The effect of mitochondrial inhibitors on membrane currents in isolated neonatal rat carotid body type I cells

The zebrafish embryo as a dynamic model of anoxia tolerance

Developing organisms depend upon a delicate balance in the supply and demand of energy to adapt to variable oxygen availability, although the essential mechanisms determining such adaptation remain elusive. In this study, we examine reversible anoxic arrest and dynamic bioenergetic transitions during zebrafish development. Our data reveal that the duration of anoxic viability corresponds to the developmental stage and anaerobic metabolic rate. Diverse chemical inhibitors of mitochondrial oxidative phosphorylation induce a similar arrest in normoxic embryos, suggesting a pathway responsive to perturbations in aerobic energy production rather than molecular oxygen. Consistent with this concept, arrest is accompanied by rapid activation of the energy-sensing AMP-activated protein kinase pathway, demonstrating a potential link between the sensing of energy status and adaptation to oxygen availability. These observations permit mechanistic insight into energy homeostasis during development that now enable genetic and small molecule screens in this vertebrate model of anoxia tolerance.

Effects of anoxia and aglycemia on cytosolic calcium regulation in rat sensory neurons

Nociceptive neurons play an important role in ischemia by sensing and transmitting information to the CNS and by secreting peptides and nitric oxide, which can have local effects. While these responses are probably primarily mediated by acid sensing channels, other events occurring in ischemia may also influence neuron function. In this study, we have investigated the effects of anoxia and anoxic aglycemia on Ca²⁺ regulation in sensory neurons from rat dorsal root ganglia. Anoxia increased [Ca²⁺]_i by evoking Ca²⁺ release from two distinct internal stores one sensitive to carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP) and one sensitive to caffeine, cyclopiazonic acid (CPA), and ryanodine [assumed to be the endoplasmic reticulum (ER)]. Anoxia also promoted progressive decline in ER Ca²⁺ content. Despite partially depolarizing mitochondria, anoxia had relatively little effect on mitochondrial Ca²⁺ uptake when neurons were depolarized but substantially delayed mitochondrial Ca²⁺ release and subsequent Ca²⁺ clearance from the cytosol on repolarization. Anoxia also reduced both sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) activity and Ca²⁺ extrusion [probably via plasma membrane Ca²⁺-ATPase (PMCA)]. Thus anoxia has multiple effects on [Ca²⁺]_i homeostasis in sensory neurons involving internal stores, mitochondrial buffering, and Ca²⁺ pumps. Under conditions of anoxic aglycemia, there was a biphasic and more profound elevation of [Ca²⁺]_i, which was associated with complete ER Ca²⁺ store emptying and progressive, and eventually complete, inhibition of Ca²⁺ clearance by PMCA and SERCA. These data clearly show that loss of oxygen, and exhaustion of glycolytic substrates, can profoundly affect many aspects of cell Ca²⁺ regulation, and this may play an important role in modulating neuronal responses to ischemia.

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.

Rat carotid body chemosensory discharge and glomus cell HIF-1α expression in vitro: regulation by a common oxygen sensor

Addition of Pco (∼350 Torr) to a normoxic medium (Po2 of ∼130 Torr) was used to investigate the relationship between carotid body (CB) sensory discharge and expression of hypoxia-inducible factor 1α (HIF-1α) in glomus cells. Afferent electrical activity measured for in vitro -perfused rat CB increased rapidly (1–2 s) with addition of high CO (Pco of ∼350 Torr; Po2 of ∼130 Torr), and this increase was fully reversed by white light. At submaximal light intensities, the extent of reversal was much greater for monochromatic light at 430 and 590 nm than for light at 450, 550, and 610 nm. This wavelength dependence is consistent with the action spectrum of the CO compound of mitochondrial cytochrome a3. Interestingly, when isolated glomus cells cultured for 45 min in the presence of high CO (Pco of ∼350 Torr; Po2 of ∼130 Torr) in the dark, the levels of HIF-1α, which turn over slowly (many minutes), increased. This increase was not observed if the cells were illuminated with white light during the incubation. Monochromatic light at 430- and 590-nm light was much more effective than that at 450, 550, and 610 nm in blocking the CO-induced increase in HIF-1α, as was the case for chemoreceptor discharge. Although the changes in HIF-1α take minutes and those for CB neural activity occur in 1–2 s, the similar responses to CO and light suggest that the oxygen sensor is the same (mitochondrial cytochrome a3).

