Reconstitution of immunopurified alveolar type II cell Na+ channel protein into planar lipid bilayers

ENaC activity requires CFTR channel function independently of phosphorylation in sweat duct

We previously showed that activation of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) Cl- conductance (gCFTR) supports parallel activation of amiloride-sensitive epithelial Na+ channel (ENaC) in the native human sweat duct. However, it is not clear whether phosphorylated CFTR, phosphorylated ENaC, or only Cl(-) -channel function is required for activation. We used basilaterally alpha-toxin-permeabilized human sweat ducts to test the hypothesis that ENaC activation depends only on Cl(-) -channel function and not on phosphorylation of either CFTR or ENaC. CFTR is classically activated by PKA plus millimolar ATP, but cytosolic glutamate activation of gCFTR is independent of ATP and phosphorylation. We show here that both phosphorylation-dependent (PKA) and phosphorylation-independent (glutamate) activation of CFTR Cl- channel function support gENaC activation. We tested whether cytosolic application of 5 mM ATP alone, phosphorylation by cAMP, cGMP, G-protein dependent kinases (all in the presence of 100 microM ATP), or glutamate could support ENaC activation in the absence of gCFTR. We found that none of these agonists activated gENaC by themselves when Cl- current (I(Cl-)) through CFTR was blocked by: 1) Cl- removal, 2) DIDS inhibition, 3) lowering the ATP concentration to 100 microM (instead of 5 mM required to support CFTR channel function), or 4) mutant CFTR (homozygous DeltaF508 CF ducts). However, Cl- gradients in the direction of absorption supported, while Cl- gradients in the direction of secretion prevented ENaC activation. We conclude that the interaction between CFTR and ENaC is dependent on activated I(Cl-) through CFTR in the direction of absorption (Cl- gradient from lumen to cell). But such activation of ENaC is independent of phosphorylation and ATP. However, reversing I(Cl-) through CFTR in the direction of secretion (Cl- gradient from cell to lumen) prevents ENaC activation even in the presence of I(Cl-) through CFTR.

Acute ENaC stimulation by cAMP in a kidney cell line is mediated by exocytic insertion from a recycling channel pool

Acute hormonal regulation of the epithelial sodium channel (ENaC) in tight epithelia increases transcellular Na⁺ transport via trafficking of intracellular channels to the apical surface. The fate of the channels removed from the apical surface following agonist washout is less clear. By repetitively stimulating polarized mouse cortical collecting duct (mCCD, _MPKCCD₁₄) epithelia, we evaluated the hypothesis that ENaC recycles through an intracellular pool to be available for reinsertion into the apical membrane. Short circuit current (I_SC), membrane capacitance (C_T), and conductance (G_T) were recorded from mCCD epithelia mounted in modified Ussing chambers. Surface biotinylation of ENaC demonstrated an increase in channel number in the apical membrane following cAMP stimulation. This increase was accompanied by a 83 ± 6% (n = 31) increase in I_SC and a 15.3 ± 1.5% (n = 15) increase in C_T. Selective membrane permeabilization demonstrated that the C_T increase was due to an increase in apical membrane capacitance. I_SC and C_T declined to basal levels on stimulus washout. Repetitive cAMP stimulation and washout (∼1 h each cycle) resulted in response fatigue; ΔI_SC decreased ∼10% per stimulation–recovery cycle. When channel production was blocked by cycloheximide, ΔI_SC decreased ∼15% per stimulation cycle, indicating that newly synthesized ENaC contributed a relatively small fraction of the channels mobilized to the apical membrane. Selective block of surface ENaC by benzamil demonstrated that channels inserted from a subapical pool made up >90% of the stimulated I_SC, and that on restimulation a large proportion of channels retrieved from the apical surface were reinserted into the apical membrane. Channel recycling was disrupted by brefeldin A, which inhibited ENaC exocytosis, by chloroquine, which inhibited ENaC endocytosis and recycling, and by latrunculin A, which blocked ENaC exocytosis. A compartment model featuring channel populations in the apical membrane and intracellular recycling pool provided an adequate kinetic description of the I_SC responses to repetitive stimulation. The model supports the concept of ENaC recycling in response to repetitive cAMP stimulation.

