Activation of the Na+-K+ pump by hyposmolality through tyrosine kinase-dependent Cl- conductance in Xenopus renal epithelial A6 cells

Physiological roles of chloride ions in bodily and cellular functions

Physiological roles of Cl-, a major anion in the body, are not well known compared with those of cations. This review article introduces: (1) roles of Cl- in bodily and cellular functions; (2) the range of cytosolic Cl- concentration ([Cl-]c); (3) whether [Cl-]c could change with cell volume change under an isosmotic condition; (4) whether [Cl-]c could change under conditions where multiple Cl- transporters and channels contribute to Cl- influx and efflux in an isosmotic state; (5) whether the change in [Cl-]c could be large enough to act as signals; (6) effects of Cl- on cytoskeletal tubulin polymerization through inhibition of GTPase activity and tubulin polymerization-dependent biological activity; (7) roles of cytosolic Cl- in cell proliferation; (8) Cl--regulatory mechanisms of ciliary motility; (9) roles of Cl- in sweet/umami taste receptors; (10) Cl--regulatory mechanisms of with-no-lysine kinase (WNK); (11) roles of Cl- in regulation of epithelial Na+ transport; (12) relationship between roles of Cl- and H+ in body functions.

Effects of Genistein on Common Kidney Diseases

Genistein is a naturally occurring phytoestrogen (soy or soybean products) that is classified as an isoflavone, and its structure is similar to that of endogenous estrogens; therefore, genistein can exert an estrogen-like effect via estrogen receptors. Additionally, genistein is a tyrosine kinase inhibitor, which enables it to block abnormal cell growth and proliferation signals through the inhibition of tyrosine kinase. Genistein is also an angiogenesis inhibitor and an antioxidant. Genistein has effects on kidney cells, some of the kidney’s physiological functions, and a variety of kidney diseases. First, genistein exerts a protective effect on normal cells by reducing the inflammatory response, inhibiting apoptosis, inhibiting oxidative stress, inhibiting remodeling, etc., but after cell injury, the protective effect of genistein decreases or even has the opposite effect. Second, genistein can regulate renin intake to maintain blood pressure balance, regulate calcium uptake to regulate Ca²⁺ and Pi balances, and reduce vasodilation to promote diuresis. Third, genistein has beneficial effects on a variety of kidney diseases (including acute kidney disease, kidney cancer, and different chronic kidney diseases), such as reducing symptoms, delaying disease progression, and improving prognosis. Therefore, this paper reviews animal and human studies on the protective effects of genistein on the kidney in vivo and in vitro to provide a reference for clinical research in the future.

Interactive Actions of Aldosterone and Insulin on Epithelial Na+ Channel Trafficking

Epithelial Na⁺ channel (ENaC) participates in renal epithelial Na⁺ reabsorption, controlling blood pressure. Aldosterone and insulin elevate blood pressure by increasing the ENaC-mediated Na⁺ reabsorption. However, little information is available on the interactive action of aldosterone and insulin on the ENaC-mediated Na⁺ reabsorption. In the present study, we tried to clarify if insulin would modify the aldosterone action on the ENaC-mediated Na⁺ reabsorption from a viewpoint of intracellular ENaC trafficking. We measured the ENaC-mediated Na⁺ transport as short-circuit currents using a four-state mathematical ENaC trafficking model in renal A6 epithelial cells with or without aldosterone treatment under the insulin-stimulated and -unstimulated conditions. We found that: (A) under the insulin-stimulated condition, aldosterone treatment (1 µM for 20 h) significantly elevated the ENaC insertion rate to the apical membrane (kI) 3.3-fold and the ENaC recycling rate (kR) 2.0-fold, but diminished the ENaC degradation rate (kD) 0.7-fold without any significant effect on the ENaC endocytotic rate (kE); (B) under the insulin-unstimulated condition, aldosterone treatment decreased kE 0.5-fold and increased kR 1.4-fold, without any significant effect on kI or kD. Thus, the present study indicates that: (1) insulin masks the well-known inhibitory action of aldosterone on the ENaC endocytotic rate; (2) insulin induces a stimulatory action of aldosterone on ENaC apical insertion and an inhibitory action of aldosterone on ENaC degradation; (3) insulin enhances the aldosterone action on ENaC recycling; (4) insulin has a more effective action on diminution of ENaC endocytosis than aldosterone.

Change in hydration indices associated with an increase in total water intake of more than 0.5 L/day, sustained over 4 weeks, in healthy young men with initial total water intake below 2 L/day

This secondary data analysis addressed gaps in knowledge about effects of chronic water intake. Longitudinal data from the Adapt Study were used to describe effects of prescribing a sustained increase in water intake relative to baseline, for 4 weeks, on multiple indices of total body water (TBW) flux, regulation, distribution, and volume in five healthy, free-living, young men, with mean total water intake initially below 2 L/day. Indices were measured weekly. Within-person fixed effect models tested for significant changes in indices over time and associations between changes in indices. Agreement between indices was described. Mixed models tested if baseline between-person differences in hydration indices modified changes in indices over time. Body water flux: The half-life of water in the body decreased significantly. Body water regulation: Serum osmolality decreased significantly. Urine anti-diuretic hormone, sodium, potassium, and osmolality decreased significantly. Plasma aldosterone and serum sodium increased significantly. Body water distribution: No significant changes were observed. Body water volume: Saliva osmolality decreased significantly. Body weight increased significantly by a mean ± SEM of 1.8% ± 0.5% from baseline over 4 weeks. Changes in indices were significantly inter-correlated. Agreement between indices changed over 4 weeks. Baseline saliva osmolality significantly modified responses to chronic water intake. The results motivate hypotheses for future studies: Chronic TBW deficit occurs in healthy individuals under daily life conditions and increases chronic disease risk; Sustained higher water intake restores TBW through gradual isotonic retention of potassium and/or sodium; Saliva osmolality is a sensitive and specific index of chronic hydration status.

Raman micro-spectroscopy as a viable tool to monitor and estimate the ionic transport in epithelial cells

The typical response to the lowering of plasma Na⁺ concentration and blood pressure in our body involves the release of aldosterone from the adrenal glands, which triggers the reabsorption of sodium in the kidney. Although the effects of aldosterone on this physiological mechanism were extensively studied in the past decades, there are still some aspects to be fully elucidated. In the present study, we propose for the first time a new approach based on Raman spectroscopy to monitor the ionic activity in aldosterone-treated A6 renal epithelial cells. This spectroscopic technique is capable of probing the cells through their thickness in a non-destructive and nimble way. The spectroscopic variations of the Raman bands associated to the O-H stretching of water were correlated to the variations of ionic concentration in the intracellular and extracellular fluids. The increase of Na⁺ concentration gradients was clearly visualized in the cytosol of aldosterone-treated cells. The enhancement of the Na⁺ current density induced by aldosterone was estimated from the variation of the ionic chemical potential across the intracellular space. In addition, the variation of the O-H Raman bands of water was used to quantify the cell thickness, which was not affected by aldosterone.

