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

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.

Anandamide modulates carotid sinus nerve afferent activity via TRPV1 receptors increasing responses to heat

Abnormal respiratory chemosensitivity is implicated in recurrent apnea syndromes, with the peripheral chemoreceptors, the carotid bodies, playing a particularly important role. Previous work suggests that supraphysiological concentrations of the endocannabinoid endovanilloid and TASK channel blocker anandamide (ANA) excite carotid bodies, but the mechanism(s) and physiological significance are unknown. Given that carotid body output is temperature-sensitive, we hypothesized that ANA stimulates carotid body chemosensory afferents via temperature-sensitive vanilloid (TRPV1) receptors. To test this hypothesis, we used the dual-perfused in situ rat preparation to confirm that independent perfusion of carotid arteries with supraphysiological concentrations of ANA strongly excites carotid sinus nerve afferents and that this activity is sufficient to increase phrenic activity. Next, using ex vivo carotid body preparations, we demonstrate that these effects are mediated by TRPV1 receptors, not CB1 receptors or TASK channels: in CB1-null mouse preparations, ANA increased afferent activity across all levels of Po(2), whereas in TRPV1-null mouse preparations, the stimulatory effect of ANA was absent. In rat ex vivo preparations, ANA's stimulatory effects were mimicked by olvanil, a nonpungent TRPV1 agonist, and suppressed by the TRPV1 antagonist AMG-9810. The specific CB1 agonist oleamide had no effect. Physiological levels of ANA had no effect alone but increased sensitivity to mild hyperthermia. AMG-9810 blocked ANA's effect on the temperature response. Immunolabeling and RT-PCR demonstrated that TRPV1 receptors are not expressed in carotid body glomus cells but reside in petrosal sensory afferents. Together, these results suggest that ANA plays a physiological role in augmenting afferent responses to mild hyperthermia by activating TRPV1 receptors on petrosal afferents.

Oxygen-coupled redox regulation of the skeletal muscle ryanodine receptor-Ca2+ release channel by NADPH oxidase 4

Physiological sensing of O(2) tension (partial O(2) pressure, pO(2)) plays an important role in some mammalian cellular systems, but striated muscle generally is not considered to be among them. Here we describe a molecular mechanism in skeletal muscle that acutely couples changes in pO(2) to altered calcium release through the ryanodine receptor-Ca(2+)-release channel (RyR1). Reactive oxygen species are generated in proportion to pO(2) by NADPH oxidase 4 (Nox4) in the sarcoplasmic reticulum, and the consequent oxidation of a small set of RyR1 cysteine thiols results in increased RyR1 activity and Ca(2+) release in isolated sarcoplasmic reticulum and in cultured myofibers and enhanced contractility of intact muscle. Thus, Nox4 is an O(2) sensor in skeletal muscle, and O(2)-coupled hydrogen peroxide production by Nox4 governs the redox state of regulatory RyR1 thiols and thereby governs muscle performance. These findings reveal a molecular mechanism for O(2)-based signaling by an NADPH oxidase and demonstrate a physiological role for oxidative modification of RyR1.

Multifocal animated imaging of changes in cellular oxygen and calcium concentrations and membrane potential within the intact adult mouse carotid body ex vivo

Carotid body (CB) type I cell hypoxia-sensing function is assumed to be based on potassium channel inhibition. Subsequent membrane depolarization initiates an intracellular calcium increase followed by transmitter release for excitation of synapses with linked nerve endings. Several reports, however, contradict this generally accepted concept by showing that type I cell oxygen-sensing properties vary significantly depending on the method of their isolation. We report therefore for the first time noninvasive mapping of the oxygen-sensing properties of type I cells within the intact adult mouse CB ex vivo by using multifocal Nipkow disk-based imaging of oxygen-, calcium- and potential-sensitive cellular dyes. Characteristic type I cell clusters were identified in the compact tissue by immunohistochemistry because of their large cell nuclei combined with positive tyrosine hydroxylase staining. The cellular calcium concentrations in these cell clusters either increased or decreased in response to reduced tissue oxygen concentrations. Under control conditions, cellular potential oscillations were uniform at ∼0.02 Hz. Under hypoxia-induced membrane depolarization, these oscillations ceased. Simultaneous increases and decreases in potential of these cell clusters resulted from spontaneous burstlike activities lasting ∼1.5 s. type I cells, identified during the experiments by cluster formation in combination with large cell nuclei, seem to respond to hypoxia with heterogeneous kinetics.

