Basolateral potassium membrane permeability of A6 cells and cell volume regulation

Hypotonic shock modulates Na(+) current via a Cl(-) and Ca(2+)/calmodulin dependent mechanism in alveolar epithelial cells

Alveolar epithelial cells are involved in Na⁺ absorption via the epithelial Na⁺ channel (ENaC), an important process for maintaining an appropriate volume of liquid lining the respiratory epithelium and for lung oedema clearance. Here, we investigated how a 20% hypotonic shock modulates the ionic current in these cells. Polarized alveolar epithelial cells isolated from rat lungs were cultured on permeant filters and their electrophysiological properties recorded. A 20% bilateral hypotonic shock induced an immediate, but transient 52% rise in total transepithelial current and a 67% increase in the amiloride-sensitive current mediated by ENaC. Amiloride pre-treatment decreased the current rise after hypotonic shock, showing that ENaC current is involved in this response. Since Cl^- transport is modulated by hypotonic shock, its contribution to the basal and hypotonic-induced transepithelial current was also assessed. Apical NPPB, a broad Cl^- channel inhibitor and basolateral DIOA a potassium chloride co-transporter (KCC) inhibitor reduced the total and ENaC currents, showing that transcellular Cl^- transport plays a major role in that process. During hypotonic shock, a basolateral Cl^- influx, partly inhibited by NPPB is essential for the hypotonic-induced current rise. Hypotonic shock promoted apical ATP secretion and increased intracellular Ca²⁺. While apyrase, an ATP scavenger, did not inhibit the hypotonic shock current response, W7 a calmodulin antagonist completely prevented the hypotonic current rise. These results indicate that a basolateral Cl^- influx as well as Ca²⁺/calmodulin, but not ATP, are involved in the acute transepithelial current rise elicited by hypotonic shock.

Physiological significance of hypotonicity-induced regulatory volume decrease: reduction in intracellular Cl- concentration acting as an intracellular signaling

Regulatory volume decrease (RVD) occurs after hypotonicity-caused cell swelling. RVD is caused by activation of ion channels and transporters, which cause effluxes of K(+), Cl(-), and H(2)O, leading to cell shrinkage. Recently, we showed that hypotonicity stimulated transepithelial Na(+) reabsorption via elevation of epithelial Na(+) channel (alpha-ENaC) expression in renal epithelia A6 cells in an RVD-dependent manner and that reduction of intracellular Cl(-) concentration ([Cl(-)](i)) stimulated the Na(+) reabsorption. These suggest that RVD would reveal its stimulatory action on the Na(+) reabsorption by reducing [Cl(-)](i). However, the reduction of [Cl(-)](i) during RVD has not been definitely analyzed due to technical difficulties involved in halide-sensitive fluorescent dyes. In the present study, we developed a new method for the measurement of [Cl(-)](i) change during RVD by using a high-resolution flow cytometer with a halide-specific fluorescent dye, N-(6-methoxyquinolyl) acetoethyl ester. The [Cl(-)](i) in A6 cells in an isotonic medium was 43.6 +/- 3.1 mM. After hypotonic shock (268 to 134 mosmol/kgH(2)O), a rapid increase of cell volume followed by RVD occurred. The RVD caused drastic diminution of [Cl(-)](i) from 43.6 to 10.8 mM. Under an RVD-blocked condition with NPPB (Cl(-) channel blocker) or quinine (K(+) channel blocker), we did not detect the reduction of [Cl(-)](i). Based on these observations, we conclude that one of the physiological significances of RVD is the reduction of [Cl(-)](i) and that RVD shows its action via reduction of [Cl(-)](i) acting as an intracellular signal regulating cellular physiological functions.

Activation of extracellular signal-regulated kinase ERK after hypo-osmotic stress in renal epithelial A6 cells

Activation of mitogen-activated protein (MAP) kinases has been reported to occur after a hypo-osmotic cell swelling in various types of cells. In renal epithelial A6 cells, the hypo-osmotic shock induced a rapid increase in the phosphorylation of an extracellular signal-regulated kinase (ERK)-like protein that was maximal 10 min after osmotic stress. Activation of ERK was significantly increased when hypo-osmotic stress was performed in the absence of extracellular Ca2+, a condition that inhibits regulatory volume decrease (RVD). Exposure of cells to PD98059, an inhibitor of the MAP kinase kinase MEK, at a concentration that fully cancelled ERK activation, did not inhibit RVD. On the contrary, RVD was abolished when osmotic shock was induced in the presence of SB203580, an inhibitor of stress-activated protein kinases (SAPKs). These results suggest that different MAP kinases are activated after hypo-osmotic stress in A6 cells. SAPKs would be involved in the control of RVD, while ERK would lead to later events, such as gene expression or energy metabolism.

