Hypotonically induced calcium release from intracellular calcium stores

Post-Anesthesia Evaluation Shows No Relationship Between Body Conditions and Salinity-Choice Propensities in Anguilla japonica Glass Eels

Anguillid eels generally exhibit catadromous migration between oceanic spawning grounds and freshwater growth habitats, but some individuals remain in coastal or estuarine saline waters for growth. This migratory plasticity had been considered to be a conditional strategy based on individual energetic status during the glass eel stage. Several studies have examined whether salinity-based habitat selection is linked to individual body conditions, but while frozen specimens of European eels showed this relationship, anesthetized samples of American eels did not. Here, we report that freezing preservation under different salinity levels influences body-condition evaluation in Japanese eels. Behavioral tests of Japanese eels did not reveal significant differences in anesthetized body conditions between those choosing saltwater and those choosing freshwater. In conclusion, the body conditions of glass-eel-stage Japanese eels are unlikely to be associated with their salinity-choice propensity.

Distinct mechanisms regulating mechanical force-induced Ca²⁺ signals at the plasma membrane and the ER in human MSCs

It is unclear that how subcellular organelles respond to external mechanical stimuli. Here, we investigated the molecular mechanisms by which mechanical force regulates Ca²⁺ signaling at endoplasmic reticulum (ER) in human mesenchymal stem cells. Without extracellular Ca²⁺, ER Ca²⁺ release is the source of intracellular Ca²⁺ oscillations induced by laser-tweezer-traction at the plasma membrane, providing a model to study how mechanical stimuli can be transmitted deep inside the cell body. This ER Ca²⁺ release upon mechanical stimulation is mediated not only by the mechanical support of cytoskeleton and actomyosin contractility, but also by mechanosensitive Ca²⁺ permeable channels on the plasma membrane, specifically TRPM7. However, Ca²⁺ influx at the plasma membrane via mechanosensitive Ca²⁺ permeable channels is only mediated by the passive cytoskeletal structure but not active actomyosin contractility. Thus, active actomyosin contractility is essential for the response of ER to the external mechanical stimuli, distinct from the mechanical regulation at the plasma membrane.

DOI:http://dx.doi.org/10.7554/eLife.04876.001

Cells receive many signals from their environment, for example, when they are compressed or pulled about by neighboring cells. Information about these ‘mechanical stimuli’ can be transmitted within the cell to trigger changes in gene expression and cell behavior.

When a cell receives a mechanical stimulus, it can activate the release of calcium ions from storage compartments within the cell, including from a compartment called the endoplasmic reticulum. Calcium ions can also enter the cell from outside via channels located in the membrane that surrounds the cell (the plasma membrane).

Kim et al. investigated how mechanical forces are transmitted in a type of human cell called mesenchymal stem cells using optical tweezers to apply a gentle force to the outside of a cell. These tweezers use a laser to attract tiny objects, in this case a bead attached to proteins in the cell's outer membrane. The cell's response to this mechanical stimulation was measured using a sensor protein that fluoresces a different color when it binds to calcium ions.

With this set-up, Kim et al. found that mesenchymal stem cells are able to transmit mechanical forces to different depths within the cell. The forces can travel deep to trigger the release of calcium ions from the endoplasmic reticulum. This process involves a network of protein fibers that criss-cross to support the structure of a cell—called the cytoskeleton—and also requires proteins that are associated with the cytoskeleton to contract. However, calcium ion entry through the plasma membrane due to a mechanical force does not require these contractile proteins—only the cytoskeleton is involved.

These results demonstrate that the transmission of mechanical signals to different depths within mesenchymal stem cells involves different components. Future work should shed light on how these mechanical signals control gene expression and the development of mesenchymal stem cells.

DOI:http://dx.doi.org/10.7554/eLife.04876.002

Measurement of intracellular Ca2+ release in intact cells using 45Ca2+

This protocol describes a technique using (45)Ca(2+) to measure the release of Ca(2+) from the intracellular stores in monolayers of intact cells cultured in 12-well 4-cm(2) clusters. The (45)Ca(2+)-flux technique described here can only be applied to cell types that adhere to plastic. We describe the loading of the stores with (45)Ca(2+), and the subsequent (45)Ca(2+) efflux.

