A novel oxygen-sensitive potassium current in rat carotid body type I cells

Carotid chemoreceptor "resetting" revisited

Carotid body (CB) chemoreceptors transduce low arterial O(2) tension into increased action potential activity on the carotid sinus nerves, which contributes to resting ventilatory drive, increased ventilatory drive in response to hypoxia, arousal responses to hypoxia during sleep, upper airway muscle activity, blood pressure control and sympathetic tone. Their sensitivity to O(2) is low in the newborn and increases during the days or weeks after birth to reach adult levels. This postnatal functional maturation of the CB O(2) response has been termed "resetting" and it occurs in every mammalian species studied to date. The O(2) environment appears to play a key role; the fetus develops in a low O(2) environment throughout gestation and initiation of CB "resetting" after birth is modulated by the large increase in arterial oxygen tension occurring at birth. Although numerous studies have reported age-related changes in various components of the O(2) transduction cascade, how the O(2) environment shapes normal CB prenatal development and postnatal "resetting" remains unknown. Viewing CB "resetting" as environment-driven (developmental) phenotypic plasticity raises important mechanistic questions that have received little attention. This review examines what is known (and not known) about mechanisms of CB functional maturation, with a focus on the role of the O(2) environment.

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.

Neuroepithelial cells of the gill and their role in oxygen sensing

A highly sensitive oxygen (O(2)) sensing mechanism is critical for the survival of all vertebrate species. In fish, this requirement is fullfilled by the neuroepithelial cells (NECs) of the gill. NECs are neurotransmitter-containing chemosensory cells that are diffusely distributed within a thin epithelial layer of the filaments and respiratory lamellae of all gill arches, and are innervated by afferent fibers from the central nervous system. In acute cell culture, NECs respond immediately, and in a dose-dependent manner, to acute changes in O(2) tension. Thus, hypoxic stimulation of gill NECs appears to initiate the production of adaptive, cardiorespiratory reflexes that contribute to the maintenance of O(2) uptake in order to meet metabolic demands. This review covers the current evidence for the status of NECs as the primary peripheral O(2) sensors in fish. We have included an overview of the phylogeny of O(2) sensing structures among vertebrate groups, and morphological and physiological evidence for the importance of NECs in O(2) sensing.

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.

Effects of exogenous hydrogen sulphide on calcium signalling, background (TASK) K channel activity and mitochondrial function in chemoreceptor cells

It has been proposed that endogenous H₂S mediates oxygen sensing in chemoreceptors; this study investigates the mechanisms by which H₂S excites carotid body type 1 cells. H₂S caused a rapid reversible increase in intracellular calcium with EC₅₀ ≈ 6 μM. This [Ca²⁺]_i response was abolished in Ca-free Tyrode. In perforated patch current clamp recordings, H₂S depolarised type 1 cells from −59 to −35 mV; this was accompanied by a robust increase in [Ca²⁺]_i. Voltage clamping at the resting membrane potential abolished the H₂S-induced rise in [Ca²⁺]_i. H₂S inhibited background K⁺ current in whole cell perforated patch and reduced background K⁺ channel activity in cell-attached patch recordings. It is concluded that H₂S excites type 1 cells through the inhibition of background (TASK) potassium channels leading to membrane depolarisation and voltage-gated Ca²⁺ entry. These effects mimic those of hypoxia. H₂S also inhibited mitochondrial function over a similar concentration range as assessed by NADH autofluorescence and measurement of intracellular magnesium (an index of decline in MgATP). Cyanide inhibited background K channels to a similar extent to H₂S and prevented H₂S exerting any further influence over channel activity. These data indicate that the effects of H₂S on background K channels are a consequence of inhibition of oxidative phosphorylation. Whilst this does not preclude a role for endogenous H₂S in oxygen sensing via the inhibition of cytochrome oxidase, the levels of H₂S required raise questions as to the viability of such a mechanism.

A Role for K2P Channels in the Operation of Somatosensory Nociceptors

The ability to sense mechanical, thermal, and chemical stimuli is critical to normal physiology and the perception of pain. Contact with noxious stimuli triggers a complex series of events that initiate innate protective mechanisms designed to minimize or avoid injury. Extreme temperatures, mechanical stress, and chemical irritants are detected by specific ion channels and receptors clustered on the terminals of nociceptive sensory nerve fibers and transduced into electrical information. Propagation of these signals, from distant sites in the body to the spinal cord and the higher processing centers of the brain, is also orchestrated by distinct groups of ion channels. Since their identification in 1995, evidence has emerged to support roles for K2P channels at each step along this pathway, as receptors for physiological and noxious stimuli, and as determinants of nociceptor excitability and conductivity. In addition, the many subtypes of K2P channels expressed in somatosensory neurons are also implicated in mediating the effects of volatile, general anesthetics on the central and peripheral nervous systems. Here, I offer a critical review of the existing data supporting these attributes of K2P channel function and discuss how diverse regulatory mechanisms that control the activity of K2P channels act to govern the operation of nociceptors.

