Intracellular potassium and macromolecular synthesis in mammalian cells

Structure and Function of Na,K-ATPase-The Sodium-Potassium Pump

Na,K-ATPase is an ubiquitous enzyme actively transporting Na-ions out of the cell in exchange for K-ions, thereby maintaining their concentration gradients across the cell membrane. Since its discovery more than six decades ago the Na-pump has been studied extensively and its vital physiological role in essentially every cell has been established. This article aims at providing an overview of well-established biochemical properties with a focus on Na,K-ATPase isoforms, its transport mechanism and principle conformations, inhibitors, and insights gained from crystal structures. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

ANXA2 Facilitates Enterovirus 71 Infection by Interacting with 3D Polymerase and PI4KB to Assist the Assembly of Replication Organelles

Similar to that of other enteroviruses, the replication of enterovirus 71 (EV71) occurs on rearranged membranous structures called replication organelles (ROs). Phosphatidylinositol 4-kinase III (PI4KB), which is required by enteroviruses for RO formation, yields phosphatidylinositol-4-phosphate (PI4P) on ROs. PI4P then binds and induces conformational changes in the RNA-dependent RNA polymerase (RdRp) to modulate RdRp activity. Here, we targeted 3D polymerase, the core enzyme of EV71 ROs, and found that the host factor Annexin A2 (ANXA2) can interact with 3D polymerase and promote the replication of EV71. Then, an experiment showed that the annexin domain of ANXA2, which possesses membrane-binding capacity, mediates the interaction of ANXA2 with EV71 3D polymerase. Further research showed that ANXA2 is localized on ROs and interacts with PI4KB. Overexpression of ANXA2 stimulated the formation of PI4P, and the level of PI4P was decreased in ANXA2-knockout cells. Furthermore, ANXA2, PI4KB, and 3D were shown to be localized to the viral RNA replication site, where they form a higher-order protein complex, and the presence of ANXA2 promoted the PI4KB-3D interaction. Altogether, our data provide new insight into the role of ANXA2 in facilitating formation of the EV71 RNA replication complex.

How potassium came to be the dominant biological cation: of metabolism, chemiosmosis, and cation selectivity since the beginnings of life

In the cytoplasm of practically all living cells, potassium is the major cation while sodium dominates in the media (seawater, extracellular fluids). Both prokaryotes and eukaryotes have elaborate mechanisms and spend significant energy to maintain this asymmetric K⁺ /Na⁺ distribution. This essay proposes an original line of evidence to explain how bacteria selected potassium at the very beginning of the evolutionary process and why it remains essential for eukaryotes.

Both monovalent cations and plectin are potent modulators of mechanical properties of keratin K8/K18 networks

Intermediate filament (IF) networks are a major contributor to cell rigidity and thus serve as vital elements to preserve the integrity of entire cell layers. Keratin K8 and K18 IFs are the basic constituents of the cytoskeleton of epithelial cells. The mechanical properties of K8/K18 networks depend on the structural arrangements of individual filaments within the network. This paper investigates the architecture of these networks in vitro under the influence of the monovalent cation potassium and that of the cytolinker protein plectin. Whereas increasing amounts of potassium ions lead to filament bundling, plectin interlinks filaments at filament intersection points but does not lead to bundle formation. The mechanics of the resulting networks are investigated by microrheology with assembled K8/K18 networks. It is shown that bundling induced by potassium ions significantly stiffens the network. Furthermore, our measurements reveal an increase in plectin-mediated keratin network rigidity as soon as an amount corresponding to more than 20% of the plectin present in cells is added to the keratin IF networks. In parallel, we investigated the influence of plectin on cell rigidity in detergent-extracted epithelial vulva carcinoma derived A431 cells in situ. These cytoskeletons, containing mostly IFs, actin filaments and associated proteins, exhibit a significantly decreased stiffness, when plectin is downregulated to ≈10% of the normal value. Therefore, we assume that plectin, via the formation of IF-IF connections and crosslinking of IFs to actin filaments, is an important contributor to cell stiffness.

Detection of pH change in cytoplasm of live myocardial ischemia cells via the ssDNA-SWCNTs nanoprobes

Myocardial ischemia is featured by a significant increase in the cytoplasm proton concentration, and such a proton change may be applied as an index for earlier ischemic heart disease diagnostics. But such a pH change in a live heart cell is difficult to monitor as a normal fluorescent probe cannot specifically transport into the cytoplasm of an ischemic cell. This is because the heart cell contains condensed myofibrils which are tight barriers for a normal probe to penetrate. We design fluorescent probes, single-strand DNA wrapped single-wall carbon nanotubes (ssDNA-SWCNTs), where the ssDNA is labeled by the dye molecule hexachloro-6-carboxyfluorescein (HEX). This nanoprobe could transport well into a live heart cell and locate in the cytoplasm to sensitively detect the intracellular pH change of myocardial ischemia. Briefly, protons neutralize the negative charges of nanoprobes in the cytoplasm. This will weaken the stability of nanoprobes and further tune their aggregation. Such aggregations induce the HEX of some nanoprobes condensed together and further result in their fluorescence quenching. The nanoprobes are advantaged in penetrating condensed myofibrils of the heart cell, and their fluorescence intensity is sensitive to the proton concentration change in the live cell cytoplasm. This new method may provide great assistance in earlier cardiopathy diagnosis in the future.

Urinary potassium loss in children with acute liver failure and acute viral hepatitis

OBJECTIVES

The aim of the present study was to determine urinary potassium (K⁺) loss (as measured by fractional excretion of K⁺ [FEK] and transtubular K⁺ gradient [TTKG]) in children with acute liver failure (ALF) and acute viral hepatitis (AVH) at the time of presentation to the hospital and day 45 of follow-up.

METHODS

Twenty-five patients with ALF and 84 patients with AVH were worked up for clinical features, liver function tests, and hepatitis viral infections and monitored for outcome. All of the patients with ALF were hospitalized. FEK and TTKG were estimated on the day patients were first seen in the hospital or hospitalized and later on day 45 of follow-up.

RESULTS

Sixty percent (15/25) of patients with ALF were hypokalemic (serum K⁺ <3.5 mEq/L) as compared with only 12% (10/84) in the AVH group (P = 0.000) at the time of presentation in the hospital. Inappropriate kaliuresis was present in 80% to 100% of hypokalemic children compared with 0% to 30% of normokalemic individuals at the time of first contact in either the ALF or AVH group. Inappropriate urinary K⁺ loss and serum K⁺ levels in the hypokalemic individuals improved as the hepatic functions recovered by day 45 of follow-up (P = 0.014-0.000). No significant change in kaliuresis was observed among normokalemic subjects between first contact and later on day 45 of follow-up (P = 0.991-0.228). Despite different physiologic mechanisms, appropriateness of kaliuresis measured by FEK and TTKG showed results in the same direction.

CONCLUSIONS

Hypokalemia and inappropriate kaliuresis observed during the acute phase of ALF and AVH reversed with clinical and biochemical recovery. In the absence of major gastrointestinal losses and renal abnormalities, there is need to investigate the contributory role of factors like hyperaldosteronism and food intake, which may have therapeutic implications.

