Ca2+ transients and Mn2+ entry in human neutrophils induced by thapsigargin

Calcium Pathways in Human Neutrophils-The Extended Effects of Thapsigargin and ML-9

In neutrophils, intracellular Ca²⁺ levels are regulated by several transporters and pathways, namely SERCA [sarco(endo)plasmic reticulum Ca²⁺-ATPase], SOCE (store-operated calcium entry), and ROCE (receptor-operated calcium entry). However, the exact mechanisms involved in the communication among these transporters are still unclear. In the present study, thapsigargin, an irreversible inhibitor of SERCA, and ML-9, a broadly used SOCE inhibitor, were applied in human neutrophils to better understand their effects on Ca²⁺ pathways in these important cells of the immune system. The thapsigargin and ML-9 effects in the intracellular free Ca²⁺ flux were evaluated in freshly isolated human neutrophils, using a microplate reader for monitoring fluorimetric kinetic readings. The obtained results corroborate the general thapsigargin-induced intracellular pattern of Ca²⁺ fluctuation, but it was also observed a much more extended effect in time and a clear sustained increase of Ca²⁺ levels due to its influx by SOCE. Moreover, it was obvious that ML-9 enhanced the thapsigargin-induced emptying of the internal stores. Indeed, ML-9 does not have this effect by itself, which indicates that, in neutrophils, thapsigargin does not act only on the influx by SOCE, but also by other Ca²⁺ pathways, that, in the future, should be further explored.

Dissecting the Ca²⁺ entry pathways induced by rotavirus infection and NSP4-EGFP expression in Cos-7 cells

Rotavirus infection modifies Ca(2+) homeostasis provoking an increase in Ca(2+) permeation, cytoplasmic Ca(2+) concentration ([Ca(2+)](cyto)), total Ca(2+) pools and, a decrease of Ca(2+) response to agonists. These effects are mediated by NSP4. The mechanism by which NSP4 deranges Ca(2+) homeostasis is not yet known. It has been proposed that the increase in [Ca(2+)](cyto) is the result of Ca(2+) release from intracellular stores, thereby activating store-operated Ca(2+) entry (SOCE). We studied the mechanisms involved in the changes of Ca(2+) permeability of the plasma membrane elicited by rotavirus infection and NSP4 expression in Cos-7 cells loaded with fura-2 or fluo-4, using inhibitors and activators of different pathways. Total depletion of ER Ca(2+) stores induced by thapsigargin or ATP was not able to elicit Ca(2+) entry in mock-infected cells to the level attained with infection or NSP4-EGFP expression. The pathway induced by NSP4-EGFP expression or infection shows properties shared by SOCE: it can be inactivated by high [Ca(2+)](cyto), is permeable to Mn(2+) and inhibited by La(3+) and the SOC inhibitor 2-aminoethoxydiphenyl borate (2-APB). Contribution of the agonist-operated channels (AOCs) to Ca(2+) entry is small and not modified by infection. The plasma membrane permeability to Ca(2+) in rotavirus infected or NSP4-EGFP expressing cells is also blocked by KB-R7943, an inhibitor of the plasma membrane Na(+)/Ca(2+) exchanger (NCX), operating in its reverse mode. In conclusion, the expression of NSP4 in infected Cos-7 cells appears to activate the NCX in reverse mode and the SOCE pathway to induce increased Ca(2+) entry.

Sulfhydryl modification induces calcium entry through IP₃-sensitive store-operated pathway in activation-dependent human neutrophils