Mechanosensitive activation of K+ channel via phospholipase C-induced depletion of phosphatidylinositol 4,5-bisphosphate in B lymphocytes

In various types of cells mechanical stimulation of the plasma membrane activates phospholipase C (PLC). However, the regulation of ion channels via mechanosensitive degradation of phosphatidylinositol 4,5-bisphosphate (PIP(2)) is not known yet. The mouse B cells express large conductance background K(+) channels (LK(bg)) that are inhibited by PIP(2). In inside-out patch clamp studies, the application of MgATP (1 mm) also inhibited LK(bg) due to the generation of PIP(2) by phosphoinositide (PI)-kinases. In the presence of MgATP, membrane stretch induced by negative pipette pressure activated LK(bg), which was antagonized by PIP(2) (> 1 microm) or higher concentration of MgATP (5 mm). The inhibition by PIP(2) was partially reversible. However, the application of methyl-beta-cyclodextrin, a cholesterol scavenger disrupting lipid rafts, induced the full recovery of LK(bg) activity and facilitated the activation by stretch. In cell-attached patches, LK(bg) were activated by hypotonic swelling of B cells as well as by negative pressure. The mechano-activation of LK(bg) was blocked by U73122, a PLC inhibitor. Neither actin depolymerization nor the inhibition of lipid phosphatase blocked the mechanical effects. Direct stimulation of PLC by m-3M3FBS or by cross-linking IgM-type B cell receptors activated LK(bg). Western blot analysis and confocal microscopy showed that the hypotonic swelling of WEHI-231 induces tyrosine phosphorylation of PLCgamma2 and PIP(2) hydrolysis of plasma membrane. The time dependence of PIP(2) hydrolysis and LK(bg) activation were similar. The presence of LK(bg) and their stretch sensitivity were also proven in fresh isolated mice splenic B cells. From the above results, we propose a novel mechanism of stretch-dependent ion channel activation, namely, that the degradation of PIP(2) caused by stretch-activated PLC releases LK(bg) from the tonic inhibition by PIP(2).

Hyperosmotic stress stimulates inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate formation independently of bis-diphosphoinositol tetrakisphosphate modulation

Hyperosmotic stress induces water diffusion out of the cell, resulting in cell shrinkage, and leading to DNA damage, cell cycle arrest, and cytoskeletal reorganization. A previous report showed that low concentrations of sorbitol (200mM) could increase up to 25-fold the concentration of InsP(8) in animal cells. Here, we investigate the effect of sorbitol (200mM) on the inositol 1,4,5-trisphosphate (InsP(3)) and inositol 1,3,4,5-tetrakisphosphate (InsP(4)) pathway. A 3- to 4-fold increase in InsP(3) and InsP(4) levels after sorbitol challenge was observed. It was prevented by the phospholipase C inhibitor U-73122 but was insensitive to the MAP kinase inhibitor U0126. We also observed an increase in the free intracellular [Ca(2+)] and the occurrence of Ca(2+) oscillations in response to sorbitol. A hyperosmotic stress could therefore affect the levels of both hyperphosphorylated inositol phosphates and InsP(3)/InsP(4)-signalling molecules.

Cell swelling-induced ATP release is tightly dependent on intracellular calcium elevations

Mechanical stresses release ATP from a variety of cells by a poorly defined mechanism(s). Using custom-designed flow-through chambers, we investigated the kinetics of cell swelling-induced ATP secretion, cell volume and intracellular calcium changes in epithelial A549 and 16HBE14o- cells, and NIH/3T3 fibroblasts. Fifty per cent hypotonic shock triggered transient ATP release from cell confluent monolayers, which consistently peaked at around 1 min 45 s for A549 and NIH/3T3, and at 3 min for 16HBE14o- cells, then declined to baseline within the next 15 min. Whereas the release time course had a similar pattern for the three cell types, the peak rates differed significantly (294 +/- 67, 70 +/- 22 and 17 +/- 2.8 pmol min(-1) (10(6) cells)(-1), for A549, 16HBE14o- and NIH/3T3, respectively). The concomitant volume changes of substrate-attached cells were analysed by a 3-dimensional cell shape reconstruction method based on images acquired from two perpendicular directions. The three cell types swelled at a similar rate, reaching maximal expansion in 1 min 45 s, but differed in the duration of the volume plateau and regulatory volume decrease (RVD). These experiments revealed that ATP release does not correlate with either cell volume expansion and the expected activation of stretch-sensitive channels, or with the activation of volume-sensitive, 5-nitro-2-(3-phenylpropylamino) benzoic acid-inhibitable anion channels during RVD. By contrast, ATP release was tightly synchronized, in all three cell types, with cytosolic calcium elevations. Furthermore, loading A549 cells with the calcium chelator BAPTA significantly diminished ATP release (71% inhibition of the peak rate), while the calcium ionophore ionomycin triggered ATP release in the absence of cell swelling. Lowering the temperature to 10 degrees C almost completely abolished A549 cell swelling-induced ATP release (95% inhibition of the peak rate). These results strongly suggest that calcium-dependent exocytosis plays a major role in mechanosensitive ATP release.

