Lack of contribution of mitochondrial electron transport to acute O(2) sensing in model airway chemoreceptors

NOX2 (gp91phox) is a predominant O2 sensor in a human airway chemoreceptor cell line: biochemical, molecular, and electrophysiological evidence

Pulmonary neuroepithelial bodies (NEBs), composed of clusters of amine [serotonin (5-HT)] and peptide-producing cells, are widely distributed within the airway mucosa of human and animal lungs. NEBs are thought to function as airway O(2)-sensors, since they are extensively innervated and release 5-HT upon hypoxia exposure. The small cell lung carcinoma cell line (H146) provides a useful model for native NEBs, since they contain (and secrete) 5-HT and share the expression of a membrane-delimited O(2) sensor [classical NADPH oxidase (NOX2) coupled to an O(2)-sensitive K(+) channel]. In addition, both native NEBs and H146 cells express different NADPH oxidase homologs (NOX1, NOX4) and its subunits together with a variety of O(2)-sensitive voltage-dependent K(+) channel proteins (K(v)) and tandem pore acid-sensing K(+) channels (TASK). Here we used H146 cells to investigate the role and interactions of various NADPH oxidase components in O(2)-sensing using a combination of coimmunoprecipitation, Western blot analysis (quantum dot labeling), and electrophysiology (patchclamp, amperometry) methods. Coimmunoprecipitation studies demonstrated formation of molecular complexes between NOX2 and K(v)3.3 and K(v)4.3 ion channels but not with TASK1 ion channels, while NOX4 associated with TASK1 but not with K(v) channel proteins. Downregulation of mRNA for NOX2, but not for NOX4, suppressed hypoxia-sensitive outward current and significantly reduced hypoxia -induced 5-HT release. Collectively, our studies suggest that NOX2/K(v) complexes are the predominant O(2) sensor in H146 cells and, by inference, in native NEBs. Present findings favor a NEB cell-specific plasma membrane model of O(2)-sensing and suggest that unique NOX/K(+) channel combinations may serve diverse physiological functions.

Differential impairment of catecholaminergic cell maturation and survival by genetic mitochondrial complex II dysfunction

The SDHD gene (subunit D of succinate dehydrogenase) has been shown to be involved in the generation of paragangliomas and pheochromocytomas. Loss of heterozygosity of the normal allele is necessary for tumor transformation of the affected cells. As complete SdhD deletion is lethal, we have generated mouse models carrying a "floxed" SdhD allele and either an inducible (SDHD-ESR strain) or a catecholaminergic tissue-specific (TH-SDHD strain) CRE recombinase. Ablation of both SdhD alleles in adult SDHD-ESR mice did not result in generation of paragangliomas or pheochromocytomas. In contrast, carotid bodies from these animals showed smaller volume than controls. In accord with these observations, the TH-SDHD mice had decreased cell numbers in the adrenal medulla, carotid body, and superior cervical ganglion. They also manifested inhibited postnatal maturation of mesencephalic dopaminergic neurons and progressive cell loss during the first year of life. These alterations were particularly intense in the substantia nigra, the most affected neuronal population in Parkinson's disease. Unexpectedly, TH(+) neurons in the locus coeruleus and group A13, also lacking the SdhD gene, were unaltered. These data indicate that complete loss of SdhD is not sufficient to induce tumorigenesis in mice. They suggest that substantia nigra neurons are more susceptible to mitochondrial damage than other catecholaminergic cells, particularly during a critical postnatal maturation period.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Hypoxia. 4. Hypoxia and ion channel function

The ability to sense and respond to oxygen deprivation is required for survival; thus, understanding the mechanisms by which changes in oxygen are linked to cell viability and function is of great importance. Ion channels play a critical role in regulating cell function in a wide variety of biological processes, including neuronal transmission, control of ventilation, cardiac contractility, and control of vasomotor tone. Since the 1988 discovery of oxygen-sensitive potassium channels in chemoreceptors, the effect of hypoxia on an assortment of ion channels has been studied in an array of cell types. In this review, we describe the effects of both acute and sustained hypoxia (continuous and intermittent) on mammalian ion channels in several tissues, the mode of action, and their contribution to diverse cellular processes.

