Characterization and localization of epithelial Na+ channels in toad urinary bladder

Vasotocin- and mesotocin-induced increases in short-circuit current across tree frog skin

In adult amphibian skin, Na(+) crosses from outside to inside. This Na(+) transport can be measured as the amiloride-blockable short-circuit current (SCC) across the skin. We investigated the effects of arginine vasotocin (AVT) and mesotocin (MT), and those of antagonists of the vasopressin and oxytocin receptors, on the SCC across Hyla japonica skin. (1) Both AVT (100 pmol/L or more) and MT (1 nmol/L or more) increased the SCC. (2) The AVT- and MT-induced increases in SCC recovered with time (downregulation). (3) These AVT/MT-induced effects were blocked by application of OPC-31260 (vasopressin V(2)-receptor antagonist). (4) The OPC-31260 concentration needed to block the AVT-induced response was lower upon post-application (after application of agonist) than upon pre-application (before application of agonist), suggesting the number of receptors may have decreased after AVT application. (5) Upon repeated application of AVT (100 pmol/L), the induced SCC increase did not differ significantly between the 1st and 2nd applications. (6) The time to reach the half-maximum value of the AVT-induced or MT-induced increase in SCC was not significantly different between washout and post-application of OPC-31260, suggesting that post-application of OPC-31260 cleared AVT and MT from their receptors. The effects of AVT, MT, and their antagonists in H. japonica, which is adapted to a terrestrial habitat, are compared with our previously published data on Rana catesbeiana (=Lithobates catesbeianus), which is adapted to a semiaquatic habitat.

Regulated sodium transport in the renal connecting tubule (CNT) via the epithelial sodium channel (ENaC)

The aldosterone-sensitive distal nephron (ASDN) includes the late distal convoluted tubule 2, the connecting tubule (CNT) and the collecting duct. The appropriate regulation of sodium (Na(+)) absorption in the ASDN is essential to precisely match urinary Na(+) excretion to dietary Na(+) intake whilst taking extra-renal Na(+) losses into account. There is increasing evidence that Na(+) transport in the CNT is of particular importance for the maintenance of body Na(+) balance and for the long-term control of extra-cellular fluid volume and arterial blood pressure. Na(+) transport in the CNT critically depends on the activity and abundance of the amiloride-sensitive epithelial sodium channel (ENaC) in the luminal membrane of the CNT cells. As a rate-limiting step for transepithelial Na(+) transport, ENaC is the main target of hormones (e.g. aldosterone, angiotensin II, vasopressin and insulin/insulin-like growth factor 1) to adjust transepithelial Na(+) transport in this tubular segment. In this review, we highlight the structural and functional properties of the CNT that contribute to the high Na(+) transport capacity of this segment. Moreover, we discuss some aspects of the complex pathways and molecular mechanisms involved in ENaC regulation by hormones, kinases, proteases and associated proteins that control its function. Whilst cultured cells and heterologous expression systems have greatly advanced our knowledge about some of these regulatory mechanisms, future studies will have to determine the relative importance of the various pathways in the native tubule and in particular in the CNT.

Regulation of epithelial Na+ transport by soluble adenylyl cyclase in kidney collecting duct cells

Alkalosis impairs the natriuretic response to diuretics, but the underlying mechanisms are unclear. The soluble adenylyl cyclase (sAC) is a chemosensor that mediates bicarbonate-dependent elevation of cAMP in intracellular microdomains. We hypothesized that sAC may be an important regulator of Na(+) transport in the kidney. Confocal images of rat kidney revealed specific immunolocalization of sAC in collecting duct cells, and immunoblots confirmed sAC expression in mouse cortical collecting duct (mpkCCD(c14)) cells. These cells exhibit aldosterone-stimulated transepithelial Na(+) currents that depend on both the apical epithelial Na(+) channel (ENaC) and basolateral Na(+),K(+)-ATPase. RNA interference-mediated 60-70% knockdown of sAC expression comparably inhibited basal transepithelial short circuit currents (I(sc)) in mpkCCD(c14) cells. Moreover, the sAC inhibitors KH7 and 2-hydroxyestradiol reduced I(sc) in these cells by 50-60% within 30 min. 8-Bromoadenosine-3',5'-cyclic-monophosphate substantially rescued the KH7 inhibition of transepithelial Na(+) current. Aldosterone doubled ENaC-dependent I(sc) over 4 h, an effect that was abolished in the presence of KH7. The sAC contribution to I(sc) was unaffected with apical membrane nystatin-mediated permeabilization, whereas the sAC-dependent Na(+) current was fully inhibited by basolateral ouabain treatment, suggesting that the Na(+),K(+)-ATPase, rather than ENaC, is the relevant transporter target of sAC. Indeed, neither overexpression of sAC nor treatment with KH7 modulated ENaC currents in Xenopus oocytes. ATPase and biotinylation assays in mpkCCD(c14) cells demonstrated that sAC inhibition decreases catalytic activity rather than surface expression of the Na(+),K(+)-ATPase. In summary, these results suggest that sAC regulates both basal and agonist-stimulated Na(+) reabsorption in the kidney collecting duct, acting to enhance Na(+),K(+)-ATPase activity.

