Alveolar type 1 cells express the alpha2 Na,K-ATPase, which contributes to lung liquid clearance.

The alveolar epithelium is composed of alveolar type 1 (AT1) and alveolar type 2 (AT2) cells, which represent approximately 95% and approximately 5% of the alveolar surface area, respectively. Lung liquid clearance is driven by the osmotic gradient generated by the Na,K-ATPase. AT2 cells have been shown to express the alpha1 Na,K-ATPase. We postulated that AT1 cells, because of their larger surface area, should be important in the regulation of active Na+ transport. By immunofluorescence and electron microscopy, we determined that AT1 cells express both the alpha1 and alpha2 Na,K-ATPase isoforms. In isolated, ouabain-perfused rat lungs, the alpha2 Na,K-ATPase in AT1 cells mediated 60% of the basal lung liquid clearance. The beta-adrenergic agonist isoproterenol increased lung liquid clearance by preferentially upregulating the alpha2 Na,K-ATPase protein abundance in the plasma membrane and activity in alveolar epithelial cells (AECs). Rat AECs and human A549 cells were infected with an adenovirus containing the rat Na,K-ATPase alpha2 gene (Adalpha2), which resulted in the overexpression of the alpha2 Na,K-ATPase protein and caused a 2-fold increase in Na,K-ATPase activity. Spontaneously breathing rats were also infected with Adalpha2, which increased alpha2 protein abundance and resulted in a approximately 250% increase in lung liquid clearance. These studies provide the first evidence that alpha2 Na,K-ATPase in AT1 cells contributes to most of the active Na+ transport and lung liquid clearance, which can be further increased by stimulation of the beta-adrenergic receptor or by adenovirus-mediated overexpression of the alpha2 Na,K-ATPase.

Alveolar fluid reabsorption is impaired by increased left atrial pressures in rats.

Cardiogenic pulmonary edema results from increased hydrostatic pressures across the pulmonary circulation. We studied active Na(+) transport and alveolar fluid reabsorption in isolated perfused rat lungs exposed to increasing levels of left atrial pressure (LAP; 0--20 cmH(2)O) for 60 min. Active Na(+) transport and fluid reabsorption did not change when LAP was increased to 5 and 10 cmH(2)O compared with that in the control group (0 cmH(2)O; 0.50 +/- 0.02 ml/h). However, alveolar fluid reabsorption decreased by approximately 50% in rat lungs in which the LAP was raised to 15 cmH(2)O (0.25 +/- 0.03 ml/h). The passive movement of small solutes ((22)Na(+) and [(3)H]mannitol) and large solutes (FITC-albumin) increased progressively in rats exposed to higher LAP. There was no significant edema in lungs with a LAP of 15 cmH(2)O when all active Na(+) transport was inhibited by hypothermia or amiloride (10(-4) M) and ouabain (5 x 10(-4) M). However, when LAP was increased to 20 cmH(2)O, there was a significant influx of fluid (-0.69 +/- 0.10 ml/h), precluding the ability to assess the rate of fluid reabsorption. In additional studies, LAP was decreased from 15 to 0 cmH(2)O in the second and third hours of the experimental protocol, which resulted in normalization of lung permeability to solutes and alveolar fluid reabsorption. These data suggest that in an increased LAP model, the changes in clearance and permeability are transient, reversible, and directly related to high pulmonary circulation pressures.

Dopamine regulates Na-K-ATPase in alveolar epithelial cells via MAPK-ERK-dependent mechanisms.

Dopamine (DA) increases lung edema clearance by regulating vectorial Na+ transport and Na-K-ATPase in the pulmonary epithelium. We studied the role of the mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (ERK) pathway in the DA regulation of Na-K-ATPase in alveolar epithelial cells (AEC). Incubation of AEC with DA resulted in a rapid stimulation of ERK activity via dopaminergic type 2 receptors. Analysis of total RNA and protein showed a 1.5-fold increase in the Na-K-ATPase beta1-subunit mRNA levels and up to a fivefold increase in beta1-subunit protein abundance after DA stimulation, which was blocked by the MAPK kinase (MEK) inhibitors PD-98059 and U-0126. Also, the DA-ERK pathway stimulated the synthesis of a green fluorescent protein reporter gene driven by the beta1-subunit promoter, which indicates that DA regulates the Na-K-ATPase beta1-subunit at the transcriptional level. The DA-mediated increase in beta1-subunit mRNA protein resulted in an increase in functional Na pumps in the basolateral membranes of alveolar type II cells. These results suggest that the MAPK-ERK pathway is an important mechanism in the regulation of Na-K-ATPase by DA in the alveolar epithelium.