Inhibition by cytoplasmic nucleotides of a new cation channel in freshly isolated human and rat type II pneumocytes

Here we report a 26- to 29-pS cation channel abundantly expressed in freshly isolated and primary cultured type II cells from rat or healthy human lungs. The channel was never spontaneously active in cell-attached patches but could be activated by cell permeabilization with beta-escin. Excised patch-clamp experiments revealed activation by Ca(2+) concentrations at the cytoplasmic side in the micromolar range. High concentrations of amiloride (>10 microM) at the extracellular side did not inhibit. The channel was equally permeable for K(+) and Na(+) but was essentially impermeable for Cl(-), Ca(2+), and Mg(2+). It was blocked by adenosine nucleotides (cytoplasmic side) with the following order of potency: AMP approximately ADP (EC(50) </= 10 microM) > ATP >> adenosine >> cyclic AMP. The blocking effect of ATP was reproduced by its nonhydrolyzable analogs AMPPNP or ATP-gamma-S. GTP did not inhibit. Cd(2+) blocked the channel with an EC(50) approximately 55.5 nM. We conclude that type II cells express a Ca(2+)-dependent, nucleotide-inhibited, nonselective, and Ca(2+)-impermeable cation channel (NSC(Ca/AMP)) with tonically suppressed activity. RT-PCR confirmed expression of TRPM4b, a channel with functional characteristics almost identical with NSC(Ca/AMP). Potential physiological roles are discussed.

cAMP-induced changes of apical membrane potentials of confluent H441 monolayers

We recorded apical membrane potentials (Va) of H441 cells [a human lung cell line exhibiting both epithelial Na+ (ENaC) and CFTR-type channels] grown as confluent monolayers, using the microelectrode technique in current-clamp mode before, during, and after perfusion of the apical membranes with 10 microM forskolin. When perfused with normal Ringer solution, the cells had a Va of -43 +/- 10 mV (means +/- SD; n = 31). Perfusion with forskolin resulted in sustained depolarization by 25.0 +/- 3.5 mV (means +/- SD; n = 23) and increased the number, open time, and the open probability of a 4.2-pS ENaC. In contrast to a previous report (Jiang J, Song C, Koller BH, Matthay MA, and Verkman AS. Am J Physiol Cell Physiol 275: C1610-C1620, 1998), no transient hyperpolarization was observed. The forskolin-induced depolarization of Va was almost totally prevented by pretreatment of monolayers with 10 microM amiloride or by substitution of Na+ ions in the bath solution with N-methyl-d-glucamine. These findings indicate that cAMP stimulation of Na+ influx across H441 confluent monolayers results from activation of an amiloride-sensitive apical Na+ conductance and not from Va hyperpolarization due to Cl- influx through CFTR-type channels.

Hypoxia and beta 2-agonists regulate cell surface expression of the epithelial sodium channel in native alveolar epithelial cells

Alveolar hypoxia may impair sodium-dependent alveolar fluid transport and induce pulmonary edema in rat and human lung, an effect that can be prevented by the inhalation of beta(2)-agonists. To investigate the mechanism of beta(2)-agonist-mediated stimulation of sodium transport under conditions of moderate hypoxia, we examined the effect of terbutaline on epithelial sodium channel (ENaC) expression and activity in cultured rat alveolar epithelial type II cells exposed to 3% O(2) for 24 h. Hypoxia reduced transepithelial sodium current and amiloride-sensitive sodium channel activity without decreasing ENaC subunit mRNA or protein levels. The functional decrease was associated with reduced abundance of ENaC subunits (especially beta and gamma) in the apical membrane of hypoxic cells, as quantified by biotinylation. cAMP stimulation with terbutaline reversed the hypoxia-induced decrease in transepithelial sodium transport by stimulating sodium channel activity and markedly increased the abundance of beta-and gamma-ENaC in the plasma membrane of hypoxic cells. The effect of terbutaline was prevented by brefeldin A, a blocker of anterograde transport. These novel results establish that hypoxia-induced inhibition of amiloride-sensitive sodium channel activity is mediated by decreased apical expression of ENaC subunits and that beta(2)-agonists reverse this effect by enhancing the insertion of ENaC subunits into the membrane of hypoxic alveolar epithelial cells.