Vasotocin has the potential to inhibit basolateral Na(+)/K (+)-pump current across isolated skin of tree frog in vitro, via its V(2)-type receptor/cAMP pathway

Adult frog skin transports Na(+) from the apical to the basolateral side across the skin. Antidiuretic hormone (ADH) is involved in the regulation of Na(+) transport in both mammals and amphibians. We investigated the effect of arginine vasotocin (AVT), the ADH of amphibians, on the short-circuit current (SCC) across intact skin and on the basolateral Na(+)/K(+)-pump current across apically nystatin-permeabilized skin of the tree frog, Hyla japonica, in which the V(2)-type ADH receptor is expressed in vitro. In intact skin, 1 pM AVT had no effect on the SCC, but 10 nM AVT was sufficient to stimulate the SCC since 10 nM and 1 microM of AVT increased the SCC 3.2- and 3.4-fold, respectively (P > 0.9). However, in permeabilized skin, AVT (1 microM) decreased the Na(+)/K(+)-pump current to 0.79 times vehicle control. Similarly, 500 microM of 8Br-cAMP increased the SCC 3.2-fold, yet 1 mM of 8Br-cAMP decreased the Na(+)/K(+)-pump current to 0.76 times vehicle control. Arachidonic acid (10(-5) M) tended to decrease the Na(+)/K(+)-pump current. To judge from these in vitro experiments, AVT has the potential to inhibit the basolateral Na(+)/K(+)-pump current via the V(2)-type receptor/cAMP pathway in the skin of the tree frog.

Phosphatidylinositol 4-phosphate 5-kinase reduces cell surface expression of the epithelial sodium channel (ENaC) in cultured collecting duct cells

Ubiquitination of ENaC subunits has been shown to negatively regulate the cell surface expression of ENaC channels. We have previously demonstrated that epsin links ubiquitinated ENaC to clathrin adaptors for clathrin-mediated endocytosis. Epsin is thought to directly modify the curvature of membranes upon binding to phosphatidylinositol 4,5-bisphosphate (PIP2) where it recruits clathrin and stimulates lattice assembly. Murine phosphatidylinositol 4-phosphate 5-kinase alpha (PI5KIalpha) has been shown to enhance endocytosis in a PIP2-dependent manner. We tested the hypothesis that PI5KIalpha-mediated PIP2 production would negatively regulate ENaC current by enhancing epsin-mediated endocytosis of the channel. Expression of PI5KIalpha decreased ENaC currents in Xenopus oocytes by 80%, entirely because of a decrease in cell surface ENaC levels. Catalytically inactive mutants of PI5Kalpha had no effect on ENaC activity. Expression of the PIP2 binding region of epsin increased ENaC current in oocytes, an effect completely reversed by co-expression of PI5KIalpha. Overexpression of epsin reduced amiloride-sensitive current in CCD cells. Overexpression of PI5KIalpha enhanced membrane PIP2 levels and reduced apical surface expression of ENaC in CCD cells, down-regulating amiloride-sensitive current. Knockdown of PI5KIalpha with isoform-specific siRNA resulted in a 4-fold enhancement of ENaC activity. PI5KIalpha localized exclusively to the apical plasma membrane domain when overexpressed in mouse CCD cells, consistent for a role in regulating PIP2 production at the apical plasma membrane. We conclude that membrane turnover events regulating ENaC surface expression and activity in oocytes and CCD cells can be regulated by PI5KIalpha.

