Role of Na+ conductance, Na(+)-H+ exchange, and Na(+)-K(+)-2Cl- symport in the regulatory volume increase of rat hepatocytes

Multiscale transport and 4D time-lapse imaging in precision-cut liver slices (PCLS)

Background

Monitoring cellular processes across different levels of complexity, from the cellular to the tissue scale, is important for understanding tissue structure and function. However, it is challenging to monitor and estimate these structural and dynamic interactions within three-dimensional (3D) tissue models.

Objective

The aim of this study was to design a method for imaging, tracking, and quantifying 3D changes in cell morphology (shape and size) within liver tissue, specifically a precision-cut liver slice (PCLS). A PCLS is a 3D model of the liver that allows the study of the structure and function of liver cells in their native microenvironment.

Methods

Here, we present a method for imaging liver tissue during anisosmotic exposure in a multispectral four-dimensional manner. Three metrics of tissue morphology were measured to quantify the effects of osmotic stress on liver tissue. We estimated the changes in the volume of whole precision cut liver slices, quantified the changes in nuclei position, and calculated the changes in volumetric responses of tissue-embedded cells.

Results

During equilibration with cell-membrane-permeating and non-permeating solutes, the whole tissue experiences shrinkage and expansion. As nuclei showed a change in position and directional displacement under osmotic stress, we demonstrate that nuclei could be used as a probe to measure local osmotic and mechanical stress. Moreover, we demonstrate that cells change their volume within tissue slices as a result of osmotic perturbation and that this change in volume is dependent on the position of the cell within the tissue and the duration of the exposure.

Conclusion

The results of this study have implications for a better understanding of multiscale transport, mechanobiology, and triggered biological responses within complex biological structures.

Severe acute respiratory syndrome coronavirus 2 may cause liver injury via Na+/H+ exchanger

The liver has many significant functions, such as detoxification, the urea cycle, gluconeogenesis, and protein synthesis. Systemic diseases, hypoxia, infections, drugs, and toxins can easily affect the liver, which is extremely sensitive to injury. Systemic infection of severe acute respiratory syndrome coronavirus 2 can cause liver damage. The primary regulator of intracellular pH in the liver is the Na⁺/H⁺ exchanger (NHE). Physiologically, NHE protects hepatocytes from apoptosis by making the intracellular pH alkaline. Severe acute respiratory syndrome coronavirus 2 increases local angiotensin II levels by binding to angiotensin-converting enzyme 2. In severe cases of coronavirus disease 2019, high angi-otensin II levels may cause NHE overstimulation and lipid accumulation in the liver. NHE overstimulation can lead to hepatocyte death. NHE overstimulation may trigger a cytokine storm by increasing proinflammatory cytokines in the liver. Since the release of proinflammatory cytokines such as interleukin-6 increases with NHE activation, the virus may indirectly cause an increase in fibrinogen and D-dimer levels. NHE overstimulation may cause thrombotic events and systemic damage by increasing fibrinogen levels and cytokine release. Also, NHE overstimulation causes an increase in the urea cycle while inhibiting vitamin D synthesis and gluconeogenesis in the liver. Increasing NHE3 activity leads to Na⁺ loading, which impairs the containment and fluidity of bile acid. NHE overstimulation can change the gut microbiota composition by disrupting the structure and fluidity of bile acid, thus triggering systemic damage. Unlike other tissues, tumor necrosis factor-alpha and angiotensin II decrease NHE3 activity in the intestine. Thus, increased luminal Na⁺ leads to diarrhea and cytokine release. Severe acute respiratory syndrome coronavirus 2-induced local and systemic damage can be improved by preventing virus-induced NHE overstimulation in the liver.

The Important Role of Ion Transport System in Cervical Cancer

Cervical cancer is a significant gynecological cancer and causes cancer-related deaths worldwide. Human papillomavirus (HPV) is implicated in the etiology of cervical malignancy. However, much evidence indicates that HPV infection is a necessary but not sufficient cause in cervical carcinogenesis. Therefore, the cellular pathophysiology of cervical cancer is worthy of study. This review summarizes the recent findings concerning the ion transport processes involved in cell volume regulation and intracellular Ca²⁺ homeostasis of epithelial cells and how these transport systems are themselves regulated by the tumor microenvironment. For cell volume regulation, we focused on the volume-sensitive Cl⁻ channels and K⁺-Cl⁻ cotransporter (KCC) family, important regulators for ionic and osmotic homeostasis of epithelial cells. Regarding intracellular Ca²⁺ homeostasis, the Ca²⁺ store sensor STIM molecules and plasma membrane Ca²⁺ channel Orai proteins, the predominant Ca²⁺ entry mechanism in epithelial cells, are discussed. Furthermore, we evaluate the potential of these membrane ion transport systems as diagnostic biomarkers and pharmacological interventions and highlight the challenges.

Scanning ion conductance microscopy for studying biological samples

Scanning ion conductance microscopy (SICM) is a scanning probe technique that utilizes the increase in access resistance that occurs if an electrolyte filled glass micro-pipette is approached towards a poorly conducting surface. Since an increase in resistance can be monitored before the physical contact between scanning probe tip and sample, this technique is particularly useful to investigate the topography of delicate samples such as living cells. SICM has shown its potential in various applications such as high resolution and long-time imaging of living cells or the determination of local changes in cellular volume. Furthermore, SICM has been combined with various techniques such as fluorescence microscopy or patch clamping to reveal localized information about proteins or protein functions. This review details the various advantages and pitfalls of SICM and provides an overview of the recent developments and applications of SICM in biological imaging. Furthermore, we show that in principle, a combination of SICM and ion selective micro-electrodes enables one to monitor the local ion activity surrounding a living cell.