Postnatal hyperoxia impairs acute oxygen sensing of rat glomus cells by reduced membrane depolarization

Previous work demonstrated that hyperoxia (30-60% O(2)) exposure in the post-natal period reduces the ventilatory response to acute hypoxia and this impairment may continue considerably beyond the period of hyperoxia exposure. Previous work from our laboratory demonstrated that 1-2 weeks of hyperoxia (60% O(2)) starting between P1 and P14: reduced the single chemoreceptor unit response to hypoxia, reduced the rise in glomus cell calcium caused by acute hypoxia and reduced hypoxia-induced catecholamine release (Donnelly 05, Donnelly 09). The present study asked whether the impairment extended to hypoxia-induced membrane depolarization, an earlier step in the transduction cascade. Perforated patch, whole-cell recordings were obtained from rat glomus cells exposed to hyperoxia from P0-P8 or P8-P15 and age-matched control groups. In both cases, hypoxia-induced membrane depolarization was significantly less in the hyperoxia treated groups compared to controls, while depolarization to 20 mM K(+) was not significantly affected. Resting membrane potential and input resistance were also not different in the hyperoxia treated groups. Whole carotid body quantitative real time PCR showed that TASK-1, TASK-3 and L-type Ca(2+) channel expression was significantly down-regulated at Hyper 8-15 compared to controls. We conclude that 1 week of postnatal hyperoxia during the early and late stage of CB maturation impairs organ function by affecting the coupling between hypoxia and glomus cell depolarization. This may be caused by altered expression of TASK1, TASK3 or L-type Ca(2+) channel gene expression. We speculate that an identification of cellular changes caused by hyperoxia may yield unique insights to the mechanism of oxygen sensing by the carotid bodies.

Ion channel regulation by the LKB1-AMPK signalling pathway: the key to carotid body activation by hypoxia and metabolic homeostasis at the whole body level

Our recent investigations provide further support for the proposal that, consequent to inhibition of mitochondrial oxidative phosphorylation, activation of AMP-activated protein kinase (AMPK) mediates carotid body excitation by hypoxia. Consistent with the effects of hypoxia, intracellular dialysis from a patch pipette of an active (thiophosphorylated) recombinant AMPK heterotrimer (α2β2γ1) or application of the AMPK activators AICAR and A769662: (1) Inhibited BK(Ca) currents and TASK K(+) currents in rat carotid body type I cells; (2) Inhibited whole-cell currents carried by KCa1.1 and TASK3, but not TASK1 channels expressed in HEK293 cells; (3) Triggered carotid body activation. Furthermore, preliminary studies using mice with conditional knockout in type I cells of the primary upstream kinase that activates AMPK in response to metabolic stresses, LKB1, appear to confirm our working hypothesis. Studies on mice with knockout of the catalytic α1 subunit and α2 subunits of AMPK, respectively, have proved equally consistent. Accumulating evidence therefore suggests that the LKB1-AMPK signalling pathway is necessary for hypoxia-response coupling by the carotid body, and serves to regulate oxygen and therefore energy supply at the whole body level.

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.

Ca2+ homeostasis and exocytosis in carotid glomus cells: role of mitochondria

In oxygen sensing carotid glomus (type 1) cells, the hypoxia-triggered depolarization can be mimicked by mitochondrial inhibitors. We examined the possibility that, other than causing glomus cell depolarization, mitochondrial inhibition can regulate transmitter release via changes in Ca(2+) dynamics. Under whole-cell voltage clamp conditions, application of the mitochondrial inhibitors, carbonyl cyanide m-chlorophenylhydrazone (CCCP) or cyanide caused a dramatic slowing in the decay of the depolarization-triggered Ca(2+) signal in glomus cells. In contrast, inhibition of the Na(+)/Ca(2+) exchanger (NCX), plasma membrane Ca(2+)-ATPase (PMCA) pump or sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) pump had much smaller effects. Consistent with the notion that mitochondrial Ca(2+) uptake is the dominant mechanism in cytosolic Ca(2+) removal, inhibition of the mitochondrial uniporter with ruthenium red slowed the decay of the depolarization-triggered Ca(2+) signal. Hypoxia also slowed cytosolic Ca(2+) removal, suggesting a partial impairment of mitochondrial Ca(2+) uptake. Using membrane capacitance measurement, we found that the increase in the duration of the depolarization-triggered Ca(2+) signal after mitochondrial inhibition was associated with an enhancement of the exocytotic response. The role of mitochondria in the regulation of Ca(2+) signal and transmitter release from glomus cells highlights the importance of mitochondria in hypoxic chemotransduction in the carotid bodies.

