Stimulation of calcium uptake in cultured astrocytes by hypoosmotic stress--effect of cyclic AMP

SRC family kinases in cell volume regulation

SRC family kinases are a group of nine cytoplasmic protein tyrosine kinases essential for many cell functions. Some appear to be ubiquitously expressed, whereas others are highly tissue specific. The ability of members of the SRC family to influence ion transport has been recognized for several years. Mounting evidence suggests a broad role for SRC family kinases in the cell response to both hypertonic and hypotonic stress, and in the ensuing regulatory volume increase or decrease. In addition, members of this tyrosine kinase family participate in the mechanotransduction that accompanies cell membrane deformation. Finally, at least one SRC family member operates in concert with the p38 MAPK to regulate tonicity-dependent gene transcription.

The involvement of calcium in the protective and toxic (nonlinear) responses of rodent and human astroglial cells

The involvement of [Ca(2+)](i) in the reactive changes of astrocytes which accompany exposure to different chemicals were studied in cultures of C6 and 1321N1 cells. Cells were exposed to up to three serial pulses of the differentiating agent dBcAMP, which induces activation-type changes in the cells. Other cells, with or without the dBcAMP treatments, were treated with a range of concentrations of the antidepressants amitriptyline and fluoxetine and the glial toxicants acrylamide and chloroquine. In some experiments the L-type voltage calcium channel blocker Nifedipine was employed. [Ca(2+)](i) was measured in populations of the cells using Fura-2AM and a charge coupled device (CCD) camera attached to a fluorescence microscope. dBcAMP induced both dose- and time-dependent changes in [ Ca(2+)](i) with increases in both the [Ca(2+)](i) oscillations and mean [Ca(2+)](i) (e.g. in C6 cells at 18 min mean [Ca(2+)](i) was 318 +/- 20nM following the single differentiating dBcAMP pulses, 489 +/- 17nM (p < 0.001) following two serial pulses, and 275 +/- 30nM (not significant) following three pulses). Therapeutic doses of fluoxetine and amitriptyline caused increases in the calcium oscillations and the mean calcium concentrations ( maximum recorded mean increase was in the C6 cells at 10min by 0.02 muM fluoxetine when [Ca(2+)](i) was 411 +/- 35nM c.f. control 254 +/- 25nM, p = 0.01). Higher (non-therapeutic) doses of both antidepressants caused significant reductions. Chloroquine and acrylamide also caused dose-dependent bi-phasic types of alterations in [Ca(2+)](i), with significant reductions at lower, sub-cytotoxic doses followed by significant increases at higher concentrations, approaching those which cause cell damage. Nifedipine treatment caused some reductions in the dBcAMP, antidepressant or toxicant-induced calcium changes, but this substance also initiated cytotoxic alterations. The findings show that both the activation-type changes (which are frequently associated with increased protective capacities) and toxic responses of C6 and 1321N1 cells to different chemical agents are associated with dose-dependent alterations in [Ca(2+)](i).

Cell volume regulation in response to hypotonicity is impaired in HeLa cells expressing a protein kinase Calpha mutant lacking kinase activity

The chloride conductance (G(Cl,swell)) that participates in the regulatory volume decrease process triggered by osmotic swelling in HeLa cells was impaired by removal of extracellular Ca(2+), depletion of intracellular Ca(2+) stores with thapsigargin, or by preloading the cells with BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid). Furthermore, overnight exposure to the phorbol ester tetradecanoyl phorbol acetate and acute incubation with inhibitors of the conventional protein kinase C (PKC) isoforms bisindolylmaleimide I and Gö6976 inhibited G(Cl,swell). Treatment of HeLa cells with U73122, a phospholipase C inhibitor, also prevented G(Cl,swell). Hypotonicity induced selective PKC alpha accumulation in the membrane/cytoskeleton fraction in fractionation experiments and translocation of a green fluorescent protein-PKC alpha fusion protein to the plasma membrane of transiently transfected HeLa cells. To further explore the role of PKCs in hypotonicity-induced G(Cl,swell), HeLa clones stably expressing either a kinase-dead dominant negative variant of the Ca(2+)-dependent PKC isoform alpha (PKC alpha K386R) or of the atypical PKC isoform zeta (PKCzeta K275W) were generated. G(Cl,swell) was significantly reduced in HeLa cells expressing the dominant negative PKC alpha mutant but remained unaltered in cells expressing dominant negative PKCzeta. These findings strongly implicate PKC alpha as a critical regulatory element that is required for efficient regulatory volume decrease in HeLa cells.