Cell spreading controls endoplasmic and nuclear calcium: a physical gene regulation pathway from the cell surface to the nucleus

Cells attaching to an extracellular matrix through integrins flatten out (spread) on the matrix, eliciting cellular signals needed for survival. We show that the shape of the nucleus changes and the nuclear calcium level increases in spreading cells. Moreover, cell spreading and osmotic stretching of isolated nuclei cause release of perinuclear Ca(2+), and patch clamping of nuclei reveals stretch-activated Ca(2+) permeable channels. Gene expression assays with myocyte enhancer factor 2, which is activated by calmodulin-dependent kinase IV, indicate that the elevation in nuclear Ca(2+) is functionally significant. We propose a mechano-transduction pathway in which spreading-induced nuclear stretching releases Ca(2+) from the perinuclear space, and the resulting Ca(2+) elevation in the nucleus provokes changes in gene expression.

Calcium mobilization evoked by hepatocellular swelling is linked to activation of phospholipase Cgamma

Recovery from swelling of hepatocytes and selected other epithelia is triggered by intracellular Ca(2+) release from the endoplasmic reticulum, which leads to fluid and electrolyte efflux through volume-sensitive K(+) and Cl(-) channels. The aim of this study was to determine the mechanisms responsible for swelling-mediated hepatocellular Ca(2+) mobilization. Swelling of HTC rat hepatoma cells, evoked by exposure to hypotonic medium, elicited transient increases in intracellular levels of inositol 1,4,5-trisphosphate (IP(3)) and cytosolic [Ca(2+)]. The latter was attenuated by inhibition of phospholipase C (PLC) with and by IP(3) receptor blockade with 2-aminoethoxydiphenyl borate, but it was unaffected by ryanodine, an inhibitor of intracellular Ca(2+)-induced Ca(2+) release channels. Hypotonic swelling was associated with a transient increase in tyrosine phosphorylation of PLCgamma, with kinetics that paralleled the increases in intracellular IP(3) levels and cytosolic [Ca(2+)]. Confocal imaging of HTC cells exposed to hypotonic medium revealed a swelling-induced association of tyrosine-phosphorylated PLCgamma with the plasma membrane. These findings suggest that activation of PLCgamma by hepatocellular swelling leads to the generation of IP(3) and stimulates discharge of Ca(2+) from the endoplasmic reticulum via activation of IP(3) receptors. By extension, these data support the concept that tyrosine phosphorylation of PLCgamma represents a critical step in adaptive responses to hepatocellular swelling.

Stretch-induced Ca(2+) release via an IP(3)-insensitive Ca(2+) channel

Various mechanical stimuli increase the intracellular Ca(2+) concentration ([Ca(2+)](i)) in vascular smooth muscle cells (VSMC). A part of the increase in [Ca(2+)](i) is due to the release of Ca(2+) from intracellular stores. We have investigated the effect of mechanical stimulation produced by cyclical stretch on the release of Ca(2+) from the intracellular stores. Permeabilized VSMC loaded with (45)Ca(2+) were subjected to 7.5% average (15% maximal) cyclical stretch. This resulted in an increase in (45)Ca(2+) rate constant by 0.126 +/- 0.0035. Inhibition of inositol 1,4,5-trisphosphate (IP(3)), ryanodine, and nicotinic acid adenine dinucleotide phosphate channels (NAADP) with 50 microg/ml heparin, 50 microM ruthenium red, and 25 microM thio-NADP, respectively, did not block the increase in (45)Ca(2+) efflux in response to cyclical stretch. However, 10 microM lanthanum, 10 microM gadolinium, and 10 microM cytochalasin D but not 10 microM nocodazole inhibited the increase in (45)Ca(2+) efflux. This supports the existence of a novel stretch-sensitive intracellular Ca(2+) store in VSMC that is distinct from the IP(3)-, ryanodine-, and NAADP-sensitive stores.

