Immunocytochemical and functional characterization of Na+ conductance in adult alveolar pneumocytes

Chloride transport-driven alveolar fluid secretion is a major contributor to cardiogenic lung edema

Alveolar fluid clearance driven by active epithelial Na(+) and secondary Cl(-) absorption counteracts edema formation in the intact lung. Recently, we showed that impairment of alveolar fluid clearance because of inhibition of epithelial Na(+) channels (ENaCs) promotes cardiogenic lung edema. Concomitantly, we observed a reversal of alveolar fluid clearance, suggesting that reversed transepithelial ion transport may promote lung edema by driving active alveolar fluid secretion. We, therefore, hypothesized that alveolar ion and fluid secretion may constitute a pathomechanism in lung edema and aimed to identify underlying molecular pathways. In isolated perfused lungs, alveolar fluid clearance and secretion were determined by a double-indicator dilution technique. Transepithelial Cl(-) secretion and alveolar Cl(-) influx were quantified by radionuclide tracing and alveolar Cl(-) imaging, respectively. Elevated hydrostatic pressure induced ouabain-sensitive alveolar fluid secretion that coincided with transepithelial Cl(-) secretion and alveolar Cl(-) influx. Inhibition of either cystic fibrosis transmembrane conductance regulator (CFTR) or Na(+)-K(+)-Cl(-) cotransporters (NKCC) blocked alveolar fluid secretion, and lungs of CFTR(-/-) mice were protected from hydrostatic edema. Inhibition of ENaC by amiloride reproduced alveolar fluid and Cl(-) secretion that were again CFTR-, NKCC-, and Na(+)-K(+)-ATPase-dependent. Our findings show a reversal of transepithelial Cl(-) and fluid flux from absorptive to secretory mode at hydrostatic stress. Alveolar Cl(-) and fluid secretion are triggered by ENaC inhibition and mediated by NKCC and CFTR. Our results characterize an innovative mechanism of cardiogenic edema formation and identify NKCC1 as a unique therapeutic target in cardiogenic lung edema.

Regulation of alveolar epithelial Na+ channels by ERK1/2 in chlorine-breathing mice

The mechanisms by which the exposure of mice to Cl(2) decreases vectorial Na(+) transport and fluid clearance across their distal lung spaces have not been elucidated. We examined the biophysical, biochemical, and physiological changes of rodent lung epithelial Na(+) channels (ENaCs) after exposure to Cl(2), and identified the mechanisms involved. We measured amiloride-sensitive short-circuit currents (I(amil)) across isolated alveolar Type II (ATII) cell monolayers and ENaC single-channel properties by patching ATII and ATI cells in situ. α-ENaC, γ-ENaC, total and phosphorylated extracellular signal-related kinase (ERK)1/2, and advanced products of lipid peroxidation in ATII cells were measured by Western blot analysis. Concentrations of reactive intermediates were assessed by electron spin resonance (ESR). Amiloride-sensitive Na(+) channels with conductances of 4.5 and 18 pS were evident in ATI and ATII cells in situ of air-breathing mice. At 1 hour and 24 hours after exposure to Cl(2), the open probabilities of these two channels decreased. This effect was prevented by incubating lung slices with inhibitors of ERK1/2 or of proteasomes and lysosomes. The exposure of ATII cell monolayers to Cl(2) increased concentrations of reactive intermediates, leading to ERK1/2 phosphorylation and decreased I(amil) and α-ENaC concentrations at 1 hour and 24 hours after exposure. The administration of antioxidants to ATII cells before and after exposure to Cl(2) decreased concentrations of reactive intermediates and ERK1/2 activation, which mitigated the decrease in I(amil) and ENaC concentrations. The reactive intermediates formed during and after exposure to Cl(2) activated ERK1/2 in ATII cells in vitro and in vivo, leading to decreased ENaC concentrations and activity.

Malnutrition impairs alveolar fluid clearance in rat lungs

Inadequate nutrition complicates the clinical course of critically ill patients, and many of these patients develop pulmonary edema. However, little is known about the effect of malnutrition on the mechanisms that resolve alveolar edema. Therefore, we studied the mechanisms responsible for the decrease in alveolar fluid clearance in rats exposed to malnutrition. Rats were allowed access to water, but not to food, for 120 h. Then, the left and right lungs were isolated for the measurement of lung water volume and alveolar fluid clearance, respectively. The rate of alveolar fluid clearance was measured by the progressive increase in the concentration of Evans blue dye that was instilled into the distal air spaces with an isosmolar 5% albumin solution over 1 h. Malnutrition decreased alveolar fluid clearance by 38% compared with controls. Amiloride (10−3M) abolished alveolar fluid clearance in malnourished rats. Either refeeding for 120 h following nutritional deprivation for 120 h or an oral supply of sodium glutamate during nutritional deprivation for 120 h restored alveolar fluid clearance to 91 and 86% of normal, respectively. Dibutyryl-cGMP, a cyclic nucleotide-gated cation channel agonist, increased alveolar fluid clearance in malnourished rats supplied with sodium glutamate. Terbutaline, a β2-adrenergic agonist, increased alveolar fluid clearance in rats under all conditions (control, malnutrition, refeeding, and glutamate-treated). These results indicate that malnutrition impairs primarily amiloride-insensitive and dibutyryl-cGMP-sensitive alveolar fluid clearance, but this effect is partially reversible by refeeding, treatment with sodium glutamate, or β-adrenergic agonist therapy.