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.

AMP-activated protein kinase mediates carotid body excitation by hypoxia

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

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.

A rotenone-sensitive site and H2O2 are key components of hypoxia-sensing in neonatal rat adrenomedullary chromaffin cells

In the perinatal period, adrenomedullary chromaffin cells (AMC) directly sense PO2 and secrete catecholamines during hypoxic stress, and this response is lost in juvenile ( approximately 2 week-old) chromaffin cells following postnatal innervation. Here we tested the hypothesis that a rotenone-sensitive O2-sensor and ROS are involved in the hypoxic response of AMC cultured from neonatal and juvenile rats. In whole-cell recordings, hypoxia (PO2=5-15 mm Hg) inhibited outward current in neonatal AMC; this response was reversed by exogenous H2O2 and mimicked and occluded by intracellular catalase (1000 units/ml), as well as the antioxidants, N-acetyl-L-cysteine (NAC; 50 microM) and Trolox (200 microM). Acute hypoxia decreased ROS levels and stimulated ATP secretion in these cells, as measured by luminol and luciferin-luciferase chemiluminescence, respectively. Of several mitochondrial electron transport chain (ETC) inhibitors tested, only rotenone, a complex I blocker, mimicked and occluded the effects of hypoxia on outward current, cellular ROS, and ATP secretion. Succinate donors, which act as complex II substrates, reversed the effects of hypoxia and rotenone in neonatal AMC. In contrast, in hypoxia-insensitive juvenile AMC, neither NAC nor rotenone stimulated ATP secretion though they both caused a decrease in ROS levels. We propose that O2-sensing by neonatal AMC is mediated by decreased ROS generation via a rotenone-sensitive site that is coupled to outward current inhibition and secretion. Interestingly, juvenile AMC display at least two modifications, i.e. an uncoupling of the O2-sensor from ROS regulation, and an apparent insensitivity of outward current to decreased ROS.

Catecholamine secretion from rat foetal adrenal chromaffin cells and hypoxia sensitivity

The adrenal medulla chromaffin cells (AMCs) secrete catecholamines in response to various types of stress. We examined the hypoxia-sensitivity of catecholamine secretion by rat foetal chromaffin cells in which the innervation by the splanchnic nerve is not established. The experiments were performed in primary cultured cells from two different ages of foetuses (F15 and F19). Membrane potential of AMCs was monitored with the patch clamp technique, and the catecholamine secretion was detected by amperometry. We found that: (1) AMCs from F19 foetuses showed hypoxia-induced catecholamine release. (2) This hypoxia-induced secretion is produced by membrane depolarization generated by an inhibition of Ca(2+)-activated K(+) current [I (K(Ca))] current. (3) Chromaffin precursor cells from F15 foetuses secrete catecholamine. The quantal release is calcium-dependent, but the size of the quantum is reduced. (4) In the precursor cells, a hypoxia-induced membrane hyperpolarization is originated by an ATP-sensitive K(+) current [I (K(ATP))] activation. (5) During the prenatal period, at F15, the percentage of the total outward current for I (K(ATP)) and I (K(Ca)) was 50 and 29.5%, respectively, whereas at F19, I (K(ATP)) is reduced to 14%, and I (K(Ca)) became 64% of the total current. We conclude that before birth, the age-dependent hypoxia response of chromaffin cells is modulated by the functional activity of K(ATP) and K(Ca) channels.

