Hypoxia induces production of nitric oxide and reactive oxygen species in glomus cells of rat carotid body

Hypoxia releases S-nitrosocysteine from carotid body glomus cells-relevance to expression of the hypoxic ventilatory response

We have provided indirect pharmacological evidence that hypoxia may trigger release of the S-nitrosothiol, S-nitroso-L-cysteine (L-CSNO), from primary carotid body glomus cells (PGCs) of rats that then activates chemosensory afferents of the carotid sinus nerve to elicit the hypoxic ventilatory response (HVR). The objective of this study was to provide direct evidence, using our capacitive S-nitrosothiol sensor, that L-CSNO is stored and released from PGCs extracted from male Sprague Dawley rat carotid bodies, and thus further pharmacological evidence for the role of S-nitrosothiols in mediating the HVR. Key findings of this study were that 1) lysates of PGCs contained an S-nitrosothiol with physico-chemical properties similar to L-CSNO rather than S-nitroso-L-glutathione (L-GSNO), 2) exposure of PGCs to a hypoxic challenge caused a significant increase in S-nitrosothiol concentrations in the perfusate to levels approaching 100 fM via mechanisms that required extracellular Ca2+, 3) the dose-dependent increases in minute ventilation elicited by arterial injections of L-CSNO and L-GSNO were likely due to activation of small diameter unmyelinated C-fiber carotid body chemoafferents, 4) L-CSNO, but not L-GSNO, responses were markedly reduced in rats receiving continuous infusion (10 μmol/kg/min, IV) of both S-methyl-L-cysteine (L-SMC) and S-ethyl-L-cysteine (L-SEC), 5) ventilatory responses to hypoxic gas challenge (10% O2, 90% N2) were also due to the activation of small diameter unmyelinated C-fiber carotid body chemoafferents, and 6) the HVR was markedly diminished in rats receiving L-SMC plus L-SEC. This data provides evidence that rat PGCs synthesize an S-nitrosothiol with similar properties to L-CSNO that is released in an extracellular Ca2+-dependent manner by hypoxia.

Mitochondria in hypoxic pulmonary hypertension, roles and the potential targets

Mitochondria are the centrol hub for cellular energy metabolisms. They regulate fuel metabolism by oxygen levels, participate in physiological signaling pathways, and act as oxygen sensors. Once oxygen deprived, the fuel utilizations can be switched from mitochondrial oxidative phosphorylation to glycolysis for ATP production. Notably, mitochondria can also adapt to hypoxia by making various functional and phenotypes changes to meet the demanding of oxygen levels. Hypoxic pulmonary hypertension is a life-threatening disease, but its exact pathgenesis mechanism is still unclear and there is no effective treatment available until now. Ample of evidence indicated that mitochondria play key factor in the development of hypoxic pulmonary hypertension. By hypoxia-inducible factors, multiple cells sense and transmit hypoxia signals, which then control the expression of various metabolic genes. This activation of hypoxia-inducible factors considered associations with crosstalk between hypoxia and altered mitochondrial metabolism, which plays an important role in the development of hypoxic pulmonary hypertension. Here, we review the molecular mechanisms of how hypoxia affects mitochondrial function, including mitochondrial biosynthesis, reactive oxygen homeostasis, and mitochondrial dynamics, to explore the potential of improving mitochondrial function as a strategy for treating hypoxic pulmonary hypertension.

The Role of Pharmacological Treatment in the Chemoreflex Modulation

From a physiological point of view, peripheral chemoreceptors (PCh) are the main sensors of hypoxia in mammals and are responsible for adaptation to hypoxic conditions. Their stimulation causes hyperventilation—to increase oxygen uptake and increases sympathetic output in order to counteract hypoxia-induced vasodilatation and redistribute the oxygenated blood to critical organs. While this reaction promotes survival in acute settings it may be devastating when long-lasting. The permanent overfunctionality of PCh is one of the etiologic factors and is responsible for the progression of sympathetically-mediated diseases. Thus, the deactivation of PCh has been proposed as a treatment method for these disorders. We review here physiological background and current knowledge regarding the influence of widely prescribed medications on PCh acute and tonic activities.

