Polarized distribution of Na(+)-H+ antiport activity in rat alveolar epithelial cells

In this study, we investigated the polarized distribution of Na(+)-H+ antiport activity in alveolar epithelial cell monolayers. Rat alveolar type II pneumocytes were grown on detachable tissue culture-treated Nuclepore filters. The membrane filters, with their adherent intact alveolar epithelial cell monolayers, were mounted in a cuvette designed to contain two fluid compartments separated by the monolayer. Cells were loaded with the pH-sensitive dye 2',7'-biscarboxyethyl-5,6-carboxylfluorescein and intracellular pH (pHi) measured spectrofluorometrically. Monolayers were studied at ambient temperature on days 3-4 in culture, coincident with the development of high tissue resistance (RT > or = 2000 omega.cm2). Cells were incubated in HCO(3-)-free Na+ buffer [(in mM) 140 NaCl, 6 HEPES, pH 7.4] and acidified by NH3 prepulse. Rates of realkalinization (JH+) were calculated as the product of the initial rate of recovery (dpHi/dt) and the intracellular buffer capacity (beta i). Under control conditions, recovery occurred with an initial JH+ of 28.4 mM/min. When 100 microM dimethylamiloride (DMA), an amiloride analogue with enhanced specificity for inhibiting the Na(+)-H+ antiporter, was present in the basolateral fluid, recovery was inhibited by > 90%. Conversely, when the monolayers were acidified in Na+ buffer containing DMA (100 microM) in the apical fluid, acidification and recovery were identical to control. Recovery from acidification was inhibited by basolateral DMA with a one-half maximal inhibitory concentration (IC50) of 100 nm and by basolateral amiloride with an IC50 of 10 microns. Recovery was completely inhibited by omission of Na+ from the basolateral fluid, but omission of Na+ from apical fluid had no effect. We conclude that Na(+)-H+ antiport activity is located exclusively on the basolateral surface of these alveolar epithelial cell monolayers, where it most likely represents the high-amiloride affinity isoform of the Na(+)-H+ antiporter, NHE-1. The Na(+)-H+ antiporter, asymmetrically distributed to the basolateral surface of the polarized alveolar epithelium, contributes to intracellular homeostasis in alveolar pneumocytes and may also play a role in signal transduction in these cells.

Defined medium for primary culture de novo of adult rat alveolar epithelial cells

Isolated type II pneumocytes grown in serum on tissue culture-treated polycarbonate filters form monolayers with characteristic bioelectric properties, and change morphologically with time in culture to resemble type I cells. Concurrently, the cells express type I cell surface epitopes, making this a potentially useful in vitro model with which to study regulation of alveolar epithelial cell function and differentiation. To define specific soluble growth factors and matrix substances that may regulate these processes, it would be preferable to culture isolated pneumocytes de novo under completely defined, serum-free conditions. In this study, we developed a completely defined serum-free medium that is capable of supporting alveolar epithelial cells in primary culture, allowing the formation of monolayers with characteristic bioelectric and phenotypic properties. Freshly isolated rat type II cells were resuspended in completely defined serum-free medium and plated de novo on polycarbonate filters. Plating efficiency, bioelectric properties, morphology, and binding of a type I cell-specific monoclonal antibody were determined as functions of time. Plating efficiency plateaus at about 14% by Day 3 in culture. Transepithelial resistance rises to high levels, peaking at 1.76 +/- 0.14 K omega-cm2 by Day 5 in culture. Short-circuit current peaks on Day 3 in culture at 2.71 +/- 0.35 microA/cm2. With time, the cells gradually become flattened with protuberant nuclei and long cytoplasmic extensions, more closely resembling type I cells, and begin to express a type I cell surface epitope.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of solute fluxes in isolated rat lungs