The role of maxiK channels in carotid body chemotransduction

MaxiK channels are a unique class of K(+) channels activated by both voltage and intracellular Ca(2+). Derived from a single gene, their diversity arises from extensive splicing, and their wide distribution has led to their implication in a large variety of cellular functions. In the carotid body, they have been proposed to contribute to the resting membrane potential of type I cells, and also to be O(2) sensitive. Thus, they have been suggested to have an important role in hypoxic chemotransduction. Their O(2) sensitivity is preserved when the channels are expressed in HEK 293 cells, permitting detailed studies of candidate mechanisms underlying hypoxic inhibition of maxiK channels. In this article, we review evidence for and against an important role for maxiK channels in chemotransduction. We also consider different mechanisms proposed to account for hypoxic channel inhibition and suggest that, although our understanding of this important physiological process has advanced significantly in recent years, there remain important, unanswered questions as to the importance of maxiK in carotid body chemoreception.

Ca2+ microdomains in smooth muscle

In smooth muscle, Ca(2+) controls diverse activities including cell division, contraction and cell death. Of particular significance in enabling Ca(2+) to perform these multiple functions is the cell's ability to localize Ca(2+) signals to certain regions by creating high local concentrations of Ca(2+) (microdomains), which differ from the cytoplasmic average. Microdomains arise from Ca(2+) influx across the plasma membrane or release from the sarcoplasmic reticulum (SR) Ca(2+) store. A single Ca(2+) channel can create a microdomain of several micromolar near (approximately 200 nm) the channel. This concentration declines quickly with peak rates of several thousand micromolar per second when influx ends. The high [Ca(2+)] and the rapid rates of decline target Ca(2+) signals to effectors in the microdomain with rapid kinetics and enable the selective activation of cellular processes. Several elements within the cell combine to enable microdomains to develop. These include the brief open time of ion channels, localization of Ca(2+) by buffering, the clustering of ion channels to certain regions of the cell and the presence of membrane barriers, which restrict the free diffusion of Ca(2+). In this review, the generation of microdomains arising from Ca(2+) influx across the plasma membrane and the release of the ion from the SR Ca(2+) store will be discussed and the contribution of mitochondria and the Golgi apparatus as well as endogenous modulators (e.g. cADPR and channel binding proteins) will be considered.

Detecting acute changes in oxygen: will the real sensor please stand up?

The majority of physiological processes proceed most favourably when O(2) is in plentiful supply. However, there are a number of physiological and pathological circumstances in which this supply is reduced either acutely or chronically. A crucial homeostatic response to such arterial hypoxaemia is carotid body excitation and a resultant increase in ventilation. Central to this response in carotid body, and many other chemosensory tissues, is the rapid inhibition of ion channels by hypoxia. Since the first direct demonstration of hypoxia-evoked depression in K(+) channel activity, the numbers of mechanisms which have been proposed to serve as the primary O(2) sensor have been almost as numerous as the experimental strategies with which to probe their nature. Three of the current favourite candidate mechanisms are mitochondria, AMP-activated kinase and haemoxygenase-2; a fourth proposal has been NADPH oxidase, but recent evidence suggests that this enzyme plays a secondary role in the O(2)-sensing process. All of these proposals have attractive points, but none can fully reconcile all of the data which have accumulated over the last two decades or so, suggesting that there may, in fact, not be a unique sensing system even within a single cell type. This latter point is key, because it implies that the ability of a cell to respond appropriately to decreased O(2) availability is biologically so important that several mechanisms have evolved to ensure that cellular function is never compromised during moderate to severe hypoxic insult.

