Dithionite increases radical formation and decreases vasoconstriction in the lung. Evidence that dithionite does not mimic alveolar hypoxia

Etherified pullulan-polyethylenimine based nanoscaffolds improved chemosensitivity of erlotinib on hypoxic cancer cells

The current research endeavor aimed to accomplish hypoxia-responsive polyethyleneimine-conjugated carboxymethyl pullulan-based co-polymer (CMP-HA-NI-PEI-NBA) bearing nitroaromatic subunits to efficiently deliver erlotinib (ERL) to reverse its hypoxia-induced resistance in cancer cells. As compared to a control co-polymer (CMP-HA-MI-PEI-BA) devoid of hypoxia-sensitive moieties, this scaffold demonstrated a hypochromic shift in the UV spectra and rapid dismantling of its self-assembled architecture upon exposure to simulated hypoxic condition. The hypoxia-responsive co-polymer encapsulated ERL with desirable loading capacity (DEE, 63.05 ± 2.59%), causing attenuated drug crystallinity. The drug release rate of the scaffold under reducing condition was faster relative to that of non-reducing environment. Their cellular uptake occurred through an energy-dependent endocytic process, which could exploit its caveolae/lipid raft-mediated internalization pathway. The ERL-loaded scaffolds more efficiently induced apoptosis and suppressed the proliferation of drug-resistant hypoxic HeLa cells than the pristine ERL. Hence, this study presented a promising drug delivery nanoplatform to overcome hypoxia-evoked ERL resistance.

Vasomotion of mice mesenteric arteries during low oxygen levels

Background

Ischemia of intestinal organs is a main cause of complications in surgical intensive care patients. Changes in the tonus of arteries contributing to vascular resistance play an important role in the determination of blood flow and thus oxygen supply of various abdominal organs. It is generally acknowledged that hypoxia itself is able to alter arterial tonus and thus blood flow.

Methods

The present study compared the effects of various degrees of hypoxia on second-order mesenteric arteries from male C57BL/6J mice. After vessel isolation and preparation, we assessed vessel diameter using an arteriograph perfusion chamber. Investigating mechanisms promoting hypoxia-induced vasodilatation, we performed experiments in Ca²⁺-containing and Ca²⁺-free solutions, and furthermore, Ca²⁺-influx was inhibited by NiCl₂, eNOS^−/−-, and TASK1^−/−-mice were investigated too.

Results

Mild hypoxia 14.4% O₂ induced, in 50% of mesenteric artery segments from wild-type (wt) mice, a vasodilatation; severe hypoxia recruited further segments responding with vasodilatation reaching 80% under anoxia. However, the extension of dilatation of luminal arterial diameter reduced from 1.96% ± 0.55 at 14.4% O₂ to 0.68% ± 0.13 under anoxia. Arteries exposed to hypoxia in Ca²⁺-free solution responded to lower oxygen levels with increasing degree of vasodilatation (0.85% ± 0.19 at 14.4% O₂ vs. 1.53% ± 0.42 at 2.7% O₂). Inhibition of voltage-gated Ca²⁺-influx using NiCl₂ completely diminished hypoxia-induced vasodilatation. Instead, all arterial segments investigated constricted. Furthermore, we did not observe altered hypoxia-induced vasomotion in eNOS^−/−- or TASK1^−/− mice compared to wt animals.

Conclusions

The present study demonstrated that hypoxic vasodilatation in mice mesenteric arteries is mediated by a NO-independent mechanism. In this experimental setting, we found evidence for Ca²⁺-mediated activation of ion channels causing hypoxic vasodilatation.

Electronic supplementary material

The online version of this article (10.1186/s40001-018-0335-8) contains supplementary material, which is available to authorized users.

Dexmedetomidine attenuates hypoxia/reoxygenation injury in primary neonatal rat cardiomyocytes

Systemic administration of dexmedetomidine provides cardioprotection against ischemia/reperfusion (I/R) injury; however, the direct effects of dexmedetomidine on cardiomyocytes have not been clarified. The present study investigated the effects of dexmedetomidine on primary neonatal rat cardiomyocytes under hypoxic/reoxygenation (H/R) conditions. In order to simulate in vivo I/R injury, primary neonatal rat cardiomyocytes were cultured under hypoxic conditions for 1 h and subsequently reoxygenated for 24 h. The effects of preconditioning with dexmedetomidine 2 h before hypoxia and postconditioning during reoxygenation were also examined. Cellular viability and activity were analyzed by monitoring the dynamic response profile of living cells using a real-time cell analyzer system. A special scaled index, defined as the normalized cell index (NCI), was used to minimize the influence of inter-experimental variations. The dose-effect curve was generated from the area under the time-course curve values of NCI. H/R exposure markedly decreased cell viability and activity. Furthermore, no cytotoxicity was associated with a clinically relevant concentration of dexmedetomidine. Preconditioning with dexmedetomidine concentration-dependently ameliorated the reductions in NCI in cardiomyocytes following H/R injury. Additionally, postconditioning with dexmedetomidine improved the reductions in NCI at concentrations between 3 and 200 nM. Finally, the effect of 3–40 nM dexmedetomidine postconditioning was greater than preconditioning. These results indicated that preconditioning and postconditioning with dexmedetomidine attenuated H/R injury in primary neonatal rat cardiomyocytes at the cellular level.

Dexmedetomidine Protects Cardiomyocytes against Hypoxia/Reoxygenation Injury by Suppressing TLR4-MyD88-NF-κB Signaling

OBJECTIVE

We previously reported that dexmedetomidine (DEX) offers cardioprotection against ischemia/reperfusion injury in rats. Here, we evaluated the role of toll-like receptors 4- (TLR4-) myeloid differentiation primary response 88- (MyD88-) nuclear factor-kappa B (NF-κB) signaling in DEX-mediated protection of cardiomyocytes using in vitro models of hypoxia/reoxygenation (H/R).