Most previous studies in isolated perfused lungs have utilized measurements of solute flow from alveolar to vascular space to characterize the barrier and transport properties of the alveolar epithelium. In this study, we measured flux of a series of nonionic hydrophilic solutes and sodium across the alveolar epithelium of the isolated rat lung from perfusate to airspace (P-->A), as well as from airspace to perfusate (A-->P). Apparent permeability-surface area products (PS) were calculated from the rates of isotope appearance downstream in either the airspace or the perfusate. Equivalent pore analysis of data for P-->A solute flow demonstrated a small pore population with radius 0.6 nm occupying 85% of the total pore area and a large pore population with radius 3.8 nm occupying 15% of the total area. Similar analysis of A-->P solute flux demonstrated a small pore population of 0.6 nm occupying 86% of the total pore area and a large pore population with radius 2.9 nm occupying 14% of total pore area. The ratio (R) of PSP-->A divided by PSA-->P was 0.8 for the nonionic hydrophilic solutes, while R for sodium was 0.5. In the presence of amiloride and ouabain, R for sucrose was unchanged while R for sodium increased to 0.8 due to a fall in PSA-->P. The difference between R for sodium and R for the passively transported solutes, and the reduction in this difference in the presence of sodium transport inhibitors, are consistent with active sodium reabsorption by the intact alveolar epithelium. Differences in measured unidirectional passive solute fluxes probably result from unequal effective surface areas for diffusion from vascular space to airspace and vice versa in the anatomically complex mammalian lung.

Evidence for amiloride-sensitive sodium channels in alveolar epithelial cells

To maintain alveolar air spaces relatively fluid free, the alveolar epithelium appears capable of vectorial transport of water and solutes. Active transepithelial transport of sodium by alveolar epithelial cell monolayers has previously been demonstrated, indicating that alveolar pneumocytes must possess ion transport mechanisms by which sodium can enter the cells apically for subsequent extrusion via Na(+)-K(+)-adenosinetriphosphatase activity at the basolateral surface. In this study, sodium entry mechanisms were investigated by directly measuring 22Na uptake into rat alveolar epithelial cells grown in primary culture. Cells exhibited increasing 22Na uptake with time over a 30-min interval. Total sodium uptake was compared in the presence and absence of several sodium transport inhibitors. Uptake was inhibited by the sodium channel blockers amiloride and benzamil but was not affected by two amiloride analogues (bromohexamethylene amiloride and dimethylamiloride) with diminished specificity for blocking sodium channels and enhanced specificity for inhibiting the Na(+)-H+ antiporter. Uptake was also unaffected by the chloride transport inhibitor bumetanide or by the absence of glucose. These data suggest that sodium uptake occurs primarily via sodium channel and that Na(+)-H+ antiport, Na(+)-K(+)-2Cl- cotransport, and Na(+)-glucose cotransport do not contribute significantly to sodium uptake under these experimental conditions. The presence of sodium channels in the alveolar epithelial cell membrane may provide the major entry mechanism by which sodium enters these cells for subsequent active extrusion, thereby effecting net salt and water reabsorption from the alveolar spaces.

Reactivity of alveolar epithelial cells in primary culture with type I cell monoclonal antibodies

An understanding of the process of alveolar epithelial cell growth and differentiation requires the ability to trace and analyze the phenotypic transitions that the cells undergo. This analysis demands specific phenotypic probes to type II and, especially, type I pneumocytes. To this end, monoclonal antibodies have been generated to type I alveolar epithelial cells using an approach designed to enhance production of lung-specific clones from a crude lung membrane preparation. The monoclonal antibodies were screened by a combination of enzyme-linked immunosorbent assay and immunohistochemical techniques, with the determination of type I cell specificity resting primarily on immunoelectron microscopic localization. Two of these new markers of the type I pneumocyte phenotype (II F1 and VIII B2) were used to analyze primary cultures of type II cells growing on standard tissue culture plastic and on a variety of substrata reported to affect the morphology of these cells in culture. On tissue culture plastic, the antibodies fail to react with early (days 1 to 3) type II cell cultures. The cells become progressively more reactive with time in culture to a plateau of approximately 6 times background by day 8, with a maximum rate of increase between days 3 and 5. This finding is consistent with the hypothesis that type II cells in primary culture undergo at least partial differentiation into type I cells. Type II cells grown on laminin, which reportedly delays the loss of type II cell appearance, and on fibronectin, which has been reported to facilitate cell spreading and loss of type II cell features, develop the type I cell markers during cultivation in vitro with kinetics similar to those on uncoated tissue culture plastic. Cells on type I collagen and on tissue culture-treated Nuclepore filters, which have been reported to support monolayers with type I cell-like morphology, also increase their expression of the II F1 and VIII B2 epitopes around days 3 to 5. Taken together with available morphologic information, these data suggest that expression of different alveolar epithelial cell phenotypic markers by type II cells in primary culture may be independently regulated. The monoclonal antibody probes described in this report should prove useful in the continued investigation of the mechanisms and regulation of alveolar epithelial cell differentiation.