Adenosine A₂a receptors and O₂ sensing in development

Reduced mitochondrial oxidative phosphorylation, via activation of adenylate kinase and the resulting exponential rise in the cellular AMP/ATP ratio, appears to be a critical factor underlying O₂ sensing in many chemoreceptive tissues in mammals. The elevated AMP/ATP ratio, in turn, activates key enzymes that are involved in physiologic adjustments that tend to balance ATP supply and demand. An example is the conversion of AMP to adenosine via 5'-nucleotidase and the resulting activation of adenosine A(₂A) receptors, which are involved in acute oxygen sensing by both carotid bodies and the brain. In fetal sheep, A(₂A) receptors associated with carotid bodies trigger hypoxic cardiovascular chemoreflexes, while central A(₂A) receptors mediate hypoxic inhibition of breathing and rapid eye movements. A(₂A) receptors are also involved in hypoxic regulation of fetal endocrine systems, metabolism, and vascular tone. In developing lambs, A(₂A) receptors play virtually no role in O₂ sensing by the carotid bodies, but brain A(₂A) receptors remain critically involved in the roll-off ventilatory response to hypoxia. In adult mammals, A(₂A) receptors have been implicated in O₂ sensing by carotid glomus cells, while central A(₂A) receptors likely blunt hypoxic hyperventilation. In conclusion, A(₂A) receptors are crucially involved in the transduction mechanisms of O₂ sensing in fetal carotid bodies and brains. Postnatally, central A(₂A) receptors remain key mediators of hypoxic respiratory depression, but they are less critical for O₂ sensing in carotid chemoreceptors, particularly in developing lambs.

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.

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

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

Presence of nicotinic, purinergic and dopaminergic receptors and the TASK-1 K+-channel in the mouse carotid body

We have characterized the mouse carotid body (CB) with special attention to nicotinic, purinergic and dopaminergic receptors as well as the TASK-1 K(+)-channel. Mouse CB sections were stained immunohistochemically and visualized using fluorescent and confocal microscopy. The CB type 1 cells contained the alpha3 (n=8), alpha4 (n=7), alpha7 (n=4) and beta2 (n=3) nicotinic acetylcholine receptor (nAChR) subunits, the ATP-receptors P2X(2) (n=15) and P2X(3) (n=9), the dopamine D(2) receptor (n=9) and the TASK-1 K(+)-channel (n=7). Here we report the presence of alpha3, alpha4, alpha7 and beta2 nAChR subunits, the D(2) receptor and the TASK-1 K(+)-channel in the mouse CB. Also, we confirm the presence of the P2X(2) and P2X(3) receptors in mouse CB. Thus, we have localized nicotinergic, purinergic and dopaminergic receptors and the TASK-1 K(+)-channel on a protein level in one species. Our data are in line with the theory that the CB chemoreceptor cell hosts an orchestra of receptor systems that ultimately modulate the response to hypoxia.

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.