METHODS

The experiments were carried out in H9C2 cells and in primary neonatal rat cardiomyocytes. Cells pretreated with vehicle or DEX were exposed to hypoxia for 1 h followed by reoxygenation for 12 h. We analyzed cell viability and lactate dehydrogenase (LDH) activity and measured tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and IL-1β mRNA levels, TLR4, MyD88, and nuclear NF-κB p65 protein expression and NF-κB p65 nuclear localization. TLR4 knock-down by TLR4 siRNA transfection and overexpression by TLR4 DNA transfection were used to further confirm our findings.

RESULTS

DEX protected against H/R-induced cell damage and inflammation, as evidenced by increased cell survival rates, decreased LDH activity, and decreased TNF-α, IL-6, and IL-1β mRNA levels, as well as TLR4 and NF-κB protein expression. TLR4 knock-down partially prevented cell damage following H/R injury, while overexpression of TLR4 abolished the DEX-mediated protective effects.

CONCLUSIONS

DEX pretreatment protects rat cardiomyocytes against H/R injury. This effect is partly mediated by TLR4 suppression via TLR4-MyD88-NF-κB signaling.

Kinetic and structural studies of aldehyde oxidoreductase from Desulfovibrio gigas reveal a dithiolene-based chemistry for enzyme activation and inhibition by H(2)O(2)

Mononuclear Mo-containing enzymes of the xanthine oxidase (XO) family catalyze the oxidative hydroxylation of aldehydes and heterocyclic compounds. The molybdenum active site shows a distorted square-pyramidal geometry in which two ligands, a hydroxyl/water molecule (the catalytic labile site) and a sulfido ligand, have been shown to be essential for catalysis. The XO family member aldehyde oxidoreductase from Desulfovibrio gigas (DgAOR) is an exception as presents in its catalytically competent form an equatorial oxo ligand instead of the sulfido ligand. Despite this structural difference, inactive samples of DgAOR can be activated upon incubation with dithionite plus sulfide, a procedure similar to that used for activation of desulfo-XO. The fact that DgAOR does not need a sulfido ligand for catalysis indicates that the process leading to the activation of inactive DgAOR samples is different to that of desulfo-XO. We now report a combined kinetic and X-ray crystallographic study to unveil the enzyme modification responsible for the inactivation and the chemistry that occurs at the Mo site when DgAOR is activated. In contrast to XO, which is activated by resulfuration of the Mo site, DgAOR activation/inactivation is governed by the oxidation state of the dithiolene moiety of the pyranopterin cofactor, which demonstrates the non-innocent behavior of the pyranopterin in enzyme activity. We also showed that DgAOR incubation with dithionite plus sulfide in the presence of dioxygen produces hydrogen peroxide not associated with the enzyme activation. The peroxide molecule coordinates to molybdenum in a η² fashion inhibiting the enzyme activity.

Aging-dependent changes in rat heart mitochondrial glutaredoxins--Implications for redox regulation

Clinical and animal studies have documented that hearts of the elderly are more susceptible to ischemia/reperfusion damage compared to young adults. Recently we found that aging-dependent increase in susceptibility of cardiomyocytes to apoptosis was attributable to decrease in cytosolic glutaredoxin 1 (Grx1) and concomitant decrease in NF-κB-mediated expression of anti-apoptotic proteins. Besides primary localization in the cytosol, Grx1 also exists in the mitochondrial intermembrane space (IMS). In contrast, Grx2 is confined to the mitochondrial matrix. Here we report that Grx1 is decreased by 50–60% in the IMS, but Grx2 is increased by 1.4–2.6 fold in the matrix of heart mitochondria from elderly rats. Determination of in situ activities of the Grx isozymes from both subsarcolemmal (SSM) and interfibrillar (IFM) mitochondria revealed that Grx1 was fully active in the IMS. However, Grx2 was mostly in an inactive form in the matrix, consistent with reversible sequestration of the active-site cysteines of two Grx2 molecules in complex with an iron–sulfur cluster. Our quantitative evaluations of the active/inactive ratio for Grx2 suggest that levels of dimeric Grx2 complex with iron–sulfur clusters are increased in SSM and IFM in the hearts of elderly rats. We found that the inactive Grx2 can be fully reactivated by sodium dithionite or exogenous superoxide production mediated by xanthine oxidase. However, treatment with rotenone, which generates intramitochondrial superoxide through inhibition of mitochondrial respiratory chain Complex I, did not lead to Grx2 activation. These findings suggest that insufficient ROS accumulates in the vicinity of dimeric Grx2 to activate it in situ.

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Glutaredoxins play key roles in cellular redox regulation, which is sensitive to aging-dependent dysregulation.

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Grx1 is diminished in the intermembrane space of mitochondria from aged heart; matrix Grx2 is increased but mostly in an inactive form.

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The inactive Grx2 is selectively activated by superoxide.

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Mitochondrial glutaredoxin changes may contribute to dysregulation of redox homeostasis during aging.

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Changes in in situ activities of heart mitochondrial Grx1 and Grx2 with aging provide mechanistic insights for future studies.

