Biophysical properties and metabolic regulation of a TASK-like potassium channel in rat carotid body type 1 cells

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

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

Acute Oxygen-Sensing via Mitochondria-Generated Temperature Transients in Rat Carotid Body Type I Cells

The Carotid Bodies (CB) are peripheral chemoreceptors that detect changes in arterial oxygenation and, via afferent inputs to the brainstem, correct the pattern of breathing to restore blood gas homeostasis. Herein, preliminary evidence is presented supporting a novel oxygen-sensing hypothesis which suggests CB Type I cell “hypoxic signaling” may in part be mediated by mitochondria-generated thermal transients in TASK-channel-containing microdomains. Distances were measured between antibody-labeled mitochondria and TASK-potassium channels in primary rat CB Type I cells. Sub-micron distance measurements (TASK-1: 0.33 ± 0.04 µm, n = 47 vs TASK-3: 0.32 ± 0.03 µm, n = 54) provided evidence for CB Type I cell oxygen-sensing microdomains. A temperature-sensitive dye (ERthermAC) indicated that inhibition of mitochondrial activity in isolated cells caused a rapid and reversible inhibition of mitochondrial thermogenesis and thus temperature in these microdomains. Whole-cell perforated-patch current-clamp electrophysiological recordings demonstrated sensitivity of resting membrane potential (Vm) to temperature: lowering bath temperature from 37°C to 24°C induced consistent and reversible depolarizations (Vm at 37°C: 48.4 ± 4.11 mV vs 24°C: 31.0 ± 5.69 mV; n = 5; p < 0.01). These data suggest that hypoxic inhibition of mitochondrial thermogenesis may play an important role in oxygen chemotransduction in the CB. A reduction in temperature within cellular microdomains will inhibit plasma membrane ion channels, influence the balance of cellular phosphorylation–dephosphorylation, and may extend the half-life of reactive oxygen species. The characterization of a thermosensory chemotransduction mechanism, that may also be used by other oxygen-sensitive cell types and may impact multiple other chemotransduction mechanisms is critical if we are to fully understand how the CBs, and potentially other oxygen-sensitive cells, respond to 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.

Higher expression of KCNK10 (TREK-2) K+ channels and their functional upregulation by lipopolysaccharide treatment in mouse peritoneal B1a cells

Innate-like CD5⁺ B1a cells localized in serous cavities are activated by innate stimuli, such as lipopolysaccharide (LPS), leading to T cell-independent antibody responses. Although ion channels play crucial roles in the homeostasis and activation of immune cells, the electrophysiological properties of B1a cells have not been investigated to date. Previously, in the mouse B cell lymphoma cells, we found that the voltage-independent two-pore-domain potassium (K2P) channels generate a negative membrane potential and drive Ca²⁺ influx. Here, we newly compared the expression and activities of K2P channels in mouse splenic follicular B (FoB), marginal zone B (MZB), and peritoneal B1a cells. Next-generation sequencing analysis showed higher levels of transcripts for TREK-2 and TWIK-2 in B1a cells than those in FoB or MZB cells. Electrophysiological analysis, using patch clamp technique, revealed higher activity of TREK-2 with the characteristic large unitary conductance (~ 250 pS) in B1a than that in FoB or MZB cells. TREK-2 activity was further increased by LPS treatment (>2 h), which was more prominent in B1a than that in MZB or FoB cells. The cytosolic Ca²⁺ concentration of B cells was decreased by high-K⁺-induced depolarization (ΔR_KCl (%)), suggesting the basal Ca²⁺ influx to be driven by negative membrane potential. The LPS treatment significantly increased the ΔR_KCl (%) in B1a, though not in FoB and MZB cells. Our study was the first to compare the K2P channels in mouse primary B cell subsets, elucidating the functional upregulation of TREK-2 and augmentation of Ca²⁺ influx by the stimulation of Toll-like receptor 4 in B1a cells.