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.

Calcium/calmodulin-modulated chloride and taurine conductances in cultured rat astrocytes

Osmotically swollen rat cerebral astrocytes develop an increased anion conductance which can mediate chloride and taurine release. We used whole cell patch clamp to study mechanisms that modulate this conductance. Astrocyte chloride conductance increased within 4 min of exposure to 200 mOsm medium and was 670+/-123% of its initial value after 15 min (mean+/-S.E.M.). This conductance was substantially reduced in 0.1 mM extracellular calcium with 20 mM BAPTA added to the electrode solution and was completely inhibited with calcium-free perfusion solution containing 1 mM EDTA (n=4). The conductance increase in 200 mOsm medium also was inhibited in a dose-dependent manner by nimodipine with a calculated K(i) of 0.31+/-0.4 microM and mean+/-S.E.M. inhibition of 84.4+/-4% at 100 microM nimodipine. In the presence of 100 microM W-7, a calmodulin antagonist, the mean+/-S.E.M. conductance increase after 15 min was 223+/-40% of the initial value while 300 microM W-7 or 100 microM trifluoperazine inhibited the conductance increase completely (n=6). With taurine as the major anion in electrode and perfusion solutions, a significant conductance increase was observed in 200 mOsm medium. This conductance increase was inhibited by 300 microM W-7 or 100 microM nimodipine. We conclude extracellular calcium influx via L-type calcium channels leads to increased astrocyte anion conductance in 200 mOsm conditions via calmodulin-dependent activation of anion channels. Efflux of anionic taurine from swollen astrocytes also may be affected by calcium influx through a similar calcium/calmodulin-dependent process.

Influence of calcium on regulatory volume decrease: role of potassium channels

In most cell types, hyposmotic swelling consistently elicits an increase in the concentration of cytosolic Ca2+ - [Ca2+]i - with contributions of extracellular and intracellular sources. The mechanisms of Ca2+ entry and release from endogenous sources are not fully clarified and may be cell specific. The ubiquity of the swelling-evoked [Ca2+]i rise makes Ca2+ a likely candidate for a role as osmotransducing signal. However, the regulatory volume decrease (RVD) which follows swelling and the osmolyte fluxes involved in this process are not always Ca2+ dependent. It was found that, with a few exceptions, in most cell types the osmosensitive Cl- efflux pathway and the swelling-activated organic osmolyte fluxes are Ca2+ independent. In contrast, Ca2+-dependent or Ca2+-independent K+ fluxes activated by swelling are detected, depending on the cell type. The close correlation found in this review between the Ca2+ dependence of RVD and that of the K+ channels activated by swelling led to the conclusion that it is the type of osmosensitive K+ pathway which largely confers the Ca2+ dependence to RVD. Interestingly, this coincidence of Ca2+-dependent K+ efflux and RVD is found predominantly in epithelial cells, whereas in nonepithelial cells both processes are largely Ca2+ independent. In these cells, the [Ca2+]i rise elicited by swelling may be an epiphenomenon.

Identification of a peptide toxin from Grammostola spatulata spider venom that blocks cation-selective stretch-activated channels

We have identified a 35 amino acid peptide toxin of the inhibitor cysteine knot family that blocks cationic stretch-activated ion channels. The toxin, denoted GsMTx-4, was isolated from the venom of the spider Grammostola spatulata and has <50% homology to other neuroactive peptides. It was isolated by fractionating whole venom using reverse phase HPLC, and then assaying fractions on stretch-activated channels (SACs) in outside-out patches from adult rat astrocytes. Although the channel gating kinetics were different between cell-attached and outside-out patches, the properties associated with the channel pore, such as selectivity for alkali cations, conductance ( approximately 45 pS at -100 mV) and a mild rectification were unaffected by outside-out formation. GsMTx-4 produced a complete block of SACs in outside-out patches and appeared specific since it had no effect on whole-cell voltage-sensitive currents. The equilibrium dissociation constant of approximately 630 nM was calculated from the ratio of association and dissociation rate constants. In hypotonically swollen astrocytes, GsMTx-4 produces approximately 40% reduction in swelling-activated whole-cell current. Similarly, in isolated ventricular cells from a rabbit dilated cardiomyopathy model, GsMTx-4 produced a near complete block of the volume-sensitive cation-selective current, but did not affect the anion current. In the myopathic heart cells, where the swell-induced current is tonically active, GsMTx-4 also reduced the cell size. This is the first report of a peptide toxin that specifically blocks stretch-activated currents. The toxin affect on swelling-activated whole-cell currents implicates SACs in volume regulation.

