Regulation of ion and fluid transport across the distal pulmonary epithelia: new insights

Abstract

SPECIAL TOPICEditorialRegulation of ion and fluid transport across the distal pulmonary epithelia: new insightsMichael A. Matthay, Michael A. Matthay American Journal of Physiology- Lung Cellular and Molecular Physiology April 2002, Volume 282 (26), Associate EditorPublished Online:01 Apr 2002https://doi.org/10.1152/ajplung.00473.2001MoreSectionsPDF (53 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmailWeChat this issue of the American Journal of Physiology-Lung Cellular and Molecular Physiologyfeatures nine articles that provide new insights into the mechanisms and the cellular pathways that regulate ion transport across the distal pulmonary epithelia in both the adult and fetal lung. Although much has been learned regarding the mechanisms that regulate fluid transport across the epithelial barrier of the normal and the injured lung (30, 31, 43), there are several major issues that remain unresolved (9). The studies published in this issue have relevance to the resolution of pulmonary edema. Clinical studies have established that patients with acute lung injury with impaired epithelial fluid clearance have a higher mortality than patients with intact (32) or maximal fluid clearance (49).The capacity of alveolar epithelial type II cells to actively transport sodium has been well established (4, 7, 9, 20, 29). However, the potential contribution of alveolar epithelial type I cells to sodium transport has been more difficult to determine, in part because these cells are difficult to isolate, and no investigators have been able to successfully create culture conditions in which they can be studied under polarized conditions. The first breakthrough came 4 years ago when Dobbs et al. (11) successfully isolated purified rat alveolar type I cells. These investigators established that freshly isolated alveolar type I cells had a high osmotic water permeability, attributable primarily to the expression of aquaporin-5, a water channel expressed on the apical surface of type I cells. Subsequent aquaporin-5 knockout studies in mice established, however, that the absence of aquaporin-5 did not impair basal or maximal alveolar fluid clearance (27) and that this channel is not critical to the formation or resolution of pulmonary edema (46). In this issue, Borok et al. (3) provide new evidence that purified alveolar type I cells express the α1- and β1-, but not the α2-, subunits of Na-K-ATPase, a finding that stands in contrast to a prior study that could only detect subunits of Na-K-ATPase in alveolar type II cells in situ (44). The current study also presents evidence for the expression of the α-subunit of the epithelial sodium channel (ENaC) in purified alveolar type I cells. The investigators also demonstrate expression of the α1-subunit of Na-K-ATPase in the rat lung on the basolateral surface of alveolar type I cells, although the intensity of the signal is less than on the basolateral surface of adjacent alveolar type II cells. There is no in situ demonstration in the lung of the subunits of ENaC, and there are no functional data on the sodium transport proteins in type I cells in this study. However, new evidence from Johnson et al. (21) indicates that all three subunits of ENaC can be localized to the apical surface of alveolar type I cells in the rat lung and that freshly isolated type I cells have amiloride-inhibitable sodium uptake. Thus, although these new studies of type I cells have important limitations, there is now suggestive evidence that alveolar epithelial type I cells may participate in active sodium transport. Although the expression of the sodium transport proteins is less evident than in type II cells, the type I cells cover 95% of the alveolar surface, so their contribution to net fluid clearance could be significant.It has been clear for many years that cAMP stimulation markedly upregulates fluid clearance in the mature lungs of most species (31, 43), including the human lung (38, 39), and that catecholamine