The ΔC splice-variant of TRPM2 is the hypertonicity-induced cation channel in HeLa cells, and the ecto-enzyme CD38 mediates its activation

Hypertonicity-induced cation channels (HICCs) are key-players in proliferation and apoptosis but their molecular correlate remains obscure. Furthermore, the activation profile of HICCs is not well defined yet. We report here that, in HeLa cells, intracellular adenosine diphosphate ribose (ADPr) and cyclic ADPr (cADPr), as supposed activators of TRPM2, elicited cation currents that were virtually identical to the osmotic activation of HICCs. Silencing of the expression of TRPM2 and of the ecto-enzyme CD38 (as a likely source of ADPr and cADPr) inhibited HICC as well as nucleotide-induced currents and, in parallel, the hypertonic volume response of cells (the regulatory volume increase, RVI) was attenuated. Quantification of intracellular cADPr levels and the systematic application of extra- vs. intracellular nucleotides indicate that the outwardly directed gradient rather than the cellular activity of ADPr and cADPr triggers TRPM2 activation, probably along with a simultaneous biotransformation of nucleotides.Cloning of TRPM2 identified the ΔC-splice variant as the molecular correlate of the HICC, which could be strongly supported by a direct comparison of the respective Ca²⁺ selectivity. Finally, immunoprecipitation and high-resolution FRET/FLIM imaging revealed the interaction of TRPM2 and CD38 in the native as well as in a heterologous (HEK293T) expression system. We propose transport-related nucleotide export via CD38 as a novel mechanism of TRPM2/HICC activation. With the biotransformation of nucleotides running in parallel, continuous zero trans-conditions are achieved which will render the system infinitely sensitive.

Kinetics of hyperosmotically stimulated Na-K-2Cl cotransporter in Xenopus laevis oocytes

A detailed study of hypertonically stimulated Na-K-2Cl cotransport (NKCC1) in Xenopus laevis oocytes was carried out to better understand the 1 K(+):1 Cl(-) stoichiometry of transport that was previously observed. In this study, we derived the velocity equations for K(+) influx under both rapid equilibrium assumptions and combined equilibrium and steady-state assumptions and demonstrate that the behavior of the equations and curves in Lineweaver-Burke plots are consistent with a model where Cl(-) binds first, followed by Na(+), a second Cl(-), and then K(+). We further demonstrate that stimulation of K(+) movement by K(+) on the trans side is an intrinsic property of a carrier that transports multiple substrates. We also demonstrate that K(+) movement through NKCC1 is strictly dependent upon the presence of external Na(+), even though only a fraction of Na(+) is in fact transported. Finally, we propose that the larger transport of K(+), as compared with Na(+), is a result of the return of partially unloaded carriers, which masks the net 1Na(+):1K(+):2Cl(-) stoichiometry of NKCC1. These data have profound implications for the physiology of Na-K-2Cl cotransport, since transport of K-Cl in some conditions seems to be uncoupled from the transport of Na-Cl.

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.

alpha-ENaC is a functional element of the hypertonicity-induced cation channel in HepG2 cells and it mediates proliferation

The molecular correlate of hypertonicity-induced cation channels (HICCs) and their role in proliferation vs. apoptosis is a matter of debate. We report in this paper that, in whole-cell patch-clamp recordings, hypertonic stress (340→450 mosM) reversibly increased the Na⁺ conductance of HepG2 cells from 0.8 to 5.8 nS. The effect was dose-dependently inhibited by flufenamate and amiloride, known blockers of HICCs, with some 50% efficiency at 300 μM. In parallel, both drugs decreased HepG2 cell proliferation [in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays and with automatic cell counting]. Small interfering RNA (siRNA) silencing of the α-subunit of the epithelial Na⁺ channel (ENaC) reduced hypertonicity-induced Na⁺ currents to 60%, whereas the rate of HepG2 cell proliferation was approximately half of that of the control. Moreover, α-ENaC siRNA inhibited the regulatory volume increase of HepG2 cells (measured with scanning acoustic microscopy) by 60%. In florescence-activated cell sorting measurements, silencing of α-ENaC led to a significant decrease in the G1 and an increase in the G2/M phase of the cell cycle, whereas the S phase was not changing. Finally (determined by a caspase 3/7 assay), HICC inhibition by flufenamate and silencing of α-ENaC increased the rate of apoptosis in HepG2 cells. It is concluded that α-ENaC is one functional element of the HICC in HepG2 cells and that the channel is an important mediator of cell proliferation; likewise, HICC blockage shifts the system from a proliferative into a rather apoptotic one. This is the first report of a role of α-ENaC in cell proliferation.

Hypertonicity-induced cation channels rescue cells from staurosporine-elicited apoptosis

Cell shrinkage is one of the earliest events during apoptosis. Cell shrinkage also occurs upon hypertonic stress, and previous work has shown that hypertonicity-induced cation channels (HICCs) underlie a highly efficient mechanism of recovery from cell shrinkage, called the regulatory volume increase (RVI), in many cell types. Here, the effects of HICC activation on staurosporine-induced apoptotic volume decrease (AVD) and apoptosis were studied in HeLa cells by means of electronic cell sizing and whole-cell patch-clamp recording. It was found that hypertonic stress reduces staurosporine-induced AVD and cell death (associated with caspase-3/7 activation and DNA fragmentation), and that this effect was actually due to activation of the HICC. On the other hand, staurosporine was found to significantly reduce osmotic HICC activation. It is concluded that AVD and RVI reflect two fundamentally distinct functional modes in terms of the activity and role of the HICC, in a shrunken cell. Our results also demonstrate, for the first time, the ability of the HICC to rescue cells from the process of programmed cell death.

The ε-Isoform of PKC Mediates the Hypertonic Activation of Cation Channels in Confluent Monolayers of Rat Hepatocytes

We were interested whether PKC alpha, delta, epsilon or zeta is the isoform actually employed in the activation of hypertonicity-induced cation channels (HICCs) in primary cultures of rat hepatocytes. Quantitative SDS-page and Western-blot experiments revealed that PKC alpha, delta and epsilon were stimulated by Indolactam V (as a DAG substitute for activation of c and nPKCs) but that only PKC delta and epsilon did respond to hypertonic stress. Furthermore, chelation of intracellular Ca(++) by BAPTA-AM did not alter HICC activation in cable-analysis experiments whereas Indolactam V as well as V8 (an Indolactam derivative specific for PKC delta and epsilon) activated HICC currents under isotonic conditions. Finally, by use of Rottlerin (as an inhibitor exhibiting a slight preference for PKC delta over epsilon) PKC epsilon could be identified as the most likely isoform responsible for the activation of the HICC.