Suppression of ribosomal function triggers innate immune signaling through activation of the NLRP3 inflammasome

Some inflammatory stimuli trigger activation of the NLRP3 inflammasome by inducing efflux of cellular potassium. Loss of cellular potassium is known to potently suppress protein synthesis, leading us to test whether the inhibition of protein synthesis itself serves as an activating signal for the NLRP3 inflammasome. Murine bone marrow-derived macrophages, either primed by LPS or unprimed, were exposed to a panel of inhibitors of ribosomal function: ricin, cycloheximide, puromycin, pactamycin, and anisomycin. Macrophages were also exposed to nigericin, ATP, monosodium urate (MSU), and poly I:C. Synthesis of pro-IL-ß and release of IL-1ß from cells in response to these agents was detected by immunoblotting and ELISA. Release of intracellular potassium was measured by mass spectrometry. Inhibition of translation by each of the tested translation inhibitors led to processing of IL-1ß, which was released from cells. Processing and release of IL-1ß was reduced or absent from cells deficient in NLRP3, ASC, or caspase-1, demonstrating the role of the NLRP3 inflammasome. Despite the inability of these inhibitors to trigger efflux of intracellular potassium, the addition of high extracellular potassium suppressed activation of the NLRP3 inflammasome. MSU and double-stranded RNA, which are known to activate the NLRP3 inflammasome, also substantially inhibited protein translation, supporting a close association between inhibition of translation and inflammasome activation. These data demonstrate that translational inhibition itself constitutes a heretofore-unrecognized mechanism underlying IL-1ß dependent inflammatory signaling and that other physical, chemical, or pathogen-associated agents that impair translation may lead to IL-1ß-dependent inflammation through activation of the NLRP3 inflammasome. For agents that inhibit translation through decreased cellular potassium, the application of high extracellular potassium restores protein translation and suppresses activation of the NLRP inflammasome. For agents that inhibit translation through mechanisms that do not involve loss of potassium, high extracellular potassium suppresses IL-1ß processing through a mechanism that remains undefined.

Nanoprobes: quantitatively detecting the femtogram level of arsenite ions in live cells

In this report, nanoprobes which could detect the femtogram level of arsenite ions in subcellular organelle of live cells are disclosed. The nanoprobes are composed of ssDNA and single-wall carbon nanotubes (SWCNTs), and the ssDNA is marked by a dye molecule. In a live cell, trace arsenite ions could interact with nanoprobes and significantly decrease the emission of the nanoprobes. With the help of a confocal microscope and cryo-electron microscopy, the lysosome target of the arsenite ion and nanoprobe is well described in high spatial resolution.

Identification of yeast genes involved in k homeostasis: loss of membrane traffic genes affects k uptake

Using the homozygous diploid Saccharomyces deletion collection, we searched for strains with defects in K(+) homeostasis. We identified 156 (of 4653 total) strains unable to grow in the presence of hygromycin B, a phenotype previously shown to be indicative of ion defects. The most abundant group was that with deletions of genes known to encode membrane traffic regulators. Nearly 80% of these membrane traffic defective strains showed defects in uptake of the K(+) homolog, (86)Rb(+). Since Trk1, a plasma membrane protein localized to lipid microdomains, is the major K(+) influx transporter, we examined the subcellular localization and Triton-X 100 insolubility of Trk1 in 29 of the traffic mutants. However, few of these showed defects in the steady state levels of Trk1, the localization of Trk1 to the plasma membrane, or the localization of Trk1 to lipid microdomains, and most defects were mild compared to wild-type. Three inositol kinase mutants were also identified, and in contrast, loss of these genes negatively affected Trk1 protein levels. In summary, this work reveals a nexus between K(+) homeostasis and membrane traffic, which does not involve traffic of the major influx transporter, Trk1.

Cardiac glycosides regulate endothelial tissue factor expression in culture

BACKGROUND

Tissue factor (TF) plays an important role in acute coronary syndromes and stent thrombosis. This study investigates whether Na(+)/K(+)-ATPase regulates TF expression in human endothelial cells.

METHODS AND RESULTS

Ouabain inhibited tumor necrosis factor (TNF)-alpha-induced endothelial TF protein expression; maximal inhibition occurred at 10(-5) mol/L, reached more than 70%, and was observed throughout the 5 hours stimulation period. The decrease in protein expression was paralleled by a reduced TF surface activity. Similarly, lowering of extracellular potassium concentration inhibited TNF-alpha-induced TF protein expression. In contrast, ouabain did not affect TNF-alpha-induced expression of full-length TF mRNA for up to 5 hours of stimulation; instead, expression of alternatively-spliced TF mRNA was upregulated after 3 and 5 hours of stimulation. Ouabain did not affect TNF-alpha-induced activation of the MAP kinases p38, extracellular signal-regulated kinase (ERK), and c-Jun terminal NH(2) kinase; activation of Akt and p70S6 kinase remained unaltered as well. Similar to the MAP kinases, ouabain did not affect TNF-alpha-induced degradation of IkappaB-alpha. Ouabain had no effect on TF protein degradation.

CONCLUSIONS

Na(+)/K(+)-ATPase is required for protein translation of endothelial TF in culture. This observation provides novel insights into posttranscriptional regulation of TF expression.

Intracellular monovalent ions as second messengers

It is generally accepted that electrochemical gradients of monovalent ions across the plasma membrane, created by the coupled function of pumps, carriers and channels, are involved in the maintenance of resting and action membrane potential, cell volume adjustment, intracellular Ca(2+ )handling and accumulation of glucose, amino acids, nucleotides and other precursors of macromolecular synthesis. In the present review, we summarize data showing that side-by-side with these classic functions, modulation of the intracellular concentration of monovalent ions in a physiologically reasonable range is sufficient to trigger numerous cellular responses, including changes in enzyme activity, gene expression, protein synthesis, cell proliferation and death. Importantly, the engagement of monovalent ions in regulation of the above-listed cellular responses occurs at steps upstream of Ca(2+) (i) and other key intermediates of intracellular signaling, which allows them to be considered as second messengers. With the exception of HCO (3) (-) -sensitive soluble adenylyl cyclase, the molecular origin of sensors involved in the function of monovalent ions as second messengers remains unknown.

Ability of Sit4p to promote K+ efflux via Nha1p is modulated by Sap155p and Sap185p

We demonstrate here that SAP155 encodes a negative modulator of K+ efflux in the yeast Saccharomyces cerevisiae. Overexpression of SAP155 decreases efflux, whereas deletion increases efflux. In contrast, a homolog of SAP155, called SAP185, encodes a positive modulator of K+ efflux: overexpression of SAP185 increases efflux, whereas deletion decreases efflux. Two other homologs, SAP4 and SAP190, are without effect on K+ homeostasis. Both SAP155 and SAP185 require the presence of SIT4 for function, which encodes a PP2A-like phosphatase important for the G1-S transition through the cell cycle. Overexpression of either the outwardly rectifying K+ channel, Tok1p, or the putative plasma membrane K+/H+ antiporter, Kha1p, increases efflux in both wild-type and sit4Delta strains. However, overexpression of the Na+-K+/H+ antiporter, Nha1p, is without effect in a sit4Delta strain, suggesting that Sit4p signals to Nha1p. In summary, the combined activities of Sap155p and Sap185p appear to control the function of Nha1p in K+ homeostasis via Sit4p.