Regulation of intracellular pH in alveolar epithelial cells

Alveolar type II epithelial cells in adult mammalian lungs actively transport salt and water, secrete surfactant, and differentiate into type I cells under normal conditions and following lung injury. It has become increasingly apparent that, like all epithelial cells, alveolar pneumocytes have evolved specialized ion transport mechanisms by which they regulate their intracellular pH (pHi). pHi is an important biological parameter in all living cells whose regulation is necessary for normal cellular homeostasis. pHi, and the ion transport mechanisms by which it is regulated, may contribute to many cellular processes, including transcellular transport, cell volume and osmolarity regulation, and intracellular transport, cell volume and osmolarity regulation, and intracellular electrolyte composition. Moreover, changes in pHi may serve as intracellular signals for biological processes such as cell growth, proliferation, and differentiation. We review herein the general principles of pHi regulation in epithelia and describe the mechanisms and effects of pHi regulation in alveolar pneumocytes. Many of the critical issues in current pulmonary research involve processes that pHi is most likely to affect, including maintenance of alveolar epithelial barrier integrity, development and maintenance of epithelial polarity, epithelial proliferation and differentiation, and regulation of transepithelial transport with respect to alveolar fluid balance in normal individuals and in those with excess alveolar fluid (i.e., pulmonary edema). Investigations into the regulation of pHi in alveolar pneumocytes and the regulatory effects of pHi in turn on other cellular processes are likely to yield information important to the understanding of lung biology and pulmonary disease.

Contribution of active Na+ and Cl- fluxes to net ion transport by alveolar epithelium

Changes in bioelectric properties of alveolar epithelial cell monolayers due to pharmacological agents such as beta-agonists, amiloride and ouabain have recently been reported. In order to determine specifically which ionic species contribute to these changes, fluxes of Na+ and Cl- across primary cultured monolayers of rat type II pneumocytes were directly measured. Monolayers were mounted in modified flux chambers and short-circuited. Unidirectional fluxes of 22Na (or 36Cl) and [14C]-mannitol were measured simultaneously. Experimental maneuvers included apical (A) exposure to 10 microM amiloride, basolateral (B) exposure to 1 mM ouabain, or basolateral exposure to 20 microM terbutaline. Results show that baseline monolayers actively reabsorb Na+ (about 0.14 micro Eq.cm-2.h-1) from the apical fluid, while mannitol and Cl- appear to traverse the alveolar epithelium passively. Active Na+ reabsorption was abolished by amiloride or ouabain, while Cl- and mannitol fluxes were unaffected. Terbutaline, on the other hand, markedly increased net absorption of Na+ and caused active transport of Cl- in the A to B direction. Passive mannitol flow was somewhat increased with terbutaline. These data indicate that active Na+ reabsorption across alveolar epithelial monolayers is dependent on intact Na+,K(+)-ATPase activity and cell Na+ entry (probably via Na+ channels), and can be stimulated by beta-agonists. Beta-agonists also cause active reabsorption of Cl- (passive under other conditions).

Na(+)-HCO3- symport modulates intracellular pH in alveolar epithelial cells

We investigated Na(+)-HCO3- cotransport as a mechanism for regulation of intracellular pH (pHi) in rat alveolar pneumocytes grown in primary culture. pHi was monitored using the fluorescent pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Cells incubated in 6 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) medium at pH 7.4 were subjected to rapid acidification by CO2 pulse. pHi recovered in the presence of Na+ with an initial rate (dpHi/dt) of 0.15 min-1, which was reduced by 67% when Na+ was replaced by choline, unaffected by substitution of gluconate for Cl-, reduced 40% in the presence of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, 500 microM), and unchanged by amiloride (1 mM). In parallel experiments, cells were incubated at pH 7.4 with 20 mM HCO3- and pHi acutely lowered by NH3 prepulse. dpHi/dt in these experiments was 0.14 min-1 in the presence of Na+ and HCO3-, and reduced 79% under Na(+)-free conditions. These data indicate the presence of a Na(+)-dependent, Cl(-)-independent, DIDS-sensitive and amiloride-insensitive mechanism of recovery from acute intracellular acidification in alveolar pneumocytes, most consistent with Na(+)-HCO3- cotransport (symport) effecting acid extrusion under these experimental conditions. This ion transport mechanism may contribute to regulation of pHi in alveolar pneumocytes, transepithelial transport of acid-base equivalents across the alveolar epithelium, and modulation of pH of alveolar fluid in adult mammalian lungs.