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.

Changes in oxygen sensitivity of TASK in carotid body glomus cells during early postnatal development

A post-natal increase in carotid body (CB) hypoxia responsiveness occurs at the level of carotid sinus nerve activity, intracellular calcium, cell membrane depolarization and hypoxic inhibition of O(2)-sensitive background K(+) conductance. TASK-1, TASK-1/3 and TASK-3 are functionally expressed in CB glomus cells, with TASK-1/3 providing the major part of the O(2)-sensitive TASK-like background K(+) conductance. Here we report the effects of graded hypoxia on TASK-like channel activity in CB glomus cells from rats aged 0 to 1, 6 to 7 and 16 to 18 days; the time frame of postnatal CB functional maturation. TASK was active in nearly all cell-attached patches and TASK activity during normoxia did not differ across ages. Hypoxia produced a progressive decrease in channel opening frequency with graded decreases in O(2) level and also produced glomus cell depolarization, as assessed by the shift in reversal potential of TASK single channel current. Hypoxic inhibition of TASK activity was least at P0-P1 and increased with age mainly between 6-7 and 16-18 days. The O(2)-sensitive TASK activity was significantly greater in glomus cells from P16 to P18 when compared to cells from P0 to P1 day old rats. These results support the hypothesis that postnatal carotid body functional maturation is due, at least in part, to changes in the sensitivity of TASK to the hypoxic signals generated in glomus cells.

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.

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.

Mechanisms for acute oxygen sensing in the carotid body

Hypoxic chemotransduction in the carotid body requires release of excitatory transmitters from type I cells that activate afferent sensory neurones. Transmitter release is dependent on voltage-gated Ca2+ entry which is evoked by membrane depolarization. This excitatory response to hypoxia is initiated by inhibition of specific O2 sensitive K+ channels, of which several types have been reported. Here, we discuss mechanisms which have been put forward to account for hypoxic inhibition of type I cell K+ channels. Whilst evidence indicates that one O2 sensitive K+ channel, BKCa, may be regulated by gasotransmitters (CO and H2S) in an O2-dependent manner, other studies now indicate that activation of AMP-activated protein kinase (AMPK) accounts for inhibition of both BKCa and 'leak' O2 sensitive K+ channels, and perhaps also other O2 sensitive K+ channels reported in different species. We propose that type I cell AMPK activation occurs as a result of inhibition of mitochondrial oxidative phosphorylation, and does not require increased production of reactive oxygen species. Thus, AMPK activation provides the basis for unifying the 'membrane' and 'mitochondrial' hypotheses, previously regarded as disparate, to account for hypoxic chemotransduction.

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

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

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.

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.

Zebrafish (Danio rerio) gill neuroepithelial cells are sensitive chemoreceptors for environmental CO2

Adult zebrafish exhibit hyperventilatory responses to absolute environmental CO(2) levels as low as 0.13% ( mmHg), more than an order of magnitude lower than the typical arterial levels (40 mmHg) monitored by the mammalian carotid body. The sensory basis underlying the ability of fish to detect and respond to low ambient CO(2) levels is not clear. Here, we show that the neuroepithelial cells (NECs) of the zebrafish gill, known to sense O(2) levels, also respond to low levels of CO(2). An electrophysiological characterization of this response using both current and voltage clamp protocols revealed that for increasing CO(2) levels, a background K(+) channel was inhibited, resulting in a partial pressure-dependent depolarization of the NEC. To elucidate the signalling pathway underlying K(+) channel inhibition, we used immunocytochemistry to show that these NECs express carbonic anhydrase (CA), an enzyme involved in CO(2) sensing in the mammalian carotid body. Further, the NEC response to CO(2) (magnitude of membrane depolarization and time required to achieve maximal response), under conditions of constant pH, was reduced by 50% by the CA-inhibitor acetazolamide. This suggests that the CO(2) detection mechanism involves an intracellular sensor that is responsive to the rate of acidification associated with the hydration of CO(2) and which does not require a change of extracellular pH. Because some cells that were responsive to increasing also responded to hypoxia with membrane depolarization, the present results demonstrate that a subset of the NECs in the zebrafish gill are bimodal sensors of CO(2) and O(2).

