Antidiuretic hormone action in A6 cells: effect on apical Cl and Na conductances and synergism with aldosterone for NaCl reabsorption

Two adjacent phosphorylation sites in the C-terminus of the channel's α-subunit have opposing effects on epithelial sodium channel (ENaC) activity

How phosphorylation of the epithelial sodium channel (ENaC) contributes to its regulation is incompletely understood. Previously, we demonstrated that in outside-out patches ENaC activation by serum- and glucocorticoid-inducible kinase isoform 1 (SGK1) was abolished by mutating a serine residue in a putative SGK1 consensus motif RXRXX(S/T) in the channel’s α-subunit (S621 in rat). Interestingly, this serine residue is followed by a highly conserved proline residue rather than by a hydrophobic amino acid thought to be required for a functional SGK1 consensus motif according to invitro data. This suggests that this serine residue is a potential phosphorylation site for the dual-specificity tyrosine phosphorylated and regulated kinase 2 (DYRK2), a prototypical proline-directed kinase. Its phosphorylation may prime a highly conserved preceding serine residue (S617 in rat) to be phosphorylated by glycogen synthase kinase 3 β (GSK3β). Therefore, we investigated the effect of DYRK2 on ENaC activity in outside-out patches of Xenopus laevis oocytes heterologously expressing rat ENaC. DYRK2 included in the pipette solution significantly increased ENaC activity. In contrast, GSK3β had an inhibitory effect. Replacing S621 in αENaC with alanine (S621A) abolished the effects of both kinases. A S617A mutation reduced the inhibitory effect of GKS3β but did not prevent ENaC activation by DYRK2. Our findings suggest that phosphorylation of S621 activates ENaC and primes S617 for subsequent phosphorylation by GSK3β resulting in channel inhibition. In proof-of-concept experiments, we demonstrated that DYRK2 can also stimulate ENaC currents in microdissected mouse distal nephron, whereas GSK3β inhibits the currents.

Cadmium-induced Inhibition of ADH-stimulated Ion Transport in Cultured Kidney-derived Epithelial Cells (A6)

An epithelial cell line (A6) derived from the distal tubule of toad kidney, was used to study the effect of cadmium (Cd2+) on the increase in active ion transport induced by antidiuretic hormone (ADH). Addition of Cd2+ (1mM) to the basolateral solution of A6 epithelia generated an immediate and transient increase in active ion transport, measured as short circuit current (SCC). This increase was not affected by prior addition of ADH. However, there was a distinct inhibition of ADH-induced stimulation of SCC in epithelia pre-treated with Cd2+. Since cAMP serves as an intracellular messenger for ADH by increasing the ion permeability of the apical membrane in A6 epithelial cells, the effects of Cd2+ on enzymes involved in cAMP metabolism were measured. The results showed that Cd2+ markedly inhibits cAMP production by inhibiting adenylate cyclase (which had been stimulated with forskolin, magnesium or a non-hydrolysed GTP-analog), indicating that Cd2+ inhibits the catalytic subunit of adenylate cyclase. Furthermore, degradation of cAMP by phosphodiesterase was not stimulated by Cd2+, also suggesting that the mechanism by which Cd2+ inhibits the ADH-induced ion transport could be through inhibition of adenylate cyclase. Taken together, these results indicate that, in addition to the well-known toxic effect on the proximal tubule, Cd2+ could also have an effect on the distal part of the kidney, where the important hormonal regulation of salt and water homeostasis takes place.

Epithelial sodium channel abundance is decreased by an unfolded protein response induced by hyperosmolality

Large shifts of osmolality occur in the kidney medulla as part of the urine concentrating mechanism. Hyperosmotic stress profoundly challenges cellular homeostasis and induces endoplasmic reticulum (ER) stress. Here, we examined the unfolded protein response (UPR) in hyperosmotically-challenged principal cells of the kidney collecting duct (CD) and show its relevance in controlling epithelial sodium channel (ENaC) abundance, responsible for the final adjustment of Na(+) excretion. Dehydration increases medullary but not cortical osmolality. Q-PCR analysis of microdissected CD of water-deprived mice revealed increased aquaporin-2 (AQP2) expression in outer medullary and cortical CD while ENaC abundance decreased in outer medullary but not cortical CD. Immunoblotting, Q-PCR and immunofluorescence revealed that hyperosmolality induced a transient ER stress-like response both ex vivo and in cultured CD principal cells and increased activity of the canonical UPR mediators PERK and ATF6. Both hyperosmolality and chemical induction of ER stress decreased ENaC expression in vitro. ENaC depletion by either stimulus was abolished by transcriptional inhibition and by the chemical chaperone 4-phenylbutyric acid and was partly abrogated by either PERK or ATF6 silencing. Our data suggest that induction of the UPR by hyperosmolality may help preserve body fluid homeostasis under conditions of dehydration by uncoupling AQP2 and ENaC abundance in outer medullary CD.

Vasopressin V2 receptors, ENaC, and sodium reabsorption: a risk factor for hypertension?

Excessive sodium reabsorption by the kidney has long been known to participate in the pathogenesis of some forms of hypertension. In the kidney, the final control of NaCl reabsorption takes place in the distal nephron through the amiloride-sensitive epithelial sodium channel (ENaC). Liddle's syndrome, an inherited form of hypertension due to gain-of-function mutations in the genes coding for ENaC subunits, has demonstrated the key role of this channel in the sodium balance. Although aldosterone is classically thought to be the main hormone regulating ENaC activity, several studies in animal models and in humans highlight the important effect of vasopressin on ENaC regulation and sodium transport. This review summarizes the effect of vasopressin V2 receptor stimulation on ENaC activity and sodium excretion in vivo. Moreover, we report the experimental and clinical data demonstrating the role of renal ENaC in water conservation at the expense of a reduced ability to excrete sodium. Acute administration of the selective V2 receptor agonist dDAVP not only increases urine osmolality and reduces urine flow rate but also reduces sodium excretion in rats and humans. Chronic V2 receptor stimulation increases blood pressure in rats, and a significant correlation was found between blood pressure and urine concentration in healthy humans. This led us to discuss how excessive vasopressin-dependent ENaC stimulation could be a risk factor for sodium retention and resulting increase in blood pressure.

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.

Endogenous ATP release inhibits electrogenic Na⁺ absorption and stimulates Cl⁻ secretion in MDCK cells

Our previous studies with a line of Madin-Darby canine kidney (MDCK) cells (FL-MDCK) transfected with FLAG-labeled alpha, beta, and gamma subunits of epithelial Na(+) channel (ENaC) showed that, although most of the short-circuit current (I (sc)) was amiloride sensitive (AS-I (sc)), there was also an amiloride-insensitive component (NS-I (sc)) due to Cl(-) secretion (Morris and Schafer, J Gen Physiol 120:71-85, 2002). In the present studies, we observed a progressive increase in NS-I (sc) and a corresponding decrease in AS-I (sc) during experiments. There was a significant negative correlation between AS-I (sc) and NS-I (sc) both in the presence and absence of treatment with cyclic adenosine monophosphate (cAMP). NS-I (sc) could be attributed to both cystic fibrosis transmembrane conductance regulator (CFTR) and a 4, 4'-diisothiocyano-2, 2'-disulfonic acid stilbene (DIDS)-sensitive Ca(2+)-activated Cl(-) channel (CaCC). Continuous perfusion of both sides of the Ussing chamber with fresh rather than recirculated bathing solutions, or addition of hexokinase (6 U/ml), prevented the time-dependent changes and increased AS-I (sc) by 40-60%, with a proportional decrease in NS-I (sc). Addition of 100 muM adenosine triphosphate (ATP) in the presence of luminal amiloride produced a transient four-fold increase in NS-I (sc) that was followed by a sustained increase of 50-60% above the basal level. ATP release from the monolayers, measured by bioluminescence, was found to occur across the apical but not the basolateral membrane, and the apical release was tripled by cAMP treatment. These data show that constitutive apical ATP release, which occurs under both basal and cAMP-stimulated conditions, underlies the time-dependent rise in Cl(-) secretion and the proportional fall in ENaC-mediated Na(+) absorption in FL-MDCK cells. Thus, endogenous ATP release can introduce a significant confounding variable in experiments with this and similar epithelial cells, and it may underlie at least some of the observed interaction between Cl(-) secretion and Na(+) absorption.