NO2 decreases paracellular resistance to ion and solute flow in alveolar epithelial monolayers

Primary cultured monolayers of rat alveolar epithelial cells grown on tissue culture-treated Nuclepore filters were exposed to 2.5 ppm nitrogen dioxide (NO2) for 2-20 min. Changes in monolayer bioelectric properties and solute permeabilities were subsequently measured. Exposure to NO2 produced a dose-dependent decrease in monolayer transepithelial electrical resistance (Rt), whereas monolayer short-circuit current was unaffected. Post-exposure monolayer permeability to 14C-sucrose (which primarily crosses alveolar epithelium via the paracellular pathway) increased markedly. That for 3H-glycerol (which permeates through both paracellular and transcellular pathways) increased to a lesser extent. Partial recovery of Rt and solute permeabilities was noted by 48-h post-exposure. The time courses of the decrease in Rt and increase in solute permeabilities were similar. These results suggest that NO2 primarily impairs passive alveolar epithelial barrier functions in vitro, probably by altering intercellular junctions, and does not appear to directly affect cell membrane active ion transport processes. When correlated with results obtained from experimental approaches, studies of in vitro alveolar epithelial monolayers may facilitate investigations of dosimetry, sites, and mechanisms of oxidant injury in the lung.

Evidence for active H+ secretion by rat alveolar epithelial cells

A plasma membrane proton-translocating adenosinetriphosphatase (ATPase) has been identified in rat alveolar pneumocytes in primary culture using the pH-sensitive fluorescent probe 2',7'-biscarboxyethyl-5,6-carboxyfluorescein. Intracellular pH (pHi) was acutely lowered by NH3 prepulse in HCO3(-)-free medium buffered with 6 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid, and its recovery was measured thereafter under control conditions, in the presence of amiloride to inhibit Na(+)-H+ antiport, and in the presence of N-ethylmaleimide (NEM), a plasma membrane H(+)-ATPase inhibitor. Initial rate of pHi recovery was reduced by 67% in the presence of amiloride, 52% in the presence of NEM, and 96% in the presence of both. Recovery was decreased but not abolished in Na(+)-free buffer, was essentially abolished when NEM was present in the absence of Na+, and was also abolished by addition of the metabolic inhibitor KCN in glucose- and Na(+)-free medium. These data suggest that alveolar epithelial cells possess a plasma membrane H(+)-ATPase. In Na(+)-containing buffer at pH 7.4, steady-state pHi was 7.50. This value was unaffected by amiloride but decreased to 7.01 in the presence of NEM, suggesting active H(+)-ATPase and inactive Na(+)-H+ antiport at steady-state pHi. We conclude that this plasma membrane proton-translocating ATPase in alveolar pneumocytes may be an important mechanism contributing to regulation of steady-state pHi, recovery from acute intracellular acidification, and modulation of extracellular alveolar fluid pH.

Type I cell-like morphology in tight alveolar epithelial monolayers

The pulmonary alveolar epithelium separates air spaces from a fluid-filled interstitium and might be expected to exhibit high resistance to fluid and solute movement. Previous studies of alveolar epithelial barrier properties have been limited due to the complex anatomy of adult mammalian lung. In this study, we characterized a model of isolated alveolar epithelium with respect to barrier transport properties and cell morphology. Alveolar epithelial cells were isolated from rat lungs and grown as monolayers on tissue culture-treated Nuclepore filters. On Days 2-6 in primary culture, monolayers were analyzed for transepithelial resistance (Rt) and processed for electron microscopy. Mean cell surface area and arithmetic mean thickness (AMT) were determined using morphometric techniques. By Day 5, alveolar epithelial cells in vitro exhibited morphologic characteristics of type I alveolar pneumocytes, with thin cytoplasmic extensions and protruding nuclei. Morphometric data demonstrated that alveolar pneumocytes in vitro develop increased surface area and decreased cytoplasmic AMT similar to young type I cells in vivo. Concurrent with the appearance of type I cell-like morphology, monolayers exhibited high Rt (greater than 1000 omega.cm2), consistent with the development of tight barrier properties. These monolayers of isolated alveolar epithelial cells may reflect the physiological and morphological properties of the alveolar epithelium in vivo.