A Phenotypic Perspective on Mammalian Oxygen Sensor Candidates

Chronic hypoxic stimulation in mammals can induce several phenotypic changes, such as polycythemia, pulmonary vascular changes, pulmonary hypertension, and carotid body (CB) enlargement. These phenotypic alterations provide a tool to test whether an oxygen sensor candidate is involved in an organism's response to environmental hypoxia. Here I evaluate the phenotypic evidence for several commonly considered oxygen sensor candidates. Germline mutations in NADPH oxidase, mitochondrial complexes I, III, IV, and heme oxygenase 2 genes cause different phenotypic consequences, suggesting distinct physiological roles rather than oxygen sensing. Germline mutations in VHL and HIF1 prolyl hydroxylase 2 genes cause polycythemia consistent with their role in oxygen homeostasis. However, it is unclear whether environmental variations affecting oxygen availability modify their phenotype, as would be expected from a defect in an oxygen sensor. Succinate dehydrogenase (SDH); mitochondrial complex II) germline mutations cause CB paragangliomas and there is evidence that the severity and the population genetics of paragangliomas may be influenced by altitude. Thus, from a phenotypic perspective, succinate dehydrogenase (SDH) appears to be a well-supported oxygen sensor candidate. It is suggested that a universal oxygen sensor candidate must be supported by evidence from multiple layers of biological complexity.

Reactive oxygen species in the control of hypoxia-inducible factor-mediated gene expression

Reactive oxygen species (ROS) have long been considered as cytotoxic. However, recent evidence indicates a prominent role of ROS as signaling molecules in the response to hormones, growth and coagulation factors, cytokines and other factors as well as to changes in oxygen tension. The hypoxia-inducible transcription factors (HIFs) are key players in the cellular response to changes in oxygen tension. Recently, HIFs have also been shown to respond to the above-mentioned non-hypoxic stimuli. In this article, the role of ROS in the regulation of HIF-1 under hypoxic and non-hypoxic conditions is summarized.

Oxygen sensing in the body

This review is divided into three parts: (a) The primary site of oxygen sensing is the carotid body which instantaneously respond to hypoxia without involving new protein synthesis, and is historically known as the first oxygen sensor and is therefore placed in the first section (Lahiri, Roy, Baby and Hoshi). The carotid body senses oxygen in acute hypoxia, and produces appropriate responses such as increases in breathing, replenishing oxygen from air. How this oxygen is sensed at a relatively high level (arterial PO2 approximately 50 Torr) which would not be perceptible by other cells in the body, is a mystery. This response is seen in afferent nerves which are connected synaptically to type I or glomus cells of the carotid body. The major effect of oxygen sensing is the increase in cytosolic calcium, ultimately by influx from extracellular calcium whose concentration is 2 x 10(4) times greater. There are several contesting hypotheses for this response: one, the mitochondrial hypothesis which states that the electron transport from the substrate to oxygen through the respiratory chain is retarded as the oxygen pressure falls, and the mitochondrial membrane is depolarized leading to the calcium release from the complex of mitochondria-endoplasmic reticulum. This is followed by influx of calcium. Also, the inhibitors of the respiratory chain result in mitochondrial depolarization and calcium release. The other hypothesis (membrane model) states that K(+) channels are suppressed by hypoxia which depolarizes the membrane leading to calcium influx and cytosolic calcium increase. Evidence supports both the hypotheses. Hypoxia also inhibits prolyl hydroxylases which are present in all the cells. This inhibition results in membrane K(+) current suppression which is followed by cell depolarization. The theme of this section covers first what and where the oxygen sensors are; second, what are the effectors; third, what couples oxygen sensors and the effectors. (b) All oxygen consuming cells have a built-in mechanism, the transcription factor HIF-1, the discovery of which has led to the delineation of oxygen-regulated gene expression. This response to chronic hypoxia needs new protein synthesis, and the proteins of these genes mediate the adaptive physiological responses. HIF-1alpha, which is a part of HIF-1, has come to be known as master regulator for oxygen homeostasis, and is precisely regulated by the cellular oxygen concentration. Thus, the HIF-1 encompasses the chronic responses (gene expression in all cells of the body). The molecular biology of oxygen sensing is reviewed in this section (Semenza). (c) Once oxygen is sensed and Ca(2+) is released, the neurotransmittesr will be elaborated from the glomus cells of the carotid body. Currently it is believed that hypoxia facilitates release of one or more excitatory transmitters from glomus cells, which by depolarizing the nearby afferent terminals, leads to increases in the sensory discharge. The transmitters expressed in the carotid body can be classified into two major categories: conventional and unconventional. The conventional neurotransmitters include those stored in synaptic vesicles and mediate their action via activation of specific membrane bound receptors often coupled to G-proteins. Unconventional neurotransmitters are those that are not stored in synaptic vesicles, but spontaneously generated by enzymatic reactions and exert their biological responses either by interacting with cytosolic enzymes or by direct modifications of proteins. The gas molecules such as NO and CO belong to this latter category of neurotransmitters and have unique functions. Co-localization and co-release of neurotransmitters have also been described. Often interactions between excitatory and inhibitory messenger molecules also occur. Carotid body contains all kinds of transmitters, and an interplay between them must occur. But very little has come to be known as yet. Glimpses of these interactions are evident in the discussion in the last section (Prabhakar).

