A large conductance, Ca2+-activated K+ channel in a human lung epithelial cell line (A549)

A549 in-silico 1.0: A first computational model to simulate cell cycle dependent ion current modulation in the human lung adenocarcinoma

Lung cancer is still a leading cause of death worldwide. In recent years, knowledge has been obtained of the mechanisms modulating ion channel kinetics and thus of cell bioelectric properties, which is promising for oncological biomarkers and targets. The complex interplay of channel expression and its consequences on malignant processes, however, is still insufficiently understood. We here introduce the first approach of an in-silico whole-cell ion current model of a cancer cell, in particular of the A549 human lung adenocarcinoma, including the main functionally expressed ion channels in the plasma membrane as so far known. This hidden Markov-based model represents the electrophysiology behind proliferation of the A549 cell, describing its rhythmic oscillation of the membrane potential able to trigger the transition between cell cycle phases, and it predicts membrane potential changes over the cell cycle provoked by targeted ion channel modulation. This first A549 in-silico cell model opens up a deeper insight and understanding of possible ion channel interactions in tumor development and progression, and is a valuable tool for simulating altered ion channel function in lung cancer electrophysiology.

Advances in the understanding of functional alterations at genetic, epigenetic or protein expression and the expanding knowledge in mechanisms modulating ion channel kinetics and thus the cells’ bioelectric properties have arisen as promising cancer biomarkers and oncological targets. Our hidden Markov-based in-silico cell model represents the electrophysiology behind proliferation of the A549 cell line, explaining the cell’s rhythmic oscillation from hyperpolarized to depolarized states of the membrane potential, able to trigger the transition between cell cycle phases. The model enables the prediction of membrane potential changes over the cell cycle provoked by targeted modulation of specific ion channels, leading to cell cycle promotion or interruption. We are encouraged that the availability of this first cancer cell model will provide profound insight into possible roles and interactions of ion channels in tumor development and progression, and may aid in the testing of research hypotheses in lung cancer electrophysiology.

The Role of KCNMB1 and BK Channels in Myofibroblast Differentiation and Pulmonary Fibrosis

The differentiation of fibroblasts into myofibroblasts is critical for the development of fibrotic disorders, including idiopathic pulmonary fibrosis (IPF). Previously, we demonstrated that fibroblasts from patients with IPF exhibit changes in DNA methylation across the genome that contribute to a profibrotic phenotype. One of the top differentially methylated genes identified in our previous study was KCNMB1, which codes for the β subunit of the large-conductance potassium (BK, also known as MaxiK or K_Ca1.1) channel. Here, we examined how the expression of KCNMB1 differed between IPF fibroblasts and normal cells, and how BK channels affected myofibroblast differentiation. Fibroblasts from patients with IPF exhibited increased expression of KCNMB1, which corresponded to increased DNA methylation within the gene body. Patch-clamp experiments demonstrated that IPF fibroblasts had increased BK channel activity. Knockdown of KCNMB1 attenuated the ability of fibroblasts to contract collagen gels, and this was associated with a loss of α-smooth muscle actin (SMA) expression. Pharmacologic activation of BK channels stimulated α-SMA expression, whereas BK channel inhibitors blocked the upregulation of α-SMA. The ability of BK channels to enhance α-SMA expression was dependent on intracellular calcium, as activation of BK channels resulted in increased levels of intracellular calcium and the effects of BK agonists were abolished when calcium was removed. Together, our findings demonstrate that epigenetic upregulation of KCNMB1 contributes to increased BK channel activity in IPF fibroblasts, and identify a newfound role for BK channels in myofibroblast differentiation.