Neurotransmitter and neuromodulatory mechanisms at peripheral arterial chemoreceptors

The control of breathing depends critically on sensory inputs to the central pattern generator of the brainstem, arising from peripheral arterial chemoreceptors located principally in the carotid bodies (CBs). The CB receptors, i.e. glomus or type I cells, are excited by chemical stimuli in arterial blood, particularly hypoxia, hypercapnia, acidosis and low glucose, which initiate corrective reflex cardiorespiratory and cardiovascular adjustments. Type I cells occur in clusters and are innervated by petrosal afferent fibres. Synaptic specializations (both chemical and electrical) occur between type I cells and petrosal terminals, and between neighbouring type I cells. This, together with the presence of a wide array of neurotransmitters and neuromodulators linked to both ionotropic and metabotropic receptors, allows for a complex modulation of CB sensory output. Studies in several laboratories over the last 20 years have provided much insight into the transduction mechanisms. More recent studies, aided by the development of a co-culture model of the rat CB, have shed light on the role of neurotransmitters and neuromodulators in shaping the afferent response. This review highlights some of these developments, which have contributed to our current understanding of information processing at CB chemoreceptors.

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.

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).

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.

Carotid body: new stimuli and new preparations--invited article

Beginning with the pioneering work of Heymans and collaborators in the 1930's, investigations into the role of the mammalian carotid body (CB) in the control of ventilation have attracted much attention. Progress for many years was restricted to the whole animal and organ level, resulting in characterization of the stimulus-response characteristics of the CB with its afferent nerve supply during exposure to chemostimuli such as low PO(2) (hypoxia), elevated PCO(2) (hypercapnia), and low pH (acidity). Major advances on the cellular and molecular mechanisms of chemotransduction occurred 20 years ago with the use of freshly-dissociated CB preparations and single cell studies using patch clamp and spectrofluorimetric techniques. This review will focus on more recent advances based on novel preparations including co-cultures of isolated CB receptor clusters and dispersed sensory or autonomic neurons, thin CB tissue slice preparations, and transgenic models. These preparations have contributed significantly, not only to our understanding of the transduction and neurotransmitter mechanisms that operate in the CB during sensory processing, but also to the identification and characterization of novel CB stimuli such as hypoglycemia. Though the complexity of this remarkable organ still belies its tiny size, these recent advances are slowly unraveling the intricacies surrounding its ability to act as a polymodal detector of blood-borne chemicals and to alter its sensitivity to patterned stimuli.

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.

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.

Intracellular traffic of the K+ channels TASK-1 and TASK-3: role of N- and C-terminal sorting signals and interaction with 14-3-3 proteins

The two-pore-domain potassium channels TASK-1 (KCNK3) and TASK-3 (KCNK9) modulate the electrical activity of neurons and many other cell types. We expressed TASK-1, TASK-3 and related reporter constructs in Xenopus oocytes, mammalian cell lines and various yeast strains to study the mechanisms controlling their transport to the surface membrane and the role of 14-3-3 proteins. We measured potassium currents with the voltage-clamp technique and fused N- and C-terminal fragments of the channels to various reporter proteins to study changes in subcellular localisation and surface expression. Mutational analysis showed that binding of 14-3-3 proteins to the extreme C-terminus of TASK-1 and TASK-3 masks a tri-basic motif, KRR, which differs in several important aspects from canonical arginine-based (RxR) or lysine-based (KKxx) retention signals. Pulldown experiments with GST fusion proteins showed that the KRR motif in the C-terminus of TASK-3 channels was able to bind to COPI coatomer. Disabling the binding of 14-3-3, which exposes the KRR motif, caused localisation of the GFP-tagged channel protein mainly to the Golgi complex. TASK-1 and TASK-3 also possess a di-basic N-terminal retention signal, KR, whose function was found to be independent of the binding of 14-3-3. Suppression of channel surface expression with dominant-negative channel mutants revealed that interaction with 14-3-3 has no significant effect on the dimeric assembly of the channels. Our results give a comprehensive description of the mechanisms by which 14-3-3 proteins, together with N- and C-terminal sorting signals, control the intracellular traffic of TASK-1 and TASK-3.

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.