Intra-carotid body inter-cellular communication

The classic peripheral chemoreflex response is a critical homeostatic mechanism. In healthy individuals, appropriate chemoreflex responses are triggered by acute activation of the carotid body - the principal chemosensory organ in mammals. However, the aberrant chronic activation of the carotid body can drive the elevated sympathetic activity underlying cardio-respiratory diseases such as hypertension, diabetes and heart failure. Carotid body resection induces intolerable side effects and so understanding how to modulate carotid body output without removing it, and whilst maintaining the physiological chemoreflex response, represents the next logical next step in the development of effective clinical interventions. By definition, excessive carotid body output must result from altered intra-carotid body inter-cellular communication. Alongside the canonical synaptic transmission from glomus cells to petrosal afferents, many other modes of information exchange in the carotid body have been identified, for example bidirectional signalling between type I and type II cells via ATP-induced ATP release, as well as electrical communication via gap junctions. Thus, herein we review the carotid body as an integrated circuit, discussing a variety of different inter-cellular signalling mechanisms and highlighting those that are potentially relevant to its pathological hyperactivity in disease with the aim of identifying novel therapeutic targets.

Ventilatory responses during and following hypercapnic gas challenge are impaired in male but not female endothelial NOS knock-out mice

The roles of endothelial nitric oxide synthase (eNOS) in the ventilatory responses during and after a hypercapnic gas challenge (HCC, 5% CO₂, 21% O₂, 74% N₂) were assessed in freely-moving female and male wild-type (WT) C57BL6 mice and eNOS knock-out (eNOS-/-) mice of C57BL6 background using whole body plethysmography. HCC elicited an array of ventilatory responses that were similar in male and female WT mice, such as increases in breathing frequency (with falls in inspiratory and expiratory times), and increases in tidal volume, minute ventilation, peak inspiratory and expiratory flows, and inspiratory and expiratory drives. eNOS-/- male mice had smaller increases in minute ventilation, peak inspiratory flow and inspiratory drive, and smaller decreases in inspiratory time than WT males. Ventilatory responses in female eNOS-/- mice were similar to those in female WT mice. The ventilatory excitatory phase upon return to room-air was similar in both male and female WT mice. However, the post-HCC increases in frequency of breathing (with decreases in inspiratory times), and increases in tidal volume, minute ventilation, inspiratory drive (i.e., tidal volume/inspiratory time) and expiratory drive (i.e., tidal volume/expiratory time), and peak inspiratory and expiratory flows in male eNOS-/- mice were smaller than in male WT mice. In contrast, the post-HCC responses in female eNOS-/- mice were equal to those of the female WT mice. These findings provide the first evidence that the loss of eNOS affects the ventilatory responses during and after HCC in male C57BL6 mice, whereas female C57BL6 mice can compensate for the loss of eNOS, at least in respect to triggering ventilatory responses to HCC.

Claudin-5 Affects Endothelial Autophagy in Response to Early Hypoxia

Hypoxic injury to cerebrovascular endothelial cells (ECs) after stroke leads to blood-brain barrier (BBB) dysfunction, which is commonly associated with disruptions of endothelial tight junctions (TJs) and increased permeability. Therefore, maintaining the structural integrity and proper function of the BBB is essential for the homeostasis and physiological function of the central nervous system (CNS). Our previous study revealed that autophagy functions on protecting the BBB by regulating the dynamics of Claudin-5, the essential TJ protein, under short-term starvation or hypoxia conditions. Here, we show that in zebrafish and in vitro cells, loss of membranous Claudin-5 conversely determine the occurrence of hypoxia-induced autophagy in cerebrovascular ECs. Absence of endothelial Claudin-5 could partly attenuate endothelial cell apoptosis caused by short-term hypoxic injury. Mechanism studies revealed that under hypoxic conditions, the existence of membranous Claudin-5 affects the stimulation of hypoxia inducible factor 1 subunit alpha (HIF-1a) and the inducible nitric oxide synthase (iNOS), which are responsible for the translocation of and endocytosis of caveole-packaged Claudin-5 into cytosol. Meanwhile, loss of Claudin-5 affects the generation of reactive oxygen species (ROS) and the downstream expression of BCL2/adenovirus E1B 19kDa protein interacting protein 3 (Bnip3). These together suppress the endothelial autophagy under hypoxia. This finding provides a theoretical basis for clarifying the mechanism of hypoxia-induced BBB injury and its potential protection mechanisms.

