Importance of cytoskeletal elements in volume regulatory responses of trout hepatocytes

Cell Volume Changes and Membrane Ruptures Induced by Hypotonic Electrolyte and Sugar Solutions

The cell volume changes induced by hypotonic electrolyte and sucrose solutions were studied in Chinese-hamster-ovary epithelial cells. The effects in the solutions with osmolarities between 32 and 315 mosM/L and distilled water were analyzed using bright-field and fluorescence confocal microscopy. The changes of the cell volume, accompanied by the detachment of cells, the formation of blebs, and the occurrence of almost spherical vesicle-like cells (“cell-vesicles”), showed significant differences in the long-time responses of the cells in the electrolyte solutions compared with the sucrose-containing solutions. A theoretical model based on different permeabilities of ions and sucrose molecules and on the action of Na⁺/K⁺-ATPase pumps is applied. It is consistent with the observed temporal behavior of the cells’ volume and the occurrence of tension-induced membrane ruptures and explains lower long-time responses of the cells in the sucrose solutions.

Role of cytoskeleton network in anisosmotic volume changes of intact and permeabilized A549 cells

Recently we found that cytoplasm of permeabilized mammalian cells behaves as a hydrogel displaying intrinsic osmosensitivity. This study examined the role of microfilaments and microtubules in the regulation of hydrogel osmosensitivity, volume-sensitive ion transporters, and their contribution to volume modulation of intact cells. We found that intact and digitonin-permeabilized A549 cells displayed similar rate of shrinkage triggered by hyperosmotic medium. It was significantly slowed-down in both cell preparations after disruption of actin microfilaments by cytochalasin B, suggesting that rapid water release by intact cytoplasmic hydrogel contributes to hyperosmotic shrinkage. In hyposmotic swelling experiments, disruption of microtubules by vinblastine attenuated the maximal amplitude of swelling in intact cells and completely abolished it in permeabilized cells. The swelling of intact cells also triggered ~10-fold elevation of furosemide-resistant (86)Rb+ (K+) permeability and the regulatory volume decrease (RVD), both of which were abolished by Ba2+. Interestingly, RVD and K+ permeability remained unaffected in cytocholasin/vinblastine treated cells demonstrating that cytoskeleton disruption has no direct impact on Ba2+-sensitive K+-channels involved in RVD. Our results show, for the first time, that the cytoskeleton network contributes directly to passive cell volume adjustments in anisosmotic media via the modulation of the water retained by the cytoplasmic hydrogel.

Functional interaction between AQP2 and TRPV4 in renal cells

We have previously demonstrated that renal cortical collecting duct cells (RCCD(1)), responded to hypotonic stress with a rapid activation of regulatory volume decrease (RVD) mechanisms. This process requires the presence of the water channel AQP2 and calcium influx, opening the question about the molecular identity of this calcium entry path. Since the calcium permeable nonselective cation channel TRPV4 plays a crucial role in the response to mechanical and osmotic perturbations in a wide range of cell types, the aim of this work was to test the hypothesis that the increase in intracellular calcium concentration and the subsequent rapid RVD, only observed in the presence of AQP2, could be due to a specific activation of TRPV4. We evaluated the expression and function of TRPV4 channels and their contribution to RVD in WT-RCCD(1) (not expressing aquaporins) and in AQP2-RCCD(1) (transfected with AQP2) cells. Our results demonstrated that both cell lines endogenously express functional TRPV4, however, a large activation of the channel by hypotonicity only occurs in cells that express AQP2. Blocking of TRPV4 by ruthenium red abolished calcium influx as well as RVD, identifying TRPV4 as a necessary component in volume regulation. Even more, this process is dependent on the translocation of TRPV4 to the plasma membrane. Our data provide evidence of a novel association between TRPV4 and AQP2 that is involved in the activation of TRPV4 by hypotonicity and regulation of cellular response to the osmotic stress, suggesting that both proteins are assembled in a signaling complex that responds to anisosmotic conditions.

