties of BK channels between PASMCs of normoxic (N) and chronic hypoxic (CH) rats and then explore key findings in human PASMCs. Polymerase chain reaction results revealed that 94.3% of transcripts encoding BKα pore proteins in PASMCs from N rats represent splice variants lacking the stress axis regulated exon insert, which confers oxygen sensitivity. Subsequent patch-clamp recordings from inside-out (I-O) patches confirmed a dense population of BK channels insensitive to hypoxia. The BK channels were highly activated by intracellular Ca²⁺ and the BKCO lithocholate; these responses require BKα-β₁ subunit coupling. PASMCs of CH rats with established PH exhibited a profound overabundance of the dominant oxygen-insensitive BKα variant. Importantly, human BK (hBK) channels in PASMCs from human donor lungs also represented the oxygen-insensitive BKα variant activated by BKCOs. The hBK channels showed significantly enhanced Ca²⁺ sensitivity compared with rat BK channels. We conclude that rat BK and hBK channels in PASMCs are oxygen-insensitive BKα-β₁ complexes highly sensitive to Ca²⁺ and the BKCO lithocholate. BK channels are overexpressed in PASMCs of a rat model of PH and may provide an abundant target for BKCOs designed to restore K⁺ efflux.

Oxygen sensing by the carotid body: mechanisms and role in adaptation to hypoxia

Oxygen (O2) is fundamental for cell and whole-body homeostasis. Our understanding of the adaptive processes that take place in response to a lack of O2(hypoxia) has progressed significantly in recent years. The carotid body (CB) is the main arterial chemoreceptor that mediates the acute cardiorespiratory reflexes (hyperventilation and sympathetic activation) triggered by hypoxia. The CB is composed of clusters of cells (glomeruli) in close contact with blood vessels and nerve fibers. Glomus cells, the O2-sensitive elements in the CB, are neuron-like cells that contain O2-sensitive K(+)channels, which are inhibited by hypoxia. This leads to cell depolarization, Ca(2+)entry, and the release of transmitters to activate sensory fibers terminating at the respiratory center. The mechanism whereby O2modulates K(+)channels has remained elusive, although several appealing hypotheses have been postulated. Recent data suggest that mitochondria complex I signaling to membrane K(+)channels plays a fundamental role in acute O2sensing. CB activation during exposure to low Po2is also necessary for acclimatization to chronic hypoxia. CB growth during sustained hypoxia depends on the activation of a resident population of stem cells, which are also activated by transmitters released from the O2-sensitive glomus cells. These advances should foster further studies on the role of CB dysfunction in the pathogenesis of highly prevalent human diseases.

Experimental Observations on the Biological Significance of Hydrogen Sulfide in Carotid Body Chemoreception

The cascade of transduction of hypoxia and hypercapnia, the natural stimuli to chemoreceptor cells, is incompletely understood. A particular gap in that knowledge is the role played by second messengers, or in a most ample term, of modulators. A recently described modulator of chemoreceptor cell responses is the gaseous transmitter hydrogen sulfide, which has been proposed as a specific activator of the hypoxic responses in the carotid body, both at the level of the chemoreceptor cell response or at the level of the global output of the organ. Since sulfide behaves in this regard as cAMP, we explored the possibility that sulfide effects were mediated by the more classical messenger. Data indicate that exogenous and endogenous sulfide inhibits adenyl cyclase finding additionally that inhibition of adenylyl cyclase does not modify chemoreceptor cell responses elicited by sulfide. We have also observed that transient receptor potential cation channels A1 (TRPA1) are not regulated by sulfide in chemoreceptor cells.

Oxidative Stress and Maxi Calcium-Activated Potassium (BK) Channels

All cells contain ion channels in their outer (plasma) and inner (organelle) membranes. Ion channels, similar to other proteins, are targets of oxidative impact, which modulates ion fluxes across membranes. Subsequently, these ion currents affect electrical excitability, such as action potential discharge (in neurons, muscle, and receptor cells), alteration of the membrane resting potential, synaptic transmission, hormone secretion, muscle contraction or coordination of the cell cycle. In this chapter we summarize effects of oxidative stress and redox mechanisms on some ion channels, in particular on maxi calcium-activated potassium (BK) channels which play an outstanding role in a plethora of physiological and pathophysiological functions in almost all cells and tissues. We first elaborate on some general features of ion channel structure and function and then summarize effects of oxidative alterations of ion channels and their functional consequences.