Washing out of lipophilic compounds induces a transient increase in the passive Ca(2+) leak in permeabilized A7r5 cells

We have investigated how the immunosuppressant drug FK506 affected the basal Ca(2+) leak in permeabilized A7r5 cells. Non-mitochondrial Ca(2+) stores loaded to steady state with Ca(2+) slowly lost their accumulated Ca(2+) during incubation in a Ca(2+)-free efflux medium. FK506 up to 100 microM had no effect on the basal Ca(2+) leak. In contrast, the rate of Ca(2+) release proceeded much faster immediately after washing out FK506. The increase in rate of Ca(2+) release after washing out of this compound depended on both its initial concentration and on the time of pre-incubation. A similar effect was also observed after removing another immunosuppressant drug (rapamycin) and after removing the inositol 1,4,5-trisphosphate receptor inhibitor xestospongin C. Since all these substances have a high octanol/H(2)O partition coefficient and accumulate in the endoplasmic reticulum membrane, we suggest that the transient increase in the basal Ca(2+) leak is due to the sudden removal of these lipophilic substances from the membrane.

Purinergic-independent calcium signaling mediates recovery from hepatocellular swelling: implications for volume regulation

Swelling of hepatocytes and other epithelia activates volume-sensitive ion channels that facilitate fluid and electrolyte efflux to restore cell volume, but the responsible signaling pathways are incompletely defined. Previous work in model HTC rat hepatoma cells has indicated that swelling elicits ATP release, which stimulates P2 receptors and activates Cl(-) channels, and that this mechanism is essential for hepatocellular volume recovery. Since P2 receptors are generally coupled to Ca(2+) signaling pathways, we determined whether hepatocellular swelling affected cytosolic [Ca(2+)], and if this involved a purinergic mechanism. Exposure of HTC cells to hypotonic media evoked an increase in cytosolic [Ca(2+)], which was followed by activation of K(+) and Cl(-) currents. Maneuvers that interfered with swelling-induced increases in cytosolic [Ca(2+)], including extracellular Ca(2+) removal and intracellular Ca(2+) store depletion with thapsigargin, inhibited activation of membrane currents and volume recovery. However, the swelling-induced increases in cytosolic [Ca(2+)] were unaffected by either extracellular ATP depletion with apyrase or blockade of P2 receptors with suramin. These findings indicate that swelling elicits an increase in hepatocellular Ca(2+), which is essential for ion channel activation and volume recovery, but that this increase does not stem from activation of volume-sensitive P2 receptors. Collectively, these observations imply that regulatory responses to hepatocellular swelling involve a dual requirement for a purinergic-independent Ca(2+) signaling cascade and a Ca(2+)-independent purinergic signaling pathway.

Hypotonic swelling-induced Ca(2+) release by an IP(3)-insensitive Ca(2+) store

Hypotonic swelling increases the intracellular Ca(2+) concentration ([Ca(2+)](i)) in vascular smooth muscle cells (VSMC). The source of this Ca(2+) is not clear. To study the source of increase in [Ca(2+)](i) in response to hypotonic swelling, we measured [Ca(2+)](i) in fura 2-loaded cultured VSMC (A7r5 cells). Hypotonic swelling produced a 40.7-nM increase in [Ca(2+)](i) that was not inhibited by EGTA but was inhibited by 1 microM thapsigargin. Prior depletion of inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) stores with vasopressin did not inhibit the increase in [Ca(2+)](i) in response to hypotonic swelling. Exposure of (45)Ca(2+)-loaded intracellular stores to hypotonic swelling in permeabilized VSMC produced an increase in (45)Ca(2+) efflux, which was inhibited by 1 microM thapsigargin but not by 50 microg/ml heparin, 50 microM ruthenium red, or 25 microM thio-NADP. Thus hypotonic swelling of VSMC causes a release of Ca(2+) from the intracellular stores from a novel site distinct from the IP(3)-, ryanodine-, and nicotinic acid adenine dinucleotide phosphate-sensitive stores.