Phosphorylation on the Ser 824 residue of TRPV4 prefers to bind with F-actin than with microtubules to expand the cell surface area

Previously, we demonstrated that the transient receptor potential vanilloid 4 (TRPV4) cation channel, a member of the TRP vanilloid subfamily, is one of the serum glucocorticoid-induced protein kinase1 (SGK1) authentic substrate proteins, and that the Ser 824 residue of TRPV4 is phosphorylated by SGK1. In this study, we demonstrated that phosphorylation on the Ser 824 residue of TRPV4 is required for its interaction with F-actin, using TRPV4 mutants (S824D; a phospho-mimicking TRPV4 mutant and S824A; a non-phosphorylatable TRPV4 mutant) and its proper subcellular localization. Additionally, we noted that the phosphorylation of the Ser824 residue promotes its single channel activity, Ca(2+) influx, protein stability, and cell surface area (expansion of plasma membrane).

Regulatory volume decrease and P receptor signaling in fish cells: mechanisms, physiology, and modeling approaches

For animal cell plasma membranes, the permeability of water is much higher than that of ions and other solutes, and exposure to hyposmotic conditions almost invariably causes rapid water influx and cell swelling. In this situation, cells deploy regulatory mechanisms to preserve membrane integrity and avoid lysis. The phenomenon of regulatory volume decrease, the partial or full restoration of cell volume following cell swelling, is well-studied in mammals, with uncountable investigations yielding details on the signaling network and the effector mechanisms involved in the process. In comparison, cells from other vertebrates and from invertebrates received little attention, despite of the fact that e.g. fish cells could present rewarding model systems given the diversity in ecology and lifestyle of this animal group that may be reflected by an equal diversity of physiological adaptive mechanisms, including those related to cell volume regulation. In this review, we therefore present an overview on the most relevant aspects known on hypotonic volume regulation presently known in fish, summarizing transporters and signaling pathways described so far, and then focus on an aspect we have particularly studied over the past years using fish cell models, i.e. the role of extracellular nucleotides in mediating cell volume recovery of swollen cells. We, furthermore, present diverse modeling approaches developed on the basis of data derived from studies with fish and other models and discuss their potential use for gaining insight into the theoretical framework of volume regulation.

Cell volume regulation following hypotonic shock in hepatocytes isolated from Sparus aurata

The response of isolated hepatocytes of Sparus aurata to hypotonic shock was studied by the aid of videometric and light scattering methods. The isolated cells exposed to a rapid change (from 370 to 260 mOsm/kg) of the osmolarity of the bathing solution swelled but thereafter underwent a decrease of cell volume tending to recovery the original size. This homeostatic response RVD (regulatory volume decrease) was inhibited in the absence of extracellular Ca²+ and in the presence of TMB8, an inhibitor of Ca²+ release from intracellular stores. It is likely that Ca²+ entry through verapamil sensitive Ca²+-channels, probably leading to a release of Ca²+ from intracellular stores, is responsible for RVD since the blocker impaired the ability of the cell to recover its volume after the hypotonic shock. RVD tests performed in the presence of various inhibitors of different transport mechanisms, such as BaCl₂, quinine, glybenclamide and bumetanide as well as in the presence of a KCl activator, NEM, led us to suggest that the recovery of cell volume in hypotonic solution is accomplished by an efflux of K+ and Cl⁻ through conductive pathways paralleled by the operation of the KCl cotransport, followed by an obliged water efflux from the cells.