Modulation of calcium-dependent chloride secretion by basolateral SK4-like channels in a human bronchial cell line

The human bronchial cell line16HBE14o- was used as a model of airway epithelial cells to study the Ca(2+)-dependent Cl(-) secretion and the identity of K(Ca) channels involved in the generation of a favorable driving force for Cl(-) exit. After ionomycin application, a calcium-activated short-circuit current ( I(sc)) developed, presenting a transient peak followed by a plateau phase. Both phases were inhibited to different degrees by NFA, glybenclamide and NPPB but DIDS was only effective on the peak phase. (86)Rb effluxes through both apical and basolateral membranes were stimulated by calcium, blocked by charybdotoxin, clotrimazole and TPA. 1-EBIO, a SK-channel opener, stimulated (86)Rb effluxes. Block of basolateral K(Ca) channels resulted in I(sc) inhibition but, while reduced, I(sc) was still observed if mucosal Cl(-) was lowered. Among SK family members, only SK4 and SK1 mRNAs were detected by RT-PCR. KCNQ1 mRNAs were also identified, but involvement of K(cAMP) channels in Cl(-) secretion was unlikely, since cAMP application had no effect on (86)Rb effluxes. Moreover, chromanol 293B or clofilium, specific inhibitors of KCNQ1 channels, had no effect on cAMP-dependent I(sc). In conclusion, two distinct components of Cl(-) secretion were identified by a pharmacological approach after a Cai2+ rise. K(Ca) channels presenting the pharmacology of SK4 channels are present on both apical and basolateral membranes, but it is the basolateral SK4-like channels that play a major role in calcium-dependent chloride secretion in 16HBE14o- cells.

Recovery of cell volume and electrolytes of A6 cells after re-establishing isotonicity following hypotonic stress

Cellular element concentrations and dry weight contents in A6 cells were determined using electron microprobe analysis to establish whether these cells exhibit a regulatory volume increase (post-RVD-RVI) when re-establishing isotonicity following a hypotonically induced regulatory volume decrease (RVD). Hypotonic stress was induced by reducing basolateral [NaCl], and hence, osmolarity fell from 260 to 140 mosmol/l. The alterations in cell volume after re-establishing isotonicity, calculated from the cellular dry weight changes, indicate within the first 2 min cell shrinkage from 120 to 76% of control, compatible with almost ideal osmometric behaviour of A6 cells, and thereafter a post-RVD-RVI to 94%. The cellular uptake of osmolytes necessary to explain the post-RVD-RVI could be accounted for solely by a gain in cellular K and Cl. The involvement of a Na-K-2Cl cotransporter in most of the KCl uptake seems plausible since basolateral bumetanide blocked KCl uptake and post-RVD-RVI. The net uptake of cations (K uptake of 185.2, Na loss of 8.2 mmol/kg dry wt) during the isotonic period exceeded the Cl uptake by 38.2 mmol/kg dry wt, suggesting the uptake of another anion and/or the alteration of cellular buffer capacity. The relatively low Na concentration maintained during the isotonic period (13.3 vs. 20.4 mmol/kg wet wt under control conditions) might favour electrolyte uptake via the Na-K-2Cl cotransporter.

Cloning and characterisation of amphibian ClC-3 and ClC-5 chloride channels

Amphibians have provided important model systems to study transepithelial transport, acid-base balance and cell volume regulation. Several families of chloride channels and transporters are involved in these functions. The purpose of this review is to report briefly on some of the characteristics of the chloride channels so far reported in amphibian epithelia, and to focus on recently cloned members of the ClC family and their possible physiological roles. The electrophysiological characterisation, distribution, localisation and possible functions are reviewed and compared to their mammalian orthologs.

Hypotonic induction of SGK1 and Na+ transport in A6 cells

Serum and glucocorticoid-regulated kinase-1 (SGK1) is a serine-threonine kinase that is regulated at the transcriptional level by numerous regulatory inputs, including mineralocorticoids, glucocorticoids, follicle-stimulating hormone, and osmotic stress. In the distal nephron, SGK1 is induced by aldosterone and regulates epithelial Na+ channel-mediated transepithelial Na+ transport. In other tissues, including liver and shark rectal gland, SGK1 is regulated by hypertonic stress and is thought to modulate epithelial Na+ channel- and Na+-K+-2Cl- cotransporter-mediated Na+ transport. In this report, we examined the regulation of SGK1 mRNA and protein expression and Na+ currents in response to osmotic stress in A6 cells, a cultured cell line derived from Xenopus laevis distal nephron. We found that in contrast to hepatocytes and rectal gland cells, hypotonic conditions stimulated SGK1 expression and Na+ transport in A6 cells. Moreover, a correlation was found between SGK1 induction and the later phase of activation of Na+ transport in response to hypotonic treatment. When A6 cells were pretreated with an inhibitor of phosphatidylinositol 3-kinase (PI3K), Na+ transport was blunted and only inactive forms of SGK1 were expressed. Surprisingly, these results demonstrate that both hypertonic and hypotonic stimuli can induce SGK1 gene expression in a cell type-dependent fashion. Moreover, these data lend support to the view that SGK1 contributes to the defense of extracellular fluid volume and tonicity in amphibia by mediating a component of the hypotonic induction of distal nephron Na+ transport.