As the first line of host defense, neutrophils are stimulated by pro-inflammatory cytokines from resting state, facilitating the execution of immunomodulatory functions in activation state. Sulfhydryl modification has a regulatory role in a wide variety of physiological functions through mediation of signaling transductions in various cell types. Recent research suggested that two kinds of sulfhydryl modification, S-nitrosylation by exogenous nitric oxide (NO) and alkylation by N-ethylmaleimide (NEM), could induce calcium entry through a non-store-operated pathway in resting rat neutrophils and DDT₁MF-2 cells, while in active human neutrophils a different process has been observed by us. In the present work, data showed that NEM induced a sharp rising of cytosolic calcium concentration ([Ca²⁺]_c) without external calcium, followed by a second [Ca²⁺]_c increase with readdition of external calcium in phorbol 12-myristate 13-acetate (PMA)-activated human neutrophils. Meanwhile, addition of external calcium did not cause [Ca²⁺]_c change of Ca²⁺-free PMA-activated neutrophils before application of NEM. These data indicated that NEM could induce believable store-operated calcium entry (SOCE) in PMA-activated neutrophils. Besides, we found that sodium nitroprusside (SNP), a donor of exogenous NO, resulted in believable SOCE in PMA-activated human neutrophils via S-nitrosylation modification. In contrast, NEM and SNP have no effect on [Ca²⁺]_c of resting neutrophils which were performed in suspension. Furthermore, 2-Aminoethoxydiphenyl borate, a reliable blocker of SOCE and an inhibitor of inositol 1,4,5-trisphosphate (IP₃) receptor, evidently abolished SNP and NEM-induced calcium entry at 75 µM, while preventing calcium release in a concentration-dependent manner. Considered together, these results demonstrated that NEM and SNP induced calcium entry through an IP₃-sensitive store-operated pathway of human neutrophils via sulfhydryl modification in a PMA-induced activation-dependent manner.

Role of transient receptor potential canonical 6 (TRPC6) in non-transferrin-bound iron uptake in neuronal phenotype PC12 cells

Cells take up transferrin-bound iron or NTBI (non-transferrin-bound iron). After treatment with NGF (nerve growth factor), PC12 cells exhibited a neuronal phenotype and an increase in the NTBI uptake (55Fe2+ or 55Fe3+). We loaded the cells with the dye calcein, whose fluorescence increases in the presence of Ca2+ but is quenched with Fe2+ or Fe3+. When examined using calcein fluorescence or radioactive iron, DAG (diacylglycerol)-stimulated NTBI entry was more in NGF-treated PC12 cells compared with untreated cells. All experiments were performed at 1.5 mM extracellular Ca2+. Nramp2 (natural-resistance-associated macrophage protein 2) mRNA expression did not change after the NGF treatment. Expression of the bivalent cation entry protein TRPC6 (transient receptor potential canonical 6) was detected only in the NGF-treated cells. To verify that increased NTBI uptake depended on TRPC6, we examined whether transfecting HEK-293 (human embryonic kidney 293) cells with TRPC6 also increased the NTBI (55Fe) uptake. We also cotransfected HEK-293 cells with two plasmids, one expressing TRPC6 and the other expressing the fluorescent protein DsRED2 to identify the transfected cells. Challenging the calcein-loaded HEK-293 cells (which intrinsically express the a1-adrenergic receptors) with phenylephrine or a cell-permeant DAG increased the fluorescence signal more rapidly in transfected cells compared with untransfected cells. However, when iron (Fe2+ and Fe3+) was added before adding phenylephrine or DAG, the fluorescence intensity decreased more rapidly in transfected cells compared with untransfected cells, thereby indicating a greater stimulation of the NTBI uptake in cells expressing TRPC6. We postulate that the increase in the NTBI entry into neuronal PC12 cells is through TRPC6, a pathway that is unique since it is receptor-stimulated. Since neuronal cells express TRPC6, this pathway may have a role in neurotoxicity.

Calcium signalling is altered in myeloid cells with a deficiency in NADPH oxidase activity

The relation of O(2.-)-production and Ca2+ homeostasis was investigated in PLB-985 cell lines and neutrophilic granulocytes from peripheral blood. In differentiated wild-type PLB-985 cells, a high level of O(2.-)-production was associated with a significant decrease in the membrane potential and the inhibition of capacitative Ca2+ entry. These correlations were not observed in gp91phox -/- cells or in cells transfected with a non-functional mutant of gp91phox (Thr341Lys). Membrane depolarization and inhibition of Ca2+ entry reappeared in cells transfected with wild-type gp91phox. These experiments demonstrate that inhibition of Ca2+ entry depends on the presence of a functional NADPH oxidase. The Ca2+ signal induced by stimulation of chemotactic receptors also showed remarkable differences: [Ca2+]ic in the sustained phase was higher in gp91phox-/- than in wild-type cells. Alteration of the Ca2+ signal was reproduced by treating peripheral blood neutrophils with the NADPH oxidase inhibitor diphenylene-iodonium. It is concluded that the deficiency in O(2.-)-production is accompanied by significant alterations of Ca2+ homeostasis in myeloid cells.