A mechanosensitive ion channel regulating cell volume

Cells respond to a hyposmotic challenge by swelling and then returning toward the resting volume, a process known as the regulatory volume decrease or RVD. The sensors for this process have been proposed to include cationic mechanosensitive ion channels that are opened by membrane tension. We tested this hypothesis using a microfluidic device to measure cell volume and the peptide GsMTx4, a specific inhibitor of cationic mechanosensitive channels. GsMTx4 had no effect on RVD in primary rat astrocytes or Madin-Darby canine kidney (MDCK) cells but was able to completely inhibit RVD and the associated Ca(2+) uptake in normal rat kidney (NRK-49F) cells in a dose-dependent manner. Gadolinium (Gd(3+)), a nonspecific blocker of many mechanosensitive channels, inhibited RVD and Ca(2+) uptake in all three cell types, demonstrating the existence of at least two types of volume sensors. Single-channel stretch-activated currents are present in outside-out patches from NRK-49F, MDCK, and astrocytes, and they are reversibly inhibited by GsMTx4. While mechanosensitive channels are involved in volume regulation, their role for volume sensing is specialized. The NRK cells form a stable platform from which to screen drugs that affect volume regulation via mechanosensory channels and as a sensitive system to clone the channel.

Cloning and regulation of expression of the Na+–Cl––taurine transporter in gill cells of freshwater Japanese eels

Our previous studies have demonstrated the hypertonic-induced expression of osmotic stress transcription factor and the regulatory volume increase (RVI)response in gill cells isolated from freshwater eels. In this study, we aimed to clone one of the organic osmolyte transporters, the Na+–Cl––taurine transporter (TauT),and to characterize its expression in anisosmotic conditions, using both in vivo and in vitro approaches. A cDNA clone encoding TauT was isolated from gill tissues of Japanese eels, Anguilla japonica. The deduced amino acid sequence shows 88–90% identity to other reported piscine TauT sequences. Our data indicated that TauT mRNA was detectable in both freshwater and seawater fish gills. The expression level of TauT mRNA increased in gills of seawater-acclimating fish. A high abundance of TauT protein was found to be localized in seawater gill chloride cells. Using primary gill cell culture, expression of the gene was induced when the ambient osmolarity was raised from 320 to 500 mosmol l–1. Hypertonic treatment of the culture caused an increase of F-actin distribution in the cell periphery. Treatment of the cells with colchicine or cytochalasin D significantly reduced TauT transcript level following hypertonic exposure. The inhibition of myosin light chain (MLC) kinase by ML-7 had a significant additive effect on hypertonic-induced TauT expression. Collectively, the data of this study reveal, for the first time, the regulation of TauT expression in gill cells of euryhaline fish. We have demonstrated the involvement of ionic strength, the cytoskeleton and MLC kinase in the regulation of TauT expression. The results shed light on the osmosensing and hyperosmotic adaption in fish gills.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Microfilament dynamics during HaCaT cell volume regulation

Cell volume is an important parameter in many physiological processes, and is closely regulated in many cell types. In those cells, swelling induced by hypotonic media is followed by an ion-driven regulatory volume decrease. In many cell types, this regulatory volume decrease requires an intact actin cytoskeleton. Therefore, we investigated the changes in the structure and polymerization state of the actin cytoskeleton in HaCaT keratinocytes during cell swelling and regulatory volume decrease. Disruption of the actin cytoskeleton by 2microM cytochalasin D inhibits regulatory volume decrease in HaCaT cells. Cells swollen in the presence of low concentrations of cytochalasin D (0.8microM, 305-250mosM) keep the elevated volume even after cytochalasin D removal. A further decrease of tonicity (250-200mosM) is again counteracted by regulatory volume decrease reaching the volume, which has been established at 250mosM. In contrast, no visible changes occurred in actin cytoskeleton morphology of EGFP-actin-transfected HaCaT cells during swelling or regulatory volume decrease. However, biochemical analysis showed an increase in total F-actin levels 90s after the onset of hypotonicity. The ratio of Triton-soluble to -insoluble actin also increased after hypotonic shock, suggesting that the measured increase in F-actin is primarily due to de novo polymerization and formation of short actin filaments, i.e., actin oligomers. These results show that a rapid reorganization of the actin cytoskeleton takes place after hypotonic treatment. This reorganization can influence signaling in response to hypotonicity either indirectly by means of sequestering or releasing actin-associated proteins, or directly by the interaction of short actin filaments with plasma membrane ion channels, and may be involved in determining a new volume set point.