Catecholamine secretion by chemical hypoxia in guinea-pig, but not rat, adrenal medullary cells: differences in mitochondria

The effects of mitochondrial inhibitors (CN(-), a complex IV inhibitor and CCCP, protonophore) on catecholamine (CA) secretion and mitochondrial function were explored functionally and biochemically in rat and guinea-pig adrenal chromaffin cells. Guinea-pig chromaffin cells conspicuously secreted CA in response to CN(-) or CCCP, but rat cells showed a little, if any, secretory response to either of them. The resting metabolic rates in rat adrenal medullae did not differ from those in guinea-pig adrenal medullae. On the other hand, the time course of depolarization of the mitochondrial membrane potential (ΔΨm) in guinea-pig chromaffin cells in response to CN(-) was slower than that in rat chromaffin cells, and this difference was abolished by oligomycin, an F1F0-ATPase inhibitor. The extent of CCCP-induced decrease in cellular ATP in guinea-pig chromaffin cells, which was indirectly measured using a Mg(2+) indicator, was smaller than that in rat chromaffin cells. Relative expression levels of F1F0-ATPase inhibitor factor in guinea-pig adrenal medullae were smaller than in rat adrenal medullae, and the opposite was true for F1F0-ATPase α subunit. The present results indicate that guinea-pig chromaffin cells secrete more CA in response to a mitochondrial inhibitor than rat chromaffin cells and this higher susceptibility in the former is accounted for by a larger extent of reversed operation of F1F0-ATPase with the consequent decrease in ATP under conditions where ΔΨm is depolarized.

A modern literature review of carbon monoxide poisoning theories, therapies, and potential targets for therapy advancement

The first descriptions of carbon monoxide (CO) and its toxic nature appeared in the literature over 100 years ago in separate publications by Drs. Douglas and Haldane. Both men ascribed the deleterious effects of this newly discovered gas to its strong interaction with hemoglobin. Since then the adverse sequelae of CO poisoning has been almost universally attributed to hypoxic injury secondary to CO occupation of oxygen binding sites on hemoglobin. Despite a mounting body of literature suggesting other mechanisms of injury, this pathophysiology and its associated oxygen centric therapies persists. This review attempts to elucidate the remarkably complex nature of CO as a gasotransmitter. While CO's affinity for hemoglobin remains undisputed, new research suggests that its role in nitric oxide release, reactive oxygen species formation, and its direct action on ion channels is much more significant. In the course of understanding the multifaceted character of this simple molecule it becomes apparent that current oxygen based therapies meant to displace CO from hemoglobin may be insufficient and possibly harmful. Approaching CO as a complex gasotransmitter will help guide understanding of the complex and poorly understood sequelae and illuminate potentials for new treatment modalities.

How does the heart sense changes in oxygen tension: a role for ion channels?

SIGNIFICANCE

Oxygen plays a key role in cellular metabolism and function. Oxygen delivery to cells is crucial, and a lack of oxygen such as that which occurs during myocardial infarction can be lethal. Cells should, therefore, be able to respond to changes in oxygen tension.

RECENT ADVANCES

Since the first studies examining the acute cellular effect of hypoxia on activation of transmitter release from glomus or type I chemoreceptor cells, it is now known that virtually all cells are able to respond to changes in oxygen tension.

CRITICAL ISSUES

Despite advances made in characterizing hypoxic responses, the identity of the "oxygen sensor" remains debated. Recently, more evidence has evolved as to how cardiac myocytes sense acute changes in oxygen. This review will examine the available evidence in support of acute oxygen-sensing mechanisms providing a brief historical perspective and then more detailed insights into the heart and the role of cardiac ion channels in hypoxic responses.

FUTURE DIRECTIONS

A further understanding of these cellular processes should result in interventions that assist in preventing the deleterious effects of acute changes in oxygen tension such as alterations in contractile function and cardiac arrhythmia.

The role of ion channels in the hypoxia-induced aggressiveness of glioblastoma

The malignancy of glioblastoma multiform (GBM), the most common and aggressive form of human brain tumors, strongly correlates with the presence of hypoxic areas, but the mechanisms controlling the hypoxia-induced aggressiveness are still unclear. GBM cells express a number of ion channels whose activity supports cell volume changes and increases in the cytosolic Ca²⁺ concentration, ultimately leading to cell proliferation, migration or death. In several cell types it has previously been shown that low oxygen levels regulate the expression and activity of these channels, and more recent data indicate that this also occurs in GBM cells. Based on these findings, it may be hypothesized that the modulation of ion channel activity or expression by the hypoxic environment may participate in the acquisition of the aggressive phenotype observed in GBM cells residing in a hypoxic environment. If this hypothesis will be confirmed, the use of available ion channels modulators may be considered for implementing novel therapeutic strategies against these tumors.