Cyclic AMP-mediated inhibition of 5-lipoxygenase translocation and leukotriene biosynthesis in human neutrophils

5-Lipoxygenase (5-LO) catalyzes the transformation of arachidonic acid to leukotrienes (LT). In stimulated human PMN, activation of 5-LO involves calcium, p38 MAP kinase (p38) phosphorylation, and translocation of 5-LO from the cytosol to nuclear membranes containing the 5-LO activating protein (FLAP). In this study, cAMP-elevating agents such as isoproterenol, prostaglandin E(2), CGS-21680 (an adenosine A(2a) receptor agonist), the type IV phosphodiesterase inhibitor RO 20-1724, the adenylate cyclase activator forskolin, and the Gs-protein activator cholera toxin all inhibited LT biosynthesis and 5-LO translocation to the nucleus in cytokine-primed human PMN stimulated with platelet-activating factor and in human PMN stimulated with the endomembrane Ca(2+)-ATPase blocker thapsigargin. Furthermore, monophosphorothioate analogs of cAMP, which activate protein kinase A (PKA), also inhibited LT biosynthesis and 5-LO translocation in stimulated cells. Treatment of PMN with CGS-21680 also prevented the phosphorylation of p38 by thapsigargin. Treatment of PMN with the PKA inhibitors H-89 and KT-5720 prevented the inhibitory effect of cAMP-elevating agents on LT biosynthesis, 5-LO translocation, and p38 phosphorylation, whereas the p38 inhibitor SB 203,580 dose-dependently inhibited arachidonic acid-induced LT biosynthesis. The 5-LO translocation was also inhibitable by the FLAP antagonist MK-0591 and correlated with LT biosynthesis in all experimental conditions tested. These results indicate that cAMP-mediated PKA activation in PMN results in the concomitant inhibition of 5-LO translocation and LT biosynthesis and support a role of p38 in the signaling pathway involved. This represents the first physiological down-regulation mechanism of 5-LO translocation in human PMN.

Cholesterol-modulating agents selectively inhibit calcium influx induced by chemoattractants in human neutrophils

The effects of cholesterol-perturbing agents on the mobilization of calcium induced upon the stimulation of human neutrophils by chemotactic factors were tested. Methyl-beta-cyclodextrin and filipin did not alter the initial peak of calcium mobilization but shortened the duration of the calcium spike that followed the addition of fMet-Leu-Phe. These agents also inhibited the influx of Mn(2+) induced by fMet-Leu-Phe or thapsigargin. Methyl-beta-cyclodextrin and filipin completely abrogated the mobilization of calcium induced by 10(-10) m platelet-activating factor, which at this concentration depends to a major extent on an influx of calcium as well as the influx of calcium induced by 10(-7) m platelet-activating factor. On the other hand, methyl-beta-cyclodextrin and filipin enhanced the mobilization of calcium induced by ligation of FcgammaRIIA, an agonist that did not induce a detectable influx of calcium. Finally, methyl-beta-cyclodextrin and filipin enhanced the stimulation of the profile of tyrosine phosphorylation, the activity of phospholipase D (PLD), and the production of superoxide anions induced by fMet-Leu-Phe. These results suggest that the calcium channels utilized by chemotactic factors in human neutrophils are either located in cholesterol-rich regions of the plasma membrane, or that the mechanisms that lead to their opening depend on the integrity of these microdomains.