Chloride channels regulate HIT cell volume but cannot fully account for swelling-induced insulin secretion

Insulin-secreting pancreatic islet beta-cells possess anion-permeable Cl- channels (I(Cl,islet)) that are swelling-activated, but the role of these channels in the cells is unclear. The Cl- channel blockers 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) and niflumic acid were evaluated for their ability to inhibit I(Cl,islet) in clonal beta-cells (HIT cells). Both drugs blocked the channel, but the blockade due to niflumic acid was less voltage-dependent than the blockade due to DIDS. HIT cell volume initially increased in hypotonic solution and was followed by a regulatory volume decrease (RVD). The addition of niflumic acid and, to a lesser extent, DIDS to the hypotonic solution potentiated swelling and blocked the RVD. In isotonic solution, niflumic acid produced swelling, suggesting that islet Cl- channels are activated under basal conditions. The channel blockers glyburide, gadolinium, or tetraethylammonium-Cl did not alter hypotonic-induced swelling or volume regulation. The Na/K/2Cl transport blocker furosemide produced cell shrinkage in isotonic solution and blocked cell swelling normally induced by hypotonic solution. Perifused HIT cells secreted insulin when challenged with hypotonic solutions. However, this could not be completely attributed to I(Cl,islet)-mediated depolarization, because secretion persisted even when Cl- channels were fully blocked. To test whether blocker-resistant secretion occurred via a distal pathway, distal secretion was isolated using 50 mmol/l potassium and diazoxide. Under these conditions, glucose-dependent secretion was blunted, but hypotonically induced secretion persisted, even with Cl- channel blockers present. These results suggest that beta-cell swelling stimulates insulin secretion primarily via a distal I(Cl,islet)-independent mechanism, as has been proposed for K(ATP)-independent glucose- and sulfonylurea-stimulated insulin secretion. Reverse transcriptase-polymerase chain reaction of HIT cell mRNA identified a CLC-3 transcript in HIT cells. In other systems, CLC-3 is believed to mediate swelling-induced outwardly rectifying Cl- channels. This suggests that the proximal effects of swelling to regulate cell volume may be mediated by CLC-3 or a closely related Cl- channel.

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.

Two mechanisms that raise free intracellular calcium in rat hippocampal neurons during hypoosmotic and low NaCl treatment

Previous studies have shown that exposing hippocampal slices to low osmolarity (pi(o)) or to low extracellular NaCl concentration ([NaCl](o)) enhances synaptic transmission and also causes interstitial calcium ([Ca(2+)](o)) to decrease. Reduction of [Ca(2+)](o) suggests cellular uptake and could explain the potentiation of synaptic transmission. We measured intracellular calcium activity ([Ca(2+)](i)) using fluorescent indicator dyes. In CA1 hippocampal pyramidal neurons in tissue slices, lowering pi(o) by approximately 70 mOsm caused "resting" [Ca(2+)](i) as well as synaptically or directly stimulated transient increases of calcium activity (Delta[Ca(2+)](i)) to transiently decrease and then to increase. In dissociated cells, lowering pi(o) by approximately 70 mOsm caused [Ca(2+)](i) to almost double on average from 83 to 155 nM. The increase of [Ca(2+)](i) was not significantly correlated with hypotonic cell swelling. Isoosmotic (mannitol- or sucrose-substituted) lowering of [NaCl](o), which did not cause cell swelling, also raised [Ca(2+)](i). Substituting NaCl with choline-Cl or Na-methyl-sulfate did not affect [Ca(2+)](i). In neurons bathed in calcium-free medium, lowering pi(o) caused a milder increase of [Ca(2+)](i), which was correlated with cell swelling, but in the absence of external Ca(2+), isotonic lowering of [NaCl](o) triggered only a brief, transient response. We conclude that decrease of extracellular ionic strength (i.e., in both low pi(o) and low [NaCl](o)) causes a net influx of Ca(2+) from the extracellular medium whereas cell swelling, or the increase in membrane tension, is a signal for the release of Ca(2+) from intracellular stores.