Role of vasopressin in rat distal colon function

The specific role of vasopressin in colonic crypt function and its possible synergistic action with aldosterone were studied. Sprague-Dawley rats fed a high-Na+ (HS; 150 mM NaCl) or a low-Na+ (LS; 150 microM NaCl) diet were deprived of water or infused with vasopressin, and some animals were treated with specific vasopressin receptor subtype V1 and V2 antagonists. The expression of the epithelial Na+ channel (ENaC), alpha-smooth muscle actin (alpha-SMA) and aquaporin-2 (AQP-2) were determined by immunolocalization in distal colonic mucosa. The pericryptal Na+ concentration was determined by confocal microscopy, using a low-affinity Na+-sensitive fluorescent dye (sodium red) and crypt permeability was measured by the rate of escape of fluorescein isothiocyanate-labelled dextran (10 kDa) from the crypt lumen into the pericryptal space in isolated rat distal colonic mucosa. A high plasma concentration of vasopressin raised alpha-SMA expression in the pericryptal sheath (P < 0.05), increased the pericryptal Na+ accumulation in this space (P < 0.01) and caused a reduction of crypt wall permeability (P < 0.01). All these effects were reversed by selective blockade of V1 and V2 receptors. No synergistic effects with aldosterone were observed. Dehydration and vasopressin infusion increased AQP-2 expression in distal colonic mucosa (P < 0.05). This action of vasopressin was prevented by tolvaptan, a specific V2 receptor antagonist (P < 0.05). It is concluded that vasopressin has trophic effects in the rat distal colon, increasing pericryptal myofibroblast growth which affects crypt absorption, and these effects are independent of the presence of aldosterone.

Regulation of NaCl transport in the renal collecting duct: lessons from cultured cells

The fine control of NaCl absorption regulated by hormones takes place in the distal nephron of the kidney. In collecting duct principal cells, the epithelial sodium channel (ENaC) mediates the apical entry of Na(+), which is extruded by the basolateral Na(+),K(+)-ATPase. Simian virus 40-transformed and "transimmortalized" collecting duct cell lines, derived from transgenic mice carrying a constitutive, conditionally, or tissue-specific promoter-regulated large T antigen, have been proven to be valuable tools for studying the mechanisms controlling the cell surface expression and trafficking of ENaC and Na(+),K(+)-ATPase. These cell lines have made it possible to identify sets of aldosterone- and vasopressin-stimulated proteins, and have provided new insights into the concerted mechanism of action of serum- and glucocorticoid-inducible kinase 1 (Sgk1), ubiquitin ligase Nedd4-2 (neural precursor cell-expressed, developmentally down-regulated protein 4-2), and 14-3-3 regulatory proteins in modulating ENaC-mediated Na(+) currents. Epidermal growth factor and induced leucine zipper protein have also been shown to repress and stimulate ENaC-dependent Na(+) absorption, respectively, by activating or repressing the mitogen-activated protein kinase externally regulated kinase(1/2). Overall, these findings have provided evidence suggesting that multiple pathways are involved in regulating NaCl absorption in the distal nephron.

Long-term effects of vasopressin on the subcellular localization of ENaC in the renal collecting system

Previous studies revealed that chronic (days) vasopressin treatment stimulates amiloride-sensitive sodium transport in isolated renal cortical collecting ducts and increases the abundance of beta- and gamma-subunits of the epithelial sodium channel (ENaC) in the kidney. The aim of the present work was to investigate in vivo the cellular basis of these effects. The long-term effect of V2 vasopressin agonist (1-deamino-8-D-arginine vasopressin (dDAVP)) on the abundance and subcellular localization of ENaC along the rat renal collecting system was determined by immunohistochemistry and laser confocal microscopy. Moreover, we studied by real-time reverse transcriptase-polymerase chain reaction the effect of vasopressin on proteins implicated in the regulation of ENaC (Nedd4-2, prostasin, Sgk1). After 5 days of administration, dDAVP markedly increased the intracellular pool of the beta- and gamma-ENaC subunits in the principal cells, with an increasing gradient from connecting tubule to the outer medullary collecting duct, but did not increase any subunit at the cell surface. The apical immunostaining of ENaC increased in response to sodium restriction, as expected, but dDAVP did not further enhance this apical labelling. dDAVP increased the gene expression of prostasin in the cortex but not that of Nedd4-2 and Sgk1. These findings suggest that the previously reported increase in sodium transport induced by sustained stimulation of vasopressin V2 receptor is probably mediated by other mechanism than an increase in the apical density of ENaC.

Stimulation of Xenopus P2Y1 receptor activates CFTR in A6 cells

Nucleotide binding to purinergic P2Y receptors contributes to the regulation of a variety of physiological functions in renal epithelial cells. Here, we investigate the regulatory mechanism of the P2Y1 receptor agonist 2-methylthioadenosine diphosphate (2-MeSADP) on Cl- transport in A6 cells, a commonly used model of the distal section of the Xenopus laevis nephron. Protein and mRNA expression analysis together with functional measurements demonstrated the basolateral location of the Xenopus P2Y1 receptor. 2-MeSADP increased intracellular [Ca2+] and cAMP and Cl- efflux, responses that were all inhibited by the specific P2Y1 receptor antagonist MRS 2179. Cl- efflux was also inhibited by the cystic fibrosis transmembrane conductance regulator (CFTR) blocker glibenclamide. Inhibition of either protein kinase A (PKA) or the binding between A-kinase-anchoring proteins (AKAPs) and the regulatory PKA RII subunit blocked the 2-MeSADP-induced activation of CFTR, suggesting that PKA mediates P2Y1 receptor regulation of CFTR through one or more AKAPs. Further, the truncation of the PDZ1 domain of the scaffolding protein Na+/H+ exchanger regulatory factor-2 (NHERF-2) inhibited 2-MeSADP-dependent stimulation of Cl- efflux, suggesting the involvement of this scaffolding protein. Activation or inhibition of PKC had no effect per se on basal Cl- efflux but potentiated or reduced the 2-MeSADP-dependent stimulation of Cl- efflux, respectively. These data suggest that the X laevis P2Y1 receptor in A6 cells can increase both cAMP/PKA and Ca2+/PKC intracellular levels and that the PKC pathway is involved in CFTR activation via potentiation of the PKA pathway.

Aldosterone and tight junctions: modulation of claudin-4 phosphorylation in renal collecting duct cells

Aldosterone classically modulates Na transport in tight epithelia such as the renal collecting duct (CD) through the transcellular route, but it is not known whether the hormone could also affect paracellular permeability. Such permeability is controlled by tight junctions (TJ) that form a size- and charge-selective barrier. Among TJ proteins, claudin-4 has been highlighted as a key element to control paracellular charge selectivity. In RCCD2 CD cells grown on filters, we have identified novel early aldosterone effects on TJ. Endogenous claudin-4 abundance and cellular localization were unaltered by aldosterone. However, the hormone promoted rapid (within 15-20 min) and transient phosphorylation of endogenous claudin-4 on threonine residues, without affecting tyrosine or serine; this event was fully developed at 10 nM aldosterone and appeared specific for aldosterone (because it is not observed after dexamethasone treatment and it depends on mineralocorticoid receptor occupancy). Within the same delay, aldosterone also promoted an increased apical-to-basal passage of 125I (a substitute for 36Cl), whereas 22Na passage was unaffected; paracellular permeability to [3H]mannitol was also reduced. Later on (45 min), a fall in transepithelial resistance was observed. These data indicate that aldosterone modulates TJ properties in renal epithelial cells.

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.