Immunolocalization and mRNA expression of the epithelial Na+ channel alpha-subunit in the kidney and urinary bladder of the marine toad, Bufo marinus, under hyperosmotic conditions

The amiloride-sensitive epithelial sodium channel (ENaC) has previously been shown to be involved in the maintenance of body fluid volume and in Na(+) absorption across the skin and urinary bladder in amphibians. However, the function and distribution of ENaC have not been clearly described in amphibian kidney. We therefore cloned the ENaC alpha-subunit cDNA from kidney of the marine toad, Bufo marinus. The ENaC mRNA and protein were abundantly expressed in the kidney and in the urinary bladder and ventral pelvic skin. In an immunohistochemical study, the ENaC alpha-subunit protein was specifically localized to the apical membrane of the principal cells but not the intercalated cells from the late distal tubule to the collecting duct in the kidney or in the apical area of cells of urinary bladder epithelia. When toads were acclimated to dry and hyper-saline environments, the levels of ENaC mRNA expression in the kidney and urinary bladder decreased under hyper-saline acclimation, but not under dry conditions. Immunohistochemical observations indicated that the levels of ENaC protein expression were much lower in the apical area of renal distal tubules and urinary bladder epithelia of hyper-saline acclimated toad compared with controls. The present study suggests that Bufo ENaC is significantly expressed and functions during Na(+) reabsorption in the apical membrane domain in the distal nephron of normal and desiccated toads. Natriuresis may be caused by decreases in ENaC expression and its trafficking to the cell surface in the distal nephron, a response to prevent excessive Na(+) reabsorption in hyper-saline-acclimated toads.

Frog atrial natriuretic peptide and cGMP activate amiloride-sensitive Na(+) channels in urinary bladder cells of Japanese tree frog, Hyla japonica

In our previous study, it was suggested that ANP and cGMP may increase Na(+) absorption in the urinary bladder of the Japanese tree frog, Hyla japonica. Thus, Na(+) transport activated by ANP was investigated electrophysiologically by using a cell-attached patch-clamp technique in freshly isolated cells from the urinary bladder. A predominant channel expressed was a low conductance Na(+) channel in the epithelial cells. The channel exhibited conductance for inward currents of 4.9 +/- 0.2 pS, long open and closed times (c.a. 190 ms), and positive reversal potential. The channel activity was decreased under the pipette solution including 10(-6) M amiloride. These characteristics were similar to those of amiloride-sensitive Na(+) channels (ENaC). Addition of 10(-9) M ANP activated and significantly increased the ENaC activity from 0.58 +/- 0.09 to 1.47 +/- 0.34. On the other hand, mean amplitudes and conductance of single channel did not change significantly after the addition of ANP. Addition of 10(-5) M 8-Br-cGMP also activated the ENaC and significantly increased the channel activity from 0.56 +/- 0.10 to 2.00 +/- 0.33. The addition of ANP failed to activate the ENaC in the presence of 10(-6) M amiloride. These results suggested that ANP and cGMP activate Na(+) transport via ENaC in the epithelial cells of frog urinary bladder.

Transport of sodium and chloride across earthworm skin in vitro

We present a new invertebrate model for the study of epithelial sodium transport in tight epithelia, the earthworm integument. Dissected segments of earthworm integument were mounted in modified Ussing chambers and perfused with either pond water (PW) or earthworm ringer solution (ERS) on the apical side. In order to investigate ion transport under near-in vivo physiological conditions, measurements were performed under current-clamp conditions by monitoring the transepithelial potential (V (T)), as well as the transepithelial resistance (R (T)). These were recorded continuously and the virtual short circuit current (I (SC)) was calculated. The integument has a high transepithelial resistance (R (T) = 9,037 +/- 502 Omega cm(2) for PW, n = 24, and 11,055 +/- 1,320 Omega cm(2) for ERS, n = 32). V (T) was -3.7 +/- 2.2 mV for PW (n = 24) and -1.5 +/- 1.0 mV for ERS (n = 32), and I (SC) was -0.57 +/- 0.30 microA/cm(2) for PW (n = 24) and -0.44 +/- 0.24 microA/cm(2) for ERS (n = 32). Only under PW, but not under ERS conditions, was there a pronounced inhibition of I (SC) by low doses of amiloride or its analogues phenamil and benzamil. The resistance of the paracellular pathway was found to be very high. The terrestrial oligochaete Lumbricus seems especially adapted to the environmental conditions because it has an ultra-tight integument and a very fast up- and down-regulation of apical Na(+) channels.