Modulation of sodium transport in fetal alveolar epithelial cells by oxygen and corticosterone

Regulation of active Na(+) transport across fetal distal lung epithelial cells (FDLE) by corticosterone (CST), corticotropin-releasing hormone (CRH), and oxygen tension may be crucial for postnatal adaptation. FDLE isolated from 19-day rat fetuses (term: 22 days) were grown on permeable supports to confluent monolayers (duration 3 days) in 2.5, 5, 12, or 20% O(2) with 5% CO(2)-balance N(2) and mounted in Ussing chambers for measurement of short-circuit currents (I(sc)). FDLE monolayers grown in 20% O(2) had significantly higher levels of total I(sc) and of their amiloride-sensitive (I(amil)) and ouabain-sensitive (I(ouab)) components than hypoxic cells. Values (microA/cm(2) +/- SE) for 2.5-5% O(2) and 20% O(2) were, respectively, I(sc) 5.3 +/- 0.2 vs. 8.4 +/- 0.3 (P < 0.001), I(amil) 3.4 +/- 0.2 vs. 4.3 +/- 0.2 (P < 0.01), and I(ouab) 3.4 +/- 0.6 vs. 9.1 +/- 0.6 (P < 0.001). Addition of CST but not CRH to the culture medium at any O(2) concentration increased I(amil). FDLE cells grown at 5% O(2) expressed significantly lower levels of alpha-, beta-, and gamma-epithelial Na(+) channel (ENaC), and of the alpha(1)-Na(+)-K(+)-ATPase, as determined by Western blotting. We conclude that higher O(2) concentrations increased total vectorial Na(+) transport, and the function of Na(+)-K(+)-ATPase and apical amiloride-sensitive Na(+) conductance, whereas CST only increased ENaC function.

Invited review: biophysical properties of sodium channels in lung alveolar epithelial cells

Amiloride-sensitive sodium channels in the lung play an important role in lung fluid balance. Particularly in the alveoli, sodium transport is closely regulated to maintain an appropriate fluid layer on the surface of the alveoli. Alveolar type II cells appear to play an important role in this sodium transport, with the role of alveolar type I cells being less clear. In alveolar type II cells, there are a variety of different amiloride-sensitive, sodium-permeable channels. This significant diversity appears to play a role in both normal lung physiology and in pathological states. In many epithelial tissues, amiloride-sensitive epithelial sodium channels (ENaC) are formed from three subunit proteins, designated alpha-, beta-, and gamma-ENaC. At least part of the diversity of sodium-permeable channels in lung arises from the assembling of different combinations of these subunits to form channels with different biophysical properties and different mechanisms for regulation. This leads to epithelial tissue in the lung, which has enormous flexibility to alter the magnitude and regulation of salt and water transport. In this review, we discuss the biophysical properties and occurrence of these various channels and some of the mechanisms for their regulation.

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.

Alveolar epithelial type I cells contain transport proteins and transport sodium, supporting an active role for type I cells in regulation of lung liquid homeostasis

Transport of lung liquid is essential for both normal pulmonary physiologic processes and for resolution of pathologic processes. The large internal surface area of the lung is lined by alveolar epithelial type I (TI) and type II (TII) cells; TI cells line >95% of this surface, TII cells <5%. Fluid transport is regulated by ion transport, with water movement following passively. Current concepts are that TII cells are the main sites of ion transport in the lung. TI cells have been thought to provide only passive barrier, rather than active, functions. Because TI cells line most of the internal surface area of the lung, we hypothesized that TI cells could be important in the regulation of lung liquid homeostasis. We measured both Na(+) and K(+) (Rb(+)) transport in TI cells isolated from adult rat lungs and compared the results to those of concomitant experiments with isolated TII cells. TI cells take up Na(+) in an amiloride-inhibitable fashion, suggesting the presence of Na(+) channels; TI cell Na(+) uptake, per microgram of protein, is approximately 2.5 times that of TII cells. Rb(+) uptake in TI cells was approximately 3 times that in TII cells and was inhibited by 10(-4) M ouabain, the latter observation suggesting that TI cells exhibit Na(+)-, K(+)-ATPase activity. By immunocytochemical methods, TI cells contain all three subunits (alpha, beta, and gamma) of the epithelial sodium channel ENaC and two subunits of Na(+)-, K(+)-ATPase. By Western blot analysis, TI cells contain approximately 3 times the amount of alphaENaC/microg protein of TII cells. Taken together, these studies demonstrate that TI cells not only contain molecular machinery necessary for active ion transport, but also transport ions. These results modify some basic concepts about lung liquid transport, suggesting that TI cells may contribute significantly in maintaining alveolar fluid balance and in resolving airspace edema.

Lack of amiloride-sensitive transport across alveolar and respiratory epithelium of iNOS(-/-) mice in vivo

The extent to which endogenously generated nitric oxide alters Na(+) transport across the mammalian alveolar epithelium in vivo has not been documented. Herein we measured alveolar fluid clearance and nasal potential differences in mice lacking the inducible form of nitric oxide synthase [iNOS; iNOS(-/-)] and their corresponding wild-type controls [iNOS(+/+)]. Alveolar fluid clearance values in iNOS(+/+) and iNOS(-/-) anesthetized mice with normal oxygenation and acid-base balance were ~30% of instilled fluid/30 min. In both groups of mice, fluid absorption was dependent on vectorial Na(+) movement. Amiloride (1.5 mM) decreased alveolar fluid clearance in iNOS(+/+) mice by 61%, whereas forskolin (50 microM) increased alveolar fluid clearance by 55% by stimulating amiloride-insensitive pathways. Neither agent altered alveolar fluid clearance in iNOS(-/-) mice. Hyperoxia upregulated iNOS expression in iNOS(+/+) mice and decreased their amiloride-sensitive component of alveolar fluid clearance but had no effect on the corresponding values in iNOS(-/-) mice. Nasal potential difference measurements were consistent with alveolar fluid clearance in that both groups of mice had similar baseline values, which were amiloride sensitive in the iNOS(+/+) but not in the iNOS(-/-) mice. These data suggest that nitric oxide produced by iNOS under basal conditions plays an important role in regulating amiloride-sensitive Na(+) channels in alveolar and airway epithelia.

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.

