Airway chemotransduction: from oxygen sensor to cellular effector

Mechanisms underlying the health benefits of intermittent hypoxia conditioning

Intermittent hypoxia (IH) is commonly associated with pathological conditions, particularly obstructive sleep apnoea. However, IH is also increasingly used to enhance health and performance and is emerging as a potent non-pharmacological intervention against numerous diseases. Whether IH is detrimental or beneficial for health is largely determined by the intensity, duration, number and frequency of the hypoxic exposures and by the specific responses they engender. Adaptive responses to hypoxia protect from future hypoxic or ischaemic insults, improve cellular resilience and functions, and boost mental and physical performance. The cellular and systemic mechanisms producing these benefits are highly complex, and the failure of different components can shift long-term adaptation to maladaptation and the development of pathologies. Rather than discussing in detail the well-characterized individual responses and adaptations to IH, we here aim to summarize and integrate hypoxia-activated mechanisms into a holistic picture of the body's adaptive responses to hypoxia and specifically IH, and demonstrate how these mechanisms might be mobilized for their health benefits while minimizing the risks of hypoxia exposure.

The role of potassium current in the pulmonary response to environmental oxidative stress

Exposure of human lung epithelial cells (A549 cell line) to the oxidant pollutant ozone (O₃) alters cell membrane currents inducing its decrease, when the cell undergoes to a voltage-clamp protocol ranging from -90 to +70mV. The membrane potential of these cells is mainly maintained by the interplay of potassium and chloride currents. Our previous studies indicated the ability of O₃ to activate ORCC (Outward Rectifier Chloride Channel) and consequently increases the chloride current. In this paper our aim was to understand the response of potassium current to oxidative stress challenge and to identify the kind potassium channel involved in O₃ induced current changes. After measuring the total membrane current using an intracellular solution with or without potassium ions, we obtained the contribution of potassium to the overall membrane current in control condition by a mathematical approach. Repeating these experiments after O₃ treatment we observed a significant decrease of I_potassium. Treatment of the cells with Iberiotoxin (IbTx), a specific inhibitor of BK channel, we were able to verify the presence and the functionality of BK channels. In addition, the administration of 4-Aminopyridine (an inhibitor of voltage dependent K channels but not BK channels) and Tetraethylammonium (TEA) before and after O₃ treatment we observed the formation of BK oxidative post-translation modifications. Our data suggest that O₃ is able to inhibit potassium current by targeting BK channel. Further studies are needed to better clarify the role of this BK channel and its interplay with the other membrane channels under oxidative stress conditions. These findings can contribute to identify the biomolecular pathway induced by O₃ allowing a possible pharmacological intervention against oxidative stress damage in lung tissue.

A Case for Hydrogen Sulfide Metabolism as an Oxygen Sensing Mechanism

The ability to detect oxygen availability is a ubiquitous attribute of aerobic organisms. However, the mechanism(s) that transduce oxygen concentration or availability into appropriate physiological responses is less clear and often controversial. This review will make the case for oxygen-dependent metabolism of hydrogen sulfide (H₂S) and polysulfides, collectively referred to as reactive sulfur species (RSS) as a physiologically relevant O₂ sensing mechanism. This hypothesis is based on observations that H₂S and RSS metabolism is inversely correlated with O₂ tension, exogenous H₂S elicits physiological responses identical to those produced by hypoxia, factors that affect H₂S production or catabolism also affect tissue responses to hypoxia, and that RSS efficiently regulate downstream effectors of the hypoxic response in a manner consistent with a decrease in O₂. H₂S-mediated O₂ sensing is then compared to the more generally accepted reactive oxygen species (ROS) mediated O₂ sensing mechanism and a number of reasons are offered to resolve some of the confusion between the two.

Eosinophil extracellular traps drive asthma progression through neuro-immune signals

Eosinophilic inflammation is a feature of allergic asthma. Despite mounting evidence showing that chromatin filaments released from neutrophils mediate various diseases, the understanding of extracellular DNA from eosinophils is limited. Here we show that eosinophil extracellular traps (EETs) in bronchoalveolar lavage fluid are associated with the severity of asthma in patients. Functionally, we find that EETs augment goblet-cell hyperplasia, mucus production, infiltration of inflammatory cells and expressions of type 2 cytokines in experimental non-infection-related asthma using both pharmaceutical and genetic approaches. Multiple clinically relevant allergens trigger EET formation at least partially via thymic stromal lymphopoietin in vivo. Mechanically, EETs activate pulmonary neuroendocrine cells via the CCDC25-ILK-PKCα-CRTC1 pathway, which is potentiated by eosinophil peroxidase. Subsequently, the pulmonary neuroendocrine cells amplify allergic immune responses via neuropeptides and neurotransmitters. Therapeutically, inhibition of CCDC25 alleviates allergic inflammation. Together, our findings demonstrate a previously unknown role of EETs in integrating immunological and neurological cues to drive asthma progression.

Diffuse idiopathic pulmonary neuroendocrine cell hyperplasia: DIPNECH

PURPOSE OF REVIEW

Diffuse idiopathic pulmonary neuroendocrine cell hyperplasia (DIPNECH) is a rare premalignant condition. Over the past decade, there has been increased recognition and reporting of DIPNECH in the literature. Currently, our understanding is that DIPNECH has a predilection to nonsmoking females around their sixth decade of life. The patients usually present with chronic cough, dyspnea, and computed tomography (CT) showing multifocal pulmonary nodules with associated mosaic attenuation. The clinic history is largely driven by constrictive obliterative bronchiolitis, which typically has an indolent course with progressive respiratory decline and difficult to treat symptoms.

RECENT FINDINGS

DIPNECH has been found to be associated with carcinoid tumors. Recent data has found that symptomatic DIPNECH patients respond to somatostatin analog (SSA). SSAs provide improvement in symptoms and pulmonary function tests. According to small studies and case series SSAs can be used in conjunction with steroids and bronchodilators for the treatment of respiratory symptoms.

SUMMARY

DINPNECH is a premalignant condition that can transform into carcinoid tumors. Although the recent data suggest the potential efficacy of SSA, further studies are needed to validate such results in prospective fashion in addition to investigating other therapeutic agents.

Constrictive bronchiolitis in diffuse idiopathic pulmonary neuroendocrine cell hyperplasia

Background

Diffuse idiopathic pulmonary neuroendocrine cell hyperplasia (DIPNECH) is characterised by multifocal proliferation of neuroendocrine cells and belongs in the spectrum of pulmonary neuroendocrine tumours. Some patients with DIPNECH develop airflow obstruction but the relationship between the two entities remains unclear.

Methods

We performed a computer-assisted search of the Mayo Clinic's electronic medical records for biopsy-proven cases of DIPNECH. We extracted clinical, pulmonary function, imaging and histopathological data along with treatments and outcomes.