Subunits alpha, beta and gamma of the epithelial Na+ channel (ENaC) are functionally related to the hypertonicity-induced cation channel (HICC) in rat hepatocytes

Specific small interfering RNA (siRNA) constructs were used to test for the functional relation of subunits alpha, beta, and gamma of the epithelial Na(+) channel (ENaC) to the hypertonicity-induced cation channel (HICC) in confluent rat hepatocytes. In current-clamp recordings, hypertonic stress (300 --> 400 mosM) increased membrane conductance from 75.4 +/- 9.4 to 91.1 +/- 11.2 pS (p < 0.001). The effect was completely blocked by 100 microM amiloride and reduced to 46, 30, and 45% of the control value by anti-alpha-, anti-beta-, and anti-gamma-rENaC siRNA, respectively. Scanning acoustic microscopy revealed an initial shrinkage of cells from 6.98 +/- 0.45 to 6.03 +/- 0.43 pl within 2 min. This passive response was then followed by a regulatory volume increase (RVI) by 0.42 +/- 0.05 pl (p < 0.001). With anti-alpha-, anti-beta-, and anti-gamma-rENaC siRNA, the volume response was reduced to 31, 31, and 36% of the reference level, respectively. It is concluded that all three subunits of the ENaC are functionally related to RVI and HICC activation in rat hepatocytes.

Permissive role of calcium on regulatory volume decrease in freshly isolated mouse cholangiocytes

Calcium (Ca2+) pathways are important in cell volume regulation in many cells, but its role in volume regulatory processes in cholangiocytes is unclear. Thus, we have investigated the role of Ca2+ in regulatory volume decrease (RVD) in cholangiocytes using freshly isolated bile duct cell clusters (BDCCs) from normal mouse. No significant increase in [Ca2+]i was observed during RVD, while ionomycin and ATP showed significant increases. Confocal imaging also showed no significant changes in the levels or distributions of intracellular Ca2+ during RVD. Cell volume study by quantitative videomicroscopy indicated that removal and chelation of extracellular Ca2+ by ethylene glycol-bis (beta-aminoethyl ether)-N,N,N-tetraacetic acid (EGTA) or administration of nifedipine did not affect RVD but verapamil significantly inhibited the RVD. Moreover, Ca2+ agonists or inhibitors of Ca2+ release from intracellular stores had no significant effect on RVD. However, 1,2-bis (2-aminophenoxy) ethane-N,N,N'N'-tetraacetic acid-AM (BAPTA-AM) showed significant decreases in [Ca2+]i and significantly inhibited RVD, which was reversed with coadministration of valinomycin, suggesting that BAPTA-AM-induced inhibition is due to potassium conductance or other cellular processes requiring permissive [Ca2+](i. These findings indicate that an increase in [Ca2+]i or extracellular Ca2+ is not required for RVD but Ca2+ has a permissive role in RVD of mouse cholangiocytes.

Regulation of osmotic stress transcription factor 1 (Ostf1) in tilapia(Oreochromis mossambicus) gill epithelium during salinity stress

Mechanisms of induction of osmotic stress transcription factor 1 (Ostf1)were analyzed in gill epithelium of tilapia exposed to salinity stress. Experiments with primary cultures of gill epithelial cells revealed that hyperosmotic Ostf1 induction was independent of systemic factors. In addition,the synthetic glucocorticoid receptor agonist dexamethasone did not affect Ostf1 levels, arguing against cortisol being the signal for Ostf1 induction during hyperosmotic stress. Exposure of primary gill cell cultures to a hyperosmotic agent that is cell permeable and non-hypertonic (glycerol) did not trigger Ostf1 induction. However, when gill cells were exposed to hypertonicity (either in the form of NaCl or other forms) Ostf1 was rapidly and significantly induced. Analysis of hnRNA and mRNA levels revealed that Ostf1 upregulation in gill cells of intact fish and primary cultures of gill epithelial cells was mediated by transient mRNA stabilization. In addition to the initial transient mRNA stabilization a subsequent transcriptional induction of Ostf1 was observed. In cultured gill cells increase in Ostf1 mRNA synthesis was stable and very potent, whereas in gill cells of intact fish this increase was transient. This observation suggests positive feedback by Ostf1 or one of its targets and negative feedback by systemic factors on Ostf1 transcription.

We conclude that Ostf1 induction in gill epithelial cells of tilapia exposed to salinity stress (1) is independent of cortisol or other systemic factors; (2) depends on hypertonicity as the signal; and (3) is based on transient mRNA stabilization. Moreover, our data on primary cell cultures show that systemic signals are necessary to prevent sustained transcriptional induction of Ostf1 during hyperosmotic stress, indicating feedback regulation and a high degree of complexity of osmosensing and signaling networks in euryhaline fishes.

Hypertonicity-induced cation channels

Whenever studied in a quantitative fashion, hypertonicity-induced cation channels (HICCs) are found to be the main mediators of regulatory volume increase. In most instances, these channels are either inhibited by amiloride (but insensitive to Gd3+ and flufenamate) or they are efficiently blocked by Gd3+ and flufenamate (but insensitive to amiloride). Of note, however, from two preparations so far a mixed type of pharmacology has also been reported. Whereas the ion selectivity of amiloride-sensitive HICCs has not been studied in much detail yet, amiloride-insensitive channels are either equally permeable to Na+, K+, Cs+ and Li+ but impermeable to N-methyl-D-glucamine (NMDG+) or they exhibit a permeability to Li+ and NMDG+ that amounts to some 50% when compared with that of Na+. Also in this respect, however, some peculiarities do exist. Concerning the actual molecular correlate, evidence was reported that HICCs may be related to the (amiloride-sensitive) epithelial Na+ channel and/or to transient receptor potential channels. Recent findings suggest that HICCs may contribute to cell proliferation, just as the K+ channels that are employed in regulatory volume decrease are mediators of the opposing process, i.e. apoptosis.

A novel hypertonicity-induced cation channel in primary cultures of human hepatocytes

In whole-cell recordings on primary cultures of human hepatocytes, we observe the hypertonic activation of a novel type of cation channel with a permeability ratio for Na(+):Li(+):K(+):Cs(+):NMDG(+) of 1:1.2:1.3:1.2:0.6. With a P(Ca)/P(Na) of 0.7 the channel is also clearly permeable to Ca(++). Most likely, the channel is Cl(-) impermeable but its activity critically depends on the extracellular Cl(-) concentration (with the half maximal effect at 88 mmol/l). With a 64% inhibition by amiloride and a complete block by flufenamate and Gd(3+) (at 100 micromol/l each), the channel may represent a molecular link between the amiloride-sensitive and insensitive channels reported so far.