Two-pore domain k(+) channels and their role in chemoreception

A number of tandem P-domain K(+)- channels (K(2)P) generate background K(+)-currents similar to those found in enteroreceptors that sense a diverse range of physiological stimuli including blood pH, carbon dioxide, oxygen, potassium and glucose. This review presents an overview of the properties of both cloned K(2)P tandem-P-domain K-channels and the endogenous chemosensitive background K-currents found in central chemoreceptors, peripheral chemoreceptors, the adrenal gland and the hypothalamus. Although the identity of many of these endogenous channels has yet to be confirmed they show striking similarities to a number of K(2)P channels especially those of the TASK subgroup. Moreover these channels seem often (albeit not exclusively) to be involved in pH and nutrient/metabolic sensing.

The role of ion channels in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is an important mechanism by which localized flow of blood in small resistance pulmonary arteries is matched to alveolar ventilation. This chapter discusses the role of several potassium and calcium channels in HPV, both in enhancing calcium influx into smooth muscle cells (SMCs) and in stimulating the release of calcium from the sarcoplasmic reticulum, thus increasing cytosolic calcium. The increase in calcium sensitivity caused by hypoxia is reviewed in Chapter 19. Particular attention is paid to the activity of the L-type calcium channels which increase calcium influx as a result of membrane depolarization and also increase calcium influx at any given membrane potential in response to hypoxia. In addition, activation of the L-type calcium channel may, in the absence of any calcium influx, cause calcium release from the sarcoplasmic reticulum. Many of these mechanisms have been reported to be involved in both HPV and in normoxic contraction of the ductus arteriosus.

Developmental maturation of chemosensitivity to hypoxia of peripheral arterial chemoreceptors--invited article

Peripheral arterial chemoreceptors, particularly the carotid body chemoreceptors, are the primary sites for the detection of hypoxia and reflexly increase ventilatory drive and behavioral arousal during hypoxic or asphyxial events. Newborn infants are at risk for hypoxic and asphyxial events during sleep, yet, the strength of the chemoreceptor responses is low or absent at birth and then progressively increases with early postnatal development. This review summarizes the available data showing that even though the "oxygen sensor" in the glomus cells has not been unequivocally identified, it is clear that development affects many of the other properties of the chemoreceptor unit (glomus cell, afferent nerve fibers and neurotransmitter profile at the synapse) that are necessary and essential for the propagation of the "sensing" response, and exposure to hypoxia, hyperoxia and nicotine can modify normal development of each of the components leading to altered peripheral chemoreceptor responses.

Modulation of O(2) sensitive K (+) channels by AMP-activated protein kinase

Hypoxic inhibition of K(+) channels in type I cells is believed to be of central importance in carotid body chemotransduction. We have recently suggested that hypoxic channel inhibition is mediated by AMP-activated protein kinase (AMPK). Here, we have further explored the modulation by AMPK of recombinant K(+) channels (expressed in HEK293 cells) whose native counterparts are considered O(2)-sensitive in the rat carotid body. Inhibition of maxiK channels by AMPK activation with AICAR was found to be independent of [Ca(2+)](i) and occurred regardless of whether the alpha subunit was co-expressed with an auxiliary beta subunit. All effects of AICAR were fully reversed by the AMPK inhibitor compound C. MaxiK channels were also inhibited by the novel AMPK activator A-769662 and by intracellular dialysis with the constitutively active, truncated AMPK mutant, T172D. The molecular identity of the O(2)-sensitive leak K(+) conductance in rat type I cells remains unclear, but shares similarities with TASK-1 and TASK-3. Recombinant TASK-1 was insensitive to AICAR. However, TASK-3 was inhibited by either AICAR or A-769662 in a manner which was reversed by compound C. These data highlight a role for AMPK in the modulation of two proposed O(2) sensitive K(+) channels found in the carotid body.

MaxiK potassium channels in the function of chemoreceptor cells of the rat carotid body

Hypoxia activates chemoreceptor cells of the carotid body (CB) promoting an increase in their normoxic release of neurotransmitters. Catecholamine (CA) release rate parallels the intensity of hypoxia. Coupling of hypoxia to CA release requires cell depolarization, produced by inhibition of O(2)-regulated K(+) channels, and Ca(2+) entering the cells via voltage-operated channels. In rat chemoreceptor cells hypoxia inhibits large-conductance, calcium-sensitive K channels (maxiK) and a two-pore domain weakly inward rectifying K(+) channel (TWIK)-like acid-sensitive K(+) channel (TASK)-like channel, but the significance of maxiK is controversial. A proposal envisions maxiK contributing to set the membrane potential (E(m)) and the hypoxic response, but the proposal is denied by authors finding that maxiK inhibition does not depolarize chemoreceptor cells or alters intracellular Ca(2+) concentration or CA release in normoxia or hypoxia. We found that maxiK channel blockers (tetraethylammonium and iberiotoxin) did not modify CA release in rat chemoreceptor cells, in either normoxia or hypoxia, and iberiotoxin did not alter the Ca(2+) transients elicited by hypoxia. On the contrary, both maxiK blockers increased the responses elicited by dinitrophenol, a stimulus we demonstrate does not affect maxiK channels in isolated patches of rat chemoreceptor cells. We conclude that in rat chemoreceptor cells maxiK channels do not contribute to the genesis of the E(m), and that their full inhibition by hypoxia, preclude further inhibition by maxiK channel blockers. We suggest that full inhibition of this channel is required to generate the spiking behavior of the cells in acute hypoxia.