Short-term facilitation of breathing upon cessation of hypoxic challenge is impaired in male but not female endothelial NOS knock-out mice

Decreases in arterial blood oxygen stimulate increases in minute ventilation via activation of peripheral and central respiratory structures. This study evaluates the role of endothelial nitric oxide synthase (eNOS) in the expression of the ventilatory responses during and following a hypoxic gas challenge (HXC, 10% O₂, 90% N₂) in freely moving male and female wild-type (WT) C57BL6 and eNOS knock-out (eNOS-/-) mice. Exposure to HXC caused an array of responses (of similar magnitude and duration) in both male and female WT mice such as, rapid increases in frequency of breathing, tidal volume, minute ventilation and peak inspiratory and expiratory flows, that were subject to pronounced roll-off. The responses to HXC in male eNOS-/- mice were similar to male WT mice. In contrast, several of the ventilatory responses in female eNOS-/- mice (e.g., frequency of breathing, and expiratory drive) were greater compared to female WT mice. Upon return to room-air, male and female WT mice showed similar excitatory ventilatory responses (i.e., short-term potentiation phase). These responses were markedly reduced in male eNOS-/- mice, whereas female eNOS-/- mice displayed robust post-HXC responses that were similar to those in female WT mice. Our data demonstrates that eNOS plays important roles in (1) ventilatory responses to HXC in female compared to male C57BL6 mice; and (2) expression of post-HXC responses in male, but not female C57BL6 mice. These data support existing evidence that sex, and the functional roles of specific proteins (e.g., eNOS) have profound influences on ventilatory processes, including the responses to HXC.

Is Carotid Body Physiological O2 Sensitivity Determined by a Unique Mitochondrial Phenotype?

The mammalian carotid body (CB) is the primary arterial chemoreceptor that responds to acute hypoxia, initiating systemic protective reflex responses that act to maintain O2 delivery to the brain and vital organs. The CB is unique in that it is stimulated at O2 levels above those that begin to impact on the metabolism of most other cell types. Whilst a large proportion of the CB chemotransduction cascade is well defined, the identity of the O2 sensor remains highly controversial. This short review evaluates whether the mitochondria can adequately function as acute O2 sensors in the CB. We consider the similarities between mitochondrial poisons and hypoxic stimuli in their ability to activate the CB chemotransduction cascade and initiate rapid cardiorespiratory reflexes. We evaluate whether the mitochondria are required for the CB to respond to hypoxia. We also discuss if the CB mitochondria are different to those located in other non-O2 sensitive cells, and what might cause them to have an unusually low O2 binding affinity. In particular we look at the potential roles of competitive inhibitors of mitochondrial complex IV such as nitric oxide in establishing mitochondrial and CB O2-sensitivity. Finally, we discuss novel signaling mechanisms proposed to take place within and downstream of mitochondria that link mitochondrial metabolism with cellular depolarization.

Oxidative stress: a key regulator of leiomyoma cell survival

OBJECTIVE

To determine the effects of attenuating oxidative stress with the use of dichloroacetate (DCA) on the expression of key redox enzymes myeloperoxidase (MPO) and inducible nitric oxide synthase (iNOS) as well as on apoptosis.

DESIGN

Prospective experimental study.

SETTING

University medical center.

PATIENT(S)

Cells established from myometrium and uterine fibroid from the same patients.

INTERVENTION(S)

Cells were exposed to normal (20% O₂) or hypoxic (2% O₂) conditions for 24 hours with or without DCA (20 μg/mL), a metabolic modulator that shifts anaerobic to aerobic metabolism.

MAIN OUTCOME MEASURE(S)

Nitrate/nitrite (iNOS activity indicator), iNOS, Bcl-2/Bax ratio, MPO, and caspase-3 activities and levels were determined by means of Greiss assay, real-time reverse-transcription polymerase chain reaction, and ELISA. Data were analyzed with the use of SPSS by means of one-way analysis of variance with Tukey post hoc analysis and independent t tests.

