Conductive sodium pathway with low affinity to amiloride in LLC-PK1 cells and other epithelia

Characterization of Na+-permeable cation channels in LLC-PK1 renal epithelial cells

In this study, the presence of Na(+)-permeable cation channels was determined and characterized in LLC-PK1 cells, a renal tubular epithelial cell line with proximal tubule characteristics derived from pig kidney. Patch-clamp analysis under cell-attached conditions indicated the presence of spontaneously active Na(+)-permeable cation channels. The channels displayed nonrectifying single channel conductance of 11 pS, substates, and an approximately 3:1 Na(+)/K(+) permeability-selectivity ratio. The Na(+)-permeable cation channels were inhibited by pertussis toxin and reactivated by G protein agonists. Cation channel activity was observed in quiescent cell-attached patches after vasopressin stimulation. The addition of protein kinase A and ATP to excised patches also induced Na(+) channel activity. Spontaneous and vasopressin-induced Na(+) channel activity were inhibited by extracellular amiloride. To begin assessing potential molecular candidates for this cation channel, both reverse transcription-PCR and immunocytochemical analyses were conducted in LLC-PK1 cells. Expression of porcine orthologs of the alphaENaC and ApxL genes were found in LLC-PK1 cells. The expression of both gene products was confirmed by immunocytochemical analysis. Although alphaENaC labeling was mostly intracellular, ApxL labeled to both the apical membrane and cytoplasmic compartments of subconfluent LLC-PK1 cells. Vasopressin stimulation had no effect on alphaENaC immunolabeling but modified the cellular distribution of ApxL, consistent with an increased membrane-associated ApxL. The data indicate that proximal tubular LLC-PK1 renal epithelial cells express amiloride-sensitive, Na(+)-permeable cation channels, which are regulated by the cAMP pathway, and G proteins. This channel activity may implicate previously reported epithelial channel proteins, although this will require further experimentation. The evidence provides new clues as to potentially relevant Na(+) transport mechanisms in the mammalian proximal nephron.

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.

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.

Na+ channels in membrane vesicles from intralobular salivary ducts

Electrical potential-driven 22Na+ fluxes were measured in membrane vesicles prepared from male and female rat submandibular intralobular ducts. A relatively temperature-independent (Q10 = 1.45 +/- 0.15), amiloride-inhibitable (mean affinity constant approximately 1 microM), rheogenic Na+ transport pathway was observed. The relative potency of amiloride analogues for inhibition of this pathway was amiloride > ethylisopropyl-amiloride > phenamil, similar to that of the "low-amiloride-affinity" Na+ channel recently observed in a number of other tissues. These results are consistent with the existence of the apical Na+ channel thought to be involved in intralobular ductal salt reabsorption. No significant difference was found in the magnitude or pharmacology of electrogenic Na+ fluxes in vesicles prepared from male and female rat intralobular ducts, suggesting that the sexual dimorphism observed in this tissue is not reflected at the level of the apical membrane Na+ channel. Amiloride-sensitive 22Na+ fluxes in intralobular ductal membranes were of the same magnitude as 22Na+ fluxes measured in similarly prepared and assayed vesicles from the toad bladder, a tissue thought to be a rich source of amiloride-sensitive Na+ channels.

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.

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

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

Sodium/proton transport by apical membranes of type-II pneumocytes

Recent studies fail to confirm the coexistence of Na+ channels and Na+/H+ exchange at the apical membranes of lower airway epithelia. Availability of plasma membrane vesicles simplifies the investigation of membrane transport processes. Apical and basolateral plasma membrane vesicles of disrupted type-II pneumocytes were fractionated upon nonlinear, continuous sucrose gradients. To investigate sodium transport, 22Na+ uptake by apical membrane vesicles was assayed in the presence and absence of transmembrane sodium diffusion potentials. Interior-negative sodium diffusion potentials promoted 22Na+ uptake 1.5-fold. Internally-directed H+ gradients or NH+4 gradients inhibited 22Na+ uptake 40-50%. Amiloride (1-1000 microM) inhibited uptake 10-79%. To investigate H+ transport, decay of transmembrane pH gradients was monitored with pH probe acridine orange. In the presence or absence of externally-directed H+ gradients, external sodium promoted internal alkalinization, except in the presence of external amiloride. These observations of amiloride-sensitive, electrogenic Na+ uptake and amiloride-sensitive, electroneutral, Na+/H+ coupling indicate coexistence of Na+ channels and Na+/H+ exchange at the apical membrane of type-II pneumocytes.