Time course of alterations in pre- and post-synaptic chemoreceptor function during developmental hyperoxia

Postnatal hyperoxia exposure reduces the carotid body response to acute hypoxia and produces a long-lasting impairment of the ventilatory response to hypoxia. The present work investigated the time course of pre- and post-synaptic alterations following exposure to hyperoxia (Fl(O2) = 0.6) for 1, 3, 5, 8 and 14 days (d) starting at postnatal day 7 (P7) as compared to age-matched controls. Hyperoxia exposure for 1d enhanced the nerve response and glomus cell calcium response to acute hypoxia, but exposure for 3-5d caused a significant reduction in both. Hypoxia-induced catecholamine release and nerve conduction velocity were significantly decreased by 5d hyperoxia. We conclude that hyperoxia exerts pre-synaptic (glomus cell calcium and secretory responses) and post-synaptic (afferent nerve excitability) actions to initially enhance and then reduce the chemoreceptor response to acute hypoxia. The parallel changes in glomus cell calcium response and nerve response suggest causality between the two and that environmental hyperoxia can affect the coupling between acute hypoxia and glomus cell calcium regulation.

Heteromeric TASK-1/TASK-3 is the major oxygen-sensitive background K+ channel in rat carotid body glomus cells

Carotid body (CB) glomus cells from rat express a TASK-like background K+ channel that is believed to play a critical role in the regulation of excitability and hypoxia-induced increase in respiration. Here we studied the kinetic behaviour of single channel openings from rat CB cells to determine the molecular identity of the 'TASK-like' K+ channels. In outside-out patches, the TASK-like background K+ channel in CB cells was inhibited >90% by a reduction of pH(o) from 7.3 to 5.8. In cell-attached patches with 140 mM KCl and 1 mM Mg2+ in the bath and pipette solutions, two main open levels with conductance levels of approximately 14 pS and approximately 32 pS were recorded at a membrane potential of -60 mV. The K+ channels showed kinetic properties similar to TASK-1 (approximately 14 pS), TASK-3 (approximately 32 pS) and TASK-1/3 heteromer (approximately 32 pS). The presence of three TASK isoforms was tested by reducing [Mg2+](o) to approximately 0 mM, which had no effect on the conductance of TASK-1, but increased those of TASK-1/3 and TASK-3 to 42 pS and 74 pS, respectively. In CB cells, the reduction of [Mg2+](o) to approximately 0 mM also caused the appearance of approximately 42 pS (TASK-1/3-like) and approximately 74 pS (TASK-3-like) channels, in addition to the approximately 14 pS (TASK-1-like) channel. The 42 pS channel was the most abundant, contributing approximately 75% of the current produced by TASK-like channels. Ruthenium red (5 microM) had no effect on TASK-1 and TASK-1/3, but inhibited TASK-3 by 87%. In CB cells, ruthenium red caused approximately 12% inhibition of TASK-like activity. Methanandamide reduced the activity of all three TASKs by 80-90%, and that of TASK-like channels in CB cell also by approximately 80%. In CB cells, hypoxia caused inhibition of TASK-like channels, including TASK-1/3-like channels. These results show that TASK-1, TASK-1/3 and TASK-3 are all functionally expressed in isolated CB cells, and that the TASK-1/3 heteromer provides the major part of the oxygen-sensitive TASK-like background K+ conductance.

General redox environment and carotid body chemoreceptor function

Carotid body (CB) chemoreceptor cells detect physiological levels of hypoxia and generate a hyperventilation, homeostatic in nature, aimed to minimize the deleterious effects of hypoxia. Intimate mechanisms involved in oxygen sensing in chemoreceptor cells remain largely unknown, but reactive oxygen species (ROS) had been proposed as mediators of this process. We have determined glutathione levels and calculated glutathione redox potential (E(GSH); indicator of the general redox environment of cells) in rat diaphragms incubated in the presence of oxidizing agents of two types: nonpermeating and permeating through cell membranes; in the latter group, unspecific oxidants and inhibitors of ROS-disposing enzymes were used. Selected concentrations of oxidizing agents were tested for their ability to modify the normoxic and hypoxic activity of chemoreceptor cells measured in vitro as their rate of release of neurotransmitters. Results evidence variable relationships between E(GSH) and the activity of chemoreceptor cells. The independence of chemoreceptor cell activity from the E(GSH) would imply that the ability of the CB to play its homeostatic role is largely preserved in any pathological or toxicological contingency causing oxidative stress. Consistent with this suggestion, it was also found that CB-mediated hypoxic hyperventilation was not altered by treatment of intact animals with agents that markedly decreased the E(GSH) in all tissues assayed.