Regulation and critical role of potassium homeostasis in apoptosis

Programmed cell death or apoptosis is broadly responsible for the normal homeostatic removal of cells and has been increasingly implicated in mediating pathological cell loss in many disease states. As the molecular mechanisms of apoptosis have been extensively investigated a critical role for ionic homeostasis in apoptosis has been recently endorsed. In contrast to the ionic mechanism of necrosis that involves Ca(2+) influx and intracellular Ca(2+) accumulation, compelling evidence now indicates that excessive K(+) efflux and intracellular K(+) depletion are key early steps in apoptosis. Physiological concentration of intracellular K(+) acts as a repressor of apoptotic effectors. A huge loss of cellular K(+), likely a common event in apoptosis of many cell types, may serve as a disaster signal allowing the execution of the suicide program by activating key events in the apoptotic cascade including caspase cleavage, cytochrome c release, and endonuclease activation. The pro-apoptotic disruption of K(+) homeostasis can be mediated by over-activated K(+) channels or ionotropic glutamate receptor channels, and most likely, accompanied by reduced K(+) uptake due to dysfunction of Na(+), K(+)-ATPase. Recent studies indicate that, in addition to the K(+) channels in the plasma membrane, mitochondrial K(+) channels and K(+) homeostasis also play important roles in apoptosis. Investigations on the K(+) regulation of apoptosis have provided a more comprehensive understanding of the apoptotic mechanism and may afford novel therapeutic strategies for apoptosis-related diseases.

c-Fos expression in ouabain-treated vascular smooth muscle cells from rat aorta: evidence for an intracellular-sodium-mediated, calcium-independent mechanism

In this study, we examined the effect of Na(+)-K(+) pump inhibition on the expression of early response genes in vascular smooth muscle cells (VSMC) as possible intermediates of the massive RNA synthesis and protection against apoptosis seen in ouabain-treated VSMC in our previous experiments. Incubation of VSMC with ouabain resulted in rapid induction of c-Fos protein expression with an approximately sixfold elevation after 2 h of incubation. c-Jun expression was increased by approximately fourfold after 12 h, whereas expression of activating transcription factor 2, cAMP/Ca(2+) response element binding protein (CREB)-1 and c-Myc was not altered. Markedly augmented c-Fos expression was also observed under Na(+)-K(+) pump inhibition in potassium-depleted medium. Na(+)-K(+) pump inhibition triggered c-Fos expression via elevation of the [Na(+)](i)/[K(+)](i) ratio. This conclusion follows from experiments showing the lack of effect of ouabain on c-Fos expression in high-potassium-low-sodium medium and from the comparison of dose responses of Na(+)-K(+) pump activity, [Na(+)](i) and [K(+)](i) content and c-Fos expression to ouabain. A fourfold increment of c-Fos mRNA was revealed 30 min following addition of ouabain to the incubation medium. At this time point, treatment with ouabain resulted in an approximately fourfold elevation of [Na(+)](i) but did not affect [K(+)](i). Augmented c-Fos expression was also observed under VSMC depolarization in high-potassium medium. Increments in both c-Fos expression and (45)Ca uptake in depolarized VSMC were abolished under inhibition of L-type Ca(2+) channels with 0.1 microM nicardipine. Ouabain did not affect the free [Ca(2+)](i) or the content of exchangeable [Ca(2+)](i). Ouabain-induced c-Fos expression was also insensitive to the presence of nicardipine and [Ca(2+)](o), as well as chelators of [Ca(2+)](o) (EGTA) and [Ca(2+)](i) (BAPTA). The effect of ouabain and serum on c-Fos expression was additive. In contrast to serum, however, ouabain failed to activate the Elk-1, serum response factor, CREB and activator protein-1 transcription factors identified within the c-Fos promoter. These results suggest that Na(+)-K(+) pump inhibition triggers c-Fos expression via [Na(+)](i)-sensitive [Ca(2+)](i)-independent transcription factor(s) distinct from factors interacting with known response elements of this gene promoter.

Inhibition of Na+,K+ pump affects nucleic acid synthesis and smooth muscle cell proliferation via elevation of the [Na+]i/[K+]i ratio: possible implication in vascular remodelling

OBJECTIVES

Na+,K+ pump inhibition is known to delay the development of apoptosis in vascular smooth muscle cells (VSMC). This study examines Na+,K+ pump involvement in the regulation of VSMC macromolecular synthesis and proliferation.

METHODS

DNA, RNA and protein synthesis in VSMC from the rat aorta was studied by the incorporation of [3H]-labelled thymidine, uridine and leucine. Cell cycle progression was estimated by flow cytometry. Intracellular Na+ and K+ content and Na+,K+ pump activity were quantified as the steady-state distribution of 22Na and 86Rb and the rate of ouabain-sensitive 86Rb uptake in Na+-loaded cells, respectively.

RESULTS

Ouabain inhibited the Na+,K+ pump with a Ki of 0.1 mmol/l. At concentrations less than 0.1 mmol/l, neither [Na+]i nor [K+]i was affected by ouabain; elevation of ouabain concentration sharply increased the [Na+]i/[K+]i ratio with a K0.5 of approximately 0.3 mmol/l. At concentrations higher than 0.1 mmol/l, ouabain time- and dose-dependently activated RNA and DNA syntheses in serum-deprived VSMC and inhibited cell cycle progression triggered by serum. In quiescent VSMC, ouabain did not affect protein synthesis, total cell number, but slightly increased the percentage of cells in the S-phase (4.25 versus 1.46%) and attenuated cell death assessed by staining with trypan blue and lactate dehydrogenase release.

CONCLUSIONS

Elevation of the [Na+]i/[K+]i ratio caused by Na+,K+ pump inhibition markedly enhances nucleic acid synthesis in quiescent VSMC and blocks cell cycle progression in serum-supplied VSMC. The relative contribution of this phenomenon as well as the anti-apoptotic action of increased [Na+]i/[K+]i ratio to vascular remodelling under augmented content of endogenous Na+,K+ pump inhibitors, seen in volume-expanded hypertension, should be investigated by in-vivo studies.

Inversion of the intracellular Na(+)/K(+) ratio blocks apoptosis in vascular smooth muscle cells by induction of RNA synthesis

This study examines the involvement of RNA and protein synthesis in the modulation of apoptosis in vascular smooth muscle cells (VSMC) by intracellular monovalent cations. In VSMC transfected with E1A adenovirus (VSMC-E1A), inversion of the [Na(+)](i)/[K(+)](i) ratio by an inhibitor of the Na(+),K(+) pump, ouabain, prevented the development of apoptosis triggered by serum withdrawal. Inhibition of apoptosis by ouabain was abolished by inhibitors of RNA and protein synthesis, actinomycin D, and cycloheximide, respectively. In VSMC-E1A, incubation with ouabain for 4 and 24 hours augmented RNA synthesis by 20% to 50% and 3-fold to 4-fold, respectively. In quiescent VSMC, the effect of ouabain and serum on RNA synthesis was additive. Ouabain did not affect the level of phosphorylation of ERK, JNK, and p38 MAP kinases and blocked apoptosis independent of the presence of the MAPK kinase inhibitors PD98059 and SB 202190. Equimolar substitution of NaCl with KCl in the incubation medium abolished the effect of ouabain on intracellular Na(+) and K(+) concentration, apoptosis, and RNA synthesis. Thus, our results demonstrate that the antiapoptotic effect of the inverted [Na(+)](i)/[K(+)](i) ratio is mediated by MAPK-independent induction of de novo synthesis of RNA species encoding inhibitor(s) of programmed cell death.