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.

cAMP increases density of ENaC subunits in the apical membrane of MDCK cells in direct proportion to amiloride-sensitive Na(+) transport

Antidiuretic hormone and/or cAMP increase Na(+) transport in the rat renal collecting duct and similar epithelia, including Madin-Darby canine kidney (MDCK) cell monolayers grown in culture. This study was undertaken to determine if that increment in Na(+) transport could be explained quantitatively by an increased density of ENaC Na(+) channels in the apical membrane. MDCK cells with no endogenous ENaC expression were retrovirally transfected with rat alpha-, beta-, and gammaENaC subunits, each of which were labeled with the FLAG epitope in their extracellular loop as described previously (Firsov, D., L. Schild, I. Gautschi, A.-M. Mérillat, E. Schneeberger, and B.C. Rossier. 1996. PROC: Natl. Acad. Sci. USA. 93:15370-15375). The density of ENaC subunits was quantified by specific binding of (125)I-labeled anti-FLAG antibody (M2) to the apical membrane, which was found to be a saturable function of M2 concentration with half-maximal binding at 4-8 nM. Transepithelial Na(+) transport was measured as the amiloride-sensitive short-circuit current (AS-I(sc)) across MDCK cells grown on permeable supports. Specific M2 binding was positively correlated with AS-I(sc) measured in the same experiments. Stimulation with cAMP (20 microM 8-p-chlorothio-cAMP plus 200 microM IBMX) significantly increased AS-I(sc) from 11.2 +/- 1.3 to 18.1 +/- 1.3 microA/cm(2). M2 binding (at 1.7 nM M2) increased in direct proportion to AS-I(sc) from 0.62 +/- 0.13 to 1.16 +/- 0.18 fmol/cm(2). Based on the concentration dependence of M2 binding, the quantity of Na(+) channels per unit of AS-I(sc) was calculated to be the same in the presence and absence of cAMP, 0.23 +/- 0.04 and 0.21 +/-0.05 fmol/microA, respectively. These values would be consistent with a single channel conductance of approximately 5 pS (typically reported for ENaC channels) only if the open probability is <0.02, i.e., less than one-tenth of the typical value. We interpret the proportional increases in binding and AS-I(sc) to indicate that the increased density of ENaC subunits in the apical membrane can account completely for the I(sc) increase produced by cAMP.

Beta-adrenergic regulation of amiloride-sensitive lung sodium channels

We investigated the mechanism by which cAMP increases sodium transport in lung epithelial cells. Alveolar type II (ATII) cells have two types of amiloride-sensitive, cation channels: a nonselective cation channel (NSC) and a highly selective channel (HSC). Exposure of ATII cells to cAMP, beta-adrenergic agonists, or other agents that increase adenylyl cyclase activity increased activity of both channel types, albeit by different mechanisms. NSC open probability (P(o)) increased severalfold when exposed to terbutaline, isoproterenol, forskolin, or cAMP analogs without any change in NSC number. In contrast, terbutaline increased HSC number with no significant change in HSC P(o). For both channels, the effect of terbutaline was blocked by propranolol and H-89, suggesting a protein kinase A (PKA) requirement for beta-adrenergic-induced changes in channel activity. Terbutaline increased cAMP levels in ATII cells, but intracellular calcium also increased. Calcium sequestration with BAPTA blocked beta-adrenergic-induced increases in NSC P(o) but did not alter HSC activity. These observations suggest that beta-adrenergic stimulation increases intracellular cAMP and activates PKA. PKA increases HSC number and increases intracellular calcium. The increase in calcium increases NSC P(o). Thus increased cAMP levels are likely to increase lung sodium transport regardless of which channel type is present.