A novel role for protein phosphatase 2A in the dopaminergic regulation of Na,K-ATPase

Stimulation of dopaminergic type 1 (D(1)) receptors increases lung edema clearance by regulating Na,K-ATPase function in the alveolar epithelium. We studied the role of serine/threonine protein phosphatases in the Na,K-ATPase regulation by D(1) agonists in A549 cells. We found that low doses of the type 1/2A protein phosphatase inhibitor okadaic acid as well as SV40 small t antigen transiently transfected into A549 cells prevented the D(1) agonist-induced increase in Na,K-ATPase activity and translocation from intracellular pools to the plasma membrane. This was associated with a rapid and transient increase in protein phosphatase 2A activity. We conclude that D(1) stimulation regulates Na,K-ATPase activity by promoting recruitment of Na,K-ATPases from intracellular pools into the basolateral membranes of A549 cells via a type 2A protein phosphatase.

Epithelial Na(+) channel (ENaC) expression in the developing normal and abnormal human perinatal lung

Impaired lung epithelial Na(+) channel (ENaC) activity at the time of birth results in respiratory distress. To investigate potential mechanisms, the ontogeny and cellular distribution of the alphaENaC subunit mRNA expression was studied in normal, immature, and abnormal (hypoplastic) human fetal lungs using nonradioisotopic in situ hybridization. Surprisingly, alphaENaC expression was detected at the embryonic stage of normal lung development (4 to 5 wk gestation) when expression was localized to the fetal lung bud epithelium. By late gestation, ENaC was expressed in the conductive and respiratory airway epithelium, serous cells, and the distal lung unit in an alveolar type II (ATII) epitheliumlike distribution. Significant alphaENaC expression was found in newborn lung diseases associated with respiratory distress. One explanation is that alphaENaC mRNA is constitutively expressed, and that activity is regulated, at least in part, at the post-transcriptional level. Alternative explanations are that the expression of the beta or gammaENaC subunits may be impaired in certain newborn lung diseases or that alternate Na(+) permeant channels or transporters are important to lung liquid absorption in humans at birth.

Biophysical properties and molecular characterization of amiloride-sensitive sodium channels in A549 cells

Amiloride-sensitive Na(+) channels, present in fetal and adult alveolar epithelial type II (ATII) cells, play a critical role in the reabsorption of fetal fluid shortly after birth and in limiting the extent of alveolar edema across the adult lung. Because of the difficulty in isolating and culturing ATII cells, there is considerable interest in characterizing the properties of ion channels and their response to injury of ATII cell-like cell lines such as A549 that derive from a human alveolar cell carcinoma. A549 cells were shown to contain alpha-, beta-, and gamma-epithelial Na(+) channel mRNAs. In the whole cell mode of the patch-clamp technique (bath, 145 mM Na(+); pipette, 145 mM K(+)), A549 cells exhibited inward Na(+) currents reversibly inhibited by amiloride, with an inhibition constant of 0.83 microM. Ion substitution studies showed that these channels were moderately selective for Na(+) (Na(+)-to-K(+) permeability ratio = 6:1). Inward Na(+) currents were activated by forskolin (10 microM) and inhibited by nitric oxide (300 nM) and cGMP. Recordings in cell-attached mode revealed the presence of an amiloride-sensitive Na(+) channel with a unitary conductance of 8.6 +/- 0.04 (SE) pS. Channel activity was increased by forskolin and decreased by nitric oxide and the cGMP analog 8-bromo-cGMP. These data demonstrate that A549 cells contain amiloride-sensitive Na(+) channels with biophysical properties similar to those of ATII cells.

Mechanisms of increased Na(+) transport in ATII cells by cAMP: we agree to disagree and do more experiments

Existing evidence supports the presence of active transport of Na(+) across the mammalian alveolar epithelium and its upregulation by agents that increase cytoplasmic cAMP levels. However, there is controversy regarding the mechanisms responsible for this upregulation. Herein we present the results of various patch-clamp studies indicating the presence of 25- to 27-pS, amiloride-sensitive, moderately selective Na(+) channels (Na(+)-to-K(+) permeability ratio = 7:1) located on the apical membranes of rat alveolar type II (ATII) cells maintained in primary culture. The addition of terbutaline to the bath solution increased the open probability of single channels present in cell-attached patches of ATII cells without affecting their conductance. A similar increase in open probability was seen after the addition of protein kinase A, ATP, and Mg(2+) to the cytoplasmic side of inside-out patches. Measurement of short-circuit currents across confluent monolayers of rat or rabbit ATII cells indicates that terbutaline and 8-(4-chlorophenylthio)-cAMP increase vectorial Na(+) transport and activate Cl(-) channels. Currently, there is a controversy as to whether the cAMP-induced increase in Na(+) transport is due solely to hyperpolarization of the cytoplasmic side of the ATII cell membrane due to Cl(-) influx or whether it results from simultaneous stimulation of both Cl(-) and Na(+) conductive pathways. Additional studies are needed to resolve this issue.

Epidermal growth factor regulation in adult rat alveolar type II cells of amiloride-sensitive cation channels

Using the patch-clamp technique, we studied the effects of epidermal growth factor (EGF) on whole cell and single channel currents in adult rat alveolar epithelial type II cells in primary culture in the presence or absence of EGF for 48 h. In symmetrical sodium isethionate solutions, EGF exposure caused a significant increase in the type II cell whole cell conductance. Amiloride (10 microM) produced approximately 20-30% inhibition of the whole cell conductance in both the presence and absence of EGF, such that EGF caused the magnitude of the amiloride-sensitive component to more than double. Northern analysis showed that alpha-, beta- and gamma-subunits of rat epithelial Na(+) channel (rENaC) steady-state mRNA levels were all significantly decreased by EGF. At the single channel level, all active inside-out patches demonstrated only 25-pS channels that were amiloride sensitive and relatively nonselective for cations (P(Na(+))/P(K(+)) approximately 1.0:0.48). Although the biophysical characteristics (conductance, open-state probability, and selectivity) of the channels from EGF-treated and untreated cells were essentially identical, channel density was increased by EGF; the modal channel per patch was increased from 1 to 2. These findings indicate that EGF increases expression of nonselective, amiloride-sensitive cation channels in adult alveolar epithelial type II cells. The contribution of rENaC to the total EGF-dependent cation current under these conditions is quantitatively less important than that of the nonselective cation channels in these cells.