The carbon monoxide donor, CORM-2, is an antagonist of ATP-gated, human P2X4 receptors

Carbon monoxide (CO) is produced endogenously by heme oxygenase (HO) enzymes. HO-1 is highly expressed in many inflammatory disease states, where it is broadly protective. The protective effects of HO-1 expression can be largely mimicked by the exogenous application of CO and CO-releasing molecules (CORMs). Despite a dearth of pharmacological tools for their study, molecular methodologies have identified P2X4 receptors as a potential anti-nociceptive drug target. P2X4 receptors are up-regulated in animal models of inflammatory pain, and their knock-down reduces pain behaviours. In these same animal models, HO-1 expression is anti-nociceptive, and we therefore investigated whether P2X4 was a target for CO and tricarbonyldichlororuthenium (II) dimer (CORM-2). Using conventional whole-cell and perforated-patch recordings of heterologously expressed human P2X4 receptors, we demonstrate that CORM-2, but not CO gas, is an inhibitor of these channels. We also investigated the role of soluble guanylate cyclase and mitochondria-derived reactive oxygen species using pharmacological inhibitors but found that they were largely unable to affect the ability of CORM-2 to inhibit P2X4 currents. A control breakdown product of CORM-2 was also without effect on P2X4. These results suggest that P2X4 receptors are not a molecular target of endogenous CO production and are, therefore, unlikely to be mediating the anti-nociceptive effects of HO-1 expression in inflammatory pain models. However, these results show that CORM-2 is an effective antagonist at human P2X4 receptors and represents a useful pharmacological tool for the study of these receptors given the current dearth of antagonists.

The role of redox changes in oxygen sensing

The specialized oxygen-sensing tissues include the carotid body and arterial smooth muscle cells in the pulmonary artery (PA) and ductus arteriosus (DA). We discuss the evidence that changes in oxygen tension are sensed through changes in redox status. "Redox" changes imply the giving or accepting of electrons. This might occur through the direct tunneling of electrons from mitochondria or redox couples to an effector protein (e.g. ion channel). Alternatively, the electron might be transferred through reactive oxygen species from mitochondria or an NADPH oxidase isoform. The PA's response to hypoxia and DA's response to normoxia result from reduction or oxidation, respectively. These opposing redox stimuli lead to K+ channel inhibition, membrane depolarization and an increase in cytosolic calcium and/or calcium sensitization that causes contraction. In the neuroendocrine cells (the type 1 cell of the carotid body, neuroepithelial body and adrenomedullary cells), the response is secretion. We examine the roles played by superoxide anion, hydrogen peroxide and the anti-oxidant enzymes in the signaling of oxygen tensions.

Molecular background of leak K+ currents: two-pore domain potassium channels

Two-pore domain K(+) (K(2P)) channels give rise to leak (also called background) K(+) currents. The well-known role of background K(+) currents is to stabilize the negative resting membrane potential and counterbalance depolarization. However, it has become apparent in the past decade (during the detailed examination of the cloned and corresponding native K(2P) channel types) that this primary hyperpolarizing action is not performed passively. The K(2P) channels are regulated by a wide variety of voltage-independent factors. Basic physicochemical parameters (e.g., pH, temperature, membrane stretch) and also several intracellular signaling pathways substantially and specifically modulate the different members of the six K(2P) channel subfamilies (TWIK, TREK, TASK, TALK, THIK, and TRESK). The deep implication in diverse physiological processes, the circumscribed expression pattern of the different channels, and the interesting pharmacological profile brought the K(2P) channel family into the spotlight. In this review, we focus on the physiological roles of K(2P) channels in the most extensively investigated cell types, with special emphasis on the molecular mechanisms of channel regulation.