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.

Perinatal PTX-sensitive G-protein expression and regulation of conductive 22Na+ transport in lung apical membrane vesicles

Using apical membrane vesicles (AMV) prepared from mature foetal and early neonatal guinea pig lung we show that pertussis toxin (PTX)-sensitive G-protein regulation of conductive 22Na+ uptake undergoes rapid changes following birth. Thus, G-protein activation by intravesicular incorporation of 100 microM GTPgammaS into vesicles resuspended in NaCl, which in late gestation stimulated uptake, consistently induced inhibition of conductive Na+ uptake into AMV prepared from neonatal lung at 4 days of age (N4) (52+/-9%, n=8, P<0.05). This response was not significantly different in the presence of the relatively impermeant anion isethionate (Ise-) (69+/-9%, n=7, P<0.05). Changes in the regulation of uptake were already detectable on the day of birth (N0) in AMV resuspended in NaCl, with GTPgammaS inducing both stimulatory and inhibitory responses. These data indicate that the processes by which 22Na+ uptake into AMV is regulated by G-proteins undergoes a change at birth and by 4 days of age, G-protein regulation of uptake occurs predominantly via modulation of co-localised Na+ channels. Intravesicular incorporation of GDPbetaS or pre-treatment with PTX did not significantly alter conductive 22Na+ uptake in the presence of NaCl or NaIse suggesting that constitutively active G-proteins are not involved in this process. Pre-treatment of AMV with PTX prevented the inhibition of conductive 22Na+ uptake by GTPgammaS (105+/-16% n=7) indicating that a PTX-sensitive G-protein mediates the inhibition of channels in neonatal AMV. Western blotting demonstrated enrichment of Gialpha1, Gialpha2, Gialpha3 and Goalpha in the apical membrane preparations. We also show that there is a significant rise in the levels of Gialpha3 during the early neonatal period providing a potential candidate for the G-protein mediated changes in regulation of conductive 22Na+ uptake in neonatal AMV.

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.

cAMP activation of chloride and fluid secretion across the rabbit alveolar epithelium

Active Na+ transport by alveolar epithelial cells has been demonstrated to contribute significantly to alveolar fluid clearance. However, the contribution of transepithelial Cl- movement to the reabsorption of isosmotic fluid across the alveolar epithelium in vivo has not been elucidated. We hypothesized that Cl- transport could be increased across the alveolar epithelium in vivo and across cultured alveolar type II cells by agents that increase intracellular cAMP (e.g., forskolin). In studies where 5% albumin in sodium methanesulfonate (a Cl--free solution) was administered into the lung, forskolin administration significantly increased intracellular influx of Cl- and fluid into the alveolar space. In vitro studies with cultured rabbit alveolar type II cell monolayers in Ussing chambers demonstrated that elevations in intracellular cAMP increase short-circuit current by increasing both Cl- secretion and Na+ reabsorption. The cystic fibrosis transmembrane conductance regulator channel blocker glibenclamide and the loop diuretic bumetanide partially decreased the forskolin-induced increase in short-circuit current. These data may explain the failure of agonist that stimulated intracellular cAMP to increase alveolar fluid clearance in the rabbit. Moreover, the data suggest that in the event Na+ absorptive pathways are damaged, transepithelial Cl- secretion and the consequent intra-alveolar fluid influx may be upregulated.

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.

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.

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.

