Sodium-dependent phosphate and alanine transports but sodium-independent hexose transport in type II alveolar epithelial cells in primary culture

Sodium-coupled neutral amino acid transporter SNAT2 counteracts cardiogenic pulmonary edema by driving alveolar fluid clearance

The constant transport of ions across the alveolar epithelial barrier regulates alveolar fluid homeostasis. Dysregulation or inhibition of Na⁺ transport causes fluid accumulation in the distal airspaces resulting in impaired gas exchange and respiratory failure. Previous studies have primarily focused on the critical role of amiloride-sensitive epithelial sodium channel (ENaC) in alveolar fluid clearance (AFC), yet activation of ENaC failed to attenuate pulmonary edema in clinical trials. Since 40% of AFC is amiloride-insensitive, Na⁺ channels/transporters other than ENaC such as Na⁺-coupled neutral amino acid transporters (SNATs) may provide novel therapeutic targets. Here, we identified a key role for SNAT2 (SLC38A2) in AFC and pulmonary edema resolution. In isolated perfused mouse and rat lungs, pharmacological inhibition of SNATs by HgCl₂ and α-methylaminoisobutyric acid (MeAIB) impaired AFC. Quantitative RT-PCR identified SNAT2 as the highest expressed System A transporter in pulmonary epithelial cells. Pharmacological inhibition or siRNA-mediated knockdown of SNAT2 reduced transport of l-alanine across pulmonary epithelial cells. Homozygous Slc38a2^-/- mice were subviable and died shortly after birth with severe cyanosis. Isolated lungs of Slc38a2^+/- mice developed higher wet-to-dry weight ratios (W/D) as compared to wild type (WT) in response to hydrostatic stress. Similarly, W/D ratios were increased in Slc38a2^+/- mice as compared to controls in an acid-induced lung injury model. Our results identify SNAT2 as a functional transporter for Na⁺ and neutral amino acids in pulmonary epithelial cells with a relevant role in AFC and the resolution of lung edema. Activation of SNAT2 may provide a new therapeutic strategy to counteract and/or reverse pulmonary edema.

Cytokine-Regulation of Na+-K+-Cl- Cotransporter 1 and Cystic Fibrosis Transmembrane Conductance Regulator-Potential Role in Pulmonary Inflammation and Edema Formation

Pulmonary edema, a major complication of lung injury and inflammation, is defined as accumulation of extravascular fluid in the lungs leading to impaired diffusion of respiratory gases. Lung fluid balance across the alveolar epithelial barrier protects the distal airspace from excess fluid accumulation and is mainly regulated by active sodium transport and Cl^- absorption. Increased hydrostatic pressure as seen in cardiogenic edema or increased vascular permeability as present in inflammatory lung diseases such as the acute respiratory distress syndrome (ARDS) causes a reversal of transepithelial fluid transport resulting in the formation of pulmonary edema. The basolateral expressed Na⁺-K⁺-2Cl^- cotransporter 1 (NKCC1) and the apical Cl^- channel cystic fibrosis transmembrane conductance regulator (CFTR) are considered to be critically involved in the pathogenesis of pulmonary edema and have also been implicated in the inflammatory response in ARDS. Expression and function of both NKCC1 and CFTR can be modulated by released cytokines; however, the relevance of this modulation in the context of ARDS and pulmonary edema is so far unclear. Here, we review the existing literature on the regulation of NKCC1 and CFTR by cytokines, and-based on the known involvement of NKCC1 and CFTR in lung edema and inflammation-speculate on the role of cytokine-dependent NKCC1/CFTR regulation for the pathogenesis and potential treatment of pulmonary inflammation and edema formation.