The role of NOX2 and "novel oxidases" in airway chemoreceptor O(2) sensing

In pulmonary neuroepithelial bodies (NEB), presumed airway chemoreceptors, classical NADPH oxidase (gp91 phox, NOX2) is co-expressed with O(2) sensitive K(+) channels (K(+)O(2)) and functions as an O(2) sensor. Here we examined related NADPH oxidase homologues "novel oxidases "(NOX 1, 3&4) and their possible involvement in O(2) sensing. For immunolocalization we used specific antibodies against various NADPH components and K(+) (O(2)) subunits to label NEB in rat /rabbit lung and NEB related H146 tumor cell line. For gene expression profiling of NEB cells microdissected from human lung, and H146 cells, we used custom MultiGene-12TM RT-PCR array that included NADPH oxidase components and homologues /accessory proteins (NOX1-4, phox-p22, p40, p47, p67, Rac1, NOXO1 and NOXA1) and K(+)O(2) channels (Kv -1.2, 1.5, 2.1, 3.1, 3.3, 3.4, 4.2, 4.3;TASK1-3). In rat lung, NOX2, NOX4, p22phox, Kv3.3 (and Kv3.4 in rabbit) and TASK1 localized to the apical plasma membrane of NEB cells, and membrane or sub-membrane regions in H146 cells. NEB and H146 cells expressed all NOX proteins except NOX3, as well as all K(+)O(2) channels, except Kv1.5 and Kv4.3. Co-immunoprecipitation using Western blot multicolor Quantum dot labeling showed NOX2 molecular complexes with Kv but not with TASK, while NOX4 associated with TASK1 but not with Kv channel proteins. Hypoxia -induced serotonin release was inhibited in H 146 cells by siRNA to NOX2, while siRNA to NOX4 had only a partial effect, implicating NOX 2 as the predominant NEB cell O(2) sensor. Present findings support NEB cell specific plasma membrane model of O(2) sensing, and suggest unique NOX/K(+)O(2) channel combinations for diverse physiological NEB functions.

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.

Pulmonary neuroendocrine cell system in pediatric lung disease-recent advances

The airway epithelium of human and animal lungs contains highly specialized pulmonary neuroendocrine cells (PNEC), distributed as solitary cells and as innervated clusters, neuroepithelial bodies (NEB). The designation "PNEC system" stems from the expression of both neural and endocrine cell phenotypes, including the synthesis and release of amine (serotonin, 5-HT) and a variety of neuropeptides (that is, bombesin). The role and function of PNEC in the lung have remained a subject of speculation for many years. During the last decade, studies using modern techniques of cellular and molecular biology revealed a complex functional role for PNEC, beginning during the early stages of lung development as modulators of fetal lung growth and differentiation and at the time of birth as airway O2 sensors involved in neonatal adaptation. Postnatally and beyond, PNEC/NEB are providers of a lung stem cell niche that is important in airway epithelial regeneration and lung carcinogenesis. The focus of this review is to present and discuss recent findings pertaining to the responses of PNEC to intrauterine environmental stimuli, ontogeny and molecular regulation of PNEC differentiation, innervation of NEB, and their role as airway chemoreceptors, including mechanisms of O2 sensing and chemotransmission of hypoxia stimulus. Abnormalities of PNEC/NEB have been reported in a variety of pediatric pulmonary disorders but the clinical significance or the mechanisms involved are unknown. The discussion on the possible role of PNEC/NEB in the pathogenesis and pathobiology of pediatric lung diseases includes congenital lung disorders, bronchopulmonary dysplasia, disorders of respiratory control, neuroendocrine hyperplasia of infancy, cystic fibrosis, bronchial asthma, and pulmonary hypertension.

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.

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.