Effect of high-NaCl or high-KCl diet on hepatic Na+- and K+-receptor sensitivity and NKCC1 expression in rats

We previously reported that the bumetanide-sensitive Na+-K+-2Cl-cotransporter (NKCC1) is involved in the hepatic Na+and K+sensor mechanism. In the present study, we examined the effects of a high-NaCl or high-KCl diet on hepatic Na+and K+receptor sensitivity and NKCC1 expression in the liver of Sprague-Dawley rats. RT-PCR and Western blots were used to measure NKCC1 mRNA and protein expression, respectively. Infusion of hypertonic NaCl or isotonic KCl + NaCl solutions into the portal vein increased hepatic afferent nerve activity (HANA) in a Na+or K+dose-dependent manner. After 4 wk on a high-NaCl or high-KCl diet, HANA responses were attenuated compared with animals fed a normal diet, and NKCC1 expression was reduced. These results show that a high-NaCl or high-KCl diet decreases NKCC1 expression in the liver, and it might cause a reduction in hepatic Na+- and K+-receptor sensitivity.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Call volume and insulin signaling

Perturbations of cell hydration as provoked by changes in ambient osmolarity or under isoosmotic conditions by hormones, second messengers, intracellular substrate accumulation, or reactive oxygen intermediates critically contribute to the physiological regulation of cell function. In general an increase in cell hydration stimulates anabolic metabolism and proliferation and provides cytoprotection, whereas cellular dehydration leads to a catabolic situation and sensitizes cells to apoptotic stimuli. Insulin produces cell swelling by inducing a net K+ and Na+ accumulation inside the cell, which results from a concerted activation of Na+/H+ exchange, Na+/K+/2Cl- symport, and the Na+/K(+)-ATPase. In the liver, insulin-induced cell swelling is critical for stimulation of glycogen and protein synthesis as well as inhibition of autophagic proteolysis. These insulin effects can largely be mimicked by hypoosmotic cell swelling, pointing to a role of cell swelling as a trigger of signal transduction. This article discusses insulin-induced signal transduction upstream of swelling and introduces the hypothesis that cell swelling as a signal amplifyer represents an essential component in insulin signaling, which contributes to the full response to insulin at the level of signal transduction and function. Cellular dehydration impairs insulin signaling and may be a major cause of insulin resistance, which develops in systemic hyperosmolarity, nutrient deprivation, uremia, oxidative challenges, and unbalanced production of insulin-counteracting hormones. Hydration changes affect cell functions at multiple levels (such as transcriptom, proteom, phosphoproteom, and the metabolom) and a system biological approach may allow us to develop a more holistic view on the hydration dependence of insulin signaling in the future.

Volume-sensitive tyrosine kinases regulate liver cell volume through effects on vesicular trafficking and membrane Na+ permeability

In liver cells, the influx of Na+ mediated by nonselective cation (NSC) channels in the plasma membrane contributes importantly to regulation of cell volume. Under basal conditions, channels are closed; but both physiologic (e.g. insulin) and pathologic (e.g. oxidative stress) stimuli that are known to stimulate tyrosine kinases are associated with large increases in membrane Na+ permeability to approximately 80 pA/pF or more. Consequently, the purpose of these studies was to evaluate whether volume-sensitive tyrosine kinases mediate cell volume increases through effects on the activity or distribution of NSC channel proteins. In HTC hepatoma cells, decreases in cell volume evoked by hypertonic exposure increased total cellular tyrosine kinase activity approximately 20-fold. Moreover, hypertonic exposure (320-400 mosM) was followed after a delay by NSC channel activation and partial recovery of cell volume toward basal values (regulatory volume increase (RVI)). The tyrosine kinase inhibitors genistein and erbstatin prevented both NSC channel activation and RVI. Similarly, hypertonic exposure resulted in an increase in p60(c-src) activity, and intracellular dialysis with recombinant p60(c-src) led to activation of NSC currents in the absence of an osmolar gradient. Utilizing FM1-43 fluorescence, exposure to hypertonic media caused a rapid increase in the rate of exocytosis of approximately 40% (p < 0.01), and genistein inhibited both exocytosis and channel activation. These findings indicate that volume-sensitive increases in p60(c-src) and/or related tyrosine kinases play a key role in the regulation of membrane Na+ permeability, suggesting that increases in the NSC conductance may be mediated in part through rapid recruitment of a distinct pool of channel-containing vesicles.

Role of cell volume in K+-induced Ca2+ signaling by rat adrenal glomerulosa cells

The involvement of cell volume in the K+-evoked Ca2+ signaling was studied in cultured rat glomerulosa cells. Previously we reported that hyposmosis (250 mOsm) increased the amplitude of T-type Ca2+ current and, accordingly, enhanced the Ca2+ response of cultured rat glomerulosa cells to K+. In the present study we found that this enhancement is not influenced by the cytoskeleton-disrupting drugs cytochalasin-D (20 microM) and colchicine (100 microM). Elevation of extracellular potassium concentration ([K+]e) from 3.6 to 4.6-8.6 mM induced cell swelling, which had slower kinetics than the Ca2+ signal. Cytoplasmic Ca2+ signal measured in single glomerulosa cells in response to stimulation with 5 mm K+ for 2 min showed two phases: after a rapid rise reaching a plateau within 20-30 sec, [Ca2+]c increased further slowly by approximately one third. When 5 mM K+ was coapplied with elevation of extracellular osmolarity from 290 to 320 mOsm, the second phase was prevented. These results indicate that cell swelling evoked by physiological elevation of [K+]e may contribute to the generation of sustained Ca2+ signals by enhancing voltage-activated Ca2+ influx.

A hypertonicity-activated nonselective conductance in single proximal tubule cells isolated from mouse kidney

The whole-cell patch-clamp technique was used to examine nonselective conductances in single proximal tubule cells isolated from mouse kidney. Single cells were isolated in either the presence or absence of a cocktail designed to stimulate cAMP. Patches were obtained with Na+ Ringer in the bath and Cs+ Ringer in the pipette. On initially achieving the whole-cell configuration, whole-cell currents were small. In cAMP-stimulated cells, with 5 mM ATP in the pipette solution, whole-cell currents increased with time. The activated current was linear, slightly cation-selective, did not discriminate between Na+ and K+ and was inhibited by 100 microM gadolinium. These properties are consistent with the activation of a nonselective conductance, designated G(NS). Activation of G(NS) was abolished with pipette AMP-PNP, ATP plus alkaline phosphatase or in the absence of ATP. In unstimulated cells G(NS) was activated by pipette ATP together with PKA. These data support the hypothesis that G(NS) is activated by a PKA-mediated phosphorylation event. G(NS) was also activated by a hypertonic shock. However, G(NS) does not appear to be involved in regulatory volume increase (RVI), as RVI was unaffected in the presence of the G(NS) blocker gadolinium. Instead, the ATP sensitivity of G(NS) suggests that it may be regulated by the metabolic state of the renal proximal tubule cell.