Molecular cloning and functional characterization of swine sodium dependent phosphate cotransporter type II b (NaPi-IIb) gene

A sodium-dependent phosphate transporter gene, NaPi-IIb, was isolated from swine small intestine using cDNA library screening method. Sequencing analysis revealed that the NaPi-IIb cDNA sequences was 2,016 bp in length and encoded an open-reading frame consisting of 671 amino acids. The cDNA showed 83.1 % sequences identity to the human NaPi-IIb and 78.7 % sequences identity to the chicken NaPi-IIb. Prediction of membrane spanning domains based on the hydrophilic and hydrophobic properties of the amino acids suggested that a putative protein had nine transmembrane domains, with both the NH(2) and COOH terminal being intracellular. By northern blot, a ~4.2 kb transcript was found to be abundantly expressed in mall intestine, lung, ovary, mammary glands, liver, kidney, salivary glands, placenta and thymus. Microinjection of swine NaPi-IIb cRNA into Xenopus oocytes demonstrated that the NaPi-IIb showed sodium-dependent Pi cotransport activity, and an approximate 31-fold increase of Pi uptake was seen in cRNA injected oocytes. The swine NaPi-IIb transporter expressed in Xenopus oocytes had a Km for Pi of ~79.35 ± 7.2 μM. Furthermore, the pH dependency characterization of swine NaPi-IIb transporter showed activation at extracellular alkaline-pH.

Dexamethasone and cyclic AMP regulate sodium phosphate cotransporter (NaPi-IIb and Pit-1) mRNA and phosphate uptake in rat alveolar type II epithelial cells

Alveolar epithelial type II (AT II) cells need phosphate (Pi) for surfactant synthesis. The Na-dependent (Na(d)) Pi transporters NaPi-IIb and Pit-1 are expressed in lung, but their expression, regulation, and function in AT II cells remain unclear. We studied NaPi-IIb and Pit-1 mRNA expression in cultured AT II cells isolated from adult rat lung, their regulation by agents known to enhance surfactant production, dexamethasone (dex) and dibutyryl cyclic AMP (cAMP), and the effects of dex and cAMP on Na(d) Pi uptake by this cell type. By Northern analysis, cultured AT II cells expressed both NaPi-IIb (4.8 and 4.0 kb) and Pit-1 (4.3 kb) mRNA. Treatment with 100 nmol/l dex for 24 h decreased the expression of both mRNAs (to 0.48 +/- 0.06 and 0.77 +/- 0.05, respectively, as compared to control), while 0.1 mmol/l cAMP stimulated NaPi-IIb (1.94 +/- 0.22) but not Pit-1 mRNA (0.90 +/- 0.05, compared to vehicle-treated cells). NaPi-IIb and Pit-1 proteins could not be identified by western analysis of plasma membrane preparations of cultured AT II cells. AT II cells take up Pi in a Na(d) manner. Uptake was slightly (to 0.78-fold of the control) decreased by 100 nmol/l dex but not affected by 0.1 mmol/l cAMP treatment. Although NaPi-IIb mRNA expression was maintained to some extent by AT II cells kept in primary culture, Pi uptake was more closely related to Pit-1 mRNA expression.

Low dietary inorganic phosphate affects the lung growth of developing mice

Inorganic phosphate (Pi) plays a critical role in diverse cellular functions, and regulating the Pi balance is accomplished by sodium-dependent Pi co-transporter (NPT). Pulmonary NPT has recently been identified in mammalian lungs. However, to date, many of the studies that have involved Pi have mainly focused on its effect on bone and kidney. Therefore, current study was performed to discover the potential effects of low Pi on the lung of developing transgenic mice expressing the renilla/firefly luciferase dual reporter gene. Two-weeks old male mice divided into 2 groups and these groups were fed either a low PI diet or a normal control diet (normal: 0.5% Pi, low: 0.1% Pi) for 4 weeks. After 4 weeks of the diet, all the mice were sacrificed. Their lungs were harvested and analyzed by performing luciferase assay, Western blotting, kinase assay and immunohistochemistry. Our results demonstrate that low Pi affects the lungs of developing mice by disturbing protein translation, the cell cycle and the expression of fibroblast growth factor-2. These results suggest that optimally regulating Pi consumption may be important to maintain health.