stimulation plays an important role in the clearance of fluid in the perinatal lung at the time of birth (2,13, 35). However, how cAMP stimulates sodium uptake across the apical membrane has not been entirely clarified. In this issue, Chen et al. (6) used single channel patch-clamp measurements of isolated rat alveolar epithelial type II cells to identify two different amiloride-sensitive sodium permeable channels: a 20-pS nonspecific cation channel and a 6-pS highly selective cation channel, which has similar properties to ENaC when all three subunits are expressed in Xenopus oocytes. In these studies, cAMP stimulation of rat alveolar type II cells activates protein kinase A, which in turn promotes an increase in the number of highly selective sodium channels without changing their open probablility. These data fit well with evidence that cAMP agonists can increase insertion of ENaC into the cell membrane (45), although the details of how transport and cell surface stability of ENaC is regulated is the subject of intense research by many investigators (37). In these studies of isolated alveolar type II cells, cAMP also stimulates an increase in intracellular calcium, an effect that increases the open probability but not the number of nonselective sodium channels, indicating that cAMP agonists can upregulate sodium uptake by all types of sodium channels, an interesting finding that fits well with experimental data from several intact lung studies (31). The exact mechanisms by which cAMP stimulation increases the density of highly selective sodium channels in the membrane is not clear, and neither are the mechanisms by which increased intracellular calcium activates the nonselective cation channels.Interestingly, also in this issue, Norlin and Folkesson (34) provide evidence that intracellular calcium may function as an important second messenger in mediating cAMP-stimulated fluid transport (by elevated endogenous catecholamines) across the distal lung epithelium of late gestational guinea pigs. These investigators have recently reported that induction of preterm labor 8 days before birth with oxytocin could enhance amiloride-senstive fluid clearance from the preterm lung, a finding of considerable significance (2). Thus there is growing evidence from both the fetal and adult lung that intracellular calcium is an important mechanism for mediating cAMP-stimulated fluid clearance from the lung.The mechanisms and pathways for cAMP-stimulated chloride secretion and absorption across the distal pulmonary epithelium have been the subject of some recent studies. One group of investigators concluded that cAMP-stimulated fluid clearance works in isolated rat alveolar type II cells by increasing chloride conductance, not by a direct effect on sodium channels (36). Others have contended that cAMP increases the overall conductance of the apical membrane for sodium, not by increasing chloride conductance (24). An alternative explanation is that the driving forces for both ions increase in parallel with a balanced uptake of the two ions and that the quantity of transport might depend on the number of open channels. Another related study in this issue by Collett et al. (8) reports that cAMP stimulation of fetal cells cultured in normoxic conditions stimulates an increase in sodium conductance. There is evidence that cAMP stimulation also increased apical chloride conductance by activating an anion channel sensitive to glibenclamide, suggesting activation of cystic fibrosis transmembrane conductance regulator (CFTR). In addition, another article in this issue by Lazrak et al. (26) demonstrates that there are functional CFTR-like channels in fetal distal lung epithelial cells. In the presence of an absorptive chloride gradient, permeabilization of the basolateral membrane reveals a cAMP-stimulated glibenclamide-sensitive apical membrane anion conductance similar to CFTR, and immunostaining provides evidence for the expression of CFTR-like channels in these fetal-derived epithelial cells. The results of all of these studies are particularly