Contribution of concentration-sensitive sodium channels to the absorption of alveolar fluid in mice

The concentration-sensitive sodium channel (Nac) is activated by an increase in the extracellular sodium concentration. Although the expression of Nac in alveolar type II epithelial cells (AEC II) has been reported previously, the physiological role of Nac in the lung has not been established. We characterized Nac expression and examined amiloride-insensitive sodium transport mediated by Nac in mouse lung. Immunofluorescence studies revealed that Nac did not colocalize with either aquaporin 5 or cystic fibrosis transmembrane conductance regulator, but partially colocalized with the epithelial sodium channel γ-subunit. Immunoelectron microscopy studies showed that Nac localized at the basolateral membrane of pulmonary microvascular endothelial cells (PMVECs). Nac mRNA and protein were expressed in PMVECs isolated from the lungs of mice. Image analysis indicated that sodium influx into the alveolar wall was dependent on increases in extracellular sodium concentration. We conclude that Nac expressed in PMVECs and AEC II contributes to the reabsorption of sodium via an amiloride-insensitive pathway during alveolar fluid clearance.

Simulation of Cl(-) Secretion in Epithelial Tissues: New Methodology Estimating Activity of Electro-Neutral Cl(-) Transporter

Transcellular Cl⁻ secretion is, in general, mediated by two steps; (1) the entry step of Cl⁻ into the cytosolic space from the basolateral space across the basolateral membrane by Cl⁻ transporters, such as Na⁺-K⁺-2Cl⁻ cotransporter (NKCC1, an isoform of NKCC), and (2) the releasing step of Cl⁻ from the cytosolic space into the luminal (air) space across the apical membrane via Cl⁻ channels, such as cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channel. Transcellular Cl⁻ secretion has been characterized by using various experimental techniques. For example, measurements of short-circuit currents in the Ussing chamber and patch clamp techniques provide us information on transepithelial ion movements via transcellular pathway, transepithelial conductance, activity (open probability) of single channel, and whole cell currents. Although many investigators have tried to clarify roles of Cl⁻ channels and transporters located at the apical and basolateral membranes in transcellular Cl⁻ secretion, it is still unclear how Cl⁻ channels/transporters contribute to transcellular Cl⁻ secretion and are regulated by various stimuli such as Ca²⁺ and cAMP. In the present study, we simulate transcellular Cl⁻ secretion using mathematical models combined with electrophysiological measurements, providing information on contribution of Cl⁻ channels/transporters to transcellular Cl⁻ secretion, activity of electro-neutral ion transporters and how Cl⁻ channels/transporters are regulated.

Insulin is involved in transcriptional regulation of NKCC and the CFTR Cl(-) channel through PI3K activation and ERK inactivation in renal epithelial cells

It is is well known that insulin stimulates glucose transport and epithelial Na(+) channel (ENaC)-mediated Na(+) reabsorption; however, the action of insulin on Cl(-) secretion is not fully understood. In this study, we investigated the action of insulin on Na(+)-K(+)-2Cl(-) cotransporter (NKCC)-mediated Cl(-) secretion in epithelial A6 cells. Interestingly, insulin treatment remarkably enhanced the forskolin-stimulated Cl(-) secretion associated with an increase in apical Cl(-) conductance by upregulating mRNA expression of both CFTR and NKCC, although insulin treatment alone had no effect on the basal Cl(-) secretion or apical Cl(-) conductance without forskolin application. We next elucidated a role of phosphoinositide 3-kinase (PI3K) in the insulin-induced enhancement of the Cl(-) secretion, since insulin actually activated PI3K, resulting in activation of Akt, a downstream molecule of PI3K. LY294002 (a PI3K inhibitor) reduced the Cl(-) secretion by suppressing mRNA expression of NKCC, whereas insulin still had a stimulatory action on mRNA expression of CFTR even in the presence of LY294002. On the other hand, we found that a MEK inhibitor (PD98059) further enhanced the insulin-stimulated CFTR mRNA expression and the Cl(-) secretion in forskolin-stimulated A6 cells and that insulin induced slight, transient activation of ERK followed by significant inactivation of ERK. These observations suggest that: (1) insulin respectively upregulates mRNA expression of NKCC and CFTR through activation of PI3K and inactivation of ERK; (2) insulin signals on mRNA expression of NKCC and CFTR are not enough to stimulate transepithelial Cl(-) secretion, but enhance the stimulatory action of cAMP on transepithelial Cl(-) secretion.

Distinct action of flavonoids, myricetin and quercetin, on epithelial Cl⁻ secretion: useful tools as regulators of Cl⁻ secretion

Epithelial Cl⁻ secretion plays important roles in water secretion preventing bacterial/viral infection and regulation of body fluid. We previously suggested that quercetin would be a useful compound for maintaining epithelial Cl⁻ secretion at a moderate level irrespective of cAMP-induced stimulation. However, we need a compound that stimulates epithelial Cl⁻ secretion even under cAMP-stimulated conditions, since in some cases epithelial Cl⁻ secretion is not large enough even under cAMP-stimulated conditions. We demonstrated that quercetin and myricetin, flavonoids, stimulated epithelial Cl⁻ secretion under basal conditions in epithelial A6 cells. We used forskolin, which activates adenylyl cyclase increasing cytosolic cAMP concentrations, to study the effects of quercetin and myricetin on cAMP-stimulated epithelial Cl⁻ secretion. In the presence of forskolin, quercetin diminished epithelial Cl⁻ secretion to a level similar to that with quercetin alone without forskolin. Conversely, myricetin further stimulated epithelial Cl⁻ secretion even under forskolin-stimulated conditions. This suggests that the action of myricetin is via a cAMP-independent pathway. Therefore, myricetin may be a potentially useful compound to increase epithelial Cl⁻ secretion under cAMP-stimulated conditions. In conclusion, myricetin would be a useful compound for prevention from bacterial/viral infection even under conditions that the amount of water secretion driven by cAMP-stimulated epithelial Cl⁻ secretion is insufficient.

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.

Hypotonic stress upregulates β- and γ-ENaC expression through suppression of ERK by inducing MKP-1

We investigated a physiological role for ERK, a member of the MAPK family, in the hypotonic stimulation of epithelial Na(+) channel (ENaC)-mediated Na(+) reabsorption in renal epithelial A6 cells. We show that hypotonic stress causes a major dephosphorylation of ERK following a rapid transient phosphorylation. PD98059 (a MEK inhibitor) increases dephosphorylated ERK and enhances the hypotonic-stress-stimulated Na(+) reabsorption. ERK dephosphorylation is mediated by MAPK phosphatase (MKP). Hypotonic stress activates p38, which in turn induces MKP-1 and to a lesser extent MKP-3 mRNA expression. Inhibition of p38 suppresses MKP-1 induction, preventing hypotonic stress from dephosphorylating ERK. Inhibition of MKP-1 and -3 by the inhibitor NSC95397 also suppresses the hypotonicity-induced dephosphorylation of ERK. NSC95397 reduces both β- and γ-ENaC mRNA expression and ENaC-mediated Na(+) reabsorption stimulated by hypotonic stress. In contrast, pretreatment with PD98059 significantly enhances mRNA and protein expression of β- and γ-ENaC even under isotonic conditions. However, PD98059 only stimulates Na(+) reabsorption in response to hypotonic stress, suggesting that ERK inactivation by itself (i.e., under isotonic conditions) is not sufficient to stimulate Na(+) reabsorption, even though ERK inactivation enhances β- and γ-ENaC expression. Based on these results, we conclude that hypotonic stress stimulates Na(+) reabsorption through at least two signaling pathways: 1) induction of MKP-1 that suppresses ERK activity and induces β- and γ-ENaC expression, and 2) promotion of translocation of the newly synthesized ENaC to the apical membrane.