Vasopressin-stimulated CFTR Cl- currents are increased in the renal collecting duct cells of a mouse model of Liddle's syndrome

Liddle's syndrome is a genetic form of hypertension linked to Na(+) retention caused by activating mutations in the COOH terminus of the beta or gamma subunit of the epithelial sodium channel (ENaC). In this study, we used the short-circuit current (I(sc)) method to investigate the effects of deamino-8-d-arginine vasopressin (dDAVP) on Na(+) and Cl(-) fluxes in primary cultures of cortical collecting ducts (CCDs) microdissected from the kidneys of mice with Liddle's syndrome carrying a stop codon mutation, corresponding to the beta-ENaC R(566) stop mutation (L) found in the original pedigree. Compared to wild-type (+/+) CCD cells, untreated L/+ and L/L CCD cells exhibited 2.7- and 4.2-fold increases, respectively, in amiloride-sensitive (Ams) I(sc), reflecting ENaC-dependent Na(+) absorption. Short-term incubation with dDAVP caused a rapid and significant increase (approximately 2-fold) in Ams I(sc) in +/+, but not in L/+ or L/L CCD cells. In sharp contrast, dDAVP induced a greater increase in 5-nitro-2-(3-phenylpropamino)benzoate (NPPB)-inhibited apical Cl(-) currents in amiloride-treated L/L and L/+ cells than in their +/+ counterparts. I(sc) recordings performed under apical ion substituted conditions revealed that the dDAVP-stimulated apical secretion of Cl(-), which was absent in cultured CCDs lacking CFTR, was 1.8-fold greater in L/+ and 3.7-fold greater in L/L CCD cells than in their +/+ CCD counterparts. After the basal membrane had been permeabilized with nystatin and a basal-to-apical Cl(-) gradient had been imposed, dDAVP also stimulated larger Cl(-) currents across L/L and L/+ CCD layers than +/+ CCD layers. These findings demonstrate that vasopressin stimulates greater apical CFTR Cl(-) conductance in the renal CCD cells of mice with Liddle's syndrome than in wild-type mice. This effect could contribute to the enhanced NaCl reabsorption observed in the distal nephron of patients with Liddle's syndrome.

Phosphatidylinositol 3,4,5-trisphosphate: an early mediator of insulin-stimulated sodium transport in A6 cells

Insulin stimulates sodium transport across A6 epithelial cell monolayers. Activation of phosphatidylinositol 3-kinase (PI 3-kinase) was suggested as an early step in the insulin-stimulated sodium reabsorption (Ref. 35). To establish that the stimulation of the PI 3-kinase signaling cascade is causing stimulation of apical epithelial Na channel, we added permeant forms of phosphatidylinositol (PI) phosphate (P) derivatives complexed with a histone carrier to A6 epithelium. Only PIP3and PI( 3 , 4 )P2but not PI( 4 , 5 )P2stimulated sodium transport, although each of them penetrated into A6 cell monolayers as assessed using fluorescent permeant phosphoinositides derivatives. By Western blot analysis of A6 cell extracts, the inositol 3-phosphatase PTEN and the protein kinase B PKB were both detected. To further establish that the stimulation of sodium transport induced by insulin is related to PIP3levels, we transfected A6 cells with human PTEN cDNA and observed a 30% decrease in the natriferic effect of insulin. Similarly, the increase in sodium transport observed by addition of permeant PIP3was also reduced by 30% in PTEN-overexpressing cells. PKB, a main downstream effector of PI 3-kinase, was phosphorylated at both Thr 308 and Ser 473 residues upon insulin stimulation of the A6 cell monolayer. PKB phosphorylation in response to insulin stimulation was reduced in PTEN-overexpressing cells. Permeant PIP3also increased PKB phosphorylation. Taken together, the present results establish that the d-3-phosphorylated phosphoinositides PIP3and PI( 3 , 4 )P2mediate the effect of insulin on sodium transport across A6 cell monolayers.

Mild dehydration, vasopressin and the kidney: animal and human studies

Water balance depends essentially on fluid intake and urine excretion. Mild dehydration and the consequent hypertonicity of the extracellular fluid induce an increase in vasopressin secretion, thus stimulating urine concentrating processes and the feeling of thirst. The osmotic threshold for the release of vasopressin is lower than that for thirst and also shows appreciable individual variation. Sustained high levels of vasopressin and low hydration induce morphological and functional changes in the kidney. However, they could also be risk factors in several renal disorders, such as chronic renal failure, diabetic nephropathy and salt-sensitive hypertension.

Acute regulation of Na/H exchanger NHE3 by adenosine A(1) receptors is mediated by calcineurin homologous protein

Adenosine is an autacoid that regulates renal Na(+) transport. Activation of adenosine A(1) receptor (A(1)R) by N(6)-cyclopentidyladenosine (CPA) inhibits the Na(+)/H(+) exchanger 3 (NHE3) via phospholipase C/Ca(2+)/protein kinase C (PKC) signaling pathway. Mutation of PKC phosphorylation sites on NHE3 does not affected regulation of NHE3 by CPA, but amino acid residues 462 and 552 are essential for A(1)R-dependent control of NHE3 activity. One binding partner of the NHE family is calcineurin homologous protein (CHP). We tested the role of NHE3-CHP interaction in mediating CPA-induced inhibition of NHE3 in opossum kidney (OK) and Xenopus laevis uroepithelial (A6) cells. Both native and transfected NHE3 and CHP are present in the same immuno-complex by co-immunoprecipitation. CPA (10(-6) M) increases CHP-NHE3 interaction by 30 - 60% (native and transfected proteins). Direct CHP-NHE3 interaction is evident by yeast two-hybrid assay (bait, NHE3(C terminus); prey, CHP); the minimal interacting region is localized to the juxtamembrane region of NHE3(C terminus) (amino acids 462-552 of opossum NHE3). The yeast data were confirmed in OK cells where truncated NHE3 (NHE3(delta552)) still shows CPA-stimulated CHP interaction. Overexpression of the polypeptide from the CHP binding region (NHE3(462-552)) interferes with the ability of CPA to inhibit NHE3 activity and to increase CHPNHE3(Full-length) interaction. Reduction of native CHP expression by small interference RNA abolishes the ability of CPA to inhibit NHE3 activity. We conclude that CHPNHE3 interaction is regulated by A(1)R activation and this interaction is a necessary and integral part of the signaling pathway between adenosine and NHE3.

Low-dose vasopressin in the treatment of septic shock in sheep

After induction of cecal perforation, 20 anesthetized sheep were randomized to be treated, when arterial blood pressure fell below 75 mm Hg, with vasopressin (fixed dose of 0.02 U/minute), norepinephrine (0.5-5 microg/kg/minute titrated to maintain mean arterial pressure between 75 and 85 mm Hg), vasopressin + norepinephrine (vasopressin at fixed dose 0.01 U/minute plus norepinephrine titrated as for norepinephrine only group), or no vasopressor (Ringer's lactate [control]). Mean arterial pressure was well maintained in all treatment groups. Superior mesenteric arterial blood flow was significantly lower in the vasopressin + norepinephrine group than in the vasopressin group. Vasopressin alone or combined with norepinephrine limited the increase in blood lactate concentration and ileal PCO2-gap compared with control and norepinephrine groups. Urine output was higher in the vasopressin group than in control and norepinephrine groups. Survival time was longer in the vasopressin (30 +/- 6 hours) and vasopressin + norepinephrine (30 +/- 3 hours) groups than in the norepinephrine group (20 +/- 1 hours, p < 0.05) and in all treatment groups than in the control group (17 +/- 2 hours, p < 0.05). Tissue injury was less severe in the vasopressin and vasopressin + norepinephrine groups than in the others. In this clinically relevant model of septic shock due to peritonitis, vasopressin administration (alone or with norepinephrine) can prolong survival.

Role of SGK in hormonal regulation of epithelial sodium channel in A6 cells

The purpose of this study was to examine the role of the serum- and glucocorticoid-induced kinase (SGK) in the activation of the epithelial sodium channel (ENaC) by aldosterone, arginine vasopressin (AVP), and insulin. We used a tetracycline-inducible system to control the expression of wild-type (SGK(wt)(T)), constitutively active (S425D mutation; SGK(S425D)(T)), or inactive (K130M mutation; SGK(K130M)(T)) SGK in A6 cells independently of hormonal stimulation. The effect of SGK expression on ENaC activity was monitored by measuring transepithelial amiloride-sensitive short-circuit current (I(sc)) of transfected A6 cell lines. Expression of SGK(wt)(T) or SGK(S425D)(T) and aldosterone stimulation have additive effects on I(sc). Although SGK could play some role in the aldosterone response, our results suggest that other mechanisms take place. SGK(S425D)(T) abrogates the responses to AVP and insulin; hence, in the signaling pathways of these hormones there is a shared step that is stimulated by SGK. Because AVP and insulin induce fusion of vesicles to the apical membrane, our results support the notion that SGK promotes incorporation of channels in the apical membrane.