Inversion of the intracellular Na+/K+ ratio blocks apoptosis in vascular smooth muscle at a site upstream of caspase-3

Long term elevation of the intracellular Na+/K+ ratio inhibits macromolecule synthesis and proliferation in the majority of cell types studied so far, including vascular smooth muscle cells (VSMC). We report here that inhibition of the Na+,K+ pump in VSMC by ouabain or a 1-h preincubation in K+-depleted medium attenuated apoptosis triggered by serum withdrawal, staurosporine, or okadaic acid. In the absence of ouabain, both DNA degradation and Caspase-3 activation in VSMC undergoing apoptosis were insensitive to modification of the extracellular Na+/K+ ratio as well as to hyperosmotic cell shrinkage. In contrast, protection of VSMC from apoptosis by ouabain was abolished under equimolar substitution of Na+o with K+o, showing that the antiapoptotic action of Na+,K+ pump inhibition was caused by inversion of the intracellular Na+/K+ ratio. Unlike VSMC, the same level of increment of the [Na+]i/[K+]i ratio caused by a 2-h preincubation of Jurkat cells with ouabain did not affect chromatin cleavage and Caspase-3 activity triggered by treatment with Fas ligand, staurosporine, or hyperosmotic shrinkage. Thus, our results show for the first time that similar to cell proliferation, maintenance of a physiologically low intracellular Na+/K+ ratio is required for progression of VSMC apoptosis.

Kinetic studies on the activation of eukaryotic peptidyltransferase by potassium

In an effort to elucidate the role of potassium ions in the formation of peptide bond, we have used the reaction between puromycin and a ribosomal complex (from rabbit reticulocytes) bearing the donor substrate, AcPhe-tRNA, prebound at the so-called P site (puromycin-reactive state). This reaction can be analyzed as a first-order reaction. At saturating concentrations of puromycin (S) the first-order rate constant (k(max)S) is a measure of the apparent catalytic rate constant of peptidyltransferase in the puromycin reaction. This k(max)S depends on the concentration of potassium ions and increases when the concentration of K+ is increased. The data suggest a kinetic model in which potassium acts as an essential activator in the puromycin reaction. A single molecule of potassium participates in the mechanism of activation. The kinetics correspond to a sequential addition of potassium and puromycin to two separate and independent sites on the ribosome. At saturating levels of both K+ and S the maximal value for the catalytic rate constant of peptidyltransferase (k(p)) is equal to 20 min(-1) at 25 degrees C.

Bisphenolic compounds that enhance cell cation transport are found in commercial phenol red

We have isolated two bisphenolic compounds (4 and 5) that have a marked effect on K+ and Na+ concentrations in human cells from commercial preparations of the pH indicator dye phenol red (phenolsulfonphthalein). We used a bioassay to identify active chromatographic fractions from the lipophilic impurities present in phenol red, and we determined the structure of two active components (4 and 5) by 1H and 13C NMR and mass spectrometry. When added to human fibroblasts in serum-free medium, the bisphenol fluorene derivative 9,9-bis(4'-hydroxyphenyl)-3-hydroxyfluorene (5) produced a rapid loss of K+ and a gain of Na+, at low concentrations, with an EC50 between 30 and 60 ng/mL (80-160 nM). The 2- and 4-hydroxy isomers of the fluorene 5 (i.e., compounds 6 and 7), prepared by synthesis, had similar activity, although compound 6 was somewhat less potent. The bisphenol xanthene derivative 9,9-bis(4'-hydroxyphenyl)xanthene (4) elicited a similar biological response but was less potent than 5-7; it also had a strong effect on cell adhesion, causing release of cells from the plastic substrate at concentrations as low as 2-5 microg/mL (5.5-14 microM). The structures of xanthene (4) and fluorene (5) bisphenols have been confirmed by synthesis from xanthone and hydroxyfluorenone, respectively, by Friedel-Crafts alkylation with phenol. In the latter case, the desired 3-hydroxyfluorene isomer was formed in situ by rearrangement of the 1-hydroxy isomer.

Role of amino acid-induced changes in ion fluxes in the regulation of hepatic protein synthesis

Alanine is a powerful stimulator of hepatic protein synthesis whose mechanism of action has not yet been ascertained. The present work aimed to elucidate whether rate changes in ion fluxes accompanying the transport of this amino acid could play a role in the stimulation of protein synthesis. In perfused livers, the utilization of alanine produced a net uptake of K+ of 1.5 mumol/min/liver, a progressively increasing efflux of Ca2+ to reach a maximum of 0.9 mumol/min/liver, and alkalization of the extracellular medium. Inhibition of Na+/K+ exchange by ouabain reversed only the uptake of K+, indicating that this is the main way for the efflux of Na+ cotransported with alanine. In isolated hepatocytes, the uptake of alanine increased the intracellular content of K+ and the cell volume. The following observations suggest that these changes, and not an increased intracellular concentration of Na+, are associated with the stimulation of protein synthesis: 1) Ouabain inhibited the alanine stimulation of L-[3H]-valine incorporation into protein without altering the basal rate of protein labeling; 2) ouabain had no effects on alanine uptake indicating that Na+ influx is not involved in the alanine stimulation of protein synthesis; 3) disruption of Na+ gradient across the plasma membrane by specific ionophores, monensin and gramicidin D, inhibited both basal and alanine-stimulated protein synthesis, but substitution of extracellular Na+ by K+ did not prevent the stimulatory action of alanine. The observation that hypotonic buffer enhanced protein synthesis to the same degree than alanine in liver cells indicates that alanine-induced cell swelling could be sufficient to stimulate protein synthesis.

Activity-dependent regulation of inwardly rectifying potassium currents in non-myelinating Schwann cells in mice

1. Voltage-gated potassium currents were recorded from freshly dissociated non-myelinating Schwann cells of sural and sympathetic nerves from 1- to 12-week-old mice using the whole-cell or a single channel variation of the patch-clamp technique. 2. All sural cells from 2-week-old mice showed inwardly rectifying potassium (Kir+) currents in whole-cell recordings. Kir+ currents were virtually undetectable in sural cells from mice more than 6 weeks old, which also showed depolarization of the resting membrane potential. On the other hand, the magnitude of Kir+ currents increased in cervical sympathetic trunk (CST) cells in parallel with an increase of cell capacitance 1-6 weeks after birth. The density of Kir+ currents in CST cells increased 1-4 weeks after birth and then stayed constant for up to 12 weeks. 3. The unitary conductance of a single Kir+ channel in CST cells was 30 pS 2-12 weeks after birth; this was recorded in a cell-attached configuration with 154 mM K+ in the pipette. The steady-state open channel probability of single Kir+ channels in CST cells decreased with membrane hyperpolarization, but was not markedly changed 2-12 weeks after birth. 4. Conduction block of CST for 5 days induced by local application of tetrodotoxin (TTX) resulted in a significant decrease in both the magnitude and the density of Kir+ currents in whole-cell recordings in CST cells rostral to the sites of TTX block. Similar changes of Kir+ currents in whole-cell recordings were observed in cells in the inferior postganglionic branch of a superior cervical ganglion after 5 days of TTX block of CST. 5. These results suggest that neuronal activity regulates the expression of functional Kir+ channels in non-myelinating Schwann cells in adult nerves. The activity-dependent regulation of the expression of glial potassium channels could play an important role in the regulation of the potassium microenvironment around active axons to maintain impulse conduction in unmyelinated fibres.

Arrest of cell cycle progression of HeLa cells in the early G1 phase in K(+)-depleted conditions and its recovery upon addition of insulin and LDL

Cell cycle progression of synchronized HeLa cells was studied by measuring labeling of the nuclei with [3H]thymidine. The progression was arrested in a chemically defined medium in which K+ was replaced by Rb+ (Rb-CDM) but was restored upon addition of insulin and/or low density lipoprotein (LDL). Cells started DNA synthesis 12 hr after addition of insulin and/or LDL, regardless of the time of arrest, suggesting their arrest early in the G1 phase. After incubation of cells in Rb-CDM containing insulin or LDL singly for 3, 6, or 9 hr, replacement of the medium by that without an addition resulted in marked delay in entry of cells into the S phase, but in its replacement by medium containing both agents, the delay was insignificant. Synthesis of bulk protein, estimated as increase in the cell volume, was not strongly inhibited. From these results we conclude that cell cycle progression of HeLa cells in K(+)-depleted CDM is arrested early in the G1 phase and that the arrest is due to lack of some protein(s) required for entry into the S phase that is synthesized in the early G1 phase.