Averting Opioid-induced Respiratory Depression without Affecting Analgesia

The ventilatory control system is highly vulnerable to exogenous administered opioid analgesics. Particularly respiratory depression is a potentially lethal complication that may occur when opioids are overdosed or consumed in combination with other depressants such as sleep medication or alcohol. Fatalities occur in acute and chronic pain patients on opioid therapy and individuals that abuse prescription or illicit opioids for their hedonistic pleasure. One important strategy to mitigate opioid-induced respiratory depression is cotreatment with nonopioid respiratory stimulants. Effective stimulants prevent respiratory depression without affecting the analgesic opioid response. Several pharmaceutical classes of nonopioid respiratory stimulants are currently under investigation. The majority acts at sites within the brainstem respiratory network including drugs that act at α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (ampakines), 5-hydroxytryptamine receptor agonists, phospodiesterase-4 inhibitors, D1-dopamine receptor agonists, the endogenous peptide glycyl-glutamine, and thyrotropin-releasing hormone. Others act peripherally at potassium channels expressed on oxygen-sensing cells of the carotid bodies, such as doxapram and GAL021 (Galleon Pharmaceuticals Corp., USA). In this review we critically appraise the efficacy of these agents. We conclude that none of the experimental drugs are adequate for therapeutic use in opioid-induced respiratory depression and all need further study of efficacy and toxicity. All discussed drugs, however, do highlight potential mechanisms of action and possible templates for further study and development.

Carotid Body Type-I Cells Under Chronic Sustained Hypoxia: Focus on Metabolism and Membrane Excitability

Chronic sustained hypoxia (CSH) evokes ventilatory acclimatization characterized by a progressive hyperventilation due to a potentiation of the carotid body (CB) chemosensory response to hypoxia. The transduction of the hypoxic stimulus in the CB begins with the inhibition of K+ currents in the chemosensory (type-I) cells, which in turn leads to membrane depolarization, Ca2+ entry and the subsequent release of one- or more-excitatory neurotransmitters. Several studies have shown that CSH modifies both the level of transmitters and chemoreceptor cell metabolism within the CB. Most of these studies have been focused on the role played by such putative transmitters and modulators of CB chemoreception, but less is known about the effect of CSH on metabolism and membrane excitability of type-I cells. In this mini-review, we will examine the effects of CSH on the ion channels activity and excitability of type-I cell, with a particular focus on the effects of CSH on the TASK-like background K+ channel. We propose that changes on TASK-like channel activity induced by CSH may contribute to explain the potentiation of CB chemosensory activity.

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 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.

Acute oxygen sensing by the carotid body: from mitochondria to plasma membrane

Maintaining oxygen homeostasis is crucial to the survival of animals. Mammals respond acutely to changes in blood oxygen levels by modulating cardiopulmonary function. The major sensor of blood oxygen that regulates breathing is the carotid body (CB), a small chemosensory organ located at the carotid bifurcation. When arterial blood oxygen levels drop in hypoxia, neuroendocrine cells in the CB called glomus cells are activated to signal to afferent nerves that project to the brain stem. The mechanism by which hypoxia stimulates CB sensory activity has been the subject of many studies over the past 90 years. Two discrete models emerged that argue for the seat of oxygen sensing to lie either in the plasma membrane or mitochondria of CB cells. Recent studies are bridging the gap between these models by identifying hypoxic signals generated by changes in mitochondrial function in the CB that can be sensed by plasma membrane proteins on glomus cells. The CB is important for physiological adaptation to hypoxia, and its dysfunction contributes to sympathetic hyperactivity in common conditions such as sleep-disordered breathing, chronic heart failure, and insulin resistance. Understanding the basic mechanism of oxygen sensing in the CB could allow us to develop strategies to target this organ for therapy. In this short review, I will describe two historical models of CB oxygen sensing and new findings that are integrating these models.

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.