Basolateral and apical A1 adenosine receptors mediate sodium transport in cultured renal epithelial (A6) cells

There are conflicting reports in the literature regarding the adenosine receptor that mediates the increase in sodium transport in the A6 cell. In this study we used specific A1 and A2 adenosine receptor agonists and antagonists, as well as two different subclones of the A6 cell, to determine which adenosine receptor mediates the increase in sodium transport. In the A6S2 subclone, basolateral and apical N6-cyclohexyladenosine (CHA), a selective A1 receptor agonist, stimulated sodium transport at a threshold concentration <10(-7) M, whereas CGS-21680, a selective A2 receptor agonist, had a threshold concentration that was at least 10(-5) M. The A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) was found to have a nonspecific effect on CHA-stimulated sodium transport, whereas the A2 receptor antagonist 8-(3-chlorostyryl)caffeine (CSC) had no effect. As with the A6S2 subclone, basolateral and apical CHA stimulated sodium transport at a nanomolar concentration in the A6C1 subclone and the threshold concentration for CGS-21680 was in the high micromolar range. Concurrent with the increase in 1 receptor in different subclones of the A6 cell, including a subclone capable of anion secretion.

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.

Molecular aspects of acute inhibition of Na(+)-H(+) exchanger NHE3 by A(2)-adenosine receptor agonists

Adenosine regulates Na(+) homeostasis by its acute effects on renal Na(+) transport. We have shown in heterologously transfected A6/C1 cells (renal cell line from Xenopus laevis) that adenosine-induced natriuresis may be effected partly via A(2) adenosine receptor-mediated inactivation of the renal brush border membrane Na(+)-H(+) exchanger NHE3. In this study we utilized A6/C1 cells stably expressing wild-type as well as mutated forms of NHE3 to assess the molecular mechanism underlying A(2)-dependent control of NHE3 function. Cell surface biotinylation combined with immunoprecipitation revealed that NHE3 is targeted exclusively to the apical domain and that the endogenous Xenopus NHE is located entirely on the basolateral side of A6/C1 transfectants. Stimulation of A(2)-adenosine receptors located on the basolateral side for 15 min with CPA (N6-cyclopentyladenosine) acutely decreased NHE3 activity (microspectrofluorimety). This effect was mimicked by 8-bromo-cAMP and entirely blocked by pharmacological inhibition of PKA (with H89) or singular substitution of two PKA target sites (serine 552 and serine 605) on NHE3. Downregulation of NHE3 activity by CPA was attributable to a reduction of NHE3 intrinsic transport activity without change in surface NHE3 protein at 15 min. At 30 min, the decrease in transport activity was associated with a decrease in apical membrane NHE3 antigen. In conclusion, two highly conserved target serine sites on NHE3 determine NHE3 modulation upon A(2)-receptor activation and NHE3 inactivation by adenosine proceeds via two phases with distinct mechanisms.

CFTR disruption impairs cAMP-dependent Cl(-) secretion in primary cultures of mouse cortical collecting ducts

The role of the cystic fibrosis transmembrane conductance regulator (CFTR) in the renal cortical collecting duct (CCD) has not yet been fully elucidated. Here, we investigated the effects of deamino-8-D-arginine vasopressin (dDAVP) and isoproterenol (ISO) on NaCl transport in primary cultured CCDs microdissected from normal [CFTR(+/+)] and CFTR-knockout [CFTR(-/-)] mice. dDAVP stimulated the benzamyl amiloride (BAm)-sensitive transport of Na(+) assessed by the short-circuit current (I(sc)) method in both CFTR(+/+) and CFTR(-/-) CCDs to a very similar degree. Apical addition of 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) or glibenclamide partially inhibited the rise in I(sc) induced by dDAVP and ISO in BAm-treated CFTR(+/+) CCDs, whereas dDAVP, ISO, and NPPB did not alter I(sc) in BAm-treated CFTR(-/-) CCDs. dDAVP stimulated the apical-to-basal flux and, to a lesser extent, the basal-to-apical flux of (36)Cl(-) in CFTR(+/+) CCDs. dDAVP also increased the apical-to-basal (36)Cl(-) flux in CFTR(-/-) CCDs but not the basal-to-apical (36)Cl(-) flux. These results demonstrate that CFTR mediates the cAMP-stimulated component of secreted Cl(-) in mouse CCD.

Vasopressin-stimulated chloride transport in transimmortalized mouse cell lines derived from the distal convoluted tubule and cortical and inner medullary collecting ducts

BACKGROUND

The fine control of NaCl absorption takes place in the distal parts of the renal tubule, but the regulation of Cl(-) transport in this region has not been fully elucidated. We have analysed the effects of dD-arginine vasopressin (dDAVP) on Cl(-) fluxes in cultured mouse distal convoluted tubule (mpkDCT), cortical collecting duct (mpkCCD) and inner medullary collecting duct (mpkIMCD) cell lines.

METHODS

RT-PCR and Western blotting were used to detect the amiloride-sensitive sodium channel (ENaC) and cystic fibrosis transmembrane conductance regulator (CFTR) mRNAs and protein in cultured mpkDCT, mpkCCD and mpkIMCD cells. Cl(-) fluxes were analysed by measuring the short-circuit current (I(sc)) and bidirectional (36)Cl(-) fluxes on confluent cells grown on filters.

RESULTS

All three cell lines expressed ENaC and CFTR and had I(sc) stimulated by dDAVP. The rise in I(sc) caused by dDAVP (10(-8) M) was inhibited by amiloride, and to a lesser extent by 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) in all three cell lines. The dDAVP-dependent I(sc) measured under apical Na(+)-free condition was reduced by Cl(-) channel blockers with a profile (NPPB>glibenclamide>DIDS), similar to that for rat CFTR. dDAVP stimulated the apical-to-basal (36)Cl(-) flux and to a lesser extent the basal-to-apical (36)Cl(-) flux under open-circuit condition in all three cultured cell lines. Adding NPPB to the apical side reduced the basal-to-apical (36)Cl(-) flux but not the opposite (36)Cl(-) flux from dDAVP-treated cells.

CONCLUSION

These results indicate that dDAVP stimulates the bi-directional flux of Cl(-), resulting in net Cl(-)absorption, in these cultured mouse distal and collecting duct cells. I(sc) experiments also suggest the presence of a minor component of electrogenic Cl(-) secretion, possibly mediated by CFTR.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

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.

Pleiotropic action of aldosterone in epithelia mediated by transcription and post-transcription mechanisms

The aldosterone-induced increase in sodium reabsorption across tight epithelia can be divided schematically into two functional phases: an early regulatory phase starting after a lag period of 20 to 60 minutes, during which the pre-existing transport machinery is activated, and a late phase (>2.5 h), which can be viewed as an anabolic action leading to a further amplification/differentiation of the Na+ transport machinery. At the transcriptional level, both early and late responses are initiated during the lag period, but the functional impact of newly synthesized regulatory proteins is faster than that of the structural ones. K-Ras2 and SGK were identified as the first early aldosterone-induced regulatory proteins in A6 epithelia. Their mRNAs also were shown to be regulated in vivo by aldosterone, and their expression (constitutively active K-Ras2 and wild-type SGK) was shown to increase the function of ENaC coexpressed in Xenopus oocytes. Recently, aldosterone was also shown to act on transcription factors in A6 epithelia: It down-regulates the mRNAs of the proliferation-promoting c-Myc, c-Jun, and c-Fos by a post-transcriptional mechanism, whereas it up-regulates that of Fra-2 (c-Fos antagonist) at the transcriptional level. Together, these new data illustrate the complexity of the regulatory network controlled by aldosterone and support the view that its early action is mediated by the induction of key regulatory proteins such as K-Ras2 and SGK. These early induced proteins are sites of convergence for different regulatory inputs, and thus, their aldosterone-regulated expression level tunes the impact of other regulatory cascades on sodium transport. This suggests mechanisms for the escape from aldosterone action.

Activation of the epithelial Na+ channel (ENaC) requires CFTR Cl- channel function

It is increasingly being recognized that cells coordinate the activity of separate ion channels that allow electrolytes into the cell. However, a perplexing problem in channel regulation has arisen in the fatal genetic disease cystic fibrosis, which results from the loss of a specific Cl- channel (the CFTR channel) in epithelial cell membranes. Although this defect clearly inhibits the absorption of Na+ in sweat glands, it is widely accepted that Na+ absorption is abnormally elevated in defective airways in cystic fibrosis. The only frequently cited explanation for this hypertransport is that the activity of an epithelial Na+ channel (ENaC) is inversely related to the activity of the CFTR Cl- channel. However, we report here that, in freshly isolated normal sweat ducts, ENaC activity is dependent on, and increases with, CFTR activity. Surprisingly, we also find that the primary defect in Cl- permeability in cystic fibrosis is accompanied secondarily by a Na+ conductance in this tissue that cannot be activated. Thus, reduced salt absorption in cystic fibrosis is due not only to poor Cl- conductance but also to poor Na+ conductance.