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.

Epithelial Sodium Channel Inhibition by AMP-activated Protein Kinase in Oocytes and Polarized Renal Epithelial Cells

The epithelial Na(+) channel (ENaC) regulates epithelial salt and water reabsorption, processes that require significant expenditure of cellular energy. To test whether the ubiquitous metabolic sensor AMP-activated kinase (AMPK) regulates ENaC, we examined the effects of AMPK activation on amiloride-sensitive currents in Xenopus oocytes and polarized mouse collecting duct mpkCCD(c14) cells. Microinjection of oocytes expressing mouse ENaC (mENaC) with either active AMPK protein or an AMPK activator inhibited mENaC currents relative to controls as measured by two-electrode voltage-clamp studies. Similarly, pharmacological AMPK activation or overexpression of an activating AMPK mutant in mpkCCD(c14) cells inhibited amiloride-sensitive short circuit currents. Expression of a degenerin mutant beta-mENaC subunit (S518K) along with wild type alpha and gamma increased the channel open probability (P(o)) to approximately 1. However, AMPK activation inhibited currents similarly with expression of either degenerin mutant or wild type mENaC. Single channel recordings under these conditions demonstrated that neither P(o) nor channel conductance was affected by AMPK activation. Moreover, expression of a Liddle's syndrome-type beta-mENaC mutant (Y618A) greatly enhanced ENaC whole cell currents relative to wild type ENaC controls and prevented AMPK-dependent inhibition. These findings indicate that AMPK-dependent ENaC inhibition is mediated through a decrease in the number of active channels at the plasma membrane (N), presumably through enhanced Nedd4-2-dependent ENaC endocytosis. The AMPK-ENaC interaction appears to be indirect; AMPK did not bind ENaC in cells, as assessed by in vivo pull-down assays, nor did it phosphorylate ENaC in vitro. In summary, these results suggest a novel mechanism for coupling ENaC activity and renal Na(+) handling to cellular metabolic status through AMPK, which may help prevent cellular Na(+) loading under hypoxic or ischemic conditions.

Reciprocal protein kinase A regulatory interactions between cystic fibrosis transmembrane conductance regulator and Na+/H+ exchanger isoform 3 in a renal polarized epithelial cell model

Although Cystic fibrosis transmembrane conductance regulator (CFTR) has been shown to regulate the activity of NHE3, the potential reciprocal interaction of NHE3 to modulate the protein kinase A (PKA)-dependent regulation of CFTR in epithelial cells is still unknown. In the present work, we describe experiments to define the interactions between CFTR and NHE3 with the regulatory, scaffolding protein, NHERF that organize their PKA-dependent regulation in a renal epithelial cell line that expresses endogenous CFTR. The expression of rat NHE3 significantly decreased PKA-dependent activation of CFTR without altering CFTR expression, and this decrease was prevented by mutation of either of the two rat NHE3 PKA target serines to alanine (S552A or S605A). Inhibition of CFTR expression by antisense treatment resulted in an acute decrease in PKA-dependent regulation of NHE3 activity. CFTR, NHE3, and ezrin were recognized by NHERF-2 but not NHERF-1 in glutathione S-transferase pull-down experiments. Ezrin may function as a protein kinase A anchoring protein (AKAP) in this signaling complex, because blocking the binding of PKA to an AKAP by incubation with the S-Ht31 peptide inhibited the PKA-dependent regulation of CFTR in the absence of NHE3. In the A6-NHE3 cells S-Ht31 blocked the PKA regulation of NHE3 whereas it now failed to affect the regulation of CFTR. We conclude that CFTR and NHE3 reciprocally interact via a shared regulatory complex comprised of NHERF-2, ezrin, and PKA.