Hydrophilic solute transport across rat alveolar epithelium

Diffusional fluxes of a series of hydrophilic nonelectrolytes (molecular radii ranging from 0.15 to 0.57 nm) were measured across the alveolocapillary barrier in the isolated perfused fluid-filled rat lung. Radiolabeled solutes were lavaged into the distal air spaces of isolated Ringer-perfused lungs, and apparent permeability-surface area products were calculated from the rates of isotope appearance in the recirculating perfusate. These data were used to estimate theoretical equivalent pore radii in the alveolar epithelium, with the assumption of diffusive flow through water-filled cylindrical pores. The alveolar epithelium is best characterized by two pore populations, with small pores (radius 0.5 nm) occupying 98.7% of total pore area and larger pores (radius 3.4 nm) occupying 1.3% of total pore area. Net water flow out of the alveolar space was measured by including an impermeant solute (dextran) in the lavage fluid and measuring its concentration in the alveolar space as a function of time. Under control conditions, net water flow averaged 167 nl/s. When 24 microM terbutaline was added to the perfusate, net water flow increased significantly to 350 nl/s (P less than 0.001). Terbutaline had no effect on the fluxes of either glycerol (which traverses the small pore pathway) or sucrose (which traverses the large pore pathway). These findings indicate that the intact mammalian alveolar epithelium is complex and highly resistant to the flow of solutes and water.

Tight monolayers of rat alveolar epithelial cells: bioelectric properties and active sodium transport

Because the pulmonary alveolar epithelium separates air spaces from a fluid-filled compartment, it is expected that this barrier would be highly resistant to the flow of solutes and water. Investigation of alveolar epithelial resistance has been limited due to the complex anatomy of adult mammalian lung. Previous efforts to study isolated alveolar epithelium cultured on porous substrata yielded leaky monolayers. In this study, alveolar epithelial cells isolated from rat lungs and grown on tissue culture-treated Nucleopore filters resulted in tight monolayers with transepithelial resistance greater than 2,000 omega.cm2. Changes in bioelectric properties of these alveolar epithelial monolayers in response to ouabain, amiloride, and terbutaline are consistent with active sodium transport across a polarized barrier. 22Na flux measurements under short-circuit conditions directly confirm net transepithelial absorption of sodium by alveolar epithelial cells in the apical to basolateral direction, comparable to the observed short-circuit current (4.37 microA/cm2). The transport properties of these tight monolayers may be representative of the characteristics of the mammalian alveolar epithelial barrier in vivo.

Velocity of CO2 exchanges in the lungs

As outlined above, investigations over the past decade have provided further insight into the kinetics of many of the component processes that affect the overall velocity of CO2 exchanges in the lungs. The evolution of our understanding of the importance and role of these reactions and transport processes has not been entirely predictable. A variety of investigations into Roughton's original hypothesis regarding the mechanism of pH equilibration in capillary blood resulted in a renewed interest in carbonic anhydrase in the lung and other tissues. These latter studies have helped better define the location and kinetic properties of lung CA and its role in the overall velocity of CO2 exchange. Concurrently, a wealth of information has emerged regarding the transport properties of the red cell membrane. This has led to a better understanding of the mechanisms and characteristics of the band 3-mediated pathway for electroneutral anion exchange. Unfortunately, most of the kinetic data for this pathway have been obtained under nonphysiological conditions, making it difficult to utilize them directly in analyses of lung CO2 exchange kinetics. The potential detrimental effect of pharmacological agents in modifying lung CA and red cell CA activity and red cell anion exchange kinetics is among the more important factors to have emerged in the past decade. Ironically, the original question of whether a significant blood pH disequilibrium is present in the arterial circulation in man in vivo remains unresolved. However, the mechanisms underlying these phenomena are now recognized to be more complex than originally appreciated.