Hypoxic pulmonary vasoconstriction in the absence of pretone: essential role for intracellular Ca2+ release

  Hypoxic pulmonary vasoconstriction (HPV) maintains blood oxygenation during acute hypoxia but contributes to pulmonary hypertension during chronic hypoxia. The mechanisms of HPV remain controversial, in part because HPV is usually studied in the presence of agonist-induced preconstriction ('pretone'). This potentiates HPV but may obscure and distort its underlying mechanisms. We therefore carried out an extensive assessment of proposed mechanisms contributing to HPV in isolated intrapulmonary arteries (IPAs) in the absence of pretone by using a conventional small vessel myograph. Hypoxia elicited a biphasic constriction consisting of a small transient (phase 1) superimposed upon a sustained (phase 2) component. Neither phase was affected by the L-type Ca2+ channel antagonists diltiazem (10 and 30 μm) or nifedipine (3 μm). Application of the store-operated Ca2+ entry (SOCE) blockers BTP2 (10 μm) or SKF96365 (50 μm) attenuated phase 2 but not phase 1, whereas a lengthy (30 min) incubation in Ca2+-free physiological saline solution similarly reduced phase 2 but abolished phase 1. No further effect of inhibition of HPV was observed if the sarco/endoplasmic reticulum Ca2+-ATPase inhibitor cyclopiazonic acid (30 μm) was also applied during the 30 min incubation in Ca2+-free physiological saline solution. Pretreatment with 10 μm ryanodine and 15 mm caffeine abolished both phases, whereas treatment with 100 μm ryanodine attenuated both phases. The two-pore channel blocker NED-19 (1 μm) and the nicotinic acid adenine dinucleotide phosphate (NAADP) antagonist BZ194 (200 μm) had no effect on either phase of HPV. The lysosomal Ca2+-depleting agent concanamycin (1 μm) enhanced HPV if applied during hypoxia, but had no effect on HPV during a subsequent hypoxic challenge. The cyclic ADP ribose antagonist 8-bromo-cyclic ADP ribose (30 μm) had no effect on either phase of HPV. Neither the Ca2+-sensing receptor (CaSR) blocker NPS2390 (0.1 and 10 μm) nor FK506 (10 μm), a drug which displaces FKBP12.6 from ryanodine receptor 2 (RyR2), had any effect on HPV. HPV was virtually abolished by the rho kinase blocker Y-27632 (1 μm) and attenuated by the protein kinase C inhibitor Gö6983 (3 μm). Hypoxia for 45 min caused a significant increase in the ratio of oxidised to reduced glutathione (GSSG/GSH). HPV was unaffected by the NADPH oxidase inhibitor VAS2870 (10 μm), whereas phase 2 was inhibited but phase 1 was unaffected by the antioxidants ebselen (100 μm) and TEMPOL (3 mm). We conclude that both phases of HPV in this model are mainly dependent on [Ca2+]i release from the sarcoplasmic reticulum. Neither phase of HPV requires voltage-gated Ca2+ entry, but SOCE contributes to phase 2. We can detect no requirement for cyclic ADP ribose, NAADP-dependent lysosomal Ca2+ release, activation of the CaSR, or displacement of FKBP12.6 from RyR2 for either phase of HPV. Sustained HPV is associated with an oxidising shift in the GSSG/GSH redox potential and is inhibited by the antioxidants ebselen and TEMPOL, consistent with the concept that it requires an oxidising shift in the cell redox state or the generation of reactive oxygen species.

Hypoxic pulmonary vasoconstriction

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

Donepezil attenuated oxygen-glucose deprivation insult by blocking Kv2.1 potassium channels

Excessive K(+) efflux via activated voltage-gated K(+) channels (Kv channels) and the consequent intracellular K(+) depletion during and after ischemia/hypoxia induced long-lasting membrane depolarization promotes neuronal apoptosis. Although it has been suggested as an important potassium channel subtype in oxidative stress induced neuron apoptosis, whether Kv2.1 mediates ischemic apoptosis remains undefined. In the present study, the role of Kv2.1 played in hypoxia/anoxia induced cell apoptosis and correlated protective effect of donepezil were evaluated. Kv2.1 transfected HEK293 cell line (Kv2.1/HEK293) was used to study oxygen-glucose deprivation (OGD) induced cell apoptosis. We found Kv2.1 transfection increased the vulnerability of HEK293 cells to OGD insult, blocking Kv2.1 potassium channel by tetraethylammonium (TEA, 10mM) could attenuated OGD induced Kv2.1/HEK293 cell apoptosis significantly. OGD slightly reduced Kv2.1 currents without affecting channel kinetic activity. However, the membrane potential of Kv2.1/HEK293 cells depolarized to around 0mV after OGD treatment, a potential which could activated Kv2.1 persistently. Donepezil blocked Kv2.1 currents in a dose-dependent manner (IC(50)=7.59μM). Under OGD condition, donepezil (30μM) effectively inhibited Kv2.1 currents by accelerating channel inactivation and decreased Kv2.1/HEK293 cell apoptosis rate. In conclusion, our study revealed both the conducting role of Kv2.1 in OGD induced cell apoptosis and the importance of Kv2.1 as a target for neuronal protection. In addition, besides anti-acetylcholinesterase activity, Kv2.1 blockade capability of donepezil may attribute to its neuroprotective effects against ischemic apoptosis.

The role of redox changes in oxygen sensing

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

The role of ion channels in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is an important mechanism by which localized flow of blood in small resistance pulmonary arteries is matched to alveolar ventilation. This chapter discusses the role of several potassium and calcium channels in HPV, both in enhancing calcium influx into smooth muscle cells (SMCs) and in stimulating the release of calcium from the sarcoplasmic reticulum, thus increasing cytosolic calcium. The increase in calcium sensitivity caused by hypoxia is reviewed in Chapter 19. Particular attention is paid to the activity of the L-type calcium channels which increase calcium influx as a result of membrane depolarization and also increase calcium influx at any given membrane potential in response to hypoxia. In addition, activation of the L-type calcium channel may, in the absence of any calcium influx, cause calcium release from the sarcoplasmic reticulum. Many of these mechanisms have been reported to be involved in both HPV and in normoxic contraction of the ductus arteriosus.

