Hypoxic pulmonary vasoconstriction: a radical view

Mitochondrial oxygen sensing of acute hypoxia in specialized cells - Is there a unifying mechanism?

Acclimation to acute hypoxia through cardiorespiratory responses is mediated by specialized cells in the carotid body and pulmonary vasculature to optimize systemic arterial oxygenation and thus oxygen supply to the tissues. Acute oxygen sensing by these cells triggers hyperventilation and hypoxic pulmonary vasoconstriction which limits pulmonary blood flow through areas of low alveolar oxygen content. Oxygen sensing of acute hypoxia by specialized cells thus is a fundamental pre-requisite for aerobic life and maintains systemic oxygen supply. However, the primary oxygen sensing mechanism and the question of a common mechanism in different specialized oxygen sensing cells remains unresolved. Recent studies unraveled basic oxygen sensing mechanisms involving the mitochondrial cytochrome c oxidase subunit 4 isoform 2 that is essential for the hypoxia-induced release of mitochondrial reactive oxygen species and subsequent acute hypoxic responses in both, the carotid body and pulmonary vasculature. This review compares basic mitochondrial oxygen sensing mechanisms in the pulmonary vasculature and the carotid body.

Effects of impaired microvascular flow regulation on metabolism-perfusion matching and organ function

Impaired tissue oxygen delivery is a major cause of organ damage and failure in critically ill patients, which can occur even when systemic parameters, including cardiac output and arterial hemoglobin saturation, are close to normal. This review addresses oxygen transport mechanisms at the microcirculatory scale, and how hypoxia may occur in spite of adequate convective oxygen supply. The structure of the microcirculation is intrinsically heterogeneous, with wide variations in vessel diameters and flow pathway lengths, and consequently also in blood flow rates and oxygen levels. The dynamic processes of structural adaptation and flow regulation continually adjust microvessel diameters to compensate for heterogeneity, redistributing flow according to metabolic needs to ensure adequate tissue oxygenation. A key role in flow regulation is played by conducted responses, which are generated and propagated by endothelial cells and signal upstream arterioles to dilate in response to local hypoxia. Several pathophysiological conditions can impair local flow regulation, causing hypoxia and tissue damage leading to organ failure. Therapeutic measures targeted to systemic parameters may not address or may even worsen tissue oxygenation at the microvascular level. Restoration of tissue oxygenation in critically ill patients may depend on restoration of endothelial cell function, including conducted responses.

The Interaction between Trolox and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic Acid on Hypoxic Pulmonary Vasoconstriction in the Isolated Rabbit Lung

BACKGROUND

The mechanism of hypoxic pulmonary vasoconstriction (HPV) is still debatable. It has been proposed that reactive oxygen species (ROS) might be involved in HPV. However, there is no special transporter for superoxide anion in the cell membrane and it may release from the cells via anion exchanger. Therefore, the aim of this study was to investigate the interaction of ROS and anion exchanger in acute HPV.

METHODS

The present study was performed in the isolated rabbit lung. After preparation, the lungs were divided into four hypoxic groups of control, Trolox (antioxidant)-treated, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, anion exchanger inhibitor)-treated, and Trolox+DIDS-treated. Pulmonary artery pressure, left atrial pressure, and lung weight were continuously registered and PVR was then calculated. PO₂, PCO₂, HCO₃^-, pH, and NO metabolites of the perfusate were measured during steady-state and at the end of experiments (30 minutes). All data were compared with ANOVA and t-test and significance was considered when P<0.05.

RESULTS

Ventilation of the lungs with hypoxic gas induced HPV in the control group. DIDS did not have a further effect on HPV compared with the control group. The combination of Trolox and DIDS decreased HPV rather than Trolox per se at 5 minutes. Furthermore, HPV was abolished in both the Trolox and Trolox+DIDS groups at 30 minutes. Concentrations of NO metabolites in the Trolox+DIDS group were more than other groups.

CONCLUSION

The present study indicates a possible interaction between ROS and anion exchanger in acute HPV. It also suggests the modulatory effect of NO at above condition.