RESULT(S)

MPO, iNOS, and nitrate/nitrite expression were higher in leiomyoma than in myometrial cells, and they were further enhanced by hypoxia in myometrial cells. Treatment with the use of DCA decreased MPO, iNOS, and nitrate/nitrite levels and negated the effect of hypoxia in both types of cells. Leiomyoma cells showed less apoptosis, as indicated by both caspase-3 activity and the Bcl-2/Bax ratio, than myometrial cells. Hypoxia further decreased apoptosis in myometrial cells with no further effect on leiomyoma cells. Treatment with DCA resulted in increased apoptosis in both types of cells, even in the presence of hypoxia.

CONCLUSION(S)

Shifting anaerobic to aerobic metabolism with the use of DCA resulted in an increase in apoptosis in leiomyoma cells and protected myometrial cells from the acquisition of the leiomyoma-like phenotype.

Moderate inhibition of mitochondrial function augments carotid body hypoxic sensitivity

A functional role for the mitochondria in acute O2 sensing in the carotid body (CB) remains undetermined. Whilst total inhibition of mitochondrial activity causes intense CB stimulation, it is unclear whether this response can be moderated such that graded impairment of oxidative phosphorylation might be a mechanism that sets and modifies the O2 sensitivity of the whole organ. We assessed NADH autofluorescence and [Ca2+]i in freshly dissociated CB type I cells and sensory chemoafferent discharge frequency in an intact CB preparation, in the presence of varying concentrations of nitrite (NO2 −), a mitochondrial nitric oxide (NO) donor and a competitive inhibitor of mitochondrial complex IV. NO2 − increased CB type I cell NADH in a manner that was dose-dependent and rapidly reversible. Similar concentrations of NO2 − raised type I cell [Ca2+]i via L-type channels in a PO2-dependent manner and increased chemoafferent discharge frequency. Moderate inhibition of the CB mitochondria by NO2 − augmented chemoafferent discharge frequency during graded hypoxia, consistent with a heightened CB O2 sensitivity. Furthermore, NO2 − also exaggerated chemoafferent excitation during hypercapnia signifying an increase in CB CO2 sensitivity. These data show that NO2 − can moderate the hypoxia sensitivity of the CB and thus suggest that O2 sensitivity could be set and modified in this organ by interactions between NO and mitochondrial complex IV.

Expression of nitric oxide-containing structures in the rat carotid body

The carotid body (CB) is a major peripheral arterial chemoreceptor organ that evokes compensatory reflex responses so as to maintain gas homeostasis. It is dually innervated by sensory fibers from petrosal ganglion (PG) neurons, and autonomic fibers from postganglionic sympathetic neurons of the superior cervical ganglion (SCG) and parasympathetic vasomotor fibers of intrinsic ganglion cells in the CB. The presence of nitric oxide (NO), a putative gaseous neurotransmitter substance in a number of neuronal and non-neuronal structures, was examined in the CB, PG and SCG of the rat using nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry, nitric oxide synthase (NOS) immunohistochemistry and retrograde tracing. One week after injecting the retrograde tracer Fast Blue (FB) in the CB, we found that a subset of perikarya in the caudal portions of the PG and SCG were FB-labeled. Histochemistry and immunohistochemistry revealed that the majority of large- and medium-sized PG and SCG cells were NADPH-d positive and displayed a strong NOS immunostaining. We also observed that many varicose nerve fibers penetrating the CB and enveloping the glomus cells and blood vessels were NADPH-d reactive and expressed the constitutive isoforms of NOS, nNOS and eNOS. In addition, some autonomic microganglion cells embedded within, or located at the periphery of the CB, and not glomus or sustentacular cells were nNOS-immunopositive while CB microvasculature expressed eNOS. The present results suggest that NO is a transmitter in the autonomic nerve endings supplying the CB and is involved in efferent chemoreceptor inhibition by a dual mechanism.