Mitochondrial translocation of alpha-synuclein is promoted by intracellular acidification

Mitochondrial dysfunction plays a central role in the selective vulnerability of dopaminergic neurons in Parkinson's disease (PD) and is influenced by both environmental and genetic factors. Expression of the PD protein alpha-synuclein or its familial mutants often sensitizes neurons to oxidative stress and to damage by mitochondrial toxins. This effect is thought to be indirect, since little evidence physically linking alpha-synuclein to mitochondria has been reported. Here, we show that the distribution of alpha-synuclein within neuronal and non-neuronal cells is dependent on intracellular pH. Cytosolic acidification induces translocation of alpha-synuclein from the cytosol onto the surface of mitochondria. Translocation occurs rapidly under artificially-induced low pH conditions and as a result of pH changes during oxidative or metabolic stress. Binding is likely facilitated by low pH-induced exposure of the mitochondria-specific lipid cardiolipin. These results imply a direct role for alpha-synuclein in mitochondrial physiology, especially under pathological conditions, and in principle, link alpha-synuclein to other PD genes in regulating mitochondrial homeostasis.

Mitochondrial metabolism, redox signaling, and fusion: a mitochondria-ROS-HIF-1alpha-Kv1.5 O2-sensing pathway at the intersection of pulmonary hypertension and cancer

Pulmonary arterial hypertension (PAH) is a lethal syndrome characterized by vascular obstruction and right ventricular failure. Although the fundamental cause remains elusive, many predisposing and disease-modifying abnormalities occur, including endothelial injury/dysfunction, bone morphogenetic protein receptor-2 gene mutations, decreased expression of the O(2)-sensitive K(+) channel (Kv1.5), transcription factor activation [hypoxia-inducible factor-1alpha (HIF-1alpha) and nuclear factor-activating T cells], de novo expression of survivin, and increased expression/activity of both serotonin transporters and platelet-derived growth factor receptors. Together, these abnormalities create a cancerlike, proliferative, apoptosis-resistant phenotype in pulmonary artery smooth muscle cells (PASMCs). A possible unifying mechanism for PAH comes from studies of fawn-hooded rats, which manifest spontaneous PAH and impaired O(2) sensing. PASMC mitochondria normally produce reactive O(2) species (ROS) in proportion to P(O2). Superoxide dismutase 2 (SOD2) converts intramitochondrial superoxide to diffusible H(2)O(2), which serves as a redox-signaling molecule, regulating pulmonary vascular tone and structure through effects on Kv1.5 and transcription factors. O(2) sensing is mediated by this mitochondria-ROS-HIF-1alpha-Kv1.5 pathway. In PAH and cancer, mitochondrial metabolism and redox signaling are reversibly disordered, creating a pseudohypoxic redox state characterized by normoxic decreases in ROS, a shift from oxidative to glycolytic metabolism and HIF-1alpha activation. Three newly recognized mitochondrial abnormalities disrupt the mitochondria-ROS-HIF-1alpha-Kv1.5 pathway: 1) mitochondrial pyruvate dehydrogenase kinase activation, 2) SOD2 deficiency, and 3) fragmentation and/or hyperpolarization of the mitochondrial reticulum. The pyruvate dehydrogenase kinase inhibitor, dichloroacetate, corrects the mitochondrial abnormalities in experimental models of PAH and human cancer, causing a regression of both diseases. Mitochondrial abnormalities that disturb the ROS-HIF-1alpha-Kv1.5 O(2)-sensing pathway contribute to the pathogenesis of PAH and cancer and constitute promising therapeutic targets.

Effects of hypoxia, anoxia, and metabolic inhibitors on KATP channels in rat femoral artery myocytes

Vascular ATP-sensitive potassium (KATP) channels have an important role in hypoxic vasodilation. Because KATP channel activity depends on intracellular nucleotide concentration, one hypothesis is that hypoxia activates channels by reducing cellular ATP production. However, this has not been rigorously tested. In this study we measured KATP current in response to hypoxia and modulators of cellular metabolism in single smooth muscle cells from the rat femoral artery by using the whole cell patch-clamp technique. KATP current was not activated by exposure of cells to hypoxic solutions (Po2 approximately 35 mmHg). In contrast, voltage-dependent calcium current and the depolarization-induced rise in intracellular calcium concentration ([Ca2+]i) was inhibited by hypoxia. Blocking mitochondrial ATP production by using the ATP synthase inhibitor oligomycin B (3 microM) did not activate current. Blocking glycolytic ATP production by using 2-deoxy-D-glucose (5 mM) also did not activate current. The protonophore carbonyl cyanide m-chlorophenylhydrazone (1 microM) depolarized the mitochondrial membrane potential and activated KATP current. This activation was reversed by oligomycin B, suggesting it occurred as a consequence of mitochondrial ATP consumption by ATP synthase working in reverse mode. Finally, anoxia induced by dithionite (0.5 mM) also depolarized the mitochondrial membrane potential and activated KATP current. Our data show that: 1) anoxia but not hypoxia activates KATP current in femoral artery myocytes; and 2) inhibition of cellular energy production is insufficient to activate KATP current and that energy consumption is required for current activation. These results suggest that vascular KATP channels are not activated during hypoxia via changes in cell metabolism. Furthermore, part of the relaxant effect of hypoxia on rat femoral artery may be mediated by changes in [Ca2+]i through modulation of calcium channel activity.

Hypoxic pulmonary vasoconstriction

Humans encounter hypoxia throughout their lives. This occurs by destiny in utero, through disease, and by desire, in our quest for altitude. Hypoxic pulmonary vasoconstriction (HPV) is a widely conserved, homeostatic, vasomotor response of resistance pulmonary arteries to alveolar hypoxia. HPV mediates ventilation-perfusion matching and, by reducing shunt fraction, optimizes systemic Po(2). HPV is intrinsic to the lung, and, although modulated by the endothelium, the core mechanism is in the smooth muscle cell (SMC). The Redox Theory for the mechanism of HPV proposes the coordinated action of a redox sensor (the proximal mitochondrial electron transport chain) that generates a diffusible mediator [a reactive O(2) species (ROS)] that regulates an effector protein [voltage-gated potassium (K(v)) and calcium channels]. A similar mechanism for regulating O(2) uptake/distribution is partially recapitulated in simpler organisms and in the other specialized mammalian O(2)-sensitive tissues, including the carotid body and ductus arteriosus. Inhibition of O(2)-sensitive K(v) channels, particularly K(v)1.5 and K(v)2.1, depolarizes pulmonary artery SMCs, activating voltage-gated Ca(2+) channels and causing Ca(2+) influx and vasoconstriction. Downstream of this pathway, there is important regulation of the contractile apparatus' sensitivity to calcium by rho kinase. Controversy remains as to whether hypoxia decreases or increases ROS and which electron transport chain complex generates the ROS (I and/or III). Possible roles for cyclic adenosine diphosphate ribose and an unidentified endothelial constricting factor are also proposed by some groups. Modulation of HPV has therapeutic relevance to cor pulmonale, high-altitude pulmonary edema, and sleep apnea. HPV is clinically exploited in single-lung anesthesia, and its mechanisms intersect with those of pulmonary arterial hypertension.