Oxygen sensing and signal transduction in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV), also known as the von Euler-Liljestrand mechanism, is an essential response of the pulmonary vasculature to acute and sustained alveolar hypoxia. During local alveolar hypoxia, HPV matches perfusion to ventilation to maintain optimal arterial oxygenation. In contrast, during global alveolar hypoxia, HPV leads to pulmonary hypertension. The oxygen sensing and signal transduction machinery is located in the pulmonary arterial smooth muscle cells (PASMCs) of the pre-capillary vessels, albeit the physiological response may be modulated in vivo by the endothelium. While factors such as nitric oxide modulate HPV, reactive oxygen species (ROS) have been suggested to act as essential mediators in HPV. ROS may originate from mitochondria and/or NADPH oxidases but the exact oxygen sensing mechanisms, as well as the question of whether increased or decreased ROS cause HPV, are under debate. ROS may induce intracellular calcium increase and subsequent contraction of PASMCs via direct or indirect interactions with protein kinases, phospholipases, sarcoplasmic calcium channels, transient receptor potential channels, voltage-dependent potassium channels and L-type calcium channels, whose relevance may vary under different experimental conditions. Successful identification of factors regulating HPV may allow development of novel therapeutic approaches for conditions of disturbed HPV.

Effects of Tapinanthus globiferus and Zanthoxylum zanthoxyloides extracts on human leukocytes in vitro

Objective:

This study aimed at investigating the genotoxicity and cytotoxicity effect of Tapinanthus globiferus and Zanthoxylum zanthoxyloides to human leukocytes. In addition, the reductive potential and the chemical composition of the two plant extracts were also determined.

Materials and Methods:

Human leukocytes were obtained from healthy volunteer donors. The genotoxicity and cytotoxicity of T. globiferus and Z. zanthoxyloides were assessed using the comet assay and trypan blue exclusion, respectively. The antioxidant activity of the plant extracts was evaluated by the reducing power assay. Furthermore, high-performance liquid chromatography-diode array detector was used to characterize and quantify the constituents of these plants.

Results:

T. globiferus (10-150 µg/mL) was neither genotoxic nor cytotoxic at the concentrations tested, suggesting that it can be consumed safely at relatively high concentrations. However, Z. zanthoxyloides showed cytoxicity and genotoxicity to human leukocytes at the highest concentration tested (150 µg/mL). In addition, the total reducing power of T. globiferus was found higher than Z. zanthoxyloides in potassium ferricyanide reduction. Both plants extract contained flavonoids (rutin and quercetin) and phenolic acids (chlorogenic and caffeic).

Conclusion:

The results obtained support the fact that some caution should be paid regarding the dosage and the frequency of use of Z. zanthoxyloides extract.

Antioxidants for the treatment of patients with severe angioproliferative pulmonary hypertension?

SIGNIFICANCE

Pathobiological mechanisms that contribute to pulmonary vasoconstriction, lung vascular remodeling, and the development of right heart failure include the generation of reactive oxygen and nitrogen species and the response of lung vascular and cardiac cells to these molecules. We review the information regarding oxidant stress balanced by antioxidant mechanisms and the role of oxidants and antioxidants in hypoxic pulmonary hypertension and their potential role in an animal model of severe pulmonary arterial hypertension (PAH).

RECENT ADVANCES

In human lung tissue from patients with idiopathic PAH, we find reduced superoxide dismutase activity and high expression of the oxidant stress markers nitrotyrosine and 8-OH-guanosine. In the Sugen 5416/chronic hypoxia model of PAH, lung tissue expression of nitrotyrosine and hemeoxygenase 1 (HO-1) is substantial, while HO-1 expression in the failing right ventricle is decreased. This model, based on administration of the VEGF receptor blocker Sugen 5416 and chronic hypoxia (Su/Hx), reproduces many of the characteristic features of severe angioobliterative human PAH. Treatment of Su/Hx rats with protandim, which nuclear factor erythroid-2 related factor (Nrf2)-dependently upregulates the expression of genes encoding antioxidant enzymes, protects against right heart failure without affecting angioobliterative PAH.

CRITICAL ISSUES

In human severe PAH, patient survival is determined by the function of the stressed right ventricle; investigation of oxidative and nitrosative stresses and their potential contribution to right heart failure is necessary.

FUTURE DIRECTIONS

Antioxidant therapeutic strategies may be of benefit in the setting of human severe PAH. Whether antioxidant strategies affect lung vascular remodeling and/or prevent right heart failure remains to be examined.