Mechanisms of bradykinin-mediated Ca(2+) signalling in canine cultured corneal epithelial cells

Experiments were designed to differentiate the mechanisms of bradykinin receptors mediating the changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) in canine cultured corneal epithelial cells (CECs). Bradykinin and Lys-bradykinin caused an initial transient peak of [Ca(2+)](i) in a concentration-dependent manner, with half-maximal stimulation (pEC(50)) obtained at 6.9 and 7.1, respectively. Pretreatment of CECs with pertussis toxin (PTX) or cholera toxin (CTX) for 24 h did not affect the bradykinin-induced [Ca(2+)](i) changes. Application of Ca(2+) channel blockers, diltiazem and Ni(2+), inhibited the bradykinin-induced Ca(2+) mobilization, indicating that Ca(2+) influx was required for the bradykinin-induced responses. Addition of thapsigargin (TG), which is known to deplete intracellular Ca(2+) stores, transiently increased [Ca(2+)](i) in Ca(2+)-free buffer, and subsequently induced Ca(2+) influx when Ca(2+) was readded to this buffer. Pretreatment of CECs with TG completely abolished bradykinin-induced initial transient [Ca(2+)](i), but had slight effect on bradykinin-induced Ca(2+) influx. Pretreatment of CECs with 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole (SKF96365) and 1-(6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione (U73122) inhibited the bradykinin-induced Ca(2+) release and Ca(2+) influx, consistent with the inhibition of receptor-gated Ca(2+) channels and phospholipase C (PLC) in CECs, respectively. These results demonstrate that bradykinin directly stimulates B(2) receptors and subsequently Ca(2+) mobilization via a PTX-insensitive G protein in canine CECs. These results suggest that bradykinin-induced Ca(2+) influx into the cells is not due to depletion of these Ca(2+) stores, as prior depletion of these pools by TG has no effect on the bradykinin-induced Ca(2+) influx that is dependent on extracellular Ca(2+) in CECs.

Control of Ca2+ influx in human neutrophils by inositol 1,4,5-trisphosphate (IP3) binding: differential effects of micro-injected IP3 receptor antagonists

Neutrophils signal Ca2+ changes in response to occupancy of G-protein-linked receptors such as the formylated peptide receptor. This Ca2+ signal is composed of two parts, inositol 1,4,5-trisphosphate (IP3)-triggered release of Ca2+ from an intracellular store and Ca2+ influx. In order to probe the relationship between these events, cytosolic free Ca2+ changes in neutrophils were monitored after micro-injection of agents which inhibit IP3 binding. Micro-injection of heparin into neutrophils totally inhibited both formylmethionyl-leucylphenylalanine-induced Ca2+ release and the subsequent Ca2+ influx. This effect was not due to prior depletion of Ca2+ stores. Furthermore, micro-injection with anti-IP3-receptor antibody also inhibited Ca2+ release. However, anti-IP3-receptor antibody and another high-molecular-mass IP3-binding antagonist, heparin-albumin conjugate, failed to inhibit the accompanying Ca2+ influx. It was concluded that two IP3-binding sites exist in neutrophils: one accessible by both heparin and the high-molecular-mass inhibitors of IP3 binding and responsible for Ca2+ release, and another inaccessible to high-molecular-mass molecules and responsible for Ca2+ influx.

Colorimetric and fluorimetric assays to quantitate micromolar concentrations of transition metals

Transition metal ions, although maintained at low concentrations, play diverse important roles in many biological processes. Two assays useful for the rapid quantification of a range of first-row transition metal ions have been developed. The colorimetric assay extends the 4-(2-pyridylazo)resorcinol assay of Hunt et al. (J. Biol. Chem. 255, 14793 (1984)) to measure nanomole quantities of Co(2+), Ni(2+), and Cu(2+) as well as Zn(2+). The fluorimetric assay takes advantage of the coordination of a number of metal ions (Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+)) by Fura-2 and can also be used to measure nanomole quantities of these ions. The assays developed here have the advantage of not requiring the extensive sample preparation necessary for other methodologies, such as atomic absorption spectroscopy and inductively coupled plasma emission spectroscopy (ICPES), while being comparable in accuracy to the detection limits of ICPES for the first-row transition metal ions. To demonstrate the effectiveness of these assays, we determined the affinity of carbonic anhydrase II (CA II), a prototypical zinc enzyme, for Ni(2+) and Cd(2+). These data indicate that CA II binds transition metals with high affinity and is much more selective for Zn(2+) over Ni(2+) or Cd(2+) than most small-molecule chelators or other metalloenzymes.

