Hypothesis: do voltage-gated H+channels in alveolar epithelial cells contribute to CO2 elimination by the lung?

2000 ◽  
Vol 278 (1) ◽  
pp. C1-C10 ◽  
Author(s):  
Thomas E. DeCoursey
Keyword(s):  
Epithelial Cells ◽  
Mammalian Cells ◽  
Alveolar Epithelium ◽  
Alveolar Epithelial Cells ◽  
Facilitated Diffusion ◽  
Electrochemical Gradient ◽  
Voltage Gated ◽  
Alveolar Epithelial ◽  
H Channels ◽  
Apical Membranes

Although alveolar epithelial cells were the first mammalian cells in which voltage-gated H+ currents were recorded, no specific function has yet been proposed. Here we consider whether H+ channels contribute to one of the main functions of the lung: CO2 elimination. This idea builds on several observations: 1) some cell membranes have low CO2permeability, 2) carbonic anhydrase is present in alveolar epithelium and contributes to CO2 extrusion by facilitating diffusion, 3) the transepithelial potential difference favors selective activation of H+ channels in apical membranes, and 4) the properties of H+ channels are ideally suited to the proposed role. H+channels open only when the electrochemical gradient for H+ is outward, imparting directionality to the diffusion process. Unlike previous facilitated diffusion models, [Formula: see text] and H+ recombine to form CO2 in the alveolar subphase. Rough quantitative considerations indicate that the proposed mechanism is plausible and indicate a significant capacity for CO2 elimination by the lung by this route. Fully activated alveolar H+ channels extrude acid equivalents at three times the resting rate of CO2 production.

Voltage-gated proton channels help regulate pHi in rat alveolar epithelium

2005 ◽  
Vol 288 (2) ◽  
pp. L398-L408 ◽  
Author(s):  
Ricardo Murphy ◽  
Vladimir V. Cherny ◽  
Deri Morgan ◽  
Thomas E. DeCoursey
Keyword(s):  
Epithelial Cells ◽  
Ph Regulation ◽  
Alveolar Epithelial Cell ◽  
Alveolar Epithelium ◽  
Alveolar Epithelial Cells ◽  
Proton Channel ◽  
Resting Cells ◽  
Proton Channels ◽  
Voltage Gated ◽  
Alveolar Epithelial

Voltage-gated proton channels are expressed highly in rat alveolar epithelial cells. Here we investigated whether these channels contribute to pH regulation. The intracellular pH (pHi) was monitored using BCECF in cultured alveolar epithelial cell monolayers and found to be 7.13 in nominally HCO3−-free solutions [at external pH (pHo) 7.4]. Cells were acid-loaded by the NH4+ prepulse technique, and the recovery was observed. Under conditions designed to eliminate the contribution of other transporters that alter pH, addition of 10 μM ZnCl2, a proton channel inhibitor, slowed recovery about twofold. In addition, the pHi minimum was lower, and the time to nadir was increased. Slowing of recovery by ZnCl2 was observed at pHo 7.4 and pHo 8.0 and in normal and high-K+ Ringer solutions. The observed rate of Zn2+-sensitive pHi recovery required activation of a small fraction of the available proton conductance. We conclude that proton channels contribute to pHi recovery after an acid load in rat alveolar epithelial cells. Addition of ZnCl2 had no effect on pHi in unchallenged cells, consistent with the expectation that proton channels are not open in resting cells. After inhibition of all known pH regulators, slow pHi recovery persisted, suggesting the existence of a yet-undefined acid extrusion mechanism in these cells.

scholarly journals Ph-Dependent Inhibition of Voltage-Gated H+ Currents in Rat Alveolar Epithelial Cells by Zn2+ and Other Divalent Cations

1999 ◽  
Vol 114 (6) ◽  
pp. 819-838 ◽  
Author(s):  
Vladimir V. Cherny ◽  
Thomas E. DeCoursey
Keyword(s):  
Epithelial Cells ◽  
Divalent Cations ◽  
Active Species ◽  
Alveolar Epithelial Cells ◽  
Protonation Site ◽  
Pipette Solution ◽  
Voltage Gated ◽  
Alveolar Epithelial ◽  
High Concentrations ◽  
H Channels