High-altitude pulmonary hypertension in cattle (brisket disease): Candidate genes and gene expression profiling of peripheral blood mononuclear cells

High-altitude pulmonary hypertension (HAPH) is a consequence of chronic alveolar hypoxia, leading to hypoxic vasoconstriction and remodeling of the pulmonary circulation. Brisket disease in cattle is a naturally occurring animal model of hypoxic pulmonary hypertension. Genetically susceptible cattle develop severe pulmonary hypertension and right heart failure at altitudes >7,000 ft. No information currently exists regarding the identity of the pathways and gene(s) responsible for HAPH or influencing severity. We hypothesized that initial insights into the pathogenesis of the disease could be discovered by a strategy of (1) sequencing of functional candidates revealed by single nucleotide polymorphism (SNP) analysis and (2) gene expression profiling of affected cattle compared with altitude-matched normal controls, with gene set enrichment analysis (GSEA) and Ingenuity pathway analysis (IPA). We isolated blood from a single herd of Black Angus cattle of both genders, aged 12-18 months, by jugular vein puncture. Mean pulmonary arterial pressures were 85.6±13 mmHg STD in the 10 affected and 35.3±1.2 mmHg STD in the 10 resistant cattle, P<0.001. From peripheral blood mononuclear cells, DNA was hybridized to an Affymetrix 10K Gene Chip SNP, and RNA was used to probe an Affymetrix Bovine genome array. SNP loci were remapped using the Btau 4.0 bovine genome assembly. mRNA data was analyzed by the Partek software package to identify sets of genes with an expression that was statistically different between the two groups. GSEA and IPA were conducted on the refined expression data to identify key cellular pathways and to generate networks and conduct functional analyses of the pathways and networks. Ten SNPs were identified by allelelic association and four candidate genes were sequenced in the cohort. Neither endothelial nitric oxide synthetase, NADH dehydrogenase, TG-interacting factor-2 nor BMPR2 were different among affected and resistant cattle. A 60-gene mRNA signature was identified that differentiated affected from unaffected cattle. Forty-six genes were overexpressed in the affected and 14 genes were downregulated in the affected cattle by at least 20%. GSEA and Ingenuity analysis identified respiratory diseases, inflammatory diseases and pathways as the top diseases and disorders (P<5.14×10(-14)), cell development and cell signaling as the top cellular functions (P<1.20×10(-08)), and IL6, TREM, PPAR, NFkB cell signaling (P<8.69×10(-09)) as the top canonical pathways associated with this gene signature. This study provides insights into differences in RNA expression in HAPH at a molecular level, and eliminates four functional gene candidates. Further studies are needed to validate and refine these preliminary findings and to determine the role of transcribed genes in the development of HAPH.

Redox signaling and reactive oxygen species in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is an essential physiological mechanism of the lung that matches blood perfusion with alveolar ventilation to optimize gas exchange. Perturbations of HPV, as may occur in pneumonia or adult respiratory distress syndrome, can cause life-threatening hypoxemia. Despite intensive research for decades, the molecular mechanisms of HPV have not been fully elucidated. Reactive oxygen species (ROS) and changes in the cellular redox state are proposed to link O2 sensing and pulmonary arterial smooth muscle cell contraction underlying HPV. In this regard, mitochondria and NAD(P)H oxidases are discussed as sources of ROS. However, there is controversy whether ROS levels decrease or increase during hypoxia. With this background we summarize the current knowledge on the role of ROS and redox state in HPV.

Role of ROS signaling in differential hypoxic Ca2+ and contractile responses in pulmonary and systemic vascular smooth muscle cells

Hypoxia causes a large increase in [Ca2+]i and attendant contraction in pulmonary artery smooth muscle cells (PASMCs), but not in systemic artery SMCs. The different responses meet the respective functional needs in these two distinct vascular myocytes; however, the underlying molecular mechanisms are not well known. We and other investigators have provided extensive evidence to reveal that voltage-dependent K+ (KV) channels, canonical transient receptor potential (TRPC) channels, ryanodine receptor Ca2+ release channels (RyRs), cyclic adenosine diphosphate-ribose, FK506 binding protein 12.6, protein kinase C, NADPH oxidase and reactive oxygen species (ROS) are the essential effectors and signaling intermediates in the hypoxic increase in [Ca2+]i in PASMCs and HPV, but they may not primarily underlie the diverse cellular responses in pulmonary and systemic vascular myocytes. Hypoxia significantly increases mitochondrial ROS generation in PASMCs, which can induce intracellular Ca2+ release by opening RyRs, and may also cause extracellular Ca2+ influx by inhibiting KV channels and activating TRPC channels, leading to a large increase in [Ca2+]i in PASMCs and HPV. In contrast, hypoxia has no or a minor effect on mitochondrial ROS generation in systemic SMCs, thereby causing no change or a negligible increase in [Ca2+]i and contraction. Further preliminary work indicates that Rieske iron-sulfur protein in the mitochondrial complex III may perhaps serve as a key initial molecular determinant for the hypoxic increase in [Ca2+]i in PASMCs and HPV, suggesting its potential important role in different cellular changes to respond to hypoxic stimulation in pulmonary and systemic artery myocytes. All these findings have greatly improved our understanding of the molecular processes for the differential hypoxic Ca2+ and contractile responses in vascular SMCs from distinct pulmonary and systemic circulation systems.