Results

Among 44 patients with DIPNECH 91% were female and the median age was 65 years (interquartile range 56–69 years); 73% were never-smokers. Overall, 38 patients (86%) had respiratory symptoms including cough (68%) and dyspnoea (30%); 45% were previously diagnosed to have asthma or COPD. Pulmonary function testing showed an obstructive pattern in 52%, restrictive pattern in 11%, mixed pattern in 9%, nonspecific pattern in 23%, and was normal in 5%. On chest computed tomography scan, 95% manifested diffuse nodules and 77% manifested mosaic attenuation. For management, 25% of patients were observed without pharmacological therapy, 55% received an inhaled bronchodilator, 41% received an inhaled corticosteroid, 32% received octreotide; systemic steroids, azithromycin, or combination chemotherapy was employed in four patients (9%). Of 24 patients with available follow-up pulmonary function tests, 50% remained stable, 33% worsened and 17% improved over a median interval of 21.3 months (interquartile range 9.7–46.9 months).

Conclusion

DIPNECH occurs mostly in women and manifests diffuse pulmonary nodules and mosaic attenuation on imaging. It is commonly associated with airflow obstruction due to constrictive bronchiolitis, which manifests limited response to current pharmacological therapy.

Diffuse idiopathic pulmonary neuroendocrine cell hyperplasia (DIPNECH) is an under-recognised cause of obstructive lung disease in women. Constrictive bronchiolitis associated with DIPNECH manifests limited response to currently employed therapies.https://bit.ly/3c3RZoe

The role of TASK-2 channels in CO2 sensing in zebrafish (Danio rerio)

Peripheral chemosensitivity in fishes is thought to be mediated by serotonin-enriched neuroepithelial cells (NECs) that are localized to the gills of adults and the integument of larvae. In adult zebrafish (Danio rerio), branchial NECs are presumed to mediate the cardiorespiratory reflexes associated with hypoxia or hypercapnia, whereas in larvae, there is indirect evidence linking cutaneous NECs to hypoxic hyperventilation and hypercapnic tachycardia. No study yet has examined the ventilatory response of larval zebrafish to hypercapnia, and regardless of developmental stage, the signaling pathways involved in CO₂ sensing remain unclear. In the mouse, a background potassium channel (TASK-2) contributes to the sensitivity of chemoreceptor cells to CO₂. Zebrafish possess two TASK-2 channel paralogs, TASK-2 and TASK-2b, encoded by kcnk5a and kcnk5b, respectively. The present study aimed to determine whether TASK-2 channels are expressed in NECs of larval zebrafish and whether they are involved in CO₂ sensing. Using immunohistochemical approaches, TASK-2 protein was observed on the surface of NECs in larvae. Exposure of larvae to hypercapnia caused cardiac and breathing frequencies to increase, and these responses were blunted in fish experiencing TASK-2 and/or TASK-2b knockdown. The results of these experiments suggest that TASK-2 channels are involved in CO₂ sensing by NECs and contribute to the initiation of reflex cardiorespiratory responses during exposure of larvae to hypercapnia.

Heme Oxygenase-2 (HO-2) as a therapeutic target: Activators and inhibitors

Heme oxygenase (HO) enzymes are involved in heme catabolism and several physiological functions. Among the different HO isoforms, HO-2 stands out for its neuroprotective properties and modulatory activity in male reproduction. However, unlike the HO-1 ligands, the potential therapeutic applications of HO-2 inhibitors/activators have not been extensively explored yet. Moreover, the physiological role of HO-2 is still unclear, mostly due to the lack of highly selective HO-2 chemical probes. To boost the interest on this intriguing target, the present review updates the knowledge on the structure-activity relationships of HO-2 inhibitors and activators, as well as their potential therapeutic applications. To the best of our knowledge, among HO-2 inhibitors, clemizole derivatives are the most selective HO-2 inhibitors reported so far (IC₅₀ HO-1 >100 μM, IC₅₀ HO-2 = 3.4 μM), while the HO-2 nonselective inhibitors described herein possess IC₅₀ HO-2 values ≤ 10 μM. Furthermore, the development of HO-2 activators, such as menadione analogues, helped to understand the critical moieties required for HO-2 activation. Recent advances in the potential therapeutic applications of HO-2 inhibitors/activators cover the fields of neurodegenerative, cardiovascular, inflammatory, and reproductive diseases further stimulating the interest towards this target.

Hypoxia and aging

Eukaryotic cells require sufficient oxygen (O₂) for biological activity and survival. When the oxygen demand exceeds its supply, the oxygen levels in local tissues or the whole body decrease (termed hypoxia), leading to a metabolic crisis, threatening physiological functions and viability. Therefore, eukaryotes have developed an efficient and rapid oxygen sensing system: hypoxia-inducible factors (HIFs). The hypoxic responses are controlled by HIFs, which induce the expression of several adaptive genes to increase the oxygen supply and support anaerobic ATP generation in eukaryotic cells. Hypoxia also contributes to a functional decline during the aging process. In this review, we focus on the molecular mechanisms regulating HIF-1α and aging-associated signaling proteins, such as sirtuins, AMP-activated protein kinase, mechanistic target of rapamycin complex 1, UNC-51-like kinase 1, and nuclear factor κB, and their roles in aging and aging-related diseases. In addition, the effects of prenatal hypoxia and obstructive sleep apnea (OSA)-induced intermittent hypoxia have been reviewed due to their involvement in the progression and severity of many diseases, including cancer and other aging-related diseases. The pathophysiological consequences and clinical manifestations of prenatal hypoxia and OSA-induced chronic intermittent hypoxia are discussed in detail.

Role of Prenatal Hypoxia in Brain Development, Cognitive Functions, and Neurodegeneration

This review focuses on the role of prenatal hypoxia in the development of brain functions in the postnatal period and subsequent increased risk of neurodegenerative disorders in later life. Accumulating evidence suggests that prenatal hypoxia in critical periods of brain formation results in significant changes in development of cognitive functions at various stages of postnatal life which correlate with morphological changes in brain structures involved in learning and memory. Prenatal hypoxia also leads to a decrease in brain adaptive potential and plasticity due to the disturbance in the process of formation of new contacts between cells and propagation of neuronal stimuli, especially in the cortex and hippocampus. On the other hand, prenatal hypoxia has a significant impact on expression and processing of a variety of genes involved in normal brain function and their epigenetic regulation. This results in changes in the patterns of mRNA and protein expression and their post-translational modifications, including protein misfolding and clearance. Among proteins affected by prenatal hypoxia are a key enzyme of the cholinergic system-acetylcholinesterase, and the amyloid precursor protein (APP), both of which have important roles in brain function. Disruption of their expression and metabolism caused by prenatal hypoxia can also result, apart from early cognitive dysfunctions, in development of neurodegeneration in later life. Another group of enzymes affected by prenatal hypoxia are peptidases involved in catabolism of neuropeptides, including amyloid-β peptide (Aβ). The decrease in the activity of neprilysin and other amyloid-degrading enzymes observed after prenatal hypoxia could result over the years in an Aβ clearance deficit and accumulation of its toxic species which cause neuronal cell death and development of neurodegeneration. Applying various approaches to restore expression of neuronal genes disrupted by prenatal hypoxia during postnatal development opens an avenue for therapeutic compensation of cognitive dysfunctions and prevention of Aβ accumulation in the aging brain and the model of prenatal hypoxia in rodents can be used as a reliable tool for assessment of their efficacy.