Catecholamines increase lung edema clearance in rats with increased left atrial pressure.

During hydrostatic pulmonary edema, active Na(+) transport and alveolar fluid reabsorption are decreased. Dopamine (DA) and isoproterenol (ISO) have been shown to increase active Na(+) transport in rat lungs by upregulating Na(+)-K(+)-ATPase in the alveolar epithelium. We studied the effects of DA and ISO in isolated rat lungs with increased left atrial pressure (Pla = 15 cmH(2)O) compared with control rats with normal Pla (Pla = 0). Alveolar fluid reabsorption decreased from control value of 0.51 +/- 0.02 to 0.27 +/- 0.02 ml/h when Pla was increased to 15 cmH(2)O (P < 0.001). DA and ISO increased the alveolar fluid reabsorption back to control levels. Treatment with the D(1) antagonist SCH-23390 inhibited the stimulatory effects of DA (0.30 +/- 0.02 ml/h), whereas fenoldopam, a specific D(1)-receptor agonist, increased alveolar fluid reabsorption in rats exposed to Pla of 15 cmH(2)O (0.47 +/- 0.04 ml/h). Propranolol, a beta-adrenergic-receptor antagonist, blocked the stimulatory effects of ISO; however, it did not affect alveolar fluid reabsorption in control or DA-treated rats. Amiloride (a Na(+) channel blocker) and ouabain (a Na(+)-K(+)-ATPase inhibitor), either alone or together, inhibited the stimulatory effects of DA. Colchicine, which disrupts the cellular microtubular transport of ion-transporting proteins to the plasma membrane, inhibited the stimulatory effects of DA, whereas the isomer beta-lumicolchicine did not block the stimulatory effects of DA. These data suggest that DA and ISO increase alveolar fluid reabsorption in a model of increased Pla by regulating active Na(+) transport in rat alveolar epithelium. The effects of DA and ISO are mediated by the activation of dopaminergic D(1) receptors and the beta-adrenergic receptors, respectively.

beta-agonists regulate Na,K-ATPase via novel MAPK/ERK and rapamycin-sensitive pathways.

We studied whether the beta-adrenergic agonist, isoproterenol (ISO), regulates Na,K-ATPase in alveolar epithelial cells (AEC) via a mitogen-activated protein kinase (MAPK)/extracellular signaling related kinase (ERK) dependent pathway. ISO increased ERK activity in AEC by 10 min via a beta-adrenergic receptor, protein kinase A (PKA)-dependent mechanism. Activation of the MAPK pathway by ISO, resulted in increased Na,K-ATPase beta1 and alpha1 subunit protein abundance in whole cell lysates, which resulted in functional Na, K-ATPases at the basolateral membranes. ISO did not change the alpha1 or beta1 mRNA steady state levels, but rapamycin, the inhibitor of the mammalian target of rapamycin, also blocked the ISO-mediated increase in Na,K-ATPase total protein abundance, suggesting a posttranscriptional regulation. We conclude that ISO, regulates the Na,K-ATPase in AEC via PKA, ERK and rapamycin-sensitive mechanisms.

beta-adrenergic stimulation restores rat lung ability to clear edema in ventilator-associated lung injury.