Inhibition by polyvalent cations is a defining characteristic of voltage-gated proton channels. The mechanism of this inhibition was studied in rat alveolar epithelial cells using tight-seal voltage clamp techniques. Metal concentrations were corrected for measured binding to buffers. Externally applied ZnCl2 reduced the H+ current, shifted the voltage-activation curve toward positive potentials, and slowed the turn-on of H+ current upon depolarization more than could be accounted for by a simple voltage shift, with minimal effects on the closing rate. The effects of Zn2+ were inconsistent with classical voltage-dependent block in which Zn2+ binds within the membrane voltage field. Instead, Zn2+ binds to superficial sites on the channel and modulates gating. The effects of extracellular Zn2+ were strongly pHo dependent but were insensitive to pHi, suggesting that protons and Zn2+ compete for external sites on H+ channels. The apparent potency of Zn2+ in slowing activation was ∼10× greater at pHo 7 than at pHo 6, and ∼100× greater at pHo 6 than at pHo 5. The pHo dependence suggests that Zn2+, not ZnOH+, is the active species. Evidently, the Zn2+ receptor is formed by multiple groups, protonation of any of which inhibits Zn2+ binding. The external receptor bound H+ and Zn2+ with pKa 6.2–6.6 and pKM 6.5, as described by several models. Zn2+ effects on the proton chord conductance–voltage (gH–V) relationship indicated higher affinities, pKa 7 and pKM 8. CdCl2 had similar effects as ZnCl2 and competed with H+, but had lower affinity. Zn2+ applied internally via the pipette solution or to inside-out patches had comparatively small effects, but at high concentrations reduced H+ currents and slowed channel closing. Thus, external and internal zinc-binding sites are different. The external Zn2+ receptor may be the same modulatory protonation site(s) at which pHo regulates H+ channel gating.

scholarly journals Temperature Dependence of Voltage-gated H+ Currents in Human Neutrophils, Rat Alveolar Epithelial Cells, and Mammalian Phagocytes

1998 ◽  
Vol 112 (4) ◽  
pp. 503-522 ◽  
Author(s):  
Thomas E. DeCoursey ◽  
Vladimir V. Cherny
Keyword(s):  
Ion Channels ◽  
Epithelial Cells ◽  
Mammalian Cells ◽  
Alveolar Epithelial Cells ◽  
Human Neutrophils ◽  
Temperature Sensitive ◽  
Whole Cell ◽  
Alveolar Epithelial ◽  
Rate Determining Step ◽  
H Channels

H+ currents in human neutrophils, rat alveolar epithelial cells, and several mammalian phagocyte cell lines were studied using whole-cell and excised-patch tight-seal voltage clamp techniques at temperatures between 6 and 42°C. Effects of temperature on gating kinetics were distinguished from effects on the H+ current amplitude. The activation and deactivation of H+ currents were both highly temperature sensitive, with a Q10 of 6–9 (activation energy, Ea, ≈ 30–38 kcal/mol), greater than for most other ion channels. The similarity of Ea for channel opening and closing suggests that the same step may be rate determining. In addition, when the turn-on of H+ currents with depolarization was fitted by a delay and single exponential, both the delay and the time constant (τact) had similarly high Q10. These results could be explained if H+ channels were composed of several subunits, each of which undergoes a single rate-determining gating transition. H+ current gating in all mammalian cells studied had similarly strong temperature dependences. The H+ conductance increased markedly with temperature, with Q10 ≥ 2 in whole-cell experiments. In excised patches where depletion would affect the measurement less, the Q10 was 2.8 at >20°C and 5.3 at <20°C. This temperature sensitivity is much greater than for most other ion channels and for H+ conduction in aqueous solution, but is in the range reported for H+ transport mechanisms other than channels; e.g., carriers and pumps. Evidently, under the conditions employed, the rate-determining step in H+ permeation occurs not in the diffusional approach but during permeation through the channel itself. The large Ea of permeation intrinsically limits the conductance of this channel, and appears inconsistent with the channel being a water-filled pore. At physiological temperature, H+ channels provide mammalian cells with an enormous capacity for proton extrusion.

scholarly journals Epithelial–Mesenchymal Transition in the Pathogenesis of Idiopathic Pulmonary Fibrosis

Medicina ◽  
2019 ◽  
Vol 55 (4) ◽  
pp. 83 ◽  
Author(s):  
Francesco Salton ◽  
Maria Volpe ◽  
Marco Confalonieri
Keyword(s):  
Epithelial Cells ◽  
Idiopathic Pulmonary Fibrosis ◽  
Pulmonary Fibrosis ◽  
Epithelial Mesenchymal Transition ◽  
Treatment Options ◽  
Alveolar Epithelium ◽  
Alveolar Epithelial Cells ◽  
Mesenchymal Transition ◽  
Alveolar Epithelial ◽  
Serious Disease