Ion channels of the lung and their role in disease pathogenesis

Maintenance of normal epithelial ion and water transport in the lungs includes providing a thin layer of surface liquid that coats the conducting airways. This airway surface liquid is critical for normal lung function in a number of ways but, perhaps most importantly, is required for normal mucociliary clearance and bacterial removal. Preservation of the appropriate level of hydration, pH, and viscosity for the airway surface liquid requires the proper regulation and function of a battery of different types of ion channels and transporters. Here we discuss how alterations in ion channel/transporter function often lead to lung pathologies.

Hyperplasia of pulmonary neuroendocrine cells in infancy and childhood

Pulmonary neuroendocrine cells (PNEC) are widely distributed throughout the airway mucosa of mammalian lung as solitary cells and as distinctive innervated clusters, neuroepithelial bodies (NEB). These cells differentiate early during lung development and are more prominent in fetal/neonatal lungs compared to adults. PNEC/NEB cells produce biogenic amine (serotonin) and a variety of peptides (i.e., bombesin) involved in regulation of lung function. During the perinatal period, NEB are thought to function as airway O(2)/CO(2) sensors. Increased numbers of PNEC/NEBs have been observed in a variety of perinatal and postnatal lung disorders. Recent advances in cellular and molecular biology of these cells, as they relate to perinatal and postnatal lung disorders associated with PNEC/NEB cell hyperplasia are reviewed and their possible role in pulmonary pathobiology discussed (WC 125).

Pulmonary neuroepithelial bodies are polymodal airway sensors: Evidence for CO2/H+ sensing

Pulmonary neuroepithelial bodies (NEB) in mammalian lungs are thought to function as airway O2 sensors that release serotonin (5-HT) in response to hypoxia. Direct evidence that NEB cells also respond to airway hypercapnia/acidosis (CO2/H+) is presently lacking. We tested the effects of CO2/H+ alone or in combination with hypoxia on 5-HT release from intact NEB cells in a neonatal hamster lung slice model. For the detection of 5-HT release we used carbon fiber amperometry. Fluorescence Ca2+ imaging method was used to assess CO2/H+-evoked changes in intracellular Ca2+. Exposure to 10 and 20% CO2 or pH 6.8–7.2 evoked significant release of 5-HT with a distinct rise in intracellular Ca2+ in hamster NEBs. This secretory response was dependent on the voltage-gated entry of extracellular Ca2+. Moreover, the combined effects of hypercapnia and hypoxia were additive. Critically, an inhibitor of carbonic anhydrase (CA), acetazolamide, suppressed CO2/H+-mediated 5-HT release. The expression of mRNAs for various CA isotypes, including CAII, was identified in NEB cells from human lung, and protein expression was confirmed by immunohistochemistry using a specific anti-CAII antibody on sections of human and hamster lung. Taken together our findings provide strong evidence for CO2/H+ sensing by NEB cells and support their role as polymodal airway sensors with as yet to be defined functions under normal and disease conditions.

How does the heart sense changes in oxygen tension: a role for ion channels?

SIGNIFICANCE

Oxygen plays a key role in cellular metabolism and function. Oxygen delivery to cells is crucial, and a lack of oxygen such as that which occurs during myocardial infarction can be lethal. Cells should, therefore, be able to respond to changes in oxygen tension.

RECENT ADVANCES

Since the first studies examining the acute cellular effect of hypoxia on activation of transmitter release from glomus or type I chemoreceptor cells, it is now known that virtually all cells are able to respond to changes in oxygen tension.

CRITICAL ISSUES

Despite advances made in characterizing hypoxic responses, the identity of the "oxygen sensor" remains debated. Recently, more evidence has evolved as to how cardiac myocytes sense acute changes in oxygen. This review will examine the available evidence in support of acute oxygen-sensing mechanisms providing a brief historical perspective and then more detailed insights into the heart and the role of cardiac ion channels in hypoxic responses.

FUTURE DIRECTIONS

A further understanding of these cellular processes should result in interventions that assist in preventing the deleterious effects of acute changes in oxygen tension such as alterations in contractile function and cardiac arrhythmia.

Hydrogen sulfide as an oxygen sensor

SIGNIFICANCE

Although oxygen (O2)-sensing cells and tissues have been known for decades, the identity of the O2-sensing mechanism has remained elusive. Evidence is accumulating that O2-dependent metabolism of hydrogen sulfide (H2S) is this enigmatic O2 sensor.

RECENT ADVANCES

The elucidation of biochemical pathways involved in H2S synthesis and metabolism have shown that reciprocal H2S/O2 interactions have been inexorably linked throughout eukaryotic evolution; there are multiple foci by which O2 controls H2S inactivation, and the effects of H2S on downstream signaling events are consistent with those activated by hypoxia. H2S-mediated O2 sensing has been demonstrated in a variety of O2-sensing tissues in vertebrate cardiovascular and respiratory systems, including smooth muscle in systemic and respiratory blood vessels and airways, carotid body, adrenal medulla, and other peripheral as well as central chemoreceptors.

CRITICAL ISSUES

Information is now needed on the intracellular location and stoichometry of these signaling processes and how and which downstream effectors are activated by H2S and its metabolites.

FUTURE DIRECTIONS

Development of specific inhibitors of H2S metabolism and effector activation as well as cellular organelle-targeted compounds that release H2S in a time- or environmentally controlled way will not only enhance our understanding of this signaling process but also provide direction for future therapeutic applications.