Mechanical ventilation with high tidal volume (HVT) causes lung injury and decreases the lung's ability to clear edema in rats. beta-adrenergic agonists increase active Na(+) transport and lung edema clearance in normal rat lungs by stimulating apical Na(+) channels and basolateral Na,K-ATPase in alveolar epithelial cells. We studied whether beta-adrenergic agonists could restore lung edema clearance in rats ventilated with HVT (40 ml/kg, peak airway pressure of 35 cm H(2)O) for 40 min. The ability of rat lungs to clear edema decreased by approximately 50% after 40 min of HVT ventilation. Terbutaline (TERB) and isoproterenol (ISO) increased lung edema clearance in control nonventilated rats (from 0.50 +/- 0. 02 ml/h to 0.81 +/- 0.04 ml/h and 0.99 +/- 0.05 ml/h, respectively) and restored the lung's ability to clear edema in HVT ventilated rats (from 0.25 +/- 0.03 ml/h to 0.64 +/- 0.02 ml/h and 0.88 +/- 0. 09 ml/h, respectively). Disruption of cell microtubular transport system by colchicine inhibited the stimulatory effects of ISO in HVT ventilated rats, whereas beta-lumicolchicine did not affect beta-adrenergic stimulation. The Na,K-ATPase alpha(1)- and beta(1)-subunit mRNA steady state levels were not affected by incubation with ISO for 60 min in alveolar type II cells isolated from control and HVT ventilated rats. The data suggest that beta-adrenergic agonists increased alveolar fluid reabsorption in rats ventilated with HVT by promoting recruitment of ion-transporting proteins from intracellular pools to the plasma membrane of alveolar epithelial cells.

Aldosterone regulates Na,K-ATPase and increases lung edema clearance in rats.

Aldosterone increases the Na,K-ATPase function in renal cells involved in active Na(+) transport. Because the alveolar type 2 (AT2) cells participate in active Na(+) transport, we studied whether aldosterone regulates the Na,K-ATPase in rat AT2 cells and whether aldosterone delivered by aerosols to spontaneously breathing rats affects edema clearance in a model of isolated-perfused lungs. The AT2 cells treated with aldosterone had increased Na,K-ATPase beta1-subunit mRNA and protein, which was associated with a 4-fold increase in the Na,K-ATPase hydrolytic activity and the ouabain-sensitive (86)Rb(+) uptake. In physiologic experiments, 24 h after aldosterone was delivered by aerosols to the rat air spaces, the active Na(+) transport and lung edema clearance increased by approximately 53% as compared with control rats and rats in which saline aerosols were delivered. The data suggest that increased active Na(+) transport and lung edema clearance induced by aldosterone is probably due to Na,K-ATPase regulation in alveolar epithelial cells. Conceivably, aldosterone may be used as a strategy to increase lung edema clearance.

Stimulation of the dopamine 1 receptor increases lung edema clearance.

We previously reported that lung edema clearance was stimulated by dopamine (DA). The purpose of this study was to determine whether the DA-mediated stimulation of edema clearance occurs via an adrenergic or dopaminergic regulation of alveolar epithelial Na, K-ATPase. When isolated perfused rat lungs were coinstilled with DA and SCH 23390 (a specific D(1) receptor antagonist), there was a dose-dependent attenuation of the stimulatory effects of DA. Coinstillation with S-sulpiride (a specific D(2) receptor antagonist) or propranolol (a beta-adrenergic antagonist) did not alter DA-stimulated clearance. Similarly, the specific dopaminergic D(1) agonist fenoldopam increased lung edema clearance, but quinpirole (a specific dopaminergic D(2) agonist) did not. (125)I-SCH 23982 binding studies suggested that D(1) receptors are expressed on alveolar type II (ATII) cells with an apparent dissociation constant (K(d)) of 4.4 nM and binding maximum (Bmax) 9.8 pmol/mg. Consistent with these results, the D(1) receptor messenger RNA (mRNA) and protein were detected in ATII cells by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis, respectively. These data demonstrate a novel mechanism involving the activation of dopaminergic D(1) receptors which mediates DA-stimulated edema removal from rat lungs.

Mechanical stretching of alveolar epithelial cells increases Na(+)-K(+)-ATPase activity.