Interdependence of Ca2+ and proton movements in trout hepatocytes

This study was undertaken to investigate possible interrelationships between Ca2+ homeostasis and pH regulation in trout hepatocytes. Exposure of cells to Ca2+ mobilizing agents ionomycin (0.5 μmol l–1) and thapsigargin (0.1 μmol l–1)induced an increase in intracellular pH (pHi) that was dependent on Ca2+ influx from the extracellular medium as well as Ca2+ release from intracellular pools. Surprisingly, this increase in pHi and intracellular Ca2+ concentration,[Ca2+]i, was not accompanied by any change in proton secretion. By contrast, removal of extracellular Ca2+(Ca2+e) using EGTA (0.5 mmol l–1)briefly increased proton secretion rate with no apparent effect on pHi, while chelation of Ca2+i using BAPTA-AM (25 μmol l–1) resulted in a drop in pHi and a sustained increase in proton secretion rate. [Ca2+]i therefore affected intracellular proton distribution and/or proton production and also affected the distribution of protons across the cell membrane. Accordingly, changes in pHi were not always compensated for by proton secretion across the cell membrane.

Alteration in pHe below and above normal values induced a slow,continuous increase in [Ca2+]i with a tendency to stabilize upon exposure to high pHe values. Rapid pHi increase induced by NH4Cl was accompanied by an elevation in[Ca2+]i from both extracellular and intracellular compartments. Ca2+e appeared to be involved in pHi regulation following NH4Cl-induced alkalinization whereas neither removal of Ca2+e nor chelation of Ca2+i affected pHi recovery following Na-propionate exposure. Similarly, [Ca2+]i increase induced by hypertonicity appeared to be a consequence of the changes in pHi as Na-free medium as well as cariporide diminished the hypertonicity-induced increase in[Ca2+]i. These results imply that a compensatory relationship between changes in pHi and proton secretion across cell plasma membrane is not always present. Consequently, calculating proton extrusion from buffering capacity and rate of pHi change cannot be taken as an absolute alternative for measuring proton secretion rate, at least in response to Ca2+ mobilizing agents.

Mechanisms of hypotonic inhibition of the sodium, proton exchanger type 1 (NHE1) in a biliary epithelial cell line (Mz-Cha-1)

AIM

To elucidate the cellular events that results in inhibition of Na(+), H(+) exchanger type 1 (NHE1) by hypotonicity.

METHODS

Intracellular pH (pH(i)) was measured in biliary epithelial cells, with the pH-sensitive fluorochrome 2',7'-bis-(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) using a spectrophotometer. Regulatory volume decrease (RVD) was analysed from confocal images. Changes in NHE1 membrane content were visualized by confocal laser scanning microscopy after transfection of Mz-Cha-1 cells with a NHE1-cMyc fusion protein.

RESULTS

In Mz-Cha-1 cells hypotonicity (-80 mmol L(-1) NaCl) inhibited endogenous Na(+), H(+) exchange. Tyrosine and serine kinase inhibitors were incapable to prevent inhibition. As several signalling pathways influence Na(+), H(+) exchange, we tested the effect of the Ca(++), Calmodulin, protein kinase C or the cAMP, protein kinase A system on inhibition of Na(+), H(+) exchange by hypotonic challenge, but neither system was involved. In contrast, cytoskeleton did influence the effect of hypotonicity. Inhibition of microtubule polymerization by colchicine prevented inhibition of NHE1, and also restored Na(+), H(+) exchange kinetics. Specific inhibition of Src kinases with PP2, attenuated pH(i) recovery rate from 1.93 +/- 0.16 pH units min(-1) (normotonic environment) to 1.02 +/- 0.50 pH units min(-1) (hypotonic environment). Membrane staining of NHE1-cMyc fusion protein was maintained after hypotonic exposure in colchicine pre-treated cells as was RVD. Microfilament inhibition by cytochalasin preserved NHE1 activity. Inhibition of phosphatidylinositol-3'-kinase was unable to restore Na(+), H(+) exchange activity.