interesting in view of new work in the intact mouse and human lung that shows that CFTR is probably necessary for cAMP-mediated removal of fluid from the distal air spaces of the lung (12).The article by Morgan et al. in this issue (33) addresses the question of whether prolonged exposure to β-adrenergic agonists could diminish the capacity of β-adrenergic agonists to stimulate fluid clearance from the intact lung. This is an important issue because several experimental studies have shown that β2-agonist therapy can enhance the resolution of alveolar edema (15, 16, 23, 40, 41). In the Morgan study, there was no effect on the ability of a β2-agonist (terbutaline) to stimulate clearance with a low dose of isoproterenol infusion in rats for 48 h, although exposure to moderate and high doses of isoproterenol decreased and then eliminated the stimulatory effect of terbutaline. In the setting of acute pulmonary edema in the intensive care unit from either hydrostatic or lung injury pulmonary edema, it is unlikely that many patients would have already been exposed to sufficient β-adrenergic stimulation (exogenous or endogenous) to eliminate their responses to aerosolized β2-agonists. Conceivably, there may be some patients with persistent shock treated with vasopressors for several days who might be less responsive to aerosolized β-agonists. Clinical studies are needed to evaluate the response to aerosolized β-adrenergic agonists on the resolution of pulmonary edema (9).Another article in this issue addresses important issues concerning the role of the alveolar epithelium in regulating pH at the alveolar level. Joseph et al. (22) provide evidence that alveolar type II cell monolayers are relatively impermeable to acid/base fluxes primarily because of impermeability of the intercellular junctions and of the apical, not the basolateral, membrane. The principal basolateral acid exit pathway is sodium-hydrogen exchange, wheres proton uptake into the cells occurs across the basolateral cell membrane by a different undetermined mechanism. Thus the alveolar epithelium can maintain an apical to basolateral air space to blood pH gradient. The potential effect of subacute to chronic acidosis on alveolar epithelial fluid transport and the resolution of alveolar edema is unknown but might be important.The mechanisms by which glucocorticoids can upregulate sodium and fluid transport in distal lung epithelia have been the subject of several studies (1, 10, 14, 18, 25, 47). In this issue, Itani et al. (19) studied a human bronchiolar epithelial cell line (H441) as well as an alveolar epithelial cell line (A549). They used molecular and biophysical methods to establish that amiloride-sensitive transport is probably mediated through ENaC channels and that glucocorticoids upregulated sodium uptake by transcriptional effects of all three subunits of ENaC plus a transcriptional effect on the serum and glucocorticoid-regulated serine/threonine protein kinase, sgk1, an interesting observation since coexpression of sgk1 with the ENaC subunits in Xenopus oocytes significantly enhances Na current (5).Although most of the articles in this issue focus on mechanisms that can upregulate fluid clearance, Mairbäurl et al. (28) examine the effect of hypoxia on sodium transport in cultured rat type II cells. This study provides evidence that hypoxia in monolayers inhibits sodium absorption by reducing the rates of both apical amiloride-sensitive sodium uptake and basolateral sodium extrusion. Although the molecular basis for these effects was not explored in this study, one recent study reported that hypoxia in intact rats for 24 h decreased fluid clearance by 50%, but there was no decrease in gene or protein expression for any of the ENaC subunits or the α1- and β1-subunits of Na-K-ATPase (17, 48). Interestingly, in that study, cAMP stimulation overcame the effect of hypoxia, an issue that was not explored in the Mairbäurl study but may have clinical importance, especially since preliminary evidence indicates that β2-adrenergic agonist therapy may reduce