Hyperoxia treatment of TREK-1/TREK-2/TRAAK-deficient mice is associated with a reduction in surfactant proteins

We previously proposed a role for the two-pore domain potassium (K2P) channel TREK-1 in hyperoxia (HO)-induced lung injury. To determine whether redundancy among the three TREK isoforms (TREK-1, TREK-2, and TRAAK) could protect from HO-induced injury, we now examined the effect of deletion of all three TREK isoforms in a clinically relevant scenario of prolonged HO exposure and mechanical ventilation (MV). We exposed WT and TREK-1/TREK-2/TRAAK-deficient [triple knockout (KO)] mice to either room air, 72-h HO, MV [high and low tidal volume (TV)], or a combination of HO + MV and measured quasistatic lung compliance, bronchoalveolar lavage (BAL) protein concentration, histologic lung injury scores (LIS), cellular apoptosis, and cytokine levels. We determined surfactant gene and protein expression and attempted to prevent HO-induced lung injury by prophylactically administering an exogenous surfactant (Curosurf). HO treatment increased lung injury in triple KO but not WT mice, including an elevated LIS, BAL protein concentration, and markers of apoptosis, decreased lung compliance, and a more proinflammatory cytokine phenotype. MV alone had no effect on lung injury markers. Exposure to HO + MV (low TV) further decreased lung compliance in triple KO but not WT mice, and HO + MV (high TV) was lethal for triple KO mice. In triple KO mice, the HO-induced lung injury was associated with decreased surfactant protein (SP) A and SPC but not SPB and SPD expression. However, these changes could not be explained by alterations in the transcription factors nuclear factor-1 (NF-1), NKX2.1/thyroid transcription factor-1 (TTF-1) or c-jun, or lamellar body levels. Prophylactic Curosurf administration did not improve lung injury scores or compliance in triple KO mice.

Genetic variation in genes encoding airway epithelial potassium channels is associated with chronic rhinosinusitis in a pediatric population

Background

Apical potassium channels regulate ion transport in airway epithelial cells and influence air surface liquid (ASL) hydration and mucociliary clearance (MCC). We sought to identify whether genetic variation within genes encoding airway potassium channels is associated with chronic rhinosinusitis (CRS).

Methods

Single nucleotide polymorphism (SNP) genotypes for selected potassium channels were derived from data generated on the Illumnia HumanHap550 BeadChip or Illumina Human610-Quad BeadChip for 828 unrelated individuals diagnosed with CRS and 5,083 unrelated healthy controls from the Children's Hospital of Philadelphia (CHOP). Statistical analysis was performed with set-based tests using PLINK, and corrected for multiple testing.

Results

Set-based case control analysis revealed the gene KCNMA1 was associated with CRS in our Caucasian subset of the cohort (598 CRS cases and 3,489 controls; p = 0.022, based on 10,000 permutations). In addition there was borderline evidence that the gene KCNQ5 (p = 0.0704) was associated with the trait in our African American subset of the cohort (230 CRS cases and 1,594 controls). In addition to the top significant SNPs rs2917454 and rs6907229, imputation analysis uncovered additional genetic variants in KCNMA1 and in KCNQ5 that were associated with CRS.

Conclusions

We have implicated two airway epithelial potassium channels as novel susceptibility loci in contributing to the pathogenesis of CRS.

Mechanism of action of novel lung edema therapeutic AP301 by activation of the epithelial sodium channel

AP301 [Cyclo(CGQRETPEGAEAKPWYC)], a cyclic peptide comprising the human tumor necrosis factor lectin-like domain (TIP domain) sequence, is currently being developed as a treatment for lung edema and has been shown to reduce extravascular lung water and improve lung function in mouse, rat, and pig models. The current paradigm for liquid homeostasis in the adult mammalian lung is that passive apical uptake of sodium via the amiloride-sensitive epithelial Na⁺ channel (ENaC) and nonselective cyclic-nucleotide-gated cation channels creates the major driving force for reabsorption of water through the alveolar epithelium in addition to other ion channels such as potassium and chloride channels. AP301 can increase amiloride-sensitive current in A549 cells as well as in freshly isolated type II alveolar epithelial cells from different species. ENaC is expressed endogenously in all of these cell types. Consequently, this study was undertaken to determine whether ENaC is the specific target of AP301. The effect of AP301 in A549 cells as well as in human embryonic kidney cells and Chinese hamster ovary cells heterologously expressing human ENaC subunits (α, β, γ, and δ) was measured in patch clamp experiments. The congener TIP peptide AP318 [Cyclo(4-aminobutanoic acid-GQRETPEGAEAKPWYD)] activated ENaC by increasing single-channel open probability. AP301 increased current in proteolytically activated (cleaved) but not near-silent (uncleaved) ENaC in a reversible manner. αβγ- or δβγ-ENaC coexpression was required for maximal activity. No increase in current was observed after deglycosylation of extracellular domains of ENaC. Thus, our data suggest that the specific interaction of AP301 with both endogenously and heterologously expressed ENaC requires precedent binding to glycosylated extracellular loop(s).

