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THIS ISSUE of the American Journal of Physiology-Lung Cellular and Molecular Physiology features 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
1- and
1-, but not the
2-,
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
-subunit of the epithelial sodium
channel (ENaC) in purified alveolar type I cells. The investigators
also demonstrate expression of the
1-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 -adrenergic agonists could diminish the capacity of
-adrenergic agonists to stimulate fluid clearance from the intact lung. This is an important issue because several experimental studies have shown that
2-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
2-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
-adrenergic stimulation (exogenous or endogenous) to
eliminate their responses to aerosolized
2-agonists.
Conceivably, there may be some patients with persistent shock treated
with vasopressors for several days who might be less responsive to aerosolized
-agonists. Clinical studies are needed to evaluate the
response to aerosolized
-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 1- and
1-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
2-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).
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ACKNOWLEDGEMENTS |
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This editorial was supported by National Heart, Lung, and Blood Institute Grant HL-51856.
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FOOTNOTES |
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10.1152/ajplung.00473.2001
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REFERENCES |
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---|
1.
Barquin, N,
Ciccolella DE,
Ridge KM,
and
Sznajder JI.
Dexamethasone upregulates the Na-K-ATPase in rat alveolar epithelial cells.
Am J Physiol Lung Cell Mol Physiol
273:
L825-L830,
1997[ISI][Medline].
2.
Bland, RD.
Loss of liquid from the lung lumen in labor: more than a simple "squeeze".
Am J Physiol Lung Cell Mol Physiol
280:
L602-L605,
2001
3.
Borok, Z,
Liebler JM,
Lubman RL,
Foster MJ,
Zhou B,
Li X,
Zabski SM,
Kim KJ,
and
Crandall ED.
Sodium transport proteins are expressed by rat alveolar epithelial type I cells.
Am J Physiol Lung Cell Mol Physiol
282:
L599-L608,
2002
4.
Cheek, JM,
Kim KJ,
and
Crandall ED.
Tight monolayers of rat alveolar epithelial cells: bioelectric properties and active sodium transport.
Am J Physiol Cell Physiol
256:
C688-C693,
1989
5.
Chen, SY,
Bhargava A,
Mastroberardino L,
Meijer OC,
Wang J,
Buse P,
Firestone GL,
Verrey F,
and
Pearce D.
Epithelial sodium channel regulated by aldosterone-induced protein sgk.
Proc Natl Acad Sci USA
96:
2514-2519,
1999
6.
Chen, X-J,
Eaton DC,
and
Jain L.
-adrenergic regulation of amiloride-sensitive lung sodium channels.
Am J Physiol Lung Cell Mol Physiol
282:
L609-L620,
2002
7.
Clerici, C.
Sodium transport in alveolar epithelial cells: modulation by O2 tension.
Kidney Int Suppl
65:
S79-S83,
1998[Medline].
8.
Collett, A,
Ramminger SJ,
Olver RE,
and
Wilson SM.
-adrenoceptor-mediated control of apical membrane conductive properties in fetal distal lung epithelia.
Am J Physiol Lung Cell Mol Physiol
282:
L621-L630,
2002
9.
Crandall, ED,
and
Matthay MA.
Alveolar epithelial transport. Basic science to clinical medicine.
Am J Respir Crit Care Med
163:
1021-1029,
2001
10.
Dagenais, A,
Denis C,
Vives MF,
Girouard S,
Masse C,
Nguyen T,
Yamagata T,
Grygorczyk C,
Kothary R,
and
Berthiaume Y.
Modulation of -ENaC and
1-Na+-K+-ATPase by cAMP and dexamethasone in alveolar epithelial cells.
Am J Physiol Lung Cell Mol Physiol
281:
L217-L230,
2001
11.
Dobbs, LG,
Gonzalez R,
Matthay MA,
Carter EP,
Allen L,
and
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 USA
95:
2991-2996,
1998
12.
Fang, XH,
Fukuda N,
Barbry P,
Sartori C,
Verkman AS,
and
Matthay MA.
Novel role for CFTR in fluid absorption from the distal airspaces of the lung.
J Gen Physiol
119:
199-207,
2002
13.
Finley, N,
Norlin A,
Baines DL,
and
Folkesson HG.
Alveolar epithelial fluid clearance is mediated by endogenous catecholamines at birth in guinea pigs.
J Clin Invest
101:
972-981,
1998
14.