the risk of developing high altitude pulmonary edema in hypoxic mountain climbers (42). From a mechanistic perspective, we need a better understanding of how hypoxia prevents normal sodium and chloride transport across alveolar epithelium. For example, does hypoxia reduce the insertion of ENaC and CFTR into the cell membrane and does it alter the function of Na-K-ATPase, and by what mechanisms? Similarly, we need to know why cAMP agonists can rapidly overcome the depressant effect of hypoxia on the fluid transport capacity of the distal lung epithelium.In summary, the articles on this issue's special topic provide valuable new insights into the mechanisms that are responsible for active reabsorption of fluid from the distal air spaces of the adult and fetal lung, an area of major physiological and clinical importance because of its relevance to unresolved lung edema in the newborn infant (2, 35) as well as the resolution of clinical pulmonary edema in adults with acute respiratory failure (32, 49).This editorial was supported by National Heart, Lung, and Blood Institute Grant HL-51856.FOOTNOTES10.1152/ajplung.00473.2001REFERENCES1 Barquin N, Ciccolella DE, Ridge KM, Sznajder JI.Dexamethasone upregulates the Na-K-ATPase in rat alveolar epithelial cells.Am J Physiol Lung Cell Mol Physiol2731997L825L830Link | ISI | Google Scholar2 Bland RD.Loss of liquid from the lung lumen in labor: more than a simple “squeeze”.Am J Physiol Lung Cell Mol Physiol2802001L602L605Link | ISI | Google Scholar3 Borok Z, Liebler JM, Lubman RL, Foster MJ, Zhou B, Li X, Zabski SM, Kim KJ, Crandall ED.Sodium transport proteins are expressed by rat alveolar epithelial type I cells.Am J Physiol Lung Cell Mol Physiol2822002L599L608Link | ISI | Google Scholar4 Cheek JM, Kim KJ, Crandall ED.Tight monolayers of rat alveolar epithelial cells: bioelectric properties and active sodium transport.Am J Physiol Cell Physiol2561989C688C693Link | ISI | Google Scholar5 Chen SY, Bhargava A, Mastroberardino L, Meijer OC, Wang J, Buse P, Firestone GL, Verrey F, Pearce D.Epithelial sodium channel regulated by aldosterone-induced protein sgk.Proc Natl Acad Sci USA96199925142519Crossref | PubMed | ISI | Google Scholar6 Chen X-J, Eaton DC, Jain L.β-adrenergic regulation of amiloride-sensitive lung sodium channels.Am J Physiol Lung Cell Mol Physiol2822002L609L620Link | ISI | Google Scholar7 Clerici C.Sodium transport in alveolar epithelial cells: modulation by O2 tension.Kidney Int Suppl651998S79S83PubMed | Google Scholar8 Collett A, Ramminger SJ, Olver RE, Wilson SM.β-adrenoceptor-mediated control of apical membrane conductive properties in fetal distal lung epithelia.Am J Physiol Lung Cell Mol Physiol2822002L621L630Link | ISI | Google Scholar9 Crandall ED, Matthay MA.Alveolar epithelial transport. Basic science to clinical medicine.Am J Respir Crit Care Med163200110211029Crossref | PubMed | ISI | Google Scholar10 Dagenais A, Denis C, Vives MF, Girouard S, Masse C, Nguyen T, Yamagata T, Grygorczyk C, Kothary R, Berthiaume Y.Modulation of α-ENaC and α1-Na+-K+-ATPase by cAMP and dexamethasone in alveolar epithelial cells.Am J Physiol Lung Cell Mol Physiol2812001L217L230Link | ISI | Google Scholar11 Dobbs LG, Gonzalez R, Matthay MA, Carter EP, Allen L, Verkman AS.Highly water permeable type I alveolar epithelial cells confer high water permeability between the airspace and vasculature in rat lung.Proc Natl Acad Sci USA95199829912996Crossref | PubMed | ISI | Google Scholar12 Fang XH, Fukuda N, Barbry P, Sartori C, Verkman AS, Matthay MA.Novel role for CFTR in fluid absorption from the distal airspaces of the lung.J Gen Physiol1192002199207Crossref | PubMed | ISI | Google Scholar13 Finley N, Norlin A, Baines DL, Folkesson HG.Alveolar epithelial fluid clearance is mediated by endogenous catecholamines at birth in guinea pigs.J Clin Invest1011998972981Crossref | PubMed | ISI | Google Scholar14 Folkesson HG, Norlin A, Wang Y, Abedinpour P, Matthay MA.Dexamethasone and thyroid hormone pretreatment upregulate alveolar epithelial fluid clearance in adult rats.J Appl Physiol882000416424Link | ISI | Google Scholar15 Frank JA, Wang Y, Osorio O, Matthay MA.