AMP-activated protein kinase underpins hypoxic pulmonary vasoconstriction and carotid body excitation by hypoxia in mammals

In order to maintain tissue partial pressure of oxygen (P(O(2))) within physiological limits, vital homeostatic mechanisms monitor O(2) supply and respond to a fall in P(O(2)) by altering respiratory and circulatory function, and the capacity of the blood to transport O(2). Two systems that are key to this process in the acute phase are the pulmonary arteries and the carotid bodies. Hypoxic pulmonary vasoconstriction is driven by mechanisms intrinsic to the pulmonary arterial smooth muscle and endothelial cells, and aids ventilation-perfusion matching in the lung by diverting blood flow from areas with an O(2) deficit to those that are rich in O(2). By contrast, a fall in arterial P(O(2)) precipitates excitation-secretion coupling in carotid body type I cells, increases sensory afferent discharge from the carotid body and thereby elicits corrective changes in breathing patterns via the brainstem. There is a general consensus that hypoxia inhibits mitochondrial oxidative phosphorylation in these O(2)-sensing cells over a range of P(O(2)) values that has no such effect on other cell types. However, the question remains as to the identity of the mechanism that underpins hypoxia-response coupling in O(2)-sensing cells. Here, I lay out the case in support of a primary role for AMP-activated protein kinase in mediating chemotransduction by hypoxia.

AMP-activated protein kinase and the regulation of Ca2+ signalling in O2-sensing cells

All cells respond to metabolic stress. However, a variety of specialized cells, commonly referred to as O2-sensing cells, are acutely sensitive to relatively small changes in PO2. Within a variety of organisms such O2-sensing cells have evolved as vital homeostatic mechanisms that monitor O2 supply and alter respiratory and circulatory function, as well as the capacity of the blood to transport O2. Thereby, arterial PO2 may be maintained within physiological limits. In mammals, for example, two key tissues that contribute to this process are the pulmonary arteries and the carotid bodies. Constriction of pulmonary arteries by hypoxia optimizes ventilation-perfusion matching in the lung, whilst carotid body excitation by hypoxia initiates corrective changes in breathing patterns via increased sensory afferent discharge to the brain stem. Despite extensive investigation, the precise mechanism(s) by which hypoxia mediates these responses has remained elusive. It is clear, however, that hypoxia inhibits mitochondrial function in O2-sensing cells over a range of PO2 that has no such effect on other cell types. This raised the possibility that AMP-activated protein kinase might function to couple mitochondrial oxidative phosphorylation to Ca2+ signalling mechanisms in O2-sensing cells and thereby underpin pulmonary artery constriction and carotid body excitation by hypoxia. Our recent investigations have provided significant evidence in support of this view.