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.

Swelling-activated cation-selective channels in A6 epithelia are permeable to large cations

Effects of basolateral monovalent cation replacements (Na+ by Li+, K+, Cs+, methylammonium, and guanidinium) on permeability to 86Rb of volume-sensitive cation channels (VSCC) in the basolateral membrane and on regulatory volume decrease (RVD), elicited by a hyposmotic shock, were studied in A6 epithelia in the absence of apical Na+ uptake. A complete and quick RVD occurred only when the cells were perfused with Na+ or Li+ saline. With both cations, hypotonicity increased basolateral 86Rb release (RblRb), which reached a maximum after 15 min and declined back to control level. When the major cation was K+, Cs+, methylammonium, or guanidinium, the RVD was abolished. Methylammonium induced a biphasic time course of cell thickness (Tc), with an initial decline of Tc followed by a gradual increase. With K+, Cs+, or guanidinium, Tc increased monotonously after the rapid initial rise evoked by the hypotonic challenge. In the presence of K+, Cs+, or methylammonium, RblRb remained high during most of the hypotonic period, whereas with guanidinium blockage of RblRb was initiated after 6 min of hypotonicity, suggesting an intracellular location of the site of action. With all cations, 0.5 mM basolateral Gd3+ completely blocked RVD and fully abolished the RblRb increase induced by the hypotonic shock. The lanthanide also blocked the additional volume increase induced by Cs+, K+, guanidinium, or methylammonium. When pH was lowered from 7. 4 to 6.0, RVD and RblRb were markedly inhibited. This study demonstrates that the VSCCs in the basolateral membrane of A6 cells are permeable to K+, Rb+, Cs+, methylammonium, and guanidinium, whereas a marked inhibitory effect is exerted by Gd3+, protons, and possibly intracellular guanidinium.

Hypotonicity activates a lanthanide-sensitive pathway for K+ release in A6 epithelia

The nature of the pathway for K+ release activated during regulatory volume decrease (RVD) in A6 epithelia was investigated by measuring cell thickness (Tc) as an index of cell volume and by probing K+ efflux with 86Rb as tracer for K+ (RRb). Cell swelling was induced by sudden reduction of basolateral osmolality (from 260 to 140 mosmol/kgH2O). Experiments were performed in the absence of Na+ transport. Apical RRb was negligible in iso- and hyposmotic conditions. On the other hand, osmotic shock increased basolateral RRb (RblRb) rapidly, reaching a maximum 7 min after the peak in Tc. Quinine (0.5 mM) completely inhibited RVD and RblRb. Also verapamil (0.2 mM) impeded volume recovery considerably; lidocaine (0.2 mM) did not exert a noticeable effect. The K+ channel blocker Ba2+ (30 mM) delayed RVD but could not prevent complete volume recovery. Cs+ inhibited RVD noticeably at concentrations <40 mM. With large Cs+ concentrations (>40 mM), the initial osmometric swelling was followed by a gradual increase of Tc, suggesting activation of Cs+ influx. Chronic exposure of the basolateral surface to 0.5 mM La3+ or Gd3+ completely abolished RVD and RblRb. Acute administration of lanthanides at the time of osmolality decrease did not affect the initial phase of RVD and reduced RblRb only slightly. Apical Gd3+ exerted an inhibitory effect on RVD and RblRb. The effect of Gd3+ should therefore be localized at an intracellular site. The role of Ca2+ entry could be excluded by failure of extracellular Ca2+ removal to inhibit volume recovery. In contrast to lanthanides, chronically and acutely administered Mg2+ (0.5 mM) inhibited RVD and RblRb by approximately 50%. These data suggest that K+ excretion during RVD occurs through a rather poorly selective pathway that does not seem to be directly activated by membrane stretch.

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.