β-adrenergic agonist therapy accelerates the resolution of hydrostatic pulmonary edema in sheep and rats.J Appl Physiol89200012551265Link | ISI | Google Scholar16 Garat C, Meignan M, Matthay MA, Luo DF, Jayr C.Alveolar epithelial fluid clearance mechanisms are intact after moderate hyperoxic lung injury in rats.Chest111199713811388Crossref | PubMed | ISI | Google Scholar17 Hardiman KM, Matalon S.Modification of sodium transport and alveolar fluid clearance by hypoxia: mechanisms and physiological implications.Am J Respir Cell Mol Biol252001538541Crossref | PubMed | ISI | Google Scholar18 Ingbar DH, Duvick S, Savick SK, Schellhase DE, Detterding R, Jamieson JD, Shannon JM.Developmental changes of fetal rat lung Na-K-ATPase after maternal treatment with dexamethasone.Am J Physiol Lung Cell Mol Physiol2721997L665L672Link | ISI | Google Scholar19 Itani OA, Auerback SD, Husted RF, Volk KA, Ageloff S, Knepper MA, Stokes JB, Thomas CP.Glucocorticoid-stimulated Na+ transport is associated with regulated ENaC and sgk1 expression.Am J Physiol Lung Cell Mol Physiol2822002L631L641Link | ISI | Google Scholar20 Jain L, Chen XJ, Malik B, Al-Khalili O, Eaton DC.Antisense oligonucleotides against the α-subunit of ENaC decrease lung epithelial cation channel activity.Am J Physiol Lung Cell Mol Physiol2761999L1046L1051Link | ISI | Google Scholar21 Johnson M, Widdicombe J, Allen L, Barbry P, and Dobbs L. Alveolar epithelial type I cells contain transport proteins and transport sodium, supporting an active role for type I cells in regulation of lung liquid hemostatis. Proc Natl Acad Sci. In press.Google Scholar22 Joseph D, Tirmizi O, Zhang X-L, Crandall ED, Lubman RL.Polarity of alveolar epithelial cell acid-base permeability.Am J Physiol Lung Cell Mol Physiol2822002L675L683Link | ISI | Google Scholar23 Lasnier JM, Wangensteen OD, Schmitz LS, Gross CR, Ingbar DH.Terbutaline stimulates alveolar fluid resorption in hyperoxic lung injury.J Appl Physiol81199617231729Link | ISI | Google Scholar24 Lazrak A, Nielsen VG, Matalon S.Mechanisms of increased Na+ transport in ATII cells by cAMP: we agree to disagree and do more experiments.Am J Physiol Lung Cell Mol Physiol2782000L233L238Link | ISI | Google Scholar25 Lazrak A, Samanta A, Venetsanou K, Barbry P, Matalon S.Modification of biophysical properties of lung epithelial Na+ channels by dexamethasone.Am J Physiol Cell Physiol2792000C762C770Link | ISI | Google Scholar26 Lazrak A, Ulrich T, Carpantanto M, Ware J, Chen L, Venglarik CJ, Matalon S.cAMP regulation of Cl− and HCO3− secretion across rat fetal distal lung epithelial cells.Am J Physiol Lung Cell Mol Physiol2822002L650L658Link | ISI | Google Scholar27 Ma T, Fukuda N, Song Y, Matthay MA, Verkman AS.Lung fluid transport in aquaporin-5 knockout mice.J Clin Invest105200093100Crossref | PubMed | ISI | Google Scholar28 Mairbäurl H, Mayer K, Kim K-J, Borok Z, Bärtsch P, Crandall ED.Hypoxia decreases active Na transport across primary rat alveolar epithelial cell monolayers.Am J Physiol Lung Cell Mol Physiol2822002L659L665Link | ISI | Google Scholar29 Matalon S.Mechanisms and regulation of ion transport in adult mammalian alveolar type II pneumocytes.Am J Physiol Cell Physiol2611991C727C738Link | ISI | Google Scholar30 Matalon S, O'Brodovich H.Sodium channels in alveolar epithelial cells: molecular characterization, biophysical properties, and physiological significance.Annu Rev Physiol611999627661Crossref | PubMed | ISI | Google Scholar31 Matthay MA, Folkesson HG, Verkman AS.Salt and water transport across alveolar and distal airway epithelium in the adult lung.Am J Physiol Lung Cell Mol Physiol2701996L487L503Link | ISI | Google Scholar32 Matthay MA, Wiener-Kronish JP.Intact epithelial barrier function is critical for the resolution of alveolar edema in humans.Am Rev Respir Dis142199012501257Crossref | PubMed | ISI | Google Scholar33 Morgan EE, Hodnichak CM, Stader SM, Maender KC, Boja JW, Folkesson HG, Maron MB.Prolonged isoproterenol infusion impairs the ability of β2-agonists to