Alveolar epithelial cells effect edema clearance by transporting Na(+) and liquid out of the air spaces. Active Na(+) transport by the basolaterally located Na(+)-K(+)-ATPase is an important contributor to lung edema clearance. Because alveoli undergo cyclic stretch in vivo, we investigated the role of cyclic stretch in the regulation of Na(+)-K(+)-ATPase activity in alveolar epithelial cells. Using the Flexercell Strain Unit, we exposed a cell line of murine lung epithelial cells (MLE-12) to cyclic stretch (30 cycles/min). After 15 min of stretch (10% mean strain), there was no change in Na(+)-K(+)-ATPase activity, as assessed by (86)Rb(+) uptake. By 30 min and after 60 min, Na(+)-K(+)-ATPase activity was significantly increased. When cells were treated with amiloride to block amiloride-sensitive Na(+) entry into cells or when cells were treated with gadolinium to block stretch-activated, nonselective cation channels, there was no stimulation of Na(+)-K(+)-ATPase activity by cyclic stretch. Conversely, cells exposed to Nystatin, which increases Na(+) entry into cells, demonstrated increased Na(+)-K(+)-ATPase activity. The changes in Na(+)-K(+)-ATPase activity were paralleled by increased Na(+)-K(+)-ATPase protein in the basolateral membrane of MLE-12 cells. Thus, in MLE-12 cells, short-term cyclic stretch stimulates Na(+)-K(+)-ATPase activity, most likely by increasing intracellular Na(+) and by recruitment of Na(+)-K(+)-ATPase subunits from intracellular pools to the basolateral membrane.

Isoproterenol improves ability of lung to clear edema in rats exposed to hyperoxia.

Exposure of adult rats to 100% O(2) results in lung injury and decreases active sodium transport and lung edema clearance. It has been reported that beta-adrenergic agonists increase lung edema clearance in normal rat lungs by upregulating alveolar epithelial Na(+)-K(+)-ATPase function. This study was designed to examine whether isoproterenol (Iso) affects lung edema clearance in rats exposed to 100% O(2) for 64 h. Active Na(+) transport and lung edema clearance decreased by approximately 44% in rats exposed to acute hyperoxia. Iso (10(-6) M) increased the ability of the lung to clear edema in room-air-breathing rats (from 0.50 +/- 0.02 to 0.99 +/- 0. 05 ml/h) and in rats exposed to 100% O(2) (from 0.28 +/- 0.03 to 0. 86 +/- 0.09 ml/h; P < 0.001). Disruption of intracellular microtubular transport of ion-transporting proteins by colchicine (0. 25 mg/100 g body wt) inhibited the stimulatory effects of Iso in hyperoxia-injured rat lungs, whereas the isomer beta-lumicolchicine, which does not affect microtubular transport, did not inhibit active Na(+) transport stimulated by Iso. Accordingly, Iso restored the lung's ability to clear edema after hyperoxic lung injury, probably by stimulation of the recruitment of ion-transporting proteins (Na(+)-K(+)-ATPase) from intracellular pools to the plasma membrane in rat alveolar epithelium.

Dopamine restores lung ability to clear edema in rats exposed to hyperoxia.

Exposure to hyperoxia causes lung injury, decreases active sodium transport and lung edema clearance in rats. Dopamine (DA) increases lung edema clearance by stimulating vectorial Na+ flux and Na, K-ATPase function in rat alveolar epithelium. This study was designed to test whether DA (10(-)5 M) would increase lung edema clearance in rats exposed to 100% O2 for 64 h. Active Na+ transport and lung edema clearance decreased by approximately 44% in rats exposed to acute hyperoxia (p < 0.001). DA increased lung edema clearance in room air breathing rats (from 0.50 +/- 0.02 to 0.75 +/- 0.06 ml/h) and in rats exposed to 100% O2 (from 0.28 +/- 0.03 to 0. 67 +/- 0.03 ml/h). Disruption of cell microtubular transport system by colchicine blocked the stimulatory effect of DA on active Na+ transport in control and hyperoxic rats, whereas the isomer beta-lumicolchicine, which does not affect cell microtubular transport, did not inhibit the stimulatory effects of dopamine. The Na,K-ATPase alpha1-subunit protein abundance increased in the basolateral membranes of alveolar type II (ATII) cells incubated with 10(-)5 M DA for 15 min, probably by recruiting Na+ pumps from intracellular pools. Colchicine, but not beta-lumicolchicine, prevented the recruitment of alpha1 subunits to the plasma membrane by DA. Accordingly, DA restored lung ability to clear edema in hyperoxic-injured rat lungs. Conceivably, dopamine induces recruitment of Na+ pumps from intracellular pools to the plasma membrane of alveolar epithelial cells and thus increases lung edema clearance.

Isoproterenol increases Na+-K+-ATPase activity by membrane insertion of alpha-subunits in lung alveolar cells.