ROS-dependent signaling mechanisms for hypoxic Ca(2+) responses in pulmonary artery myocytes

Hypoxic exposure causes pulmonary vasoconstriction, which serves as a critical physiologic process that ensures regional alveolar ventilation and pulmonary perfusion in the lungs, but may become an essential pathologic factor leading to pulmonary hypertension. Although the molecular mechanisms underlying hypoxic pulmonary vasoconstriction and associated pulmonary hypertension are uncertain, increasing evidence indicates that hypoxia can result in a significant increase in intracellular reactive oxygen species concentration ([ROS](i)) through the mitochondrial electron-transport chain in pulmonary artery smooth muscle cells (PASMCs). The increased mitochondrial ROS subsequently activate protein kinase C-epsilon (PKCepsilon) and NADPH oxidase (Nox), providing positive mechanisms that further increase [ROS](i). ROS may directly cause extracellular Ca(2+) influx by inhibiting voltage-dependent K(+) (K(V)) channels and opening of store-operated Ca(2+) (SOC) channels, as well as intracellular Ca(2+) release by activating ryanodine receptors (RyRs), leading to an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) and associated contraction. In concert with ROS, PKCepsilon may also affect K(V) channels, SOC channels, and RyRs, contributing to hypoxic Ca(2+) and contractile responses in PASMCs.

Thoracoscopic bleb resection using two-lung ventilation anesthesia with low tidal volume for primary spontaneous pneumothorax

BACKGROUND

We performed thoracoscopic surgery for pneumothorax using two-lung ventilation with low tidal volume and evaluated the feasibility and safety of this procedure.

METHODS

Forty-six patients (mean age, 23.6 +/- 10.47 years) each with a primary spontaneous pneumothorax underwent wedge resection with chemical and mechanical pleurodesis. Two-lung ventilation anesthesia was performed with a single-lumen endotracheal tube, and the tidal volume was reduced to 4 mL/kg; the respiratory rate was increased to 24 cycles/min. Airway pressure, end-tidal CO(2), and the results of blood gas analysis were obtained right after endotracheal intubation and during the operation, and were compared.

RESULTS

The tidal volume was 496.2 +/- 94.33 mL at anesthesia induction, which decreased to 243.9 +/- 34.43 mL during the two-lung ventilation. In 5 patients, the tidal volume was additionally decreased by 32.5 +/- 12.58 mL (p = 0.014) to obtain an optimal working field. The differences between the airway pressure, pH, partial pressure of carbon dioxide, and partial pressure of oxygen were significant between the two measurement times. However, all of the values of the arterial blood gas analysis were within normal range. The oxygen saturation (99.9% +/- 0.69% versus 99.8 +/- 0.72%; p = 0.160) and end-tidal CO(2) (33.2 +/- 3.74 mm Hg versus 34.1 +/- 4.19 mm Hg; p = 0.157) were not significantly different. The time from intubation before the incision was 17.1 +/- 4.18 minutes, the operation time was 31.9 +/- 14.48 minutes, and the total anesthesia time was 65.8 +/- 15.02 minutes.

CONCLUSIONS

Thoracoscopic surgery for primary spontaneous pneumothorax using two-lung ventilation with low tidal volume was technically feasible.

Heterogeneous gene expression and functional activity of ryanodine receptors in resistance and conduit pulmonary as well as mesenteric artery smooth muscle cells

BACKGROUND

Hypoxia causes heterogeneous contractile responses in resistance and conduit pulmonary as well as systemic (mesenteric) artery smooth muscle cells (RPASMCs, CPASMCs and MASMCs), but the underlying mechanisms are largely unknown. In this study, we aimed to investigate whether the gene expression and functional activity of ryanodine receptors (RyRs) would be different in these 3 cell types.