increase alveolar liquid clearance.Am J Physiol Lung Cell Mol Physiol2822002L666L674Link | ISI | Google Scholar34 Norlin A, Folkesson HG.Ca2+-dependent stimulation of alveolar fluid clearance in near-term fetal guinea pigs.Am J Physiol Lung Cell Mol Physiol2822002L642L649Link | ISI | Google Scholar35 O'Brodovich H.Fetal lung liquid secretion: insights using the tools of inhibitors and genetic knock-out experiments.Am J Respir Cell Mol Biol252001810Crossref | PubMed | ISI | Google Scholar36 O'Grady SM, Jiang X, Ingbar DH.Cl channel activation is necessary for stimulation of Na transport in adult alveolar epithelial cells.Am J Physiol Lung Cell Mol Physiol2782000L239L244Link | ISI | Google Scholar37 Rotin D, Kanelis V, Schild L.Trafficking and cell surface stability of ENaC.Am J Physiol Renal Physiol2812001F391F399Link | ISI | Google Scholar38 Sakuma T, Folkesson HG, Suzuki S, Okaniwa G, Fujimura S, Matthay MA.β-adrenergic agonist stimulated alveolar fluid clearance in ex vivo human and rat lungs.Am J Respir Crit Care Med1551997506512Crossref | PubMed | ISI | Google Scholar39 Sakuma T, Okaniwa G, Nakada T, Nishimura T, Fujimura S, Matthay MA.Alveolar fluid clearance in the resected human lung.Am J Respir Crit Care Med1501994305310Crossref | PubMed | ISI | Google Scholar40 Saldias FJ, Comellas A, Ridge KM, Lecuona E, Sznajder JI.Isoproterenol improves ability of lung to clear edema in rats exposed to hyperoxia.J Appl Physiol8719993035Link | ISI | Google Scholar41 Saldias FJ, Lecuona E, Comellas AP, Ridge KM, Sznajder JI.Dopamine restores lung ability to clear edema in rats exposed to hyperoxia.Am J Respir Crit Care Med1591999626633Crossref | PubMed | ISI | Google Scholar42 Sartori C, Lipp E, Duplain H, Egli M, Hutter D, Alleman Y, Nicod P, Scherrer U.Prevention of high altitude pulmonary edema by β-adrenergic stimulation of the alveolar transepithelial sodium transport (Abstract).Am J Resp Crit Care Med1612000415AGoogle Scholar43 Saumon G, Basset G.Electrolyte and fluid transport across the mature alveolar epithelium.J Appl Physiol741993115Link | ISI | Google Scholar44 Schneeberger EE, McCarthy KM.Cytochemical localization of Na+-K+-ATPase in rat type II pneumocytes.J Appl Physiol60198615841589Link | ISI | Google Scholar45 Snyder PM.Liddle's syndrome mutations disrupt cAMP-mediated translocation of the epithelial Na+ channel to the cell surface.J Clin Invest10520004553Crossref | PubMed | ISI | Google Scholar46 Song Y, Fukuda N, Bai C, Ma T, Matthay MA, Verkman AS.Role of aquaporins in alveolar fluid clearance in neonatal and adult lung, and in oedema formation following acute lung injury in mice.J Physiol (Lond)5252000771779Crossref | Google Scholar47 Tchepichev S, Ueda J, Canessa C, Rossier BC, O'Brodovich H.Lung epithelial Na channel subunits are differentially regulated during development and by steroids.Am J Physiol Cell Physiol2691995C805C812Link | ISI | Google Scholar48 Vivona M, Matthay MA, Friedlander G, Clerici C.Hypoxia reduces alveolar epithelial sodium and fluid transport in rats: reversal by β2-adrenergic agonist treatment.Am J Resp Cell Mol Biol252001554561Crossref | PubMed | ISI | Google Scholar49 Ware LB, Matthay MA.Alveolar fluid clearance is impaired in the majority of patients with acute lung injury and the acute respiratory distress syndrome.Am J Respir Crit Care Med163200113761383Crossref | PubMed | ISI | Google Scholar Download PDF Back to Top Next FiguresReferencesRelatedInformationCited ByHuman Hemorrhagic Pulmonary Leptospirosis: Pathological Findings and Pathophysiological Correlations12 August 2013 | PLoS ONE, Vol. 8, No. 8Cell Culture Models of the Respiratory Tract Relevant to Pulmonary Drug DeliveryJournal of Aerosol Medicine, Vol. 18, No. 2Mechanisms of ventilator-induced lung injury in premature infantsSeminars in Neonatology, Vol. 7, No. 5 More from this issue > Volume 282Issue 4April 2002Pages L595-L598 Copyright & PermissionsCopyright © 2002 the American Physiological Societyhttps://doi.org/10.1152/ajplung.00473.2001History Published online 1 April 2002 Published in print 1 April 2002 Metrics