CONCLUSION

We conclude that regulation of Na(+), H(+) exchange during RVD is mediated by cytoskeletal elements. This receptor independent pathway is regulated by Src.

Hypotonicity causes actin reorganization and recruitment of the actin-binding ERM protein moesin in membrane protrusions in collecting duct principal cells

Hypotonicity-induced cell swelling is characterized by a modification in cell architecture associated with actin cytoskeleton remodeling. The ezrin/radixin/moesin (ERM) family proteins are important signal transducers during actin reorganization regulated by the monomeric G proteins of the Rho family. We report here that in collecting duct CD8 cells hypotonicity-induced cell swelling resulted in deep actin reorganization, consisting of loss of stress fibers and formation of F-actin patches in membrane protrusions where the ERM protein moesin was recruited. Cell swelling increased the interaction between actin and moesin and induced the transition of moesin from an oligomeric to a monomeric functional conformation, characterized by both the COOH- and NH2-terminal domains being exposed. In this conformation, which is stabilized by phosphorylation of a conserved threonine in the COOH-terminal domain by PKC or Rho kinase, moesin can bind interacting proteins. Interestingly, hypotonic stress increased the amount of threonine-phosphorylated moesin, which was prevented by the PKC-α inhibitor Gö-6976 (50 nM). In contrast, the Rho kinase inhibitor Y-27632 (1 μM) did not affect the hypotonicity-induced increase in phosphorylated moesin. The present data represent the first evidence that hypotonicity-induced actin remodeling is associated with phosphorylated moesin recruitment at the cell border and interaction with actin.

Spectrally and spatially resolved fluorescence lifetime imaging in living cells: TRPV4-microfilament interactions

Time- and space-correlated single photon counting method has been used to demonstrate the interactions of cation channel "transient receptor potential vanilloid 4" (TRPV4) and microfilaments. Living cells co-expressing TRPV4-CFP and actin-YFP, when excited for the donor molecules (CFP) exhibited an emission peak at 527 nm and decrease of the lifetime in the wavelength band 460-490 nm; corresponding to resonance energy transfer to YFP. CFP fluorescence decay was fitted best by a dual mode decay model. Considering the average lifetime of the donor, both in the presence and absence of acceptor yielded an apparent FRET efficiency of approximately 20%. This is rather high placing the minimum distance of chromophores in the two fluorescent proteins in the range of 4 nm. Thus, this study shows for the first time that TRPV4 and actin intimately associate within living cells. The significance of this finding for cell volume regulation is highlighted.

Expression of the calcium-binding protein S100A4 is markedly up-regulated by osmotic stress and is involved in the renal osmoadaptive response

Proteomic analysis of Inner Medullary Collecting Duct (IMCD3) cells adapted to increasing levels of tonicity (300, 600, and 900 mosmol/kg H(2)O) by two-dimensional difference gel electrophoresis and mass spectrometry revealed several proteins as yet unknown to be up-regulated in response to hypertonic stress. Of these proteins, one of the most robustly up-regulated (22-fold) was S100A4. The identity of the protein was verified by high pressure liquid chromatography-mass spectrometry. Western blot analysis confirmed increased expression with increased tonicity, both acute and chronic. S100A4 protein expression was further confirmed by immunocytochemical analysis. Cells grown in isotonic conditions showed complete absence of immunostaining, whereas chronically adapted IMCD3 cells had uniform cytoplasmic localization. The protein is also regulated in vivo as in mouse kidney tissues S100A4 expression was many -fold greater in the papilla as compared with the cortex and increased further in the papilla upon 36 h of thirsting. Increased expression of S100A4 was also observed in the medulla and papilla, but not the cortex of a human kidney. Data from Affymetrix gene chip analysis and quantitative PCR also revealed increased S100A4 message in IMCD3 cells adapted to hypertonicity. The initial expression of message increased at 8-10 h following exposure to acute sublethal hypertonic stress (550 mosmol/kg H(2)O). Protein and message half-life in IMCD3 cells were 85.5 and 6.8 h, respectively. Increasing medium tonicity with NaCl, sucrose, mannitol, and choline chloride stimulated S100A4 expression, whereas urea did not. Silencing of S100A4 expression using a stable siRNA vector (pSM2; Open Biosystems) resulted in a 48-h delay in adaptation of IMCD3 cells under sublethal osmotic stress, suggesting S100A4 is involved in the osmoadaptive response. In summary, we describe the heretofore unrecognized up-regulation of a small calcium-binding protein, both in vitro and in vivo, whose absence profoundly delays osmoadaptation and slows cellular growth under hypertonic conditions.