K+ channels regulate ENaC expression via changes in promoter activity and control fluid clearance in alveolar epithelial cells

Active Na+ absorption by alveolar ENaC is the main driving force of liquid clearance at birth and lung edema resorption in adulthood. We have demonstrated previously that long-term modulation of KvLQT1 and KATP K+ channel activities exerts sustained control in Na+ transport through the regulation of ENaC expression in primary alveolar type II (ATII) cells. The goal of the present study was: 1) to investigate the role of the alpha-ENaC promoter, transfected in the A549 alveolar cell line, in the regulation of ENaC expression by K+ channels, and 2) to determine the physiological impact of K+ channels and ENaC modulation on fluid clearance in ATII cells. KvLQT1 and KATP channels were first identified in A549 cells by PCR and Western blotting. We showed, for the first time, that KvLQT1 activation by R-L3 (applied for 24 h) increased alpha-ENaC expression, similarly to KATP activation by pinacidil. Conversely, pharmacological KvLQT1 and KATP inhibition or silencing with siRNAs down-regulated alpha-ENaC expression. Furthermore, K+ channel blockers significantly decreased alpha-ENaC promoter activity. Our results indicated that this decrease in promoter activity could be mediated, at least in part, by the repressor activity of ERK1/2. Conversely, KvLQT1 and KATP activation dose-dependently enhanced alpha-ENaC promoter activity. Finally, we noted a physiological impact of changes in K+ channel functions on ERK activity, alpha-, beta-, gamma-ENaC subunit expression and fluid absorption through polarized ATII cells. In summary, our results disclose that K+ channels regulate alpha-ENaC expression by controlling its promoter activity and thus affect the alveolar function of fluid clearance.

Benzimidazolones enhance the function of epithelial Na⁺ transport

BACKGROUND AND PURPOSE

Pharmacological enhancement of vectorial Na⁺ transport may be useful to increase alveolar fluid clearance. Herein, we investigated the influence of the benzimidazolones 1-ethyl-1,3-dihydro-2-benzimidazolone (1-EBIO), 5,6-dichloro-1-EBIO (DC-EBIO) and chlorzoxazone on vectorial epithelial Na⁺ transport.

EXPERIMENTAL APPROACH

Effects on vectorial Na⁺ transport and amiloride-sensitive apical membrane Na⁺ permeability were determined by measuring short-circuit currents (I(SC)) in rat fetal distal lung epithelial (FDLE) monolayers. Furthermore, amiloride-sensitive membrane conductance and the open probability of epithelial Na⁺ channels (ENaC) were determined by patch clamp experiments using A549 cells.

KEY RESULTS

I(SC) was increased by approximately 50% after addition of 1-EBIO, DC-EBIO and chlorzoxazone. With permeabilized basolateral membranes in the presence of a 145:5 apical to basolateral Na⁺ gradient, the benzimidazolones markedly increased amiloride-sensitive I(SC). 5-(N-Ethyl-N-isopropyl)amiloride-induced inhibition of I(SC) was not affected. The benzamil-sensitive I(SC) was increased in benzimidazolone-stimulated monolayers. Pretreating the apical membrane with amiloride, which inhibits ENaC, completely prevented the stimulating effects of benzimidazolones on I(SC). Furthermore, 1-EBIO (1 mM) and DC-EBIO (0.1 mM) significantly increased (threefold) the open probability of ENaC without influencing current amplitude. Whole cell measurements showed that DC-EBIO (0.1 mM) induced an amiloride-sensitive increase in membrane conductance.

CONCLUSION AND IMPLICATIONS

Benzimidazolones have a stimulating effect on vectorial Na⁺ transport. The antagonist sensitivity of this effect suggests the benzimidazolones elicit this action by activating the highly selective ENaC currents. Thus, the results demonstrate a possible new strategy for directly enhancing epithelial Na⁺ transport.