Folkesson, HG,
Norlin A,
Wang Y,
Abedinpour P,
and
Matthay MA.
Dexamethasone and thyroid hormone pretreatment upregulate alveolar epithelial fluid clearance in adult rats.
J Appl Physiol
88:
416-424,
2000
15.
Frank, JA,
Wang Y,
Osorio O,
and
Matthay MA.
-adrenergic agonist therapy accelerates the resolution of hydrostatic pulmonary edema in sheep and rats.
J Appl Physiol
89:
1255-1265,
2000
16.
Garat, C,
Meignan M,
Matthay MA,
Luo DF,
and
Jayr C.
Alveolar epithelial fluid clearance mechanisms are intact after moderate hyperoxic lung injury in rats.
Chest
111:
1381-1388,
1997
17.
Hardiman, KM,
and
Matalon S.
Modification of sodium transport and alveolar fluid clearance by hypoxia: mechanisms and physiological implications.
Am J Respir Cell Mol Biol
25:
538-541,
2001
18.
Ingbar, DH,
Duvick S,
Savick SK,
Schellhase DE,
Detterding R,
Jamieson JD,
and
Shannon JM.
Developmental changes of fetal rat lung Na-K-ATPase after maternal treatment with dexamethasone.
Am J Physiol Lung Cell Mol Physiol
272:
L665-L672,
1997
19.
Itani, OA,
Auerback SD,
Husted RF,
Volk KA,
Ageloff S,
Knepper MA,
Stokes JB,
and
Thomas CP.
Glucocorticoid-stimulated Na+ transport is associated with regulated ENaC and sgk1 expression.
Am J Physiol Lung Cell Mol Physiol
282:
L631-L641,
2002
20.
Jain, L,
Chen XJ,
Malik B,
Al-Khalili O,
and
Eaton DC.
Antisense oligonucleotides against the -subunit of ENaC decrease lung epithelial cation channel activity.
Am J Physiol Lung Cell Mol Physiol
276:
L1046-L1051,
1999
21.
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.
22.
Joseph, D,
Tirmizi O,
Zhang X-L,
Crandall ED,
and
Lubman RL.
Polarity of alveolar epithelial cell acid-base permeability.
Am J Physiol Lung Cell Mol Physiol
282:
L675-L683,
2002
23.
Lasnier, JM,
Wangensteen OD,
Schmitz LS,
Gross CR,
and
Ingbar DH.
Terbutaline stimulates alveolar fluid resorption in hyperoxic lung injury.
J Appl Physiol
81:
1723-1729,
1996
24.
Lazrak, A,
Nielsen VG,
and
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 Physiol
278:
L233-L238,
2000
25.
Lazrak, A,
Samanta A,
Venetsanou K,
Barbry P,
and
Matalon S.
Modification of biophysical properties of lung epithelial Na+ channels by dexamethasone.
Am J Physiol Cell Physiol
279:
C762-C770,
2000
26.
Lazrak, A,
Ulrich T,
Carpantanto M,
Ware J,
Chen L,
Venglarik CJ,
and
Matalon S.
cAMP regulation of Cl and HCO
27.
Ma, T,
Fukuda N,
Song Y,
Matthay MA,
and
Verkman AS.
Lung fluid transport in aquaporin-5 knockout mice.
J Clin Invest
105:
93-100,
2000
28.
Mairbäurl, H,
Mayer K,
Kim K-J,
Borok Z,
Bärtsch P,
and
Crandall ED.
Hypoxia decreases active Na transport across primary rat alveolar epithelial cell monolayers.
Am J Physiol Lung Cell Mol Physiol
282:
L659-L665,
2002
29.
Matalon, S.
Mechanisms and regulation of ion transport in adult mammalian alveolar type II pneumocytes.
Am J Physiol Cell Physiol
261:
C727-C738,
1991
30.
Matalon, S,
and
O'Brodovich H.
Sodium channels in alveolar epithelial cells: molecular characterization, biophysical properties, and physiological significance.
Annu Rev Physiol
61:
627-661,
1999[ISI][Medline].
31.
Matthay, MA,
Folkesson HG,
and
Verkman AS.
Salt and water transport across alveolar and distal airway epithelium in the adult lung.
Am J Physiol Lung Cell Mol Physiol
270:
L487-L503,
1996
32.
Matthay, MA,
and
Wiener-Kronish JP.