An impurity in phenol red opens an ion channel in cultured human cells

Human fibroblast, bladder carcinoma, and breast carcinoma cells in commercial serum-free media or balanced salt solutions rapidly lose K+ and gain Na+. This rapid K+ loss is caused by one or more impurities in phenol red. Adding serum or albumin to media or to balanced salts prevents K+ loss. Quinine also prevents part of this loss in fibroblasts and breast carcinoma cells, suggesting that the impurity acts on an ion channel.

Acrylamide disrupts elemental composition and water content of rat tibial nerve. II. Schwann cells and myelin

The effects of subchronic and subacute acrylamide (ACR) intoxication on elemental composition (Na, P, S, Cl, K, Ca, Mg) and water content of Schwann cell body cytoplasm and myelin were assessed in rat tibial nerve. Electron probe X-ray microanalysis demonstrated that, in control rats, peripheral nerve glia and myelin exhibited highly characteristic distributions of elements and water and that ACR intoxication was associated with disruption of this normal subcellular distribution. When rats were intoxicated with ACR by either the oral (2.8 mM in drinking water for 15, 22, 30, and 60 days) or the intraperitoneal (50 mg/kg/day x5 and 10 days) route, an exposure-dependent loss of cytoplasmic Na, K, P, Cl, Mg, and water regulation was detected in Schwann cell cytoplasm. Maximum development of elemental deregulation occurred after 30 days of oral ACR exposure and 10 days of ip treatment. The cytoplasmic elements involved and their corresponding quantitative changes were similar regardless of the route of ACR intoxication. Analysis of myelin revealed that both oral and parenteral ACR exposure caused early, persistent increases in dry weight Na, P, and water content. However, Cl dry weight concentrations were increased by oral exposure and decreased by ip ACR injection. Results of this study indicate that ACR intoxication is associated with a significant disturbance of subcellular element and water distribution in tibial nerve Schwann cells and myelin. The pattern of elemental disruption is typical of reversible cell damage and, therefore, Schwann cell injury might play a role in the expression of ACR neurotoxicity.

A role for glial cells in activity-dependent central nervous plasticity? Review and hypothesis

Activity-dependent plasticity relies on changes in neuronal transmission that are controlled by coincidence or noncoincidence of presynaptic and postsynaptic activity. These changes may rely on modulation of neural transmission or on structural changes in neuronal circuitry. The present overview summarizes experimental data that support the involvement of glial cells in central nervous activity-dependent plasticity. A role for glial cells in plastic changes of synaptic transmission may be based on modulation of transmitter uptake or on regulation of the extracellular ion composition. Both mechanisms can be initiated via neuronal-glial information transfer by potassium ions, transmitters, or other diffusible factor originating from active neurons. In addition, the importance of changes in neuronal circuitry in many model systems of activity-dependent plasticity is summarized. Structural changes in neuronal connectivity can be influenced or mediated by glial cells via release of growth or growth permissive factors on neuronal activation, and by active displacement and subsequent elimination of axonal boutons. A unifying hypothesis that integrates these possibilities into a model of activity-dependent plasticity is proposed. In this model glial cells interact with neurons to establish plastic changes; while glial cells have a global effect on plasticity, neuronal mechanisms underlie the induction and local specificity of the plastic change. The proposed hypothesis not only explains conventional findings on activity-dependent plastic changes, but offers an intriguing possibility to explain several paradoxical findings from studies on CNS plasticity that are not yet fully understood. Although the accumulated data seem to support the proposed role for glial cells in plasticity, it has to be emphasized that several steps in the proposed cascades of events require further detailed investigation, and several "missing links" have to be addressed by experimental work. Because of the increasing evidence for glial heterogeneity (for review see Wilkin et al., 1990) it seems to be of great importance to relate findings on glial populations to the developmental stage and topographical origin of the studied cells. The present overview is intended to serve as a guideline for future studies and to expand the view of "neuro" physiologists interested in activity-dependent plasticity. Key questions that have to be addressed relate to the mechanisms of release of growth and growth-permissive factors from glial cells and neuronal-glial information transfer. It is said that every complex problem has a simple, logical, wrong solution. Future studies will reveal the contribution of the proposed simple and logical solution to the understanding of central nervous plasticity.

Elemental composition and water content of myelinated axons and glial cells in rat central nervous system

The distribution of elements (e.g. Na, Cl, K) and water in CNS cells is unknown. Therefore, electron probe X-ray microanalysis (EPMA) was used to measure water content and concentrations (mmol/kg dry or wet weight) of Na, Mg, P, S, Cl, K and Ca in morphological compartments of myelinated axons and glial cells from rat optic nerve and cervical spinal cord white matter. Axons in both CNS regions exhibited similar water content (approximately 90%), and relatively high concentrations (wet and dry weight) of K with low Na and Ca levels. The K content of axons was related to diameter, i.e. small axons in spinal cord and optic nerve had significantly less (25-50%) K than larger diameter axons from the same CNS region. The elemental composition of spinal cord mitochondria was similar to corresponding axoplasm, whereas the water content (75%) of these organelles was substantially lower than that of axoplasm. In glial cell cytoplasm of both CNS areas, P and K (wet and dry weight) were the most abundant elements and water content was approximately 75%. CNS myelin had predominantly high P levels and the lowest water content (33-55%) of any compartment measured. The results of this study demonstrate that each morphological compartment of CNS axons and glia exhibits a characteristic elemental composition and water content which might be related to the structure and function of that neuronal region.

Disruption of cellular elements and water in neurotoxicity: studies using electron probe X-ray microanalysis

Regulation of elements and water in nerve cells is a complex, multifaceted process which appears to be vulnerable to neurotoxic events. However, much of our knowledge concerning the potential role of elements in nerve cell injury is limited by the relatively gross level of corresponding analyses. If we are to confirm and understand the proposed role, more precise and detailed information is needed. As indicated in this commentary, research employing electron probe microanalysis and digital X-ray imaging has begun to provide this necessary information. Recent EPMA studies of nerve and glial cells in the peripheral and central nervous systems have shown that each cell type and their corresponding morphologic compartments exhibit unique distributions of elements and water. The use of microprobe analysis has allowed us to document precisely how elements and water redistribute in morphological compartments of damaged nerve cells. Accumulating evidence from EPMA studies suggests that, rather than being an epiphenomenon, intracellular changes in diffusible elements might mediate the functional and structural consequences of neurotoxic insult. It is also evident from this research that elements other than Ca might play a pertinent role in the injury response and that changes in intraneuronal elemental composition might develop according to a specific temporal pattern, e.g., transection-induced sequential alterations in axonal K, Na, Cl, and Ca. Therefore, rather than conducting end-point studies, longitudinal investigations are necessary to define the sequential pattern of elemental perturbation associated with a given neurotoxic event. Such research can also help identify the role of individual elements in the injury response. Future microprobe studies should be combined with measurements of ion levels (e.g., using fura-2 or ion selective electrodes) to provide a comprehensive and dynamic view of elemental deregulation. In addition, parallel biochemical studies should be performed to determine mechanisms of elemental disruption and possible biochemical and metabolic consequences of this disruption. Although evidence presented in this commentary suggests that each type of neurotoxic event produces a characteristic pattern of decompartmentalization, further work is necessary to confirm this possibility. Finally, based on a presumed involvement of elements in nerve injury, efforts are currently underway in several laboratories to develop appropriate pharmacological therapies for certain chemical- and trauma-induced neuropathological conditions (Dretchen et al., 1986; El-Fawal et al., 1989; Beattie et al., 1989).(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulating effects of insulin and low-density lipoprotein on cell growth and macromolecular syntheses of HeLa cells cultured in K(+)-depleted medium