Hypotonicity activates transcription through ERK-dependent and -independent pathways in renal cells

Acute hypotonic shock (50% dilution of medium with sterile water, but not with isotonic NaCl) activated the extracellular signal response kinase (ERK) mitogen-activated protein (MAP) kinases in renal medullary cells, as measured by Western analysis with a phospho-ERK-specific antibody and by in vitro kinase assay of epitope-tagged ERKs immunoprecipitated from stable HA-ERK transfectants. Hypotonicity also activated the transcription factor and ERK substrate Elk-1 in a partially PD-98059-sensitive fashion, as assessed by chimeric reporter gene assay. Consistent with these data, hypotonic stress activated transcription of the immediate-early gene transcription factor Egr-1 in a partially PD-98059-sensitive fashion. Hypotonicity-inducible Egr-1 transcription was mediated in part through 5'-flanking regions containing serum response elements and in part through the minimal Egr-1 promoter. Elimination of the Ets motifs adjacent to key regulatory serum response elements in the Egr-1 promoter diminished the effect of hypotonicity but failed to abolish it. Interestingly, hypotonicity also transiently activated p38 and c-Jun NH2-terminal kinase 1, as determined by immunoblotting with anti-phospho-MAP kinase antibodies. Taken together, these data strongly suggest that hypotonicity activates immediate-early gene transcription in renal medullary cells via MAP kinase kinase-dependent and -independent mechanisms.

Modulation of the swelling-activated amino acid channel of Leishmania major promastigotes by protein kinases

Leishmania promastigotes respond to hypotonic challenges by a mechanism of regulatory volume decrease (RVD), whereby anionic amino acid channels (HAAC) are hypotonically-activated and intracellular amino acids are released from the cells. Irrespective of the experimental conditions, restoration of isotonicity triggered an immediate blockage of the amino acid release. Both the speed and amplitude of the response depended on the hypotonic stimulus and on the operation of intracellular signaling mechanisms. The initial (5 s) hypotonic-induced release of amino acids (ri) and the steady state levels of amino acids attained (5 min) or amplitude (A), were markedly affected by modulators of protein kinase C: phorbol 12-myristate 13-acetate, 1-oleoyl-2-acetylglycerol and phorbol 12,13-diacetate whereas staurosporine and the related analog, bis-indolylmaleimide I (GF-109203.X) inhibited the RVD response. Agonists of cAMP-dependent protein kinase A such as forskolin or (8-(4-chlorophenylthio))-adenosine-3',5'cyclic-monophosphate enhanced the speed of the response but had little effect on its amplitude. Neither 4alpha-phorbol 12,13-didecanoate,1,9-dideoxyforskolin nor genistein, tamoxifen or thapsigargin had any apparent effect on either parameter tested. The most striking stimulation of hypotonic-induced amino acid release was exerted by arachidonic acid or by its non-metabolizable analog, 5,8,11,14-eicosatetraynoic acid (ETYA). These agents caused a major increase in the initial rate of amino acid release as well as a higher amplitude of the response, both of which were markedly inhibited by an anion channel blocker. The present studies indicate not only that hypotonicity is an obligatory and dominant component in HAAC activation, but implicate specific second messengers in the modulation of the RVD response. The modes of activation or attenuation of HAAC activity apparently differ for PKC and PKA modulators as well as for arachidonic acid. The involvement of Ca2+ in HAAC was studied in hypotonic challenged cells which were treated with intracellular Ca2+-chelators or Ca2+-free medium. These cells showed a lag in AA release and a modest inhibition of the amplitude. The inhibition of HAAC was markedly increased when cells were treated with the ionophore A23187 in Ca2+-free media. The HAAC activity was accompanied by a significant increase in internal Ca2+ when performed in Ca2+-containing medium (from 88+/-9 to 179+/-22 nM) but by no significant change when measured in Ca2+-free medium. These studies indicate that although Ca2+ might be involved in the early activation phase of HAAC, it is either not absolutely required or its action might be associated with localized events.