Vasopressin stimulates long-term net chloride secretion in cortical collecting duct cells

The classical short-term effect (within minutes) of arginine vasopressin (AVP) consists in increasing sodium, chloride and water transport in kidney cells. More recently, long-term actions (several hours) of the hormone have been evidenced on water and sodium fluxes, due to transcriptional enhancement in the expression of their transporters. The present study demonstrates that AVP is also responsible for a long-term increase in net chloride secretion. In the RCCD(1) rat cortical collecting duct cell line, 10(-8) M AVP induced, after several hours, an increase in net (36)Cl(-) secretion. This delayed effect of AVP was inhibited by basal addition of 10(-4) M bumetanide and apical addition of 10(-4) M glibenclamide, suggesting chloride entry at the basal membrane through a Na(+)/K(+)/2Cl(-) and apical secretion through a chloride conductance. An original acute cell permeabilization method was developed to allow for entry of antibodies directed against the regulatory region (R) of the cystic fibrosis transmembrane regulator (CFTR) into the cells. This procedure led to a complete and specific blocking of the long-term net chloride secretion induced by AVP. Finally, it was observed that CFTR transcripts steady-state level was significantly increased by AVP treatment. Besides the well-documented short-term effect of AVP on chloride transport, these results provide evidence that in RCCD(1) cells, AVP induces a delayed increase in transepithelial net chloride secretion that is mediated by a Na(+)/K(+)/2Cl(-) co-transporter and CFTR.

Control of apical membrane chloride permeability in the renal A6 cell line by nucleotides

1. The effect of extracellular nucleotides applied on the apical side of polarised A6 cells grown on permeant filters was investigated by measuring the changes in (i) the 36Cl efflux through the apical membranes, (ii) the intracellular chloride concentrations (aCli, measured with N-(6-methoxyquinolyl) acetoethyl ester, MQAE), (iii) ICl, the short-circuit current in the absence of Na+ transport and (iv) the characteristics of the apical chloride channels using a patch-clamp approach. 2. ATP or UTP (0.1-500 microM) transiently stimulated ICl. The sequence of purinergic agonist potencies was UTP = ATP > ADP >> the P2X-selective agonist beta,gamma-methylene ATP = the P2Y-selective agonist 2-methylthioATP. Suramin (100 microM) as the P2Y antagonist Reactive Blue 2 (10 microM) had no effect on the UTP (or ATP)-stimulated current. These findings are consistent with the presence of P2Y2-like receptors located on the apical membranes of A6 cells. Apical application of adenosine also transiently increased ICl. This effect was blocked by theophylline while the UTP-stimulated ICl was not. The existence of a second receptor, of the P1 type is proposed. 3. ATP (or UTP)-stimulated ICl was blocked by apical application of 200 microM N-phenylanthranilic acid (DPC) or 100 microM niflumic acid while 100 microM glibenclamide was ineffective. 4. Ionomycin and thapsigargin both transiently stimulated ICl; the nucleotide stimulation of ICl was not suppressed by pre-treatment with these agents. Chlorpromazin (50 microM), a Ca2+-calmodulin inhibitor strongly inhibited the stimulation of ICl induced either by apical UTP or by ionomycin application. BAPTA-AM pre-treatment of A6 cells blocked the UTP-stimulated ICl. Niflumic acid also blocked the ionomycin stimulated ICl. 5. A fourfold increase in 36Cl effluxes through the apical membranes was observed after ATP or UTP application. These increases of the apical chloride permeability could also be observed when following aCli changes. Apical application of DPC (1 mM) or 5-nitro-2(3-phenylpropylamino)benzoic acid (NPPB; 500 microM) produced an incomplete inhibition of 36Cl effluxes through the apical membranes in ATP-stimulated and in untreated monolayers. 6. In single channel patch-clamp experiments, an apical chloride channel with a unitary single channel conductance of 7.3 +/- 0.6 pS (n = 12) was usually observed. ATP application induced the activation of one or more of these channels within a few minutes. 7. These results indicate that multiple purinergic receptor subtypes are present in the apical membranes of A6 cells and that nucleotides can act as modulators of Cl- secretion in renal cells.

Immortalized kidney epithelial cells as tools for hormonally regulated ion transport studies

The development of transgenic mice carrying the simian virus-40 large T antigen gene or the temperature-sensitive simian virus-40 large T antigen gene, either alone or placed under the control of the 5'-regulatory regions of tissue-specific or ubiquitous genes, has permitted the production of differentiated, polarized kidney epithelial cells. This review covers the immortalized cell lines issued from the various parts of the renal tubule and, in particular, the recently established collecting duct cell lines that have been used as ex-vivo cell models to analyze the regulation of ion transport processes by hormones.

Regulation of Na+ pump function by aldosterone is alpha-subunit isoform specific

1. During its early 'genomic' phase of action (< 3 h), aldosterone activates pre-existing Na+ pumps (Na+,K+-ATPase) in epithelia formed by Xenopus laevis A6 kidney cells. 2. To test whether this action also applies to pumps containing mammalian alpha-subunits of different isoforms, we generated A6 cell lines expressing the naturally ouabain-resistant rat alpha1 subunit or the rat alpha2* and alpha3* subunits made ouabain resistant by site-directed mutagenesis. 3. Cell lines were obtained which expressed the exogenous alpha-subunits in active, basolateral Na+ pumps, such that ouabain-resistant pump current (Ip) could be measured following apical permeabilization with amphotericin B. 4. The inhibition constants (Ki) for ouabain of the current carried by the pumps containing exogenous rat alpha-subunits were similar to those reported previously for ATPase activity inhibition. The apparent Michaelis constant (Km) for Na+ (K+ replacement) was slightly higher for pumps containing the rat alpha1 than for those containing the alpha2* subunit (34.9 +/- 1.9 versus 26.3 +/- 2.6 mM). 5. At a Na+ concentration of 10 mM, aldosterone (2.5 h) increased the pump current carried by endogenous pumps as well as that carried by pumps containing the exogenous rat alpha1 subunit (by 1.8- to 2.2-fold). In contrast, the current carried by pumps containing the exogenous rat alpha2* subunit remained unchanged. 6. The fact that this early transcriptionally mediated activation of Na+ pumps by aldosterone is specific for pumps containing an alpha1 subunit should permit the identification in this subunit of structures involved in its regulation.

Regulation of Cl- secretion in seawater fish (Dicentrarchus labrax) gill respiratory cells in primary culture

1. Primary cultures of sea bass (Dicentrarchus labrax) gill cells grown on permeable membranes form a highly differentiated tight epithelium composed of respiratory-like cells. This preparation was also found to provide a functional model for investigating the hormonal regulation of Cl- secretion. 2. In control conditions, i.e. in the absence of hormones or other stimuli, the cultured epithelium showed a short-circuit current (Isc) of 8.8 +/- 0.4 microA cm-2, a transepithelial potential (Vt) of 28.6 +/- 0.6 mV (serosal side positive), and a transepithelial resistance (Rt) of 5026 +/- 127 Omega cm2. Addition of 50 nM PGE2 caused a stimulation of Isc, Vt and transepithelial conductance, Gt. The increase in Isc was probably due to the elevation in Cl- secretion, since it could be correlated with the stimulation of serosal to mucosal 36Cl- flux. Application of the neurohypophyseal peptide arginine vasotocin (AVT; 50 nM) or the beta-adrenergic agonist isoproterenol (isoprenaline; 0. 5 microM) evoked a stimulation in Cl- secretion, as was shown by the increases in Isc and Gt. The excitatory effect of isoproterenol followed by the inhibitory action of propranolol, a beta-adrenergic antagonist, suggested the presence of beta-adrenergic receptors. Noradrenaline (0.1 microM) elicited a reduction in Isc, Vt and Gt, which was counterbalanced by the addition of phentolamine, an alpha-adrenergic antagonist. This suggested an activation of alpha-adrenergic receptors. 3. This study provides evidence for hormonal control of the Cl- secretion in sea bass gill respiratory cells in culture, involving AVT, prostaglandin (PGE2), and beta- and alpha-adrenergic receptors.