Neurotransmitters in carotid body function: the case of dopamine--invited article

The carotid body (CB) is the main peripheral chemoreceptor. The present model of CB chemoreception states that glomus (type I) cells are the primary receptors, which are synaptically connected to the nerve terminals of the petrosal ganglion neurons. In response to hypoxia, hypercapnia and acidosis, glomus cells release one (or more) transmitter(s) which, acting on the nerve terminals of chemosensory neurons, increases the afferent discharge. Among several molecules present in glomus cells, dopamine, acetylcholine and 5'-adenosine-triphosphate have been proposed to be the excitatory transmitters in the CB. Beside these putative excitatory transmitters, other molecules modulate the chemosensory process through direct actions on glomus cells and/or by producing tonic effects on CB blood vessels. In this review, we focus on the role played by dopamine in the CB chemoreception, with emphasis on the open question if the reported differences on its actions on the generation of afferent chemosensory activity reflect true species differences. The available data suggest that dopamine may play a modulatory role within the cat CB, while in the rabbit CB, dopamine is an excitatory transmitter. Therefore, the reported differences on the actions of exogenously applied dopamine and its participation on the generation of afferent chemosensory activity appear to reflect true species differences.

Mechanisms of oxygen sensing: a key to therapy of pulmonary hypertension and patent ductus arteriosus

Specialized tissues that sense acute changes in the local oxygen tension include type 1 cells of the carotid body, neuroepithelial bodies in the lungs, and smooth muscle cells of the resistance pulmonary arteries and the ductus arteriosus (DA). Hypoxia inhibits outward potassium current in carotid body type 1 cells, leading to depolarization and calcium entry through L-type calcium channels. Increased intracellular calcium concentration ([Ca+ +]i) leads to exocytosis of neurotransmitters, thus stimulating the carotid sinus nerve and respiration. The same K+ channel inhibition occurs with hypoxia in pulmonary artery smooth muscle cells (PASMCs), causing contraction and providing part of the mechanism of hypoxic pulmonary vasoconstriction (HPV). In the SMCs of the DA, the mechanism works in reverse. It is the shift from hypoxia to normoxia that inhibits K+ channels and causes normoxic ductal contraction. In both PA and DA, the contraction is augmented by release of Ca+ + from the sarcoplasmic reticulum, entry of Ca+ + through store-operated channels (SOC) and by Ca+ + sensitization. The same three 'executive' mechanisms are partly responsible for idiopathic pulmonary arterial hypertension (IPAH). While vasoconstrictor mediators constrict both PA and DA and vasodilators dilate both vessels, only redox changes mimic oxygen by having directly opposite effects on the K+ channels, membrane potential, [Ca(++)]i and tone in the PA and DA. There are several different hypotheses as to how redox might alter tone, which remain to be resolved. However, understanding the mechanism will facilitate drug development for pulmonary hypertension and patent DA.

A role for TASK-1 (KCNK3) channels in the chemosensory control of breathing

Acid-sensitive K+ channels of the tandem P-domain K+-channel family (TASK-1 and TASK-3) have been implicated in peripheral and central respiratory chemosensitivity; however, because of the lack of decisive pharmacological agents, the final proof of the role of the TASK channel in the chemosensory control of breathing has been missing. In the mouse, TASK-1 and TASK-3 channels are dispensable for central respiratory chemosensitivity (Mulkey et al., 2007). Here, we have used knock-out animals to determine whether TASK-1 and TASK-3 channels play a role in the carotid body function and chemosensory control of breathing exerted by the carotid body chemoreceptors. Ventilatory responses to hypoxia (10% O2 in inspired air) and moderate normoxic hypercapnia (3-6% CO2 in inspired air) were significantly reduced in TASK-1 knock-out mice. In contrast, TASK-3-deficient mice showed responses to both stimuli that were similar to those developed by their wild-type counterparts. TASK-1 channel deficiency resulted in a marked reduction of the hypoxia (by 49%)- and CO2 (by 68%)-evoked increases in the carotid sinus nerve chemoafferent discharge recorded in the in vitro superfused carotid body/carotid sinus nerve preparations. Deficiency in both TASK-1 and TASK-3 channels increased baseline chemoafferent activity but did not cause a further reduction of the carotid body chemosensory responses. These observations provide direct evidence that TASK-1 channels contribute significantly to the increases in the carotid body chemoafferent discharge in response to a decrease in arterial P(O2) or an increase in P(CO2)/[H+]. TASK-1 channels therefore play a key role in the control of ventilation by peripheral chemoreceptors.