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.

Antisense oligonucleotides against the alpha-subunit of ENaC decrease lung epithelial cation-channel activity

Amiloride-sensitive Na+ transport by lung epithelia plays a critical role in maintaining alveolar Na+ and water balance. It has been generally assumed that Na+ transport is mediated by the amiloride-sensitive epithelial Na+ channel (ENaC) because molecular biology studies have confirmed the presence of ENaC subunits alpha, beta, and gamma in lung epithelia. However, the predominant Na+-transporting channel reported from electrophysiological studies by most laboratories is a nonselective, high-conductance channel that is very different from the highly selective, low-conductance ENaC reported in other tissues. In our laboratory, single-channel recordings from apical membrane patches from rat alveolar type II (ATII) cells in primary culture reveal a nonselective cation channel with a conductance of 20.6 +/- 1.1 pS and an Na+-to-K+ selectivity of 0.97 +/- 0.07. This channel is inhibited by submicromolar concentrations of amiloride. Thus there is some question about the relationship between the gene product observed with single-channel methods and the cloned ENaC subunits. We have employed antisense oligonucleotide methods to block the synthesis of individual ENaC subunit proteins (alpha, beta, and gamma) and determined the effect of a reduction in the subunit expression on the density of the nonselective cation channel observed in apical membrane patches on ATII cells. Treatment of ATII cells with antisense oligonucleotides inhibited the production of each subunit protein; however, single-channel recordings showed that only the antisense oligonucleotide targeting the alpha-subunit resulted in a significant decrease in the density of nonselective cation channels. Inhibition of the beta- and gamma-subunit proteins alone or together did not cause any changes in the observed channel density. There were no changes in open probability or other channel characteristics. These results support the hypothesis that the alpha-subunit of ENaC alone or in combination with some protein other than the beta- or gamma-subunit protein is the major component of lung alveolar epithelial cation channels.

Sodium channels in alveolar epithelial cells: molecular characterization, biophysical properties, and physiological significance

At birth, fetal distal lung epithelial (FDLE) cells switch from active chloride secretion to active sodium (Na+) reabsorption. Sodium ions enter the FDLE and alveolar type II (ATII) cells mainly through apical nonselective cation and Na(+)-selective channels, with conductances of 4-26 pS (picoSiemens) in FDLE and 20-25 pS in ATII cells. All these channels are inhibited by amiloride with a 50% inhibitory concentration of < 1 microM, and some are also inhibited by [N-ethyl-N-isopropyl]-2'-4'-amiloride (50% inhibitory concentration of < 1 microM). Both FDLE and ATII cells contain the alpha-, beta-, and gamma-rENaC (rat epithelial Na+ channels) mRNAs; reconstitution of an ATII cell Na(+)-channel protein into lipid bilayers revealed the presence of 25-pS Na+ single channels, inhibited by amiloride and [N-ethyl-N-isopropyl]-2'-4'-amiloride. A variety of agents, including cAMP, oxygen, glucocorticoids, and in some cases Ca2+, increased the activity and/or rENaC mRNA levels. The phenotypic properties of these channels differ from those observed in other Na(+)-absorbing epithelia. Pharmacological blockade of alveolar Na+ transport in vivo, as well as experiments with newborn alpha-rENaC knock-out mice, demonstrate the importance of active Na+ transport in the reabsorption of fluid from the fetal lung and in reabsorbing alveolar fluid in the injured adult lung. Indeed, in a number of inflammatory diseases, increased production of reactive oxygen-nitrogen intermediates, such as peroxynitrite (ONOO-), may damage ATII and FDLE Na+ channels, decrease Na+ reabsorption in vivo, and thus contribute to the formation of alveolar edema.

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.

Adrenergic stimulation of Na+ transport across alveolar epithelial cells involves activation of apical Cl- channels

Alveolar epithelial cells were isolated from adult Sprague-Dawley rats and grown to confluence on membrane filters. Most of the basal short-circuit current (Isc; 60%) was inhibited by amiloride (IC50 0. 96 microM) or benzamil (IC50 0.5 microM). Basolateral addition of terbutaline (2 microM) produced a rapid decrease in Isc, followed by a slow recovery back to its initial amplitude. When Cl- was replaced with methanesulfonic acid, the basal Isc was reduced and the response to terbutaline was inhibited. In permeabilized monolayer experiments, both terbutaline and amiloride produced sustained decreases in current. The current-voltage relationship of the terbutaline-sensitive current had a reversal potential of -28 mV. Increasing Cl- concentration in the basolateral solution shifted the reversal potential to more depolarized voltages. These results were consistent with the existence of a terbutaline-activated Cl- conductance in the apical membrane. Terbutaline did not increase the amiloride-sensitive Na+ conductance. We conclude that beta-adrenergic stimulation of adult alveolar epithelial cells results in an increase in apical Cl- permeability and that amiloride-sensitive Na+ channels are not directly affected by this stimulation.

Augmentation of lung liquid clearance via adenovirus-mediated transfer of a Na,K-ATPase beta1 subunit gene

Previous studies have suggested that alveolar Na,K-ATPases play an important role in active Na+ transport and lung edema clearance. We reasoned that overexpression of Na,K-ATPase subunit genes could increase Na,K-ATPase function in lung epithelial cells and edema clearance in rat lungs. To test this hypothesis we produced replication deficient human type 5 adenoviruses containing cDNAs for the rat alpha1 and beta1 Na,K-ATPase subunits (adMRCMValpha1 and adMRCMVbeta1, respectively). As compared to controls, adMRCMVbeta1 increased beta1 subunit expression and Na,K-ATPase function by 2. 5-fold in alveolar type 2 epithelial cells and rat airway epithelial cell monolayers. No change in Na,K-ATPase function was noted after infection with adMRCMValpha1. Rat lungs infected with adMRCMVbeta1, but not adMRCMValpha1, had increased beta1 protein levels and lung liquid clearance 7 d after tracheal instillation. Alveolar epithelial permeability to Na+ and mannitol was mildly increased in animals infected with adMRCMVbeta1 and a similar Escherichia coli lacZ-expressing virus. Our data shows, for the first time, that transfer of the beta1 Na,K-ATPase subunit gene augments Na,K-ATPase function in epithelial cells and liquid clearance in rat lungs. Conceivably, overexpression of Na,K-ATPases could be used as a strategy to augment lung liquid clearance in patients with pulmonary edema.