Adenosine inhibits the transfected Na+-H+ exchanger NHE3 in Xenopus laevis renal epithelial cells (A6/C1)

1. Adenosine influences the vectorial transport of Na+ and HCO3- across kidney epithelial cells. However, its action on effector proteins, such as the Na+-H+ exchanger NHE3, an epithelial brush border isoform of the Na+-H+ exchanger (NHE) gene family, is not yet defined. 2. The present study was conducted in Xenopus laevis distal nephron A6 epithelia which express both an apical adenosine receptor of the A1 type (coupled to protein kinase C (PKC)) and a basolateral receptor of the A2 type (coupled to protein kinase A (PKA)). The untransfected A6 cell line expresses a single NHE type (XNHE) which is restricted to the basolateral membrane and which is activated by PKA. 3. A6 cell lines were generated which express exogenous rat NHE3. Measurements of side-specific pHi recovery from acid loads in the presence of HOE694 (an inhibitor with differential potency towards individual NHE isoforms) detected an apical resistant Na+-H+ exchange only in transfected cell lines. The sensitivity of the basolateral NHE to HOE694 was unchanged, suggesting that exogenous NHE3 was restricted to the apical membrane. 4. Stimulation of the apical A1 receptor with N 6-cyclopentyladenosine (CPA) inhibited both apical NHE3 and basolateral XNHE. These effects were mimicked by the addition of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) and partially prevented by the PKC inhibitor calphostin C which also blocked the effect of PMA. 5. Stimulation of the basolateral A2 receptor with CPA inhibited apical NHE3 and stimulated basolateral XNHE. These effects were mimicked by 8-bromo-cAMP and partially prevented by the PKA inhibitor H89 which entirely blocked the effect of 8-bromo-cAMP. 6. In conclusion, CPA inhibits rat NHE3 expressed apically in A6 epithelia via both the apical PKC-coupled A1 and the basolateral PKA-coupled A2 adenosine receptors.

Ras pathway activates epithelial Na+ channel and decreases its surface expression in Xenopus oocytes

The small G protein K-Ras2A is rapidly induced by aldosterone in A6 epithelia. In these Xenopus sodium reabsorbing cells, aldosterone rapidly activates preexisting epithelial Na+ channels (XENaC) via a transcriptionally mediated mechanism. In the Xenopus oocytes expression system, we tested whether the K-Ras2A pathway impacts on XENaC activity by expressing XENaC alone or together with XK-Ras2A rendered constitutively active (XK-Ras2AG12V). As a second control, XENaC-expressing oocytes were treated with progesterone, a sex steroid that induces maturation of the oocytes similarly to activated Ras. Progesterone or XK-Ras2AG12V led to oocyte maturation characterized by a decrease in surface area and endogenous Na+ pump function. In both conditions, the surface expression of exogenous XENaC's was also decreased; however, in comparison with progesterone-treated oocytes, XK-ras2AG12V-coinjected oocytes expressed a fivefold higher XENaC-mediated macroscopic Na+ current that was as high as that of control oocytes. Thus, the Na+ current per surface-expressed XENaC was increased by XK-Ras2AG12V. The chemical driving force for Na+ influx was not changed, suggesting that XK-Ras2AG12V increased the mean activity of XENaCs at the oocyte surface. These observations raise the possibility that XK-Ras2A, which is the first regulatory protein known to be transcriptionally induced by aldosterone, could play a role in the control of XENaC function in aldosterone target cells.

Peptide YY inhibits vasopressin-stimulated chloride secretion in inner medullary collecting duct cells

mIMCD-k2 cells are derived from the inner medullary collecting duct of a mouse and exhibit electrogenic sodium absorption and cAMP- and vasopressin (AVP)-stimulated electrogenic chloride secretion [N. L. Kizer, B. Lewis, and B. A. Stanton. Am. J. Physiol. 268 (Renal Fluid Electrolyte Physiol. 37): F347-F355, 1995; and N. L. Kizer, D. Vandorpe, B. Lewis, B. Bunting, J. Russell, and B. A. Stanton. Am. J. Physiol. 268 (Renal Fluid Electrolyte Physiol. 37): F854-F861, 1995]. The purpose of the present study was to determine how peptide YY (PYY) affects electrogenic Na+ and Cl- current in mIMCD-k2 cells. Short-circuit currents (Isc) were measured across monolayers of mIMCD-k2 cells mounted in Ussing-type chambers. PYY did not alter baseline Isc, nor did it alter Isc in chloride-free conditions, indicating no effect on electrogenic sodium transport. Baseline chloride current in these cells is low; therefore, chloride short-circuit current (IClsc) was stimulated with AVP (10 nM) added to the basolateral surface and 10 microM amiloride added to the apical surface. Although apical applications of PYY had no effect, basolateral application of PYY caused attenuation of IClsc, with the maximal inhibitory dose (100 nM) causing 52 +/- 1.3% inhibition (IC50 = 0.11 nM). Inhibition by PYY of IClsc is mediated through the Y2 receptor subtype, as PYY-(3-36) was the only PYY analog tested that caused inhibition and was equipotent to PYY. Inhibition by PYY of IClsc was abolished following incubation with pertussis toxin. We also show that PYY inhibits AVP-stimulated cAMP accumulation, with a maximal inhibitory dose (100 nM) causing a 38% +/- 6% inhibition (IC50 = 0.16 nM), comparable to inhibition by PYY of IClsc. We conclude that PYY acts through either Gi or Go to inhibit adenylate cyclase activity, leading to a decrease in AVP-stimulated chloride current.

Cl- transport by cystic fibrosis transmembrane conductance regulator (CFTR) contributes to the inhibition of epithelial Na+ channels (ENaCs) in Xenopus oocytes co-expressing CFTR and ENaC

1. Epithelial Na+ channels (ENaCs) are inhibited by the cystic fibrosis transmembrane conductance regulator (CFTR) when CFTR is activated by protein kinase A. Since cAMP-dependent activation of CFTR Cl- conductance is defective in cystic fibrosis (CF), ENaC currents are not inhibited by CFTR. This could explain the enhanced Na+ conductance found in CF. In the present study, we examined possible mechanisms of interaction between CFTR and ENaC co-expressed in Xenopus oocytes. 2. The magnitude of CFTR Cl- currents activated by 3-isobutyl-1-methylxanthine (IBMX) in oocytes co-expressing either wild-type or mutant CFTR and ENaC determined the degree of downregulation of ENaC currents. 3. The ability of CFTR to inhibit ENaC currents was significantly reduced either when extracellular Cl- was replaced by poorly conductive anions, e.g. SCN- or gluconate, or when CFTR was inhibited by diphenylamine-carboxylate (DPC, 1 mmol l-1). 4. Downregulation of ENaC was more pronounced at positive when compared with negative clamp voltages. This suggests that outward currents, i.e. influx of Cl- through activated CFTR most effectively downregulated ENaC. 5. Activation of endogenous Ca2+-activated Cl- currents by 1 micromol l-1 ionomycin did not inhibit ENaC current. This suggests that inhibition of ENaC mediated by Cl- currents may be specific to CFTR. 6. The present findings indicate that downregulation of ENaC by CFTR is correlated to the ability of CFTR to conduct Cl-. The data have implications for how epithelia switch from NaCl absorption to NaCl secretion when CFTR is activated by secretagogues.

Microtubule disruption inhibits AVT-stimulated Cl- secretion but not Na+ reabsorption in A6 cells

The effects of microtubule disruption on arginine vasotocin (AVT)-stimulated Na+ and Cl- transport were studied in A6 cells by measuring short-circuit currents (Isc) across cell layers grown in tissue culture on permeable supports. Microtubule disruption inhibited an AVT-stimulated secretory Cl- current but did not prevent activation of amiloride-sensitive Na+ transport. This AVT-stimulated secretory Cl- current was significantly inhibited by glibenclamide, an inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR). Reverse transcription of RNA isolated from A6 cells followed by polymerase chain reaction (PCR) using primers designed to amplify a portion of the R-domain of CFTR cloned from Xenopus laevis skin and immunocytochemistry demonstrated the presence of CFTR in A6 cells and an apparent recruitment of cytoplasmic CFTR to the apical cell surface after AVT stimulation. In contrast, indirect immunofluorescent labeling of Na+ channels using a polyclonal antibody raised against a biochemically isolated Na+ channel complex from bovine renal medulla labeled the apical plasma membrane but failed to demonstrate intracellular labeling of Na+ channels (except in subconfluent cells) or recruitment of Na+ channels to the apical membrane region after AVT stimulation.