Role of K₂p channels in stimulus-secretion coupling

Two-pore domain K(+) (K2P) channels are involved in a variety of physiological processes by virtue of their high basal activity and sensitivity to various biological stimuli. One of these processes is secretion of hormones and transmitters in response to stimuli such as hypoxia, acidosis, and receptor agonists. The rise in intracellular [Ca(2+)] ([Ca(2+)]i) that is critical for the secretory event can be achieved by several mechanisms: (a) inhibition of resting (background) K(+) channels, (b) activation of Na(+)/Ca(2+)-permeable channels, and (c) release of Ca(2+) from intracellular stores. Here, we discuss the role of TASK and TREK in stimulus-secretion mechanisms in carotid body chemoreceptor cells and adrenal medullary/cortical cells. Studies show that stimuli such as hypoxia and acidosis cause cell depolarization and transmitter/hormone secretion by inhibition of TASK or TREK. Subsequent elevation of [Ca(2+)]i produced by opening of voltage-dependent Ca(2+) channels then activates a Na(+)-permeable cation channel, presumably to help sustain the depolarization and [Ca(2+)]i. Agonists such as angiotensin II may elevate [Ca(2+)]i via multiple mechanisms involving both inhibition of TASK/TREK and Ca(2+) release from internal stores to cause aldosterone secretion. Thus, inhibition of resting (background) K(+) channels and subsequent activation of voltage-gated Ca(2+) channels and Na(+)-permeable non-selective cation channels may be a common ionic mechanism that lead to hormone and transmitter secretion.

Gasotransmitter regulation of ion channels: a key step in O2 sensing by the carotid body

Carotid bodies detect hypoxia in arterial blood, translating this stimulus into physiological responses via the CNS. It is long established that ion channels are critical to this process. More recent evidence indicates that gasotransmitters exert powerful influences on O₂ sensing by the carotid body. Here, we review current understanding of hypoxia-dependent production of gasotransmitters, how they regulate ion channels in the carotid body, and how this impacts carotid body function.

A review on hemeoxygenase-2: focus on cellular protection and oxygen response

Hemeoxygenase (HO) system is responsible for cellular heme degradation to biliverdin, iron, and carbon monoxide. Two isoforms have been reported to date. Homologous HO-1 and HO-2 are microsomal proteins with more than 45% residue identity, share a similar fold and catalyze the same reaction. However, important differences between isoforms also exist. HO-1 isoform has been extensively studied mainly by its ability to respond to cellular stresses such as hemin, nitric oxide donors, oxidative damage, hypoxia, hyperthermia, and heavy metals, between others. On the contrary, due to its apparently constitutive nature, HO-2 has been less studied. Nevertheless, its abundance in tissues such as testis, endothelial cells, and particularly in brain, has pointed the relevance of HO-2 function. HO-2 presents particular characteristics that made it a unique protein in the HO system. Since attractive results on HO-2 have been arisen in later years, we focused this review in the second isoform. We summarize information on gene description, protein structure, and catalytic activity of HO-2 and particular facts such as its cellular impact and activity regulation. Finally, we call attention on the role of HO-2 in oxygen sensing, discussing proposed hypothesis on heme binding motifs and redox/thiol switches that participate in oxygen sensing as well as evidences of HO-2 response to hypoxia.

Oxygen sensing by the carotid body: is it all just rotten eggs?

SIGNIFICANCE

Ventilatory responses to hypoxia are initiated by the carotid body, where inhibition of specific K(+) channels causes cell depolarization, voltage-gated Ca(2+) influx, and neurotransmitter release. The identity of the upstream oxygen (O2) sensor is still controversial.

RECENT ADVANCES

The activity of BKCa channels is regulated by O2, carbon monoxide (CO), and hydrogen sulfide (H2S), suggesting that integration of these signals may be crucial to the physiological response of this tissue. BKCa is colocalized with hemeoxygenase-2, an enzyme that generates CO in the presence of O2, and CO is a BKCa channel opener. Reduced CO during hypoxia results in channel closure, conferring a degree of O2 sensitivity to the BKCa channel. Conversely, H2S is a potent BKCa inhibitor. H2S is produced endogenously by cystathionine-β-synthase and cystathionine-γ-lyase in the rat carotid body, and its intracellular concentration is dependent upon the balance between its enzymatic generation and its mitochondrial breakdown. During hypoxia, mitochondrial oxidation of H2S in many tissues is reduced, leading to hypoxia-evoked rises in its concentration. This may be sufficient to inhibit K(+) channels and lead to carotid body excitation.