Gene regulation in the adaptive process to hypoxia in lung epithelial cells

Lung alveolar epithelial cells are normally very well oxygenated but may be exposed to hypoxia in many pathological conditions such as pulmonary edema, acute respiratory distress syndrome, chronic obstructive pulmonary diseases, or in some environmental conditions such ascent to high altitude. The ability of alveolar epithelial cells to cope with low oxygen tensions is crucial to maintain the structural and functional integrity of the alveolar epithelium. Alveolar epithelial cells appear to be remarkably tolerant to oxygen deprivation as they are able to maintain adequate cellular ATP content during prolonged hypoxic exposure when mitochondrial oxidative phosphorylation is limited. This property mostly relies on the ability of the cells to rapidly modify their gene expression program, stimulating the expression of genes involved in anaerobic energy supply and repressing expression of genes involved in some ATP-consuming cellular processes. This adaptive strategy of the cells is mostly, but not entirely, dependent on the expression of hypoxia-inducible factors (HIFs), known to be responsible for orchestrating a large number of hypoxia-sensitive genes. This review focuses on the role of HIF isoforms expressed in alveolar epithelial cells exposed to hypoxia and on the specific hypoxic gene regulation that takes place in alveolar epithelial cells either through HIF-dependent or -independent pathways.

Glucose transport in the lung and its role in liquid movement

Glucose concentration in the liquid present in the alveolar/airway lumen is the consequence of the balance between removal by lung epithelial cells and entry from the plasma or lung interstitium through the paracellular pathway. Glucose removal is mediated by active, Na(+) -dependent, cotransport and results in transepithelial Na(+) transport and liquid absorption in animals with significant rates of luminal glucose uptake and when luminal glucose concentration is high enough. Cotransport kinetics predicted a low luminal glucose concentration at the steady state, and foetal lung fluid and adult alveolar epithelial lining fluid glucose concentrations were indeed found lower than plasma. When luminal glucose concentration is low, the glucose-dependent part of transepithelial Na(+) transport is abated and alveolar liquid clearance reduced. A means to refuel this mechanism of liquid absorption would be to increase glucose entry in alveolar spaces through an increase in paracellular permeability. This hypothesis was modelled, and experimental data were found to acceptably agree with predictions.

Fluid transport across cultured rat alveolar epithelial cells: a novel in vitro system

Previous studies have used fluid-instilled lungs to measure net alveolar fluid transport in intact animal and human lungs. However, intact lung studies have two limitations: the contribution of different distal lung epithelial cells cannot be studied separately, and the surface area for fluid absorption can only be approximated. Therefore, we developed a method to measure net vectorial fluid transport in cultured rat alveolar type II cells using an air-liquid interface. The cells were seeded on 0.4-microm microporous inserts in a Transwell system. At 96 h, the transmembrane electrical resistance reached a peak level (1,530 +/- 115 Omega.cm(2)) with morphological evidence of tight junctions. We measured net fluid transport by placing 150 microl of culture medium containing 0.5 microCi of (131)I-albumin on the apical side of the polarized cells. Protein permeability across the cell monolayer, as measured by labeled albumin, was 1.17 +/- 0.34% over 24 h. The change in concentration of (131)I-albumin in the apical fluid was used to determine the net fluid transported across the monolayer over 12 and 24 h. The net basal fluid transport was 0.84 microl.cm(-2).h(-1). cAMP stimulation with forskolin and IBMX increased fluid transport by 96%. Amiloride inhibited both the basal and stimulated fluid transport. Ouabain inhibited basal fluid transport by 93%. The cultured cells retained alveolar type II-like features based on morphologic studies, including ultrastructural imaging. In conclusion, this novel in vitro system can be used to measure net vectorial fluid transport across cultured, polarized alveolar epithelial cells.