Genistein restores functional interactions between Delta F508-CFTR and ENaC in Xenopus oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR), in addition to its Cl(-) channel properties, has regulatory interactions with other epithelial ion channels including the epithelial Na(+) channel (ENaC). Both the open probability and surface expression of wild type CFTR Cl(-) channels are increased significantly when CFTR is co-expressed in Xenopus oocytes with alphabetagamma-ENaC, and conversely, the activity of ENaC is inhibited following wild type CFTR activation. Using the Xenopus oocyte expression system, a lack of functional regulatory interactions between DeltaF508-CFTR and ENaC was observed following activation of DeltaF508-CFTR by forskolin and isobutylmethylxanthine (IBMX). Whole cell currents in oocytes expressing ENaC alone decreased in response to genistein but increased in response to a combination of forskolin and IBMX followed by genistein. In contrast, ENaC currents in oocytes co-expressing ENaC and DeltaF508-CFTR remained stable following stimulation with forskolin/IBMX/genistein. Furthermore, co-expression of DeltaF508-CFTR with ENaC enhanced the forskolin/IBMX/genistein-mediated activation of DeltaF508-CFTR. Our data suggest that genistein restores regulatory interactions between DeltaF508-CFTR and ENaC and that combinations of protein repair agents, such as 4-phenylbutyrate and genistein, may be necessary to restore DeltaF508-CFTR function in vivo.

Cyclic AMP increases cell surface expression of functional Na,K-ATPase units in mammalian cortical collecting duct principal cells

Cyclic AMP (cAMP) stimulates the transport of Na(+) and Na,K-ATPase activity in the renal cortical collecting duct (CCD). The aim of this study was to investigate the mechanism whereby cAMP stimulates the Na,K-ATPase activity in microdissected rat CCDs and cultured mouse mpkCCD(c14) collecting duct cells. db-cAMP (10(-3) M) stimulated by 2-fold the activity of Na,K-ATPase from rat CCDs as well as the ouabain-sensitive component of (86)Rb(+) uptake by rat CCDs (1.7-fold) and cultured mouse CCD cells (1.5-fold). Pretreatment of rat CCDs with saponin increased the total Na,K-ATPase activity without further stimulation by db-cAMP. Western blotting performed after a biotinylation procedure revealed that db-cAMP increased the amount of Na,K-ATPase at the cell surface in both intact rat CCDs (1.7-fold) and cultured cells (1.3-fold), and that this increase was not related to changes in Na,K-ATPase internalization. Brefeldin A and low temperature (20 degrees C) prevented both the db-cAMP-dependent increase in cell surface expression and activity of Na,K-ATPase in both intact rat CCDs and cultured cells. Pretreatment with the intracellular Ca(2+) chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid also blunted the increment in cell surface expression and activity of Na,K-ATPase caused by db-cAMP. In conclusion, these results strongly suggest that the cAMP-dependent stimulation of Na,K-ATPase activity in CCD results from the translocation of active pump units from an intracellular compartment to the plasma membrane.

Non-coordinate regulation of endogenous epithelial sodium channel (ENaC) subunit expression at the apical membrane of A6 cells in response to various transporting conditions

In many epithelial tissues in the body (e.g. kidney distal nephron, colon, airways) the rate of Na(+) reabsorption is governed by the activity of the epithelial Na(+) channel (ENaC). ENaC activity in turn is regulated by a number of factors including hormones, physiological conditions, and other ion channels. To begin to understand the mechanisms by which ENaC is regulated, we have examined the trafficking and turnover of ENaC subunits in A6 cells, a polarized, hormonally responsive Xenopus kidney cell line. As previously observed by others, the half-life of newly synthesized ENaC subunits was universally short ( approximately 2 h). However, the half-lives of alpha- and gamma-ENaC subunits that reached the apical cell surface were considerably longer (t(12) > 24 h), whereas intriguingly, the half-life of cell surface beta-ENaC was only approximately 6 h. We then examined the effects of various modulators of sodium transport on cell surface levels of individual ENaC subunits. Up-regulation of ENaC-mediated sodium conductance by overnight treatment with aldosterone or by short term incubation with vasopressin dramatically increased cell surface levels of beta-ENaC without affecting alpha- or gamma-ENaC levels. Conversely, treatment with brefeldin A selectively decreased the amount of beta-ENaC at the apical membrane. Short term treatment with aldosterone or insulin had no effect on cell surface amounts of any subunits. Subcellular fractionation revealed a selective loss of beta-ENaC from early endosomal pools in response to vasopressin. Our data suggest the possibility that trafficking and turnover of individual ENaC subunits at the apical membrane of A6 cells is non-coordinately regulated. The selective trafficking of beta-ENaC may provide a mechanism for regulating sodium conductance in response to physiological stimuli.