Catecholamines promote lung edema clearance via beta-adrenergic-mediated stimulation of active Na+ transport across the alveolar epithelium. Because alveolar epithelial type II cell Na+-K+-ATPase contributes to vectorial Na+ flux, the present study was designed to investigate whether Na+-K+-ATPase undergoes acute changes in its catalytic activity in response to beta-adrenergic-receptor stimulation. Na+-K+-ATPase activity increased threefold in cells incubated with 1 microM isoproterenol for 15 min, which also resulted in a fourfold increase in the cellular levels of cAMP. Forskolin (10 microM) also stimulated Na+-K+-ATPase activity as well as ouabain binding. The increase in Na+-K+-ATPase activity was abolished when cells were coincubated with a cAMP-dependent protein kinase inhibitor. This stimulation, however, was not due to protein kinase-dependent phosphorylation of the Na+-K+-ATPase alpha-subunit; rather, it was the result of an increased number of alpha-subunits recruited from the late endosomes into the plasma membrane. The recruitment of alpha-subunits to the plasma membrane was prevented by stabilizing the cortical actin cytoskeleton with phallacidin or by blocking anterograde transport with brefeldin A but was unaffected by coincubation with amiloride. In conclusion, isoproterenol increases Na+-K+-ATPase activity in alveolar type II epithelial cells by recruiting alpha-subunits into the plasma membrane from an intracellular compartment in an Na+-independent manner.

Time course of active and passive liquid and solute movement in the isolated perfused rat lung model.

The isolated perfused rat lung model (IL) is used to study alveolar epithelial transport properties. Most of the previous studies have been done over a short period of time and have not used the same preparation as a control and intervention group. We evaluated whether the IL preparation could be used for a prolonged period of time (5 h) and studied the rates of active Na+ transport, lung liquid clearance, and passive movement of solutes. Active Na+ transport and lung liquid clearance were stable from 1 to 5 h. The passive movement of small solutes (Na+, mannitol) did not change significantly, and albumin movement increased slightly at the fifth hour. Total RNA isolated from IL after 5 h was intact, and the Na+-K+-ATPase activity in alveolar type II cells isolated at the end of 5-h experiments was equal to Na+-K+-ATPase function from freshly isolated alveolar type II cells. Finally, we measured the stimulatory effect of the beta-adrenergic-agonist terbutaline and the inhibitory effect of the Na+-K+-ATPase-antagonist ouabain by using the same animal as a control. Accordingly, the isolated perfused lung model is functionally stable for at least 5 h, and it could be utilized to evaluate the effect of different interventions by using the same preparation.

Epidermal growth factor increases lung liquid clearance in rat lungs.

Epidermal growth factor (EGF) has been reported to stimulate the proliferation of epithelial cells and increase Na+ flux and Na+-K+-ATPase function in alveolar epithelial cell monolayers. Increases in Na+-K+-ATPase in alveolar type II cells (AT2) have been associated with increased active Na+ transport and lung edema clearance across the rat alveolar epithelium in a model of proliferative lung injury. Thus we tested whether administration of aerosolized EGF to rat lungs would increase active Na+ transport and lung liquid clearance. Sixteen adult Sprague-Dawley male rats were randomized to three groups. To a group of six rats, an aerosol generated from 20 microgram of EGF in saline was delivered to the lungs, to a second group of five rats only aerosolized saline was delivered, and a third group of five rats without treatment served as the control. Forty-eight hours postaerosolization of rat lungs with EGF there was an approximately 40% increase in active Na+ transport and lung liquid clearance compared with control rats, in the absence of changes in 22Na+, [3H]mannitol, and albumin permeabilities. The Na+-K+-ATPase activity in AT2 cells harvested from these lungs was increased in rats that received aerosolized EGF compared with AT2 cells from both control rats and rats receiving aerosolized saline. These results support the hypothesis that in vivo delivery of EGF aerosols upregulates alveolar epithelial Na+-K+-ATPase and increases lung liquid clearance in rats.