Hyposmotic stress causes ATP release and stimulates Na,K-ATPase activity in porcine lens

Purinergic receptors in lens epithelium suggest lens function can be altered by chemical signals from aqueous humor or the lens itself. Here we show release of ATP by intact porcine lenses exposed to hyposmotic solution (200 mOsm). 18α-glycyrrhetinic acid (AGA) added together with probenecid eliminated the ATP increase. N-ethylmaleimide (200 µM), an exocytotic inhibitor, had no significant effect on ATP increase. Lenses exposed to hyposmotic solution displayed a ~400% increase of propidium iodide (PI) entry into the epithelium. The increased ability of PI (MW 668) to enter the epithelium suggests possible opening of connexin and/or pannexin hemichannels. This is consistent with detection of connexin 43, connexin 50, and pannexin 1 in the epithelium and the ability of AGA + probenecid to prevent ATP release. Na,K-ATPase activity doubled in the epithelium of lenses exposed to hyposmotic solution. The increase of Na,K-ATPase activity did not occur when apyrase was used to prevent extracellular ATP accumulation or when AGA + probenecid prevented ATP release. The increase of Na,K-ATPase activity was inhibited by the purinergic P2 antagonist reactive blue-2 and pertussis toxin, a G-protein inhibitor, but not by the P2X antagonist PPADS. Hyposmotic solution activated Src family kinase (SFK) in the epithelium, judged by Western blot. The SFK inhibitor PP2 abolished both SFK activation and the Na,K-ATPase activity increase. In summary, hyposmotic shock-induced ATP release is sufficient to activate a purinergic receptor- and SFK-dependent mechanism that stimulates Na,K-ATPase activity. The responses might signify an autoregulatory loop initiated by mechanical stress or osmotic swelling.

Regulation of epithelial sodium transport via epithelial Na+ channel

Renal epithelial Na⁺ transport plays an important role in homeostasis of our body fluid content and blood pressure. Further, the Na⁺ transport in alveolar epithelial cells essentially controls the amount of alveolar fluid that should be kept at an appropriate level for normal gas exchange. The epithelial Na⁺ transport is generally mediated through two steps: (1) the entry step of Na⁺ via epithelial Na⁺ channel (ENaC) at the apical membrane and (2) the extrusion step of Na⁺ via the Na⁺, K⁺-ATPase at the basolateral membrane. In general, the Na⁺ entry via ENaC is the rate-limiting step. Therefore, the regulation of ENaC plays an essential role in control of blood pressure and normal gas exchange. In this paper, we discuss two major factors in ENaC regulation: (1) activity of individual ENaC and (2) number of ENaC located at the apical membrane.

Capsaicinoids regulate airway anion transporters through Rho kinase- and cyclic AMP-dependent mechanisms

To investigate the effects of capsaicinoids on airway anion transporters, we recorded and analyzed transepithelial currents in human airway epithelial Calu-3 cells. Application of capsaicin (100 μM) attenuated vectorial anion transport, estimated as short-circuit currents (I(SC)), before and after stimulation by forskolin (10 μM) with concomitant reduction of cytosolic cyclic AMP (cAMP) levels. The capsaicin-induced inhibition of I(SC) was also observed in the response to 8-bromo-cAMP (1 mM, a cell-permeable cAMP analog) and 3-isobutyl-1-methylxanthine (1 mM, an inhibitor of phosphodiesterases). The capsaicin-induced inhibition of I(SC) was attributed to suppression of bumetanide (an inhibitor of the basolateral Na(+)-K(+)-2 Cl(-) cotransporter 1)- and 4,4'-dinitrostilbene-2,2'-disulfonic acid (an inhibitor of basolateral HCO(3)(-)-dependent anion transporters)-sensitive components, which reflect anion uptake via basolateral cAMP-dependent anion transporters. In contrast, capsaicin potentiated apical Cl(-) conductance, which reflects conductivity through the cystic fibrosis transmembrane conductance regulator, a cAMP-regulated Cl(-) channel. All these paradoxical effects of capsaicin were mimicked by capsazepine. Forskolin application also increased phosphorylated myosin phosphatase target subunit 1, and the phosphorylation was prevented by capsaicin and capsazepine, suggesting that these capsaicinoids assume aspects of Rho kinase inhibitors. We also found that the increments in apical Cl(-) conductance were caused by conventional Rho kinase inhibitors, Y-27632 (20 μM) and HA-1077 (20 μM), with selective inhibition of basolateral Na(+)-K(+)-2 Cl(-) cotransporter 1. Collectively, capsaicinoids inhibit cAMP-mediated anion transport through down-regulation of basolateral anion uptake, paradoxically accompanied by up-regulation of apical cystic fibrosis transmembrane conductance regulator-mediated anion conductance. The latter is mediated by inhibition of Rho-kinase, which is believed to interact with actin cytoskeleton.

Regulation of paracellular Na+ and Cl(-) conductances by hydrostatic pressure

The effect of hydrostatic pressure on the paracellular ion conductance (Gp) composed of the Na(+) conductance (G(Na)) and the Cl(-) conductance (G(Cl)) has been Investigated. Gp, G(Na) and G(Cl) were time-dependently increased after applying an osmotic gradient generated by NaCl with basolateral hypotonicity. Hydrostatic pressure (1-4cm H2O) applied from the basolateral side enhanced the osmotic gradient-induced increase in Gp, G(Na) and G(Cl) in a magnitude-dependent manner, while the hydrostatic pressure applied from the apical side diminished the osmotic gradient-induced increase in Gp, G(Na) and G(Cl). How the hydrostatic pressure influences Gp, G(Na) and G(Cl) under an isosmotic condition was also investigated. Gp, G(Na) and G(Cl) were stably constant under a condition with basolateral application of sucrose canceling the NaCl-generated osmotic gradient (an isotonic condition). Even under this stable condition, the basolaterally applied hydrostatic pressure drastically elevated Gp, G(Na) and G(Cl), while apically applied hydrostatic pressure had little effect on Gp, G(Na) or G(Cl). Taken together, these observations suggest that certain factors controlled by the basolateral osmolality and the basolaterally applied hydrostatic pressure mainly regulate the Gp, G(Na) and G(Cl).

Regulation of paracellular ion conductances by NaCl gradients in renal epithelial cells

In the present study, we clarified how the NaCl gradient across the epithelial cells regulates the paracellular ion conductance. Under isotonic conditions, the absorption-directed NaCl gradient elevated the paracellular conductances of Na(+) (G(Na)) and Cl(-) (G(Cl)), while the secretion-directed NaCl gradient diminished the G(Na) and G(Cl). We further investigated the paracellular ionic conductances of NMDG (G(NMDG)) and gluconate (G(gluconate)) by replacing Na(+) with NMDG or Cl(-) with gluconate. The G(NMDG) was lower than the G(Na) and the replacement of Na(+) with NMDG decreased the G(Cl). The G(gluconate) was lower than the G(Cl) and the replacement of Cl(-) with gluconate also decreased the G(Na). These observations suggest the interaction of cations and anions on paracellular ionic conductances; i.e., cations affect paracellular anion conductances and anions affect paracellular cation conductances.