Luminal cholinergic signalling in airway lining fluid: a novel mechanism for activating chloride secretion via Ca²⁺-dependent Cl⁻ and K⁺ channels

BACKGROUND AND PURPOSE

Recent studies detected the expression of proteins involved in cholinergic metabolism in airway epithelial cells, although the function of this non-neuronal cholinergic system is not known in detail. Thus, this study focused on the effect of luminal ACh as a regulator of transepithelial ion transport in epithelial cells.

EXPERIMENTAL APPROACH

RT-PCR experiments were performed using mouse tracheal epithelial cells for ChAT and organic cation transporter (OCT) transcripts. Components of tracheal airway lining fluid were analysed with desorption electrospray ionization (DESI) MS. Effects of nicotine on mouse tracheal epithelial ion transport were examined with Ussing-chamber experiments.

KEY RESULTS

Transcripts encoding ChAT and OCT1-3 were detected in mouse tracheal epithelial cells. The DESI experiments identified ACh in the airway lining fluid. Luminal ACh induced an immediate, dose-dependent increase in the transepithelial ion current (EC₅₀: 23.3 µM), characterized by a transient peak and sustained plateau current. This response was not affected by the Na⁺-channel inhibitor amiloride. The Cl⁻-channel inhibitor niflumic acid or the K⁺-channel blocker Ba²⁺ attenuated the ACh effect. The calcium ionophore A23187 mimicked the ACh effect. Luminal nicotine or muscarine increased the ion current. Experiments with receptor gene-deficient animals revealed the participation of muscarinic receptor subtypes M₁ and M₃.

CONCLUSIONS AND IMPLICATIONS

The presence of luminal ACh and activation of transepithelial ion currents by luminal ACh receptors identifies a novel non-neuronal cholinergic pathway in the airway lining fluid. This pathway could represent a novel drug target in the airways.

Ketamine inhibits lung fluid clearance through reducing alveolar sodium transport

Ketamine is a broadly used anaesthetic for analgosedation. Accumulating clinical evidence shows that ketamine causes pulmonary edema with unknown mechanisms. We measured the effects of ketamine on alveolar fluid clearance in human lung lobes ex vivo. Our results showed that intratracheal instillation of ketamine markedly decreased the reabsorption of 5% bovine serum albumin instillate. In the presence of amiloride (a specific ENaC blocker), fluid resolution was not further decreased, suggesting that ketamine could decrease amiloride-sensitive fraction of AFC associated with ENaC. Moreover, we measured the regulation of amiloride-sensitive currents by ketamine in A549 cells using whole-cell patch clamp mode. Our results suggested that ketamine decreased amiloride-sensitive Na+ currents (ENaC activity) in a dose-dependent fashion. These data demonstrate that reduction in lung ENaC activity and lung fluid clearance following administration of ketamine may be the crucial step of the pathogenesis of resultant pulmonary edema.

Essential structural features of TNF-α lectin-like domain derived peptides for activation of amiloride-sensitive sodium current in A549 cells

The amiloride-sensitive epithelial sodium channel (ENaC) plays a prominent role in sodium uptake from alveolar fluid and is the major component in alveolar fluid clearance in normal and diseased lungs. The lectin-like domain of TNF-α has been shown to activate amiloride-sensitive sodium uptake in type II alveolar epithelial cells. Therefore, several synthetic peptides that mimic the lectin-like domain of TNF-α (TIP) were synthesized and their ability to enhance sodium current through ENaC was studied in A549 cells with the patch clamp technique. Our data suggest that a free positively charged N-terminal amino group on residue 1 and/or a free negatively charged carboxyl group on residue 17 of the TIP peptide is essential for the ENaC-activating effect. Ventilation strategies apart, no standard treatment exists for pulmonary permeability edema. Therefore, novel therapies activating sodium uptake from the alveolar fluid via ENaC could improve clinical outcome.