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

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

Molecular aspects of structure, gating, and physiology of pH-sensitive background K2P and Kir K+-transport channels

K(+) channels fulfill roles spanning from the control of excitability to the regulation of transepithelial transport. Here we review two groups of K(+) channels, pH-regulated K2P channels and the transport group of Kir channels. After considering advances in the molecular aspects of their gating based on structural and functional studies, we examine their participation in certain chosen physiological and pathophysiological scenarios. Crystal structures of K2P and Kir channels reveal rather unique features with important consequences for the gating mechanisms. Important tasks of these channels are discussed in kidney physiology and disease, K(+) homeostasis in the brain by Kir channel-equipped glia, and central functions in the hearing mechanism in the inner ear and in acid secretion by parietal cells in the stomach. K2P channels fulfill a crucial part in central chemoreception probably by virtue of their pH sensitivity and are central to adrenal secretion of aldosterone. Finally, some unorthodox behaviors of the selectivity filters of K2P channels might explain their normal and pathological functions. Although a great deal has been learned about structure, molecular details of gating, and physiological functions of K2P and Kir K(+)-transport channels, this has been only scratching at the surface. More molecular and animal studies are clearly needed to deepen our knowledge.

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

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

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.

Mg²⁺ coordinating dynamics in Mg:ATP fueled motor proteins

The coordination of Mg(2+) with the triphosphate group of adenosine triphosphate (ATP) in motor proteins is investigated using data mining and molecular dynamics. The possible coordination structures available from crystal data for actin, myosin, RNA polymerase, DNA polymerase, DNA helicase, and F1-ATPase are verified and investigated further by molecular dynamics. Coordination states are evaluated using structural analysis and quantified by radial distribution functions, coordination numbers, and pair interaction energy calculations. The results reveal a diverse range of both transitory and stable coordination arrangements between Mg(2+) and ATP. The two most stable coordinating states occur when Mg(2+) coordinates two or three oxygens from the triphosphate group of ATP. Evidence for five-site coordination is also reported involving water in addition to the triphosphate group. The stable states correspond to a pair interaction energy of either ∼-2750 kJ/mol or -3500 kJ/mol. The role of water molecules in the hydration shell surrounding Mg(2+) is also reported.

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.

Effects of modulators of AMP-activated protein kinase on TASK-1/3 and intracellular Ca(2+) concentration in rat carotid body glomus cells

Acute hypoxia depolarizes carotid body chemoreceptor (glomus) cells and elevates intracellular Ca(2+) concentration ([Ca(2+)]i). Recent studies suggest that AMP-activated protein kinase (AMPK) mediates these effects of hypoxia by inhibiting the background K(+) channels such as TASK. Here we studied the effects of modulators of AMPK on TASK activity in cell-attached patches. Activators of AMPK (1mM AICAR and 0.1-0.5mM A769662) did not inhibit TASK activity or cause depolarization during acute (10min) or prolonged (2-3h) exposure. Hypoxia inhibited TASK activity by ∼70% in cells pretreated with AICAR or A769662. Both AICAR and A769662 (15-40min) failed to increase [Ca(2+)]i in glomus cells. Compound C (40μM), an inhibitor of AMPK, showed no effect on hypoxia-induced inhibition of TASK. AICAR and A769662 phosphorylated AMPKα in PC12 cells, and Compound C blocked the phosphorylation. Our results suggest that AMPK does not affect TASK activity and is not involved in hypoxia-induced elevation of intracellular [Ca(2+)] in isolated rat carotid body glomus cells.

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.

Oxygen and mitochondrial inhibitors modulate both monomeric and heteromeric TASK-1 and TASK-3 channels in mouse carotid body type-1 cells

In rat arterial chemoreceptors, background potassium channels play an important role in maintaining resting membrane potential and promoting depolarization and excitation in response to hypoxia or acidosis. It has been suggested that these channels are a heterodimer of TASK-1 and TASK-3 based on their similarity to heterologously expressed TASK-1/3 fusion proteins. In this study, we sought to confirm the identity of these channels through germline ablation of Task-1 (Kcnk3) and Task-3 (Kcnk9) in mice. Background K-channels were abundant in carotid body type-1 cells from wild-type mice and comparable to those previously described in rat type-1 cells with a main conductance state of 33 pS. This channel was absent from both Task-1(-/-) and Task-3(-/-) cells. In its place we observed a larger (38 pS) K(+)-channel in Task-1(-/-) cells and a smaller (18 pS) K(+)-channel in Task-3(-/-) cells. None of these channels were observed in Task-1(-/-)/Task-3(-/-) double knock-out mice. We therefore conclude that the predominant background K-channel in wild-type mice is a TASK-1/TASK-3 heterodimer, whereas that in Task-1(-/-) mice is TASK-3 and, conversely, that in Task-3(-/-) mice is TASK-1. All three forms of TASK channel in type-1 cells were inhibited by hypoxia, cyanide and the uncoupler FCCP, but the greatest sensitivity was seen in TASK-1 and TASK-1/TASK-3 channels. In summary, the background K-channel in type-1 cells is predominantly a TASK-1/TASK-3 heterodimer. Although both TASK-1 and TASK-3 are able to couple to the oxygen and metabolism sensing pathways present in type-1 cells, channels containing TASK-1 appear to be more 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.