In utero cannabinoid exposure alters breathing and the response to hypoxia in newborn mice

Cannabis is one of the most commonly used recreational drugs at ages highly correlated with potential pregnancy. Endocannabinoid signalling regulates important stages of neuronal development. When cannabinoid receptors, which are widely distributed through the nervous system, are activated by exogenous cannabinoids, breathing in adult rats is depressed. Here, we show that, in newborn mice, endocannabinoids, through the activation of cannabinoid receptor type 1 (CB1 R), participate in the modulation of respiration and its control. Blocking CB1 Rs at birth suppressed the brake exerted by endocannabinoids on ventilation in basal and in hypoxic conditions. The number of apnoeas and their duration were also minimized by activation of CB1 Rs in normoxic and in hypoxic conditions. However, prenatal cannabis intoxication, caused by a daily injection of WIN55,212-2, in pregnant mice durably modified respiration of the offspring, as shown by hyperventilation in basal conditions, an altered chemoreflex in response to hypoxia, and longer apnoeas. When CB1 Rs were blocked in WIN55,212-2 treated newborns, persistent hyperventilation was still observed, which could partly be explained by a perturbation of the central respiratory network. In fact, in vitro medullary preparations from WIN55,212-2 treated pups, free of peripheral or of supramedullary structures, showed an altered fictive breathing frequency. In conclusion, the endocannabinoid pathway at birth seems to modulate breathing and protect the newborn against apnoeas. However, when exposed prenatally to an excess of cannabinoid, the breathing neuronal network in development seems to be modified, probably rendering the newborn more vulnerable in the face of an unstable environment.

Recent advances and contraversies on the role of pulmonary neuroepithelial bodies as airway sensors

Pulmonary neuroepithelial bodies are polymodal sensors widely distributed within the airway mucosa of mammals and other species. Neuroepithelial body cells store and most likely release serotonin and peptides as transmitters. Neuroepithelial bodies have a complex innervation that includes vagal sensory afferent fibers and dorsal root ganglion fibers. Neuroepithelial body cells respond to a number of intraluminal airway stimuli, including hypoxia, hypercarbia, and mechanical stretch. This article reviews recent findings in the cellular and molecular biology of neuroepithelial body cells and their potential role as airway sensors involved in the control of respiration, particularly during the perinatal period. Alternate hypotheses and areas of controversy regarding potential function as mechanosensory receptors involved in pulmonary reflexes are discussed.

Modulation of K2P3.1 (TASK-1), K2P9.1 (TASK-3), and TASK-1/3 heteromer by reactive oxygen species

Reactive oxygen species (ROS) generated by mitochondria or NADPH oxidase have been implicated in the inhibition of K(+) current by hypoxia in chemoreceptor cells. As TASKs are highly active background K(+) channels in these cells, we studied the role of ROS in hypoxia-induced inhibition of TASKs. In HeLa cells expressing TASKs, H(2)O(2) applied to inside-out patches activated TASK-1, TASK-3, and TASK-1/3 heteromer starting at ~16 mM. When applied to cell-attached or outside-out patches, 326 mM H(2)O(2) did not affect TASK activity. Other K(2P) channels (TREK-1, TREK-2, TASK-2, TALK-1, TRESK) were not affected by H(2)O(2) (tested up to 326 mM). A reducing agent (dithiothreitol) and a cysteine-modifying agent (2-aminoethyl methanethiosulfonate hydrobromide) had no effect on basal TASK activity and did not block the H(2)O(2)-induced increase in channel activity. A TASK mutant in which the C-terminus of TASK-3 was replaced with that of TREK-2 showed a normal sensitivity to H(2)O(2). Xanthine/xanthine oxidase mixture used to generate superoxide radical showed no effect on TASK-1, TASK-3, and TASK-1/3 heteromer from either side of the membrane, but it strongly activated TASK-2 from the extracellular side. Acute H(2)O(2) (32-326 mM) exposure did not affect hSlo1/b1(BK) expressed in HeLa cells and BK in carotid body glomus cells. In carotid body glomus cells, adrenal cortical cells, and cerebellar granule neurons that show abundant hypoxia-sensitive TASK activity, H(2)O(2) (>16 mM) activated the channels only when applied intracellularly, similar to that observed with cloned TASKs. These findings show that ROS do not support or inhibit TASK and BK activity and therefore are unlikely to be the hypoxic signal that causes cell excitation via inhibition of these K(+) channels.

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.

Hydrogen sulfide and oxygen sensing in the cardiovascular system

Vertebrate cardiorespiratory homeostasis is inextricably dependent upon specialized cells that provide feedback on oxygen status in the tissues, blood, and on occasion, environment. These "oxygen sensing" cells include chemoreceptors and oxygen-sensitive chromaffin cells that initiate cardiorespiratory reflexes, vascular smooth muscle cells that adjust perfusion to metabolism or ventilation, and other cells that condition themselves in response to episodic hypoxia. Identification of how these cells sense oxygen and transduce this into the appropriate physiological response has enormous clinical applicability, but despite intense research there is no consensus regarding the initial hypoxia-effector coupling mechanism. This review examines an alternative mechanism of oxygen sensing using oxidation of endogenously produced hydrogen sulfide (H(2)S) as the O(2)-sensitive couple. Support for this hypothesis includes the similarity of effects of hypoxia and H(2)S on a variety of tissues, augmentation of hypoxic responses by precursors of H(2)S production and their inhibition by inhibitors of H(2)S synthesis, and the rapid consumption of H(2)S by O(2) in the range of intracellular/mitochondrial Po(2). These studies also indicate that, under normoxic conditions, it is doubtful that free H(2)S has longer than a transient existence in tissue or extracellular fluid.

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.

[K+ channels and lung epithelial physiology]

Transcripts of more than 30 different K(+) channels have been detected in the respiratory epithelium lining airways and alveoli. These channels belong to the 3 main classes of K(+) channels, i.e. i) voltage-dependent or calcium-activated, 6 transmembrane segments (TM), ii) 2-pores 4-TM and iii) inward-rectified 2-TM channels. The physiological and functional significance of this high molecular diversity of lung epithelial K(+) channels is not well understood. Surprisingly, relatively few studies are focused on K(+) channel function in lung epithelial physiology. Nevertheless, several studies have shown that KvLQT1, KCa and K(ATP) K(+) channels play a crucial role in ion and fluid transport, contributing to the control of airway and alveolar surface liquid composition and volume. K(+) channels are involved in other key functions, such as O(2) sensing or the capacity of the respiratory epithelia to repair after injury. This mini-review aims to discuss potential functions of lung K(+) channels.