Role of nitric oxide-containing factors in the ventilatory and cardiovascular responses elicited by hypoxic challenge in isoflurane-anesthetized rats

Exposure to hypoxia elicits changes in mean arterial blood pressure (MAP), heart rate, and frequency of breathing (fR). The objective of this study was to determine the role of nitric oxide (NO) in the cardiovascular and ventilatory responses elicited by brief exposures to hypoxia in isoflurane-anesthetized rats. The rats were instrumented to record MAP, heart rate, and fR and then exposed to 90 s episodes of hypoxia (10% O2, 90% N2) before and after injection of vehicle, the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), or the inactive enantiomer D-NAME (both at 50 μmol/kg iv). Each episode of hypoxia elicited a decrease in MAP, bidirectional changes in heart rate (initial increase and then a decrease), and an increase in fR. These responses were similar before and after injection of vehicle or D-NAME. In contrast, the hypoxia-induced decreases in MAP were attenuated after administration of L-NAME. The initial increases in heart rate during hypoxia were amplified whereas the subsequent decreases in heart rate were attenuated in L-NAME-treated rats. Finally, the hypoxia-induced increases in fR were virtually identical before and after administration of L-NAME. These findings suggest that NO factors play a vital role in the expression of the cardiovascular but not the ventilatory responses elicited by brief episodes of hypoxia in isoflurane-anesthetized rats. Based on existing evidence that NO factors play a vital role in carotid body and central responses to hypoxia in conscious rats, our findings raise the novel possibility that isoflurane blunts this NO-dependent signaling.

Advances in the Pathogenesis of Adhesion Development: The Role of Oxidative Stress

Over the past several years, there has been increasing recognition that pathogenesis of adhesion development includes significant contributions of hypoxia induced at the site of surgery, the resulting oxidative stress, and the subsequent free radical production. Mitochondrial dysfunction generated by surgically induced tissue hypoxia and inflammation can lead to the production of reactive oxygen and nitrogen species as well as antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase which when optimal have the potential to abrogate mitochondrial dysfunction and oxidative stress, preventing the cascade of events leading to the development of adhesions in injured peritoneum. There is a significant cross talk between the several processes leading to whether or not adhesions would eventually develop. Several of these processes present avenues for the development of measures that can help in abrogating adhesion formation or reformation after intraabdominal surgery.

Ventilatory responses during and following exposure to a hypoxic challenge in conscious mice deficient or null in S-nitrosoglutathione reductase

Exposure to a hypoxic challenge increases ventilation in wild-type (WT) mice that diminish during the challenge (roll-off) whereas return to room air causes an increase in ventilation (short-term facilitation, STF). Since plasma and tissue levels of ventilatory excitant S-nitrosothiols such as S-nitrosoglutathione (GSNO) increase during hypoxia, this study examined whether (1) the initial increase in ventilation is due to generation of GSNO, (2) roll-off is due to increased activity of the GSNO degrading enzyme, GSNO reductase (GSNOR), and (3) STF is limited by GSNOR activity. Initial ventilatory responses to hypoxic challenge (10% O(2), 90% N(2)) were similar in WT, GSNO+/- and GSNO-/- mice. These responses diminished markedly during hypoxic challenge in WT mice whereas there was minimal roll-off in GSNOR+/- and GSNOR-/- mice. Finally, STF was greater in GSNOR+/- and GSNOR-/- mice than in WT mice (especially females). This study suggests that GSNOR degradation of GSNO is a vital step in the expression of ventilatory roll-off and that GSNOR suppresses STF.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Carotid body remodelling in l-NAME-induced hypertension in the rat

The carotid body (CB) is a chemoreceptor organ located at the bifurcation of the common carotid artery. It is made up of the carotid glomus, a structure containing type 1 cells surrounded by type 2 cells. The aim of this study was to evaluate the morphological changes of the CB and carotid glomus in the rat model of l-NAME-induced hypertension. Male Wistar rats were divided in two groups: control untreated rats (C) and rats receiving l-NAME 40 mg/kg/day (LN) for 6 weeks. At the end of the experiment, the systolic blood pressure was 63% higher in the LN group compared with the C group. Morphometric analysis showed that the area of the CB was 29% greater in the LN group compared with the C group. The density of nuclei in the CB was similar between groups, but it was 31% less in the carotid glomus of the LN group. Cells in the CB of the LN group displayed cytoplasmic vacuolation and expressed several biogenic amines. There were more elastic fibres, proteoglycans and collagen fibres in the LN group compared with the C group. Immunohistochemistry showed increased expression of nuclear factor kB, substance P, vascular endothelial growth factor and neuronal nitric oxide synthase in the LN group, while expression of the protein gene product 9.5 was decreased. l-NAME alters cell morphology and the expression of extracellular matrix molecules in the CB and carotid glomus in rats with l-NAME-induced hypertension.

