Protein kinase A phosphorylation and G protein regulation of type II pneumocyte Na+ channels in lipid bilayers

Effect of cytosolic pH on epithelial Na+ channel in normal and cystic fibrosis sweat ducts

The activities of cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel and the amiloride-sensitive epithelial Na(+) channel (ENaC) are acutely coordinated in the sweat duct. However, the mechanisms responsible for cross-talk between these ion channels are unknown. Previous studies indicated that luminal pH of sweat ducts varies over 3 pH units and that the cytoplasmic pH affects both CFTR and ENaC. Therefore, using basolaterally alpha-toxin-permeabilized apical membrane preparations of sweat ducts as an experimental system, we tested the hypothesis that the cytosolic pH may mediate the cross-talk between CFTR and ENaC. We showed that while luminal pH had no effect, cytosolic pH acutely affected ENaC activity. That is, acidic pH inhibited, while basic pH activated, ENaC. pH regulation of ENaC appears to be independent of CFTR or endogenous kinase activities because basic pH independently stimulated ENaC (1) in normal ducts even when CFTR was deactivated, (2) in CF ducts that lack CFTR in the plasma membranes and (3) after blocking endogenous kinase activity with staurosporine. Considering the evidence of Na(+)/H(+) exchange (NHE) activity as shown by the expression of mRNA and function of NHE in the basolateral membrane of the sweat duct, we postulate that changes in cytosolic Na(+) ([Na(+)]( i )) may alter cytosolic pH (pH( i )) as salt loads into the cell during electrolyte absorption. These changes may play a role in coordinating the activities of ENaC and CFTR during transepithelial salt transport.

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.

Phosphatase inhibitors increase the open probability of ENaC in A6 cells

We studied the cellular phosphatase inhibitors okadaic acid (OKA), calyculin A, and microcystin on the epithelial sodium channel (ENaC) in A6 renal cells. OKA increased the amiloride-sensitive current after approximately 30 min with maximal stimulation at 1-2 h. Fluctuation analysis of cell-attached patches containing a large number of ENaC yielded power spectra with corner frequencies in untreated cells almost two times as large as in cells pretreated for 30 min with OKA, implying an increase in single channel open probability (P(o)) that doubled after OKA. Single channel analysis showed that, in cells pretreated with OKA, P(o) and mean open time approximately doubled. Two other phosphatase inhibitors, calyculin A and microcystin, had similar effects on P(o) and mean open time. An analog of OKA, okadaone, that does not inhibit phosphatases had no effect. Pretreatment with 10 nM OKA, which blocks protein phosphatase 2A (PP2A) but not PP1 in mammalian cells, had no effect even though both phosphatases are present in A6 cells. Several proteins were differentially phosphorylated after OKA, but ENaC subunit phosphorylation did not increase. We conclude that, in A6 cells, there is an OKA-sensitive phosphatase that suppresses ENaC activity by altering the phosphorylation of a regulatory molecule associated with the channel.

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.

Roles of the C termini of alpha -, beta -, and gamma -subunits of epithelial Na+ channels (ENaC) in regulating ENaC and mediating its inhibition by cytosolic Na+

The amiloride-sensitive epithelial Na(+) channels (ENaC) in the intralobular duct cells of mouse mandibular glands are inhibited by the ubiquitin-protein ligase, Nedd4, which is activated by increased intracellular Na(+). In this study we have used whole-cell patch clamp methods in mouse mandibular duct cells to investigate the role of the C termini of the alpha-, beta-, and gamma-subunits of ENaC in mediating this inhibition. We found that peptides corresponding to the C termini of the beta- and gamma-subunits, but not the alpha-subunit, inhibited the activity of the Na(+) channels. This mechanism did not involve Nedd4 and probably resulted from the exogenous C termini interfering competitively with the protein-protein interactions that keep the channels active. In the case of the C terminus of mouse beta-ENaC, the interacting motif included betaSer(631), betaAsp(632), and betaSer(633). In the C terminus of mouse gamma-ENaC, it included gammaSer(640). Once these motifs were deleted, we were able to use the C termini of beta- and gamma-ENaC to prevent Nedd4-mediated down-regulation of Na(+) channel activity. The C terminus of the alpha-subunit, on the contrary, did not prevent Nedd4-mediated inhibition of the Na(+) channels. We conclude that mouse Nedd4 interacts with the beta- and gamma-subunits of ENaC.