Rotavirus infection induces an increase in intracellular calcium concentration in human intestinal epithelial cells: role in microvillar actin alteration

Rotaviruses, which infect mature enterocytes of the small intestine, are recognized as the most important cause of viral gastroenteritis in young children. We have previously reported that rotavirus infection induces microvillar F-actin disassembly in human intestinal epithelial Caco-2 cells (N. Jourdan, J. P. Brunet, C. Sapin, A. Blais, J. Cotte-Laffitte, F. Forestier, A. M. Quero, G. Trugnan, and A. L. Servin, J. Virol. 72:7228-7236, 1998). In this study, to determine the mechanism responsible for rotavirus-induced F-actin alteration, we investigated the effect of infection on intracellular calcium concentration ([Ca(2+)](i)) in Caco-2 cells, since Ca(2+) is known to be a determinant factor for actin cytoskeleton regulation. As measured by quin2 fluorescence, viral replication induced a progressive increase in [Ca(2+)](i) from 7 h postinfection, which was shown to be necessary and sufficient for microvillar F-actin disassembly. During the first hours of infection, the increase in [Ca(2+)](i) was related only to an increase in Ca(2+) permeability of plasmalemma. At a late stage of infection, [Ca(2+)](i) elevation was due to both extracellular Ca(2+) influx and Ca(2+) release from the intracellular organelles, mainly the endoplasmic reticulum (ER). We noted that at this time the [Ca(2+)](i) increase was partially related to a phospholipase C (PLC)-dependent mechanism, which probably explains the Ca(2+) release from the ER. We also demonstrated for the first time that viral proteins or peptides, released into culture supernatants of rotavirus-infected Caco-2 cells, induced a transient increase in [Ca(2+)](i) of uninfected Caco-2 cells, by a PLC-dependent efflux of Ca(2+) from the ER and by extracellular Ca(2+) influx. These supernatants induced a Ca(2+)-dependent microvillar F-actin alteration in uninfected Caco-2 cells, thus participating in rotavirus pathogenesis.

Inhibition of interleukin-8-activated human neutrophil chemotaxis by thapsigargin in a calcium- and cyclic AMP-dependent way

Chemotactic migration of human neutrophils, induced by interleukin-8 (IL-8) or other activators, was inhibited by thapsigargin in the high nanomolar range. The degree of inhibition depended on the type of activator. Other inhibitors of Ca(2+)-ATPases associated with intracellular calcium stores, such as cyclopiazonic acid and 2,5-di-(tert-butyl)-1,4-benzohydroquinone, equally inhibited IL-8-activated migration. Inhibition of migration by thapsigargin and the other ATPase inhibitors occurred only in the presence of extracellular Ca2+; migration was not inhibited in the presence of EGTA. La3+ reversed thapsigargin-induced inhibition to a large degree; other calcium channel blockers gave a partial reversal (econazole, verapamil, and SK&F 96365) or had no effect (gadolinium chloride and Ni2+). Using electroporated cells and Ca buffers, it was shown that inhibition started at about 0.2 microM and was complete at a cytosolic Ca concentration of about 2 microM. It appears that under certain conditions the thapsigargin-induced influx of extracellular calcium, causing relatively high local calcium concentrations, initiates or permits a process which may be detrimental to chemotactic migration. Cyclic AMP (cAMP; adenosine 3',5'-cyclic monophosphate) is probably involved in this process, because thapsigargin increased the cAMP level and cAMP inhibited IL-8-activated migration in a calcium-dependent way. The hypothesis that cAMP is involved in the effect of thapsigargin on migration is supported by the finding that very low concentrations of thapsigargin stimulate neutrophil migration in the absence of other chemoattractants. The results suggest that thapsigargin causes a (compartmentalized) increase in cAMP, which results in a calcium-dependent modulation of migration.