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.

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.

Characterization of an ATP-sensitive K(+) channel in rat carotid body glomus cells

Carotid body glomus (CB) cells express different types of K(+) channels such as TASK, BK, and Kv channels, and hypoxia has been shown to inhibit these channels. Here we report the presence of a ∼72-pS channel that has not been described previously in CB cells. In cell-attached patches with 150 mM K(+) in the pipette and bath solutions, TASK-like channels were present (∼15 and ∼36-pS). After formation of inside-out patches, a 72-pS channel became transiently active in ∼18% of patches. The 72-pS channel was K(+)-selective, inhibited by 2-4 mM ATP and 10-100 μM glybenclamide. The 72-pS channel was observed in CB cells isolated from newborn, 2-3 week and 10-12 week-old rats. Reverse transcriptase-PCR and immunocytochemistry showed that Kir6.1, Kir6.2, SUR1 and SUR2 were expressed in CB glomus cells as well as in non-glomus cells. Acute hypoxia (∼15 mmHg O(2)) inhibited TASK-like channels but failed to activate the 72-pS channel in cell-attached CB cells. K(+) channel openers (diazoxide, pinacidil, levcromakalim), sodium cyanide and removal of extracellular glucose also did not activate the 72-pS channel in the cell-attached state. The hypoxia-induced elevation of intracellular [Ca(2+)] was unchanged by glybenclamide or diazoxide. NaCN-induced increase in [Ca(2+)] was not affected by 10 μM glybenclamide but inhibited by 100 μM glybenclamide. Acute glucose deprivation did not elevate [Ca(2+)] in the presence or absence of glybenclamide. These results show that an ATP-sensitive K(+) channel is expressed in the plasma membrane of CB cells, but is not activated by short-term metabolic inhibition. The functional relevance of the 72-pS channel remains to be determined.

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

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

Carotid body chemosensory responses in mice deficient of TASK channels

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

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

AMP-activated protein kinase inhibits TREK channels

AMP-activated protein kinase (AMPK) is a serine/threonine kinase activated by conditions that increase the AMP : ATP ratio. In carotid body glomus cells, AMPK is thought to link changes in arterial O(2) with activation of glomus cells by inhibition of unidentified background K(+) channels. Modulation by AMPK of individual background K(+) channels has not been described. Here, we characterize effects of activated AMPK on recombinant TASK-1, TASK-3, TREK-1 and TREK-2 background K(+) channels expressed in HEK293 cells. We found that TREK-1 and TREK-2 channels but not TASK-1 or TASK-3 channels are inhibited by AMPK. AMPK-mediated inhibition of TREK involves key serine residues in the C-terminus that are also known to be important for PKA and PKC channel modulation; inhibition of TREK-1 requires Ser-300 and Ser-333 and inhibition of TREK-2 requires Ser-326 and Ser-359. Metabolic inhibition by sodium azide can also inhibit both TREK and TASK channels. The effects of azide on TREK occlude subsequent channel inhibition by AMPK and are attenuated by expression of a dominant negative catalytic subunit of AMPK (dnAMPK), suggesting that metabolic stress modulates TREK channels by an AMPK mechanism. By contrast, inhibition of TASK channels by azide was unaffected by expression of dnAMPK, suggesting an AMPK-independent mechanism. In addition, prolonged exposure (6-7 min) to hypoxia ( = 11 +/- 1 mmHg) inhibits TREK channels and this response was blocked by expression of dnAMPK. Our results identify a novel modulation of TREK channels by AMPK and indicate that select residues in the C-terminus of TREK are points of convergence for multiple signalling cascades including AMPK, PKA and PKC. To the extent that carotid body O(2) sensitivity is dependent on AMPK, our finding that TREK-1 and TREK-2 channels are inhibited by AMPK suggests that TREK channels may represent the AMPK-inhibited background K(+) channels that mediate activation of glomus cells by hypoxia.