METHODS

RyR mRNA expression, Ca(2+) sparks and [Ca(2+)](i) were measured by real-time quantitative RT-PCR, laser scanning confocal microscopy and wide-field fluorescence microscopy, respectively.

RESULTS

All 3 RyR subtype (RyR1, RyR2 and RyR3) mRNAs are expressed in RPASMCs, CPASMCs and MASMCs, but their expression levels are different. Spontaneous Ca(2+) sparks (functional events of RyRs) show distinct frequency, amplitude, duration, size and kinetics in these 3 cell types. Similarly, activation of RyRs by caffeine, 4-chloro-m-cresol or high K(+) induces differential Ca(2+) release. Moreover, hypoxia-induced increase in [Ca(2+)](i) is largest in MASMCs relative to CPSAMCs and smallest in RPASMCs.

CONCLUSION

This study provides comprehensive evidence that RyRs are heterogeneous in gene expression and functional activity in RPASMCs, CPASMCs and MASMCs, which may contribute to the diversity of excitation-contraction coupling and hypoxic Ca(2+) responses in different vascular smooth muscle cells.

One-lung ventilation: For how long?

OBJECTIVE

Lung injury induced by one-lung ventilation is rare, but it is a condition that may result in high mortality. This study evaluates the effects of one-lung ventilation and occlusion time on collapsed and contralateral lungs.

METHODS

Sprague-Dawley rats were allocated randomly into 7 groups consisting of 6 animals each: sham; O1, 1 hour of occlusion/2 hours of re-expansion; C1, 3 hours of mechanical ventilation control; O2, 2 hours of occlusion/2 hours of re-expansion; C2, 4 hours of mechanical ventilation control; O3, 3 hours of occlusion/2 hours of re-expansion; and C3, 5 hours of mechanical ventilation control groups. In the occlusion groups, the left lung was collapsed by bronchial occlusion. Malondialdehyde activity was determined in the blood, and myeloperoxidase and malondialdehyde activity was determined in the collapsed and contralateral lungs. Lung tissues were also examined histopathologically.

RESULTS

Malondialdehyde and myeloperoxidase levels rose as occlusion duration increased. This increase was greater in the occlusion groups than that in their own control groups. Increases were significant in the O2 compared with the O1 groups (P < .005). Histologically, tissue damage increased as occlusion time rose injury in collapsed and contralateral lungs. Injury was greater in the occlusion groups than injury in their own control groups (P < .005).

CONCLUSIONS

Our findings show that biochemical and histopathologic injury occur in collapsed and contralateral lungs in one-lung ventilation, and this injury increases as occlusion time rises. We believe that occlusion and occlusion time-related injury should be borne in mind in the clinic under conditions requiring the application of one-lung ventilation.

The role of NADPH oxidase in carotid body arterial chemoreceptors

O(2)-sensing in the carotid body occurs in neuroectoderm-derived type I glomus cells where hypoxia elicits a complex chemotransduction cascade involving membrane depolarization, Ca(2+) entry and the release of excitatory neurotransmitters. Efforts to understand the exquisite O(2)-sensitivity of these cells currently focus on the coupling between local P(O2) and the open-closed state of K(+)-channels. Amongst multiple competing hypotheses is the notion that K(+)-channel activity is mediated by a phagocytic-like multisubunit enzyme, NADPH oxidase, which produces reactive oxygen species (ROS) in proportion to the prevailing P(O2). In O(2)-sensitive cells of lung neuroepithelial bodies (NEB), multiple studies confirm that ROS levels decrease in hypoxia, and that E(M) and K(+)-channel activity are indeed controlled by ROS produced by NADPH oxidase. However, recent studies in our laboratories suggest that ROS generated by a non-phagocyte isoform of the oxidase are important contributors to chemotransduction, but that their role in type I cells differs fundamentally from the mechanism utilized by NEB chemoreceptors. Data indicate that in response to hypoxia, NADPH oxidase activity is increased in type I cells, and further, that increased ROS levels generated in response to low-O(2) facilitate cell repolarization via specific subsets of K(+)-channels.