Synergism between hypotonically induced calcium release and fatty acyl-CoA esters induced calcium release from intracellular stores

The non-mitochondrial Ca2+ stores in permeabilized A7r5 cells responded to a decrease in Mg-ATP concentration with a pronounced Ca2+ release if 20 microM CoA was present. This release was rather specific for the preincubation or removal of ATP. ATP gamma S was much less effective and AMP-PNP, GTP, ITP, CTP, UTP, ADP, AMP, adenosine and adenine had no effect. CoA activated with an EC50 of 6 microM. Dephospho-CoA was a less effective cofactor and desulfo-CoA was ineffective. The release induced by Mg-ATP removal did not occur in the presence of 2% fatty acid-free bovine serum albumin and did not develop at 4 degrees C. All these findings suggest that CoA had to be acylated by endogenous fatty-acyl-CoA synthetase to become effective. Myristoyl- and palmitoyl-CoA esters were identified as the most effective cofactors for the release. Ca2+ release induced by removing Mg-ATP did not occur if the osmolality of the medium was kept constant by addition of mannitol, sucrose, KCl, MgCl2 or Mg-GTP, indicating that the decrease in tonicity was the trigger for the release. Mg-ATP plus CoA also synergized with Ca2+ release induced by a hypotonic shock imposed by diluting the medium with H2O. Osmolality changes induced by decreasing the Mg-ATP concentration were more effective in releasing Ca2+ than equal decreases in concentration of all solutes. We conclude that fatty acyl-CoA esters sensitize the hypotonically induced Ca2+ release from the non-mitochondrial Ca2+ stores.

Syk-dependent and -independent signaling cascades in B cells elicited by osmotic and oxidative stress

It was found that Syk protein-tyrosine kinase is rapidly activated in B cells after H2O2 treatment (oxidative stress) or increased extracellular NaCl concentration (osmotic stress) as well as in response to B cell receptor activation. In this study we examined the involvement of Syk in responses elicited by these types of extracellular stress, particularly Ca2+ responses and c-Jun amino-terminal kinase (JNK) activation, using a chicken B cell line, DT40, as well as the DT40-derived mutant DT40/Syk(-), which does not express Syk. Osmotic stress evokes increases in [Ca2+]i by stimulating an extracellular Ca2+ influx in both DT40 and DT40/Syk(-) cells. In comparison, oxidative stress elicits an increase in [Ca2+]i by stimulating both an extracellular Ca2+ influx and Ca2+ release from internal stores in DT40 cells, but this Ca2+ response is partially abolished in DT40/Syk(-) cells, indicating that the oxidative stress-induced Ca2+ response is at least partly dependent on Syk. Interestingly, the depletion of Ca2+ results in a significantly decreased level of Syk activation in DT40 cells stimulated by oxidative but not osmotic stress. Furthermore, JNK is activated to different extents by these two types of stress. The extent of JNK activation in DT40/Syk(-) cells in response to osmotic stress is comparable to that observed in DT40 cells. Intriguingly, oxidative stress-induced JNK activation is significantly compromised in DT40/Syk(-) cells. Collectively, these results indicate that both the Ca2+ response and JNK activity induced by oxidative stress are partly dependent on Syk, whereas those induced by osmotic stress are independent of Syk.

Kinetics of the non-specific calcium leak from non-mitochondrial calcium stores in permeabilized A7r5 cells

We have investigated the detailed kinetics of the passive Ca2+ leak from non-mitochondrial Ca2+ stores in permeabilized A7r5 cells. The decrease in the content of stored Ca2+ in the presence of 2 microM thapsigargin deviated from a single-exponential curve in the initial phase of the efflux. The deviation persisted after correcting this efflux for passively bound Ca2+. The non-single-exponential nature of the spontaneous release also occurred when the initial store Ca2+ content was reduced to 40% of its original value by pretreatment with 200 nM inositol 1,4,5-trisphosphate (InsP3). The passive Ca2+ leak could be modelled by two exponential curves with discrete rate constants of 0.06 min-1 and 0.98 min-1, and not by any other type of non-exponential decay. We concluded that individual store units are heterogeneous with respect to their passive Ca2+ permeability. This non-exponential nature of the passive Ca2+ release is unrelated to the non-single-exponential InsP3-induced Ca2+ release.