[K+ channels and lung epithelial physiology]

Transcripts of more than 30 different K(+) channels have been detected in the respiratory epithelium lining airways and alveoli. These channels belong to the 3 main classes of K(+) channels, i.e. i) voltage-dependent or calcium-activated, 6 transmembrane segments (TM), ii) 2-pores 4-TM and iii) inward-rectified 2-TM channels. The physiological and functional significance of this high molecular diversity of lung epithelial K(+) channels is not well understood. Surprisingly, relatively few studies are focused on K(+) channel function in lung epithelial physiology. Nevertheless, several studies have shown that KvLQT1, KCa and K(ATP) K(+) channels play a crucial role in ion and fluid transport, contributing to the control of airway and alveolar surface liquid composition and volume. K(+) channels are involved in other key functions, such as O(2) sensing or the capacity of the respiratory epithelia to repair after injury. This mini-review aims to discuss potential functions of lung K(+) channels.

Molecular diversity and function of K+ channels in airway and alveolar epithelial cells

Multiple K(+) channels are expressed in the respiratory epithelium lining airways and alveoli. Of the three main classes [1) voltage-dependent or Ca(2+)-activated, 6-transmembrane domains (TMD), 2) 2-pores 4-TMD, and 3) inward-rectified 2-TMD K(+) channels], almost 40 different transcripts have already been detected in the lung. The physiological and functional significance of this high molecular diversity of lung epithelial K(+) channels is intriguing. As detailed in the present review, K(+) channels are located at both the apical and basolateral membranes in the respiratory epithelium, where they mediate K(+) currents of diverse electrophysiological and regulatory properties. The main recognized function of K(+) channels is to control membrane potential and to maintain the driving force for transepithelial ion and liquid transport. In this manner, KvLQT1, KCa and K(ATP) channels, for example, contribute to the control of airway and alveolar surface liquid composition and volume. Thus, K(+) channel activation has been identified as a potential therapeutic strategy for the resolution of pathologies characterized by ion transport dysfunction. K(+) channels are also involved in other key functions in lung physiology, such as oxygen-sensing, inflammatory responses and respiratory epithelia repair after injury. The purpose of this review is to summarize and discuss what is presently known about the molecular identity of lung K(+) channels with emphasis on their role in lung epithelial physiology.

Regulation of ENaC and CFTR expression with K+channel modulators and effect on fluid absorption across alveolar epithelial cells

In a recent study (Leroy C, Dagenais A, Berthiaume Y, and Brochiero E. Am J Physiol Lung Cell Mol Physiol 286: L1027–L1037, 2004), we identified an ATP-sensitive K+(KATP) channel in alveolar epithelial cells, formed by inwardly rectifying K+channel Kir6.1/sulfonylurea receptor (SUR)2B subunits. We found that short applications of KATP, voltage-dependent K+channel KvLQT1, and calcium-activated K+(KCa) channel modulators modified Na+and Cl−currents in alveolar monolayers. In addition, it was shown previously that a KATPopener increased alveolar liquid clearance in human lungs by a mechanism possibly related to epithelial sodium channels (ENaC). We therefore hypothesized that prolonged treatment with K+channel modulators could induce a sustained regulation of ENaC activity and/or expression. Alveolar monolayers were treated for 24 h with inhibitors of KATP, KvLQT1, and KCachannels identified by PCR. Glibenclamide and clofilium (KATPand KvLQT1 inhibitors) strongly reduced basal transepithelial current, amiloride-sensitive Na+current, and forskolin-activated Cl−currents, whereas pinacidil, a KATPactivator, increased them. Interestingly, K+inhibitors or membrane depolarization (induced by valinomycin in high-K+medium) decreased α-, β-, and γ-ENaC and CFTR mRNA. α-ENaC and CFTR proteins also declined after glibenclamide or clofilium treatment. Conversely, pinacidil augmented ENaC and CFTR mRNAs and proteins. Since alveolar fluid transport was found to be driven, at least in part, by Na+transport through ENaC, we tested the impact of K+channel modulators on fluid absorption across alveolar monolayers. We found that glibenclamide and clofilium reduced fluid absorption to a level similar to that seen in the presence of amiloride, whereas pinacidil slightly enhanced it. Long-term regulation of ENaC and CFTR expression by K+channel activity could benefit patients with pulmonary diseases affecting ion transport and fluid clearance.