The effects of oxidative stress on the liver and ileum in rats caused by one-lung ventilation

BACKGROUND AND AIM

Reactive oxygen radicals that cause remote organ injury are increased after the one-lung ventilation frequently used in thoracic surgery. The aim of this study was to examine the effects of one-lung ventilation on the liver and ileum.

MATERIALS AND METHODS

Thirty rats were divided into five groups: a sham group; 3- and 4-h mechanical ventilation groups; and 1- and 2-h left lung collapse/2-h re-expansion groups (n = 6 for each group). In the collapse groups, the left lung was collapsed by bronchial occlusion for 1 and 2 h and then re-expanded and ventilated for an additional 2 h. At the end of the study, serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels were determined to assess liver functions. Myeloperoxidase (MPO) and malondialdehyde (MDA) activity were determined in the liver and ileum tissues. The tissues were also examined by light and electron microscope. Apoptosis was assessed using terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling (TUNEL) assay.

RESULTS

Plasma ALT and AST, tissue MDA, and MPO activities in both tissues were significantly higher in the 2-h collapse/2-h re-expansion group than in the 4-h mechanical ventilation group (P < 0.05). Moreover, the levels were significantly higher in the 2-h collapse group compared to the 1-h collapse group (P < 0.016). Tissue damage and apoptotic index were most prominent in the 2-h collapse/2-h re-expansion group.

CONCLUSION

Our findings showed that one-lung ventilation causes tissue damage in the liver and ileum and that this damage increases as occlusion duration rises.

Calcium signaling phenomena in heart diseases: a perspective

Ca(2+) is a major intracellular messenger and nature has evolved multiple mechanisms to regulate free intracellular (Ca(2+))(i) level in situ. The Ca(2+) signal inducing contraction in cardiac muscle originates from two sources. Ca(2+) enters the cell through voltage dependent Ca(2+) channels. This Ca(2+) binds to and activates Ca(2+) release channels (ryanodine receptors) of the sarcoplasmic reticulum (SR) through a Ca(2+) induced Ca(2+) release (CICR) process. Entry of Ca(2+) with each contraction requires an equal amount of Ca(2+) extrusion within a single heartbeat to maintain Ca(2+) homeostasis and to ensure relaxation. Cardiac Ca(2+) extrusion mechanisms are mainly contributed by Na(+)/Ca(2+) exchanger and ATP dependent Ca(2+) pump (Ca(2+)-ATPase). These transport systems are important determinants of (Ca(2+))(i) level and cardiac contractility. Altered intracellular Ca(2+) handling importantly contributes to impaired contractility in heart failure. Chronic hyperactivity of the beta-adrenergic signaling pathway results in PKA-hyperphosphorylation of the cardiac RyR/intracellular Ca(2+) release channels. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the beta-adrenergic receptor, protein kinase C, Gq, and the down stream effectors such as mitogen activated protein kinases pathways, and the Ca(2+) regulated phosphatase calcineurin. A number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocytes. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underlie heart diseases. Recent progress in molecular cardiology makes it possible to envision a new therapeutic approach to heart failure (HF), targeting key molecules involved in intracellular Ca(2+) handling such as RyR, SERCA2a, and PLN. Controlling these molecular functions by different agents have been found to be beneficial in some experimental conditions.

Detection of reactive oxygen species in isolated, perfused lungs by electron spin resonance spectroscopy

Background

The sources and measurement of reactive oxygen species (ROS) in intact organs are largely unresolved. This may be related to methodological problems associated with the techniques currently employed for ROS detection. Electron spin resonance (ESR) with spin trapping is a specific method for ROS detection, and may address some these technical problems.

Methods

We have established a protocol for the measurement of intravascular ROS release from isolated buffer-perfused and ventilated rabbit and mouse lungs, combining lung perfusion with the spin probe l-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine (CPH) and ESR spectroscopy. We then employed this technique to characterize hypoxia-dependent ROS release, with specific attention paid to NADPH oxidase-dependent superoxide formation as a possible vasoconstrictor pathway.