Identification of subdomains in NADPH oxidase-4 critical for the oxygen-dependent regulation of TASK-1 K+ channels

Hypoxic inhibition of K+ current is a critical O2-sensing mechanism. Previously, it was demonstrated that the cooperative action of TASK-1 and NADPH oxidase-4 (NOX4) mediated the O2-sensitive K+ current response. Here we addressed the O2-sensing mechanism of NOX4 in terms of TASK-1 regulation. In TASK-1 and NOX4-coexpressing human embryonic kidney 293 cells, hypoxia (5% O2) decreased the amplitude of TASK-1 current (hypoxia-DeltaI(TASK-1)). To examine whether reactive oxygen species (ROS) mediate the hypoxia-DeltaI(TASK-1), we treated the cells with carbon monoxide (CO) which is known to reduce ROS generation from the heme-containing NOX4. Unexpectedly, CO failed to mimic hypoxia in TASK-1 regulation, rather blocked the hypoxia-DeltaI(TASK-1). Moreover, the hypoxia-DeltaI(TASK-1) was neither recovered by H2O2 treatment nor prevented by antioxidant such as ascorbic acid. However, the hypoxia-DeltaI(TASK-1) was noticeably attenuated by succinyl acetone, a heme synthase inhibitor. To further evaluate the role of heme, we constructed and expressed various NOX4 mutants, such as HBD(-) lacking the heme binding domain, NBD(-) lacking the NADPH binding domain, FBD(-) lacking the FAD binding domain, and HFBD(-) lacking both heme and FAD domains. The hypoxia-DeltaI(TASK-1) was significantly reduced in HBD(-)-, FBD(-)-, or HFBD(-)-expressing cells, versus wild-type NOX4-expressing cells. However, NBD(-) did not affect the TASK-1 response to hypoxia. We also found that p22 is required for the NOX4-dependent TASK-1 regulation. These results suggest that O2 binding with NOX4 per se controls TASK-1 activity. In this process, the heme moiety and FBD seem to be responsible for the NOX4 regulation of TASK-1, and p22 might support the NOX4-TASK-1 interaction.

Molecular diversity and function of K+ channels in airway and alveolar epithelial cells

Multiple K(+) channels are expressed in the respiratory epithelium lining airways and alveoli. Of the three main classes [1) voltage-dependent or Ca(2+)-activated, 6-transmembrane domains (TMD), 2) 2-pores 4-TMD, and 3) inward-rectified 2-TMD K(+) channels], almost 40 different transcripts have already been detected in the lung. The physiological and functional significance of this high molecular diversity of lung epithelial K(+) channels is intriguing. As detailed in the present review, K(+) channels are located at both the apical and basolateral membranes in the respiratory epithelium, where they mediate K(+) currents of diverse electrophysiological and regulatory properties. The main recognized function of K(+) channels is to control membrane potential and to maintain the driving force for transepithelial ion and liquid transport. In this manner, KvLQT1, KCa and K(ATP) channels, for example, contribute to the control of airway and alveolar surface liquid composition and volume. Thus, K(+) channel activation has been identified as a potential therapeutic strategy for the resolution of pathologies characterized by ion transport dysfunction. K(+) channels are also involved in other key functions in lung physiology, such as oxygen-sensing, inflammatory responses and respiratory epithelia repair after injury. The purpose of this review is to summarize and discuss what is presently known about the molecular identity of lung K(+) channels with emphasis on their role in lung epithelial physiology.

Hydrogen sulfide and oxygen sensing: implications in cardiorespiratory control

Although all cells are variously affected by oxygen, a few have the responsibility of monitoring oxygen tensions and initiating key homeostatic responses when P(O2) falls to critical levels. These ;oxygen-sensing' cells include the chemoreceptors in the gills (neuroepithelial cells), airways (neuroepithelial bodies) and vasculature (carotid bodies) that initiate cardiorespiratory reflexes, oxygen sensitive chromaffin cells associated with systemic veins or adrenal glands that regulate the rate of catecholamine secretion, and vascular smooth muscle cells capable of increasing blood flow to systemic tissues, or decreasing it through the lungs. In spite of intense research, and enormous clinical applicability, there is little, if any, consensus regarding the mechanism of how these cells sense oxygen and transduce this into the appropriate physiological response. We have recently proposed that the metabolism of hydrogen sulfide (H2S) may serve as an 'oxygen sensor' in vertebrate vascular smooth muscle and preliminary evidence suggests it has similar activity in gill chemoreceptors. In this proposed mechanism, the cellular concentration of H2S is determined by the simple balance between constitutive H2S production in the cytoplasm and H2S oxidation in the mitochondria; when tissue oxygen levels fall the rate of H2S oxidation decreases and the concentration of biologically active H2S in the tissue increases. This commentary briefly describes the oxygen-sensitive tissues in fish and mammals, delineates the current hypotheses of oxygen sensing by these tissues, and then critically evaluates the evidence for H2S metabolism in oxygen sensing.

Functional live cell imaging of the pulmonary neuroepithelial body microenvironment

Pulmonary neuroepithelial bodies (NEBs) are densely innervated groups of neuroendocrine cells invariably accompanied by Clara-like cells. Together with NEBs, Clara-like cells form the so-called "NEB microenvironment," which recently has been assigned a potential pulmonary stem cell niche. Conclusive data on the nature of physiological stimuli for NEBs are lacking. This study aimed at developing an ex vivo mouse lung vibratome slice model for confocal live cell imaging of physiological reactions in identified NEBs and surrounding epithelial cells. Immunohistochemistry of fixed slices demonstrated that NEBs are almost completely shielded from the airway lumen by tight junction-linked Clara-like cells. Besides the unambiguous identification of NEBs, the fluorescent dye 4-Di-2-ASP allowed microscopic identification of ciliated cells, Clara cells, and Clara-like cells in live lung slices. Using the mitochondrial uncoupler FCCP and a mitochondrial membrane potential indicator, JC-1, increases in 4-Di-2-ASP fluorescence in NEB cells and ciliated cells were shown to represent alterations in mitochondrial membrane potential. Changes in the intracellular free calcium concentration ([Ca2+](i)) in NEBs and surrounding airway epithelial cells were simultaneously monitored using the calcium indicator Fluo-4. Application (5 s) of 50 mM extracellular potassium ([K+](o)) evoked a fast and reproducible [Ca2+](i) increase in NEB cells, while Clara-like cells displayed a delayed (+/- 4 s) [Ca2+](i) increase, suggestive of an indirect, NEB-mediated activation. The presented approach opens interesting new perspectives for unraveling the functional significance of pulmonary NEBs in control lungs and disease models, and for the first time allows direct visualization of local interactions within the NEB microenvironment.