Sodium-dependent lysine flux across bullfrog alveolar epithelium

Amino acid transport across the alveolar epithelial barrier was studied by measuring radiolabeled lysine fluxes across bullfrog lungs in an Ussing chamber. In the absence of a transmural electrical gradient, L-[14C]lysine was instilled into the upstream reservoir and the rate of appearance of the radiolabel in the downstream reservoir was determined. Two lungs from the same animal were used simultaneously to determine tracer fluxes both into and out of the alveolar bath. Results showed that the radiolabel flux measured in the alveolar to the pleural direction was greater than that measured in the opposite direction in the presence of sodium in the bathing fluids. The net flux of L-[14C]lysine was saturable with [Na+], with an apparent transport coefficient (Kt) of 28 mM for Na+. Hill analysis of [14C]lysine flux vs. [Na+] indicated a coupling ratio of 1:1 between sodium and radiolabeled L-lysine. Total L-lysine flux as a function of [L-lysine] was also saturable, with Kt of 7.3 mM for L-lysine. Ouabain significantly decreased absorptive (alveolar-to-pleural) radiolabel flux, while slightly increasing the flux observed in the opposite direction. L-leucine completely inhibited absorptive net flux of L-[14C]lysine. alpha-Methylaminoisobutyric acid (MeAIB), on the other hand, only slightly reduced net flux of L-[14C]lysine from the control value. The presence of a net absorptive, Na+-dependent amino acid flux across the alveolar epithelial barrier indicates that the tissue is capable of removing amino acids and sodium from the alveolar fluid by a coupled cotransport mechanism, which may be important for both protein metabolism and fluid balance by alveolar epithelium.

Cl-/HCO3- exchange modulates intracellular pH in rat type II alveolar epithelial cells

The role of an anion exchange pathway in modulating intracellular pH (pHi) under steady-state and alkaline load conditions was investigated in confluent monolayers of rat type II alveolar epithelial cells using the pH-sensitive fluorescent probe 2'-7'-biscarboxy-ethyl-5,6-carboxylfluorescein. Under steady-state conditions in the presence of 25 mM HCO3-, 5% CO2 at pHo 7.4, pHi was 7.32 in a Na+-replete medium and 7.33 in the absence of Na+. Steady-state pHi was 7.19 in a nominally HCO3(-)-free medium at pHo 7.4, and 7.52 in a Cl(-)-free medium, with both values significantly different from that obtained in the presence of both HCO3- and Cl-. Monolayers in which pHi was rapidly elevated by removal of HCO3-/CO2 from the bathing medium demonstrated an absolute requirement for Cl- to recover toward base-line pHi. The Km of Cl- for the external site of the exchange pathway was 11 +/- 1 mM. Recovery of pHi from the alkaline load in the presence of Cl- was inhibited 60% by the stilbene derivative 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. Removal of Cl- from the medium of cells bathed in HCO3-/CO2 resulted in a rapid increment in pHi which returned to base line when Cl- was reintroduced into the bathing medium. In contrast, pHi was not perturbed by removal or addition of Cl- to monolayers bathed in a 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid-buffered medium, indicating that HCO3- was the preferred species for transport. Recovery of pHi from an alkaline load was not affected by the presence or absence of Na+. These findings define the transport pathway as Na+-independent Cl-/HCO3- exchange. This pathway contributes importantly to determining resting pHi of pneumocytes and enables the cell to recover from an alkaline load.

Effects of culture conditions on susceptibility of alveolar epithelial cell monolayers to NO2

The effects of nitrogen dioxide (NO2) exposure on primary cultured monolayers of rat type II pneumocytes were investigated as a function of the isolation and culture conditions. Monolayers were cultured in Eagle's minimum essential medium (MEM) and in MEM supplemented with Ham's F-12; in some experiments, the initial cell suspension was also replated after 3 h. Both supplementation of the basal medium and replating increased the sensitivity of the monolayers to NO2, as measured by reduction in dome formation of plastic dishes 24 h post-exposure. These findings suggest that comparisons of in vitro toxicologic observations may be complicated by the effects of specific experimental conditions.