Results

While perfusing lungs with CPH over a range of inspired oxygen concentrations (1–21 %), the rate of CP^• formation exhibited an oxygen-dependence, with a minimum at 2.5 % O₂. Addition of superoxide dismutase (SOD) to the buffer fluid illustrated that a minor proportion of this intravascular ROS leak was attributable to superoxide. Stimulation of the lungs by injection of phorbol-12-myristate-13-acetate (PMA) into the pulmonary artery caused a rapid increase in CP^• formation, concomitant with pulmonary vasoconstriction. Both the PMA-induced CPH oxidation and the vasoconstrictor response were largely suppressed by SOD. When the PMA challenge was performed at different oxygen concentrations, maximum superoxide liberation and pulmonary vasoconstriction occurred at 5 % O₂. Using a NADPH oxidase inhibitor and NADPH-oxidase deficient mice, we illustrated that the PMA-induced superoxide release was attributable to the stimulation of NADPH oxidases.

Conclusion

The perfusion of isolated lungs with CPH is suitable for detection of intravascular ROS release by ESR spectroscopy. We employed this technique to demonstrate that 1) PMA-induced vasoconstriction is caused "directly" by superoxide generated from NADPH oxidases and 2) this pathway is pronounced in hypoxia. NADPH oxidases thus may contribute to the hypoxia-dependent regulation of pulmonary vascular tone.

Hypoxic pulmonary vasoconstriction: redox events in oxygen sensing

Recently, the mitochondria have become the focus of attention as the site of O(2) sensing underlying hypoxic pulmonary vasoconstriction (HPV). However, two disparate models have emerged to explain how mitochondria react to a decrease in Po(2). One model proposes that a drop in Po(2) decreases the rate of mitochondrial reactive oxygen species (ROS) generation, resulting in a decrease in oxidant stress and an accumulation of reducing equivalents. The resulting shift of the cytosol to a reduced state causes the inhibition of voltage-dependent potassium channels, membrane depolarization, and the influx of calcium through voltage-gated (L-type) calcium channels. A second and opposing model suggests that hypoxia triggers a paradoxical increase in a mitochondrial-induced ROS signal. The resulting shift of the cytosol to an oxidized state triggers the release of intracellular calcium stores, recruitment of calcium channels in the plasma membrane, and activation of contraction. This article summarizes the potential involvement of a mitochondria-induced ROS signal in these two very different models.

Mobilization of sarcoplasmic reticulum stores by hypoxia leads to consequent activation of capacitative Ca2+ entry in isolated canine pulmonary arterial smooth muscle cells

Capacitative Ca2+ entry (CCE) has been speculated to contribute to Ca2+ influx during hypoxic pulmonary vasoconstriction (HPV). The aim of the present study was to directly test if acute hypoxia causes intracellular Ca2+ concentration ([Ca2+]i) rises through CCE in canine pulmonary artery smooth muscle cells (PASMCs). In PASMCs loaded with fura-2, hypoxia produced a transient rise in [Ca2+]i in Ca2+-free solution, indicating Ca2+ release from the intracellular Ca2+ stores. Subsequent addition of 2 mm Ca2+ in hypoxia elicited a sustained rise in [Ca2+]i, which was partially inhibited by 10 microm nisoldipine. The dihydropyridine-insensitive rise in [Ca2+]i was due to increased Ca2+ influx, because it was abolished in Ca2+-free solution and hypoxia was shown to significantly enhance the rate of Mn2+ quench of fura-2 fluorescence. The dihyropyridine-insensitive rise in [Ca2+]i and the increased rate of Mn2+ quench of fura-2 fluorescence were inhibited by 50 microm SKF 96365 and 500 microm Ni2+, but not by 100 microm La3+ or 100 microm Gd3+, exhibiting pharmacological properties characteristic of CCE. In addition, predepletion of the intracellular Ca2+ stores inhibited the rise in [Ca2+]i induced by hypoxia. These results provide the first direct evidence that acute hypoxia, by causing Ca2+ release from the intracellular stores, activates CCE in isolated canine PASMCs, which may contribute to HPV.

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

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

Dehydroepiandrosterone (DHEA) prevents and reverses chronic hypoxic pulmonary hypertension

Pulmonary artery (PA) hypertension was studied in a chronic hypoxic-pulmonary hypertension model (7-21 days) in the rat. Increase in PA pressure (measured by catheterism), cardiac right ventricle hypertrophy (determined by echocardiography), and PA remodeling (evaluated by histology) were almost entirely prevented after oral dehydroepiandrosterone (DHEA) administration (30 mg/kg every alternate day). Furthermore, in hypertensive rats, oral administration, or intravascular injection (into the jugular vein) of DHEA rapidly decreased PA hypertension. In PA smooth muscle cells, DHEA reduced the level of intracellular calcium (measured by microspectrofluorimetry). The effect of DHEA appears to involve a large conductance Ca2+-activated potassium channel (BKCa)-dependent stimulatory mechanism, at both function and expression levels (isometric contraction and Western blot), via a redox-dependent pathway. Voltage-gated potassium (Kv) channels also may be involved because the antagonist 4-amino-pyridine blocked part of the DHEA effect. The possible pathophysiological and therapeutic significance of the results is discussed.