Actin cytoskeleton architecture and signaling in osmosensing

Since the early days of cell volume regulation research, the role of actin cytoskeleton organization and rearrangement has attracted specific interest. Rapid modifications in actin dynamics and architecture have been described. They were shown to regulate cell volume changes, as well as regulatory volume decrease in a large variety of cell types, including hepatocytes, lymphocytes, fibroblasts, myocytes, and various tumor cells. Using microscopic and biochemical analyses, modifications of actin organization and polymerization dynamics were studied. This chapter summarizes the molecular approaches applied so far for the quantitative assessment of actin cytoskeleton dynamics in the various cell types. It demonstrates that rapid modifications of actin cytoskeleton dynamics regulated by specific signaling pathways play a functional role in cell volume regulation. It is concluded that studying actin polymerization dynamics and signaling represents a challenging tool for the understanding of osmosensing and osmosignaling regulation in cellular physiology.

Signalling pathways involved in hypertonicity- and acidification-induced activation of Na+/H+ exchange in trout hepatocytes

In trout hepatocytes, hypertonicity and cytosolic acidification are known to stimulate Na+/H+ exchanger (NHE) activity, which contributes to recovery of cell volume and intracellular pH (pHi),respectively. The present study investigated the signalling mechanisms underlying NHE activation under these conditions. Exposing trout hepatocytes to cariporide, a specific inhibitor of NHE-1, decreased baseline pHi,completely blocked the hypertonicity-induced increase of pHi and reduced the hypertonicity-induced proton secretion by 80%. Changing extracellular pH (pHe)above and below normal values, and allowing cells to adjust pHi accordingly,significantly delayed alkalinization during hypertonic exposure, whereas following an acid load an enhanced pHi recovery with increasing pHe was seen. Chelating Ca2+, and thereby preventing the hypertonicity-induced increase in intracellular Ca2+ ([Ca2+]i), significantly diminished hypertonic elevation of pHi, indicating that Ca2+signalling might be involved in NHE activation. A reduction in alkalinization and proton secretion was also observed in the presence of the protein kinase A(PKA) inhibitor H-89 or the calmodulin (CaM) inhibitor calmidazolium. A complete inhibition of hypertonic- and acidification-induced changes of pHi concurrent with an increase in hypertonically induced proton efflux was seen with the protein kinase C (PKC) inhibitor chelerythrine. Recovery of pHi following sodium propionate addition was reduced by more than 60% in the presence of cariporide, was sensitive to PKA inhibition, and tended to be reduced by CaM inhibition. In conclusion, we showed that NHE-1 is the main acid secretion mechanism during hypertonicity and recovery following acid loading. In addition, Ca2+-, PKA- and CaM-dependent pathways are involved in NHE-1 activation for recovery of cell volume and pHi. On the other hand, PKC appeared to have an impact on NHE-independent pathways affecting intracellular acid-base homeostasis.