Hypoxia and heme oxygenases: oxygen sensing and regulation of expression

Heme is an essential molecule for life, as it is involved in sensing and using oxygen. Heme must be synthesized and degraded within an individual nucleated cell. Physiologic heme degradation is catalyzed by two functional isozymes of heme oxygenase, heme oxygenase-1 (HO-1) and HO-2, yielding carbon monoxide, iron, and biliverdin, an immediate precursor to bilirubin. HO-1 is an inducible enzyme, but the expression level of HO-2 is maintained in a narrow range. Characteristically, human HO-1 contains no Cys residue, whereas human HO-2 contains three Cys residues, each of which might be involved in heme binding. These features suggest separate physiologic roles of HO-1 and HO-2. Recent studies have shown that the expression levels of HO-1 and HO-2 are reduced under hypoxia, depending on the cell types. Moreover, we have proposed HO-2 as a potential O(2) sensor, because HO-2-deficient mice show hypoxemia and a blunted hypoxic ventilatory response with normal hypercapnic ventilatory response. HO-2-deficient mice also show hypertrophy of the pulmonary venous myocardium and enlargement of the carotid body. These morphometric changes are attributable to chronic hypoxemia. Here, we update the understanding of the regulation of HO-1 and HO-2 expression and summarize the regulatory role of HO-2 in the intercellular communication.

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.

Chronic inhalation of carbon monoxide: effects on the respiratory and cardiovascular system at doses corresponding to tobacco smoking

Carbon monoxide (CO) is a dangerous poison in high concentrations, but the long-term effects of low doses of CO, as in the gaseous component of tobacco smoke, are not well known. The aims of our study were to evaluate the long-term effects of inhaled CO on the respiratory and cardiovascular system at doses corresponding to tobacco smoking and its effect on tumourigenesis and pulmonary neuroendocrine (NE) cells. Female Wistar rats were exposed to either CO (200 ppm) for 20 h/day (n=51) or air (n=26) for 72 weeks. Carboxyhaemoglobin was 14.7+/-0.3% in CO exposed animals and 0.3+/-0.1% in controls. In the lungs, no signs of pathology similar to that associated with cigarette smoking were observed, and no differences in number of pulmonary NE cells were observed between the groups. Chronic CO inhalation induced a 20% weight increase of the right ventricle (p=0.001) and a 14% weight increase of the left ventricle and interventricular septum (p<0.001). Histological examination of the myocardium did not reveal any signs of scarring. In the aorta and femoral artery, no signs of atherosclerosis were observed in CO exposed rats. No exposure related carcinogenic effects were observed. Spontaneous tumours were identified in 29% of CO exposed animals and in 28% of the controls. Our results suggest that low dose CO exposure is probably not responsible for the respiratory pathology associated with tobacco smoking. The effects on the cardiovascular system seem to involve myocardial hypertrophy, but not atherogenesis.

Functional facets of the pulmonary neuroendocrine system

Pulmonary neuroendocrine cells (PNECs) have been around for 60 years in the scientific literature, although phylogenetically they are ancient. Their traditionally ascribed functions include chemoreception and regulation of lung maturation and growth. There is recent evidence that neuroendocrine (NE) differentiation in the lung is regulated by genes and pathways that are conserved in the development of the nervous system from Drosophila to humans (such as achaete-scute homolog-1), or implicated in the carcinogenesis of the nervous or NE system (such as the retinoblastoma tumor suppressor gene). In addition, complex neural networks are in place to regulate chemosensory and other functions. Even solitary PNECs appear to be innervated. For the first time ever, we have mouse models for lung NE carcinomas, including the most common and virulent small cell lung carcinoma. Moreover, PNECs may be important for inflammatory responses, and pivotal for lung stem cell niches. These discoveries signify an exciting new era for PNECs and are likely to have therapeutic and diagnostic applications.

Current concepts for the surgical management of carotid body tumor

BACKGROUND

Carotid body tumor (CBT) is a rare lesion of the neuroendocrine system. Chronic hypoxia has long been recognized as an etiology of CBT and other paragangliomas. Recent biogenetic discoveries reveal that mutations in oxygen-sensing genes are another etiology, accounting for approximately 35% of cases, and that these 2 etiologies are probably additive.

DATA SOURCES

(1) A retrospective analysis of fifteen cases of CBT in a 6-year period occurring in the mountains of Southern Appalachia; (2) an extensive review of the literature on the surgery of CBT and on the expansive biogenetic understanding of the disease.

CONCLUSIONS

Improved imaging, vascular surgical techniques, and understanding of the disease have vastly improved outcomes for patients. The necessities for long-term follow-up and appropriate genetic testing and counseling of patients and their families are documented. Surgeon and institutional competence are critical in achieving maximal outcomes.

Persistent tachypnea of infancy is associated with neuroendocrine cell hyperplasia

We sought to determine the clinical course and histologic findings in lung biopsies from a group of children who presented with signs and symptoms of interstitial lung disease (ILD) without identified etiology. Patients were identified from the pathology files at the Texas Children's Hospital who presented below age 2 years with persistent tachypnea, hypoxia, retractions, or respiratory crackles, and with nonspecific and nondiagnostic lung biopsy findings. Age-matched lung biopsy controls were also identified. Their clinical courses were retrospectively reviewed. Biopsies were reviewed, and immunostaining with antibodies to neuroendocrine cells was done. Fifteen pediatric ILD patients and four control patients were identified for inclusion in the study. Clinically, the mean onset of symptoms was 3.8 months (range, 0-11 months). Radiographs demonstrated hyperinflation, interstitial markings, and ground-glass densities. Oxygen was initially required for prolonged periods, and medication trials did not eliminate symptoms. After a mean of 5 years, no deaths had occurred, and patients had improved. On review of the lung biopsies, all had a similar appearance, with few abnormalities noted. Immunostaining with antibodies to neuroendocrine cell products showed consistently increased bombesin staining. Subsequent morphometric analysis showed that immunoreactivity for bombesin and serotonin was significantly increased over age-matched controls. In conclusion, we believe this may represent a distinct group of pediatric patients defined by the absence of known lung diseases, clinical signs and symptoms of ILD, and idiopathic neuroendocrine cell hyperplasia of infancy. These findings may be important for the evaluation of ILD in young children.

Selective visualisation of neuroepithelial bodies in vibratome slices of living lung by 4-Di-2-ASP in various animal species

Pulmonary neuroepithelial bodies (NEBs) are extensively innervated organoid groups of neuroendocrine cells that lie in the epithelium of intrapulmonary airways. Our present understanding of the morphology of NEBs is comprehensive, but direct physiological studies have so far been challenging because the extremely diffuse distribution of NEBs makes them inaccessible in vivo and because a reliable in vitro model is lacking. Our aim has been to optimise an in vitro method based on vibratome slices of living lungs, a model that includes NEBs, the surrounding tissues and at least part of their complex innervation. This in vitro model offers satisfactory access to pulmonary NEBs, provided that they can be differentiated from other tissue elements. The model was first optimised for living rat lung slices. Neutral red staining, reported to stain rabbit NEBs, proved unsuccessful in rat slices. On the other hand, the styryl pyridinium dye, 4-(4-diethylaminostyryl)-N-methylpyridinium iodide (4-Di-2-ASP), showed brightly fluorescent cell groups, reminiscent of NEBs, in the airway epithelium of living lung slices from rat. In addition, nerve fibres innervating the NEBs were labelled. The reliable and specific labelling of pulmonary NEBs by 4-Di-2-ASP was corroborated by immunostaining for protein gene-product 9.5. Live cell imaging and propidium iodide staining further established the acceptable viability of 4-Di-2-ASP-labelled NEB cells in lung slices, even over long periods. Importantly, the in vitro model and 4-Di-2-ASP staining procedure for pulmonary NEBs appeared to be equally reproducible in mouse, hamster and rabbit lungs. Diverse immunocytochemical procedures could be applied to the lung slices providing an opportunity to combine physiological and functional morphological studies. Such an integrated approach offers additional possibilities for elucidating the function(s) of pulmonary NEBs in health and disease.