Oxygen-sensing pathway for SK channels in the ovine adrenal medulla

1. The intracellular pathways that modulate the opening of oxygen-sensitive ion channels during periods of hypoxia are poorly understood. Different tissues appear to use either NADPH oxidase or a rotenone-sensitive mechanism as an oxygen sensor. The aim of the present study was to identify the oxygen-sensing pathway in the oxygen-sensitive sheep adrenal medullary chromaffin cell (AMCC). 2. The whole-cell patch-clamp technique was used to measure K+ currents in dissociated adult ovine chromaffin cells as well as SK channel currents expressed in the H4IIE cell line. 3. Diphenyliodonium, an inhibitor of NADPH oxidase, had no effect on the hypoxia-evoked closure of K+ channels in primary AMCC, whereas the mitochondrial inhibitor rotenone abolished the hypoxia-evoked response. Both these compounds significantly reduced K+ current amplitude under normoxic conditions. 4. One possible mechanism through which the oxygen sensor may modulate K+ channel activity is by altering the redox state of the cell. In sheep AMCC, altering the redox state by the addition of H2O2 to the extracellular solution increased K+ conductance. 5. The oxygen-sensitive K+ (Ko2) channels in sheep chromaffin cells are from the SK family and the whole-cell conductance of cells expressing mouse SK2 or SK3, but not human SK1, was increased by H2O2 and decreased by the reducing agent dithiothreitol. 6. These studies show that, in sheep AMCC, Ko2 channels are modulated via a rotenone-sensitive mechanism and that alteration of the cellular redox state mimics the change produced by alterations in Po2. In a heterologous expression system, SK2 and SK3 channels, the channels that initiate hypoxia-evoked changes in AMCC function, are modulated appropriately by changes in cellular redox state.

Mitochondrial function and carotid body transduction

Carotid body chemoreceptors respond to a decrease in arterial oxygen tension by increasing spiking activity on the sinus nerve. Our understanding of the oxygen-transducing ability of the organ arose from studies in the 1930s intended to understand how metabolic poisons stimulated breathing. Since that time, an intimate link between energy state and hypoxia sensing has been assumed and forms the basis of the metabolic hypothesis of oxygen sensing. This hypothesis is supported by studies demonstrating a loss of mitochondrial potential in carotid body cells at oxygen tensions that cause no change in cells from other tissues. Although the nature of the coupling between mitochondrial function and nerve excitation remains unresolved, experimental evidence supports roles for (1) release of mitochondrial calcium stores, (2) modulation of membrane channels that are linked to mitochondrial complexes I and IV, and (3) generation of signaling intermediates, such as reactive oxygen species (ROS) from complex I and III of the electron transport chain. If the mitochondrion is the oxygen-sensing site for peripheral chemoreceptors, then there exists the potential ability to manipulate, perhaps pharmacologically, the sensing function by alterations in expression of uncoupler proteins or chemicals that can alter the affinity of cytochrome oxidase for oxygen. Such manipulation may be useful for the treatment of hypoventilation syndromes or high altitude accommodation.

O2 sensing at the mammalian carotid body: why multiple O2 sensors and multiple transmitters?

Carotid bodies are the sensory organs for detecting systemic hypoxia and the ensuing reflexes prevent the development of tissue/cellular hypoxia. Although every mammalian cell responds to hypoxia, O2 sensing by the carotid body is unique in that it responds instantaneously (within seconds) to even a modest drop in arterial PO2. Sensing hypoxia in the carotid body requires an initial transduction step involving O2 sensor(s) and transmitter(s) for subsequent activation of the afferent nerve ending. This brief review focuses on: (a) whether the transduction involves 'single' or 'multiple' O2 sensors; (b) the identity of the excitatory transmitter(s) responsible for afferent nerve activation by hypoxia; and (c) whether inhibitory transmitters have any functional role. The currently proposed O2 sensors include various haem-containing proteins, and a variety of O2-sensitive K+ channels. It is proposed that the transduction involves an ensemble of, and interactions between, haem-containing proteins and O2-sensitive K+-channel proteins functioning as a 'chemosome'; the former for conferring sensitivity to wide range of PO2 values and the latter for the rapidity of the response. Hypoxia releases both excitatory and inhibitory transmitters from the carotid body. ATP is emerging as an important excitatory transmitter for afferent nerve activation by hypoxia. Whereas the inhibitory messengers act in concert with excitatory transmitters like a 'push-pull' mechanism to prevent over excitation, conferring the 'slowly adapting' nature of the afferent nerve activation during prolonged hypoxia. Further studies are needed to test the interactions between putative O2 sensors and excitatory and inhibitory transmitters in the carotid body.