Intracellular calcium plays a role as the second messenger of hypotonic stress in gene regulation of SGK1 and ENaC in renal epithelial A6 cells

In A6 cells, a renal cell line derived from Xenopus laevis, hypotonic stress stimulates the amiloride-sensitive Na(+) transport. Hypotonic action on Na(+) transport consists of two phases, a nongenomic early phase and a genomic delayed phase. Although it has been reported that, during the genomic phase, hypotonic stress stimulates transcription of Na(+) transport-related genes, such as serum- and glucocorticoid-inducible kinase 1 (SGK1) and subunits of the epithelial Na(+) channel (ENaC), increasing Na(+) transport, the mechanism remains unknown. We focused the present study on the role of intracellular Ca(2+) in hypotonicity-induced SGK1 and ENaC subunit transcription. Since hypotonic stress raises intracellular Ca(2+) concentration in A6 cells, we hypothesized that Ca(2+)-dependent signals participate in the genomic action. Using real-time quantitative RT-PCR and Western blot techniques and measuring short-circuit currents, we observed that 1) BAPTA-AM and W7 blunted the hypotonicity-induced expression of SGK1 mRNA and protein, 2) ionomycin dose dependently stimulated expression of SGK1 mRNA and protein under an isotonic condition and the time course of the stimulatory effect of ionomycin on SGK1 mRNA was remarkably similar to that of hypotonic action on SGK1 mRNA, 3) hypotonic stress stimulated transcription of three ENaC subunits in an intracellular Ca(2+)-dependent manner, and 4) BAPTA-AM retarded the delayed phase of hypotonic stress-induced Na(+) transport but had no effect on the early phase. These observations indicate for the first time that intracellular Ca(2+) plays a role as the second messenger in hypotonic stress-induced Na(+) transport by stimulating transcription of SGK1 and ENaC subunits.

Epithelial Na+ channel and ion transport in human nasal polyp and paranasal sinus mucosa

The purpose of the present study is to characterize the ENaC-mediated Na+ absorption in human upper airway epithelia, nasal cavity, and paranasal sinus. To perform the purpose, we obtained epithelial cells from human nasal polyp (NP) and paranasal sinus mucosa (PSM) by endoscopic surgery. We measured the short-circuit current (I(sc)) sensitive to benzamil (a specific ENaC blocker). The benzamil-sensitive I(sc) (Na+ absorption) in NP was larger than that in PSM. The mRNA expression of three subunits of ENaC was as follows: alpha>beta>gamma in both tissue, NP and MS. The mRNA expression of gamma subunit of ENaC in NP was larger than that in PSM, but no difference of mRNA expression of alpha or beta ENaC subunit between NP and PSM was detected. We found correlation of the Na+ absorption to mRNA expression of gamma ENaC in NP and PSM. Forskolin diminished the Na+ absorption associated with an increase in Cl- secretion. These observations suggest that: (1) human NP absorbs more ENaC-mediated Na+ than PSM, (2) expression of gamma ENaC in plays a key role in the ENaC-mediated Na+ absorption in NP and PSM, and (3) cAMP diminishes the ENaC-mediated Na+ absorption by stimulating Cl- secretion (diminution of driving force for Na+ absorption) in NP and PSM.

Aldosterone-induced modification of osmoregulated ENaC trafficking

Aldosterone and osmotic stress are well known to regulate the epithelial Na(+) channel (ENaC)-mediated Na(+) transport in renal epithelial cells. However, we have no information on how aldosterone and osmotic stress interact on stimulation of ENaC-mediated Na(+) transport in renal epithelium. In the present report, we studied how application of aldosterone (1 microM for 1 day) modifies the action of hypotonic stress on the ENaC-mediated Na(+) transport in renal A6 epithelial cells by measuring the benzamil (a specific inhibitor for ENaC)-sensitive short-circuit current. The present study suggests that: (1) most ENaCs in cells without aldosterone treatment are translocated to Golgi apparatus, (2) major parts of aldosterone-generated ENaCs are located at the endoplasmic reticulum, (3) aldosterone diminishes the endocytosis rate of ENaCs from the apical membrane without any significant changes in the insertion rate of ENaCs into the apical membrane, and (4) application of sucrose after hypotonic stress stimulates the endocytosis of ENaCs, and elongates the functional life time of ENaCs by enhancing recycle of ENaCs into the endoplasmic reticulum in a retrograde manner.

Involvement of p38 MAPK in hypotonic stress-induced stimulation of beta- and gamma-ENaC expression in renal epithelium

We investigated a role of p38 MAPK in the regulation of transepithelial Na(+) reabsorption by chronic application (20-24h) of hypotonicity (hypotonic stress) in renal epithelial A6 cells. Pretreatment with a specific p38 MAPK inhibitor (SB202190) significantly reduced the chronic hypotonicity-stimulated transepithelial Na(+) reabsorption by diminishing the Na(+) entry through epithelial Na(+) channel (ENaC) in the apical membrane and the Na(+) extrusion via the Na(+)/K(+) ATPase (pump), although the rate limiting step was still the Na(+) entry step. We further examined whether the inhibitory effects of SB202190 on the transepithelial Na(+) reabsorption is caused through suppression of mRNA expression of ENaC participating in the transepithelial Na(+) reabsorption as the Na(+) entry pathway. The chronic hypotonicity increased the mRNA expression of alpha-, beta-, and gamma-subunits of ENaC. Moreover, we found that inhibition of p38 MAPK by SB202190 diminished the mRNA expression of beta- and gamma-ENaC but not alpha-ENaC. Based on these observations, it is suggested that the chronic hypotonicity stimulates the renal transepithelial Na(+) reabsorption by upregulating the mRNA expression of beta- and gamma-ENaC via a p38 MAPK-dependent pathway.

Action of neltenexine on anion secretion in human airway epithelia

Neltenexine has been applied to human lung diseases such as chronic obstructive pulmonary disease (COPD) as a mucolytic agent. However, we have no information on the neltenexine action in bronchial epithelial cells. We studied the neltenexine action on the ion transport in human submucosal serous Calu-3 cells. Under a hyper-secreting condition caused by terbutaline (a beta2-adrenergic agonist), neltenexine diminished anion secretion by inhibiting the Cl- and HCO3- uptake via Na+/K+/2Cl- cotransporter and Na+/HCO3- cotransporter without blockade of the cystic fibrosis transmembrane conductance regulator (CFTR) channel, and also diminished anion secretion via stimulation of Cl-/HCO3- exchanger, which facilitates the extrusion of more CFTR-permeant anion, Cl-, with the uptake of less CFTR-permeant anion, HCO3-. Thus, neltenexine reduced the hyper-secretion to keep an appropriate fluid level in the airway, providing a possibility that neltenexine can be an effective drug in airway obstructive diseases by decreasing the airway resistance under a hyper-secreting condition.