Lung epithelial fluid transport and the resolution of pulmonary edema

The discovery of mechanisms that regulate salt and water transport by the alveolar and distal airway epithelium of the lung has generated new insights into the regulation of lung fluid balance under both normal and pathological conditions. There is convincing evidence that active sodium and chloride transporters are expressed in the distal lung epithelium and are responsible for the ability of the lung to remove alveolar fluid at the time of birth as well as in the mature lung when pathological conditions lead to the development of pulmonary edema. Currently, the best described molecular transporters are the epithelial sodium channel, the cystic fibrosis transmembrane conductance regulator, Na+-K+-ATPase, and several aquaporin water channels. Both catecholamine-dependent and -independent mechanisms can upregulate isosmolar fluid transport across the distal lung epithelium. Experimental and clinical studies have made it possible to examine the role of these transporters in the resolution of pulmonary edema.

Glucose transporter gene expression in freshly isolated and cultured rat pneumocytes

Alveolar epithelium in situ takes up luminal glucose by cotransport with sodium. Cultured alveolar type II pneumocytes have only sodium-independent glucose uptake. It is unclear which isoforms are responsible for glucose transport in these cells and why sodium-glucose cotransport activity disappears during culture. GLUT1, GLUT4, GLUT5 and SGLT1 mRNA were detected in freshly isolated rat alveolar type II cells by reverse transcriptase-polymerase chain reaction. We show that SGLT1 mRNA was 90% lower in cells cultured in plastic wells for 2 or 4 days than in freshly isolated cells. mRNAs coding for the facilitated transporters were reduced from 40% (GLUT1) and 75% (GLUT4 and GLUT5) in cultured cells. Cells cultured at the air-liquid interface better preserved their phenotype as attested by significantly higher surfactant-associated protein mRNA levels. However, these cells had no higher GLUT1 and SGLT1 gene expression. Thus, alveolar type II cells lose sodium-glucose cotransport activity in part because of a decrease in mRNA levels. These changes in gene expression and/or mRNA stability may be an additional consequence of the shift towards the type I cell phenotype observed in cultured type II pneumocytes.

Selectivity properties of a Na-dependent amino acid cotransport system in adult alveolar epithelial cells

We investigated the amino acid specificity of a Na-dependent amino acid cotransport system that contributes to transepithelial Na absorption in the apical membrane of cultured adult rat alveolar epithelial cell monolayers. Short-circuit current was increased by basic, uncharged polar, and nonpolar amino acids but not by L-aspartic acid or L-proline. EC(50) values for L-lysine and L-histidine were 0.16 and 0.058 mM, respectively. The L-lysine-stimulated short-circuit current was Na dependent, with a concentration causing a half-maximal stimulation by Na of 44.24 mM. L-Serine, L-glutamine, and L-cysteine had EC(50) values of 0.095, 0.25, and 0.12 mM, respectively. L-Alanine had the highest affinity, with an EC(50) of 0.027 mM. We conclude that monolayer cultures of adult rat alveolar epithelial cells possess a broad-specificity Na-dependent amino acid cotransport system with properties consistent with system B(0,+). We suggest that this cotransport system plays a critical role in recycling of constituent amino acids that make up glutathione, thus ensuring efficient replenishment of this important antioxidant within the alveolar fluid.

Isolation and localization of type IIb Na/Pi cotransporter in the developing rat lung

Differential display analysis of rat lung at different developmental stages identified a fragment, HG80, which appeared on embryonic day 16.5 and thereafter. A full-length cDNA derived from a cDNA library of newborn rat lung probed with HG80 was the rat counterpart of sodium-dependent phosphate transporter type IIb and was designated rNaPi IIb. In situ hybridization showed that rNaPi IIb was expressed in type II alveolar cells, suggesting a role in the synthesis of surfactant in the alveoli. The time-dependent changes in localization of this gene in the developing lung and its possible use as a type II pneumocyte marker are discussed.