Regulation of oxygen sensing by ion channels

O(2) sensing is of critical importance for cell survival and adaptation of living organisms to changing environments or physiological conditions. O(2)-sensitive ion channels are major effectors of the cellular responses to hypoxia. These channels are preferentially found in excitable neurosecretory cells (glomus cells of the carotid body, cells in the neuroepithelial bodies of the lung, and neonatal adrenal chromaffin cells), which mediate fast cardiorespiratory adjustments to hypoxia. O(2)-sensitive channels are also expressed in the pulmonary and systemic arterial smooth muscle cells where they participate in the vasomotor responses to low O(2) tension (particularly in hypoxic pulmonary vasoconstriction). The mechanisms underlying O(2) sensing and how the O(2) sensors interact with the ion channels remain unknown. Recent advances in the field give different support to the various current hypotheses. Besides the participation of ion channels in acute O(2) sensing, they also contribute to the gene program developed under chronic hypoxia. Gene expression of T-type calcium channels is upregulated by hypoxia through the same hypoxia-inducible factor-dependent signaling pathway utilized by the classical O(2)-regulated genes. Alteration of acute or chronic O(2) sensing by ion channels could participate in the pathophysiology of human diseases, such as sudden infant death syndrome or primary pulmonary hypertension.

Oxygen sensing and oxidant/redox-related pathways

What is the nature of the oxygen sensor(s) and how do organisms sense variations in oxygen? A progressive rise of oxidative stress due to the altered reduction-oxidation (redox) homeostasis appears to be one of the hallmarks of the processes that regulate gene transcription. Dynamic changes in oxygen homeostasis and its close association with redox equilibrium, therefore, constitute a signaling mechanism for the expression/activation of oxygenes. This variation subsequently regulates the compartmentalization and functioning of HIF-1alpha and NF-kappaB. In addition, oxygen-evoked regulation of HIF-1alpha and NF-kappaB is closely coupled with intracellular redox state, such that modulating redox equilibrium affects their responsiveness at the molecular level (expression/transactivation). Interestingly, are these particular transcription factors potential oxygen sensors? The basic components of the intracellular oxidative/redox machinery and its crucial regulation of oxygen- and redox-sensitive transcription factors may help understand the network of oxygen sensing mechanisms and redox-related pathways.

A Fenton reaction at the endoplasmic reticulum is involved in the redox control of hypoxia-inducible gene expression

It has been proposed that hydroxyl radicals (.OH) generated in a perinuclear iron-dependent Fenton reaction are involved in O(2)-dependent gene expression. Thus, it was the aim of this study to localize the cellular compartment in which the Fenton reaction takes place and to determine whether scavenging of.OH can modulate hypoxia-inducible factor 1 (HIF-1)-dependent gene expression. The Fenton reaction was localized by using the nonfluorescent dihydrorhodamine (DHR) 123 that is irreversibly oxidized to fluorescent rhodamine 123 while scavenging.OH together with gene constructs allowing fluorescent labeling of mitochondria, endoplasmic reticulum (ER), Golgi apparatus, peroxisomes, or lysosomes. A 3D two-photon confocal laser scanning microscopy showed.OH generation in distinct hot spots of perinuclear ER pockets. This ER-based Fenton reaction was strictly pO(2)-dependent. Further colocalization experiments showed that the O(2)-sensitive transcription factor HIF-1alpha was present at the ER under normoxia, whereas HIF-1alpha was present only in the nucleus under hypoxia. Inhibition of the Fenton reaction by the.OH scavenger DHR attenuated HIF-prolyl hydroxylase activity and interaction with von Hippel-Lindau protein, leading to enhanced HIF-1alpha levels, HIF-1alpha transactivation, and activated expression of the HIF-1 target genes plasminogen activator inhibitor 1 and heme oxygenase 1. Further,.OH scavenging appeared to enhance redox factor 1 (Ref-1) binding and, thus, recruitment of p300 to the transactivation domain C because mutation of the Ref-1 binding site cysteine 800 abolished DHR-induced transactivation. Thus, the localized Fenton reaction appears to impact the expression of hypoxia-regulated genes by means of HIF-1alpha stabilization and coactivator recruitment.