Hypotonicity stimulates renal epithelial sodium transport by activating JNK via receptor tyrosine kinases

We previously reported that hypotonic stress stimulated transepithelial Na(+) transport via a pathway dependent on protein tyrosine kinase (PTK; Niisato N, Van Driessche W, Liu M, Marunaka Y. J Membr Biol 175: 63-77, 2000). However, it is still unknown what type of PTK mediates this stimulation. In the present study, we investigated the role of receptor tyrosine kinase (RTK) in the hypotonic stimulation of Na(+) transport. In renal epithelial A6 cells, we observed inhibitory effects of AG1478 [an inhibitor of the EGF receptor (EGFR)] and AG1296 [an inhibitor of the PDGF receptor (PDGFR)] on both the hypotonic stress-induced stimulation of Na(+) transport and the hypotonic stress-induced ligand-independent activation of EGFR. We further studied whether hypotonic stress activates members of the MAP kinase family, ERK1/2, p38 MAPK, and JNK/SAPK, via an RTK-dependent pathway. The present study indicates that hypotonic stress induced phosphorylation of ERK1/2 and JNK/SAPK, but not p38 MAPK, that the hypotonic stress-induced phosphorylation of ERK1/2 and JNK/SAPK was diminished by coapplication of AG1478 and AG1296, and that only JNK/SAPK was involved in the hypotonic stimulation of Na(+) transport. A further study using cyclohexamide (a protein synthesis inhibitor) suggests that both RTK and JNK/SAPK contributed to the protein synthesis-independent early phase in hypotonic stress-induced Na(+) transport, but not to the protein synthesis-dependent late phase. The present study also suggests involvement of phosphatidylinositol 3-kinase (PI3-kinase) in RTK-JNK/SAPK cascade-mediated Na(+) transport. These observations indicate that 1) hypotonic stress activates JNK/SAPK via RTKs in a ligand-independent pathway, 2) the RTK-JNK/SAPK cascade acts as a mediator of hypotonic stress for stimulation of Na(+) transport, and 3) PI3-kinase is involved in the RTK-JNK/SAPK cascade for the hypotonic stress-induced stimulation of Na(+) transport.

Epithelial ion transport of human nasal polyp and paranasal sinus mucosa

Nasal cavity and paranasal sinus have various functions. However, little information is available on ion transport in these upper airway epithelia. In the present study, we measured the anion secretion and the anion channel activity to characterize the ion transport in epithelial cells prepared from human paranasal sinus mucosa (PSM) and nasal polyp (NP). To estimate the anion secretion and the anion channel activity, we measured the short-circuit current (Isc) and the transepithelial conductance (Gt) sensitive to NPPB (a Cl(-) channel blocker). The NPPB-sensitive Isc in PSM was larger than that in NP, correlating to the NPPB-sensitive Gt (Cl(-) channel activity). Forskolin stably elevated the NPPB-sensitive Isc associated with an increase in the NPPB-sensitive Gt in PSM and NP. UTP transiently stimulated the Isc associated with an elevation of Gt in PSM and NP. The stimulatory action of UTP on Isc and Gt was diminished by application of NPPB but not benzamil in PSM and NP, suggesting that UTP induced the NPPB-sensitive Isc (Cl(-) secretion) and Gt (Cl(-) channel activity). These observations suggest that in human PSM and NP, cAMP stably stimulates anion secretion by activating the Cl(-) (anion) channels, and that UTP just transiently elevates anion secretion via activation of some Cl(-) (anion) channels.

Ambroxol-induced modification of ion transport in human airway Calu-3 epithelia

Ambroxol is often used as a mucolytic agent in various lung diseases. However, it is unclear how ambroxol acts on bronchial epithelial cells. To clarify the action of ambroxol, we studied the effects of ambroxol on the ion transport in human Calu-3 cells, a human submucosal serous cell line, measuring the transepithelial short-circuit current and conductance across monolayers of Calu-3 cells. Ambroxol of 100 microM diminished the terbutaline (a beta2-adrenergic agonist)-stimulated Cl-/HCO3(-)-dependent secretion without any decreases in the conductance of cystic fibrosis transmembrane conductance regulator (CFTR) channel locating on the apical membrane. On the other hand, under the basal (unstimulated) condition ambroxol increased the Cl(-)-dependent secretion with no significant change in the apical CFTR channel conductance and decreased the HCO3- secretion associated with a decrease in the apical CFTR channel conductance. Ambroxol had no major action on the epithelial Na+ channel (ENaC) or the ENaC-mediated Na+ absorption. These results indicate that in Calu-3 cells: (1) under the basal (unstimulated) condition ambroxol increases Cl- secretion by stimulating the entry step of Cl- and decreases HCO3- secretion by diminishing the activity of the CFTR channel and/or the Na+/HCO3(-)-dependent cotransporter, (2) under the adrenergic agonist-stimulated condition, ambroxol decreases Cl- secretion by acting on the Cl-/HCO3- exchanger, and (3) ambroxol has a more powerful action than the adrenergic agonist on the Cl-/HCO3- exchanger, leading fluid secretion to a moderately stimulated level from a hyper-stimulated level.

Quercetin and NPPB-induced diminution of aldosterone action on Na+ absorption and ENaC expression in renal epithelium

In renal epithelial A6 cells, aldosterone applied for 24 h increased the transepithelial Cl- secretion over 30-fold due to activation of the Na+/K+/2Cl- cotransporter and stimulated the transepithelial Na+ absorption, activity of epithelial Na+ channel (ENaC), and alpha-ENaC mRNA expression. The stimulatory action of aldosterone on the transepithelial Na+ absorption, ENaC activity, and alpha-ENaC mRNA expression was diminished by 24h-pretreatment with quercetin (an activator of Na+/K+/2Cl- cotransporter participating in Cl- entry into the cytosolic space) or 5-nitro 2-(3-phenylpropylamino)benzoate (NPPB) (a blocker of Cl- channel participating in Cl- release from the cytosolic space), while 24h-pretreatment with bumetanide (a blocker of Na+/K+/2Cl- cotransporter) enhanced the stimulatory action of aldosterone on transepithelial Na+ absorption. On the other hand, under the basal (aldosterone-unstimulated) condition, quercetin, NPPB or bumetanide had no effect on transepithelial Na+ absorption, activity of ENaC or alpha-ENaC mRNA expression. These observations suggest that although aldosterone shows overall its stimulatory action on ENaC (transepithelial Na+ transport), aldosterone has an inhibitory action on ENaC (transepithelial Na+ transport) via activation of the Na+/K+/2Cl- cotransporter, and that modification of activity of Cl- transporter/channel participating in the transepithelial Cl- secretion influences the aldosterone-stimulated ENaC (transepithelial Na+ transport).

Synergic action of insulin and genistein on Na+/K+/2Cl − cotransporter in renal epithelium

Transepithelial Cl(-) secretion in polarized renal A6 cells is composed of two steps: (1) Cl(-) entry step across the basolateral membrane mediated by Na(+)/K(+)/2Cl(-) cotransporter (NKCC) and (2) Cl(-) releasing step across the apical membrane via cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. We estimated CFTR Cl(-) channel activity and transcellular Cl(-) secretion by measuring 5-nitro 2-(3-phenylpropylamino)benzoate (NPPB, a blocker of CFTR Cl(-) channel)-sensitive transepithelial conductance (Gt) and short-circuit current (Isc), respectively. Pretreatment with 1 microM insulin for 24 h had no effects on NPPB-sensitive Gt or Isc. On the other hand, in A6 cells treated with carbobenzoxy-L-leucyl-leucyl-L-leucinal (MG132; 100 microM for 2 h) that inhibits endocytosis of proteins at the plasma membrane into the cytosolic space, insulin pretreatment increased the NPPB-sensitive Isc with no effects on NPPB-sensitive Gt. Genistein (100 microM) induced sustained increases in NPPB-sensitive Gt and Isc, which were diminished by brefeldin A (a blocker of protein translocation to Golgi apparatus from endoplasmic reticulum). Co-application of insulin and genistein synergically stimulated the NPPB-sensitive Isc without any effects on NPPB-sensitive Gt. These observations suggest that: (1) insertion and endocytosis of NKCC are stimulated by insulin, (2) the insulin-induced stimulation of NKCC insertion into the basolateral membrane is offset by the stimulatory action on NKCC endocytosis from the basolateral membrane, (3) genistein stimulates insertion of both CFTR Cl(-) channel into the apical membrane and NKCC into the basolateral membrane, and (4) insulin and genistein synergically stimulated NKCC insertion into the basolateral membrane.