Hypoxia regulates gene expression of alveolar epithelial transport proteins

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

Hypoxia upregulates activity and expression of the glucose transporter GLUT1 in alveolar epithelial cells

Alveolar epithelial cells (AEC) are directly exposed to high alveolar O(2) tension. Many pulmonary disorders are associated with a decrease in alveolar O(2) tension and AEC need to develop adaptative mechanisms to cope with O(2) deprivation. Under hypoxia, because of inhibition of oxidative phosphorylation, adenosine triphosphate supply is dependent on the ability of cells to increase anaerobic glycolysis. In this study we show that under hypoxia, primary rat AEC maintained their energy status close to that of normoxic cells through increasing anaerobic glycolysis. We therefore examined the effect of hypoxia on glucose transport and evaluated the mechanisms of this regulation. Hypoxia induced a stimulation of Na-independent glucose transport, as shown by the increase in 2-deoxy-D-glucose (DG) uptake. This increase was dependent on time and O(2) concentration: maximal at 0% O(2) for 18 h, and reversible after hypoxic cells were allowed to recover in normoxia. Concomitantly, exposure of AEC to hypoxia (18 h 0% O(2)) induced a 3-fold increase of glucose transporter GLUT1 at both protein and messenger RNA (mRNA) levels. To determine whether the increase in GLUT1 mRNA level was dependent on O(2) deprivation per se or resulted from decrease of oxidative phosphorylation, we examined in normoxic cells the effects of cobalt chloride and Na azide, respectively. Cobalt chloride (100 microM) and Na azide (1 mM) increased both mRNA levels and DG uptake, mimicking the effect of hypoxia. Electrophoretic mobility shift assays revealed a hypoxic and a cobalt chloride induction of a hypoxia-inducible factor (HIF) that bound to the sequence of nucleotides, corresponding to a hypoxia-inducible element upstream of the GLUT1 gene. AEC also expressed this factor under nonhypoxic conditions. Together, our results demonstrate that AEC increased glucose transport in response to hypoxia by regulating GLUT1 gene-encoding protein. This regulation likely occurred at the transcriptional level through the activation of an HIF, the nature of which remains to be elucidated.

Expression of type II Na-P(i) cotransporter in alveolar type II cells

Type II Na-P(i) cotransporters (type IIa and type IIb) represent apically located Na-P(i) cotransporters in epithelia of proximal tubules (type IIa) and small intestine (type IIb). Here we provide evidence that the type IIb (but not the type IIa) Na-P(i) cotransporter is also expressed in the lung. With the use of immunohistochemistry, location of the type IIb protein was found exclusively in the apical membrane of type II cells of the alveolar epithelium. Such a location of the type IIb cotransporter suggests an involvement in the reuptake of phosphate necessary for the synthesis of surfactant. A possible regulation of the abundance of the type IIb cotransporter in the lung was studied after adaptation of mice to a low-P(i) diet. After a chronic adaptation to a low-P(i) diet, no changes in the type IIb protein and the type IIb transcript were observed. These results exclude dietary intake of phosphate as a regulatory factor of the type IIb Na-P(i) cotransporter in alveolar type II cells.

Cloning and functional characterization of a sodium-dependent phosphate transporter expressed in human lung and small intestine

A cDNA clone with 53% amino acid identity to the human type II sodium-dependent phosphate transporter (NaPi-3) was isolated from human small intestine and lung. Functional characterization in Xenopus laevis oocytes showed this cDNA to encode a sodium-dependent phosphate transporter. The electrogenic response is similar to that found in other type II transporters but an inverse pH dependence was observed. By Northern blot, a 4.2-kb transcript was found to be abundantly expressed in lung and, to a lesser degree, in several other tissues of epithelial origin including small intestine, pancreas, prostate, and kidney. This transcript encompasses a 2.073-kb open reading frame which is most closely related (78% amino acid identity) to the mouse sodium-dependent phosphate transporter IIb isoform. This novel transporter, designated human NaPi-3b (Genbank AF111856), appears to be an isoform of the mammalian renal type II co-transporter family.