Modulation of hepatocellular swelling-activated K+currents by phosphoinositide pathway-dependent protein kinase C

K+channels participate in the regulatory volume decrease (RVD) accompanying hepatocellular nutrient uptake and bile formation. We recently identified KCNQ1 as a molecular candidate for a significant fraction of the hepatocellular swelling-activated K+current ( IKVol). We have shown that the KCNQ1 inhibitor chromanol 293B significantly inhibited RVD-associated K+flux in isolated perfused rat liver and used patch-clamp techniques to define the signaling pathway linking swelling to IKVolactivation. Patch-electrode dialysis of hepatocytes with solutions that maintain or increase phosphatidylinositol 4,5-bisphosphate (PIP2) increased IKVol, whereas conditions that decrease cellular PIP2decreased IKVol. GTP and AlF4−stimulated IKVoldevelopment, suggesting a role for G proteins and phospholipase C (PLC). Supporting this, the PLC blocker U-73122 decreased IKVoland inhibited the stimulatory response to PIP2or GTP. Protein kinase C (PKC) is involved, because K+current was enhanced by 1-oleoyl-2-acetyl- sn-glycerol and inhibited after chronic PKC stimulation with phorbol 12-myristate 13-acetate (PMA) or the PKC inhibitor GF 109203X. Both IKVoland the accompanying membrane capacitance increase were blocked by cytochalasin D or GF 109203X. Acute PMA did not eliminate the cytochalasin D inhibition, suggesting that PKC-mediated IKVolactivation involves the cytoskeleton. Under isotonic conditions, a slowly developing K+current similar to IKVolwas activated by PIP2, lipid phosphatase inhibitors to counter PIP2depletion, a PLC-coupled α1-adrenoceptor agonist, or PKC activators and was depressed by PKC inhibition, suggesting that hypotonicity is one of a set of stimuli that can activate IKVolthrough a PIP2/PKC-dependent pathway. The results indicate that PIP2indirectly activates hepatocellular KCNQ1-like channels via cytoskeletal rearrangement involving PKC activation.

Intracellular pH modulates taste receptor cell volume and the phasic part of the chorda tympani response to acids

The relationship between cell volume and the neural response to acidic stimuli was investigated by simultaneous measurements of intracellular pH (pHi) and cell volume in polarized fungiform taste receptor cells (TRCs) using 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) in vitro and by rat chorda tympani (CT) nerve recordings in vivo. CT responses to HCl and CO2 were recorded in the presence of 1 M mannitol and specific probes for filamentous (F) actin (phalloidin) and monomeric (G) actin (cytochalasin B) under lingual voltage clamp. Acidic stimuli reversibly decrease TRC pHi and cell volume. In isolated TRCs F-actin and G-actin were labeled with rhodamine phalloidin and bovine pancreatic deoxyribonuclease-1 conjugated with Alexa Fluor 488, respectively. A decrease in pHi shifted the equilibrium from F-actin to G-actin. Treatment with phalloidin or cytochalasin B attenuated the magnitude of the pHi-induced decrease in TRC volume. The phasic part of the CT response to HCl or CO2 was significantly decreased by preshrinking TRCs with hypertonic mannitol and lingual application of 1.2 mM phalloidin or 20 microM cytochalasin B with no effect on the tonic part of the CT response. In TRCs first treated with cytochalasin B, the decrease in the magnitude of the phasic response to acidic stimuli was reversed by phalloidin treatment. The pHi-induced decrease in TRC volume induced a flufenamic acid-sensitive nonselective basolateral cation conductance. Channel activity was enhanced at positive lingual clamp voltages. Lingual application of flufenamic acid decreased the magnitude of the phasic part of the CT response to HCl and CO2. Flufenamic acid and hypertonic mannitol were additive in inhibiting the phasic response. We conclude that a decrease in pHi induces TRC shrinkage through its effect on the actin cytoskeleton and activates a flufenamic acid-sensitive basolateral cation conductance that is involved in eliciting the phasic part of the CT response to acidic stimuli.