Methylation increases the open probability of the epithelial sodium channel in A6 epithelia

We used single channel methods on A6 renal cells to study the regulation by methylation reactions of epithelial sodium channels. 3-Deazaadenosine (3-DZA), a methyltransferase blocker, produced a 5-fold decrease in sodium transport and a 6-fold decrease in apical sodium channel activity by decreasing channel open probability (P(o)). 3-Deazaadenosine also blocked the increase in channel open probability associated with addition of aldosterone. Sodium channel activity in excised "inside-out" patches usually decreased within 1-2 min; in the presence of S-adenosyl-l-methionine (AdoMet), activity persisted for 5-8 min. Sodium channel mean time open (t(open)) before and after patch excision was higher in the presence of AdoMet than in untreated excised patches but less than t(open) in cell-attached patches. Sodium channel activity in excised patches exposed to both AdoMet and GTP usually remained stable for more than 10 min, and P(o) and the number of active channels per patch were close to values in cell-attached patches from untreated cells. These findings suggest that a methylation reaction contributes to the activity of epithelial sodium channels in A6 cells and is directed to some regulatory element closely connected with the channel, whose activity also depends on the presence of intracellular GTP.

Epithelial sodium channels regulate cystic fibrosis transmembrane conductance regulator chloride channels in Xenopus oocytes

The cystic fibrosis transmembrane conductance regulator (CFTR), in addition to its well defined Cl(-) channel properties, regulates other ion channels. CFTR inhibits epithelial Na(+) channel (ENaC) currents in many epithelial and nonepithelial cells. Because modulation of net NaCl reabsorption has important implications in extracellular fluid volume homeostasis and airway fluid volume and composition, we investigated whether this regulation was reciprocal by examining whether ENaC regulates CFTR. Co-expression of human (h) CFTR and mouse (m) alphabetagammaENaC in Xenopus oocytes resulted in a significant, 3.7-fold increase in whole-cell hCFTR Cl(-) conductance compared with oocytes expressing hCFTR alone. The forskolin/3-isobutyl-1-methylxanthine-stimulated whole-cell conductance in hCFTR-mENaC co-injected oocytes was amiloride-insensitive, indicating an inhibition of mENaC following hCFTR activation, and it was blocked by DPC (diphenylamine-2-carboxylic acid) and was DIDS (4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid)-insensitive. Enhanced hCFTR Cl(-) conductance was also observed when either the alpha- or beta-subunit of mENaC was co-expressed with hCFTR, but this was not seen when CFTR was co-expressed with the gamma-subunit of mENaC. Single Cl(-) channel analyses showed that both CFTR Cl(-) channel open probability and the number of CFTR Cl(-) channels detected per patch increased when hCFTR was co-expressed with alphabetagammamENaC. We conclude that in addition to acting as a regulator of ENaC, CFTR activity is regulated by ENaC.

Liddle's syndrome mutations disrupt cAMP-mediated translocation of the epithelial Na(+) channel to the cell surface

The epithelial Na(+) channel (ENaC) plays a critical role in Na(+) absorption, and mutations in this channel cause diseases of Na(+) homeostasis, including a genetic form of hypertension (Liddle's syndrome). To investigate cAMP-mediated stimulation of ENaC, alpha, beta, and gammaENaC were coexpressed in Fischer rat thyroid epithelia to generate apical Na(+) channels and transepithelial Na(+) current. cAMP agonists stimulated Na(+) current by 70%. Following covalent modification of cysteines introduced into ENaC, cAMP increased the rate of appearance of unmodified channels at the cell surface. In addition, cAMP increased the fluorescent labeling of ENaC at the apical cell surface. Inhibition of vesicle trafficking by incubating epithelia at 15 degrees C prevented the cAMP-mediated stimulation of ENaC. These results suggest that cAMP stimulates Na(+) absorption in part by increasing translocation of ENaC to the cell surface. Stimulation of ENaC by cAMP was dependent on a sequence (PPPXY) in the COOH terminus of each subunit. This sequence is the target for mutations that cause Liddle's syndrome, suggesting that cAMP-mediated translocation of ENaC to the cell surface is defective in this genetic form of hypertension.