Protease modulation of the activity of the epithelial sodium channel expressed in Xenopus oocytes

We have investigated the effect of extracellular proteases on the amiloride-sensitive Na+ current (INa) in Xenopus oocytes expressing the three subunits alpha, beta, and gamma of the rat or Xenopus epithelial Na+ channel (ENaC). Low concentrations of trypsin (2 microg/ml) induced a large increase of INa within a few minutes, an effect that was fully prevented by soybean trypsin inhibitor, but not by amiloride. A similar effect was observed with chymotrypsin, but not with kallikrein. The trypsin-induced increase of INa was observed with Xenopus and rat ENaC, and was very large (approximately 20-fold) with the channel obtained by coexpression of the alpha subunit of Xenopus ENaC with the beta and gamma subunits of rat ENaC. The effect of trypsin was selective for ENaC, as shown by the absence of effect on the current due to expression of the K+ channel ROMK2. The effect of trypsin was not prevented by intracellular injection of EGTA nor by pretreatment with GTP-gammaS, suggesting that this effect was not mediated by G proteins. Measurement of the channel protein expression at the oocyte surface by antibody binding to a FLAG epitope showed that the effect of trypsin was not accompanied by an increase in the channel protein density, indicating that proteolysis modified the activity of the channel present at the oocyte surface rather than the cell surface expression. At the single channel level, in the cell-attached mode, more active channels were observed in the patch when trypsin was present in the pipette, while no change in channel activity could be detected when trypsin was added to the bath solution around the patch pipette. We conclude that extracellular proteases are able to increase the open probability of the epithelial sodium channel by an effect that does not occur through activation of a G protein-coupled receptor, but rather through proteolysis of a protein that is either a constitutive part of the channel itself or closely associated with it.

Vasopressin potentiates mineralocorticoid selectivity by stimulating 11 beta hydroxysteroid deshydrogenase in rat collecting duct

Arginine vasopressin (AVP) and corticosteroid hormones are involved in sodium reabsorption regulation in the renal collecting duct. Synergy between AVP and aldosterone has been well documented, although its mechanism remains unclear. Both aldosterone and glucocorticoid hormones bind to the mineralocorticoid receptor (MR), and mineralocorticoid selectivity depends on the MR-protecting enzyme 11 beta hydroxysteroid deshydrogenase (11-HSD), which metabolizes glucocorticoids into derivatives with low affinity for MR. We have investigated whether the activity of 11-HSD could be influenced by AVP and corticosteroid hormones. This study shows that in isolated rat renal collecting ducts, AVP increases 11-HSD catalytic activity. This effect is maximal at 10(-8) M AVP (a concentration clearly above the normal physiological range of AVP concentrations) and involves the V2 receptor pathway, while activation of protein kinase C or changes in intracellular calcium are ineffective. The stimulatory effect of AVP on 11-HSD is largely reduced after adrenalectomy, and is selectively restored by infusion of aldosterone, not glucocorticoids. We conclude that this synergy between AVP and aldosterone in controlling the activity of 11-HSD is likely to play a pivotal role in resetting mineralocorticoid selectivity, and hence sodium reabsorption capacities of the renal collecting duct.

Cross talk of bumetanide-sensitive and HCO3--dependent transporters activated by IBMX in renal epithelial A6 cells

We studied cAMP-dependent regulation of ion transport in aldosterone-untreated renal epithelial A6 cells by measuring short-circuit current (Isc). Biphasic increases in Isc, a transient phase followed by a sustained one, were elicited in response to 1 mm 3-isobutyl-1-methylxanthine (IBMX, an inhibitor of phosphodiesterase) which increased cytosolic cAMP concentration. IBMX increased the apical Cl- conductance. The sustained phase of Isc induced by IBMX was reduced by 50 microM bumetanide (Na+/K+/2 Cl- cotransporter inhibitor) or 100 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, an inhibitor of Cl-/HCO3- exchanger). Under the normal condition, the inhibitory effect of bumetanide was much larger than that of DIDS. On the other hand, under a low Cl- condition, the effect of DIDS was more effective than that of bumetanide. Further, under a Cl--free condition Na+/HCO3- symporter contributed to the IBMX-generated Isc. Taken together, our observations suggest that in A6 cells (i) IBMX stimulates Cl- secretion associated with an increase in apical Cl- conductances, (ii) the ionic components to generate the IBMX-induced Isc are mainly maintained by bumetanide-sensitive Na+/K+/2 Cl- cotransporter and DIDS-sensitive Cl-/HCO3- exchanger, (iii) Cl-/HCO3- exchanger coupled to Na+/HCO3- symporter under a low-Cl- condition or Na+/HCO3- symporter under a Cl--free condition contributes to the IBMX-induced Isc, compensating for diminishment of the Na+/K+/2Cl- cotransporter-mediated Cl- secretion, (iv) IBMX increases Cl- and HCO3- conductances in the apical membrane.

Phorbol 12-myristate 13-acetate down-regulates Na,K-ATPase independent of its protein kinase C site: decrease in basolateral cell surface area

The effect of protein kinase C (PKC) stimulation on the pump current (Ip) generated by the Na,K-ATPase was measured in A6 epithelia apically permeabilized with amphotericin B. Phorbol 12-myristate 13-acetate (PMA) produced a decrease in Ip carried by sodium pumps containing the endogenous Xenopus laevis or transfected Bufo marinus alpha 1 subunits (approximately 30% reduction within 25 min, maximum after 40 min) independent of the PKC phosphorylation site (T15A/S16A). In addition to this major effect of PMA, which was independent of the intracellular sodium concentration and was prevented by the PKC inhibitor bisindolylmaleimide GF 109203X (BIM), another BIM-resistant, PKC site-independent decrease was observed when the Ip was measured at low sodium concentrations (total reduction approximately 50% at 5 mM sodium). Using ouabain binding and cell surface biotinylation, stimulation of PKC was shown to reduce surface Na,K-ATPase by 14 to 20% within 25 min. The same treatment stimulated fluid phase endocytosis sevenfold and decreased by 16.5% the basolateral cell surface area measured by transepithelial capacitance measurements. In conclusion, PKC stimulation produces a decrease in sodium pump function which can be attributed, to a large extent, to a withdrawal of sodium pumps from the basolateral cell surface independent of their PKC site. This reduction of the number of sodium pumps is parallel to a decrease in basolateral membrane area.

pH modulates cAMP-induced increase in Na+ transport across frog skin epithelium

Apical membrane potential (Va), fractional apical membrane resistance (FRa), and/or intracellular pH (pHi) were measured in principal cells of isolated frog (Rana pipiens) skin with microelectrodes under short-circuit conditions. Apical exposure to 0.33 mM 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (cAMP) depolarized Va, decreased FRa and increased short-circuit current (Isc). cAMP-induced 50% larger effects on Va and Isc at external pH (pHo) of 8.0 than at pHo 6.4. Increasing pHo from 6.4 to 8.0 in presence of cAMP further depolarized Va and increased Isc. cAMP-induced effects on Va and Isc were observed in the absence of Cl- and HCO3- and in the presence of 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or 10 microM 5-(N-ethyl-N-isopropyl)amiloride (EIPA) or 1 microM 5-(N-methyl-N-isobutyl)amiloride (MIA). These data indicate that Na(+)-H+ exchange, Cl(-)-HCO3- exchange, and electrogenic Na(+)-(HCO3-)n cotransport are not involved in cAMP-induced increase in Isc. Apical exposure to 2 mM Cd2+ or Zn2+ depolarized Va, decreased FRa, increased Isc and increased pHi. In HCO(3-)-free solutions containing DIDS, unilateral replacement of apical Cl- by NO3- induced a fast transient depolarization of Va and an increase in Isc. These data suggest that potential-dependent changes in pHi are involved in increases in Isc. However, when changes in Va were minimized by pretreating the basolateral membrane with 25 or 75 mM K+, the cAMP-induced increase in Isc was not blocked. These data indicate that changes in pHi do not play a strict regulatory role but are only permissive in cAMP-induced effects on Isc.