Cystic fibrosis transmembrane conductance regulator inverts protein kinase A-mediated regulation of epithelial sodium channel single channel kinetics

Abnormal regulation of ion channels by members of the ABC transport protein superfamily has been implicated in hyperinsulinemic hypoglycemia and in excessive Na+ absorption by airway epithelia in cystic fibrosis (CF). How ABC proteins regulate ion conductances is unknown, but must generally involve either the number or activity of specific ion channels. Here we report that the cystic fibrosis transmembrane conductance regulator (CFTR), which is defective in CF, reverses the regulation of the activity of single epithelial sodium channels (ENaC) by cAMP. ENaC expressed alone in fibroblasts responded to activation of cAMP-dependent protein kinase with increased open probability (Po) and mean open time, whereas ENaC co-expressed with CFTR exhibited decreased Po and mean open time under conditions optimal for PKA-mediated protein phosphorylation. Thus, CFTR regulates ENaC at the level of single channel gating, by switching the response of single channel Po to cAMP from an increase to a decrease.

Diversity and regulation of amiloride-sensitive Na+ channels

Amiloride-sensitive Na+ channels play a vital role in many important physiological processes such as delineation of the final urine composition, sensory transduction, and whole-body Na+ homeostasis. These channels display a wide range of biophysical properties, and are regulated by cAMP-mediated second messenger systems. The first of these channels has recently been cloned. This cloned amiloride-sensitive Na+ channel is termined ENaC (Epithelial Na+ Channel) and, in heterologous cellular expression systems, displays a single channel conductance of 4 to 7 pS, a high PNa/PK (> 10), a high amiloride sensitivity (Ki(amil) = 150 nM), and relatively long open and closed times. ENaC may form the core conduction element of many of these functionally diverse forms of Na+ channel. The kinetic and regulatory differences between these channels may be due, in large measure, to unique polypeptides that associate with the core element, forming a functional channel unit.

Regulation of epithelial sodium channels by the cystic fibrosis transmembrane conductance regulator

Cystic fibrosis airway epithelia exhibit enhanced Na+ reabsorption in parallel with diminished Cl- secretion. We tested the hypothesis that the cystic fibrosis transmembrane conductance regulator (CFTR) directly affects epithelial Na+ channel activity by co-incorporating into planar lipid bilayers immunopurified bovine tracheal CFTR and either heterologously expressed rat epithelial Na+ channel ( alpha,b eta,gamma-rENaC) or an immunopurified bovine renal Na+ channel protein complex. The single channel open probability (Po) of rENaC was decreased by 24% in the presence of CFTR. Protein kinase A (PKA) plus ATP activated CFTR, but did not have any effect on rENaC. CFTR also decreased the extent of elevation of the renal Na+ channel Po following PKA-mediated phosphorylation. Moreover, the presence of CFTR prohibited the inward rectification of the gating of this renal Na+ channel normally induced by PKA-mediated phosphorylation, thus down-regulating inward Na+ current. This interaction between CFTR and Na+ channels occurs independently of whether or not wild-type CFTR is conducting anions. However, the nonconductive CFTR mutant, G551D CFTR, cannot substitute for the wild-type molecule. Our results indicate that CFTR can directly down-regulate single Na+ channel activity, thus accounting, at least in part, for the observed differences in Na+ transport between normal and cystic fibrosis-affected airway epithelia.

Triple-barrel organization of ENaC, a cloned epithelial Na+ channel

A cloned rat epithelial Na+ channel (rENaC) was studied in planar lipid bilayers. Two forms of the channel were examined: channels produced by the alpha subunit alone and those formed by alpha, beta, and gamma subunits. The protein was derived from two sources: either from in vitro translation reaction followed by Sephadex column purification or from heterologous expression in Xenopus oocytes and isolation of plasma membranes. We found that either alpha-rENaC alone or alpha- in combination with beta- and gamma-rENaC, produced highly Na(+)-selective (PNa/PK = 10), amiloride-sensitive (Kamili = 170 nM), and mechanosensitive cation channels in planar bilayers. alpha-rENaC displayed a complicated gating mechanism: there was a nearly constitutively open 13-picosiemens (pS) state and a second 40-pS level that was achieved from the 13-pS level by a 26-pS transition. alpha-, beta-, gamma-rENaC showed primarily the 13-pS level. alpha-rENaC and alpha,beta,gamma-rENaC channels studied by patch clamp displayed the same gating pattern, albeit with > 2-fold lowered conductance levels, i.e. 6 and 18 pS, respectively. Upon treatment of either channel with the sulfhydryl reducing agent dithiothreitol, both channels fluctuated among three independent 13-pS sublevels. Bathing each channel with a high salt solution (1.5 M NaCl) produced stochastic openings of 19 and 38 pS in magnitude between all three conductance levels. Different combinations of alpha-, beta-, and gamma-rENaC in the reconstitution mixture did not produce channels of intermediate conductance levels. These findings suggest that functional ENaC is composed of three identical conducting elements and that their gating is concerted.