Behavior of junction channels between rat glomus cells during normoxia and hypoxia

The activity of gap junction channels between cultured and clustered carotid body glomus cells of the rat was studied with dual voltage clamping during normoxia (PO(2) 300 Torr) and hypoxia induced by sodium dithionite (Na(2)S(2)O(4)) or 100% N(2). Na(2)S(2)O(4) reduced the saline PO(2) to approximately 10 Torr, whereas 100% N(2) reduced ambient O(2) to approximately 60 Torr. The following observations were made. 1) In normoxia, the intercellular macroconductance (G(j) = 3.0 +/- 1.01 ns, mean +/- SE) was changed unevenly (increased and decreased) under hypoxic conditions by either agent, although N(2) produced the largest changes. 2) The intercellular microconductances of the channels (g(j) = 104.44 +/- 10.16 pS under normoxic conditions) significantly decreased in 100% N(2) but showed depressions and enhancements in Na(2)S(2)O(4). 3) The conductance of single-junction channels (SChs), calculated as g(j) variance/mean g(j), yielded a mean of approximately 17.6 pS. Larger values were obtained with manual measurements of the data (approximately 34 pS). Hypoxic hypoxia (induced by 100% N(2)) significantly depressed the conductance of SChs when calculated from digitized records or from manual measurements. Hypoxia induced by Na(2)S(2)O(4) did not significantly change junctional conductance. 4) The number of intercellular channels, calculated as g(j)/SCh g(j), had a mean of approximately 452 (range 1 to 2,471). During N(2)-induced hypoxia, this number significantly decreased to approximately 84 but remained unchanged during Na(2)S(2)O(4) hypoxia. 5) The mean open time of junction channels varied from 4 to 30 ms in different experiments, having an overall mean of mu = 11.33 +/- 0.33 ms. This value was significantly reduced by 100% N(2) but was not changed by Na(2)S(2)O(4). 6) Intracellular calcium ([Ca(2+)](i)), 46.2 +/- 4.84 nM under normoxia, significantly increased to 77.32 +/- 11.27 nM with Na(2)S(2)O(4) and to 66.39 +/- 11.64 nM with 100% N(2). It is concluded that 100% N(2) uncouples glomus cells by significantly reducing intercellular macro- and microconductances. Hypoxia induced by Na(2)S(2)O(4) had variable effects. The coupling effects of hypoxia may depend on, or be aided by, increases in [Ca(2+)](i) and/or intracellular pH changes. However, secreted transmitters and ATP plus the effects of hypoxia on second messengers and other cytoplasmic components may also play an important role in this phenomenon.

LTRPC2 Ca2+-permeable channel activated by changes in redox status confers susceptibility to cell death

Redox status changes exert critical impacts on necrotic/apoptotic and normal cellular processes. We report here a widely expressed Ca2+-permeable cation channel, LTRPC2, activated by micromolar levels of H2O2 and agents that produce reactive oxygen/nitrogen species. This sensitivity of LTRPC2 to redox state modifiers was attributable to an agonistic binding of nicotinamide adenine dinucleotide (beta-NAD+) to the MutT motif. Arachidonic acid and Ca2+ were important positive regulators for LTRPC2. Heterologous LTRPC2 expression conferred susceptibility to death on HEK cells. Antisense oligonucleotide experiments revealed physiological involvement of "native" LTRPC2 in H2O2- and TNFalpha-induced Ca2+ influx and cell death. Thus, LTRPC2 represents an important intrinsic mechanism that mediates Ca2+ and Na+ overload in response to disturbance of redox state in cell death.

Hydrogen peroxide--an intracellular signal in the pulmonary circulation: involvement in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is a regulatory feature of the pulmonary circulation that ensures consistent matching of perfusion to ventilation in the normal lung. However, under pathophysiological conditions, HPV contributes to the elevated pulmonary arterial pressure inherent to numerous disease states. Consequently, control of HPV is an avenue of potential therapy for such conditions. This review discusses the role of hydrogen peroxide (H(2)O(2)) as an intracellular signal in the pulmonary circulation, concentrating on the potential involvement of H(2)O(2) in HPV and in the control of pulmonary arterial tone. Sites of hypoxic pulmonary arterial H(2)O(2) production include the mitochondrial electron transport chain, a microsomal electron transport chain containing an NADH oxidoreductase and alternatively, a membrane-bound NADPH oxidase. Each of these sources of H(2)O(2) and the effect of hypoxia on the production of reactive oxygen species are considered. The review also discusses the variance in vascular reactivity of H(2)O(2), which is described to elicit both pulmonary arterial vasoconstriction and dilatation at varying concentrations. The redox capabilities of H(2)O(2) are also considered. The relevance of all of these actions of H(2)O(2) are also assessed as potential pharmacological targets for the future development of therapy for lung diseases that are characterised by some degree of HPV and in the pathogenesis of pulmonary diseases in which reactive oxygen species are implicated.