CO2 transduction mechanisms in avian intrapulmonary chemoreceptors: experiments and models

Intrapulmonary chemoreceptors (IPC) are neurons that sense tonic and phasic CO2 stimuli in the lungs of birds and diapsid reptiles. IPC are different from most other vertebrate respiratory CO2 receptors because: (1) they are stimulated by low PCO2 and inhibited by high PCO2, (2) they have extremely rapid response characteristics, (3) their CO2 sensitivity is nearly abolished by intracellular inhibitors of carbonic anhydrase, and (4) their CO2 sensitivity is strongly depressed by inhibiting Na+/H+ antiport exchange. Experimental evidence suggests that IPC respond to intracellular pH, not CO2 directly, and that intracellular pH and IPC discharge are determined by a kinetic balance between CO2 hydration/dehydration rates, transmembrane acid/base exchange rates, and intracellular buffering. We review experimental evidence for and against various mechanisms of IPC CO2 chemotransduction, present a conceptual and mathematical model of the proposed mechanisms, and compare this model to CO2 transduction in other respiratory chemoreceptors.

Neuroepithelial bodies not connected to pulmonary slowly adapting stretch receptors

Neuroepithelial bodies (NEBs) are believed to be connected with one of the known types of airway receptors. The present studies determined whether NEB afferents are pulmonary slowly adapting stretch receptors (SARs). NEBs are immunoreactive with antibodies against protein gene product (PGP) 9.5 and calcitonin gene-related peptide (CGRP), whereas SARs are reactive with antibody to Na(+)/K(+)-ATPase. Using histochemical staining in combination with confocal microscopy, we compared the morphology of NEBs and SARs in the rat. Our results show that NEBs and SARs are different in location, size, and shape. Double staining of airway tissues for PGP (or CGRP) plus Na(+)/K(+)-ATPase shows that NEBs and SARs do not co-localize. In addition, we electrophysiologically recorded single-unit activity of SARs from the cervical vagus nerve, identified their receptive fields, dissected them into blocks, and then double-stained and examined the receptor structures. We found that the blocks contain the SAR, but not NEB structures. Thus, we conclude that NEBs are not connected to SARs.

Airway mechanosensors

A mechanosensory unit is a functional unit that contains multiple receptors (or encoders) with different characteristics, including rapidly adapting receptors, slowly adapting receptors, and deflation-activated-receptors. Each is capable of sensing different aspects of lung mechanics. The sensory unit is both a transducer and a processor. Significant information integration occurs at the intra-encoder and inter-encoder levels. Within an encoder, the information is encoded as analog signals and integrated by amplitude modulation. Information from each single stretch-activated channel is processed through several levels of temporal and spatial summation, producing a generator potential that encodes averaged overall information within the encoder. This analog signal is transformed into a digital signal in the form of action potentials that are encoded as frequency (frequency modulation). These all-or-none propagated action potentials from different encoders interact through a competitive selection mechanism. Such inter-encoder interaction may occur at several levels, because of the fractal nature of the sensory unit. Inter-encoder interaction retains representative information but eliminates redundant information, resulting in the final output to the central nervous system, where multiple decoders specific for different variables decipher the encoded information for further processing.

Cellular mechanisms involved in CO(2) and acid signaling in chemosensitive neurons

An increase in CO(2)/H(+) is a major stimulus for increased ventilation and is sensed by specialized brain stem neurons called central chemosensitive neurons. These neurons appear to be spread among numerous brain stem regions, and neurons from different regions have different levels of chemosensitivity. Early studies implicated changes of pH as playing a role in chemosensitive signaling, most likely by inhibiting a K(+) channel, depolarizing chemosensitive neurons, and thereby increasing their firing rate. Considerable progress has been made over the past decade in understanding the cellular mechanisms of chemosensitive signaling using reduced preparations. Recent evidence has pointed to an important role of changes of intracellular pH in the response of central chemosensitive neurons to increased CO(2)/H(+) levels. The signaling mechanisms for chemosensitivity may also involve changes of extracellular pH, intracellular Ca(2+), gap junctions, oxidative stress, glial cells, bicarbonate, CO(2), and neurotransmitters. The normal target for these signals is generally believed to be a K(+) channel, although it is likely that many K(+) channels as well as Ca(2+) channels are involved as targets of chemosensitive signals. The results of studies of cellular signaling in central chemosensitive neurons are compared with results in other CO(2)- and/or H(+)-sensitive cells, including peripheral chemoreceptors (carotid body glomus cells), invertebrate central chemoreceptors, avian intrapulmonary chemoreceptors, acid-sensitive taste receptor cells on the tongue, and pain-sensitive nociceptors. A multiple factors model is proposed for central chemosensitive neurons in which multiple signals that affect multiple ion channel targets result in the final neuronal response to changes in CO(2)/H(+).

Hypoxemia and blunted hypoxic ventilatory responses in mice lacking heme oxygenase-2

Heme oxygenase (HO) catalyzes physiological heme degradation and consists of two structurally related isozymes, HO-1 and HO-2. Here we show that HO-2-deficient (HO-2(-/-)) mice exhibit hypoxemia and hypertrophy of the pulmonary venous myocardium associated with increased expression of HO-1. The hypertrophied venous myocardium may reflect adaptation to persistent hypoxemia. HO-2(-/-) mice also show attenuated ventilatory responses to hypoxia (10% O2) with normal responses to hypercapnia (10% CO2), suggesting the impaired oxygen sensing. Importantly, HO-2(-/-) mice exhibit normal breathing patterns with normal arterial CO2 tension and retain the intact alveolar architecture, thereby excluding hypoventilation and shunting as causes of hypoxemia. Instead, ventilation-perfusion mismatch is a likely cause of hypoxemia, which may be due to partial impairment of the lung chemoreception probably at pulmonary artery smooth muscle cells. We therefore propose that HO-2 is involved in oxygen sensing and responsible for the ventilation-perfusion matching that optimizes oxygenation of pulmonary blood.