Acid-sensing ion channels contribute to transduction of extracellular acidosis in rat carotid body glomus cells

Carotid body chemoreceptors sense hypoxemia, hypercapnia, and acidosis and play an important role in cardiorespiratory regulation. The molecular mechanism of pH sensing by chemoreceptors is not clear, although it has been proposed to be mediated by a drop in intracellular pH of carotid body glomus cells, which inhibits a K+ current. Recently, pH-sensitive ion channels have been described in glomus cells that respond directly to extracellular acidosis. In this study, we investigated the possible molecular mechanisms of carotid body pH sensing by recording the responses of glomus cells isolated from rat carotid body to rapid changes in extracellular pH using the whole-cell patch-clamping technique. Extracellular acidosis evoked transient inward current in glomus cells that was inhibited by the acid-sensing ion channel (ASIC) blocker amiloride, absent in Na+-free bathing solution, and enhanced by either Ca2+-free buffer or addition of lactate. In addition, ASIC1 and ASIC3 were shown to be expressed in rat carotid body by quantitative PCR and immunohistochemistry. In the current-clamp mode, extracellular acidosis evoked both a transient and sustained depolarizations. The initial transient component of depolarization was blocked by amiloride, whereas the sustained component was eliminated by removal of K+ from the pipette solution and partially blocked by the TASK (tandem-p-domain, acid-sensitive K+ channel) blockers anandamide and quinidine. The results provide the first evidence that ASICs may contribute to chemotransduction of low pH by carotid body chemoreceptors and that extracellular acidosis directly activates carotid body chemoreceptors through both ASIC and TASK channels.

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.

Modulation of TASK-like background potassium channels in rat arterial chemoreceptor cells by intracellular ATP and other nucleotides

The carotid body's physiological role is to sense arterial oxygen, CO(2) and pH. It is however, also powerfully excited by inhibitors of oxidative phosphorylation. This latter observation is the cornerstone of the mitochondrial hypothesis which proposes that oxygen is sensed through changes in energy metabolism. All of these stimuli act in a similar manner, i.e. by inhibiting a background TASK-like potassium channel (K(B)) they induce membrane depolarization and thus neurosecretion. In this study we have evaluated the role of ATP in modulating K(B) channels. We find that K(B) channels are strongly activated by MgATP (but not ATP(4)(-)) within the physiological range (K(1/2) = 2.3 mm). This effect was mimicked by other Mg-nucleotides including GTP, UTP, AMP-PCP and ATP-gamma-S, but not by PP(i) or AMP, suggesting that channel activity is regulated by a Mg-nucleotide sensor. Channel activation by MgATP was not antagonized by either 1 mm AMP or 500 microm ADP. Thus MgATP is probably the principal nucleotide regulating channel activity in the intact cell. We therefore investigated the effects of metabolic inhibition upon both [Mg(2+)](i), as an index of MgATP depletion, and channel activity in cell-attached patches. The extent of increase in [Mg(2+)](i) (and thus MgATP depletion) in response to inhibition of oxidative phosphorylation were consistent with a decline in [MgATP](i) playing a prominent role in mediating inhibition of K(B) channel activity, and the response of arterial chemoreceptors to metabolic compromise.

AMP-activated protein kinase and chemotransduction in the carotid body

AMP-activated protein kinase (AMPK) is a key component of a kinase cascade that regulates energy balance at the cellular level. Our recent research has raised the possibility that AMPK may also function to couple hypoxic inhibition of mitochondrial oxidative phosphorylation to O(2)-sensitive K(+) channel inhibition and hence underpin carotid body type I cell excitation. Thus, in addition to maintaining the cellular energy state AMPK may act as the primary metabolic sensor and effector of hypoxic chemotransduction in type I cells. These findings provide a unifying link between two previously separate theories pertaining to O(2)-sensing in the carotid body, namely the 'membrane hypothesis' and the 'mitochondrial hypothesis'. Furthermore, our data suggest that in addition to its effects at the cellular level the AMPK signalling cascade can mediate vital physiological mechanisms essential for meeting the metabolic needs of the whole organism.