Impact of beta-adrenergic agonist on Na+ channel and Na+-K+-ATPase expression in alveolar type II cells

It has been shown that short-term (hours) treatment with beta-adrenergic agonists can stimulate lung liquid clearance via augmented Na+ transport across alveolar epithelial cells. This increase in Na+ transport with short-term beta-agonist treatment has been explained by activation of the Na+ channel or Na+-K+-ATPase by cAMP. However, because the effect of sustained stimulation (days) with beta-adrenergic agonists on the Na+ transport mechanism is unknown, we examined this question in cultured rat alveolar type II cells. Na+-K+-ATPase activity was increased in these cells by 10(-4) M terbutaline in an exposure time-dependent manner over 7 days in culture. This increased activity was also associated with an elevation in transepithelial current that was inhibited by amiloride. The enzyme's activity was also augmented by continuous treatment with dibutyryl-cAMP (DBcAMP) for 5 days. This increase in Na+-K+-ATPase activity by 10(-4) M terbutaline was associated with an increased expression of alpha1-Na+-K+-ATPase mRNA and protein. beta-Adrenergic agonist treatment also enhanced the expression of the alpha-subunit of the epithelial Na+ channel (ENaC). These increases in gene expression were inhibited by propranolol. Amiloride also suppressed this long-term effect of terbutaline and DBcAMP on Na+-K+-ATPase activity. In conclusion, beta-adrenergic agonists enhance the gene expression of Na+-K+-ATPase, which results in an increased quantity and activity of the enzyme. This heightened expression is also associated with augmented ENaC expression. Although the cAMP system is involved, the inhibition of enhanced enzyme activity with amiloride suggests that increased Na+ entry at the apical surface plays a role in this process.

Differential regulation of Na+ and Cl- conductances by PTX-sensitive G proteins in fetal lung apical membrane vesicles

In apical membrane vesicles (AMV) prepared from late gestation fetal guinea pig lung we show that conductive 22Na+ uptake is modulated by at least two pathways involving pertussis toxin (PTX)-sensitive G proteins. Intravesicular incorporation of 100 microM GTPgammaS into vesicles resuspended in NaCl caused a significant stimulation (P<0. 05) of conductive Na+ uptake in AMV to 150+/-10% (n=10) of control, whereas GDPbetaS reduced uptake to 65+/-9% (n=4) of control. This contrasting response to GTPgammaS and GDPbetaS is characteristic of a G protein mediated pathway. GTPgammaS induced a significantly smaller stimulation, 125+/-8% (n=5) of control, in the presence of the relatively impermeant anion isethionate (Ise-). Taken together, these data indicate modulation of both Na+ and Cl- channels in the apical membrane by co-localised G protein(s). Treatment with PTX stimulated conductive 22Na+ uptake to 171+/-20% (n=13) of control in AMV resuspended in NaCl, but did not have a significant effect, 94+/-19% of control, in the presence of NaIse indicating the existence of tonic activation of Cl- channels in these AMV under resting conditions. As the combined effects of PTX and GTPgammaS diminished uptake, we propose that the G protein(s) responsible for Na+ channel activation in response to GTPgammaS is PTX-sensitive and that additional PTX-insensitive G proteins might also modulate 22Na+ uptake in these AMV. The presence of Gialpha1, Gialpha2, Gialpha3 and Goalpha in this apical membrane preparation was confirmed by PTX catalysed [32P]ADP-dependent ribosylation and Western blotting. Incubation of AMV with 200 microM DTT caused an inhibition of conductive Na+ uptake in AMV resuspended in NaCl or NaIse to 66+/-8% (n=11) and 64+/-8% (n=6) of control respectively. Pre-treatment with DTT did not affect the ability of GTPgammaS to stimulate conductive Na+ uptake suggesting that the regulation of 22Na+ uptake in late gestation guinea pig fetal lung AMV is unlikely to involve an associated regulatory protein.

Overexpression of the Na+,K+-ATPase alpha1 subunit increases Na+,K+-ATPase function in A549 cells

We hypothesized that viral mediated transfer of Na+,K+-ATPase subunit genes to alveolar epithelial cells to overexpress Na+, K+-ATPase could increase Na+,K+-ATPase function. We produced replication-deficient human type 5 adenoviruses that contained cytomegalovirus (CMV)-driven cDNAs for the rat alpha1 and beta1 subunits of Na+,K+-ATPase (AdMRCMValpha1 and AdMRCMVbeta1, respectively). These viruses were used to transduce human adenocarcinoma cells (A549) in culture. Na+,K+-ATPase function was increased by 2.5-fold in the AdMRCMValpha1-infected cells. Sham and AdMRCMVbeta1-infected cells, and cells infected by a CMV-driven beta-galactosidase-expressing adenovirus, had no increases in Na+, K+-ATPase activity. A549 cells infected with multiplicities of infection of 10-200 of AdMRCMValpha1 demonstrated expression of a rat alpha1 mRNA and increased alpha1 protein; no change in beta1 message or protein was noted. Ouabain sensitivity was measured in A549 cells following infection with AdMRCMValpha1. In contrast to controls, AdMRCMValpha1-infected cells demonstrated two IC50s. The first was similar to the IC50s of the controls; the second IC50 was 2 logs greater than the first, consistent with the presence of both the rat and human alpha1 isozymes. These results demonstrate for the first time that adenoviruses can be used to augment Na+,K+-ATPase function.