Alveolar epithelial barrier. Role in lung fluid balance in clinical lung injury

Several studies have established that transport of sodium from the air spaces to the lung interstitium is a primary mechanism driving alveolar fluid clearance, although further work is needed to determine the role of chloride in vectorial fluid transport across the alveolar epithelium. Although there are significant differences among species in the basal rates of sodium and fluid transport, the basic mechanism seems to depend on sodium uptake by channels on the apical membrane of alveolar type II cells, followed by extrusion of sodium on the basolateral surface by Na,K-ATPase. This process can be upregulated by several catecholamine-dependent and independent mechanisms. The identification of water channels expressed in lung, together with the high water permeabilities, suggest a potential role for channel-mediated water movement between the air space and capillary compartments, although definitive evidence will depend on the results of transgenic mouse knock-out studies. The application of this new knowledge regarding salt and water transport in alveolar epithelium in relation to pathologic conditions has been successful in clinically relevant experimental studies, as well as in a few clinical studies. The studies of exogenous and endogenous catecholamine regulation of alveolar fluid clearance are a good example of how new insights into the basic mechanisms of alveolar sodium and fluid transport can be translated to clinically relevant experimental studies. Exogenous catecholamines can increase the rate of alveolar fluid clearance in several species, including the human lung, and it is also apparent that release of endogenous catecholamines can upregulate alveolar fluid clearance in animals with septic or hypovolemic shock. It is possible that therapy with beta-adrenergic agonists might be useful to accelerate the resolution of alveolar edema in some patients. In some patients, the extent of injury to the alveolar epithelial barrier may be too severe for beta-adrenergic agonists to enhance the resolution of alveolar edema, although some experimental studies indicate that alveolar fluid clearance can be augmented in the presence of moderately severe lung injury. A longer-term upregulation of alveolar epithelial fluid transport might be achieved by strategies that accelerate the proliferation of alveolar type II cells repopulating the injured epithelium in clinical lung injury. More clinical research is needed to evaluate the strategies that can upregulate alveolar epithelial fluid transport with both short-term therapy (i.e., beta-agonists) and more sustained, longer-term effects of epithelial mitogens such as keratinocyte growth factor. These approaches may be useful in reducing mortality in the acute respiratory distress syndrome.

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.

Peroxynitrite inhibits amiloride-sensitive Na+ currents in Xenopus oocytes expressing alpha beta gamma-rENaC

We examined the effect of peroxynitrite (ONOO-) on the cloned rat epithelial Na+ channel (alpha beta gamma-rENaC) expressed in Xenopus oocytes. 3-Morpholinosydnonimine (SIN-1) was used to concurrently generate nitric oxide (.NO) and superoxide (O2-.), which react to form ONOO-, a species known to promote protein nitration and oxidation. Under control conditions, oocytes displayed an amiloride-sensitive whole cell conductance of 7.4 +/- 2.8 (SE) microS. When incubated at 18 degrees C with SIN-1 (1 mM) for 2 h (final ONOO- concentration = 10 microM), the amiloride-sensitive conductance was reduced to 0.8 +/- 0.5 microS. To evaluate whether the observed inhibition was due to ONOO-, as opposed to .NO, we also exposed oocytes to SIN-1 in the presence of urate (500 microM), a scavenger of ONOO- and superoxide dismutase, which scavenges O2-., converting SIN-1 from an ONOO- to an .NO donor. Under these conditions, conductance values remained at control levels following SIN-1 treatment. Tetranitromethane, an agent that oxidizes sulfhydryl groups at pH 6, also inhibited the amiloride-sensitive conductance. These data suggest that oxidation of critical sulfhydryl groups within rENaC by ONOO- directly inhibits channel activity.