CRITICAL ISSUES

Carotid body function is heavily dependent upon regulated production and breakdown of CO and H2S and integration of signals from these newly emerging gasotransmitters, in combination with several other proposed mechanisms, may refine, or even define, responses of this tissue to hypoxia.

FUTURE DIRECTIONS

Since several other sensors have been postulated, the challenge of future research is to begin to integrate each in a unifying mechanism, as has been attempted for the first time herein.

Mitochondrial but not plasmalemmal BK channels are hypoxia-sensitive in human glioma

Tumor cells are resistant to hypoxia but the underlying mechanism(s) of this tolerance remain poorly understood. In healthy brain cells, plasmalemmal Ca(2+)-activated K(+) channels ((plasma)BK) function as oxygen sensors and close under hypoxic conditions. Similarly, BK channels in the mitochondrial inner membrane ((mito)BK) are also hypoxia sensitive and regulate reactive oxygen species production and also permeability transition pore formation. Both channel populations are therefore well situated to mediate cellular responses to hypoxia. In tumors, BK channel expression increases with malignancy, suggesting these channels contribute to tumor growth; therefore, we hypothesized that the sensitivity of (plasma)BK and/or (mito)BK to hypoxia differs between glioma and healthy brain cells. To test this, we examined the electrophysiological properties of (plasma)BK and (mito)BK from a human glioma cell line during normoxia and hypoxia. We observed single channel activities in whole cells and isolated mitoplasts with slope conductance of 199 ± 8 and 278 ± 10 pA, respectively. These currents were Ca(2+)- and voltage-dependent, and were inhibited by the BK channel antagonist charybdotoxin (0.1 μM). (plasma)BK could only be activated at membrane potentials >+40 mV and had a low open probability (NPo) that was unchanged by hypoxia. Conversely, (mito)BK were active across a range of membrane potentials (-40 to +40 mV) and their NPo increased during hypoxia. Activating (plasma)BK, but not (mito)BK induced cell death and this effect was enhanced during hypoxia. We conclude that unlike in healthy brain cells, glioma (mito)BK channels, but not (plasma)BK channels are oxygen sensitive.

Oxygen sensitivity of mitochondrial function in rat arterial chemoreceptor cells

The mechanism of oxygen sensing in arterial chemoreceptors is unknown but has often been linked to mitochondrial function. A common criticism of this hypothesis is that mitochondrial function is insensitive to physiological levels of hypoxia. Here we investigate the effects of hypoxia (down to 0.5% O2) on mitochondrial function in neonatal rat type-1 cells. The oxygen sensitivity of mitochondrial [NADH] was assessed by monitoring autofluorescence and increased in hypoxia with a P50 of 15 mm Hg (1 mm Hg = 133.3 Pa) in normal Tyrode or 46 mm Hg in Ca(2+)-free Tyrode. Hypoxia also depolarised mitochondrial membrane potential (m, measured using rhodamine 123) with a P50 of 3.1, 3.3 and 2.8 mm Hg in normal Tyrode, Ca(2+)-free Tyrode and Tyrode containing the Ca(2+) channel antagonist Ni(2+), respectively. In the presence of oligomycin and low carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone (FCCP; 75 nm) m is maintained by electron transport working against an artificial proton leak. Under these conditions hypoxia depolarised m/inhibited electron transport with a P50 of 5.4 mm Hg. The effects of hypoxia upon cytochrome oxidase activity were investigated using rotenone, myxothiazol, antimycin A, oligomycin, ascorbate and the electron donor tetramethyl-p-phenylenediamine. Under these conditions m is maintained by complex IV activity alone. Hypoxia inhibited cytochrome oxidase activity (depolarised m) with a P50 of 2.6 mm Hg. In contrast hypoxia had little or no effect upon NADH (P50 = 0.3 mm Hg), electron transport or cytochrome oxidase activity in sympathetic neurons. In summary, type-1 cell mitochondria display extraordinary oxygen sensitivity commensurate with a role in oxygen sensing. The reasons for this highly unusual behaviour are as yet unexplained.