Idiopathic pulmonary fibrosis (IPF) is a serious disease of the lung, which leads to extensive parenchymal scarring and death from respiratory failure. The most accepted hypothesis for IPF pathogenesis relies on the inability of the alveolar epithelium to regenerate after injury. Alveolar epithelial cells become apoptotic and rare, fibroblasts/myofibroblasts accumulate and extracellular matrix (ECM) is deposited in response to the aberrant activation of several pathways that are physiologically implicated in alveologenesis and repair but also favor the creation of excessive fibrosis via different mechanisms, including epithelial–mesenchymal transition (EMT). EMT is a pathophysiological process in which epithelial cells lose part of their characteristics and markers, while gaining mesenchymal ones. A role for EMT in the pathogenesis of IPF has been widely hypothesized and indirectly demonstrated; however, precise definition of its mechanisms and relevance has been hindered by the lack of a reliable animal model and needs further studies. The overall available evidence conceptualizes EMT as an alternative cell and tissue normal regeneration, which could open the way to novel diagnostic and prognostic biomarkers, as well as to more effective treatment options.

Silica directly increases permeability of alveolar epithelial cells

1990 ◽  
Vol 68 (4) ◽  
pp. 1354-1359 ◽  
Author(s):  
R. K. Merchant ◽  
M. W. Peterson ◽  
G. W. Hunninghake
Keyword(s):  
Epithelial Cells ◽  
Cell Injury ◽  
Alveolar Epithelial Cell ◽  
Alveolar Epithelium ◽  
Alveolar Epithelial Cells ◽  
Dependent Manner ◽  
Alveolar Epithelial ◽  
Lactate Dehydrogenase Release ◽  
Capillary Membrane

Alveolar epithelial cell injury and increased alveolar-capillary membrane permeability are important features of acute silicosis. To determine whether silica particles contribute directly to this increased permeability, we measured paracellular permeability of rat alveolar epithelium after exposure to silica, in vitro, using markers of the extracellular space. Silica (Minusil) markedly increased permeability in a dose- and time-dependent manner. This was not the result of cytolytic injury, because lactate dehydrogenase release from monolayers exposed to silica was not increased. Pretreatment of the silica with serum, charged dextrans, or aluminum sulfate blocked the increase in permeability. Scanning electron microscopy demonstrated adherence of the silica to the surface of the alveolar epithelial cells. Thus silica can directly increase permeability of alveolar epithelium.

scholarly journals Mechanisms of EGF-induced stimulation of sodium reabsorption by alveolar epithelial cells

1998 ◽  
Vol 275 (1) ◽  
pp. C82-C92 ◽  
Author(s):  
Spencer I. Danto ◽  
Zea Borok ◽  
Xiao-Ling Zhang ◽  
Melissa Z. Lopez ◽  
Paryus Patel ◽  
...  
Keyword(s):  
Epithelial Cells ◽  
Alveolar Epithelium ◽  
Alveolar Epithelial Cells ◽  
Cell Labeling ◽  
Northern Analysis ◽  
Sodium Reabsorption ◽  
Alveolar Epithelial ◽  
Subunit Mrna ◽  
Epidermal Growth ◽  
Stimulation Of

We investigated the effects of epidermal growth factor (EGF) on active Na+ absorption by alveolar epithelium. Rat alveolar epithelial cells (AEC) were isolated and cultivated in serum-free medium on tissue culture-treated polycarbonate filters. mRNA for rat epithelial Na+ channel (rENaC) α-, β-, and γ-subunits and Na+ pump α1- and β1-subunits were detected in day 4 monolayers by Northern analysis and were unchanged in abundance in day 5 monolayers in the absence of EGF. Monolayers cultivated in the presence of EGF (20 ng/ml) for 24 h from day 4 to day 5 showed an increase in both α1 and β1Na+ pump subunit mRNA but no increase in rENaC subunit mRNA. EGF-treated monolayers showed parallel increases in Na+ pump α1- and β1-subunit protein by immunoblot relative to untreated monolayers. Fixed AEC monolayers demonstrated predominantly membrane-associated immunofluorescent labeling with anti-Na+ pump α1- and β1-subunit antibodies, with increased intensity of cell labeling for both subunits seen at 24 h following exposure to EGF. These changes in Na+ pump mRNA and protein preceded a delayed (>12 h) increase in short-current circuit (measure of active transepithelial Na+transport) across monolayers treated with EGF compared with untreated monolayers. We conclude that EGF increases active Na+ resorption across AEC monolayers primarily via direct effects on Na+ pump subunit mRNA expression and protein synthesis, leading to increased numbers of functional Na+ pumps in the basolateral membranes.