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

Inhibitors of mitochondrial energy metabolism have long been known to be potent stimulants of the carotid body, yet their mechanism of action remains obscure. We have therefore investigated the effects of rotenone, myxothiazol, antimycin A, cyanide (CN(-)) and oligomycin on isolated carotid body type I cells. All five compounds caused a rapid rise in intracellular Ca(2+), which was inhibited on removal of extracellular Ca(2+). Under current clamp conditions rotenone and CN(-) caused a rapid membrane depolarization and elevation of [Ca(2+)](i). Voltage clamping cells to -70 mV substantially attenuated this rise in [Ca(2+)](i). Rotenone, cyanide, myxothiazol and oligomycin significantly inhibited resting background K(+) currents. Thus rotenone, myxothiazol, cyanide and oligomycin mimic the effects of hypoxia in that they all inhibit background K(+) current leading to membrane depolarization and voltage-gated calcium entry. Hypoxia, however, failed to have any additional effect upon membrane currents in the presence of CN(-) or rotenone or the mitochondrial uncoupler p-trifluoromethoxyphenyl hydrazone (FCCP). Thus not only do mitochondrial inhibitors mimic the effects of hypoxia, but they also abolish oxygen sensitivity. These observations suggest that there is a close link between oxygen sensing and mitochondrial function in type I cells. Mechanisms that could account for this link and the actions of mitochondrial inhibitors are discussed.

Effects of reducing agents on glutathione metabolism and the function of carotid body chemoreceptor cells

Two current hypotheses of O2 sensing in the carotid body (CB) chemoreceptors suggest participation of oxygen reactive (ROS) species, but they are mechanistically opposed. One postulates that hypoxia decreases ROS levels; the other that hypoxia increases them. Yet, both propose that the ensuing alteration in the cellular redox environment is the key signal triggering hypoxic chemoreception. Since the glutathione redox pair is the main cellular buffer for ROS and the main determinant of the general redox environment of the cells, a way to test whether ROS participate in chemoreception is to determine glutathione levels and to correlate them with the activity of CB chemoreceptor cells. We found that hypoxia does not alter the glutathione reduction potential but that it activates chemoreceptor cell neurosecretion. Incubation of tissues with reduced glutathione increases the glutathione-reducing potential but does not activate chemoreceptor cells in normoxia nor does it modify hypoxic activation. Like reduced glutathione, N-acetylcysteine promoted a general reducing environment in the cells without alteration of chemoreceptor cell activity. N-(mercaptopropionyl)-glycine, like the two previous agents, increases the reduction potential of glutathione. In contrast, the compound activated chemoreceptor cells in normoxia, promoting a dose- and Ca(2+)-dependent neurosecretion and a potentiation of the hypoxic responses. The existence of multiple relationships between glutathione reduction potential in the cells and their activity indicates that the general cellular redox environment is not a factor determining chemoreceptor cell activation. It cannot be excluded that the local redox environments of restricted microdomain(s) in the cells with specific regulating mechanisms are important signals for chemoreceptor cell activity.

Mechanisms for Hypoxia Detection in O2-Sensitive Cells

Since O(2) is the bare necessity for multicellular organisms, they develop multiple protective mechanisms against hypoxia. Mammals will adapt to hypoxia in short and long terms. The short-term responses include enhancement of the respiratory and cardiac functions, adrenaline secretion from adrenal medullary cells, and pulmonary vasoconstriction, whereas the long-term response is the increase in erythropoietin production with the consequent increase in red blood cells. Although much work has been done to elucidate molecular mechanisms for O(2)-sensing for the last ten years, the majority of the mechanisms remain unclear. We will review mechanisms proposed for hypoxia detection in carotid body type I cells, pulmonary artery smooth muscle, adrenal medullary cells, and liver cells, with the special focus on adrenal medullary cells.

Effects of mitochondrial inhibitors and uncouplers on hypoxic vasoconstriction in rabbit lungs