Thiazide-sensitive Na-Cl cotransport mediates NaCl absorption in amphibian distal tubule

To find out the mechanism(s) underlying NaCl absorption in the distal tubule of Necturus, we devised a variant of the split-drop technique. Following injection an oil column, subsequently split by a NaCl solution isotonic to plasma, a double-barrelled microelectrode (conventional/selective to Na+ or to Cl- ions) recorded Na+ (alpha Na) or Cl- (alpha Cl) activity and transepithelial potential (Vte). Paired control/low-Na+ solutions yielded reabsorptive half-times (t1/2) of 0.68 +/- 0.11 min and 7.6 +/- 1.8 min respectively; corresponding Vte values were -22.2 +/- 4.0 mV and -7.6 +/- 1.9 mV. t1/2 values of control versus low-Cl- solutions were 0.77 +/- 0.32 min and 6.5 +/- 1.7 min respectively, whereas respective Vte values were not different from one another: -23.8 +/- 4.3 mV versus -18.8 +/- 5.5 mV. Nominally K(+)-free solutions or bumetanide, 10 mumol/l, did not alter t1/2 or Vte, with regard to the paired control. Amiloride, 5 mumol/l or 2 mmol/l, failed to decrease t1/2 or to lower Vte; apparently, the role of a Na+/H+ antiport does not contribute significantly to NaCl absorption. Furosemide, 0.1 mmol/l, reduced t1/2 by 54% with regard to the control state. Determination of t1/2 as a function of increasing hydrochlorothiazide concentrations revealed apical high- and low-affinity sites, estimated at 0.56 mumol/l and 0.115 mmol/l respectively. Taken together these observations indicate that NaCl absorption is predominantly carried out by an electroneutral Na(+)-Cl- cotransport.

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

The purpose of this study was to document the existence, assess the spatial localization, and characterize some of the transport properties of proteins antigenically related to epithelial Na+ channels in freshly isolated rabbit and rat alveolar type II (ATII) cells. ATII cells, isolated by elastase digestion of lung tissue and purified by density-gradient centrifugation, were incubated with polyclonal antibodies raised against Na+ channel protein purified from beef kidney papilla (NaAb), followed by a secondary antibody (goat antirabbit immunoglobulin G conjugated to fluorescein isothiocyanate). Rat ATII cells exhibited specific staining with NaAb at the level of the plasma membrane, which, in most cells, colocalized with that of the lectin Maclura pomiferra agglutinin, an apical surface marker. In Western blots, NaAb specifically recognized a 135 +/- 10-kDa protein in rat ATII membrane vesicles. When patch clamped in the whole cell mode using symmetrical solutions (150 mM Na+ glutamate), ATII cells exhibited outwardly rectified Na+ currents that were diminished by amiloride (10-100 microM) instilled into the bath solution. Ion substitution studies showed that the conductive pathways were three times more permeable to Na+ than K+. Amiloride, benzamil, and 5-(N-ethyl-N-isopropyl)-2',4'-amiloride were equally effective in diminishing 22Na+ flux into rabbit and rat ATII cells (45% inhibition at 100 microM, with IC50 of approximately 1 microM for all inhibitors). Tetraethylammonium chloride (10 mM) or BaCl2 (2 mM), well-known K+ channel blockers, had no effect on 22Na+ uptake. These results indicate that ATII cells express an amiloride-sensitive Na+ conductance, probably a channel, with a lower affinity for amiloride and its structural analogues than the well-established amiloride-sensitive Na+ channels found in bovine renal papila and cultured amphibian A6 kidney cells.

Mechanisms and regulation of ion transport in adult mammalian alveolar type II pneumocytes

The adult alveolar epithelium consists of type I and type II (ATII) pneumocytes that form a tight barrier, which severely restricts the entry of lipid-insoluble molecules from the interstitial to the alveolar space. Current in vivo and in vitro evidence indicates that the alveolar epithelium is also an absorptive epithelium, capable of transporting Na+ from the alveolar lumen, which is bathed by a small amount of epithelial lining fluid, to the interstitial space. The in situ localization of Na(+)-K(+)-ATPase activity in ATII cells and the fact that these cells are involved in a number of crucial functions, such as surfactant secretion and alveolar remodeling after injury, led investigators to examine their transport characteristics. Radioactive flux studies, in both freshly isolated and cultured cells, and bioelectric measurements in ATII cells grown on porous supports indicate that they transport Na+ according to the Koefoed-Johnsen and Ussing model of epithelial transport. Na+ enters the apical membrane, because of the favorable electrochemical gradient, through Na+ cotransporters, a Na(+)-H+ antiport, and cation channels and is pumped across the basolateral membrane by a ouabain-sensitive Na(+)-K+ pump. Na+ transport is enhanced by substances that increase intracellular adenosine 3',5'-cyclic monophosphate. In addition to Na+ transporters, ATII cells contain several transporters that regulate their intracellular pH, including a H(+)-ATPase, which may explain the low pH of the epithelial lining fluid. The absorptive properties of ATII cells may play an important role in regulating the degree of alveolar fluid in health and disease.