Sensing hypoxia: physiology, genetics and epigenetics

The carotid body is a sensory organ for detecting arterial blood O2 levels and reflexly mediates systemic cardiac, vascular and respiratory responses to hypoxia. This article presents a brief review of the roles of gaseous messengers in the sensory transduction at the carotid body, genetic and epigenetic influences on hypoxic sensing and the role of the carotid body chemoreflex in cardiorespiratory diseases. Type I (also called glomus) cells, the site of O2 sensing in the carotid body, express haem oxygenase-2 and cystathionine-γ-lyase, the enzymes which catalyse the generation of CO and H2S, respectively. Physiological studies have shown that CO is an inhibitory gas messenger, which contributes to the low sensory activity during normoxia, whereas H2S is excitatory and mediates sensory stimulation by hypoxia. Hypoxia-evoked H2S generation in the carotid body requires the interaction of cystathionine-γ-lyase with haem oxygenase-2, which generates CO. Hypoxia-inducible factors 1 and 2 constitute important components of the genetic make-up in the carotid body, which influence hypoxic sensing by regulating the intracellular redox state via transcriptional regulation of pro- and antioxidant enzymes. Recent studies suggest that developmental programming of the carotid body response to hypoxia involves epigenetic changes, e.g. DNA methylation of genes encoding redox-regulating enzymes. Emerging evidence implicates heightened carotid body chemoreflex in the progression of autonomic morbidities associated with cardiorespiratory diseases, such as sleep-disordered breathing with apnoea, congestive heart failure and essential hypertension.

Oxygen-sensitive reduction in Ca²⁺-activated K⁺ channel open probability in turtle cerebrocortex

In response to low ambient oxygen levels the western painted turtle brain undergoes a large depression in metabolic rate which includes a decrease in neuronal action potential frequency. This involves the arrest of N-methyl-D-aspartate receptor (NMDAR) and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPAR) currents and paradoxically an increase in γ-aminobutyric acid receptor (GABAR) currents in turtle cortical neurons. In a search for other oxygen-sensitive channels we discovered a Ca(2+)-activated K(+) channel (K(Ca)) that exhibited a decrease in open time in response to anoxia. Single-channel recordings of K(Ca) activity were obtained in cell-attached and excised inside-out patch configurations from neurons in cortical brain sheets bathed in either normoxic or anoxic artificial cerebrospinal fluid (aCSF). The channel has a slope conductance of 223pS, is activated in response to membrane depolarization, and is controlled in a reversible manner by free [Ca(2+)] at the intracellular membrane surface. In the excised patch configuration anoxia had no effect on K(Ca) channel open probability (P(open)); however, in cell-attached mode, there was a reversible fivefold reduction in P(open) (from 0.5 ± 0.05 to 0.1 ± 0.03) in response to 30-min anoxia. The inclusion of the potent protein kinase C (PKC) inhibitor chelerythrine prevented the anoxia-mediated decrease in P(open) while drip application of a phorbol ester PKC activator decreased P(open) during normoxia (from normoxic 0.4 ± 0.05 to phorbol-12-myristate-13-acetate (PMA) 0.1 ± 0.02). Anoxia results in a slight depolarization of turtle pyramidal neurons (∼8 mV) and an increase in cytosolic [Ca(2+)]; therefore, K(Ca) arrest is likely important to prevent Ca(2+) activation during anoxia and to reduce the energetic cost of maintaining ion gradients. We conclude that turtle pyramidal cell Ca(2+)-activated K(+) channels are oxygen-sensitive channels regulated by cytosolic factors and are likely the reptilian analog of the mammalian large conductance Ca(2+)-activated K(+) channels (BK channels).

Modulation of K2P3.1 (TASK-1), K2P9.1 (TASK-3), and TASK-1/3 heteromer by reactive oxygen species