The influence of the intracellular K+ concentration on the effects of growth factors (insulin, EGF, hydrocortisone, and transferrin) and LDL on growth of HeLa cells was investigated. Upon replacement of K+ in a chemically defined medium (K(+)-CDM) by Rb+ (Rb(+)-CDM), about 80% of the intracellular K+ was replaced by Rb+ within 24 h, but showed no further change in the next 24 h, irrespective of addition of dialyzed calf serum (5%) or growth factors to the medium. In Rb(+)-CDM, cell growth and DNA synthesis were greatly suppressed, although cell viability was not significantly altered for 72 h. The suppression of cell growth was partially restored by addition of serum, insulin (5 micrograms/ml), or LDL (2.5 mg/ml) to Rb(+)-CDM. A combination of serum and insulin or insulin and LDL stimulated cell growth to approximately the level in K(+)-CDM without any addition, but a combination of serum and LDL did not have more effect than that of serum alone. Unexpectedly, other factors were ineffective in stimulating growth in Rb(+)-CDM. In Rb(+)-CDM, the effect of insulin was lost in 24-48 h, whereas that of LDL persisted for at least 96 h. Insulin and LDL also enhanced growth in K(+)-CDM. After cessation of cell growth in Rb(+)-CDM for 24 h, addition of insulin and/or LDL markedly restored cell growth and DNA synthesis. Therefore, insulin and LDL may stimulate certain mechanisms required for cell growth that can operate in K(+)-deficient conditions.

Dark-rearing retards the maturation of astrocytes in restricted layers of cat visual cortex

The cat visual cortex develops its mature appearance, i.e., its circuitry and neuronal morphology, during a limited period of postnatal development under the influence of visual experience. The critical period for cortical plasticity, which normally extends from the third to seventh postnatal week, can be prolonged by raising animals in total darkness. The prolongation of the critical period by dark-rearing is restricted to the cortical layers except layer IV. Besides the influence of afferent activity on the physiology of cortical cells and on the interconnectivity of thalamo-cortical afferents, visual experience has also been shown to affect the development of glial cells. The present study investigates the effects of dark-rearing on astroglial characteristics as determined by immunostaining for glial fibrillary acidic protein (GFAP) and the S-100 protein. The data reveal a retardation of astrocytic maturation in dark-reared animals, shown by a reduced presence of GFAP immunoreactivity compared to light-experienced animals. The density of astrocytic cell bodies positive for S-100 is unaffected by dark-rearing, suggesting that astroglial proliferation does not rely on afferent activity. However, punctate S-100 staining in the neuropil, which has been shown to reflect astrocytic processes, was also reduced in certain cortical layers in dark-reared animals. The effects of dark-rearing on the expression of GFAP and S-100 were restricted to the cortical layers except layer IV, i.e., those layers that reveal a prolongation of the critical period for cortical plasticity following dark-rearing. It is concluded that astrocytic maturation in the visual cortex is influenced by neuronal activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular compartmentalization of potassium

The evidence that there is intracellular compartmentalization of potassium is indirect but diverse. Intracellular electrode measurement of potassium activity, 42K radioisotope studies, and more recently 39K nuclear magnetic resonance (NMR) all support such compartmentalization. The use of rubidium to apparently shift potassium between sites with different NMR characteristics (visibility) is strong evidence for such compartmentalization. The evidence that intracellular compartmentalization of potassium is of (patho) physiological significance is also indirect. Postulated roles for regulation of intracellular K+ activity, perhaps by control of compartmentalization, include enzyme activity, protein synthesis, and cell growth. There is also evidence that compartmentalization of potassium may contribute to the maintenance of a stable intracellular environment following potassium loading. The apparent magnetic field dependence of the visibility of K+ by 39K NMR offers the opportunity to explore further the phenomenon of compartmentalization.

Control of protein synthesis by extracellular Na+ in cultured fibroblasts

In chick embryo fibroblasts (CEFs), a partial substitution of extracellular Na+ with other cations or carbohydrates decreased the intracellular Na+ content without altering the K+ level. Concomitantly, a significant decrease in the serum-dependent rate of protein synthesis occurred. This phenomenon appeared to be quickly reversible upon reconstitution of the correct extracellular Na+ concentration in the culture medium. The presence of a transcriptional inhibitor such as actinomycin D during the treatment did not inhibit the reversibility of the phenomenon. The presence in the culture medium of K+ in such excess as to dissipate the membrane potential did not alter the observed relationship between the protein synthesis rate and the internal Na+ content. Analysis of the amino acid pool indicated that the observed inhibition of the rate of protein synthesis in CEFs incubated in low Na+ medium was not caused by an unbalanced availability of intracellular amino acids. In addition, intracellular pH, as estimated by the measurement of the equilibrium distribution of benzoic acid, did not show any significant alteration in cells incubated in the presence of bicarbonate buffer and in low extracellular Na+. Moreover, the relationship between the rate of protein synthesis and the internal Na+ content was still observed in CEFs cultured in bicarbonate-containing media, but at lower or higher than physiological pH. Analysis by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) of the proteins synthesized by CEFs cultured at a reduced extracellular Na+ concentration showed that specific alterations of gene expression occurred.

Role of dietary magnesium and/or manganese variables on Ehrlich ascites tumor-bearing mice

Female Swiss albino mice were placed on seven dietary regimens for five weeks. These regimens differed only in magnesium and/or manganese contents. At the end of the feeding period, the animals were inoculated with Ehrlich ascites tumor. Ten days after transplantation, Ehrlich ascites carcinoma (EAC) cells were harvested, and all animals were killed. EAC cells and plasma samples were subjected to several biochemical tests. The results suggest several conclusions. 1. Dietary supplements of magnesium and/or manganese have no effect on retarding tumor growth in vivo. 2. Dietary restriction of manganese and combined magnesium and manganese gave promising effects on retarding tumor growth in vivo. 3. Dietary magnesium deficiency, per se, had no significant effect on tumor regression in vivo. 4. In contrast to in vitro studies, manganese supplementation appeared to exert no effect on tumor progression in vivo. 5. Magnesium supplementation seemed to have no effect on tumor progression in vivo, which is in agreement with in vitro studies.