Vagal innervation of the air sacs in a songbird, Taenopygia guttata

The air sacs of birds are thin-walled chambers connected to the lung that act as bellows in the ventilatory mechanism. Physiological evidence exists to suggest that they may contain receptors that are innervated by the vagus nerve, but no morphological study has examined the vagal innervation of these putative structures. To do this, we injected the cervical vagus nerve with choleragenoid and examined the innervation of the air sacs using light and confocal microscopy. We identified vagally innervated structures in the air sac wall that resemble the neuroepithelial bodies (NEBs) described in the airways of many vertebrates. Although NEBs have been proposed to have a dual chemoreceptive and mechanoreceptive role, their specific function in the air sacs of birds remains unclear.

Functional Neuroanatomy of the Sensorimotor Control of Singing

Reviews of the songbird vocal control system frequently begin by describing the forebrain nuclei and pathways that form anterior and posterior circuits involved in song learning and song production, respectively. They then describe extratelencephalic projections upon the brainstem respiratory-vocal system in a manner suggesting, quite erroneously, that this system is itself well understood. One aim of this chapter is to demonstrate how limited is our understanding of that system. I begin with an overview of the neural network for the motor control of song production, with a particular emphasis on brainstem structures, including the tracheosyringeal motor nucleus (XIIts), which innervates the syrinx, and nucleus retroambigualis (RAm), which projects upon XIIts and upon spinal motor neurons innervating expiratory muscles. I describe the sources of afferent projections to XIIts and RAm and discuss their probable role in coordinating the bilateral activity of respiratory and syringeal muscles during singing. I then consider the routes by which sensory feedback, which could arise from numerous structures involved in singing, might access the song system to guide song learning, maintain accurate song production, and inform the song system of the requirements for air. I describe possible routes of access of auditory feedback, which is known to be necessary for song learning and maintenance, and identify potential sites of interaction with somatosensory and visceral feedback that could arise from the syrinx, the lungs and air sacs, and the upper vocal tract, including the jaw. I conclude that the incorporation of brainstem-based respiratory-vocal variables is likely to be a necessary next step in the construction of more sophisticated models of the control of vocalization.

Physiological and pathological responses to hypoxia

As the average age in many countries steadily rises, heart infarction, stroke, and cancer become the most common causes of death in the 21st century. The causes of these disorders are many and varied and include genetic predisposition and environmental influences, but they all share a common feature in that limitation of oxygen availability participates in the development of these pathological conditions. However, cells and organisms are able to trigger an adaptive response to hypoxic conditions that is aimed to help them to cope with these threatening conditions. This review provides a description of several systems able to sense oxygen concentration and of the responses they initiate both in the acute and also in long-term hypoxia adaptation. The role of hypoxia in three pathological conditions, myocardial and cerebral ischemia as well as tumorigenesis, is briefly discussed.

System-specific O2 sensitivity of the tandem pore domain K+ channel TASK-1

Hypoxic inhibition of TASK-1, a tandem pore domain background K+ channel, provides a critical link between reduced O2 levels and physiological responses in various cell types. Here, we examined the expression and O2 sensitivity of TASK-1 in immortalized adrenomedullary chromaffin (MAH) cells. In physiological (asymmetrical) K+ solutions, 3 μM anandamide or 300 μM Zn2+ inhibited a strongly pH-sensitive current. Under symmetrical K+ conditions, the anandamide- and Zn2+-sensitive K+ currents were voltage independent. These data demonstrate the functional expression of TASK-1, and cellular expression of this channel was confirmed by RT-PCR and Western blotting. At concentrations that selectively inhibit TASK-1, anandamide and Zn2+ were without effect on the magnitude of the O2-sensitive current or the hypoxic depolarization. Thus TASK-1 does not contribute to O2 sensing in MAH cells, demonstrating the failure of a known O2-sensitive K+ channel to respond to hypoxia in an O2-sensing cell. These data demonstrate that, ultimately, the sensitivity of a particular K+ channel to hypoxia is determined by the cell, and we propose that this is achieved by coupling distinct hypoxia signaling systems to individual channels. Importantly, these data also reiterate the indirect O2 sensitivity of TASK-1, which appears to require the presence of an intracellular mediator.

GABAB receptor activation augments TASK-1 in MAH cells and mediates autoreceptor feedback during hypoxia

Previously, we demonstrated an autoregulatory feedback loop in the rat carotid body (CB), involving presynaptic GABA(B) receptor-mediated activation of the background K(+) channel TASK-1. Here, we examined the effects of the selective GABA(B) receptor agonist baclofen on K(+) currents in immortalised adrenomedullary chromaffin (MAH) cells, which share the same sympathoadrenal lineage as CB type I cells. Under symmetrical K(+) conditions, 50 microM baclofen enhanced a K(+) current which was linear and reversed close to 0 mV. Under physiological K(+) conditions, baclofen enhanced outward K(+) current and caused membrane hyperpolarisation, effects inhibited by 100 nM CGP 55845. Current enhancement was virtually abolished in the presence of 300 microM Zn(2+), a selective inhibitor of TASK-1. When recording membrane potential from MAH cells in clusters, hypoxic depolarisation was augmented by 100 nM CGP 55845. These data demonstrate that GABA(B) receptors mediate autoreceptor feedback in the adrenal medulla presumably via TASK-1, demonstrating a common autoregulatory feedback pathway in neurosecretory, chemosensitive cells.

Reduced number of intrinsic pulmonary nitrergic neurons in Fawn-Hooded rats as compared to control rat strains

The Fawn-Hooded rat (FHR) strain reveals a congenital predisposition to primary (idiopathic) pulmonary hypertension (PPH), and can therefore be regarded as an animal model in which to study possible mechanisms underlying an inherited susceptibility to pulmonary hypertension. Pulmonary hypertension can be induced in FHRs after a short exposure to mild hypoxia, presumably because of an altered peripheral oxygen sensitivity. Given the presence of pulmonary nitrergic neurons in rat lungs, the observed link between airway hypoxia and the expression of pulmonary neuronal nitric oxide synthase (nNOS), and the fact that nNOS appears to be involved in peripheral chemoreceptor sensitivity, we examined the intrinsic pulmonary nitrergic innervation in the FHR. In the present study the number of intrapulmonary nitrergic nerve cell bodies, detected by NADPH diaphorase (NADPHd) histochemistry, was quantified in the FHR and three control rat strains. Compared to the control rat strains, the FHR lungs revealed a highly significantly lower number of intrinsic nitrergic neurons, while no apparent differences were found in the number of enteric nitrergic neurons in the esophagus. In conclusion, the possible links between neuronal NO, hypersensitivity to airway hypoxia, and the development of PPH clearly deserve further investigation.