Biochemical status of renal epithelial Na+ channels determines apparent channel conductance, ion selectivity, and amiloride sensitivity

Purified bovine renal papillary Na+ channels, when reconstituted into planar lipid bilayers, reside in three conductance states: a 40-pS main state, and two subconductive states (12-13 pS and 24-26 pS). The activity of these channels is regulated by phosphorylation and by G-proteins. Protein kinase A (PKA)-induced phosphorylation increased channel activity by increasing the open state time constants from 160 +/- 30 (main conductance), and 15 +/- 5 ms (both lower conductances), respectively, to 365 +/- 30 ms for all of them. PKA phosphorylation also altered the closed time of the channel from 250 +/- 30 ms to 200 +/- 35 ms, thus shifting the channel into a lower-conductance, long open time mode. PKA phosphorylation increased the PNa:PK of the channel from 7:1 to 20:1, and shifted the amiloride inhibition curve to the right (apparent K(i)amil from 0.7 to 20 microM). Pertussis toxin-induced ADP-ribosylation of either phosphorylated of either phosphorylated or nonphosphorylated channels decreased the PNa:PK to 2:1 and 4:1, respectively, and altered K(i)amil to 8 and 2 microM for phosphorylated and nonphosphorylated channels, respectively. GTP-gamma-S treatment of either phosphorylated or nonphosphorylated channels resulted in an increase of PNa:PK to 30:1 and 10:1, respectively, and produced a leftward shift in the amiloride dose-response curve, altering K(i)amil to 0.5 and 0.1 microM, respectively. These results suggest that amiloride-sensitive renal Na+ channel biophysical characteristics are not static, but depend upon the biochemical state of the channel protein and/or its associated G-protein.

Increased expression and activity of sodium channels in alveolar type II cells of hyperoxic rats

We investigated the cellular and molecular events associated with the increase in sodium transport across the alveolar epithelium of rats exposed to hyperoxia (85% O2 for 7 days followed by 100% O2 for 4 days). Alveolar type II (ATII) cell RNA was isolated and probed with a cDNA for one of the rat colonic epithelial sodium channel subunits (alpha rENaC). The alpha rENaC mRNA (3.7-kb transcript) increased 3-fold in ATII cell RNA isolated from rats exposed to 85% O2 for 7 days and 6-fold after 4 days of subsequent exposure to 100% O2. In situ hybridization revealed increased expression of alpha rENaC mRNA transcripts in both airway and alveolar epithelial cells of hyperoxic rats. When immunostained with a polyclonal antibody to kidney sodium channel protein, ATII cells from hyperoxic rats exhibited a significant increase in the amount of immunogenic protein present in both the plasma membrane and the cytoplasm. When patched in the whole-cell mode, ATII cells from hyperoxic rats exhibited amiloride and 5-(N-ethyl-N-isopropyl)-2',4'-amiloride (EIPA)-sensitive currents that were 100% higher compared with those obtained from air-breathing rats. Single-channel sodium currents (mean conductance of 25 pS) were seen in ATII cells patched in both the inside-out and cell-attached modes. The number and open probability of these channels increased significantly during exposure to hyperoxia. Exposure to sublethal hyperoxia up-regulated both alpha rENaC mRNA and the functional expression of sodium channels in ATII cells.