Hypoxic pulmonary vasoconstriction (HPV) matches lung perfusion to ventilation for optimizing pulmonary gas exchange; however, the underlying mechanism has not yet been fully elucidated. Lung nitric oxide (NO) generation appears to be involved in this process. Recently, mitochondria have been proposed as oxygen sensors, with HPV signaling via a hypoxia-induced increase in the generation of reactive oxygen species derived from mitochondrial complex III and escaping through an anion channel into the cytoplasm. In addition, complex II has been suggested to be specifically involved in hypoxia-dependent generation of reactive oxygen species in the lung. We investigated the effects of several mitochondrial inhibitors and uncouplers on the strength of HPV, and asked for their capacity to mimic HPV during normoxia in isolated buffer-perfused rabbit lungs. Specificity of the agents for HPV was tested by comparison of their effects on non-hypoxia-induced vasoconstriction, elicited by the thromboxane mimetic U46619. Interference with NO metabolism was determined by performing parallel studies with blocked lung NO generation and by measurement of exhaled NO. Rotenone, 3-nitroproprionic acid, and myxothiazol dose-dependently inhibited HPV without being mimics of HPV during normoxia. The inhibitory effect of these agents was only partly specific for HPV by comparison with U46619-induced vasoconstriction. During pre-blocked lung NO synthesis, the selectivity for HPV inhibition was increased for rotenone, but largely lost for myxothiazol. 2-tenoyltrifluoroacetone resulted in an unspecific inhibition of HPV as compared with U46619-induced vasoconstriction. 1-methyl-4-phenylpyridinium iodide and 2-heptyl-4-hydroxyquinoline-N-oxide specifically suppressed HPV and increased normoxic vascular tone. Antimycin A suppressed HPV, an effect being specific in lungs with intact NO synthesis and only partly specific while blocking NO. However, this agent did not mimic HPV during normoxia, as may be expected for interference with the mitochondrial electron transport downstream in complex III. The uncouplers 2,4-dinitrophenol (DNP, 10-200 microM) and carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP, 1-3 microM) induced sustained vasoconstriction during normoxia, with enhancement of HPV by DNP at low and suppression of HPV for both agents at high concentrations. The anion channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid inhibited HPV and U46619-induced vasoconstriction with identical dose-response curves. These findings suggest that mitochondria are in some manner involved in the regulation of HPV in intact rabbit lungs. The hypothesis that enhanced superoxide leak at complex III of mitochondria represents the underlying mechanism of acute HPV is supported by the rotenone and 2-heptyl-4-hydroxyquinoline-N-oxide data, but partly contradicted by the findings with 1-methyl-4-phenylpyridinium iodide, antimycin A, DNP, and FCCP. Further studies are mandatory to clarify the link between mitochondrial respiratory chain and hypoxic pulmonary vasoconstriction.

Oxygen and glucose sensing by carotid body glomus cells

Carotid body glomus cells sense hypoxia through the inhibition of plasmalemmal K(+) channels, which leads to Ca(2+) influx and transmitter release. Although the mechanism of O(2) sensing remains enigmatic, it does not seem to depend on cellular redox status or inhibition of mitochondrial electron transport. Hypoxia inducible factors appear to be necessary for the expression of the O(2) sensor and carotid body remodeling in chronic hypoxia, but are not directly involved in acute O(2) sensing. Glomus cells are also rapidly activated by reductions of glucose concentration due to inhibition of K(+) channels. These cells function as combined O(2) and glucose sensors that help to prevent neuronal damage by acute hypoxia and/or hypoglycemia.

Rotenone selectively occludes sensitivity to hypoxia in rat carotid body glomus cells

Carotid body glomus cells release transmitters in response to hypoxia due to the increase of excitability resulting from inhibition of O2 -regulated K+ channels. However, the mechanisms involved in the detection of changes of O2 tension are unknown. We have studied the interaction between glomus cell O2 sensitivity and inhibition of the mitochondrial electron transport chain (ETC) in a carotid body thin slice preparation in which catecholamine release from intact single glomus cells can be monitored by amperometry. Inhibition of the mitochondrial ETC at proximal and distal complexes induces external Ca2+-dependent catecholamine secretion. At saturating concentration of the ETC inhibitors, the cellular response to hypoxia is maintained. However, rotenone, a complex I blocker, selectively occludes the responsiveness to hypoxia of glomus cells in a dose-dependent manner. The effect of rotenone is mimicked by 1-methyl-4-phenylpyridinium ion (MPP+), an agent that binds to the same site as rotenone, but not by complex I inhibitors acting on different sites. In addition, the effect of rotenone is not prevented by incubation of the cells with succinate, a substrate of complex II. These data strongly suggest that sensitivity to hypoxia of carotid body glomus cells is not linked in a simple way to mitochondrial electron flow and that a rotenone (and MPP+)-sensitive molecule critically participates in acute oxygen sensing in the carotid body.