Regulatory volume decrease in a renal distal tubular cell line (A6). I. Role of K+ and Cl-

Changes in volume of A6 epithelial cells were monitored by recording cell thickness (Tc). The response of Tc to a reduction of the basolateral osmolality from 260 to 140 mosmol/kg was recorded while transepithelial Na+ transport was inhibited by 20 microM amiloride. With Cl--containing bathing media, this osmotic challenge elicited a rapid rise in Tc followed by a regulatory volume decrease (RVD). Substitution of SO4(2-) or gluconate for Cl- markedly reduced the RVD, whereas cells completely maintained their ability to regulate their volume after replacing Cl- by NO3(-). A conductive pathway for Cl- excretion is suggested, which is insensitive to NPPB [5-nitro-2-(3-phenylpropylamino)benzoic acid], an inhibitor of some types of Cl- channels. Ba2+ (5 or 20 mM) reduced the RVD. A more pronounced inhibition of the RVD was obtained with 500 microM quinine, a potent blocker of volume-activated K+ channels. K+-induced depolarization of the basolateral membranes of tissues incubated with SO4(2-)-containing solutions completely abolished the RVD. Noise analysis in the presence of Ba2+ showed the activation of an apical K+ conductive pathway. These results demonstrate that cell volume regulation is controlled by processes involving Cl- and K+ excretion through conductive pathways.

Volume regulation in a toad epithelial cell line: role of coactivation of K+ and Cl- channels

1. We have measured changes in cell volume, membrane potential and ionic currents in distal nephron A6 cells following a challenge with hypotonic solutions (HTS). 2. The volume increase induced by HTS is compensated by a regulatory volume decrease (RVD), which is inhibited by both 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) and quinine. Quinine (500 microM) completely blocked RVD, whereas 100 microM NPPB delayed and attenuated RVD. 3. The resting potential in A6 cells was -52.3 +/- 4.8 mV (n = 53), and shifted to -35.1 +/- 2.2 mV (n = 33) during HTS. 4. Resting membrane current in A6 cells was 0.35 +/- 0.12 pA pF-1 at -80 mV and 0.51 +/- 0.16 pA pF-1 at +80 mV (n = 5). During cell swelling these values increased to 11.5 +/- 1.1 and 29.3 +/- 2.8 pA pF-1 (n = 29), respectively. 5. Quinine (500 microM) completely blocked the HTS-activated current at -15 mV, the reversal potential for Cl- currents, but exerted only a small block at -100 mV (K+ equilibrium potential). NPPB (100 microM) inhibited the current at both potentials almost to the same extent. The HTS-induced net current reversed at -41 +/- 2.5 mV (n = 15), which is close to the measured resting potential during HTS. 6. The quinine-insensitive current reversed near the Cl- equilibrium potential. The quinine-sensitive current reversed near the K+ equilibrium potential. The respective conductances activated by HTS at the zero-current potential were 2.1 +/- 0.7 nS for K+ and 5.2 +/- 1.3 nS for Cl- (n = 15). 7. Single channel analysis unveiled activation of at least two different channels during HTS. A 36 pS channel reversing at the Cl- equilibrium potential showed increased open probability at depolarized potentials. HTS also activated a K+ channel with a 29 pS conductance in high-K+ extracellular solutions (130 mM) or 12 pS in 2.5 mM K+. 8. This coactivation of K+ and Cl- channels shifts the membrane potential towards a value between EK and ECl (the reversal potentials for K+ and Cl-), where a net efflux of Cl- (Cl- inward current) and K+ (K+ outward current) under zero-current conditions occurs. Block of either the K+ or the Cl- conductance will shift the zero-current potential towards the equilibrium potential of the unblocked channel, preventing net efflux of osmolytes and RVD. This coactivation of K+ and Cl- currents causes a shift of osmolytes out of the cells, which almost completely accounts for the observed RVD.

Na/Ca exchange in the basolateral membrane of the A6 cell monolayer: role in Cai homeostasis

The presence of a Na/Ca exchanger in A6 cells was investigated by measuring intracellular calcium (Cai) fluctuations and the 45Ca fluxes through the basolateral membranes (blm) of the cell monolayer. Removal of Na+ from the medium produced a transient increase in Cai followed by a regulatory phase returning Cai to control levels in 3-4 min, this phase being greatly accelerated (< 60 s) by NaCl addition (apparent Km of approximately 5 mM Na+). The Cai increase was only found with the Na(+)-free medium on the basolateral side of the cell monolayer. A twofold increase in the 45Ca influx was observed under these conditions. In Ca(2+)- depleted cells, the initial Cai increase after Ca2+ addition to the medium was greater when the putative Na/Ca exchanger was not functioning (i.e. in a Na(+)-free medium). 45Ca effluxes through the blm of the monolayer were greatly and transiently increased by a Na(+)-free medium on the serosal side and blocked by orthovanadate (1 mM). The Cai increased induced by a hypo-osmotic shock was greater in cells bathed in a Na(+)-medium, conditions expected to block the activity of the Na/Ca exchanger. These findings support the hypothesis that a Na/Ca exchanger is present on the blm of A6 cells and affirm its role in Cai homeostasis in steady-state conditions and following osmotic shock. In addition, a Ca2+ pump also located on the blm and Ca2+ stores sensitive to inositol 1,4,5-trisphosphate were found to be implicated in Cai homeostasis.