Acid-sensitive ion channels and receptors

Acidosis is a noxious condition associated with inflammation, ischaemia or defective acid containment. As a consequence, acid sensing has evolved as an important property of afferent neurons with unmyelinated and thinly myelinated nerve fibres. Protons evoke multiple currents in primary afferent neurons, which are carried by several acid-sensitive ion channels. Among these, acid-sensing ion channels (ASICs) and transient receptor potential (TRP) vanilloid-1 (TRPV1) ion channels have been most thoroughly studied. ASICs survey moderate decreases in extracellular pH, whereas TRPV1 is activated only by severe acidosis resulting in pH values below 6. Two-pore-domain K(+) (K(2P)) channels are differentially regulated by small deviations of extra- or intracellular pH from physiological levels. Other acid-sensitive channels include TRPV4, TRPC4, TRPC5, TRPP2 (PKD2L1), ionotropic purinoceptors (P2X), inward rectifier K(+) channels, voltage-activated K(+) channels, L-type Ca(2+) channels, hyperpolarization-activated cyclic nucleotide gated channels, gap junction channels, and Cl(-) channels. In addition, acid-sensitive G protein coupled receptors have also been identified. Most of these molecular acid sensors are expressed by primary sensory neurons, although to different degrees and in various combinations. Emerging evidence indicates that many of the acid-sensitive ion channels and receptors play a role in acid sensing, acid-induced pain and acid-evoked feedback regulation of homeostatic reactions. The existence and apparent redundancy of multiple pH surveillance systems attests to the concept that acid-base regulation is a vital issue for cell and tissue homeostasis. Since upregulation and overactivity of acid sensors appear to contribute to various forms of chronic pain, acid-sensitive ion channels and receptors are considered as targets for novel analgesic drugs. This approach will only be successful if the pathological implications of acid sensors can be differentiated pharmacologically from their physiological function.

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.

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.

Hydrogen sulfide as an oxygen sensor in trout gill chemoreceptors

O2 chemoreceptors elicit cardiorespiratory reflexes in all vertebrates, but consensus on O2-sensing signal transduction mechanism(s) is lacking. We recently proposed that hydrogen sulfide (H2S) metabolism is involved in O2 sensing in vascular smooth muscle. Here, we examined the possibility that H2S is an O2 sensor in trout chemoreceptors where the first pair of gills is a primary site of aquatic O2 sensing and the homolog of the mammalian carotid body. Intrabuccal injection of H2S in unanesthetized trout produced a dose-dependent bradycardia and increased ventilatory frequency and amplitude similar to the hypoxic response. Removal of the first, but not second, pair of gills significantly inhibited H2S-mediated bradycardia, consistent with the loss of aquatic chemoreceptors. mRNA for H2S-synthesizing enzymes, cystathionine beta-synthase and cystathionine gamma-lyase, was present in branchial tissue. Homogenized gills produced H2S enzymatically, and H2S production was inhibited by O2, whereas mitochondrial H2S consumption was O2 dependent. Ambient hypoxia did not affect plasma H2S in unanesthetized trout, but produced a PO2-dependent increase in a sulfide moiety suggestive of increased H2S production. In isolated zebrafish neuroepithelial cells, the putative chemoreceptive cells of fish, both hypoxia and H2S, produced a similar approximately 10-mV depolarization. These studies are consistent with H2S involvement in O2 sensing/signal transduction pathway(s) in chemoreceptive cells, as previously demonstrated in vascular smooth muscle. This novel mechanism, whereby H2S concentration ([H2S]) is governed by the balance between constitutive production and oxidation, tightly couples tissue [H2S] to PO2 and may provide an exquisitely sensitive, yet simple, O2 sensor in a variety of tissues.