Postoperative adhesion development following cesarean and open intra-abdominal gynecological operations: a review

In this review, we discuss the pathophysiology of adhesion development, the impact of physiological changes associated with pregnancy on markers of adhesion development, and the clinical implications of adhesion development following cesarean delivery (CD). Although peritoneal adhesions develop after the overwhelming majority of intra-abdominal and pelvic surgery, there is evidence in the literature that suggests that patients having CD may develop adhesions less frequently. However, adhesions continue to be a concern after CD, and are likely significant, albeit on average less than after gynecological operations, but with potential to cause significant delay in the delivery of the baby with serious, lifelong consequences. Appreciation of the pathophysiology of adhesion development described herein should allow a more informed approach to the rapidly evolving field of intra-abdominal adhesions and should serve as a reference for an evidence-based approach to consideration for the prevention and treatment of adhesions.

Real-time in vivo simultaneous measurements of nitric oxide and oxygen using an amperometric dual microsensor

This paper reports a real-time study of the codynamical changes in the release of endogenous nitric oxide (NO) and oxygen (O(2)) consumption in a rat neocortex in vivo upon electrical stimulation using an amperometric NO/O(2) dual microsensor. Electrical stimulation induced transient cerebral hypoxia due to the increased metabolic demands that were not met by the blood volume inside the stimulated cortical region. A NO/O(2) dual microsensor was successfully used to monitor the pair of real-time dynamic changes in the tissue NO and O(2) contents. At the onset of electrical stimulation, there was an immediate decrease in the cortical tissue O(2) followed by a subsequent increase in the cortical tissue NO content. The averages of the maximum normalized concentration changes induced by the stimulation were a 0.41 (±0.04)-fold decrease in the O(2) and a 3.6 (±0.9)-fold increase in the NO concentrations when compared with the corresponding normalized basal levels. The peak increase in NO was always preceded by the peak decrease in O(2) in all animals (n = 11). The delay between the maximum decrease in O(2) and the maximum increase in NO varied from 3.1 to 54.8 s. This rather wide variation in the temporal associations was presumably attributed to the sparse distribution of NOS-containing neurons and the individual animal's differences in brain vasculatures, which suggests that a sensor with fine spatial resolution is needed to measure the location-specific real-time NO and O(2) contents. In summary, the developed NO/O(2) dual microsensor is effective for measuring the NO and O(2) contents in vivo. This study provides direct support for the dynamic role of NO in regulating the cerebral hemodynamics, particularly related to the tissue oxygenation.

Mitochondrial complex III: an essential component of universal oxygen sensing machinery?

Oxygen is necessary for the survival of mammalian cells. In order to maintain adequate cellular oxygenation, mammals have evolved multiple acute and long-term adaptive responses to hypoxia. These include hypoxic increases in erythropoiesis, pulmonary vasoconstriction and carotid body neurosecretion. Collectively, these responses help maintain oxygen homeostasis as oxygen levels remain scarce. There are multiple effectors proposed to underlie these diverse responses to hypoxia including PHD2, AMPK, NADPH oxidases, and mitochondrial complex III. Here I propose a model wherein complex III is integral to oxygen sensing in regulating diverse response to hypoxia.

Lifespan and oxidative stress show a non-linear response to atmospheric oxygen in Drosophila

Oxygen provides the substrate for most ATP production, but also serves as a source of reactive oxygen species (ROS), which can induce cumulative macromolecular oxidative damage and cause aging. Pure oxygen atmospheres (100 kPa) are known to strongly reduce invertebrate lifespan and induce aging-related physiological changes. However, the nature of the relationship between atmospheric oxygen, oxidative stress, and lifespan across a range of oxygen levels is poorly known. Developmental responses are likely to play a strong role, as prior research has shown strong effects of rearing oxygen level on growth, size and respiratory system morphology. In this study, we examined (1) the effect of oxygen on adult longevity and (2) the effect of the oxygen concentration experienced by larvae on adult lifespan by rearing Drosophila melanogaster in three oxygen atmospheres throughout larval development (10, 21 and 40 kPa), then measuring the lifespan of adults in five oxygen tensions (2, 10, 21, 40, 100 kPa). We also assessed the rate of protein carbonyl production for flies kept at 2, 10, 21, 40 and 100 kPa as adults (all larvae reared in normoxia). The rearing of juveniles in varying oxygen treatments affected lifespan in a complex manner, and the effect of different oxygen tensions on adult lifespan was non-linear, with reduced longevity and heightened oxidative stress at extreme high and low atmospheric oxygen levels. Moderate hypoxia (10 kPa) extended maximum, but not mean lifespan.