Reactive oxygen species (ROS) generated by mitochondria or NADPH oxidase have been implicated in the inhibition of K(+) current by hypoxia in chemoreceptor cells. As TASKs are highly active background K(+) channels in these cells, we studied the role of ROS in hypoxia-induced inhibition of TASKs. In HeLa cells expressing TASKs, H(2)O(2) applied to inside-out patches activated TASK-1, TASK-3, and TASK-1/3 heteromer starting at ~16 mM. When applied to cell-attached or outside-out patches, 326 mM H(2)O(2) did not affect TASK activity. Other K(2P) channels (TREK-1, TREK-2, TASK-2, TALK-1, TRESK) were not affected by H(2)O(2) (tested up to 326 mM). A reducing agent (dithiothreitol) and a cysteine-modifying agent (2-aminoethyl methanethiosulfonate hydrobromide) had no effect on basal TASK activity and did not block the H(2)O(2)-induced increase in channel activity. A TASK mutant in which the C-terminus of TASK-3 was replaced with that of TREK-2 showed a normal sensitivity to H(2)O(2). Xanthine/xanthine oxidase mixture used to generate superoxide radical showed no effect on TASK-1, TASK-3, and TASK-1/3 heteromer from either side of the membrane, but it strongly activated TASK-2 from the extracellular side. Acute H(2)O(2) (32-326 mM) exposure did not affect hSlo1/b1(BK) expressed in HeLa cells and BK in carotid body glomus cells. In carotid body glomus cells, adrenal cortical cells, and cerebellar granule neurons that show abundant hypoxia-sensitive TASK activity, H(2)O(2) (>16 mM) activated the channels only when applied intracellularly, similar to that observed with cloned TASKs. These findings show that ROS do not support or inhibit TASK and BK activity and therefore are unlikely to be the hypoxic signal that causes cell excitation via inhibition of these K(+) channels.

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.

Carbon monoxide (CO) and hydrogen sulfide (H(2)S) in hypoxic sensing by the carotid body

Carotid bodies are sensory organs for monitoring arterial blood oxygen (O(2)) levels, and the ensuing reflexes maintain cardio-respiratory homeostasis during hypoxia. This article provides a brief update of the role of carbon monoxide (CO) and hydrogen sulfide (H(2)S) in hypoxic sensing by the carotid body. Glomus cells, the primary site of O(2) sensing in the carotid body express heme oxygenase-2 (HO-2), a CO catalyzing enzyme. HO-2 is a heme containing enzyme and has high affinity for O(2). Hypoxia inhibits HO-2 activity and reduces CO generation. Pharmacological and genetic approaches suggest that CO inhibits carotid body sensory activity. Stimulation of carotid body activity by hypoxia may reflect reduced formation of CO. Glomus cells also express cystathionine γ-lyase (CSE), an H(2)S generating enzyme. Exogenous application of H(2)S donors, like hypoxia, stimulate the carotid body activity and CSE knockout mice exhibit severely impaired sensory excitation by hypoxia, suggesting that CSE catalyzed H(2)S is an excitatory gas messenger. Hypoxia increases H(2)S generation in the carotid body, and this response was attenuated or absent in CSE knockout mice. HO inhibitor increased and CO donor inhibited H(2)S generation. It is proposed that carotid body response to hypoxia requires interactions between HO-2-CO and CSE-H(2)S systems.

Contribution of BK(Ca) channels of neurons in rostral ventrolateral medulla to CO-mediated central regulation of respiratory rhythm in medullary slices of neonatal rats

We recently described that carbon monoxide (CO) participated in the regulation of rhythmic respiration in medullary slices. The present study was undertaken to further assess whether the large-conductance calcium-activated potassium channels (BK(Ca) channels) are involved in the CO-mediated central regulation of respiratory rhythm in medullary slices. The rhythmic discharge of hypoglossal rootlets of medullary slices of neonatal rats was recorded. We observed that blocking BK(Ca) channels could partially abolish the effects of CO on the rhythmic bursts of hypoglossal rootlets. With whole-cell patch-clamp recording technique, we further observed that CO could reversibly augment potassium current density of the neurons in the rostral ventrolateral medulla. The CO-induced increase in potassium current was entirely blocked by the pretreatment of slices with BK(Ca) channels blocker; whereas blockade of CO generation with zinc protoporphyrin-IX produced an opposite response. Altogether, these data indicate that BK(Ca) channels of the neurons in neonatal rostral ventrolateral medulla could be activated by CO and involved in CO-mediated central regulation of respiratory rhythm in medullary slices.

The carotid body in Sudden Infant Death Syndrome

The aim of the present study is to provide a review of cytochemical, clinical and experimental data indicating disruption of perinatal carotid body maturation as one of the possible mechanisms underlying SIDS pathogenesis. SIDS victims have been reported to show alterations in respiratory regulation which may partly be ascribed to peripheral arterial chemoreceptors. Carotid body findings in SIDS victims, although not entirely confirmed by other authors, have included reductions in glomic tissue volume and cytoplamic granules of type I cells, changes in cytological composition (higher percentages of progenitor and type II cells) and increases in dopamine and noradrenaline contents. Prematurity and environmental factors, such as exposure to tobacco smoke, substances of abuse, hyperoxia and continuous or intermittent hypoxia, increase the risk of SIDS and are known to affect carotid body functional and structural maturation adversely, supporting a role for peripheral arterial chemoreceptors in SIDS.