Hypertonic activation of phospholemman in solitary rat hepatocytes in primary culture

Under hypertonic conditions, solitary rat hepatocytes in primary culture shrink and subsequently exhibit a distinct regulatory volume increase (RVI). Reverse-transcribed polymerase chain reaction and 5' and 3' RACE (rapid amplification of cDNA ends) techniques reveal that these cells express phospholemman (PLM). In whole-cell recordings, the hypertonic activation of a channel is observed that resembles PLM with respect to unitary conductance (600-700 pS), gating pattern, and non-selectivity for Na(+) over K(+). In Xenopus oocytes expressing hepatocyte PLM, hypertonic stress induces a non-selective cation conductance and noise analysis reveals the activation of a channel with a unitary conductance of approximately 700 pS. These results suggest a role of PLM in the RVI of rat hepatocytes.

Expression and regulation of the Na(+)/K(+)/2Cl(-) cotransporter NKCC1 in rat liver and human HuH-7 hepatoma cells

The expression of sodium potassium chloride cotransporter 1 (NKCC1) was studied in different liver cell types. NKCC1 was found in rat liver parenchymal and sinusoidal endothelial cells and in human HuH-7 hepatoma cells. NKCC1 expression in rat hepatic stellate cells increased during culture-induced transformation in the myofibroblast-like phenotype. NKCC1 inhibition by bumetanide increased alpha(1)-smooth muscle actin expression in 2-day-cultured hepatic stellate cells but was without effect on basal and platelet-derived-growth-factor-induced proliferation of the 14-day-old cells. In perfused rat liver the NKCC1 made a major contribution to volume-regulatory K(+) uptake induced by hyperosmolarity. Long-term hyperosmotic treatment of HuH-7 cells by elevation of extracellular NaCl or raffinose concentration but not hyperosmotic urea or mannitol profoundly induced NKCC1 mRNA and protein expression. This was antagonized by the compatible organic osmolytes betaine or taurine. The data suggest a role of NKCC1 in stellate cell transformation, hepatic volume regulation, and long-term adaption to dehydrating conditions.

Effects of the ionophores valinomycin, ionomycin and gramicidin A on the element compartmentation in cultured rat hepatocytes

The element compartmentation in cultured rat hepatocytes was studied by electron probe X-ray microanalysis of freeze-dried cryosections after exposure of the cells to the ionophores valinomycin, ionomycin or gramicidin A. The most striking effect of these ionophores is the decrease of the intracellular potassium/sodium ratio from values of approximately 10 under control conditions to values below 1 after application of the ionophores. Changes of sodium, potassium and chloride are similar in cytoplasm and nucleus. However, elemental changes are delayed or impeded in mitochondria with respect to the surrounding cytoplasm. The water portion of cytoplasm and mitochondria slightly increases. Besides that, each ionophore has specific effects on the intracellular ion distribution. As compared to gramicidin A and ionomycin, valinomycin does not change the intracellular chloride content. Ionomycin induces calcium accumulation in mitochondria. The cytotoxic effects of the studied ionophores on the intracellular element distribution are more complex than supposed from their ion selective properties in membranes.

The cellular hydration state: a critical determinant for cell death and survival

Alterations in cellular hydration not only contribute to metabolic regulation, but also critically determine the cellular response to different kinds of stress. Whereas cell swelling triggers anabolic pathways and protects cells from heat and oxidative challenge, cellular dehydration contributes to insulin resistance and catabolism and increases the cellular susceptibility to stress-induced damage. Intracellular accumulation of organic osmolytes, cell cycle delay and the expression of heat shock proteins provide cellular tolerance to hyperosmolarity and protect against stressors under dehydrating conditions. This article discusses some mechanisms by which alterations in cell hydration contribute to cytoprotection or cell damage. In addition, the close relationship between osmotic and oxidative stress and the contribution of isoosmotic shrinkage to apoptotic cell death are considered.

p38 MAP kinase modulates liver cell volume through inhibition of membrane Na+ permeability

In hepatocytes, Na+ influx through nonselective cation (NSC) channels represents a key point for regulation of cell volume. Under basal conditions, channels are closed, but both physiologic and pathologic stimuli lead to a large increase in Na+ and water influx. Since osmotic stimuli also activate mitogen-activated protein (MAP) kinase pathways, we have examined regulation of Na+ permeability and cell volume by MAP kinases in an HTC liver cell model. Under isotonic conditions, there was constitutive activity of p38 MAP kinase that was selectively inhibited by SB203580. Decreases in cell volume caused by hypertonic exposure had no effect on p38, but increases in cell volume caused by hypotonic exposure increased p38 activity tenfold. Na+ currents were small when cells were in isotonic media but could be increased by inhibiting constitutive p38 MAP kinase, thereby increasing cell volume. To evaluate the potential inhibitory role of p38 more directly, cells were dialyzed with recombinant p38alpha and its upstream activator, MEK-6, which substantially inhibited volume-sensitive currents. These findings indicate that constitutive p38 activity contributes to the low Na+ permeability necessary for maintenance of cell volume, and that recombinant p38 negatively modulates the set point for volume-sensitive channel opening. Thus, functional interactions between p38 MAP kinase and ion channels may represent an important target for modifying volume-sensitive liver functions.

In rat hepatocytes, the hypertonic activation of Na(+) conductance and Na(+)-K(+)-2Cl(-) symport--but not Na(+)-H(+) antiport--is mediated by protein kinase C

1. The initial event in the regulatory volume increase (RVI) of rat hepatocytes is an import of extracellular Na(+) via Na(+) conductance, Na(+)-K(+)-2Cl(-) symport, and Na(+)-H(+) antiport. 2. Here, the protein kinase inhibitors staurosporine (100 nmol l(-1)) and bis-indolyl-maleimide I (400 nmol l(-1)) were used to test for a possible contribution of protein kinase C (PKC) to the hypertonic activation of these transporters in confluent primary cultures. 3. Stimulation of Na(+) conductance was monitored: (i) by use of a differential approach based on Na(+) fluxes, (ii) by means of cable analysis, and (iii) in experiments with low Na(+) pulses. All three experimental protocols in concert demonstrated a block of the activation of Na(+) conductance by staurosporine and bis-indolyl-maleimide I. 4. In addition, both compounds significantly reduced the hypertonic activation of Na(+)-K(+)-2Cl(-) symport (quantified on the basis of furosemide-sensitive (86)Rb(+) uptake) to approximately 30 %. 5. In contrast, neither staurosporine nor bis-indolyl-maleimide I had any detectable effect on the hypertonicity-induced alkalinization of cell pH via Na(+)-H(+) antiport (determined fluorometrically). 6. Staurosporine and bis-indolyl-maleimide I completely blocked the RVI of rat hepatocytes (quantified by means of confocal laser-scanning microscopy). The high efficiency of the block suggests an additional inhibitory effect of both compounds on the activity of Na(+)/K(+)-ATPase (determined as ouabain-sensitive (86)Rb(+) uptake). 7. It is concluded that the hypertonic activation of rat hepatocyte Na(+) conductance and Na(+)-K(+)-2Cl(-) symport--but not Na(+)-H(+) antiport--is probably mediated by PKC.