Distinct effects of oxygen on surfactant protein B expression in bronchiolar and alveolar epithelium

1992 ◽  
Vol 262 (1) ◽  
pp. L32-L39 ◽  
Author(s):  
K. A. Wikenheiser ◽  
S. E. Wert ◽  
J. R. Wispe ◽  
M. Stahlman ◽  
M. D'Amore-Bruno ◽  
...  
Keyword(s):  
Epithelial Cells ◽  
Alveolar Epithelium ◽  
Surfactant Protein ◽  
Alveolar Epithelial Cells ◽  
Cell Protein ◽  
Altered Expression ◽  
Alveolar Epithelial ◽  
Surfactant Protein B ◽  
Bronchiolar Epithelium ◽  
Protein B

Hyperoxia causes severe lung injury in association with altered expression of surfactant proteins and lipids. To test whether oxygen induces surfactant protein B (SP-B) expression in specific respiratory epithelial cells, adult B6C3F1 and FVB/N mice were exposed to room air or 95% oxygen for 1–5 days. Northern blot analysis demonstrated an 8- to 10-fold increase in SP-B mRNA after 3 days that was maintained thereafter. In situ hybridization localized SP-B mRNA to bronchial, bronchiolar, and alveolar epithelial cells. Hyperoxia was associated with increased SP-B mRNA, noted primarily in the bronchiolar epithelium and decreased SP-B mRNA in the alveolar epithelium. After 5 days, central regions of lung parenchyma were nearly devoid of SP-B mRNA, while SP-B mRNA was maintained in alveolar cell populations close to vascular structures. To determine whether increased bronchiolar expression of SP-B mRNA during hyperoxia was a specific response, the abundance of CC10 mRNA (a Clara cell protein) was assessed. CC10 mRNA was detected in tracheal, bronchial, and bronchiolar, but not alveolar epithelium and was decreased upon exposure to hyperoxia. Immunocytochemistry demonstrated that SP-B proprotein was detected in bronchial, bronchiolar, and alveolar epithelial cells with staining increased in the bronchial and bronchiolar epithelium upon exposure to hyperoxia. SP-B gene expression in the respiratory epithelium is regulated at a pretranslational level and occurs in a cell specific manner during hyperoxic injury in the mouse.

scholarly journals Hypoxia regulates gene expression of alveolar epithelial transport proteins

2000 ◽  
Vol 88 (5) ◽  
pp. 1890-1896 ◽  
Author(s):  
Christine Clerici ◽  
Michael A. Matthay
Keyword(s):  
Gene Expression ◽  
Epithelial Cells ◽  
Alveolar Epithelium ◽  
Transport Proteins ◽  
Alveolar Epithelial Cells ◽  
Type I ◽  
Type Ii ◽  
Atp Content ◽  
Glut 1 ◽  
Alveolar Epithelial

Alveolar hypoxia occurs during ascent to high altitude but is also commonly observed in many acute and chronic pulmonary disorders. The alveolar epithelium is directly exposed to decreases in O2tension, but a few studies have evaluated the effects of hypoxia on alveolar cell function. The alveolar epithelium consists of two cell types: large, flat, squamous alveolar type I and cuboidal type II (ATII). ATII cells are more numerous and have a number of critical functions, including transporting ions and substrates required for many physiological processes. ATII cells express 1) membrane proteins used for supplying substrates required for cell metabolism and 2) ion transport proteins such as Na+channels and Na+-K+-ATPase, which are involved in the vectorial transport of Na+from the alveolar to interstitial spaces and therefore drive the resorption of alveolar fluid. This brief review focuses on gene expression regulation of glucose transporters and Na+transport proteins by hypoxia in alveolar epithelial cells. Cells exposed to severe hypoxia (0% or 3% O2) for 24 h upregulate the activity and expression of the glucose transporter GLUT-1, resulting in preservation of ATP content. Hypoxia-induced increases in GLUT-1 mRNA levels are due to O2deprivation and inhibition of oxidative phosphorylation. This regulation occurs at the transcriptional level through activation of a hypoxia-inducible factor. In contrast, hypoxia downregulates expression and activity of Na+channels and Na+-K+-ATPase in cultured alveolar epithelial cells. Hypoxia induces time- and concentration-dependent decreases of α-, β-, and γ-subunits of epithelial Na+channel mRNA and β1- and α1-subunits of Na+-K+-ATPase, effects that are completely reversed after reoxygenation. The mechanisms by which O2deprivation regulates gene expression of Na+transport proteins are not fully elucidated but likely involve the redox status of the cell. Thus hypoxia regulates gene expression of transport proteins in cultured alveolar epithelial type II cells differently, preserving ATP content.