Acute oxygen sensing in cellular models: relevance to the physiology of pulmonary neuroepithelial and carotid bodies

The combination of studies in native tissues and immortalised model systems during the last decade has made possible a deeper understanding of the physiology and functional morphology of arterial and airway oxygen sensors. Complementary and overlapping information from these earlier studies has allowed a detailed description of the cellular events that link decreased environmental oxygen to the release of physiologically important vasoactive transmitters. Since these basic pathways have now been defined functionally, what remains to be determined is the molecular identity of the specific proteins involved in the signal transduction pathways, and how these proteins interact to produce a full physiological response. With these goals clearly in sight, we have embarked upon a strategy that is a novel combination of proteomics and functional genomics. It is hoped this strategy will enable us to develop and refine the initial models in order to understand more completely the process of oxygen sensing in health and disease.

Airway chemotransduction: from oxygen sensor to cellular effector

The process of sensing, transducing, and acting on environmental cues is critical to normal physiologic function. Furthermore, dysfunction of this process can lead to the development of disease. This is especially true of the homeostatic mechanisms that have evolved to maintain the carriage of O2 to respiring tissues during acute hypoxic challenge. During periods of reduced O2 availability, three major mechanisms act conjointly to increase ventilation and optimize the ventilation-perfusion ratio throughout the lung by directing pulmonary blood flow to better ventilated areas of the lung. These mechanisms are as follows: (1) increased carotid sinus nerve discharge rate to the respiratory centers of the brain, (2) intrinsic hypoxic vasoconstriction of pulmonary resistance vessels, and (3) potential local and central modulation via stimulation of neuroepithelial bodies of the lung. The key to the rapid response to the O2 signal is the ability of each of these tissues to sense acutely the changes in PO2, to transduce the signal, and for cellular effectors to initiate compensatory mechanisms that will offset rapidly the reduction in PO2 before O2 availability to tissues is compromised. This review concentrates on the signal transduction mechanism that links altered PO2 to depolarization in the recently proposed airway chemosensory element, the neuroepithelial body (and its immortalized cellular counterpart, the H146 cell line), and discusses the pertinent similarities and differences that exist between airway, carotid body, and pulmonary arteriolar O2 sensing.

Hypoxia, energy state and pulmonary vasomotor tone

Vasomotor responses to hypoxia constitute a fundamental adaptation to a commonly encountered stress. It has long been suspected that changes in cellular energetics may modulate both hypoxic systemic artery vasodilatation (HSV) and hypoxic pulmonary artery vasoconstriction (HPV). Although limitation of energy has been shown to underlie hypoxic relaxation in some smooth muscles, the response to hypoxia in vascular smooth muscle does not appear to be a simple function of energy stores, but instead may involve perturbations of ATP or energy delivery to mechanisms controlling muscle force, and/or changes associated with anaerobic metabolism. Recent work in pulmonary vascular smooth muscle has demonstrated that energy stores are maintained during hypoxic pulmonary vasoconstriction, and that this is dependent on glucose availability and up-regulation of glycolysis. There is increasing evidence that glycolysis is preferentially coupled to a variety of membrane associated ATP dependent processes, including the Na(+) pump, Ca(2+)-ATPase, and possibly some protein kinases. These and other mechanisms may influence excitation-contraction coupling in both systemic and pulmonary arteries by effects on intracellular Ca(2+) and/or Ca(2+) sensitivity. Hypoxia has also been postulated to have major effects on other cytosolic second messenger systems including phosphatidylinositol pathways, cell redox state and mitochondrial reactive oxygen species production. This review examines the relationship between energy state, anaerobic respiration and hypoxic vasomotor tone, with a particular emphasis on hypoxic pulmonary vasoconstriction.