Metabolic inhibition with cyanide induces calcium release in pulmonary artery myocytes and Xenopus oocytes

We examined the effects of metabolic inhibition on intracellular Ca(2+) release in single pulmonary arterial smooth muscle cells (PASMCs). Severe metabolic inhibition with cyanide (CN, 10 mM) increased intracellular calcium concentration ([Ca(2+)](i)) and activated Ca(2+)-activated Cl(-) currents [I(Cl(Ca))] in PASMCs, responses that were greatly inhibited by BAPTA-AM or caffeine. Mild metabolic inhibition with CN (1 mM) increased spontaneous transient inward currents and Ca(2+) sparks in PASMCs. In Xenopus oocytes, CN also induced Ca(2+) release and activated I(Cl(Ca)), and these responses were inhibited by thapsigargin and cyclopiazonic acid to deplete sarcoplasmic reticulum (SR) Ca(2+), whereas neither heparin nor anti-inositol 1,4,5-trisphosphate receptor (IP(3)R) antibodies affected CN responses. In both PASMCs and oocytes, CN-evoked Ca(2+) release was inhibited by carbonyl cyanide m-chlorophenylhydrazone (CCCP) and oligomycin or CCCP and thapsigargin. Whereas hypoxic stimuli resulted in Ca(2+) release in pulmonary but not mesenteric artery myocytes, CN induced release in both cell types. We conclude that metabolic inhibition with CN increases [Ca(2+)](i) in both pulmonary and systemic artery myocytes by stimulating Ca(2+) release from the SR and mitochondria.

O(2) sensing in hypoxic pulmonary vasoconstriction: the mitochondrial door re-opens

The identity of the O(2) sensor underlying the hypoxic pulmonary vasoconstriction (HPV) response has been sought for more than 50 years. Recently, the mitochondria have again come into sharp focus as the cellular organelle responsible for triggering the events that culminate in pulmonary artery constriction. Studies from different laboratories propose two disparate models to explain how mitochondria react to a decrease in P(O(2)). One model proposes that hypoxia slows or inhibits mitochondrial electron transport resulting in the accumulation of reducing equivalents and a decrease in the generation of reactive oxygen species (ROS). This is proposed to activate a redox-sensitive pathway leading to pulmonary vasoconstriction. A second and opposing model suggests that hypoxia triggers a paradoxical increase in mitochondrial ROS generation. This increase would then lead to the activation of an oxidant-sensitive signaling transduction pathway leading to HPV. This article summarizes the potential involvement of mitochondria in these two very different models.

Ca(2+) release from ryanodine-sensitive store contributes to mechanism of hypoxic vasoconstriction in rat lungs

Studies of thapsigargin, cyclopiazonic acid, and ryanodine in isolated pulmonary arteries and smooth muscle cells suggest that release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))- and/or ryanodine-sensitive sarcoplasmic reticulum Ca(2+) stores is a component of the mechanism of acute hypoxic pulmonary vasoconstriction (HPV). However, the actions of these agents on HPV in perfused lungs have not been reported. Thus we tested effects of thapsigargin and cyclopiazonic acid, inhibitors of sarcoplasmic reticulum Ca(2+)-ATPase, and of ryanodine, an agent that either locks the ryanodine receptor open or blocks it, on HPV in salt solution-perfused rat lungs. After inhibition of cyclooxygenase and nitric oxide synthase, thapsigargin (10 nM) and cyclopiazonic acid (5 microM) augmented the vasoconstriction to 0% but not to 3% inspired O(2). Relatively high concentrations of ryanodine (100 and 300 microM) blunted HPV in nitric oxide synthase-inhibited lungs. The results indicate that release of Ca(2+) from the ryanodine-sensitive, but not the IP(3)-sensitive, store, contributes to the mechanism of HPV in perfused rat lungs and that Ca(2+)-ATPase-dependent Ca(2+) buffering moderates the response to severe hypoxia.