Nitric oxide inhibits Na+ absorption across cultured alveolar type II monolayers

We examined the mechanisms by which nitric oxide (.NO) decreased vectorial Na+ transport across confluent monolayers of rat alveolar type II (ATII) cells grown on permeable supports. Amiloride (10 microM) applied to the apical side of monolayers inhibited approximately 90% of the equivalent (Ieq) and the short-circuit (Isc) current, with an half-maximal inhibitory concentration (IC50) of 0.85 microM, indicating that Na+ entry into ATII cells occurred through amiloride-sensitive Na+ channels. .NO generated by spermine NONOate and papa NONOate added to both sides of the monolayers decreased Ieq and increased transepithelial resistance in a concentration-dependent fashion (IC50 = 0.4 microM .NO). These changes were prevented or reversed by addition of oxyhemoglobin (50 microM). Incubation of ATII monolayers with 8-bromoguanosine 3',5'-cyclic monophosphate (400 microM) had no effect on transepithelial Na+ transport. When the basolateral membranes of ATII cells were permeabilized with amphotericin B (10 microM) in the presence of a mucosal-to-serosal Na+ gradient (145:25 mM), .NO (generated by 100 microM papa NONOate) inhibited approximately 60% of the amiloride-sensitive Isc. In addition, after permeabilization of the apical membranes, .NO inhibited the Isc [a measure of Na(+)-K(+)-adenosinetriphosphatase (ATPase) activity] by approximately 60%. We concluded that .NO at noncytotoxic concentrations decreased Na+ absorption across cultured ATII monolayers by inhibiting both the amiloride-sensitive Na+ channels and Na(+)-K(+)-ATPase through guanosine 3',5'-cyclic monophosphate-independent mechanisms.

Surfactant protein A mediates mycoplasmacidal activity of alveolar macrophages

Mycoplasma pneumoniae is a leading cause of pneumonia and exacerbates other respiratory diseases in humans. We investigated the potential role of surfactant protein (SP) A in antimycoplasmal defense using alveolar macrophages (AMs) from C57BL/6NCr (C57BL) mice, which are highly resistant to infections of Mycoplasma pulmonis. C57BL AMs, activated with interferon (IFN)-gamma and incubated with SP-A (25 micrograms/ml) at 37 degrees C, produced significant amounts of nitric oxide (.NO; nitrate and nitrite production = 1.1 microM.h-1.10(5) AMs-1) and effected an 83% decrease in mycoplasma colony-forming units (CFUs) by 6 h postinfection. Preincubation of AMs with the inducible nitric oxide synthase inhibitor NG-monomethyl-L-arginine abolished .NO production and SP-A-mediated killing of mycoplasmas. No decrease in CFUs was seen when IFN-gamma-activated macrophages were infected with mycoplasmas in the absence of SP-A despite significant .NO production (nitrate and nitrite production = 0.6 microM.h-1.10(5) AMs-1). These results demonstrate that SP-A mediates killing of mycoplasmas by AMs, possibly through an .NO-dependent mechanism.

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.

Dopamine stimulates sodium transport and liquid clearance in rat lung epithelium

Pulmonary edema clearance is driven primarily by active sodium transport out of the alveoli, mediated predominantly by apical sodium channels and the basolateral NA,K-ATPase. We postulated that dopamine, analogous to its effects in other transporting epithelia, could regulate these sodium transport mechanisms and affect lung liquid clearance. We therefore studied the effects of dopamine on sodium transport and liquid clearance in isolated perfused rat lungs. Instillation of dopamine into the airways caused a dose-dependent increase in liquid clearance from isolated rat lungs of up to 33% above control values at 10(-8) to 10(-4) M concentrations. 10(-6) M amiloride, which selectively inhibits apical sodium channels, decreased basal liquid clearance by 34% but did not inhibit the dopamine-mediated stimulation of lung liquid clearance. Instillation of 10(-4) M amiloride into rat airways, which inhibits other sodium transport mechanisms non-selectively, decreased basal lung liquid clearance by 49% and inhibited the dopamine-mediated stimulation of lung liquid clearance. Perfusion of rat lungs with 5 x 10(-4) M ouabain to specifically inhibit Na,K-ATPase reduced both basal clearance (by 55%) and the dopamine-stimulated increase in lung fluid clearance. Conceivably, the stimulation of lung liquid clearance by dopamine is due to a modulation of Na,K-ATPase in the pulmonary epithelium.

Rat lung alveolar type II cell line maintains sodium transport characteristics of primary culture

Culture of primary alveolar type II cells has been widely used to investigate the Na+ transport characteristics of alveolar epithelium. However, this model was restricted by early morphological and physiological dedifferentiation in culture. Recently, a cell line has been obtained by transfection of neonatal type II cells with the simian virus SV40 large T antigen gene (SV40-T2). SV40-T2 cells have retained proliferative characteristics of the primary type II cells (Clement et al., 1991, Exp. Cell Res., 196:198-205.) In the present study, we have characterized Na+ transport pathways in SV40-T2 cells. SV40-T2 cells retained most cardinal properties of the original alveolar epithelial cells. Na+ entry occurred, as in primary cultures, through both Na(+)-cotransporters and amiloride-sensitive Na+ channels. SV40-T2 cells expressed Na(+)-phosphate. Na(+)-amino acid and Na(+)-K(+)-Cl cotransports which are quantitatively similar to that of primary cultures. The existence of amiloride-sensitive Na+ channels was supported by molecular and functional data. SV40-T2 expressed the cloned alpha- and gamma-mRNAs for the rat epithelial Na+ channel (rENaC), whereas beta subunit was not detected, and 22Na+ influx was significantly inhibited by 10 microM amiloride. Na+, which enters SV40-T2 cells, is extruded through a Na+, K(+)-ATPase: mRNA for alpha 1 and beta 1 isoforms of Na+, K(+)-ATPase were present and Na+, K(+)-ATPase activity was evidenced either on intact cells by the presence of a ouabain-sensitive component of 86Rb+ influx or on cell homogenates by the measurement of ouabain-inhibitable ATP hydrolysis. These results indicate that SV40-T2 cell line displays most of the Na+ transport characteristics of well-differentiated primary cells in the first days of culture. We conclude that the SV40-T2 cell line provides a model of differentiated alveolar type II cells and may be a powerful tool to study, in vitro, the modulation of Na+ transport in pathophysiological conditions.