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.

Chemoreceptor hypersensitivity, sympathetic excitation, and overexpression of ASIC and TASK channels before the onset of hypertension in SHR

RATIONALE

Increased sympathetic nerve activity has been linked to the pathogenesis of hypertension in humans and animal models. Enhanced peripheral chemoreceptor sensitivity which increases sympathetic nerve activity has been observed in established hypertension but has not been identified as a possible mechanism for initiating an increase in sympathetic nerve activity before the onset of hypertension.

OBJECTIVE

We tested this hypothesis by measuring the pH sensitivity of isolated carotid body glomus cells from young spontaneously hypertensive rats (SHR) before the onset of hypertension and their control normotensive Wistar-Kyoto (WKY) rats.

METHODS AND RESULTS

We found a significant increase in the depolarizing effect of low pH in SHR versus WKY glomus cells which was caused by overexpression of 2 acid-sensing non-voltage-gated channels. One is the amiloride-sensitive acid-sensing sodium channel (ASIC3), which is activated by low pH and the other is the 2-pore domain acid-sensing K(+) channel (TASK1), which is inhibited by low pH and blocked by quinidine. Moreover, we found that the increase in sympathetic nerve activity in response to stimulation of chemoreceptors with sodium cyanide was markedly enhanced in the still normotensive young SHR compared to control WKY rats.

CONCLUSIONS

Our results establish a novel molecular basis for increased chemotransduction that contributes to excessive sympathetic activity before the onset of hypertension.

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.

Heme oxygenase is necessary for the excitatory response of cultured neonatal rat rostral ventrolateral medulla neurons to hypoxia

Heme oxygenase has been linked to the oxygen-sensing function of the carotid body, pulmonary vasculature, cerebral vasculature, and airway smooth muscle. We have shown previously that the cardiorespiratory regions of the rostral ventrolateral medulla are excited by local hypoxia and that heme oxygenase-2 (HO-2) is expressed in the hypoxia-chemosensitive regions of the rostral ventrolateral medulla (RVLM), the respiratory pre-Bötzinger complex, and C1 sympathoexcitatory region. To determine whether heme oxygenase is necessary for the hypoxic-excitation of dissociated RVLM neurons (P1) cultured on confluent medullary astrocytes (P5), we examined their electrophysiological responses to hypoxia (NaCN and low Po(2)) using the whole-cell perforated patch clamp technique before and after blocking heme oxygenase with tin protoporphyrin-IX (SnPP-IX). Following the electrophysiological recording, immunocytochemistry was performed on the recorded neuron to correlate the electrophysiological response to hypoxia with the expression of HO-2. We found that the responses to NaCN and hypoxia were similar. RVLM neurons responded to NaCN and low Po(2) with either depolarization or hyperpolarization and SnPP-IX blocked the depolarization response of hypoxia-excited neurons to both NaCN and low Po(2) but had no effect on the hyperpolarization response of hypoxia-depressed neurons. Consistent with this observation, HO-2 expression was present only in the hypoxia-excited neurons. We conclude that RVLM neurons are excited by hypoxia via a heme oxygenase-dependent mechanism.

The RCK1 high-affinity Ca2+ sensor confers carbon monoxide sensitivity to Slo1 BK channels

Carbon monoxide (CO) is a lethal gas, but it is also increasingly recognized as a physiological signaling molecule capable of regulating a variety of proteins. Among them, large-conductance Ca(2+)- and voltage-gated K(+) (Slo1 BK) channels, important in vasodilation and neuronal firing, have been suggested to be directly stimulated by CO. However, the molecular mechanism of the stimulatory action of CO on the Slo1 BK channel has not been clearly elucidated. We report here that CO reliably and repeatedly activates Slo1 BK channels in excised membrane patches in the absence of Ca(2+) in a voltage-sensor-independent manner. The stimulatory action of CO on the Slo1 BK channel requires an aspartic acid and two histidine residues located in the cytoplasmic RCK1 domain, and the effect persists under the conditions known to inhibit the conventional interaction between CO and heme in other proteins. We propose that CO acts as a partial agonist for the high-affinity divalent cation sensor in the RCK1 domain of the Slo1 BK channel.