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.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates the oxygen sensing type I (glomus) cells of rat carotid bodies via reduction of a background TASK-like K+current

Pituitary adenylate cyclase-activating polypeptide (PACAP)-deficient mice are prone to sudden neonatal death and have reduced respiratory response to hypoxia. Here we found that PACAP-38 elevated cytosolic [Ca(2+)] ([Ca(2+)](i)) in the oxygen sensing type I cells but not the glial-like type II (sustentacular) cells of the rat carotid body. This action of PACAP could not be mimicked by vasoactive intestinal peptide but was abolished by PACAP 6-38, implicating the involvement of PAC(1) receptors. H89, a protein kinase A (PKA) inhibitor attenuated the PACAP response. Simultaneous measurement of membrane potential and [Ca(2+)](i) showed that the PACAP-mediated [Ca(2+)](i) rise was accompanied by depolarization and action potential firing. Ni(2+), a blocker of voltage-gated Ca(2+) channels (VGCC) or the removal of extracellular Ca(2+) reversibly inhibited the PACAP-mediated [Ca(2+)](i) rise. In the presence of tetraethylammonium (TEA) and 4-aminopyridine (4-AP), PACAP reduced a background K(+) current. Anandamide, a blocker of TWIK-related acid-sensitive K(+) (TASK)-like K(+) channel, occluded the inhibitory action of PACAP on K(+) current. We conclude that PACAP, acting via the PAC(1) receptors coupled PKA pathway inhibits a TASK-like K(+) current and causes depolarization and VGCC activation. This stimulatory action of PACAP in carotid type I cells can partly account for the role of PACAP in respiratory disorders.

Orthodromic spike generation from electrical stimuli in the rat carotid body: implications for the afferent spike generation process

Carotid body chemoreceptors respond to a decrease in arterial partial pressure of O(2) with an increase in sinus nerve action potential (AP) activity which initiates a number of protective reflexes. The spike generation process is unresolved but is generally considered to be caused by a synaptic depolarizing potential (SDP) in the nerve endings caused by release of an excitatory transmitter from the glomus cell, which is a secretory cell that is presynaptic to the nerve terminals. To detect the purported SDPs, stimulating electrodes were placed at sites within the carotid body from which orthodromic APs could be evoked at low threshold currents. The probability of AP generation as a function of stimulus current was fitted well to a Boltzmann distribution. Subthreshold electrical stimuli which were expected to summate with subthreshold SDPs, failed, in all instances, to evoke APs at the expected probability. When the stimulus was gated to the occurrence of a spontaneous AP, no change in electrical threshold was observed as the delay between the spontaneous AP and electrical stimulus was increased, despite the presumed disappearance of an SDP in the post-AP period. Decreases in spontaneous AP generation rate, caused by hyperoxia, were associated with only slight changes in the mean orthodromic stimulus threshold, but with a significant increase in slope of the Boltzmann function, suggesting a decrease in the variance of nerve terminal excitability during hyperoxia. These results suggest that AP generation is not due to SDP events; rather, AP generation is likely to be due to a process endogenous to the nerve terminals that modulates the variability of nerve terminal excitability.

AMP-activated protein kinase mediates carotid body excitation by hypoxia

Early detection of an O2 deficit in the bloodstream is essential to initiate corrective changes in the breathing pattern of mammals. Carotid bodies serve an essential role in this respect; their type I cells depolarize when O2 levels fall, causing voltage-gated Ca2+ entry. Subsequent neurosecretion elicits increased afferent chemosensory fiber discharge to induce appropriate changes in respiratory function (1). Although depolarization of type I cells by hypoxia is known to arise from K+ channel inhibition, the identity of the signaling pathway has been contested, and the coupling mechanism is unknown (2). We tested the hypothesis that AMP-activated protein kinase (AMPK) is the effector of hypoxic chemotransduction. AMPK is co-localized at the plasma membrane of type I cells with O2-sensitive K+ channels. In isolated type I cells, activation of AMPK using 5-aminoimidazole-4-carboxamide riboside (AICAR) inhibited O2-sensitive K+ currents (carried by large conductance Ca2+-activated (BKCa) channels and TASK (tandem pore, acid-sensing potassium channel)-like channels, leading to plasma membrane depolarization, Ca2+ influx, and increased chemosensory fiber discharge. Conversely, the AMPK antagonist compound C reversed the effects of hypoxia and AICAR on type I cell and carotid body activation. These results suggest that AMPK activation is both sufficient and necessary for the effects of hypoxia. Furthermore, AMPK activation inhibited currents carried by recombinant BKCa channels, whereas purified AMPK phosphorylated thealpha subunit of the channel in immunoprecipitates, an effect that was stimulated by AMP and inhibited by compound C. Our findings demonstrate a central role for AMPK in stimulus-response coupling by hypoxia and identify for the first time a link between metabolic stress and ion channel regulation in an O2-sensing system.