Basolateral membrane chloride permeability of A6 cells: implication in cell volume regulation

The permeability to Cl- of the basolateral membrane (blm) was investigated in renal (A6) epithelial cells, assessing their role in transepithelial ion transport under steady-state conditions (isoosmotic) and following a hypoosmotic shock (i.e. in a regulatory volume decrease, RVD). Three different complementary studies were made by measuring: (1) the Cl- transport rates (delta F/Fo s-1 (x10(-3))), where F is the fluorescence of N-(6-methoxyquinoyl) acetoethyl ester, MQAE, and Fo the maximal fluorescence (x10(-3)) of both membranes by following the intracellular Cl- activities (ai Cl-, measured with MQAE) after extracellular Cl- substitution (2) the blm 86Rb and 36Cl uptakes and (3) the cellular potential and Cl- current using the whole-cell patch-clamp technique to differentiate between the different Cl- transport mechanisms. The permeability of the blm to Cl- was found to be much greater than that of the apical membranes under resting conditions: aiCl- changes were 5.3 +/- 0.7 mM and 25.5 +/- 1.05 mM (n = 79) when Cl- was substituted by NO3(-) in the media bathing apical and basolateral membranes. The Cl- transport rate of the blm was blocked by bumetanide (100 microM) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 50 microM) but not by N-phenylanthranilic acid (DPC, 100 microM). 86Rb and 36Cl uptake experiments confirmed the presence of a bumetanide- and a NPPB-sensitive Cl- pathway, the latter being approximately three times more important than the former (Na/K/2Cl cotransporter). Appli-cation of a hypoosmotic medium to the serosal side of the cell increased delta F/Fo s-1 (x10(-3)) after extracellular Cl- substitution (1.03 +/- 0.10 and 2.45 +/- 0.17 arbitrary fluorescent units s-1 for isoosmotic and hypoosmotic conditions respectively, n = 11); this delta F/Fo s-1 (x10(-3)) increase was totally blocked by serosal NPPB application; on the other hand, cotransporter activity was decreased by the hypoosmotic shock. Cellular Ca2+ depletion had no effect on delta F/Fo s-1 (x10(-3)) under isoosmotic conditions, but blocked the delta F/Fo s-1 (x10(-3)) increase induced by a hypoosmotic stress. Under isotonic conditions the measured cellular potential at rest was -37.2 +/- 4.0 mV but reached a maximal and transient depolarization of -25.1 +/- 3.7 mV (n = 9) under hypoosmotic conditions. The cellular current at a patch-clamping cellular potential of -85 mV (close to the Nernst equilibrium potential for K+) was blocked by NPPB and transiently increased by hypoosmotic shock (&ap;50% maximum increase). This study demonstrates that the major component of Cl- transport through the blm of the A6 monolayer is a conductive pathway (NPPB-sensitive Cl- channels) and not a Na/K/2Cl cotransporter. These channels could play a role in transepithelial Cl- absorption and cell volume regulation. The increase in the blm Cl- conductance, inducing a depolarization of these membranes, is proposed as one of the early events responsible for the stimulation of the 86Rb efflux involved in cell volume regulation.

Side-specific Toxic Effects on the Membranes of Cultured Renal Epithelial Cells (A6)

- A cultured epithelial cell line from toad kidney (A6) was used to investigate side-specific toxicity related to the apical (outer) and basolateral (inner) membranes of epithdia. Well-known inhibitors and stimulators of ion transport were used to show that the ion transport proteins are asymmetrically distributed: the apical membrane contains sodium and chloride channels and the basolateral membrane contains Na+/K+ pumps, Na+/Cl- co-transporters, potassium channels and receptors for antidiuretic hormone The data demonstrate that the cellular toxicity of chemicals decreases when they are added to the apical side, illustrating that the epithelium acts as a functional barrier. However, the side-specific toxicity was more pronounced for ions and water-soluble molecules than for organic solvents, indicating that A6 epitheha can be used to distinguish between drugs that target specific membrane proteins and those that target membrane lipids. Furthermore, the cell line could be used to pick up chemicals that, at low concentrations inhibit sodium absorption and chloride secretion, without having any effect on cellular toxicity.

Aldosterone interaction on sodium transport and chloride permeability: influence of epithelial structure

The effects of aldosterone on sodium transport and chloride permeability were investigated by electrophysiology in two structurally distinct epithelial used as models for the distal renal tubule: the A6 cell monolayer as compared with the amphibian skin epithelium (ASE). Short-circuit current (Isc) and transepithelial conductance (Gt) were measured in A6 monolayers incubated overnight with(out) aldosterone. Cell and shunt conductances (Gcell and Gsh) were also determined, as well as the conductive nature of the chloride pathway. These parameters were correlated with sodium and chloride fluxes in A6 cells (JNa and JCl) and compared with the data recorded across ASE (Bufo marinus). The existence of a cAMP-dependent chloride secretory pathway in A6 cells was also investigated upon exposition to arginine vasopressin (AVP) or oxytocin. When A6 monolayers were incubated with aldosterone, Gt significantly increased with respect to control preparations; this increase resulted solely from an increase in Gcell, and was reflected by a 3-fold increase in Isc. There was a significant relationship between Isc and Gcell, as well as between Isc and JNa in both control and aldosterone-stimulated preparations. The A6 clone used was devoid of cAMP-dependent chloride secretory activity and was unresponsive to AVP or oxytocin. Thus, comparison between ASE and A6 preparations revealed two major differences: unlike ASE, (i) aldosterone has no effect on Gsh and (ii) no conductive reabsorptive chloride pathway is operative in A6 monolayers tested. In addition, cobalt had no effect on electrical parameters of A6 monolayers. These observations show that difference in epithelial structure is reflected in terms of electrophysiological response to aldosterone, which suggests that cell heterogeneity could be a prerequisite for observing a conductive reabsorptive chloride pathway in aldosterone-responsive, sodium-transporting epithelia.

Aldosterone modulates sodium kinetics of Na,K-ATPase containing an alpha 1 subunit in A6 kidney cell epithelia

Short-term aldosterone (10(-6) M, 2.5 h) induces in A6-C1 cell epithelia an increase in Na transport, which is due to the in situ activation of the apical Na channel and, presumably, the basolateral Na pump (Na,K-ATPase). We have now directly measured the effect of aldosterone on the transport activity of endogenous Na pumps and hybrid Na pumps containing an exogenous alpha 1 subunit by measuring the pump current (Ip) across epithelia apically permeabilized with amphotericin B. Aldosterone (2.5 h) had no significant early effect on the maximal Ip, nor on the Na concentration required for half-maximal activation. In contrast, it increased the Ip at physiological intracellular Na concentrations (1.7-fold at 5 mM Na). This effect was blocked by the protein synthesis inhibitor cycloheximide. Hybrid pumps containing the transfected cardiotonic steroid-resistant alpha 1 subunit of Bufo marinus were also stimulated by aldosterone (2.5 h). A long aldosterone treatment (4 days) increased the maximal Ip produced by the endogenous pumps 1.5 to 2.1-fold. In conclusion, aldosterone acts on Na pumps containing an alpha 1 subunit in two ways. During its early phase of action it stimulates their transport activity by increasing their apparent Na affinity at physiological intracellular Na concentrations. In the long term it produces an increase in the maximal transport capacity, which corresponds to the known increase in the number of Na pumps.

Anion secretion by the inner medullary collecting duct. Evidence for involvement of the cystic fibrosis transmembrane conductance regulator

It is well established that the terminal renal collecting duct is capable of electrogenic Na+ absorption. The present experiments examined other active ion transport processes in primary cultures of the rat inner medullary collecting duct. When the amiloride analogue benzamil inhibited electrogenic Na+ absorption, cAMP agonists stimulated a transmonolayer short circuit current that was not dependent on the presence of Na+ in the apical solution, but was dependent on the presence of Cl- and HCO3-. This current was not inhibited by the loop diuretic bumetanide, but was inhibited by ouabain, an inhibitor of the Na+/K+ pump. The current was reduced by anion transport inhibitors, with a profile similar to that seen for inhibitors of the cystic fibrosis transmembrane conductance regulator (CFATR) Cl- channel. Using several PCR strategies, we demonstrated fragments of the predicted lengths and sequence identity with the rat CFTR. Using whole-cell patch-clamp analysis, we demonstrated a cAMP-stimulated Cl- current with characteristics of the CFTR. We conclude that the rat inner medullary collecting duct has the capacity to secrete anions. It is highly likely that the CFTR Cl- channel is involved in this process.