Oxygen sensors in context

The ability to adapt to changes in the availability of O2 provides a critical advantage to all O2-dependent lifeforms. In mammals it allows optimal matching of the O2 requirements of the cells to ventilation and O2 delivery, underpins vital changes to the circulation during the transition from fetal to independent, air-breathing life, and provides a means by which dysfunction can be limited or prevented in disease. Certain tissues such as the carotid body, pulmonary circulation, neuroepithelial bodies and fetal adrenomedullary chromaffin cells are specialised for O2 sensing, though most others show for example alterations in transcription of specific genes during hypoxia. A number of mechanisms are known to respond to variations in PO2 over the physiological range, and have been proposed to fulfil the function as O2 sensors; these include modulation of mitochondrial oxidative phosphorylation and a number of O2-dependent synthetic and degradation pathways. There is however much debate as to their relative importance within and between specific tissues, whether their O2 sensitivity is actually appropriate to account for their proposed actions, and in particular their modus operandi. This review discusses our current understanding of how these mechanisms may operate, and attempts to put them into the context of the actual PO2 to which they are likely to be exposed. An important point raised is that the overall O2 sensitivity (P50) of any O2-dependent mechanism does not necessarily correlate with that of its O2 sensor, as the coupling function between the two may be complex and non-linear. In addition, although the bulk of the evidence suggests that mitochondria act as the key O2 sensor in carotid body, pulmonary artery and chromaffin cells, the signalling mechanisms by which alterations in their function are translated into a response appear to differ fundamentally, making a global unified theory of O2 sensing unlikely.

The TASK background K2P channels: chemo- and nutrient sensors

Specialized chemo- and nutrient-sensing cells share a common electrophysiological mechanism by transducing low O(2), high CO(2) and low glucose stimuli into a compensatory cellular response: the closing of background K(+) channels encoded by the K(2P) subunits. Inhibition of the TASK K(2P) channels by extracellular acidosis leads to an increased excitability of brainstem respiratory neurons. Moreover, hypoxic down-modulation of TASK channels is implicated in the activation of glomus cells in the carotid body. Stimulation of both types of cell leads to an enhanced ventilation and to cardiocirculatory adjustments. Differential modulation of TASK channels by acidosis and high glucose alters excitability of the hypothalamic orexin neurons, which influence arousal, food seeking and breathing. These recent results shed light on the role of TASK channels in sensing physiological stimuli.

Biophysical, pharmacological, and functional characteristics of cloned and native mammalian two-pore domain K+ channels

The mammalian family of two-pore domain K+ (K2P) channel proteins are encoded by 15 KCNK genes and subdivided into six subfamilies on the basis of sequence similarities: TWIK, TREK, TASK, TALK, THIK, and TRESK. K2P channels are expressed in cells throughout the body and have been implicated in diverse cellular functions including maintenance of the resting potential and regulation of excitability, sensory transduction, ion transport, and cell volume regulation, as well as metabolic regulation and apoptosis. In recent years K2P channel isoforms have been identified as important targets of several widely employed drugs, including: general anesthetics, local anesthetics, neuroprotectants, and anti-depressants. An important goal of future studies will be to identify the basis of drug actions and channel isoform selectivity. This goal will be facilitated by characterization of native K2P channel isoforms, their pharmacological properties and tissue-specific expression patterns. To this end the present review examines the biophysical, pharmacological, and functional characteristics of cloned mammalian K2P channels and compares this information with the limited data available for native K2P channels in order to determine criteria which may be useful in identifying ionic currents mediated by native channel isoforms and investigating their pharmacological and functional characteristics.