Selective inhibition by bis(2-chloroethyl)methylamine (nitrogen mustard) of the Na+/K+/Cl- cotransporter of murine L1210 leukemia cells

Incubation of L1210 murine leukemia cells in vitro with 10 microM of the bifunctional alkylating agent bis(2-chloroethyl)methylamine (nitrogen mustard, HN2) for 10 min brought about a fall of more than 99.9% in their ability to form colonies when the cells were suspended in 0.5% nutrient agar. Incubation with HN2 also inhibited the influx of the potassium congener 86Rb+ to exponentially proliferating L1210 cells in a concentration-dependent manner. This inhibition was specific and was accounted for by a reduction of a diuretic-sensitive component of 86Rb+ influx, identified in the preceding paper (Wilcock, C. and Hickman, J.A. (1988) Biochim. Biophys. Acta 946, 359-367) as being mediated by a Na+/K+/Cl- cotransporter. Inhibition by 10 microM HN2 was complete after a 3-h incubation. There was no inhibition at this time of the ouabain-sensitive component of 86Rb+ influx, mediated by Na+/K+-ATPase. After 3 h of incubation with 10 microM HN2 there was also no change in the membrane potential of the treated cells as measured by the distribution of the [3H]TPMP+, no decrease in cellular ATP concentration and no change in intracellular pH, and the ability of the cells to exclude the vital dye Trypan blue was not significantly different from control values. These effects of HN2, therefore, appeared to follow lethal damage, but precede cell death. In the stationary phase of L1210 cell growth, the component of HN2 and diuretic-sensitive K+ influx to L1210 cells was reduced, whilst the component constituting the HN2-insensitive ouabain-sensitive sodium pump was increased. The monofunctional alkylating agent MeHN1 (2-chloroethyldimethylamine) which cannot cross-link cellular targets and has no antitumor activity, did not inhibit 86Rb+ influx to L1210 cells when incubated at equimolar or equitoxic concentrations to HN2. Intracellular potassium concentration was maintained close to control values of 138 +/- 10 mM in HN2-treated cells because of an approx. 35% fall in cell volume. The results suggest that the Na+/K+/Cl- cotransporter is a selectively inhibitable target for HN2, and the lesion is discussed with reference to the cytotoxic effects of this agent.

Characterization of Na+ transport in normal human fibroblasts and neoplastic H.Ep.2 cells and the role of inhibitin

Na+ transport was characterized in normal human fibroblasts and neoplastic H.Ep.2 cells in order to investigate the role of the endogenous peptidic factor 'inhibitin' that is secreted by a variety of neoplastic cells (including H.Ep.2) and inhibits Na+/Na+ exchange in human erythrocytes. Although active (Na+,K+-ATPase mediated) Na+ fluxes were similar in the two cell types, H.Ep.2 cells maintained higher intracellular Na+ concentration (26 mM) compared to fibroblasts (12 mM). An analysis of passive Na+ fluxes showed a difference in the handling of Na+ via ouabain and bumetanide-insensitive transport between the two cell types: H.Ep.2 cells achieved net Na+ influx via an amiloride-sensitive pathway that was only demonstrated in fibroblasts when 10% fetal calf serum (FCS) was present. Kinetic studies were undertaken to investigate the interaction between Na+ flux via Na+/H+ and Na+/Na+ exchanges. For this purpose, an outwardly directed Na+ gradient was created by loading the cells with Na+ (Nai greater than 100 mM) to activate the reverse functioning of Na+/H+ exchange (i.e., Na+out H+in). The rates of ouabain- and bumetanide-insensitive Na+ efflux were measured over a range of extracellular Na+ concentrations (Na+o 14-140 mM). In the presence of 10% FCS, the two cell types showed different responses: in fibroblasts the Na+ efflux rate showed an inverse correlation with extracellular Na+ concentration, while H.Ep.2 cells significantly increased their rate of Na+ efflux as extracellular Na+ concentration increased. So although the thermodynamic force would direct net Na+ efflux when Na+i greater than Na+o, H.Ep.2 cells were under kinetic control to perform Na+/Na+ exchange. When exogenous inhibitin was tested on fibroblasts, the steady-state intracellular Na+ concentration increased from 14 to 19 mM (p less than 0.01). In Na+-loaded fibroblasts, serum-stimulated Na+ efflux was partially inhibitin sensitive and the maximal inhibitory effect was seen when extracellular Na+ concentration was 14 mM and presumably the Na+/H+ exchanger operating in the reverse mode. This study demonstrated that, in contrast to fibroblasts, H.Ep.2 cells have a modified Na+/H+ exchange system whereby it acts in the Na+in H+out mode without exogenous growth factor activation and resists functioning in the reversed mode. It is proposed that inhibitin is the endogenous modifier of this transport system in H.Ep.2 cells with the result that H.Ep.2 cells maintain a higher concentration of intracellular Na+ compared to fibroblasts.

K+ efflux in NIH mouse 3T3 cells and transformed derivatives: dependence on extracellular Ca2+ and phorbol esters

In culture medium deficient in Ca2+, NIH mouse 3T3 cells lose K+, gain Na+, and stop growing. A marked increase in the rate of K+ efflux accounts for this loss; Na+, K+-ATPase pump activity increases but does not fully compensate for enhanced K+ efflux. Phorbol esters and cycloheximide inhibit K+ loss in Ca2+-deficient medium. Phorbol esters inhibit K+ efflux from human fibroblasts as well, even at physiological levels of Ca2+. Two cell lines derived from NIH-3T3, one transformed by a simian virus 40 deletion mutant, the other by the polyoma virus oncogene encoding the middle-sized tumor antigen, retain K+ and can multiply in medium with low Ca2+. Efflux of K+ from these cells is relatively insensitive to reduced Ca2+ concentration, phorbol esters, and cycloheximide. The results suggest the following hypothesis: a channel, nonselective for K+ and Na+, opens when NIH-3T3 cells are in Ca2+-deficient medium; the channel is controlled by the receptor for phorbol ester (protein kinase C) and may also be regulated by a short-lived protein.

Effects of anisosmotic medium on cell volume, transmembrane potential and intracellular K+ activity in mouse hepatocytes

Mouse hepatocytes in primary monolayer culture (4 hr) were exposed for 10 min at 37 degrees C to anisosmotic medium of altered NaCl concentration. Hepatocytes maintained constant relative cell volume (experimental volume/control volume) as a function of external medium relative osmolality (control mOsm/experimental mOsm) ranging from 0.8 to 1.5. In contrast, the relative cell volume fit a predicted Boyle-Van't Hoff plot when the experiment was done at 4 degrees C. Mouse liver slices were used for electrophysiologic studies, in which hepatocyte transmembrane potential (Vm) and intracellular K+ activity (aik) were recorded continuously by open-tip and liquid ion-exchanger ion-sensitive glass microelectrodes, respectively. Liver slices were superfused with control and then with anisosmotic medium of altered NaCl concentration. Vm increased (hyperpolarized) with hypoosmotic medium and decreased (depolarized) with hyperosmotic medium, and in [10(experimental Vm/control Vm)] was a linear function of relative osmolality (control mOsm/experimental mOsm) in the range 0.8-1.5. The aik did not change when medium osmolality was decreased 40-70 mOsm from control of 280 mOsm. Similar hypoosmotic stress in the presence of either 60 mM K+ or 1 mM quinine HCl or at 27 degrees C resulted in no change in Vm compared with a 20-mV increase in Vm without the added agents or at 37 degrees C. We conclude that mouse hepatocytes maintain their volume and aik in response to anisosmotic medium; however, Vm behaves as an osmometer under these conditions. Also, increases in Vm by hypoosmotic stress were abolished by conditions or agents that inhibit K+ conductance.

Selective inhibition of protein synthesis by hypertonic medium in Marituba (Bunyaviridae) virus-infected L-A9 cells

Elevation of the NaCl concentration in the growth medium of L-A9 cells caused an inhibition of the protein synthesis accompanied by a complete breakdown of polyribosomes. However, a complete recovery of the rate of protein synthesis was observed when isotonicity was restored. In Marituba virus infected cells, protein synthesis became resistant to hypertonic treatment. Under hypertonic conditions cellular protein synthesis was selectively suppressed and an enhancement of virus proteins was observed. Analysis of the virus specific proteins by polyacrylamide gel electrophoresis revealed that the synthesis of G1 was unalterable, and N was stimulated.