Differential expression and regulation of Na(+)/H(+) exchanger isoforms in rabbit parietal and mucous cells

Several Na(+)/H(+) exchanger (NHE) isoforms are expressed in the stomach, and NHE1 and NHE2 knockout mice display gastric mucosal atrophy. This study investigated the cellular distribution of the NHE isoforms NHE1, NHE2, NHE3, and NHE4 in rabbit gastric epithelial cells and their regulation by intracellular pH (pH(i)), hyperosmolarity, and an increase in cAMP. Semiquantitative RT-PCR and Northern blot experiments showed high NHE1 and NHE2 mRNA levels in mucous cells and high NHE4 mRNA levels in parietal and chief cells. Fluorescence optical measurements in cultured rabbit parietal and mucous cells using the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein and NHE isoform-specific inhibitors demonstrated that in both cell types, intracellular acidification activates NHE1 and NHE2, whereas hyperosmolarity activates NHE1 and NHE4. The relative contribution of the different isoforms to pH(i)- and hyperosmolarity-activated Na(+)/H(+) exchange in the different cell types paralleled their relative expression levels. cAMP elevation also stimulated NHE4, whereas an increase in osmolarity above a certain threshold further increased NHE1 and not NHE4 activity. We conclude that in rabbit gastric epithelium, NHE1 and NHE4 regulate cell volume and NHE1 and NHE2 regulate pH(i). The high NHE1 and NHE2 expression levels in mucous cells may reflect their special need for pH(i) regulation during high gastric acidity. NHE4 is likely involved in volume regulation during acid secretion.

Insulin resistance induced by loop diuretics and hyperosmolarity in perfused rat liver

Insulin-induced cell swelling was recently suggested to reflect an independent signal for metabolic insulin effects such as inhibition of hepatic proteolysis, which is transmitted at the level of autophagosome formation via p38MAPK activation [Häussinger et al., Gastroenterology 116 (1999), 921-935]. Here, the role of insulin-induced cell swelling in the overall context of insulin signalling towards proteolysis inhibition was studied in perfused rat liver. Loop diuretics and hyperosmolarity, which impair insulin-stimulated cell swelling, strongly blunt Erk-2 and p38MAPK activation as well as proteolysis inhibition by insulin, but are without effect on insulin-induced tyrosine phosphorylation of IR-beta and IRS-1. Inhibitors of phosphatidylinositol-3-kinase (PI3-kinase) also block insulin-induced cell swelling, MAP kinase activation and proteolysis inhibition, but the antiproteolytic response to hypoosmolarity remains unaffected. We suggest that PI3-kinase-mediated cell swelling induced by insulin is required to amplify the insulin signal to MAP kinases and thus proteolysis regulation. The perturbation of insulin-induced cell swelling may be of pathophysiological relevance for the development of insulin resistance in clinical situations associated with hyperosmotic dehydration and loop diuretic treatment.

The epithelial Na(+) channel (ENaC) is related to the hypertonicity-induced Na(+) conductance in rat hepatocytes

The epithelial Na(+) channel (ENaC) is composed of the subunits alpha, beta, and gamma [Canessa et al., Nature 367 (1994) 463-467] and typically exhibits a high affinity to amiloride [Canessa et al., Nature 361 (1993) 467-470]. When expressed in Xenopus oocytes, conflicting results were reported concerning the osmo-sensitivity of the channel [Ji et al., Am. J. Physiol. 275 (1998) C1182-C1190; Hawayda and Subramanyam, J. Gen. Physiol. 112 (1998) 97-111; Rossier, J. Gen. Physiol. 112 (1998) 95-96]. Rat hepatocytes were the first system in which amiloride-sensitive sodium currents in response to hypertonic stress were reported [Wehner et al., J. Gen. Physiol. 105 (1995) 507-535; Wehner et al., Physiologist 40 (1997) A-4]. Moreover, all three ENaC subunits are expressed in these cells [Böhmer et al., Cell. Physiol. Biochem. 10 (2000) 187-194]. Here, we injected specific antisense oligonucleotides directed against alpha-rENaC into single rat hepatocytes in confluent primary culture and found an inhibition of hypertonicity-induced Na(+) currents by 70%. This is the first direct evidence for a role of the ENaC in cell volume regulation.

Genistein inhibits osmotic activation of Na(+) /H( +) exchange in human platelets

The osmotic shrinkage is an important activator of the Na(+)/H( *) exchanger. The intracellular signaling mechanisms by which shrinkage changes intracellular pH have not been fully elucidated. In human platelets, the removal of calcium did not prevent the osmotic activation of the exchanger. The increase of pH(i) after an hyperosmotic stress was reduced by W-7 (63 micromol l(-1)), and by ML-7 (25 micromol l(-1)), inhibitors of responses mediated by calmodulin or by myosin light chain kinase, but the high concentrations needed suggested that non-specific effects could be involved. Although the exchanger was quiescent during preincubation in hypertonic sodium free solutions, some steps of the signal transduction chain that links the shrinkage to the exchanger activation suffers a modification. Therefore, upon exposure to isotonic sodium-containing media, the rate of recovery from acid loads was increased. The presence of genistein (100 micromol l( -1)) during the preincubation inhibited this activation of Na(+)/H( +) exchanger. We propose that shrinkage induce activation of tyrosine kinases, which in turn leads to the activation of Na(+)/H(+) exchanger and contributes to the restoration of cell volume in human platelets.