Autocrine and paracrine actions of ATP in rat carotid body

Carotid bodies are peripheral chemoreceptors that detect lowering of arterial blood O(2) level. The carotid body comprises clusters of glomus (type I) cells surrounded by glial-like sustentacular (type II) cells. Hypoxia triggers depolarization and cytosolic [Ca(2+)] ([Ca(2+)](i)) elevation in glomus cells, resulting in the release of multiple transmitters, including ATP. While ATP has been shown to be an important excitatory transmitter in the stimulation of carotid sinus nerve, there is considerable evidence that ATP exerts autocrine and paracrine actions in carotid body. ATP acting via P2Y(1) receptors, causes hyperpolarization in glomus cells and inhibits the hypoxia-mediated [Ca(2+)](i) rise. In contrast, adenosine (an ATP metabolite) triggers depolarization and [Ca(2+)](i) rise in glomus cells via A(2A) receptors. We suggest that during prolonged hypoxia, the negative and positive feedback actions of ATP and adenosine may result in an oscillatory Ca(2+) signal in glomus cells. Such mechanisms may allow cyclic release of transmitters from glomus cells during prolonged hypoxia without causing cellular damage from a persistent [Ca(2+)](i) rise. ATP also stimulates intracellular Ca(2+) release in sustentacular cells via P2Y(2) receptors. The autocine and paracrine actions of ATP suggest that ATP has important roles in coordinating chemosensory transmission in the carotid body.

Therapeutic potential of heme oxygenase-1/carbon monoxide in lung disease

Heme oxygenase (HO), a catabolic enzyme, provides the rate-limiting step in the oxidative breakdown of heme, to generate carbon monoxide (CO), iron, and biliverdin-IXα. Induction of the inducible form, HO-1, in tissues is generally regarded as a protective mechanism. Over the last decade, considerable progress has been made in defining the therapeutic potential of HO-1 in a number of preclinical models of lung tissue injury and disease. Likewise, tissue-protective effects of CO, when applied at low concentration, have been observed in many of these models. Recent studies have expanded this concept to include chemical CO-releasing molecules (CORMs). Collectively, salutary effects of the HO-1/CO system have been demonstrated in lung inflammation/acute lung injury, lung and vascular transplantation, sepsis, and pulmonary hypertension models. The beneficial effects of HO-1/CO are conveyed in part through the inhibition or modulation of inflammatory, apoptotic, and proliferative processes. Recent advances, however, suggest that the regulation of autophagy and the preservation of mitochondrial homeostasis may serve as additional candidate mechanisms. Further preclinical and clinical trials are needed to ascertain the therapeutic potential of HO-1/CO in human clinical disease.

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.

Carbon monoxide: an emerging regulator of ion channels

Carbon monoxide is rapidly emerging as an important cellular messenger, regulating a wide range of physiological processes. Crucial to its role in both physiology and disease is its ability differentially to regulate several classes of ion channels, including examples from calcium-activated K(+) (BK(Ca)), voltage-activated K(+) (K(v)) and Ca(2+) channel (L-type) families, ligand-gated P2X receptors (P2X2 and P2X4), tandem P domain K(+) channels (TREK1) and the epithelial Na(+) channel (ENaC). The mechanisms by which CO regulates these ion channels are still unclear and remain somewhat controversial. However, available structure-function studies suggest that a limited range of amino acid residues confer CO sensitivity, either directly or indirectly, to particular ion channels and that cellular redox state appears to be important to the final integrated response. Whatever the molecular mechanism by which CO regulates ion channels, endogenous production of this gasotransmitter has physiologically important roles and is currently being explored as a potential therapeutic.

Reciprocal regulation of Ca²+-activated outward K+ channels of Pyrus pyrifolia pollen by heme and carbon monoxide

• The regulation of plant potassium (K+) channels has been extensively studied in various systems. However, the mechanism of their regulation in the pollen tube is unclear. • In this study, the effects of heme and carbon monoxide (CO) on the outward K+ (K+(out)) channel in pear (Pyrus pyrifolia) pollen tube protoplasts were characterized using a patch-clamp technique. • Heme (1 μM) decreased the probability of K+(out) channel opening without affecting the unitary conductance, but this inhibition disappeared when heme was co-applied with 10 μM intracellular free Ca²+. Conversely, exposure to heme in the presence of NADPH increased channel activity. However, with tin protoporphyrin IX treatment, which inhibits hemeoxygenase activity, the inhibition of the K+(out) channel by heme occurred even in the presence of NADPH. CO, a product of heme catabolism by hemeoxygenase, activates the K+(out) channel in pollen tube protoplasts in a dose-dependent manner. The current induced by CO was inhibited by the K+ channel inhibitor tetraethylammonium. • These data indicate a role of heme and CO in reciprocal regulation of the K+(out) channel in pear pollen tubes.