Regulation of Shaker-type potassium channels by hypoxia. Oxygen-sensitive K+ channels in PC12 cells

Little is known about the molecular composition of the O2-sensitive K+ (Ko2) channels. The possibility that these channels belong to the Shaker subfamily (Kv1) of voltage-dependent K+ (Kv) channels has been raised in pulmonary artery (PA) smooth muscle cells. Numerous findings suggest that the Ko2 channel in PC12 cells is a Kv1 channel, formed by the Kv1.2 alpha subunit. The Ko2 channel in PC12 cells is a slow-inactivating voltage-dependent K+ channel of 20 pS conductance. Other Kv channels, also expressed in PC12 cells, are not inhibited by hypoxia. Selective up-regulation by chronic hypoxia of the Kv1.2 alpha subunit expression correlates with an increase O2-sensitivity of the K+ current. Other Kv1 alpha subunit genes encoding slow-inactivating Kv channels, such as Kv1.3, Kv2.1, Kv3.1 and Kv3.2 are not modulated by chronic hypoxia. The Ko2 current in PC12 cells is blocked by 5 mM externally applied tetraethylammonium chloride (TEA) and by charydbotoxin (CTX). The responses of the Kv1.2 K+ channel to hypoxia have been studied in the Xenopus oocytes and compared to those of Kv2.1, also proposed as Ko2 channel in PA smooth muscle cells. Two-electrode voltage clamp experiments show that hypoxia induces inhibition of K+ current amplitude only in oocytes injected with Kv1.2 cRNA. These data indicate that Kv1.2 K+ channels are inhibited by hypoxia.

NADPH oxidase: a universal oxygen sensor?

NADPH oxidase is classically regarded as a key enzyme of neutrophils, where it is involved in the pathogenic production of reactive oxygen species. However, NADPH oxidase-like enzymes have recently been identified in non-neutrophil cells, supporting a separate role for NADPH-oxidase derived oxygen species in oxygen sensitive processes. This article reviews the current literature surrounding the potential role of NADPH oxidase in the oxygen sensing processes which underlie hypoxic pulmonary vasoconstriction, systemic vascular smooth muscle proliferation, carotid and airways chemoreceptor activation, erythropoietin gene expression, and oxytropic responses of plant cells.

O2-sensitive K+ channels: role of the Kv1.2 -subunit in mediating the hypoxic response

One of the early events in O2 chemoreception is inhibition of O2-sensitive K+ (KO2) channels. Characterization of the molecular composition of the native KO2 channels in chemosensitive cells is important to understand the mechanism(s) that couple O2 to the KO2 channels. The rat phaeochromocytoma PC12 clonal cell line expresses an O2-sensitive voltage-dependent K+ channel similar to that recorded in other chemosensitive cells. Here we examine the possibility that the Kv1.2 alpha-subunit comprises the KO2 channel in PC12 cells. Whole-cell voltage-clamp experiments showed that the KO2 current in PC12 cells is inhibited by charybdotoxin, a blocker of Kv1.2 channels. PC12 cells express the Kv1.2 alpha-subunit of K+ channels: Western blot analysis with affinity-purified anti-Kv1.2 antibody revealed a band at approximately 80 kDa. Specificity of this antibody was established in Western blot and immunohystochemical studies. Anti-Kv1.2 antibody selectively blocked Kv1.2 current expressed in the Xenopus oocyte, but had no effect on Kv2.1 current. Anti-Kv1.2 antibody dialysed through the patch pipette completely blocked the KO2 current, while the anti-Kv2.1 and irrelevant antibodies had no effect. The O2 sensitivity of recombinant Kv1.2 and Kv2.1 channels was studied in Xenopus oocytes. Hypoxia inhibited the Kv1.2 current only. These findings show that the KO2 channel in PC12 cells belongs to the Kv1 subfamily of K+ channels and that the Kv1.2 alpha-subunit is important in conferring O2 sensitivity to this channel.

Effects of hypoxia and dithionite on catecholamine release from isolated type I cells of the rat carotid body

1. Amperometric recordings were conducted to investigate the ability of hypoxia and anoxia to evoke quantal catecholamine secretion from isolated type I cells of the rat carotid body. 2. Hypoxia (PO2 8-14 mmHg) consistently failed to evoke catecholamine secretion from type I cells, when cells were perfused either at room temperature (21-24 C) or at 35-37 C, and regardless of whether Hepes- or HCO3-/CO2-buffered solutions were used. 3. Elevating extracellular [K+] caused concentration-dependent secretion from individual type I cells, with a threshold concentration of approximately 25 mM. In the presence of this level of extracellular K+, hypoxia (PO2 8-14 mmHg) caused a marked enhancement of secretion which was fully blocked by 200 microM Cd2+, a non-specific blocker of voltage-gated Ca2+ channels. 4. Anoxia (N2-equilibrated solution containing 0.5 mM dithionite) evoked exocytosis from type I cells when extracellular [K+] was 5 mM. This secretion was completely inhibited by removal of extracellular Ca2+, but was not significantly affected by Cd2+ (200 microM), Ni2+ (2 mM), Zn2+ (1 mM) or nifedipine (2 microM). Secretion was also observed when 0.5 mM dithionite was added to air-equilibrated solutions. 5. Anoxia also evoked secretion from chemoreceptive phaeochromocytoma (PC12) cells, which was wholly Ca2+ dependent, but unaffected by Cd2+ (200 microM). 6. Our results suggest that hypoxia can evoke catecholamine secretion from isolated type I cells, but only in the presence of elevated extracellular [K+]. This may be due to the cells being relatively hyperpolarized following dissociation. In addition, we have shown that dithionite evokes catecholamine release regardless of PO2 levels, and this release is due mainly to an artefactual Ca2+ influx pathway activated in the presence of dithionite.