Amiloride-blockable Ca2+-activated Na+-permeant channels in the fetal distal lung epithelium

The Na+ transport function of alveolar epithelium represents an important mechanism for clearance of fluid in air space at birth. I observed the activity of two types of amiloride-blockable Na+-permeant cation channels in the apical membrane of fetal distal lung epithelium cultured on permeable filters for 2 days after harvesting of the cells from Wistar rats of 20 days gestation (term = 22 days). One type was a nonselective cation (NSC) channel and had a linear current/voltage (I/V) relationship with a single-channel conductance of 26.9 +/- 0.8 pS (n = 5). The other type was highly Na+ selective (i.e. Na+ channel) and had an inwardly rectifying I/V relationship with a single-channel conductance of 11.8 +/- 0.2 pS (n = 5) around resting membrane potential. The NSC channel was more frequently observed (1.37 +/- 0.15 per patch membrane; n = 73) than the Na+ channel (0.15 +/- 0.40 per patch membrane; n = 73). However, the open probability of the NSC channel was smaller than that of the Na+ channel. Both types of the channels were activated by cytosolic Ca2+, however the sensitivity to cytosolic Ca2+ was much higher in the Na+ channel than in the NSC channel. Furthermore, both types of the channels were blocked by amiloride or benzamil. The half-maximal inhibitory concentration (IC50) of amiloride or benzamil of the Na+ channel was 1-2 microM, while that of NSC channel was less than 1 microM. Both channels were activated by insulin.

Insulin-activated amiloride-blockable nonselective cation and Na+ channels in the fetal distal lung epithelium

1. The apical membrane of fetal distal lung epithelium had two types of amiloride-blockable Na(+)-permeant cation channels; (1) nonselective cation (NSC) channel with a single channel conductance of 27 pS and (2) Na+ channel with a single channel conductance of 12 pS around resting membrane potential. 2. The IC50 of amiloride to the Na+ channel was 1-2 microM, while the IC50 of amiloride to the NSC channel was less 1 microM. The open probability of the Na+ channel was about 10-fold larger than that of the NSC channel. 3. Insulin (100 nM) increased the open probability of both channels.

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.

Structure and function of amiloride-sensitive Na+ channels

A new molecular biological epoch in amiloride-sensitive Na+ channel physiology has begun. With the application of these new techniques, undoubtedly a plethora of new information and new questions will be forthcoming. First and foremost, however, is the question of how many discrete amiloride-sensitive Na+ channels exist. This question is important not only for elucidating structure-function relationships, but also for developing strategies for pharmacological or, ultimately, genetic intervention in such diseases as obstructive nephropathy, Liddle's syndrome, or salt-sensitive hypertension where amiloride-sensitive Na+ channel dysfunction has been implicated [17, 62]. Epithelia Na+ channels purified from kidney are multimeric. However, it is not yet clear which subunits are regulatory and which participate directly as a part of the Na+ conducting core and what is the nature of the gate. The combination of electrophysiologic techniques such as patch clamp and the ability to study reconstituted channels in planar lipid bilayers along with molecular biology techniques to potentially manipulate the individual subunits should provide the answers to questions that have puzzled physiologists for decades. It seems clear that the robust versatility of the channel in responding to a wide range of differing and potentially synergistic regulatory inputs must be a function of its multimeric structure and relation to the cytoskeleton. Multiple mechanisms of regulation imply multiple regulatory sites. This hypothesis has been validated by the demonstration that enzymatic carboxyl methylation and phosphorylation have both individual and synergistic effects on the purified channel in planar lipid bilayers. Of the multiple mechanisms proposed for channel regulation, evidence is now available to support the ideas that channels may be activated (or inactivated) by direct modifications including phosphorylation and carboxyl methylation, by activation or association of regulatory proteins such as G proteins, and by recruitment from subapical membrane domains. The observation that channel gating is achieved primarily through regulation of open probability without alterations in conductance may simplify future understanding of the molecular events involved in gating once the regulatory sites have been identified. As more Na+ channels or Na+ channel subunits are cloned from different epithelia, it will become possible to piece together the puzzle of epithelial Na+ channels. It is interesting to observe that renal Na+ channel proteins contain a subunit which falls into the 70 kD range. This size protein is in the range reported for the aldosterone-induced proteins [12, 46, 153].(ABSTRACT TRUNCATED AT 400 WORDS)

Conductive cation transport in apical membrane vesicles prepared from fetal lung

In order to characterise the apically-located conductive cation pathway of the type II pneumocyte, apical plasma membranes were prepared from mature fetal guinea pig lung. The protocol yielded purified apical membranes that enriched 19-fold with the brush border enzyme marker alkaline phosphatase; there was no significant contamination with other cellular membranes. A technique for imposing an outwardly-directed electrochemical Na+ gradient was used to amplify conductive 22Na+ uptake into vesicles. Uptake of 22Na+ was time-dependent, proportional to the magnitude of the Na+ gradient, specific and sensitive to the amiloride analogues phenamil and EIPA (apparent minimum Ki values of 50 nM and 10 microM, respectively, with maximum uptake inhibition of 42% and 39% at 100 microM). Uptake experiments in which the outwardly-directed Na+ gradient was replaced by outwardly-directed gradients of small monovalent cations and molecular cations were performed. The Na+/K+ permeability ratio was 1.2:1, and over the extended range of small monovalent cations, a permeability sequence of Na+ > K+ > Li+ > Rb+ > Cs+ was observed, indicating the presence of fixed negative charge in or spatially close to the pore. The molecular cation permeability sequence of NH4+ > methylamine+ > dimethylamine+ > choline+ > N-methyl-D-glucamine+ > tetraethylammonium+ > tetramethylammonium+, after transformation, gives an estimate of 8 A for the conducting pore diameter. These data are consistent with the presence in the apical membrane of fetal type II pneumocytes of a cation specific channel with low Na+ selectivity and amiloride sensitivity.