Agonist-induced cytoplasmic volume changes in cultured rabbit parietal cells

Concomitant Na(+)/H(+) and Cl(-)/HCO(3)(-) exchange activation occurs during stimulation of acid secretion in cultured rabbit parietal cells, possibly related to a necessity for volume regulation during the secretory process. We investigated whether cytoplasmic volume changes occur during secretagogue stimulation of cultured rabbit parietal cells. Cells were loaded with the fluorescent dye calcein, and the calcein concentration within a defined cytoplasmic volume was recorded by confocal microscopy. Forskolin at 10(-5) M, carbachol at 10(-4) M, and hyperosmolarity (400 mosmol) resulted in a rapid increase in the cytoplasmic dye concentration by 21 +/- 6, 9 +/- 4, and 23 +/- 5%, respectively, indicative of cell shrinkage, followed by recovery to baseline within several minutes, indicative of regulatory volume increase (RVI). Depolarization by 5 mM barium resulted in a decrease of the cytoplasmic dye concentration by 10 +/- 2%, indicative of cell swelling, with recovery within 15 min, and completely prevented forskolin- or carbachol-induced cytoplasmic shrinkage. Na(+)/H(+) exchange inhibitors slightly reduced the initial cell shrinkage and significantly slowed the RVI, whereas 100 microM bumetanide had no significant effect on either parameter. We conclude that acid secretagoguges induce a rapid loss of parietal cell cytoplasmic volume, followed by RVI, which is predominantly mediated by Na(+)/H(+) and Cl(-)/HCO(3)(-) exchange.

Analysis of Multivariables during Porcine Liver Digestion to Improve Hepatocyte Yield and Viability for Use in Bioartificial Liver Support Systems

In order to achieve optimal BALSS function, preparation of porcine hepatocytes with high yield, viability, and P450 activity is known to be important. To date hepatocyte yields have varied from 0.58 × 10¹⁰ to 3.45 × 10 and viabilities from 75% to 95% within and between laboratories, even when using the same digestion methods and procedures, indicating that hepatocyte isolation during porcine liver digestion is not fully optimized. The aim of this work was to identify the critical parameters affecting cell recovery during porcine liver harvesting by investigating 21 variables involved in the process, including pig body and liver weight, different digestion times of perfusates, pH, a range of concentrations of sodium and chloride in EDTA, and collagenase perfusates. Univariate and multivariate analysis of a retrospective study (n = 23) revealed that low perfusate pH during the process of digestion had a positive effect on hepatocyte yield (p < 0.05), while high (relative) concentrations of sodium and chloride in the perfusates had significant negative effects on hepatocyte viability (both p < 0.05). Sodium and chloride had narrow optimal ranges for achieving a >90% viability. These findings were then tested in a prospective study (n = 10) and further verified. High hepatocyte viabilities (91.8 ± 1.6%, p = 0.036) and yields (2.56 ± 0.48 × 10¹⁰) were achieved consistently, and P450IA1 activity was increased after sodium and chloride concentrations and pH in the perfusates were controlled. The physiological mechanism by which sodium and chloride affects hepatocyte viability during porcine liver digestion is discussed.

Functional interactions between oxidative stress, membrane Na(+) permeability, and cell volume in rat hepatoma cells

BACKGROUND & AIMS

Oxidative stress leads to a rapid initial loss of liver cell volume, but the adaptive mechanisms that serve to restore volume have not been defined. This study aimed to evaluate the functional interactions between oxidative stress, cell volume recovery, and membrane ion permeability.

METHODS

In HTC rat hepatoma cells, oxidative stress was produced by exposure to H(2)O(2) or D-alanine plus D-amino acid oxidase (40 U/mL).

RESULTS

Oxidative stress resulted in a rapid decrease in relative cell volume to 0.85 +/- 0.06. This was followed by an approximately 100-fold increase in membrane cation permeability and partial volume recovery to 0.97 +/- 0.05 of original values. The volume-sensitive conductance was permeable to Na(+) approximately K(+) >> Tris(+), and whole-cell current density at -80 mV increased from -1.2 pA/pF at 10(-5) mol/L H(2)O(2) to -95.1 pA/pF at 10(-2) mol/L H(2)O(2). The effects of H(2)O(2) were completely inhibited by dialysis of the cell interior with reduced glutathione, and were markedly enhanced by inhibition of glutathione synthase.

CONCLUSIONS

These findings support the presence of dynamic functional interactions between cell volume, oxidative stress, and membrane Na(+) permeability. Stress-induced Na(+) influx may represent a beneficial adaptive response that partially restores cell volume over short periods, but sustained cation influx could contribute to the increase in intracellular [Na(+)] and [Ca(2+)] associated with cell injury and necrosis.

Evidence for Gd(3+) inhibition of membrane ATP permeability and purinergic signaling

Extracellular ATP functions as an important autocrine and paracrine signal that modulates a broad range of cell and organ functions through activation of purinergic receptors in the plasma membrane. Because little is known of the cellular mechanisms involved in ATP release, the purpose of these studies was to evaluate the potential role of the lanthanide Gd(3+) as an inhibitor of ATP permeability and to assess the physiological implications of impaired purinergic signaling in liver cells. In rat hepatocytes and HTC hepatoma cells, increases in cell volume stimulate ATP release, and the localized increase in extracellular ATP increases membrane Cl(-) permeability and stimulates cell volume recovery through activation of P(2) receptors. In cells in culture, spontaneous ATP release, as measured by a luciferin-luciferase-based assay, was always detectable under control conditions, and extracellular ATP concentrations increased 2- to 14-fold after increases in cell volume. Gd(3+) (200 microM) inhibited volume-sensitive ATP release by >90% (P < 0.001), inhibited cell volume recovery from swelling (P < 0.01), and uncoupled cell volume from increases in membrane Cl(-) permeability (P < 0.01). Moreover, Gd(3+) had similar inhibitory effects on ATP release from other liver and epithelial cell models. Together, these findings support an important physiological role for constitutive release of ATP as a signal coordinating cell volume and membrane ion permeability and suggest that Gd(3+) might prove to be an effective inhibitor of ATP-permeable channels once they are identified.