Does AMP-activated protein kinase couple inhibition of mitochondrial oxidative phosphorylation by hypoxia to calcium signaling in O2-sensing cells?

Specialized O2-sensing cells exhibit a particularly low threshold to regulation by O2 supply and function to maintain arterial pO2 within physiological limits. For example, hypoxic pulmonary vasoconstriction optimizes ventilation-perfusion matching in the lung, whereas carotid body excitation elicits corrective cardio-respiratory reflexes. It is generally accepted that relatively mild hypoxia inhibits mitochondrial oxidative phosphorylation in O2-sensing cells, thereby mediating, in part, cell activation. However, the mechanism by which this process couples to Ca2+ signaling mechanisms remains elusive, and investigation of previous hypotheses has generated contrary data and failed to unite the field. We propose that a rise in the cellular AMP/ATP ratio activates AMP-activated protein kinase and thereby evokes Ca2+ signals in O2-sensing cells. Co-immunoprecipitation identified three possible AMP-activated protein kinase subunit isoform combinations in pulmonary arterial myocytes, with alpha1 beta2 gamma1 predominant. Furthermore, their tissue-specific distribution suggested that the AMP-activated protein kinase-alpha1 catalytic isoform may contribute, via amplification of the metabolic signal, to the pulmonary selectivity required for hypoxic pulmonary vasoconstriction. Immunocytochemistry showed AMP-activated protein kinase-alpha1 to be located throughout the cytoplasm of pulmonary arterial myocytes. In contrast, it was targeted to the plasma membrane in carotid body glomus cells. Consistent with these observations and the effects of hypoxia, stimulation of AMP-activated protein kinase by phenformin or 5-aminoimidazole-4-carboxamide-riboside elicited discrete Ca2+ signaling mechanisms in each cell type, namely cyclic ADP-ribose-dependent Ca2+ mobilization from the sarcoplasmic reticulum via ryanodine receptors in pulmonary arterial myocytes and transmembrane Ca2+ influx into carotid body glomus cells. Thus, metabolic sensing by AMP-activated protein kinase may mediate chemotransduction by hypoxia.

Hemeoxygenase-2 as an O2 sensor in K+ channel-dependent chemotransduction

The physiological response of the carotid body is critically dependent upon oxygen-sensing by potassium channels expressed in glomus cells. One such channel is the large conductance, voltage- and calcium-dependent potassium channel, BK(Ca). Although it is well known that a decrease in oxygen evokes glomus cell depolarization, voltage-gated calcium entry, and transmitter release, the molecular identity of the upstream oxygen sensor has been the subject of some controversy for decades. Recently, we have demonstrated that hemeoxygenase-2 associates tightly with recombinant BK(Ca) and that activity of this enzyme confers oxygen sensitivity to the BK(Ca) channel complex. Similar observations were also made in native channels recorded from carotid body glomus cells, suggesting that hemoxygenase-2 functions as an oxygen sensor of native and recombinant BK(Ca) channels.

Role of mitochondria in the regulation of hypoxia-inducible factor-1α in the rat carotid body glomus cells