Hypoxia increases the dependence of glioma cells on glutathione

Glutathione (GSH) is an essential antioxidant responsible for the maintenance of intracellular redox homeostasis. As tumors outgrow their blood supply and become hypoxic, their redox homeostasis is challenged by the production of nitric oxide and reactive oxygen species (ROS). In gliomas, the sustained import of L-cystine via the L-cystine/L-glutamate exchanger, system x(c)(-), is rate-limiting for the synthesis of GSH. We show that hypoxia causes a significant increase in NO and ROS but without affecting glioma cell growth. This is explained by a concomitant increase in the utilization of GSH, which is accompanied by an increase in the cell-surface expression of xCT, the catalytic subunit of system x(c)(-), and L-cystine uptake. Growth was inhibited when GSH synthesis was blocked by buthionine sulfoximine (BSO), an inhibitor of the enzyme required for GSH synthesis, or when cells were deprived of L-cystine. These findings suggest that glioma cells show an increased requirement for GSH to maintain growth under hypoxic conditions. Therefore, approaches that limit GSH synthesis such as blocking system x(c)(-) may be considered as an adjuvant to radiation or chemotherapy.

Mechanisms for acute oxygen sensing in the carotid body

Hypoxic chemotransduction in the carotid body requires release of excitatory transmitters from type I cells that activate afferent sensory neurones. Transmitter release is dependent on voltage-gated Ca2+ entry which is evoked by membrane depolarization. This excitatory response to hypoxia is initiated by inhibition of specific O2 sensitive K+ channels, of which several types have been reported. Here, we discuss mechanisms which have been put forward to account for hypoxic inhibition of type I cell K+ channels. Whilst evidence indicates that one O2 sensitive K+ channel, BKCa, may be regulated by gasotransmitters (CO and H2S) in an O2-dependent manner, other studies now indicate that activation of AMP-activated protein kinase (AMPK) accounts for inhibition of both BKCa and 'leak' O2 sensitive K+ channels, and perhaps also other O2 sensitive K+ channels reported in different species. We propose that type I cell AMPK activation occurs as a result of inhibition of mitochondrial oxidative phosphorylation, and does not require increased production of reactive oxygen species. Thus, AMPK activation provides the basis for unifying the 'membrane' and 'mitochondrial' hypotheses, previously regarded as disparate, to account for hypoxic chemotransduction.

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

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

Exposure to polychlorinated biphenyls enhances lipid peroxidation in human normal peritoneal and adhesion fibroblasts: a potential role for myeloperoxidase

Nitric oxide, superoxide, and lipid peroxidation (LPO) produced under oxidative stress may contribute to the development of postoperative adhesions. The objective of this study was to determine the effects of polychlorinated biphenyls (PCBs) on LPO, superoxide dismutase, myeloperoxidase (MPO), and nitrite/nitrate in human normal peritoneal and adhesion fibroblasts. PCB treatment reduced inducible nitric oxide synthase (iNOS) expression as well as levels of nitrite/nitrate in both cell lines. Although there was no difference in iNOS expression between the two cell lines, adhesion fibroblasts manifested lower basal levels of MPO compared to normal peritoneal fibroblasts. There was a reduction in MPO expression and its activity in response to PCB treatment in normal peritoneal fibroblasts; however, this effect was minimal in adhesion fibroblasts. Moreover, adhesion fibroblasts manifested higher levels of LPO compared to normal peritoneal fibroblasts, whereas PCB treatment increased LPO levels in both cell types. We conclude that PCBs promote the development of the adhesion phenotype by generating an oxidative stress environment. This is evident by lower iNOS, MPO, and nitrite/nitrate and a simultaneous increase in LPO. Loss of MPO activity, possibly through a mechanism involving MPO heme depletion and free iron release, is yet another source of oxidative stress.