Ecotropic murine leukemia virus-induced fusion of murine cells

Extensive fusion occurs upon cocultivation of murine fibroblasts producing ecotropic murine leukemia viruses (MuLVs) with a large variety of murine cell lines in the presence of the polyene antibiotic amphotericin B, the active component of the antifungal agent Fungizone. The resulting polykaryocytes contain nuclei from both infected and uninfected cells, as evidenced by autoradiographic labeling experiments in which one or the other parent cell type was separately labeled with [3H]thymidine and fused with an unlabeled parent. This cell fusion specifically requires the presence of an ecotropic MuLV-producing parent and is not observed for cells producing xenotropic, amphotropic, or dualtropic viruses. Mouse cells infected with nonecotropic viruses retain their sensitivity toward fusion, whereas infection with ecotropic viruses abrogates the fusion of these cells upon cocultivation with other ecotropic MuLV-producing cells. Nonmurine cells lacking the ecotropic gp70 receptor are not fused under similar conditions. Fusion is effectively inhibited by monospecific antisera to gp70, but not by antisera to p15(E), and studies with monoclonal antibodies identify distinct amino- and carboxy-terminal gp70 regions which play a role in the fusion reaction. The enhanced fusion which occurs in the presence of amphotericin B provides a rapid and sensitive assay for the expression of ecotropic MuLVs and should facilitate further mechanistic studies of MuLV-induced fusion of murine cells.

Ion channels in rabbit cultured fibroblasts

Large outward currents are recorded with the whole-cell patch-clamp technique on depolarization of rabbit cultured fibroblasts. Our findings suggest that these outward currents consist of two voltage-dependent components, one of which also depends on cytoplasmic calcium concentration. Total replacement of external Cl- by the large anion ascorbate does not affect the amplitude of the currents, indicating that both components must be carried by K+. Consistent with these findings with whole-cell currents, in single channel recordings from fibroblasts we found that most patches contain high-conductance potassium-selective channels whose activation depends on both membrane potential and the calcium concentration at the cytoplasmic surface of the membrane. In a smaller number of patches, a second population of high-conductance calcium-independent potassium channels is observed having different voltage-dependence. The calcium- and voltage-dependence suggest that these two channels correspond with the two components of outward current seen in the whole-cell recordings. The single channel conductance of both channels in symmetrical KCl (150 mM) is 260-270 pS. Both channels are highly selective for K+ over both Na+ and Cl-. The conductance of the channels when outward current is carried by Rb+ is considerably smaller than when it is carried by K+. Some evidence is adduced to support the hypothesis that these potassium channel populations may be involved in the control of cell proliferation.

Effects of K+-deficiency and serum supplementation on protein turnover and nucleic acid synthesis in HeLa cells

When most of the K+ in a chemically defined medium was replaced with Rb+, cell growth of HeLa cells was strongly inhibited. The growth was partially but significantly restored by an addition of 5% dialyzed calf serum to the medium. The inhibition of cell growth in Rb+-substituted medium was partly due to suppression of protein synthesis by K+ deficiency, but the key mechanism of inhibition is still unknown. Rb+ substitution did not influence protein degradation or nucleic acid synthesis. The restoration of cell growth on addition of serum took place chiefly through stimulation of DNA synthesis. Protein and RNA syntheses were not affected by addition of serum, and serum-induced prevention of protein degradation was less in Rb+-substituted medium than in normal K+ medium.

Rapid changes in bidirectional K+ fluxes preceding DMSO-induced granulocytic differentiation of HL-60 human leukemic cells

When grown in medium containing 5 mM potassium and 140 mM sodium, HL-60, a human promyelocytic cell line, maintained a steady-state intracellular K+ concentration of 145 mmol/L cells and a steady-state intracellular Na+ concentration of 30 mmol/L cells. Nearly 90% of the unidirectional 42K+ influx could be inhibited by the cardiac glycoside ouabain with a Ki of 5 X 10(-8) M. This ouabain-sensitive component of influx rose as a saturating function of the extracellular K+ concentration with a K1/2 of 0.85 mM. The component of 42K+ influx resistant to ouabain inhibition was a linear function of the extracellular K+ concentration and was insensitive to inhibition by the diuretic furosemide. Unidirectional K+ efflux followed first order kinetics with a half-time of 55 min. Addition of 1.5% dimethyl sulfoxide (DMSO) to a culture of HL-60 cells allowed two population doublings followed by the cessation of growth without an impairment of cell viability. Beginning 2 to 3 days after DMSO addition, the cells underwent a dramatic reduction in volume (from 925 microns 3 to 500 microns 3) and began to take on the morphological features of mature granulocytes. Throughout this process of differentiation there was no change in the intracellular sodium or potassium concentration. However, immediately following the addition of DMSO to a culture of cells, there began an immediate, coordinated reduction in bidirectional K+ flux. The initial rate of the ouabain-sensitive component of K+ influx fell with a half-time of 11 h to a final rate, at 6 days induction, equal to one ninth that of the uninduced control, and over the same period, the rate constant for K+ efflux fell with a half-time of 14 h to a final value one fourth that of the uninduced control. The rapidity with which these flux changes occur raises the possibility that they play some role in the control of subsequent events in the process of differentiation.

Relationship of muscle growth in vitro to sodium pump activity and transmembrane potential

Serum stimulates embryonic avian skeletal muscle growth in vitro and the growth-related processes of amino acid transport and protein synthesis. Serum also stimulates myotube Na pump activity (measured as ouabain-sensitive rubidium-86 uptake) for at least 2 h after serum addition. Serum-stimulated growth depends on this Na pump activity since ouabain added at the same time as serum totally inhibits the growth responses. The relationship of myotube growth, Na pump activity, and transmembrane potential was studied to determine whether serum-stimulated Na pump activation and growth are coupled by long-term membrane hyperpolarization. When myotube amino acid transport and protein synthesis are prestimulated by serum, ouabain was found to have little inhibitory effect, indicating that the already stimulated growth-related processes are not tightly coupled to continued Na pump activity. Serum-stimulated protein synthesis is tightly coupled to Na pump activity, but only during the first 5-10 min after serum addition. When myotube transmembrane potentials were measured using the lipophilic cation tetraphenylphosphonium, serum at concentrations that stimulate myotube growth and Na pump activity was found to have little effect on the cell's transmembrane potential. Furthermore, partial depolarization of the myotubes with 12- to 55-mM extracellular potassium does not prevent serum stimulation of myotube growth. Monensin was found to hyperpolarize the myotubes, but causes myotube atrophy. These results indicate that although Na pump activity is associated with initiation of serum-stimulated myotube growth, continued Na pump activity is not essential, and there is little relationship between myotube growth and the myotube's transmembrane potential.

Action of oligomycin on cultured mammalian cells. Permeabilization to translation inhibitors

Oligomycin, an inhibitor of ATP synthesis, has been used as a model to study the effects of ATP depletion on macromolecular synthesis and modification of membrane permeability. Protein synthesis is totally blocked by the antibiotic, whereas RNA and DNA synthesis are less inhibited. Different concentrations of monovalent and divalent cations do not revert the inhibition of protein synthesis. Measurement of cellular ATP and 86Rb+ content indicate that the blockade of translation depends on the ATP content. A significant decrease in cellular ATP does not lead to the reduction of monovalent ions in the cell, although hyperpolarization of the cell membrane does take place. An increased membrane permeability to some inhibitors develops when the cells are hyperpolarized by oligomycin.