Halothane Directly Modifies Na+ and K+ Channel Activities in Cultured Human Alveolar Epithelial Cells

During inhalational anesthesia, halogenated gases are in direct contact with the alveolar epithelium, in which they may affect transepithelial ion and fluid transport. The effects of halogenated gases in vivo on epithelial Na+ and K+ channels, which participate in alveolar liquid clearance, remain unclear. In the present study, the effects of halothane (1, 2, and 4% atm) on ion-channel function in cultured human alveolar cells were investigated using the patch-clamp technique. After exposure to 4% halothane, amiloride-sensitive whole-cell inward currents increased by 84+/-22%, whereas tetraethylammonium-sensitive outward currents decreased by 63+/-7%. These effects, which occurred within 30 s, remained for 30-min periods of exposure to the gas, were concentration-dependent, and were reversible upon washout. Pretreatment with amiloride prevented 90+/-7% of the increase in inward currents without change in outward currents, consistent with an activation of amiloride-sensitive epithelial sodium channels. Tetraethylammonium obliterated 90+/-9% of the effect of halothane on outward currents, without change in inward currents, indicating inhibition of Ca2+-activated K+ channels. These channels were identified in excised patches to be small-conductance Ca2+-activated K+ channels. These effects of halothane were not modified after the inhibition of cytosolic phospholipase A2 by aristolochic acid. Exposure of the cells to either trypsin or to low Na+ completely prevented the increase in amiloride-sensitive currents induced by halothane, suggesting a release of Na+ channels self-inhibition. Thus, halothane modifies differentially and independently Na+ and K+ permeabilities in human alveolar cells.

Molecular identity and function in transepithelial transport of K(ATP) channels in alveolar epithelial cells

K(+) channels play a crucial role in epithelia by repolarizing cells and maintaining electrochemical gradient for Na(+) absorption and Cl(-) secretion. In the airway epithelium, the most frequently studied K(+) channels are KvLQT1 and K(Ca). A functional role for K(ATP) channels has been also suggested in the lung, where K(ATP) channel openers activate alveolar clearance and attenuate ischemia-reperfusion injury. However, the molecular identity of this channel is unknown in airway and alveolar epithelial cells (AEC). We adopted an RT-PCR strategy to identify, in AEC, cDNA transcripts for Kir channels (Kir6.1 or 6.2) and sulfonylurea receptors (SUR1, 2A, or 2B) forming K(ATP) channels. Only Kir6.1 and SUR2B were detected in freshly isolated and cultured alveolar cells. To determine the physiological role of K(+) channels in the transepithelial transport of alveolar monolayers, we studied the effect, on total short-circuit currents (I(sc)), of basolateral application of glibenclamide, an inhibitor of K(ATP) channels, as well as clofilium, charybdotoxin, clotrimazole, and iberiotoxin, inhibitors of KvLQT1 and K(Ca) channels, respectively. Interestingly, activity of the three types of K(+) channels was detected, since all tested inhibitors decreased I(sc). Furthermore, these K(+) channel inhibitors reduced amiloride-sensitive Na(+) currents (mediated by ENaC) and completely abolished stimulation of Cl(-) currents by forskolin. Conversely, pinacidil, an activator of K(ATP) channels, increased Na(+) and Cl(-) transepithelial transport by 33-35%. These results suggest the presence, in AEC, of a K(ATP) channel, formed from Kir6.1 and SUR2B subunits, which plays a physiological role, with KvLQT1 and K(Ca) channels, in Na(+) and Cl(-) transepithelial transport.

Hypoxia-induced inhibition of whole cell membrane currents and ion transport of A549 cells