Modulation of lung liquid clearance by isoproterenol in rat lungs.

beta-Adrenergic agonists have been reported to increase lung liquid clearance by stimulating active Na+ transport across the alveolar epithelium. We studied mechanisms by which beta-adrenergic isoproterenol (Iso) increases lung liquid clearance in isolated perfused fluid-filled rat lungs. Iso perfused through the pulmonary circulation at concentrations of 10(-4) to 10(-8) M increased lung liquid clearance compared with that of control lungs (P < 0.01). The increase in lung liquid clearance was inhibited by the beta-antagonist propranolol (10(-5) M), the Na(+)-channel blocker amiloride (10(-4) M), and the antagonist of Na-K-ATPase, ouabain (5 x 10(-4) M). Colchicine, which inhibits cell microtubular transport of ion-transporting proteins to the plasma membrane, blocked the stimulatory effects of Iso on active Na+ transport, whereas the isomer lumicolchicine, which does not affect cell microtubular transport, did not inhibit Na+ transport. In parallel with these changes, the Na-K-ATPase alpha 1-subunit protein abundance and activity increased in alveolar type II cells stimulated by 10(-6) M Iso. Colchicine blocked the stimulatory effect of Iso and the recruitment of Na-K-ATPase alpha 1-protein to the basolateral membrane of alveolar type II cells. Accordingly, Iso increased active Na+ transport and lung liquid clearance by stimulation of beta-adrenergic receptors and probably by upregulation of apical Na+ channels and basolateral Na-K-ATPase mechanisms. Recruitment from intracellular pools and microtubular transport of Na+ pumps to the plasma membrane participate in beta-adrenergic stimulation of lung liquid clearance in rat lungs.

Dexamethasone upregulates the Na-K-ATPase in rat alveolar epithelial cells.

Previous studies in kidney, heart, and liver cells have demonstrated that dexamethasone regulates the expression of Na-K-ATPase. In the lungs, Na-K-ATPase has been reported in alveolar epithelial type II (ATII) cells and is thought to participate in active Na+ transport and lung edema clearance. The aim of this study was to determine whether Na-K-ATPase would be regulated by dexamethasone in cultured rat ATII cells. Regulation of the Na-K-ATPase by dexamethasone could lead to a greater understanding of its role in active Na+ transport and lung edema clearance. Rat ATII cells were isolated, plated for 24 h, and exposed to 10(-7) and 10(-8) M dexamethasone. These cells were harvested at 0, 3, 6, 12, and 24 h after dexamethasone exposure for determination of steady-state Na-K-ATPase mRNA transcript levels, protein expression, and function. The steady-state Na-K-ATPase beta1-mRNA transcript levels increased in ATII cells 6, 12, and 24 h after dexamethasone exposure (P < 0.05). However, the steady-state alpha1-mRNA transcript levels were unchanged. The protein expression for the alpha1- and beta1-subunits increased in ATII cells exposed to dexamethasone compared with controls in association with a temporal increase in Na-K-ATPase function after dexamethasone exposure. These results suggest that dexamethasone regulates Na-K-ATPase in ATII cells possibly by transcriptional, translational, and posttranslational mechanisms.

Differential expression of Na-K-ATPase isoforms in rat alveolar epithelial cells.

Lung Na-K-ATPase has been shown to contribute to vectorial Na+ transport and edema clearance. The alpha 1- and beta 1-Na-K-ATPase subunits have been localized to alveolar type II (ATII) cells, and the alpha 2-Na-K-ATPase has been reported in rat lung homogenates. Expression of Na-K-ATPase alpha 1-, alpha 2-, and beta 1-subunits was investigated in rat ATII cells cultured for 7 days, a period during which they lose their phenotypic markers and differentiate to an alveolar type I (ATI)-like cell phenotype. Differentiation of ATII cells to an ATI-like phenotype resulted in a decrease of alpha 1- and an increase of alpha 2-mRNA and protein abundance without changes in the beta 1-subunit. Thus ATI-like cells exhibited a mixture of alpha 1- and alpha 2-isoforms. Nuclear run-on analysis suggests that these changes were transcriptionally regulated. The existence of the distinct functional classes of Na-K-ATPase in ATII and ATI-like cells was confirmed by ouabain inhibition of Na-K-ATPase activity. Ouabain inhibition of ATII cells was consistent with expression of the alpha 1-isozyme [50% inhibitory concentration (IC50) = 4 x 10(-5) M], whereas, in ATI-like cells, it was consistent with the presence of both alpha 1- and alpha 2-isozymes (IC50 = 9.0 x 10(-5) and 1.5 x 10(-7) M, respectively); [3H]ouabain binding studies corroborated these findings. Our results indicate that, during ATII cell cytodifferentiation with time in culture, there is a shift in isoform composition that may reflect physiological functions of alveolar epithelial cells.