Uptake of bromosulfophthalein via SO2-4/OH- exchange increases the K+ conductance of rat hepatocytes

In confluent primary cultures of rat hepatocytes, micromolar concentrations of bromosulfophthalein (BSP) lead to a sizeable hyperpolarization of membrane voltage. The effect is a saturable function of BSP concentration yielding an apparent value of 226 micromol/l and a Vmax of -10.3 mV. The BSP-induced membrane hyperpolarization is inhibited by the K+ channel blocker Ba2+, and in cable-analysis and ion-substitution experiments it becomes evident that the effect is due to a significant increase in cell membrane K+ conductance. Voltage changes were attenuated by the simultaneous administration of SO2-4, succinate, and cholate (cis-inhibition) and increased after preincubation with SO2-4 and succinate (trans-stimulation), suggesting that the effect occurs via BSP uptake through the known SO2-4/OH- exchanger. Microfluorometric measurements reveal that BSP-induced activation of K+ conductance is not mediated by changes in cell pH, cell Ca2+, or cell volume. It is concluded that K+ channel activation by BSP (as well as by DIDS and indocyanine green) may reflect a physiological mechanism linking the sinusoidal uptake of certain anions to their electrogenic canalicular secretion.

Aquaporin-1 expression in proximal tubule epithelial cells of human kidney is regulated by hyperosmolarity and contrast agents

Primary cells of renal proximal tubule epithelium (S1 segment) of human kidney (HRPTE cells) up-regulate aquaporin-1 (AQP-1) expression in response to hyperosmolarity. NaCl and D(+)-raffinose increased (2-2.5 fold) AQP-1 expression when medium osmolarity was 400 and 500 mOsm/kg.H2O. Urea did not have this effect. Unlike our previous findings with mIMCD-3 cells, vasopressin (10(-8)M) did not affect AQP-1 expression in HRPTE cells in isosmolar or NaCl-enriched hyperosmolar conditions. Furthermore, HRPTE cells increased (3-4 fold) AQP-1 expression when exposed to hyperosmolar Reno-60 and Hypaque-76 (diatrizoates, ionic) contrast agents at 400 and 500 mOsm/kg.H2O. Isosmolar (290 mOsm/kg H2O) Visipaque (iodixanol, non-ionic) at 10% (v/v) concentrations also increased AQP-1 expression, and 25% v/v of Visipaque rendered morphological alterations of HRPTE cells and a 3-fold increase in AQP-1 expression after 24h exposure. Finally, semi-quantitative RT-PCR of HRPTE cells subjected to various isosmolar or hyperosmolar conditions demonstrated up-regulation of AQP-1 mRNA and protein levels. Our results suggest AQP-1 up-regulation in HRPTE cells exposed to environmental stresses such as hyperosmolarity and high doses of isosmolar contrast agents.

Epithelial sodium channel regulated by aldosterone-induced protein sgk

Sodium homeostasis in terrestrial and freshwater vertebrates is controlled by the corticosteroid hormones, principally aldosterone, which stimulate electrogenic Na+ absorption in tight epithelia. Although aldosterone is known to increase apical membrane Na+ permeability in target cells through changes in gene transcription, the mechanistic basis of this effect remains poorly understood. The predominant early effect of aldosterone is to increase the activity of the epithelial sodium channel (ENaC), although ENaC mRNA and protein levels do not change initially. Rather, the open probability and/or number of channels in the apical membrane are greatly increased by unknown modulators. To identify hormone-stimulated gene products that modulate ENaC activity, a subtracted cDNA library was generated from A6 cells, a stable cell line of renal distal nephron origin, and the effect of candidates on ENaC activity was tested in a coexpression assay. We report here the identification of sgk (serum and glucocorticoid-regulated kinase), a member of the serine-threonine kinase family, as an aldosterone-induced regulator of ENaC activity. sgk mRNA and protein were strongly and rapidly hormone stimulated both in A6 cells and in rat kidney. Furthermore, sgk stimulated ENaC activity approximately 7-fold when they were coexpressed in Xenopus laevis oocytes. These data suggest that sgk plays a central role in aldosterone regulation of Na+ absorption and thus in the control of extracellular fluid volume, blood pressure, and sodium homeostasis.

Osmotic pressure regulates alpha beta gamma-rENaC expressed in Xenopus oocytes

The hypothesis that amiloride-sensitive Na+ channels (ENaC) are involved in cell volume regulation was tested. Anisosmotic ND-20 media (ranging from 70 to 450 mosM) were used to superfuse Xenopus oocytes expressing alpha beta gamma-rat ENaC (alpha beta gamma-rENaC). Whole cell currents were reversibly dependent on external osmolarity. Under conditions of swelling (70 mosM) or shrinkage (450 mosM), current amplitude decreased and increased, respectively. In contrast, there was no change in current amplitude of H2O-injected oocytes to the above osmotic insults. Currents recorded from alpha beta gamma-rENaC-injected oocytes were not sensitive to external Cl- concentration or to the K+ channel inhibitor BaCl2. They were sensitive to amiloride. The concentration of amiloride necessary to inhibit one-half of the maximal rENaC current expressed in oocytes (Ki; apparent dissociation constant) decreased in swollen cells and increased in shrunken oocytes. The osmotic pressure-induced Na+ currents showed properties similar to those of stretch-activated channels, including inhibition by Gd3+ and La3+, and decreased selectivity for Na+. alpha beta gamma-rENaC-expressing oocytes maintained a nearly constant cell volume in hypertonic ND-20. The present study is the first demonstration that alpha beta gamma-rENaC heterologously expressed in Xenopus oocytes may contribute to oocyte volume regulation following shrinkage.

Cardiac cell volume: crystal clear or murky waters? A comparison with other cell types

The osmolarity of bodily fluids is strictly controlled so that most cells do not experience changes in osmotic pressure under normal conditions, but osmotic changes can occur in pathological states such as ischemia, septic shock, and diabetic coma. The primary effect of a change in osmolarity is to acutely alter cell volume. If the osmolarity around a cell is decreased, the cell swells, and if increased, it shrinks. In order to tolerate changes in osmolarity, cells have evolved volume regulatory mechanisms activated by osmotic challenge to normalise cell volume and maintain normal function. In the heart, osmotic stress is encountered during a period of myocardial ischemia when metabolites such as lactate accumulate intracellularly and to a certain degree extracellularly, and cause cell swelling. This swelling may be exacerbated further on reperfusion when the hyperosmotic extracellular milieu is replaced by normosmotic blood. In this review, we describe the theory and mechanisms of volume regulation, and draw on findings in extracardiac tissues, such as kidney, whose responses to osmotic change are well characterised. We then describe cell volume regulation in the heart, with particular emphasis on the effect of myocardial ischemia. Finally, we describe the consequences of osmotic cell swelling for the cell and for the heart, and discuss the implications for antiarrhythmic drug efficacy. Using computer modelling, we have summated the changes induced by cell swelling, and predict that swelling will shorten the action potential. This finding indicates that cell swelling is an important component of the response to ischemia, a component modulating the excitability of the heart.