In excitable cells, hypoxia inhibits K channels, causes membrane depolarization, and initiates complex adaptive mechanisms. It is unclear whether K channels of alveolar epithelial cells reveal a similar response to hypoxia. A549 cells were exposed to hypoxia during whole cell patch-clamp measurements. Hypoxia reversibly inhibited a voltage-dependent outward current, consistent with a K current, because tetraethylamonium (TEA; 10 mM) abolished this effect; however, iberiotoxin (0.1 microM) does not. In normoxia, TEA and iberiotoxin inhibited whole cell current (-35%), whereas the K-channel inhibitors glibenclamide (1 microM), barium (1 mM), chromanol B293 (10 microM), and 4-aminopyridine (1 mM) were ineffective. (86)Rb uptake was measured to see whether K-channel modulation also affected transport activity. TEA, iberiotoxin, and 4-h hypoxia (1.5% O(2)) inhibited total (86)Rb uptake by 40, 20, and 35%, respectively. Increased extracellular K also inhibited (86)Rb uptake in a dose-dependent way. The K-channel opener 1-ethyl-2-benzimidazolinone (1 mM) increased (86)Rb uptake by 120% in normoxic and hypoxic cells by activation of Na-K pumps (+60%) and Na-K-2Cl cotransport (+170%). However, hypoxic transport inhibition was also seen in the presence of 1-ethyl-2-benzimidazolinone, TEA, and iberiotoxin. These results indicate that hypoxia, membrane depolarization, and K-channel inhibition decrease whole cell membrane currents and transport activity. It appears, therefore, that a hypoxia-induced change in membrane conductance and membrane potential might be a link between hypoxia and alveolar ion transport inhibition.

Chloride and potassium channel function in alveolar epithelial cells

Electrolyte transport across the adult alveolar epithelium plays an important role in maintaining a thin fluid layer along the apical surface of the alveolus that facilitates gas exchange across the epithelium. Most of the work published on the transport properties of alveolar epithelial cells has focused on the mechanisms and regulation of Na(+) transport and, in particular, the role of amiloride-sensitive Na(+) channels in the apical membrane and the Na(+)-K(+)-ATPase located in the basolateral membrane. Less is known about the identity and role of Cl(-) and K(+) channels in alveolar epithelial cells, but studies are revealing important functions for these channels in regulation of alveolar fluid volume and ionic composition. The purpose of this review is to examine previous work published on Cl(-) and K(+) channels in alveolar epithelial cells and to discuss the conclusions and speculations regarding their role in alveolar cell transport function.

Large conductance Ca2+-activated K+ channels sense acute changes in oxygen tension in alveolar epithelial cells

The rise in alveolar oxygen tension (PO(2)) that occurs as the newborn infant takes its first breaths induces removal of liquid from the lung lumen due to ion transport across the alveolar epithelium and the activity of alveolar Na(+) channel (ENaC). In the present study, we have aimed to identify an ion conductance in alveolar epithelial A549 cells that responds to acute changes in PO(2). Variation in PO(2) did not affect single-channel ENaC activity. However, in these cells we have detected single-channel conductance having properties similar to those of large conductance Ca(2+)-activated K(+) (BK(Ca)) channels. Reverse transcriptase-polymerase chain reaction and Western blotting demonstrated presence of alpha-BKCa channel subunit and iberiotoxin, a blocker of BK(Ca) channels, inhibited whole cell K(+) current. Chronic changes in PO(2) did not affect expression, recruitment, or function of BK(Ca) channels in A549 cells. In contrast, acute changes of PO(2) regulated the BK(Ca) channel activity by controlling the channel mean open time. This effect of PO(2) was insensitive to inhibitor of flavoproteins, diphenylene iodinium. In addition, decrease in PO(2) and iberiotoxin induced membrane depolarization and Ca(2+) oscillations in A549 cells. We conclude that BK(Ca) channels serve as oxygen sensors in human alveolar A549 epithelial cells.

Exploring the open pore of the potassium channel from Streptomyces lividans

The tetrameric potassium channel from Streptomyces lividans (KcsA) embedded in planar bilayers exhibits the following electrophysiological characteristics: (i) K+ ions can cross the pore in a highly hydrated state (nH2O > or = 6), (ii) the selectivity for K+ exceeds that for Na+ ions by 11 times, and both Ca2+ and Mg2+ are permeant, (iii) the internal side is blocked by Ba2+ ions in a voltage-dependent manner, (iv) intrinsic rectification is due to gating, depending on the direction of the electric field, (v) the internal side is pH-sensitive, and (vi) the open pore has a diameter of approximately 5.8 A. In conclusion, our results show that ion conduction and selectivity of KcsA cannot easily be reconciled with the properties deduced from the rigid crystal structure [Doyle et al., Science 280 (1998) 69-77], which must be concluded to have the pore trapped in its closed state.