Epithelial sodium conductance in rabbit preimplantation trophectodermal cells

We examined the development of epithelial Na+ conductance in 6- and 7-day post coitus (p.c.) preimplantation rabbit embryos using the whole-cell patch-clamp technique on dissociated rabbit trophectodermal cells and by immunocytochemical localization using a polyclonal antibody directed against subunits of an apical epithelial Na+ channel on the intact blastocyst. In Day 6 and 7 p.c. trophectodermal cells, we observed an outwardly rectified whole-cell Na+ current. The current-voltage characteristics did not differ between the 6- and the 7-day p.c. cells. Replacement of Na+ with the impermeant cation N-methyl-D-glucamine in the pipette or bath reduced outward currents and inward currents, respectively, indicating that the current was Na(+)-dependent. Treatment of 7-day p.c. cells with 100 microM amiloride, benzamil, or ethylisopropyl amiloride (EIPA) blocked the whole-cell currents within 5 min. However, the current of the Day 6 p.c. embryo was not blocked by amiloride. The amiloride block at Day 7 p.c. was only partially reversible after 15 min of continuous perfusion of the bath with an amiloride-free solution. The apparent dissociation constant (Ki) for amiloride, benzamil, and EIPA was 12, 50, and 16 microM, respectively, when measured 5 min after drug addition. Immunolocalization studies of blastocysts with a polyclonal antibody raised against a high amiloride affinity Na+ channel isolated from bovine kidney revealed no specific binding to the trophectodermal cells at Day 6 p.c.(ABSTRACT TRUNCATED AT 250 WORDS)

Epithelial ion transport in the fetal and perinatal lung

The lung is a complex organ whose intrauterine development depends on many factors, one of which is a continuous secretion of large volumes of Cl(-)-enriched fluid by the pulmonary epithelium. At birth this fluid must be cleared, and it is now known that this process depends in large part on active Na+ transport by the pulmonary epithelium. Only recently has it been possible to culture some of the different lung epithelial cells so that it is possible to investigate the role of individual epithelial cell types, their individual cellular transport mechanisms, and how these are affected by developmental lung maturity.

Effects of divalent cations and pH on amiloride-sensitive Na+ fluxes into luminal membrane vesicles from pars recta of rabbit proximal tubule

The effect of Ca2+, Cd2+, Ba2+, Mg2+ and pH on the renal epithelial Na(+)-channel was investigated by measuring the amiloride-sensitive 22Na+ fluxes into luminal membrane vesicles from pars recta of rabbit proximal tubule. It was found that intravesicular Ca2+ as well as extravesicular Ca2+ substantially lowered the channel-mediated flux. Amiloride sensitive Na+ uptake was nearly completely blocked by 10 microM Ca2+ at pH 7.4. The inhibitory effect of Ca2+ was dependent on pH. Thus, 10 microM Ca2+ produced 90% inhibition of 22Na+ uptake at pH 7.4, and only 40% inhibition at pH 7.0. The tracer fluxes measured in the absence of Ca2+ were pH independent over the range from 7.0 to 7.4. All the cations Ca2+, Cd2+, Ba2+ except Mg2+ inhibited the 22Na+ influx drastically when added extravesicularly in millimolar concentrations. The cations Cd2+, Ba2+ and Mg2+ in the same concentrations intravesicularly inhibited the 22Na+ influx only slightly. A millimolar concentration of Ca2+ intravesicularly blocked the amiloride-sensitive 22Na+ flux completely. The data indicate that Ca2+ inhibits Na+ influx specifically by binding to sites composed of one or several deprotonated groups on the channel proteins.

Increased Na(+)-H+ antiporter activity in apical membrane vesicles from mutant LLC-PK1 cells

In whole cell experiments, the PKE20 mutant of the renal epithelial cell line LLC-PK1 displays a severalfold elevation of Na(+)-H+ antiporter activity at the apical surface (J.G. Haggerty, N. Agarwal, R.F. Reilly, E. A. Adelberg, and C.W. Slayman. Proc. Natl. Acad. Sci. USA 85: 6797-6801, 1988). The present study was undertaken to explore the properties of the mutant at the membrane level. Apical membrane vesicles were prepared by the magnesium-aggregation technique, with a similar enrichment (ca. 10-fold) of the marker enzyme gamma-glutamyltranspeptidase in vesicles from parent and mutant cell lines. In both cases, 22Na influx was stimulated by an inside-acid pH gradient, inhibited by ethylisopropylamiloride (EIPA), and unaffected by valinomycin, indicating that it was mediated by Na(+)-H+ antiport. Quantitatively, PKE20 vesicles showed a 4.2-fold increase in the maximal velocity of Na(+)-H+ antiporter activity compared with the parent, with only minor increases in the activity of two other Na(+)-dependent transporters (14-56% for alpha-methylglucoside and L-glutamate). Dose-response curves for EIPA indicated that the increased Na(+)-H+ antiport activity in PKE20 vesicles was due to an increased activity of the relatively amiloride-resistant form of the Na(+)-H+ antiporter with little or no change in the amiloride-sensitive form. No differences in polypeptide composition of the two vesicle preparations could be detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Taken together, the results indicate that the mutation in PKE20 is expressed at the membrane level and is specific for the relatively amiloride-resistant Na(+)-H+ antiporter.(ABSTRACT TRUNCATED AT 250 WORDS)