Intermittent hypoxia: cause of or therapy for systemic hypertension?

During acute episodes of hypoxia, chemoreceptor-mediated sympathetic activity increases heart rate, cardiac output, peripheral resistance and systemic arterial pressure. However, different intermittent hypoxia paradigms produce remarkably divergent effects on systemic arterial pressure in the post-hypoxic steady state. The hypertensive effects of obstructive sleep apnea (OSA) vs. the depressor effects of therapeutic hypoxia exemplify this divergence. OSA, a condition afflicting 15-25% of American men and 5-10% of women, has been implicated in the pathogenesis of systemic hypertension and is a major risk factor for heart disease and stroke. OSA imposes a series of brief, intense episodes of hypoxia and hypercapnia, leading to persistent, maladaptive chemoreflex-mediated activation of the sympathetic nervous system which culminates in hypertension. Conversely, extensive evidence in animals and humans has shown controlled intermittent hypoxia conditioning programs to be safe, efficacious modalities for prevention and treatment of hypertension. This article reviews the pertinent literature in an attempt to reconcile the divergent effects of intermittent hypoxia therapy and obstructive sleep apnea on hypertension. Special emphasis is placed on research conducted in the nations of the former Soviet Union, where intermittent hypoxia conditioning programs are being applied therapeutically to treat hypertension in patients. Also reviewed is evidence regarding mechanisms of the pro- and anti-hypertensive effects of intermittent hypoxia.

Enhanced nitric oxide-mediated chemoreceptor inhibition and altered cyclic GMP signaling in rat carotid body following chronic hypoxia

Multiple studies have shown that chronic hypoxia (CH) elicits a time-dependent upregulation of carotid body chemoreceptor sensitivity in mammals. In the present study, we demonstrate that enhanced excitation is accompanied by a parallel increase of nitric oxide (NO)-dependent inhibition, which acts via a CH-induced modification of the normal mechanism in O(2)-sensitive type I cells. The NO synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME), elicits a progressively larger increase in carotid sinus nerve (CSN) chemoreceptor activity following incremental increases in CH exposure lasting 1-16 days. The inhibitory effect of the NO donor, S-nitroso-N-acetyl-penicillamine (SNAP), on CSN activity is enhanced following CH. However, the activation of soluble guanylate cyclase (sGC) by SNAP, assessed via production of cGMP, is impaired, along with decreased expression of sGC mRNA transcript. Inhibition of hypoxia-evoked Ca(2+) responses by SNAP is mediated via a cGMP/protein kinase G (PKG)-dependent mechanism in normal type I cells that is sensitive to the PKG inhibitor KT-5823, but following CH, inhibitory responses are minimally sensitive to PKG inhibition. The data are consistent with the hypothesis that CH hampers cGMP-mediated inhibition of type I cells in favor of an alternative mechanism.

Increased endogenous nitric oxide release by iron chelation and purinergic activation in the rat carotid body

We examined the hypothesis that hypoxic chemotransduction with stabilization of HIF-1 and activation of purinoceptors stimulate the endogenous NO production in the rat carotid body. The effects of blockade of purinoceptors with suramin, or blockade of HIF-1α hydroxylation by suppressing prolyl hydroxylase (PAH) activity on the endogenous NO release measured electrochemically by microsensor inserted into the isolated carotid body superfused with bicarbonate-buffer were examined. Suramin did not change the resting NO level under normoxic conditions but it significantly decreased the hypoxia-induced NO elevation in a dose-dependent manner. Suramin (100μM) blocked the NO response to acute hypoxia by 53%. Intracellular iron chelator, ciclopirox olamine (CPX) significantly increased the resting NO release close to the hypoxic level, which was reversed by FeSO₄ or blocked by L-NMMA. Also, PAH inhibition with dimethy-loxalylglycine (DMOG) moderately increased the resting NO release. In the presence of CPX and DMOG the resting NO release was increased to the hypoxic level. Collectively, results suggest that iron chelation and purinoceptor stimulation play a role in the hypoxic chemotransduction for an increase in the endogenous NO production in the rat carotid body.