Integrative biology of an embryonic respiratory behaviour in pond snails:the `embryo stir-bar hypothesis'

Embryos of freshwater snails undergo direct development from single cell to juvenile inside egg masses that are deposited on vegetation and other substratum in pond, lake and stream habitats. Helisoma trivolvis, a member of the Planorbidae family of basommatophoran snails, has served as a model for studying the developmental and physiological roles for neurotransmitters during embryogenesis. Early studies revealed that H. trivolvis embryos from stage E15 to E30, the period between gastrulation and the trochophore–juvenile transition, display a cilia-driven behaviour consisting of slow basal rotation and transient periods of rapid rotation. The discovery of a bilateral pair of early serotonergic neurons,named ENC1, which project an apical process to the embryo surface and basal neurites to ciliated cells, prompted the hypothesis that each ENC1 is a dual-function sensory and motor neuron mediating a physiological embryonic response. This article reviews our past and present studies and addresses questions concerning this hypothesis, including the following. (1) What environmental signal regulates ENC1 activity and rotational behaviour? (2)Does ENC1 function as both a primary sensory and motor neuron underlying the rotational behaviour? (3) What are the sensory transduction mechanisms? (4)How does ENC1 regulate ciliary beating? (5) Do other basommatophoran species have similar neural–ciliary pathways and behavioural responses? (6) How is the behaviour manifest in the dynamic natural environment? In this review,we introduce the `embryo stir-bar hypothesis', which proposes that embryonic rotation is a hypoxia-sensitive respiratory behaviour responsible for mixing the egg capsule fluid, thereby enhancing delivery of environmental oxygen to the embryo.

Age-dependent involvement of ATP-sensitive potassium channel Kir6.2 in hypoxic ventilatory depression of mouse

In order to examine whether ATP-sensitive potassium channel Kir6.2 is involved in hypoxic ventilatory responses, especially in hypoxic ventilatory depression (HVD), and whether the involvement shows age-dependence, we measured the hypoxic ventilatory response in the Kir6.2-knockout mouse (Kir6.2-/-) in an unanesthetized unrestrained state by means of pressure plethysmography in the 2nd and 4th postnatal weeks, and compared the response with that of its wild type counterpart, the C57BL6/J mouse. In the 4th postnatal week, but not in the 2nd week, the Kir6.2-/- exhibited a larger and longer initial augmentation and a weaker subsequent depression of respiratory frequency and ventilation in response to hypoxia (FIO(2)=0.12 in N(2)). These findings suggest that Kir6.2 is involved in HVD of the mouse at a certain point during the postnatal development.

TASK channels determine pH sensitivity in select respiratory neurons but do not contribute to central respiratory chemosensitivity

Central respiratory chemoreception is the mechanism by which the CNS maintains physiologically appropriate pH and PCO2 via control of breathing. A prominent hypothesis holds that neural substrates for this process are distributed widely in the respiratory network, especially because many neurons that make up this network are chemosensitive in vitro. We and others have proposed that TASK channels (TASK-1, K(2P)3.1 and/or TASK-3, K(2P)9.1) may serve as molecular sensors for central chemoreception because they are highly expressed in multiple neuronal populations in the respiratory pathway and contribute to their pH sensitivity in vitro. To test this hypothesis, we examined the chemosensitivity of two prime candidate chemoreceptor neurons in vitro and tested ventilatory responses to CO2 using TASK channel knock-out mice. The pH sensitivity of serotonergic raphe neurons was abolished in TASK channel knock-outs. In contrast, pH sensitivity of neurons in the mouse retrotrapezoid nucleus (RTN) was fully maintained in a TASK null background, and pharmacological evidence indicated that a K+ channel with properties distinct from TASK channels contributes to the pH sensitivity of rat RTN neurons. Furthermore, the ventilatory response to CO2 was completely retained in single or double TASK knock-out mice. These data rule out a strict requirement for TASK channels or raphe neurons in central respiratory chemosensation. Furthermore, they indicate that a non-TASK K+ current contributes to chemosensitivity of RTN neurons, which are profoundly pH-sensitive and capable of driving respiratory output in response to local pH changes in vivo.

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.

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.