Effects of nitrogen dioxide on alveolar epithelial barrier properties

This study analyzed the effects of nitrogen dioxide (NO2) on alveolar epithelial permeability and transport properties. Primary cultured monolayers of rat Type II pneumocytes, cultured on both nonporous and porous surfaces, were used as models of isolated alveolar epithelium for in vitro exposure to nitrogen dioxide. The effects of nitrogen dioxide exposure for monolayers cultured on nonporous substrata were monitored by observing the changes in the net volume of fluid under the monolayer; for cells cultured on porous substrata, alterations in tissue bioelectric properties were noted. As a first step, primary cultured monolayers of rat Type II pneumocytes plated on nonporous plastic Petri dishes were used to investigate the effects of nitrogen dioxide on alveolar epithelial barrier properties. Such monolayers form fluid filled domes that are thought to result from active solute transport from medium to substratum, with water following passively. We used dome formation as a transport marker. Five-day-old cultures were directly exposed to 30 ppm NO2 in 5 percent CO2 in air at 25 degrees C, by cyclically tilting culture plates from side to side, so that both halves of the monolayer were exposed during each cycle. Exposures consisted of 10 cycles of four minutes each (two minutes per side), for a cell exposure time of 20 minutes. Control plates were simultaneously exposed to 5 percent CO2 in air under identical conditions. One day after the exposure, nitrogen dioxide-exposed monolayers exhibited significant decreases in dome density and individual dome volume, compared to the controls. By 48 hours post-exposure, differences between nitrogen dioxide-exposed and control monolayers were less, but remained significant. These results showed that short-term sublethal exposures to nitrogen dioxide produce a decrease in dome formation in Type II alveolar epithelial cell monolayers. This finding is most likely due to a decrease in the active transepithelial sodium transport rate, or an increase in the permeability of cell membranes or tight junctions, or both. Addition of vitamin E-containing liposomes to the culture media 24 hours pre-exposure did not affect the nitrogen dioxide-induced decrease in dome formation, indicating that under these circumstances no protective effect was provided by the antioxidant.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for active sodium transport across alveolar epithelium of isolated rat lung

We have previously presented evidence that cultured alveolar epithelial cell monolayers actively transport sodium from medium to substratum, a process that can be inhibited by sodium transport blockers and stimulated by beta-agonists. In this study, the isolated perfused rat lung was utilized in order to investigate the presence of active sodium transport by intact adult mammalian alveolar epithelium. Radioactive tracers (22Na and [14C]sucrose) were instilled into the airways of isolated Ringer-perfused rat lungs whose weight was continuously monitored. The appearance of isotopes in the recirculated perfusate was measured, and fluxes and apparent permeability-surface area products were determined. A pharmacological agent (amiloride, ouabain, or terbutaline) was added to the perfusate during each experiment after a suitable control period. Amiloride and ouabain resulted in decreased 22Na fluxes and a faster rate of lung weight gain. Terbutaline resulted in increased 22Na flux and a more rapid rate of lung weight loss. [14C]sucrose fluxes were unchanged by the presence of these pharmacological agents. These data are most consistent with the presence of a regulable active component of sodium transport across intact mammalian alveolar epithelium that leads to removal of sodium from the alveolar space, with anions and water following passively. Regulation of the rate of sodium and fluid removal from the alveolar space may play an important role in the prevention and/or resolution of alveolar pulmonary edema.

Characterization of Na+-H+ antiport in type II alveolar epithelial cells

The presence of a Na+-H+ exchange pathway in the plasma membrane of type II alveolar epithelial cells was explored using the pH-sensitive fluorescent probe 2,7-biscarboxyethyl-5,6-carboxyfluorescein (BCECF) to monitor changes in cytosolic pH. Freshly prepared pneumocytes suspended in medium at pH 7.4 had an intracellular pH of 7.07 +/- 0.07. Acid-loaded cells equilibrated in sodium-free buffer showed rapid cytoplasmic alkalinization when exposed to sodium. This response to sodium was inhibited greater than 90% by 10(-4) M amiloride. The presence of the K+ ionophore, valinomycin, had no effect on the rate of Na+-dependent alkalinization, indicating the electroneutrality of the system. Li+ partially supported the alkalinization process, but other monovalent cations, notably K+, Rb+, and Cs+, were without effect. Kinetic analysis for Na+ at the external binding site yielded KNat (dissociation constant) = 62 +/- 3 mM. Hill equation analysis of the data derived a Hill coefficient (n) = 1.2 +/- 0.1 for Na+, consistent with a 1:1 stoichiometry for Na+ and H+ for the transporter. The Ki for amiloride inhibition of proton efflux at the external locus was 0.45 microM. These findings define the transport pathway as Na+-H+ antiport, with kinetic parameters somewhat similar to those described for other cell types. Antiport activity was detected at intracellular pH (pHi) values of 6.8 or below, with no activity observed at pHi 7.0-7.2. It is suggested that Na+-H+ exchange is a major mechanism whereby pneumocytes recover from an acid load and that this transport pathway may play an important role in vectorial reabsorption of Na+ from the alveolar air spaces.