A revisit to O2 sensing and transduction in the carotid body chemoreceptors in the context of reactive oxygen species biology

Oxygen-sensing and transduction in purposeful responses in cells and organisms is of great physiological and medical interest. All animals, including humans, encounter in their lifespan many situations in which oxygen availability might be insufficient, whether acutely or chronically, physiologically or pathologically. Therefore to trace at the molecular level the sequence of events or steps connecting the oxygen deficit with the cell responses is of interest in itself as an achievement of science. In addition, it is also of great medical interest as such knowledge might facilitate the therapeutical approach to patients and to design strategies to minimize hypoxic damage. In our article we define the concepts of sensors and transducers, the steps of the hypoxic transduction cascade in the carotid body chemoreceptor cells and also discuss current models of oxygen- sensing (bioenergetic, biosynthetic and conformational) with their supportive and unsupportive data from updated literature. We envision oxygen-sensing in carotid body chemoreceptor cells as a process initiated at the level of plasma membrane and performed by a hemoprotein, which might be NOX4 or a hemoprotein not yet chemically identified. Upon oxygen-desaturation, the sensor would experience conformational changes allosterically transmitted to oxygen regulated K+ channels, the initial effectors in the transduction cascade. A decrease in their opening probability would produce cell depolarization, activation of voltage dependent calcium channels and release of neurotransmitters. Neurotransmitters would activate the nerve endings of the carotid body sensory nerve to convey the information of the hypoxic situation to the central nervous system that would command ventilation to fight hypoxia.

Mechanism of inhibition by hydrogen sulfide of native and recombinant BKCa channels

Recent evidence suggests that H(2)S contributes to activation of the carotid body by hypoxia by inhibiting K(+) channels. Here, we determine both the molecular identity of the K(+) channel target within the carotid body and the biophysical characteristics of the H(2)S-evoked inhibition by analyzing native rat and human recombinant BK(Ca) channel activity in voltage-clamped, inside-out membrane patches. Rat glomus cells express the enzymes necessary for the endogenous generation of H(2)S, cystathionine-beta-synthase and cystathionine-gamma-lyase. H(2)S inhibits native carotid body and human recombinant BK(Ca) channels with IC(50) values of around 275 microM. Inhibition by H(2)S is rapid and reversible, works by a mechanism which is distinct from that suggested for CO gas regulation of this channel and does not involve an interaction with either the "Ca bowl" or residues distal to this Ca(2+)-sensing domain. These data show that BK(Ca) is a K(+) channel target of H(2)S, and suggest a mechanism to explain the H(2)S-dependent component of O(2) sensing in the carotid body.

A crucial role for hydrogen sulfide in oxygen sensing via modulating large conductance calcium-activated potassium channels

Hydrogen sulfide (H(2)S) is an important signaling molecule produced from L-cysteine by cystathionine beta-synthetase (CBS) or cystathionine gamma-lyase (CSE). Here we examined the role of H(2)S in the oxygen-sensing function of the carotid body chemoreceptors, where the large conductance Ca(2+)-activated potassium channel (BK(Ca)) plays a key role. In the isolated mouse carotid body/sinus nerve preparations, the H(2)S donor, NaHS, excited the chemoreceptor afferent nerves in a concentration-dependent manner that was reversed by carbon monoxide donor. The NaHS-evoked excitation was abolished by removing extracellular Ca(2+), or using Cd(2+), pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid and hexomethonium, suggesting that H(2)S evokes release of ATP/ACh from type I glomus cells of the carotid body. The chemoreceptor afferent activation by hypoxia was decreased remarkably using CBS inhibitors, amino oxyacetic acid (AOAA) and hydroxylamine, but not CSE inhibitors, propargylglycine and beta-cyano-L-alanine, despite expression of both enzymes in type I glomus cells. In these cells, the BK(Ca) currents were inhibited by hypoxia and such inhibition was mimicked by NaHS and diminished by AOAA. Finally, mice hyperventilated in response to hypoxia, which was prevented by CBS inhibitors. These data suggest that H(2)S plays a crucial role in mediating the response of carotid body chemoreceptors to hypoxia via modulating the BK(Ca) channels.