Sensing hypoxia in the carotid body: from stimulus to response

The carotid body is a peripheral sensory organ that can transduce modest falls in the arterial PO2 (partial pressure of oxygen) into a neural signal that provides the afferent limb of a set of stereotypic cardiorespiratory reflexes that are graded according to the intensity of the stimulus. The stimulus sensed is tissue PO2 and this can be estimated to be around 50 mmHg during arterial normoxia, falling to between 10–40 mmHg during hypoxia. The chemoafferent hypoxia stimulus-response curve is exponential, rising in discharge frequency with falling PO2, and with no absolute threshold apparent in hyperoxia. Although the oxygen sensor has not been definitely identified, it is believed to reside within type I cells of the carotid body, and presently two major hypotheses have been put forward to account for the sensing mechanism. The first relies upon alterations in the cell energy status that is sensed by the cytosolic enzyme AMPK (AMP-activated protein kinase) subsequent to hypoxia-induced increases in the cellular AMP/ATP ratio during hypoxia. AMPK is localized close to the plasma membrane and its activation can inhibit both large conductance, calcium-activated potassium (BK) and background, TASK-like potassium channels, inducing membrane depolarization, voltage-gated calcium entry and neurosecretion of a range of transmitter and modulator substances, including catecholamines, ATP and acetylcholine. The alternative hypothesis considers a role for haemoxygenase-2, which uses oxygen as a substrate and may act to gate an associated BK channel through the action of its products, carbon monoxide and possibly haem. It is likely however, that these and other hypotheses of oxygen transduction are not mutually exclusive and that each plays a role, via its own particular sensitivity, in shaping the full response of this organ between hyperoxia and anoxia.

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.

Oxygen sensing in the carotid body and its relation to heart failure

This brief review first touches on the origins of the earth's oxygen. It then identifies and locates the principal oxygen sensor in vertebrates, the carotid body (CB). The CB is unique in that in human subjects, it is the only sensor of lower than normal levels in the partial pressure of oxygen (hypoxia, HH). Another oxygen sensor, the aortic bodies, are mostly vestigial in higher vertebrates. At least they play a much smaller role than the CB. In such an important role, the many reflexes in response to CB stimulation by HH are presented. After briefly reviewing what CB stimulation does, the next topic is to describe how the CB chemotransduces HH into neural signals to the brain. Several mechanisms are known, but critical steps in the mechanisms of chemosensation and chemotransduction are still under investigation. Finally, a brief glance at the operation of the CB in chronic heart failure patients is presented. Specifically, the role of nitric oxide, NO, is discussed.

A new look at the respiratory stimulant doxapram

A number of life-threatening clinical disorders may be amenable to treatment with a drug that can stimulate respiratory drive. These include acute respiratory failure secondary to chronic obstructive pulmonary disease, post-anesthetic respiratory depression, and apnea of prematurity. Doxapram has been available for over forty years for the treatment of these conditions and it has a low side effect profile compared to other available agents. Generally though, the use of doxapram has been limited to these clinical niches involving patients in the intensive care, post-anesthesia care and neonatal intensive care units. Recent basic science studies have made considerable progress in understanding the molecular mechanism of doxapram's respiratory stimulant action. Although it is unlikely that doxapram will undergo a clinical renaissance based on this new understanding, it represents a significant advance in our knowledge of the control of breathing.

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

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

Ionic currents underlying the response of rat dorsal vagal neurones to hypoglycaemia and chemical anoxia

A proportion of dorsal vagal neurones (DVN) are glucosensors. These cells respond to brief hypoglycaemia either with a K(ATP) channel-mediated hyperpolarization or with depolarization owing to an as yet unknown mechanism. K(ATP) currents are observed not only during hypoglycaemia, but also in response to mitochondrial inhibition. Here we show that similarly to the observations for K(ATP) currents, both hypoglycaemia and inhibition of mitochondrial function elicited a small inward current that persisted in TTX in DVN of rat brainstem slices. Removal of glucose from the bath solution induced this inward current within 50 +/- 4 s in one subpopulation of DVN and in 279 +/- 36 s in another subpopulation. No such subpopulations were observed for the response to mitochondrial inhibition. Biophysical analysis revealed that mitochondrial inhibition or hypoglycaemia inhibited an openly rectifying K+ conductance in 25% of DVN. In the remaining cells, either an increase in conductance, with a reversal potential between -58 and +10 mV, or a parallel inward shift of the holding current was observed. This current most probably resulted from inhibition of the Na+-K+-ATPase and/or the opening of an ion channel. Recordings with electrodes containing 145 mm instead of 5 mm Cl- failed to shift the reversal potential of the inward current, indicating that a Cl- channel was not involved. In summary, glucosensing and non-glucosensing DVN appear to use common electrical pathways to respond to mitochondrial inhibition and to hypoglycaemia. We suggest that differences in glucose metabolism rather than differences in the complement of ion channels distinguish these two cell types.