Hypoxia-inducible factor-1alpha (HIF-1alpha) protein, a heterodimeric transcription factor that regulates transcriptional activation of several genes, is involved in adaptive responses to hypoxia. Earlier, we have reported that in carotid body (CB), the peripheral oxygen sensing organ, HIF-1alpha is up-regulated during hypoxia. One model proposes that an intact mitochondrial respiratory chain is necessary for this regulation of HIF-1alpha. To test this hypothesis in the CB glomus cells, we studied the effect of mitochondrial electron transport chain (ETC) inhibitors: rotenone (complex I; 1 microM), malonate (complex II; 0.5 M), antimycin A (complex III; 1 microg/ml), sodium azide (complex IV; 5 mM), and uncoupler of oxidative phosphorylation: carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP; 1 mM) on HIF-1alpha expression during normoxia and hypoxia. Inhibitors and uncoupler of mitochondrial ETC abrogated hypoxia-induced HIF-1alpha expression in isolated glomus cells significantly (P < 0.001). Effect of rotenone during hypoxia was abolished by succinate (4 mM), a substrate for complex II. Further, HIF-1alpha expression was not altered by any of these mitochondrial inhibitors during normoxia. Taken together, these results strongly indicate that a functional mitochondrial ETC is required for the stabilization of HIF-1alpha, and further the connection between HIF-1alpha and mitochondria in CB oxygen sensing is reiterated.

Hypoxic Augmentation of Ca2+ Channel Currents Requires a Functional Electron Transport Chain

The incidence of Alzheimer disease is increased following ischemic episodes, and we previously demonstrated that following chronic hypoxia (CH), amyloid beta (Abeta) peptide-mediated increases in voltage-gated L-type Ca(2+) channel activity contribute to the Ca(2+) dyshomeostasis seen in Alzheimer disease. Because in certain cell types mitochondria are responsible for detecting altered O(2) levels we examined the role of mitochondrial oxidant production in the regulation of recombinant Ca(2+) channel alpha(1C) subunits during CH and exposure to Abeta-(1-40). In wild-type (rho(+)) HEK 293 cells expressing recombinant L-type alpha(1C) subunits, Ca(2+) currents were enhanced by prolonged (24 h) exposure to either CH (6% O(2)) or Abeta-(1-40) (50 nm). By contrast the response to CH was absent in rho(0) cells in which the mitochondrial electron transport chain (ETC) was depleted following long term treatment with ethidium bromide or in rho(+) cells cultured in the presence of 1 microm rotenone. CH was mimicked in rho(0) cells by the exogenous production of O2(-.). by xanthine/xanthine oxidase. Furthermore Abeta-(1-40) enhanced currents in rho(0) cells to a degree similar to that seen in cells with an intact ETC. The antioxidants ascorbate (200 microm) and Trolox (500 microm) ablated the effect of CH in rho(+) cells but were without effect on Abeta-(1-40)-mediated augmentation of Ca(2+) current in rho(0) cells. Thus oxidant production in the mitochondrial ETC is a critical factor, acting upstream of amyloid beta peptide production in the up-regulation of Ca(2+) channels in response to CH.

O2 sensing by recombinant TWIK-related halothane-inhibitable K+ channel-1 background K+ channels heterologously expressed in human embryonic kidney cells

Hypoxic inhibition of K+ channels provides a link between low O2 and cell function, and in glossopharyngeal neurons hypoxic inhibition of a TWIK-related halothane-inhibitable K+ channel-1 (THIK-1)-like background K+ channel regulates neuronal function. In the present study, we examined directly the O2 sensitivity of recombinant THIK-1 channels, expressed in human embryonic kidney (HE293) cells. THIK-1 expression conferred a moderately outwardly rectifying halothane-inhibited and arachidonic acid-potentiated K+ current and invoked a strongly hyperpolarized resting membrane potential. Endogenous K+ currents in untransfected cells were unaffected by either agent. Hypoxia (P(O2), 20 mmHg) reversibly inhibited THIK-1 currents and caused membrane depolarization, effects that were occluded by halothane. Neither the mitochondrial complex I inhibitors rotenone, myxothiazol and sodium cyanide, nor the NADPH oxidase inhibitors diphenylene iodonium and phenylarsine oxide, were effective in inhibiting the O2-sensitivity of THIK-1. Thus, hypoxic inhibition of THIK-1 occurs by a mechanism dissimilar to that which regulates the activity of other members of the background K+ channel family. Given the O2 sensitivity of THIK-1 channels and their abundant expression in the CNS, we raise for the first time the possibility of a physiological and/or pathological role for these channels during brain ischemia.