Hyperbaric oxygenation upregulates rat lung Na,K-ATPase.

Exposure of rats to hyperoxia is associated with increased active Na+ transport in rat lungs and increased Na,K-adenosine triphosphate (ATPase) expression in alveolar epithelial cells. Hyperbaric oxygenation (HBO) has been reported to act as an accelerated model of hyperoxic cell damage. Sublethal and intermittent exposure to HBO, however, has been suggested to upregulate endogenous protective mechanisms. In the present study, we tested whether short-term HBO, prior to inducing lung injury, would upregulate lung Na,K-ATPase. The results show that HBO, either intermittent or single 2.5 h exposure, increased lung Na,K-ATPase alpha-1 and beta-1 messenger ribonucleic acid (mRNA) transcript levels up to fourfold. Na,K-ATPase activity in lungs of rats exposed to HBO increased twofold during the first 2 h following removal from the hyperbaric chamber, and remained elevated for up to 6 h following HBO. Conceivably, the increase in Na,K-ATPase activity following HBO is due to an increase in activity from a basal to a higher rate, or possibly due to recruitment/translocation of Na,K-ATPases from inner membranes to the plasma membrane.

Lung liquid clearance and Na,K-ATPase during acute hyperoxia and recovery in rats.

Lung liquid clearance, epithelial permeability for Na+, mannitol and albumin, as well as Na,K-ATPase activity in alveolar type 2 (AT2) cells were studied during the acute and the recovery phase of hyperoxic lung injury. Rats exposed to 100% oxygen for 64 h were studied at 0, 7 and 14 d after removal from the hyperoxic chamber and compared with control rats breathing room air. In the isolated-perfused, liquid-filled rat lung, the albumin flux from the perfusate into the air spaces increased immediately after the oxygen exposure (220 +/- 56 mg/h) and returned to control values (28 +/- 7 mg/h) after 7 and 14 d of recovery. The small solutes (Na+ and mannitol) flux across the alveolar epithelium normalized only after 14 d of recovery in room air. Active Na+ transport and lung liquid clearance were reduced by approximately 45% immediately after oxygen exposure when compared with control values, increased by approximately 56% above control values after 7 d of recovery, and returned to control values after 14 d of recovery. Paralleling these changes the Na,K-ATPase activity decreased by approximately 41% in AT2 cells isolated from rats after 64 h of breathing 100% O2 and increased by approximately 25% after the rats recovered in room air for 7 d. These results suggest that alveolar epithelial Na,K-ATPase may contribute in the recovery from the hyperoxic lung injury by participating in the clearance of lung edema.

Mechanisms of lung liquid clearance during hyperoxia in isolated rat lungs.

Sodium transport across the lung epithelium is predominantly effected by apical amiloride-sensitive Na+ channels and basolaterally located ouabain-sensitive Na,K-ATPases. Previously, we reported that subacute hyperoxia caused an increase in active Na+ transport in rat lungs paralleling Na,K-ATPase upregulation in alveolar Type 2 cells isolated from the same lungs. In the present study we set out to quantify the amiloride-sensitive Na+ flux and ouabain-sensitive active Na+ transport in the isolated-perfused, fluid-filled lung model from rats exposed to 85% O2 for 7 d compared with normoxic control rats. We found increased transpulmonary albumin flux and permeability to small solutes (Na+ and mannitol) in hyperoxic rat lungs compared with controls. Amiloride (10(-5) M) instilled into rat airspaces inhibited active Na+ transport by approximately 62% in control rat lungs and by approximately 87% in lungs from rats exposed to hyperoxia, without further changing permeability for Na+ and mannitol. Ouabain (10(-5)M) perfused through the pulmonary circulation decreased active Na+ transport by approximately 40% in normal rat lungs and by approximately 52% in lungs from rats exposed to hyperoxia. We conclude that active Na+ transport and edema clearance are increased in the subacute hyperoxic lung injury in rats, caused in part by the upregulation of amiloride-sensitive apical Na+ channels and alveolar epithelial Na,K-ATPases. Conceivably, the upregulation of alveolar epithelial Na+ channels and Na,K-ATPases protects against the effects of lung injury in this model by contributing to effective edema clearance.