Hypoxia downregulates expression and activity of epithelial sodium channels in rat alveolar epithelial cells

Decrease in alveolar oxygen tension may induce acute lung injury with pulmonary edema. We investigated whether, in alveolar epithelial cells, expression and activity of epithelial sodium (Na) channels and Na,K-adenosine triphosphatase, the major components of transepithelial Na transport, were regulated by hypoxia. Exposure of cultured rat alveolar cells to 3% and 0% O2 for 18 h reduced Na channel activity estimated by amiloride-sensitive 22Na influx by 32% and 67%, respectively, whereas 5% O2 was without effect. The decrease in Na channel activity induced by 0% O2 was time-dependent, significant at 3 h of exposure and maximal at 12 and 18 h. It was associated with a time-dependent decline in the amount of mRNAs encoding the alpha-, beta-, and gamma-subunits of the rat epithelial Na channel (rENaC) and with a 42% decrease in alpha-rENaC protein synthesis as evaluated by immunoprecipitation after 18 h of exposure. The 0% O2 hypoxia also caused a time-dependent decrease in (1) ouabain-sensitive 86Rubidium influx in intact cells, (2) the maximal velocity of Na,K-ATPase on crude homogenates, and (3) alpha1- and beta1-Na,K-ATPase mRNA levels. Levels of rENaC and alpha1-Na,K-ATPase mRNA returned to control values within 48 h of reoxygenation, and this was associated with complete functional recovery. We conclude that hypoxia induced a downregulation of expression and activity of epithelial Na channels and Na,K-ATPase in alveolar cells. Subsequent decrease in Na reabsorption by alveolar epithelium could participate in the maintenance of hypoxia-induced alveolar edema.

Evidence for Na-K-Cl cotransport in alveolar epithelial cells: effect of phorbol ester and osmotic stress

We have investigated the presence of Na-K-Cl cotransport in alveolar type II cells using uptake of 86Rb. Several data support the presence of a Na-K-Cl cotransport in these cells. First, a large fraction of ouabain-resistant 86Rb uptake was inhibited by bumetanide and furosemide. Second, bumetanide-sensitive 86Rb uptake required the presence of Na+ and Cl- in the incubation medium; dependency on extracellular Na+ and K+ was hyperbolic, with a Km of 14.6 mM and 8.3 mM, respectively, while dependency on extracellular Cl- was sigmoidal, which suggests a 1:1:2 stoichiometry. Third, a fraction of amiloride-insensitive 22Na influx was deeply inhibited by bumetanide. 22Na influx was dependent on the presence of extracellular K+ and Cl-. Since Na-K-Cl activity dramatically decreased with time in culture, further characterization of the cotransport on polarized cells could not be performed. The phorbol ester PMA inhibited Na-K-Cl cotransport in a time- and concentration-dependent manner. This inhibition was mimicked by oleoylacetylglycerol, dioctanoylglycerol, and the diacylglycerol kinase inhibitor R59022, and was reversed by an antagonist of PKC, staurosporine. Since the Na-K-Cl cotransport has been reported to be involved in cell volume regulation, we investigated its modulation by changes in extracellular osmolarity. Na-K-Cl activity was increased after a two-step procedure: swelling in hypotonic medium followed by shrinking in hypertonic medium. Under these conditions, cotransport activity increased whenever PKC activity was up- or downregulated, which suggests that the cell volume-induced modulation of the cotransport is independent from the PKC activity. Though we were not able to determine the polarity of the cotransport, it may also be involved in the absorptive function of alveolar type II cells, and would provide an alternate pathway for sodium entry.

Lipopolysaccharides stimulate Na-dependent transport in alveolar cells and protect against oxidant injury

We have evaluated the effect of lipopolysaccharides (LPS), endotoxins from gram negative bacteria, on sodium-coupled amino acid and phosphate transport by alveolar epithelial type II cells and on their alteration induced by oxidants. Alveolar type II cells were obtained by enzymatic digestion of rat lung and grown for 24 h prior to incubation with LPS and then exposed or not exposed to H2O2 (2.5 mM; 20 min). LPS (10 micrograms/ml, 24 h) induced a significant increase in the Na-dependent component of alanine and phosphate uptake while they decreased Na,K-ATPase activity measured by ouabain-sensitive 86Rb influx. We showed that this stimulatory effect i) was independent from macrophage products since it was not mimicked either by supernatant of LPS-treated alveolar macrophages or by pretreatment with tumor necrosis factor and/or interleukin 1 and ii) was dependent on protein synthesis since it was abolished by protein synthesis inhibitors cycloheximide and actinomycin D. Moreover, LPS blunted H2O2-induced decrease of Na-dependent alanine and phosphate uptake. This protective effect of LPS against H2O2 injury i) was independent of macrophage products, ii) was abolished by cycloheximide, and iii) was not associated with either changes in extracellular H2O2 clearance or catalase and glutathione peroxidase activities. We conclude that, in alveolar type II cells, LPS stimulate sodium-coupled transport by a process involving protein synthesis and partially prevent H2O2-induced decrease of Na-coupled transport without discernible change in antioxidant activities.