Effects of dextran-bound inhibitors on carbonic anhydrase activity in isolated rat lungs

Effects of macromolecular Prontosil-dextran inhibitors (PD) on carbonic anhydrase (CA) activity in isolated rat lungs were studied. Isolated lungs were perfused with Krebs-Ringer bicarbonate (KRB) solutions containing no inhibitor, PD 100,000 (mol wt 100,000), PD 5,000 (mol wt 5,000), or low-molecular-weight inhibitors (Prontosil or acetazolamide). The time course of effluent perfusate pH equilibration was measured in a stop-flow pH electrode apparatus. Pulmonary CO2 excretion (Vco2) was monitored by continuously recording expired CO2 concentration. The lungs were ventilated with room air and perfused at 37 degrees C with KRB prebubbled with 5% CO2- 20% O2- 75% N2. The results obtained show that both the low-molecular-weight inhibitors and PD's caused postcapillary pH disequilibria (delta pH) in effluent perfusate. However, only acetazolamide and Prontosil caused a reduction in Vco2. These results suggest that there is an intravascular CA, presumably associated with endothelial cell membranes, that is accessible to all inhibitors used and is responsible in part for equilibration of the CO2- HCO3- -H+ reactions in the perfusate but, under the conditions used, does not affect CO2 excretion; and there is an extravascular (possibly intracellular) CA that can be inhibited by low-molecular-weight inhibitors, is primarily responsible for enhanced CO2 transfer across the alveolar-capillary barrier (perhaps via facilitation of CO2 diffusion), and is in part responsible for pH equilibration.

Effects of terbutaline on sodium transport in isolated perfused rat lung

We have previously presented evidence that cultured alveolar epithelial cell monolayers actively transport sodium from medium to substratum, and that this process can be stimulated by beta-agonists. In this study the isolated perfused rat lung was utilized to investigate sodium transport across intact mammalian alveolar epithelium. Radioisotopic tracer(s) (22Na and/or [14C]sucrose) were instilled into the airways of isolated Ringer-perfused rat lungs. The appearance of isotope(s) in the recirculated perfusate was measured and a permeability-surface area product was calculated. Pharmacological agent(s) (terbutaline and/or propranolol) were present in the instillate or were added to the perfusate during the experiments. Terbutaline alone, whether in the instillate or perfusate, caused a significant increase in 22Na flux. This increase was prevented by the presence of propranolol. [14C]sucrose fluxes were unaffected by the presence of terbutaline. These data are consistent with the presence of an active component of sodium transport across intact mammalian alveolar epithelium that leads to removal of sodium from the alveolar space.

Sodium-amino acid cotransport by type II alveolar epithelial cells

Type II alveolar epithelial cell monolayers have been shown to actively transport sodium (Na+). Coupling to amino acid uptake could be an important mechanism for Na+ entry into these cells. This study demonstrates the presence of such a coupled cotransport mechanism in the plasma membrane of isolated type II cells by use of the nonmetabolizable amino acid analogue alpha-methylaminoisobutyric acid (MeAIB). Transport of MeAIB in 137 mM Na+ is saturable, with the uptake constant (Vmax) equaling 13.9 pmol X mg prot-1 X s-1 and the Michaelis-Menten constant (Km) equaling 0.13 mM. In the presence of Na+, MeAIB is accumulated against a concentration gradient. MeAIB uptake in the absence of Na+ is linear with MeAIB concentration, as expected for simple diffusion. The Hill coefficient for Na+-MeAIB cotransport is 1.11, suggesting a 1:1 stoichiometry. Proline inhibits Na+-MeAIB cotransport, with Ki equaling 0.5 mM. These findings suggest that Na+-amino acid cotransport may be an important pathway for Na+ (and/or amino acid) uptake into type II alveolar epithelial cells.