Biophysical and Pharmacological Characteristics of Native Two-Pore Domain TASK Channels in Rat Adrenal Glomerulosa Cells

Multiple genes of the TASK subfamily of two-pore domain K(+) channels are reported to be expressed in rat glomerulosa cells. To determine which TASK isoforms contribute to native leak channels controlling resting membrane potential, patch-clamp studies were performed to identify biophysical and pharmacological characteristics of macroscopic and unitary K(+) currents diagnostic of recombinant TASK channel isoforms. Results indicate K(+) conductance (gK(+)) is mediated almost exclusively by a weakly voltage-dependent (leak) K(+) channel closely resembling TASK-3. Leak channels exhibited a unitary conductance approximating that expected for TASK-3 under the recording conditions employed, brief mean open times and a voltage-dependent open probability. Extracellular H(+) induced voltage-independent inhibition of gK(+), exhibiting an IC(50) of 56 nM: (pH 7.25) and a Hill coefficient of 0.75. Protons inhibited leak channel open probability (P(o)) by promoting a long-lived closed state (tau > 500 ms). Extracellular Zn(2+) mimicked the effects of H(+); inhibition of gK(+) exhibited an IC(50) of 41 microM: with a Hill coefficient of 1.26, inhibiting channel gating by promoting a long-lived closed state. Ruthenium red (5 microM: ) inhibited gK(+) by 75.6% at 0 mV. Extracellular Mg(2+) induced voltage-dependent block of gK(+), inhibiting unitary current amplitude without affecting mean open time. Bupivacaine induced voltage-dependent block of gK(+), exhibiting IC(50) values of 116 microM: at -100 mV and 28 microM: at 40 mV with Hill coefficients of 1 at both potentials. Halothane induced a voltage-independent stimulation of gK(+) primarily by decreasing the leak channel closed-state dwell time.

Adenosine stimulates depolarization and rise in cytoplasmic [Ca2+] in type I cells of rat carotid bodies

During hypoxia, the level of adenosine in the carotid bodies increases as a result of ATP catabolism and adenosine efflux via adenosine transporters. Using Ca2+imaging, we found that adenosine, acting via A2Areceptors, triggered a rise in cytoplasmic [Ca2+] ([Ca2+]i) in type I (glomus) cells of rat carotid bodies. The adenosine response could be mimicked by forskolin (but not its inactive analog), and could be abolished by the PKA inhibitor H89. Simultaneous measurements of membrane potential (perforated patch recording) and [Ca2+]ishowed that the adenosine-mediated [Ca2+]irise was accompanied by depolarization. Ni2+, a voltage-gated Ca2+channel (VGCC) blocker, abolished the adenosine-mediated [Ca2+]irise. Although adenosine was reported to inhibit a 4-aminopyridine (4-AP)-sensitive K+current, 4-AP failed to trigger any [Ca2+]irise, or to attenuate the adenosine response. In contrast, anandamide, an inhibitor of the TWIK-related acid-sensitive K+-1 (TASK-1) channels, triggered depolarization and [Ca2+]irise. The adenosine response was attenuated by anandamide but not by tetraethylammonium. Our results suggest that adenosine, acting via the adenylate cyclase and PKA pathways, inhibits the TASK-1 K+channels. This leads to depolarization and activation of Ca2+entry via VGCC. This excitatory action of adenosine on type I cells may contribute to the chemosensitivity of the carotid body during hypoxia.

Postnatal development of carotid body glomus cell response to hypoxia

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