Effect of dibutyryl cyclic AMP on the kinetics of myo-inositol transport in cultured astrocytes

Dibutyryl cyclic AMP (dBcAMP) is known to induce maturation and differentiation in astrocytes. As myo-inositol is an important osmoregulator in astrocytes, we examined the effects of maturation and biochemical differentiation on the kinetic properties of myo-inositol transport. Treatment of astrocytes with dBcAMP significantly decreased the Vmax of myo-inositol uptake, but the effect on Km was not significant. The myo-inositol content of astrocytes was significantly decreased in cells treated for 5 days with dBcAMP as compared with untreated controls. Maximum suppression of myo-inositol uptake occurred 7 days after exposure of astrocytes to dBcAMP; this was gradually reversible when dBcAMP was removed from the medium. After exposure to hypertonic medium for 6 h, mRNA expression of the myo-inositol co-transporter was diminished by approximately 36% in astrocytes treated with dBcAMP as compared with untreated cells. It appears that myo-inositol transporters in astrocytes treated with dBcAMP are either decreased in number or inactivated during maturation and differentiation, suggesting that the stage of differentiation and biochemical maturation of astrocytes is an important factor in osmoregulation.

Stimulation of pepsinogen release from chief cells by Helicobacter pylori: evidence for a role of calcium and calmodulin

To define the mechanisms by which Helicobacter pylori stimulates pepsinogen secretion, the in vitro release of pepsinogen was studied using a preparation of pig chief cell monolayers. Helicobacter pylori induced a time- and concentration-dependent release of pepsinogen into the medium, with about a three-fold increase in pepsinogen secretion over controls found after 45 min of incubation. 3x10(7) H. pylori produced 50% of the maximal response found at a H. pylori count of 2x10(8). The action of H. pylori did not depend on the presence of the vacuolating toxin (vacA) and the cytotoxin-associated protein (cagA). Dibutyryl-cAMP and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate also markedly stimulated pepsinogen secretion and enhanced the stimulatory effect of H. pylori. Helicobacter pylori-stimulated pepsinogen release was inhibited by lanthanum and the calmodulin antagonist W-7, but not by the L-type Ca2+ channel blocker nifedipine, TMB-8, an agent that blocks the release of Ca2+ from intracellular stores, the protein kinase C inhibitor staurosporine and the protein kinase A inhibitor H-8. It is suggested that H. pylori directly stimulates pepsinogen release from gastric chief cells and that this effect is mediated via the calcium/calmodulin messenger branch.

Isoform-specific regulation of the Na+/Ca2+ exchanger in rat astrocytes and neurons by PKA

The Na+/Ca2+ exchanger is a major transporter of Ca2+ in neurons and glial cells. The Na+/Ca2+ exchanger gene NCX1 expresses tissue-specific isoforms of the Na+/Ca2+ exchanger, and the isoforms have been examined here quantitatively using primary cultures of astrocytes and neurons. We present a PCR-based quantitative method, quantitative end-labeled reverse transcription-PCR (QERT-PCR), to determine the relative amounts of the NCX1 isoforms present in these cells. Six exons (A, B, C, D, E, and F) are alternatively spliced to produce the known NCX1 isoforms. Three exon B-containing isoforms (BDEF, BDF, and BD) are the predominant transcripts in primary rat cortical astrocytes and in C6 glioma cells. In contrast, exon A-containing isoforms (ADF and AD) are the predominant transcripts in primary rat hippocampal neurons. Functional differences between full-length constructs of NCX1 containing either the astrocyte isoform BD or the neuron isoform AD were examined in a Xenopus oocyte expression system. Although both isoforms function normally, the activity of the AD isoform can be increased 39% by activation of protein kinase A (PKA), whereas that of the BD isoform is not affected. We conclude that specific NCX1 isoforms are expressed in distinct patterns in astrocytes and neurons. Furthermore, the activity of a neuronal (but not glial) isoform of the Na+/Ca2+ exchanger can be altered by the activation of the PKA pathway.