Intact epithelial barrier function is critical for the resolution of alveolar edema in humans.
Am Rev Respir Dis
142:
1250-1257,
1990[ISI][Medline].
33.
Morgan, EE,
Hodnichak CM,
Stader SM,
Maender KC,
Boja JW,
Folkesson HG,
and
Maron MB.
Prolonged isoproterenol infusion impairs the ability of 2-agonists to increase alveolar liquid clearance.
Am J Physiol Lung Cell Mol Physiol
282:
L666-L674,
2002
34.
Norlin, A,
and
Folkesson HG.
Ca2+-dependent stimulation of alveolar fluid clearance in near-term fetal guinea pigs.
Am J Physiol Lung Cell Mol Physiol
282:
L642-L649,
2002
35.
O'Brodovich, H.
Fetal lung liquid secretion: insights using the tools of inhibitors and genetic knock-out experiments.
Am J Respir Cell Mol Biol
25:
8-10,
2001
36.
O'Grady, SM,
Jiang X,
and
Ingbar DH.
Cl channel activation is necessary for stimulation of Na transport in adult alveolar epithelial cells.
Am J Physiol Lung Cell Mol Physiol
278:
L239-L244,
2000
37.
Rotin, D,
Kanelis V,
and
Schild L.
Trafficking and cell surface stability of ENaC.
Am J Physiol Renal Physiol
281:
F391-F399,
2001
38.
Sakuma, T,
Folkesson HG,
Suzuki S,
Okaniwa G,
Fujimura S,
and
Matthay MA.
-adrenergic agonist stimulated alveolar fluid clearance in ex vivo human and rat lungs.
Am J Respir Crit Care Med
155:
506-512,
1997[Abstract].
39.
Sakuma, T,
Okaniwa G,
Nakada T,
Nishimura T,
Fujimura S,
and
Matthay MA.
Alveolar fluid clearance in the resected human lung.
Am J Respir Crit Care Med
150:
305-310,
1994[Abstract].
40.
Saldias, FJ,
Comellas A,
Ridge KM,
Lecuona E,
and
Sznajder JI.
Isoproterenol improves ability of lung to clear edema in rats exposed to hyperoxia.
J Appl Physiol
87:
30-35,
1999
41.
Saldias, FJ,
Lecuona E,
Comellas AP,
Ridge KM,
and
Sznajder JI.
Dopamine restores lung ability to clear edema in rats exposed to hyperoxia.
Am J Respir Crit Care Med
159:
626-633,
1999
42.
Sartori, C,
Lipp E,
Duplain H,
Egli M,
Hutter D,
Alleman Y,
Nicod P,
and
Scherrer U.
Prevention of high altitude pulmonary edema by -adrenergic stimulation of the alveolar transepithelial sodium transport (Abstract).
Am J Resp Crit Care Med
161:
415A,
2000.
43.
Saumon, G,
and
Basset G.
Electrolyte and fluid transport across the mature alveolar epithelium.
J Appl Physiol
74:
1-15,
1993[Abstract].
44.
Schneeberger, EE,
and
McCarthy KM.
Cytochemical localization of Na+-K+-ATPase in rat type II pneumocytes.
J Appl Physiol
60:
1584-1589,
1986
45.
Snyder, PM.
Liddle's syndrome mutations disrupt cAMP-mediated translocation of the epithelial Na+ channel to the cell surface.
J Clin Invest
105:
45-53,
2000
46.
Song, Y,
Fukuda N,
Bai C,
Ma T,
Matthay MA,
and
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)
525:
771-779,
2000
47.
Tchepichev, S,
Ueda J,
Canessa C,
Rossier BC,
and
O'Brodovich H.
Lung epithelial Na channel subunits are differentially regulated during development and by steroids.
Am J Physiol Cell Physiol
269:
C805-C812,
1995[Abstract].
48.
Vivona, M,
Matthay MA,
Friedlander G,
and
Clerici C.
Hypoxia reduces alveolar epithelial sodium and fluid transport in rats: reversal by 2-adrenergic agonist treatment.
Am J Resp Cell Mol Biol
25:
554-561,
2001
49.
Ware, LB,
and
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 Med
163:
1376-1383,
2001
Michael A. Matthay, Associate Editor American Journal of Physiology- Lung Cellular and Molecular Physiology April 2002, Volume 282 (26) |