Association of the epithelial sodium channel with Apx and alpha-spectrin in A6 renal epithelial cells

Recent molecular cloning of the epithelial sodium channel (ENaC) provides the opportunity to identify ENaC-associated proteins that function in regulating its cell surface expression and activity. We have examined whether ENaC is associated with Apx (apical protein Xenopus) and the spectrin-based membrane cytoskeleton in Xenopus A6 renal epithelial cells. We have also addressed whether Apx is required for the expression of amiloride-sensitive Na(+) currents by cloned ENaC. Sucrose density gradient centrifugation of A6 cell detergent extracts showed co-sedimentation of xENaC, alpha-spectrin, and Apx. Immunoblot analysis of proteins co-immunoprecipitating under high stringency conditions from peak Xenopus ENaC/Apx-containing gradient fractions indicate that ENaC, Apx, and alpha-spectrin are associated in a macromolecular complex. To examine whether Apx is required for the functional expression of ENaC, alphabetagamma mENaC cRNAs were coinjected into Xenopus oocytes with Apx sense or antisense oligodeoxynucleotides. The two-electrode voltage clamp technique showed there was a marked reduction in amiloride-sensitive current in oocytes coinjected with antisense oligonucleotides when to compared with oocytes coinjected with sense oligonucleotides. These studies indicate that ENaC is associated in a macromolecular complex with Apx and alpha-spectrin in A6 cells and suggest that Apx is required for the functional expression of ENaC in Xenopus epithelia.

Structure and hormone responsiveness of the gene encoding the alpha-subunit of the rat amiloride-sensitive epithelial sodium channel

The rat amiloride-sensitive epithelial sodium channel (rENaC) is the rate-limiting step for vectorial transport of Na+ across tight epithelia. The complex is composed of three subunits, alpha, beta, and gamma. Expression of the subunits has been shown to be tissue-specific and developmentally and hormonally regulated. To study mechanisms involved in transcriptional regulation of alpharENaC, we determined the genomic organization of the alpharENaC gene. By 5' rapid amplification of cDNA ends and primer extension, two transcriptional start sites were detected 453 base pairs (bp) apart, resulting in alternative 5' untranslated region (UTR) lengths of 515 or 62 bp. The longer 5' UTR is more prevalent in fetal lung than in adult lung or kidney. The 5' untranslated and coding regions are contained within 12 exons, with the translation start site located within the first exon. Sequence analysis of approximately 1,500 bp of 5' flanking DNA identified putative binding sites for transcription factors PEA3, SP1, AP-1, nuclear factor-kappaB, and thyroid and glucocorticoid receptors. alpharENaC promoter-reporter gene constructs produced low levels of reporter gene activity in transiently transfected cells, which could be increased by dexamethasone (DEX) treatment. Tri-iodothyronine treatment alone had no effect but potentiated stimulation by DEX.

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.

Expression of the cystic fibrosis phenotype in a renal amphibian epithelial cell line

Mutations in a Cl- channel (cystic fibrosis transmembrane conductance regulator or CFTR) are responsible for the cystic fibrosis (CF) phenotype. Increased Na+ transport rates are observed in CF airway epithelium, and recent studies suggest that this is due to an increase in Na+ channel open probability (Po). The Xenopus renal epithelial cell line, A6, expresses both cAMP-activated 8-picosiemen (pS) Cl- channels and amiloride-sensitive 4-pS Na+ channels, and provides a model system for examining the interactions of CFTR and epithelial Na+ channels. A6 cells express CFTR mRNA, as demonstrated by reverse transcriptase-polymerase chain reaction and partial sequence analysis. A phosphorothioate antisense oligonucleotide, complementary to the 5' end of the open reading frame of Xenopus CFTR, was used to inhibit functional expression of CFTR in A6 cells. Parallel studies utilized the corresponding sense oligonucleotide as a control. CFTR protein expression was markedly reduced in cells incubated with the antisense oligonucleotide. Incubation of A6 cells with the antisense oligonucleotide led to inhibition of forskolin-activated amiloride-insensitive short circuit current (Isc). After a 30-min exposure to 10 microM forskolin, 8-pS Cl- channel activity was detected in only 1 of 31 (3%) cell-attached patches on cells treated with antisense oligonucleotide, compared to 5 of 19 (26%) patches from control cells. A shift in the single-channel current-voltage relationship derived from antisense-treated cells was also consistent with a reduction in Cl- reabsorption. Both amiloride-sensitive Isc and Na+ channel Po were significantly increased in antisense-treated, forskolin-stimulated A6 cells, when compared with forskolin-stimulated controls. These data suggest that the regulation of Na+ channels by CFTR is not limited to respiratory epithelia and to epithelial cells in culture overexpressing CFTR and epithelial Na+ channels.