Hypoxia, metabolic inhibition, and isolated rat mesenteric tone: influence of arterial diameter

The present study examined and compared the effects of severe hypoxia (induced by either substitution of O(2) in the gassing medium with N(2) or by addition of the O(2) scavenger sodium dithionite) and metabolic inhibition (induced by addition of sodium cyanide) on the tone of isolated rat mesenteric resistance vessels. The influence of vessel diameter and the endothelium on responses to these maneuvers was investigated. Hypoxia (due to both substitution of O(2) with N(2) and by addition of 2 mM sodium dithionite) caused near maximal relaxation of all tissues studied. Addition of 10 mM dithionite, however, produced a significantly smaller response. Two mM cyanide also relaxed mesenteric arteries. In small vessels a near maximal relaxation to cyanide was observed, however, in larger vessels the relaxation to metabolic inhibition was significantly less than that observed to hypoxia. Increasing the concentration of cyanide had no further effect on responses. All responses were found to be endothelium-independent. Thus, as the effects of hypoxia and cyanide are not always similar, care must be taken when extrapolating the effects of metabolic inhibition to those of hypoxia. The results of this study suggest that, in large mesenteric vessels at least, an "O(2) sensing step," in addition to effects of metabolism, may be involved in the hypoxic response.

Anoxic decrease in potassium outward currents of hippocampal cultured neurons in absence and presence of dithionite

The effect of brief anoxia on voltage dependent K(+)-currents of hippocampal cultured neurons was studied. The oxygen scavenger dithionite (hydrosulphite) was previously used for creating zero oxygen pressure. However, dithionite consumes O(2) in parallel with generation of superoxide radicals and is a strongly reducing agent. In this study anoxia was produced by perfusion of the neurons with a solution bubbled with nitrogen for 1 h using a chamber with an argon layer isolating the anoxic bath flow from atmospheric oxygen in presence and absence of dithionite. Oxygen partial pressure of dithionite-free solution was determined by oxygen dependent quenching of the phosphorescence of Pd-coproporphyrin to be 0.15+/-0. 02 Torr (values are given as mean+/-S.D., n=6). Slow (I(K))- and fast (I(A))-inactivating K(+)-currents were measured with the patch clamp technique in the whole cell configuration. Exposure of the neurons to anoxia reversibly decreased the amplitude of I(K) at a test pulse of 0 mV to 77+/-12% (n=7) in absence and to 83+/-7% (n=6) in presence of 2 mM dithionite; the amplitude of I(A) decreased to 78+/-11% in absence and to 82+/-9% in presence of 2 mM dithionite. Voltage dependence of activation and inactivation shifted 5 min after exposure to anoxia reversibly by about 6 mV in depolarizing direction. The decay times of inactivation were insensitive to anoxia. Dithionite had no significant effects on K(+)-currents. In 15 of 21 neurons not employed for analysis on K(+)-currents, a reversible increase in holding current under dithionite was observed. In absence of dithionite in 4 of 19 neurons the holding current reversibly increased during anoxia. Although dithionite does not affect K(+)-currents, changes in holding current show that the dithionite may affect neurons independently of oxygen deprivation.

O2 sensing is preserved in mice lacking the gp91 phox subunit of NADPH oxidase

The rapid response to hypoxia in the pulmonary artery (PA), carotid body, and ductus arteriosus is partially mediated by O2-responsive K+ channels. K+ channels in PA smooth muscle cells (SMCs) are inhibited by hypoxia, causing membrane depolarization, increased cytosolic calcium, and hypoxic pulmonary vasoconstriction. We hypothesize that the K+ channels are not themselves "O2 sensors" but rather respond to the reduced redox state created by hypoxic inhibition of candidate O2 sensors (NADPH oxidase or the mitochondrial electron transport chain). Both pathways shuttle electrons from donors, down a redox gradient, to O2. Hypoxia inhibits these pathways, decreasing radical production and causing cytosolic accumulation of unused, reduced, freely diffusible electron donors. PASMC K+ channels are redox responsive, opening when oxidized and closing when reduced. Inhibitors of NADPH oxidase (diphenyleneiodonium) and mitochondrial complex 1 (rotenone) both inhibit PASMC whole-cell K+ current but lack the specificity to identify the O2-sensor pathway. We used mice lacking the gp91 subunit of NADPH oxidase [chronic granulomatous disease (CGD) mice] to assess the hypothesis that NADPH oxidase is a PA O2-sensor. In wild-type lungs, gp91 phox and p22 phox subunits are present (relative expression: macrophages > airways and veins > PASMCs). Deletion of gp91 phox did not alter p22 phox expression but severely inhibited activated O2 species production. Nonetheless, hypoxia caused identical inhibition of whole-cell K+ current (in PASMCs) and hypoxic pulmonary vasoconstriction (in isolated lungs) from CGD vs. wild-type mice. Rotenone vasoconstriction was preserved in CGD mice, consistent with a role for the mitochondrial electron transport chain in O2 sensing. NADPH oxidase, though a major source of lung radical production, is not the pulmonary vascular O2 sensor in mice.

Sodium hydrosulfite contractions of smooth muscle are calcium and myosin phosphorylation independent

In an effort to further understand the processes underlying hypoxic pulmonary vasoconstriction, we examined the mechanism by which sodium hydrosulfite (Na2S2O4), a potent reducing agent and oxygen scavenger, induces smooth muscle contraction. In rat pulmonary arterial strips, sodium hydrosulfite (10 mM) induced contractions that were 65.9 +/- 12.8% of the response to 60 mM KCl (n = 9 segments). Contractions were not inhibited by nisoldipine (5 microM) or by repeated stimulation with caffeine (10 mM), carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (10 microM), or cyclopiazonic acid (10 microM), all of which eliminated responses to contractile agonists. Maximum force generation after exposure to sodium hydrosulfite was 0.123 +/- 0.013 mN in the presence of 1.8 mM calcium and 0.127 +/- 0.015 mN in the absence of calcium. Sodium hydrosulfite contractions in pulmonary arterial segments were not due to the generation of H2O2 and occurred in the presence of chelerythrine (10 microM), which blocked phorbol ester contractions, and solution hyperoxygenation. Similar contractile responses were obtained in rat aortic and tracheal smooth muscles. Finally, contractions occurred in the complete absence of an increase in myosin light chain phosphorylation. Therefore sodium hydrosulfite-induced smooth muscle contraction is not specific to pulmonary arterial smooth muscle, is independent of calcium and myosin light chain phosphorylation, and is not mediated by either hypoxia or protein kinase C.