Volume regulation and the efflux of amino acids from cells in incubated slices of rat cerebral cortex. II. Dependence on Ca2+ ions

The efflux of gamma-aminoisobutyric acid (GABA) and L-glutamate from pre-loaded cells in rat cerebral cortical slices has been studied during interventions designed to affect the availability of intracellular Ca2+ during hyposmotic swelling and membrane depolarization due to raised extracellular K+. Calmodulin-dependent acceleration of amino acid efflux in hyposmotic media, with cell swelling less than would be predicted on the basis of perfect osmometric behaviour (see Ref. [1]), was unaffected by Ca-ionophore in the presence of external Ca2+ or by the omission of external Ca2+, but was suppressed by pre-exposure of slices to thapsigargin (2 microM), which is reported to deplete cytosolic Ca2+, and by TMB-8 (0.5 mM), which blocks release of Ca2+ from internal stores. TMB-8 also led to significant cell swelling. The effects of TMB-8 were reversed by Ca-ionophore. Raised external K+ (54 mM) led to accelerated amino acid efflux which required calmodulin activation and was blocked by (i) omission of external Ca2+, (ii) the voltage-sensitive Ca2+ channel blocker nifedipine (1 microM), (iii) the anion transport inhibitor DIDS (25 microM for GABA, 100 microM for L-glutamate, see Ref. [1]), and (iv) the -SH group acetylator N-ethylmaleimide. TMB-8 was without effect in high K+ media. These results suggest that while enhanced amino acids efflux probably occurs through the same population of Ca/calmodulin-dependent, DIDS-sensitive pathways following hyposmotic shock or membrane depolarization, the source of Ca2+ ions required for the activation of these pathways may depend upon which of these acceleratory stimuli is applied.

Neuronal and glial handling of glutamate and glutamine during hypoosmotic stress: a biochemical and quantitative immunocytochemical analysis using the rat cerebellum as a model

Biochemical and immunocytochemical analyses were performed to resolve how glutamate and glutamine are handled in rat cerebellar cortex in acute hypoosmotic stress. Rats were subjected to a 15-20% reduction in plasma osmolality by intraperitoneal injection of distilled water and then perfusion fixed after 4 or 8 h survival. Some rats in the latter group had their plasma isoosmolality restored by injections of hypertonic saline 4 h prior to perfusion. Water loading caused a pronounced increase in the tissue level of glutamine and an equimolar decrease in the level of glutamate after 4 h survival. The increase in glutamine was transient, as judged by analyses at 8 h survival. Light microscopic immunocytochemistry revealed a pronounced enhancement of the glutamine immunolabelling of glial cells (Golgi epithelial cells and astrocytes), including their perivascular end feet, and quantitative immunogold analyses at the electron microscopic level showed that this enhancement reflected a 50% increase in the intracellular concentration of fixed glutamine. Since water loading was associated with glial swelling this change corresponded to a several-fold increase in the glial content of glutamine. There was a modest reduction in the overall staining intensity for glutamate. The biochemical and immunocytochemical changes were reversed upon restoration of plasma osmolality by hypertonic saline. These findings suggest that hypoosmotic stress causes an increased conversion of glutamate to glutamine in glial cells and that the latter amino acid is subsequently lost from the tissue. The flux of glutamate carbon skeletons through the glutamine synthetase pathway in glia, prior to an efflux to the systemic circulation, may explain how glutamate, and excitatory transmitter and potential toxin, can be used as an organic osmolyte in brain tissue.

Inhibition by dihydropyridines of regulatory volume decrease and osmolyte fluxes in cultured astrocytes is unrelated to extracellular calcium

The 1,4-dihydropyridines (DHP), nimodipine (NMD) and nitrendipine (NTD) were potent blockers of regulatory volume decrease (RVD) and the volume-associated release of [3H]taurine and chloride (measured as 125I) in 2-weeks cultured rat cerebellar astrocytes. The IC50 were 30 microM and 29 microM for taurine efflux and 26 and 27 microM for C1 efflux for NMD and NTD, respectively. Inhibition by DHP was independent of extracellular Ca, as the effect was the same in media with 1 mM Ca or without Ca and 0.5 mM EGTA. DHP did not affect the basal (isosmotic) release of [3H]taurine or 125I inhibition by DHP (measured only on [3H]taurine efflux) was the same in 3-4 weeks cultured cerebellar astrocytes, 2-4 weeks cultured cortical astrocytes and 2-weeks cultured cerebellar astrocytes treated with dibutyril cAMP. Diltiazem (50 microM) and verapamil (100 microM) failed to inhibit RVD or osmolyte efflux.