Sodium-coupled transport of glucose by plasma membranes of type II pneumocytes

Sodium-dependent absorption of alveolar fluid promotes efficient gas exchange. In animal models, alveolar glucose stimulates phlorizin-sensitive, Na(+)-dependent fluid absorption. It is hypothesized that Na+/glucose cotransporters are localized to apical membranes of type II pneumocytes. Enriched apical and basolateral plasma membrane vesicles were isolated from adult bovine type II pneumocytes. Uptakes of 22Na+ and [3H]glucose by enriched apical and basolateral vesicles were monitored over time. Following addition of external glucose (75 mM), 22Na+ uptake by mannitol-loaded, apically-enriched vesicles was significantly increased over controls. Substitution of interior-negative charge gradients for internally directed Na+ gradients increased glucose-dependent Na+ uptakes even greater. By contrast, external glucose did not significantly promote 22Na+ uptake by enriched basolateral vesicles. External Na+ (75 mM) significantly increased [3H]glucose uptakes by enriched apical vesicles with evidence of overshoot. Phlorizin (100 microM) inhibited both glucose-coupled 22Na+ uptakes and Na(+)-coupled [3H]glucose uptakes. These observations support localization of electrogenic, Na+/glucose cotransporters to enriched apical membranes of mature type II pneumocytes.

Expression cloning of human and rat renal cortex Na/Pi cotransport

We have isolated two cDNA clones, NaPi-2 and NaPi-3, by screening rat kidney cortex and human kidney cortex cDNA libraries, respectively, for expression of sodium-dependent phosphate transport in Xenopus laevis oocytes. Substrate specificity and a detailed kinetic analysis (Na, Pi, H+ concentrations) suggested that expressed uptake activities relate to proximal tubular brush border membrane Na/Pi cotransport. NaPi-2 cDNA contains 2464 bp encoding a protein of 637 aa; NaPi-3 cDNA contains 2573 bp encoding a protein of 639 aa. NaPi-2- and NaPi-3-deduced protein sequences show high homology to each other but are different from the protein sequence deduced from the previously cloned NaPi-1 cDNA (from rabbit proximal tubules). Hydropathy profile predictions suggest at least eight membrane-spanning regions in NaPi-2/3-related proteins. In vitro translation results in proteins of the expected size and suggests glycosylation. Northern blot analysis shows corresponding mRNA species (approximately 2.7 kb) in kidney cortex of various species but no hybridization with RNAs isolated from a variety of other tissues (including intestinal segments); a hybridization signal (approximately 4.8 kb) was observed only in the lung (human). We conclude that we have structurally identified two closely related proteins most likely involved in human and rat renal brush border Na/Pi cotransport.

GLUT 1-glucose transporter protein in adult and fetal mouse lung

We observed approximately 45-50 kD GLUT 1 protein in mouse lung homogenates and demonstrated a greater abundance in fetus compared to adult. In situ immunohistochemical analysis demonstrated GLUT 1 expression only in the perineural sheath of nerves. While the trapped fetal red blood cells expressed GLUT 1 abundantly, adult red blood cells were devoid of GLUT 1. No GLUT 1 was evident in fetal and adult lung alveolar and bronchiolar epithelial cells, vascular endothelial cells and the lung mesenchymal elements. Thus, GLUT 1 is not the major lung glucose transporter.