Extracellular Ca2+regulates the stimulation of Na+transport in A6 renal epithelia

We investigated the involvement of intracellular and extracellular Ca2+in the stimulation of Na+transport during hyposmotic treatment of A6 renal epithelia. A sudden osmotic decrease elicits a biphasic stimulation of Na+transport, recorded as increase in amiloride-sensitive short-circuit current ( Isc) from 3.4 ± 0.4 to 24.0 ± 1.3 μA/cm2( n = 6). Changes in intracellular Ca2+concentration ([Ca2+]i) were prevented by blocking basolateral Ca2+entry with Mg2+and emptying the intracellular Ca2+stores before the hyposmotic challenge. This treatment did not noticeably affect the hypotonicity-induced stimulation of Isc. However, the absence of extracellular Ca2+severely attenuated Na+transport stimulation by the hyposmotic shock, and Iscmerely increased from 2.2 ± 0.3 to 4.8 ± 0.7 μA/cm2. Interestingly, several agonists of the Ca2+-sensing receptor, Mg2+(2 mM), Gd3+(0.1 mM), neomycin (0.1 mM), and spermine (1 mM) were able to substitute for extracellular Ca2+. When added to the basolateral solution, these agents restored the stimulatory effect of the hyposmotic solutions on Iscin the absence of extracellular Ca2+to levels that were comparable to control conditions. None of the above-mentioned agonists induced a change in [Ca2+]i. Quinacrine, an inhibitor of PLA2, overruled the effect of the agonists on Na+transport. In conclusion, we suggest that a Ca2+-sensing receptor in A6 epithelia mediates the stimulation of Na+transport without the interference of changes in [Ca2+]i.

Cross talk of cAMP and flavone in regulation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel and Na+/K+/2Cl- cotransporter in renal epithelial A6 cells

We have reported that in renal epithelial A6 cells flavones stimulate the transepithelial Cl- secretion by activating the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel and/or the Na+/K+/2Cl- cotransporter. On the other hand, it has been established that cAMP activates the CFTR Cl- channel and the Na+/K+/2Cl- cotransporter. However, no information is available on the interaction between cAMP and flavones on stimulation of the CFTR Cl- channel and the Na+/K+/2Cl- cotransporter. To clarify the interaction between cAMP and flavones, we studied the regulatory mechanism of the CFTR Cl- channel and the Na+/K+/2Cl- cotransporter by flavones (apigenin, luteolin, kaempherol, and quercetin) under the basal and cAMP-stimulated conditions in renal epithelial A6 cells. Under the basal (cAMP-unstimulated) condition, these flavones stimulated the Cl- secretion by activating the Na+/K+/2Cl- cotransporter without any significant effects on the CFTR Cl- channel activity. On the other hand, these flavones diminished the activity of the cAMP-stimulated Na+/K+/2Cl- cotransporter without any significant effects on the CFTR Cl- channel activity. Interestingly, the level of the flavone-induced Cl- secretion under the basal condition was identical to that under the cAMP-stimulated condition. Based on these results, it is suggested that although both cAMP and flavones activate the Na+/K+/2Cl- cotransporter, these flavones have more powerful effects than cAMP on the Na+/K+/2Cl- cotransporter.

Evidence That Tyrphostins AG10 and AG18 Are Mitochondrial Uncouplers That Alter Phosphorylation-dependent Cell Signaling

Receptor agonists that initiate fluid secretion in salivary gland epithelial cells also increase protein phosphorylation. To assess contributions of tyrosine phosphorylation to secretion, changes in muscarinic receptor-initiated secretion (estimated from sodium pump-dependent increases in oxygen consumption) were measured in parotid acinar cells exposed to tyrosine kinase inhibitors. However, like the mitochondrial uncoupler carbonyl cyanide p-trifluoromethoxyphenyl hydrazone, tyrphostins AG10 and AG18 increased the rate of oxygen consumption and reduced cellular ATP by approximately 90% in the absence of the muscarinic agonist carbachol, indicating that these tyrphostins uncouple mitochondria. Exposure of isolated mitochondria to five structurally related tyrphostins demonstrated that their relative potencies as uncouplers differed from their in vitro kinase-inhibitory potencies due to different molecular requirements for the two effects. AG10 and AG18 blocked parotid phosphorylation events only at concentrations that reduced ATP content. The tyrosine kinase inhibitor genistein reduced ATP content by 15-20% and weakly uncoupled isolated mitochondria, but its inhibition of carbachol-mediated protein kinase Cdelta tyrosine phosphorylation and ERK1/2 activation appeared attributable to blocking tyrosine kinases directly. Carbachol itself rapidly reduced ATP content by 15-20%. Carbachol, 3'-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (P2X(7) receptor agonist), AG10, AG18, and carbonyl cyanide p-trifluoromethoxyphenyl hydrazone rapidly activated the fuel sensor AMP-activated protein kinase (AMPK); however, only AMPK activation by carbachol and BzATP was due to sodium pump stimulation. AG10 and AG18 also activated AMPK and/or uncoupled mitochondria in PC12, HeLa, and HEK293 cells. These studies demonstrate that some tyrosine kinase inhibitors produce cellular effects that are mechanistically different from their primary in vitro characterizations and, as do salivary secretory stimuli, promote rapid metabolic alterations that initiate secondary signaling events.

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.

H89, an inhibitor of protein kinase A (PKA), stimulates Na+ transport by translocating an epithelial Na+ channel (ENaC) in fetal rat alveolar type II epithelium

The present study was performed to clarify the effect of H89, an inhibitor of cAMP-activated protein kinase (protein kinase A; PKA), on Na(+) absorption in fetal rat alveolar type II epithelium. H89 stimulated the Na(+) absorption by increasing the open probability (Po) and number of a nonselective cation (NSC) channel composed of four alpha subunits of epithelial Na(+) channel (ENaC). Brefeldin A (BFA), an inhibitor of intracellular protein translocation, blocked the stimulatory action of H89 on the Na(+) absorption by interrupting the action of H89 on the Po and number of the NSC channel. H85, an inactive form of H89, showed an effect similar to H89, suggesting that H89 does not show its effect by inhibiting PKA, but acts on the channel depending the structure. These observations indicate that: (1) the H89 induced increase in number of the channel at the apical membrane is due to translocation of alpha subunit of ENaC to the apical membrane, (2) the elevation of Po of the channel is mediated through translocation of a protein activating alpha subunit of ENaC, and (3) the effect of H89 is dependent on its structure without any relation to PKA.