Quantitation of nitrotyrosine levels in lung sections of patients and animals with acute lung injury

Activated alveolar macrophages and epithelial type II cells release both nitric oxide and superoxide which react at near diffusion-limited rate (6.7 x 10(9) M-1s-1) to form peroxynitrite, a potent oxidant capable of damaging the alveolar epithelium and pulmonary surfactant. Peroxynitrite, but not nitric oxide or superoxide, readily nitrates phenolic rings including tyrosine. We quantified the presence of nitrotyrosine in the lungs of patients with the adult respiratory distress syndrome (ARDS) and in the lungs of rats exposed to hyperoxia (100% O2 for 60 h) using quantitative immunofluorescence. Fresh frozen or paraffin-embedded lung sections were incubated with a polyclonal antibody to nitrotyrosine, followed by goat anti-rabbit IgG coupled to rhodamine. Sections from patients with ARDS (n = 5), or from rats exposed to hyperoxia (n = 4), exhibited a twofold increase of specific binding over controls. This binding was blocked by the addition of an excess amount of nitrotyrosine and was absent when the nitrotyrosine antibody was replaced with nonimmune IgG. In additional experiments we demonstrated nitrotyrosine formation in rat lung sections incubated in vitro with peroxynitrite, but not nitric oxide or reactive oxygen species. These data suggest that toxic levels of peroxynitrite may be formed in the lungs of patients with acute lung injury.

Na(+)-H+ antiport detected through hydrogen ion currents in rat alveolar epithelial cells and human neutrophils

Voltage-activated H(+)-selective currents were studied in cultured adult rat alveolar epithelial cells and in human neutrophils using the whole-cell configuration of the patch-clamp technique. The H+ conductance, gH, although highly selective for protons, was modulated by monovalent cations. In Na+ and to a smaller extent in Li+ solutions, H+ currents were depressed substantially and the voltage dependence of activation of the gH shifted to more positive potentials, when compared with the "inert" cation tetramethylammonium (TMA+). The reversal potential of the gH, Vrev, was more positive in Na+ solutions than in inert ion solutions. Amiloride at 100 microM inhibited H+ currents in the presence of all cations studied except Li+ and Na+, in which it increased H+ currents and shifted their voltage-dependence and Vrev to more negative potentials. The more specific Na(+)-H+ exchange inhibitor dimethylamiloride (DMA) at 10 microM similarly reversed most of the suppression of the gH by Na+ and Li+. Neither 500 microM amiloride nor 200 microM DMA added internally via the pipette solution were effective. Distinct inhibition of the gH was observed with 1% [Na+]o, indicating a mechanism with high sensitivity. Finally, the effects of Na+ and their reversal by amiloride were large when the proton gradient was outward (pHo parallel pHi 7 parallel 5.5), smaller when the proton gradient was abolished (pH 7 parallel 7), and absent when the proton gradient was inward (pH 6 parallel 7). We propose that the effects of Na+ and Li+ are due to their transport by the Na(+)-H+ antiporter, which is present in both cell types studied. Electrically silent H+ efflux through the antiporter would increase pHi and possibly decrease local pHo, both of which modulate the gH in a similar manner: reducing the H+ currents at a given potential and shifting their voltage-dependence to more positive potentials. A simple diffusion model suggests that Na(+)-H+ antiport could deplete intracellular protonated buffer to the extent observed. Evidently the Na(+)-H+ antiporter functions in perfused cells, and its operation results in pH changes which can be detected using the gH as a physiological sensor. Thus, the properties of the gH can be exploited to study Na(+)-H+ antiport in single cells under controlled conditions.

The lung amiloride-sensitive Na+ channel: biophysical properties, pharmacology, ontogenesis, and molecular cloning

Water balance in the lung is controlled via active Na+ and Cl- transport. Electrophysiological measurements on lung epithelial cells demonstrated the presence of a Na+ channel that is inhibited by amiloride (K0.5 = 90 nM) and some of its derivatives such as phenamil (K0.5 = 19 nM) and benzamil (K0.5 = 14 nM) but not by ethylisopropylamiloride. An amiloride-sensitive Na+ channel of 4 pS was recorded from outside-out patches excised from the apical membrane. This channel is highly selective for Na+ (PNa+/PK+ > or = to 10). Isolation of a human lung cDNA led to the primary structure of the lung Na+ channel. The corresponding protein is 669 residues long and has two large hydrophobic domains. An amiloride-sensitive Na(+)-selective current apparently identical to the one observed in lung epithelial cells was recorded after expression of the cloned channel in oocytes. The level of the mRNA for the Na+ channel was highly increased from fetal to newborn and adult stages. This observation indicates that the increased Na+ reabsorption that occurs at birth as a necessary event to pass to an air-breathing environment is probably associated with control of transcription of this Na+ channel. The human gene for the lung Na+ channel was mapped on chromosome 12p13.

Immunocytochemical localization of amiloride-sensitive sodium channels in the lower intestine of the hen

We have used polyclonal antibodies generated against purified bovine renal amiloride-sensitive Na+ channels to localize amiloride-sensitive Na+ channels within the lower intestine (colon and coprodeum) of the hen. These antibodies cross-reacted with two polypeptides exhibiting M(r)'s of 235 and 150 kDa on immunoblots of detergent-solubilized apical membrane fractions from both the colon and coprodeum. The apparent molecular masses of theses polypeptides are in agreement with the M(r)'s of 2 of the subunits of the renal high amiloride-affinity Na+ channel, namely the alpha and the beta (= amiloride binding) subunits. The cellular distribution of Na+ channels was determined by immunoperoxidase and indirect immunofluorescence cytochemical techniques. The apical (luminal) membrane and cytoplasm of villar principal cells in both colon and coprodeum exhibited immunoreactivity, whereas goblet cells were negative. Both principal and goblet cells of the crypts were also negative. We conclude that the amiloride-sensitive Na+ channels are localized to the principal cells of the intestinal villi and that these cells are responsible for intestinal Na+ absorption.