Impairment of cation transport in A549 cells and rat alveolar epithelial cells by hypoxia

1997 ◽  
Vol 273 (4) ◽  
pp. L797-L806 ◽  
Author(s):  
Heimo Mairbäurl ◽  
Ralf Wodopia ◽  
Sigrid Eckes ◽  
Susanne Schulz ◽  
Peter Bärtsch
Keyword(s):  
Epithelial Cells ◽  
A549 Cells ◽  
Alveolar Epithelium ◽  
Cell Types ◽  
Cation Transport ◽  
Alveolar Epithelial Cells ◽  
Alveolar Type ◽  
Water Reabsorption ◽  
Alveolar Epithelial

A reduced cation reabsorption across the alveolar epithelium decreases water reabsorption from the alveoli and could diminish clearing accumulated fluid. To test whether hypoxia restricts cation transport in alveolar epithelial cells, cation uptake was measured in rat lung alveolar type II pneumocytes (AII cells) in primary culture and in A549 cells exposed to normoxia and hypoxia. In AII and A549 cells, hypoxia caused a[Formula: see text]-dependent inhibition of the Na-K pump, of Na-K-2Cl cotransport, and of total and amiloride-sensitive22Na uptake. Nifedipine failed to prevent hypoxia-induced transport inhibition in both cell types. In A549 cells, the inhibition of the Na-K pump and Na-K-2Cl cotransport occurred within ∼30 min of hypoxia, was stable >20 h, and was reversed by 2 h of reoxygenation. There was also a reduction in cell membrane-associated Na-K-ATPase and a decrease in Na-K-2Cl cotransport flux after full activation with calyculin A, indicating a decreased transport capacity. [14C]serine incorporation into cell proteins was reduced in hypoxic A549 cells, but inhibition of protein synthesis with cycloheximide did not reduce ion transport. In AII and A549 cells, ATP levels decreased slightly, and ADP and the ATP-to-ADP ratio were unchanged after 4 h of hypoxia. In A549 cells, lactate, intracellular Na, and intracellular K were unchanged. These results indicate that hypoxia inhibits apical Na entry pathways and the basolateral Na-K pump in A549 cells and rat AII pneumocytes in culture, indicating a hypoxia-induced reduction of transepithelial Na transport and water reabsorption by alveolar epithelium. If similar changes occur in vivo, the impaired cation transport across alveolar epithelial cells might contribute to the formation of hypoxic pulmonary edema.

scholarly journals Differential effects of claudin-3 and claudin-4 on alveolar epithelial barrier function

2011 ◽  
Vol 301 (1) ◽  
pp. L40-L49 ◽  
Author(s):  
Leslie A. Mitchell ◽  
Christian E. Overgaard ◽  
Christina Ward ◽  
Susan S. Margulies ◽  
Michael Koval
Keyword(s):  
Epithelial Cells ◽  
Tight Junction ◽  
Barrier Function ◽  
Alveolar Epithelium ◽  
Alveolar Epithelial Cells ◽  
Epithelial Barrier ◽  
Tight Junction Proteins ◽  
Epithelial Barrier Function ◽  
Alveolar Epithelial ◽  
Junction Proteins

Alveolar barrier function depends critically on the claudin family tight junction proteins. Of the major claudins expressed by alveolar epithelial cells, claudin (Cldn)-3 and Cldn-4 are the most closely related by amino acid homology, yet they differ dramatically in the pattern of expression. Previously published reports have shown that Cldn-3 is predominantly expressed by type II alveolar epithelial cells; Cldn-4 is expressed throughout the alveolar epithelium and is specifically upregulated in response to acute lung injury. Using primary rat alveolar epithelial cells transduced with yellow fluorescent protein-tagged claudin constructs, we have identified roles for Cldn-3 and Cldn-4 in alveolar epithelial barrier function. Surprisingly, increasing expression of Cldn-3 decreased alveolar epithelial barrier function, as assessed by transepithelial resistance and dye flux measurements. Conversely, increasing Cldn-4 expression improved alveolar epithelial transepithelial resistance compared with control cells. Other alveolar epithelial tight junction proteins were largely unaffected by increased expression of Cldn-3 and Cldn-4. Taken together, these results demonstrate that, in the context of the alveolar epithelium, Cldn-3 and Cldn-4 have different effects on paracellular permeability, despite significant homology in their extracellular loop domains.

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