Hypotonic stimulation of the Na+ active transport in frog skeletal muscle: role of the cytoskeleton

Hypotonicity produces a marked activation of the Na+ pump in frog sartorius muscle. The increase in net Na+ efflux under hypotonic conditions occurs despite the reductions in [Na+]i that are due to fibre swelling and Na+ loss. The pump density (ouabain binding) increases not only upon reduction of the medium osmotic pressure (pi) from its normal value (pi = 1) to one-half (pi = 0.5), but also in muscles that are returned to pi = 1 after equilibration in pi = 2 medium. The equilibration in pi = 2 medium does not affect pump density. Ouabain-binding increments cannot be ascribed to a rise in the Na+-K+ exchange rate of a fixed number of pumps: they also occurred in the continued presence of a saturating concentration of ouabain (50 microM). Under those conditions, the pi = 1 pi = 0.5 transfer produced a 43 % increase in pump sites, while the pi = 2 pi = 1 transfer induced a rise of 46 %. Actinomycin D did not alter the stimulation of Na+ extrusion elicited by hypotonicity, suggesting that de novo synthesis of pumps was not involved in the increase of the apparent number of pump sites. Disruption of microtubules by colchicine (100 microM) and intermediate filaments by acrylamide (4 mM) did not alter the hypotonic effect. Likewise, genistein (100 microM), a specific inhibitor of tyrosine kinase, did not affect significantly the hypotonic response. Microfilament-disrupting agents like cytochalasin B (5 microM) and latrunculin B (10 microM) reduced the increase in Na+ efflux induced by pi = 1 pi = 0.5 transfer by about 35 % and 72 %, respectively. Latrunculin B reduced the increases in pump density generated by pi = 1 pi = 0.5 and pi = 2 pi = 1 transfers by about 79 % and 91 %, respectively. The results suggest that the membrane stretch due to hypotonic fibre volume increase would promote a microfilament-mediated insertion of submembranous spare Na+ pumps in the sarcolemma and, consequently, the rise in active Na+ transport.

Calmodulin-dependent regulation of hypotonicity-induced translocation of ENaC in renal epithelial A6 cells

Hypotonicity stimulates translocation of epithelial Na(+) channel (ENaC) to the apical membrane from the intracellular store site of ENaC by activating protein tyrosine kinase (PTK) in renal epithelial A6 cells. Based upon the fact that calmodulin shows its action on other enzymes through PTK caused phosphorylation of tyrosine residues of calmodulin itself, we studied whether a calmodulin-dependent pathway is involved in the action of hypotonicity on ENaC. W7, an antagonist of calmodulin, diminished the stimulatory action of hypotonicity on ENaC, irrespective of W7 treatment before or after application of hypotonicity. Calmodulin is known to regulate three pathways: (1) protein phosphatase 2B (PP2B), (2) Ca(2+)/calmodulin-dependent protein kinase II (CaMK II), and (3) myosin light chain kinase (MLCK). Pretreatment with cyclosporin A, an inhibitor of PP2B, did not influence the hypotonicity action on ENaC. The hypotonicity action on ENaC was partially inhibited by pretreatment with KN93, an inhibitor of CaMK II, but not by addition of KN93 after hypotonic stimulation had been applied. ML-7, an inhibitor of MLCK, showed the action similar to KN93. These observations indicate that: (1) the hypotonicity-induced translocation of ENaC depends on CaMK II and MLCK and (2) ENaC translocated to the apical membrane by hypotonicity is maintained in its activity and/or stability at the apical membrane through a calmodulin-dependent pathway.

Activation of CFTR Cl(-) channel by tyrphostins via a protein tyrosine kinase-independent pathway in forskolin-stimulated renal epithelial A6 cells

We studied effects of tyrphostin A23 (an inhibitor of protein tyrosine kinase; PTK) and tyrphostin A63 (an inactive analog of tyrphostin A23) on forskolin-activated cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels and Cl(-) secretion in renal epithelial A6 cells. Tyrphostin A23 and A63 had no effects on the basal CFTR Cl(-) channel and Cl(-) secretion. However, under the forskolin-stimulated condition, tyrphostin A23 and A63 stimulated Cl(-) secretion by activating CFTR Cl(-) channels. These observations suggest that: 1) tyrphostin A23 and A63 stimulate the cAMP-activated CFTR Cl(-) channel via a PTK-independent, structure-dependent mechanism, and 2) tyrphostin A23 and A63 do not stimulate the basal CFTR Cl(-) channel. These lead us to an idea that: 1) cAMP might cause a conformational change of CFTR Cl(-) channel which is accessible by tyrphostins, and 2) tyrphostins would stimulate translocation of the cAMP-modified channel to the apical membrane by binding to the channel.

Effects of Ca(2+) channel blockers on amiloride-sensitive Na(+) permeable channels and Na(+) transport in fetal rat alveolar type II epithelium

A beta-adrenergic agonist (beta-agonist), terbutaline, stimulated amiloride-sensitive Na(+) absorption in fetal rat alveolar type II epithelium, contributing to the clearance of lung fluid. Cytosolic Ca(2+) plays an important role in terbutaline-stimulated Na(+) absorption, since Ca(2+)-activated, amiloride-sensitive Na(+)-permeable channels are involved in transcellular Na(+) absorption and terbutaline stably elevates the cytosolic Ca(2+) concentration by stimulating Ca(2+) influx. Therefore, we studied whether Ca(2+) channel blockers (Ni(2+), verapamil, and nifedipine) affect terbutaline-stimulated transcellular Na(+) absorption. Ni(2+) partially blocked the channel responsible for the terbutaline-stimulated Na(+) absorption at the Na(+) entry pathway across the apical membrane of the epithelium, but did not diminish the terbutaline-stimulated transcellular Na(+) absorption. By measuring the capacity of the Na(+),K(+)-pump activity, we determined that the rate-limiting step of the terbutaline-stimulated transcellular Na(+) absorption was the extrusion step across the basolateral membrane by the Na(+),K(+)-pump. The other Ca(2+) channel blockers, verapamil and nifedipine, had effects identical to those of Ni(2+). Based upon these observations, we conclude that, in the beta-agonist-stimulated fetal rat alveolar type II epithelium, Ca(2+) channel blockers diminish amiloride-sensitive channels, but do not affect transcellular Na(+) absorption, since under the beta-agonist-stimulated condition the Na(+),K(+)-pump is the rate-limiting step in Na(+) transport.

Novel effects of minocycline on Ca(2+)-dependent Cl(-) secretion in human airway epithelial Calu-3 cells

The present study concerns previously unreported effects of the antibiotic minocycline on the transepithelial Cl(-) transport in Calu-3 cells, which display electrophysiological properties consistent with human airway serous cells. Basolateral 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 200 microM) augmented Cl(-) secretion, which was detected as a 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, 100 microM, a Cl(-) channel blocker)-sensitive short-circuit current (I(sc)). The DIDS-induced I(sc) was composed of Ca(2+)-activated K(+) (K(Ca)) channel-dependent and -independent components. The former was selectively inhibited by 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra(acetoxymethyl)ester (BAPTA/AM, 10 microM), charybdotoxin (ChTx, 100 nM), clotrimazole (10 microM), basolateral Ca(2+) removal, and basolateral minocycline (IC(50) = 20 microM). The latter was attenuated by basolateral BaCl (5 mM). In contrast, forskolin (10 microM)-induced I(sc), which is insensitive to BAPTA/AM and ChTx, was unaffected by minocycline (100 microM). ATP-induced I(sc) was partially inhibited by basolateral but not by apical minocycline. I(sc) due to basolateral application of ionomycin (1 microM) was markedly suppressed by NPPB and basolateral Ca(2+) removal. These inhibitory effects were mimicked by minocycline applied only from the basolateral side of the monolayer. In the basolateral absence of Ca(2+), 1-ethyl-2-benzimdazolinone (500 microM), a K(Ca) channel opener, generated a sustained I(sc) sensitive to ChTx. Minocycline had no significant effect on the ChTx-sensitive component of the I(sc). It is concluded that minocycline inhibits K(Ca) channel-dependent Cl(-) secretion via a blockade of Ca(2+) influx across the basolateral membrane from the extracellular side.