Chronic hypoxia reduces adenosine A2A receptor-mediated inhibition of calcium current in rat PC12 cells via downregulation of protein kinase A

1. Adenosine has been shown to decrease Ca2+ current (ICa) and attenuate the hypoxia-induced enhancement of intracellular free Ca2+ ([Ca2+]i) in oxygen-sensitive rat phaeochromocytoma (PC12) cells. These effects are mediated via the adenosine A2A receptor and protein kinase A (PKA). The current study was undertaken to determine the effects of adenosine on Ca2+ current and hypoxia-induced change in [Ca2+]i during chronic hypoxia. 2. Whole cell patch-clamp studies revealed that the effect of adenosine on ICa was significantly reduced when PC12 cells were exposed to hypoxia (10 % O2) for 24 and 48 h. 3. Ca2+ imaging studies using fura-2 revealed that the anoxia-induced increase in [Ca2+]i was significantly enhanced when PC12 cells were exposed to 10 % O2 for up to 48 h. In contrast, the inhibitory effects of adenosine on anoxia-induced elevation of [Ca2+]i was significantly blunted in PC12 cells exposed to hypoxia for 48 h. 4. Northern blot analysis revealed that mRNA for the A2A receptor, which is the only adenosine receptor subtype expressed in PC12 cells, was significantly upregulated by hypoxia. Radioligand binding analysis with [3H]CGS21680, a selective A2A receptor ligand, showed that the number of adenosine A2A receptor binding sites was similarly increased during exposure to 10% O2 for 48 h. 5. PKA enzyme activity was significantly inhibited when PC12 cells were exposed to 10% O2 for 24 and 48 h. However, we found that hypoxia failed to induce change in adenosine- and forskolin-stimulated adenylate cyclase enzyme activity. Chronic hypoxia also did not alter the immunoreactivity level of the G protein Gsalpha, an effector of the A2 signalling pathway. 6. Whole cell patch-clamp analysis showed that the effect of 8-bromo-cAMP, an activator of PKA, on ICa was significantly attenuated during 48 h exposure to 10% O2.7. We conclude therefore that the reduced effect of adenosine on ICa and [Ca2+]i in PC12 cells exposed to chronic hypoxia is due to hypoxia-induced downregulation of PKA. This mechanism may serve to reduce the negative feedback on ICa and [Ca2+]i by adenosine and therefore maintain enhanced membrane excitability of PC12 cells during long-term hypoxia.

Selective inhibition of a slow-inactivating voltage-dependent K+ channel in rat PC12 cells by hypoxia

1. Electrophysiological (single-channel patch clamp) and molecular biological experiments (reverse transcriptase-polymerase chain reaction) were performed to attempt to identify the O2-sensitive K+ channel in rat phaeochromocytoma (PC12) cells. 2. Four types of K+ channels were recorded in PC12 cells: a small-conductance K+ channel (14 pS), a calcium-activated K+ channel (KCa; 102 pS) and two K+ channels with similar conductance (20 pS). These last two channels differed in their time-dependent inactivation: one was a slow-inactivating channel, while the other belonged to the family of fast transient K+ channels. 3. The slow-inactivating 20 pS K+ channel was inhibited by hypoxia. Exposure to hypoxia produced a 50% reduction in channel activity (number of active channels in the patch x open probability). Hypoxia had no effect on the 20 pS transient K+ channels, whereas reduced O2 stimulated the KCa channels. 4. The genes encoding the alpha-subunits of slow-inactivating K+ channels for two members of the Shaker subfamily of K+ channels (Kv1.2 and Kv1.3) together with the Kv2.1, Kv3.1 and Kv3.2 channel genes were identified in PC12 cells. 5. The expression of the Shaker Kv1.2, but none of the other K+ channel genes, increased in cells exposed to prolonged hypoxia (18 h). The same cells were more responsive to a subsequent exposure to hypoxia (35% inhibition of K+ current measured in whole-cell voltage clamp) compared with the cells maintained in normoxia (19% inhibition). 6. These results indicate that the O2-sensitive K+ channel in PC12 cells is a 20 pS slow-inactivating K+ channel that is upregulated by hypoxia. This channel appears to belong to the Shaker subfamily of voltage-gated K+ channels.

Diversity of response in vascular smooth muscle cells to changes in oxygen tension

Hypoxia causes pulmonary vasoconstriction (HPV), but also dilation of systemic vessels and the ductus arteriosus. In the adult animal. HPV is initiated by inhibition of potassium current (IK) in the smooth muscle cells of small resistance arteries, which results in membrane depolarization and calcium entry through voltage-gated calcium channels. The oxygen-sensitive channels that initiate HPV are 4-aminopyridine (4-AP)-sensitive delayed rectifier channels (KDR), the most prominent of which has a conductance of 37 pS. In the fetus, hypoxia causes pulmonary vasoconstriction through inhibition of a calcium-sensitive potassium channel (KCa). In smooth muscle cells from the rabbit ductus arteriosus, which dilates in response to hypoxia, whole-cell potassium current is reversibly enhanced, rather than inhibited, by hypoxia. The principal oxygen-sensitive channel is inhibited by 4-AP and has a conductance of about 58 pS. There are morphological and electrophysiological differences between individual pulmonary artery smooth muscle cells, for example, in some cells IK is predominantly carried by KDR channels and in others by KCa channels. KDR cells are more common in the resistance pulmonary arteries and KCa in the conduit arteries. Responses of specific vessels (conduit, resistance; pulmonary, systemic, ductus) at different stages of development (fetal, neonatal and adult) to changes in oxygen tension may be determined by the distribution of a variety of ion channels in the smooth muscle cells.