Forskolin activation of apical Cl- channel and Na+/K+/2Cl- cotransporter via a PTK-dependent pathway in renal epithelium

Forskolin induced the transepithelial Cl- transport (secretion) by activating the apical Cl- channel and basolateral Na+/K+/2Cl- cotransporter in renal epithelial A6 cells via an increase in cytosolic cAMP concentration. The cAMP activation of apical Cl- channel and Na+/K+/2Cl- cotransporter was partially mediated through a protein kinase A (PKA)-dependent pathway, but a PKA-independent pathway was also suggested to be involved in the cAMP activation. Therefore, we assessed a possibility of involvement of protein tyrosine kinase (PTK)-dependent pathway as a PKA-independent pathway in the cAMP activation by applying a PTK inhibitor, tyrphostin A23 (AG18). Tyrphostin A23 abolished the forskolin-induced transepithelial Cl- secretion by partially diminishing the activity of the Cl- channel and completely inhibiting the Na+/K+/2Cl- cotransporter. Further, forskolin increased phosphorylation of protein tyrosine, suggesting that cAMP activates PTK. These observations suggest that cAMP activates the Cl- channel and the Na+/K+/2Cl- cotransporter by activating PTK.

Blocking action of cytochalasin D on protein kinase A stimulation of a stretch-activated cation channel in renal epithelial A6 cells

We have shown that the apical membrane of renal epithelial A6 cells has a 29-pS stretch-activated nonselective cation (NSC) channel, which is activated by cytosolic cyclic AMP (cAMP) (J Gen Physiol 1997;110:327-36). In general, downstream signalings of cAMP are mediated through a cAMP-activated protein kinase (protein kinase A, PKA)-dependent pathway. Therefore, to study if the channel is activated by a PKA-dependent pathway, we applied a PKA catalytic subunit directly to the channel from the cytosolic surface in cytosol-free excised inside-out patches, using the single channel recording (patch clamp) technique. Application of PKA catalytic subunit with 2 mM ATP increased the open probability (P(o)) of the channel from 0.11 +/- 0.04 to 0.58 +/- 0.10 (mean +/- SD, N = 11, P < 0.001). The channel has a gating kinetics "C(L) <--> C(S) <--> O, " where C(L,) C(S,) and O are the long closed state, the short closed state, and the open state, respectively. PKA influenced the communication of the channel between C(L) and C(S) without affecting the communication between C(S) and O, leading the channel to only stay in C(S) and O. The PKA-induced increase in P(o) was attributable to the interruption of communication between C(L) and C(S) or to the reduction of time the channel stays in C(L.) Pretreatment with cytochalasin D (Cyt-D), an inhibitor of the polymerization of actin filaments, blocked the stimulatory effect of PKA on the channel. These observations suggest that phosphorylation of polymerized actin filaments regulates the gating kinetics of a stretch-activated channel in renal epithelium.

CFTR-Mediated anion conductance regulates Na(+)-K(+)-pump activity in Calu-3 human airway cells

We studied the role of CFTR in the Na(+)-K(+)-pump activity of Calu-3 human airway cells. To estimate the Na(+)-K(+)-pump activity on the basolateral membrane, the ouabain-sensitive component of the short-circuit current (Isc) was measured after permeabilization of the apical membrane with nystatin, a Na(+) ionophore. The Na(+)-K(+)-pump activity was diminished by a selective CFTR blocker (glybenclamide) or nonspecific Cl(-) channel inhibitors (NPPB and DPC) but not by outwardly rectifying Cl(-) channel blockers (DNDS, DIDS). Augmentation of anion conductance by 8-bromo-cyclic AMP (8Br-cAMP, 1 mM) potentiated the Na(+)-K(+)-pump activity that was reduced by blocking CFTR or by the replacement of Cl(-) with gluconate, a less membrane-permeant anion. The Na(+)-K(+)-pump activity was unaffected by the replacement of Cl(-) with NO(-)(3) that has equal permeability through the CFTR. These results suggest that the anion movement through the CFTR may contribute to the Na(+)-K(+)-pump activity in Calu-3 cells by regulating the rate of Na(+) entry.

The effects of PO2 upon transepithelial ion transport in fetal rat distal lung epithelial cells

1. Isolated rat fetal distal lung epithelial (FDLE) cells were cultured (for 48 h) at PO2 levels between 23 and 142 mmHg. Higher PO2 levels between 23 and 142 mmHg. Higher PO2 was associated with increased short circuit current (ISC) and increased abundance of the Na+ channel protein alpha-ENaC. PO2 had no effect upon ISC remaining after apical application of amiloride (10 microM). 2. Studies of cells maintained (for 48 h) at PO2 levels of 23 mmHg or 100 mmHg, and subsequently nystatin permeabilized (50 microM), showed that high PO2 increased Na+ pump capacity. This response was apparent 24 h after PO2 was raised whilst it took 48 h for the rise in ISC seen in intact cells to become fully established. Both parameters were unaffected by raising PO2 for only 30 min. 3. Basolateral application of isoprenaline (10 microM) did not affect ISC in cells maintained at 23 mmHg but evoked progressively larger responses at higher PO2. The response seen at 142 mmHg was larger than at 100 mmHg, the normal physiological alveolar PO2. 4. Isoprenaline had no effect on Na+ pump capacity at PO2 levels of 23 mmHg or 100 mmHg, but stimulated Na+ extrusion at 142 mmHg. Increasing PO2 above normal physiological levels thus allows the Na+ pump to be controlled by isoprenaline. This may explain the enhanced sensitivity to isoprenaline seen under these slightly hyperoxic conditions. 5. Changes in PO2 mimicking those occurring at birth thus exert profound influence over Na+ transport in FDLE cells and the Na+ pump could be an important locus at which this control is exercised.

Cell swelling activates stress-activated protein kinases, p38 MAP kinase and JNK, in renal epithelial A6 cells

Osmotic shock is well recognized as one of the factors activating stress-activated protein kinases (SAPKs), p38 MAP kinase and c-Jun N-terminal kinases (JNKs). In renal epithelial A6 cells, hypo-osmotic shock transiently activated SAPKs with maximal activation at 5 min. A6 cells showed a regulatory volume decrease (RVD) after swelling when the cells were exposed to a hypo-osmotic solution. In contrast, activation of SAPKs was maintained over 90 min after hypo-osmotic shock in the presence of 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, a Cl(-) channel blocker), which completely blocked the RVD and kept the cells continuously swelling. Exposure of the cells to a high K(+) iso-osmotic solution containing nystatin, which induces continuous cell swelling, also continuously activated SAPKs. Furthermore, membrane deformation induced by chlorpromazine activated SAPKs. These results suggest that changes in membrane tension by cell swelling or chlorpromazine, but not osmolality, are important steps for activation of SAPKs in A6 cells.