Ni2+ transport by the human Na+/Ca2+ exchanger expressed in Sf9 cells

The mechanism of Ni2+ block of the Na+/Ca2+ exchanger was examined in Sf 9 cells expressing the human heart Na+/Ca2+ exchanger (NCX1-NACA1). As predicted from the reported actions of Ni2+, its application reduced extracellular Na+-dependent changes in intracellular Ca2+ concentration (measured by fluo 3 fluorescence changes). However, contrary to expectation, the reduced fluorescence was accompanied by measured 63Ni2+ entry. The 63Ni2+ entry was observed in Sf 9 cells expressing the Na+/Ca2+ exchanger but not in control cells. The established sequential transport mechanism of the Na+/Ca2+ exchanger could be compatible with these results if one of the two ion translocation steps is blocked by Ni2+ and the other permits Ni2+ translocation. We conclude that, because Ni2+ entry was inhibited by extracellular Ca2+ and enhanced by extracellular Na+, the Ca2+ translocation step moved Ni2+, whereas the Na+ translocation step was inhibited by Ni2+. A model is presented to discuss these findings.

Bradykinin-induced phosphoinositide hydrolysis and Ca2+ mobilization in canine cultured tracheal epithelial cells

1. Experiments were designed to differentiate the mechanisms and subtype of kinin receptors mediating the changes in intracellular Ca2+ concentration ([Ca2+]i) induced by bradykinin (BK) in canine cultured tracheal epithelial cells (TECs). 2. BK and Lys-BK caused an initial transient peak of [Ca2+]i in a concentration-dependent manner, with half-maximal stimulation (pEC50) obtained at 7.70 and 7.23, respectively. 3. Kinin B2 antagonists Hoe 140 (10 nM) and [D-Arg0, Hyp3, Thi5,8, D-Phe7]-BK (1 microM) had high affinity in antagonizing BK-induced Ca2+ response with pKB values of 8.90 and 6.99, respectively. 4. Pretreatment of TECs with pertussis toxin (100 ng ml(-1)) or cholera toxin (10 microg ml(-1)) for 24 h did not affect the BK-induced IP accumulation and [Ca2+]i changes in TECs. 5. Removal of Ca2+ by the addition of EGTA or application of Ca2+-channel blockers, verapamil, diltiazem, and Ni2+, inhibited the BK-induced IP accumulation and Ca2+ mobilization, indicating that Ca2+ influx was required for the BK-induced responses. 6. Addition of thapsigargin (TG), which is known to deplete intracellular Ca2+ stores, transiently increased [Ca2+]i in Ca2+-free buffer and subsequently induced Ca2+ influx when Ca2+ was re-added to this buffer. Pretreatment of TECs with TG completely abolished BK-induced initial transient [Ca2+]i, but had slight effect on BK-induced Ca2+ influx. 7. Pretreatment of TECs with SKF96365 and U73122 inhibited the BK-induced Ca2+ influx and Ca2+ release, consistent with the inhibition of receptor-gated Ca2+-channels and phospholipase C in TECs, respectively. 8. These results demonstrate that BK directly stimulates kinin B2 receptors and subsequently phospholipase C-mediated IP accumulation and Ca2+ mobilization via a pertussis toxin-insensitive G protein in canine TECs. These results also suggest that BK-induced Ca2+ influx into the cells is not due to depletion of these Ca2+ stores, as prior depletion of these pools by TG has no effect on the BK-induced Ca2+ influx that is dependent on extracellular Ca2+ in TECs.

Dissociation of intracellular Ca2+ release and Ca2+ entry response to 5-hydroxytryptamine in cultured canine tracheal smooth muscle cells

The relationship between the agonist-sensitive Ca2+ pool and those discharged by the Ca2+ -ATPase inhibitor thapsigargin (TG) were investigated in canine tracheal smooth muscle cells (TSMCs). In fura-2-loaded TSMCs, 5-hydroxytryptamine (5-HT) stimulated a rapid increase in intracellular Ca2+ ([Ca2+]i), followed by a sustained plateau phase that was dependent on extracellular Ca2+. In such cells, TG produced a concentration-dependent increase in [Ca2+]i, which remained elevated over basal level for several minutes and was substantially attenuated in the absence of extracellular Ca2+. Application of 5-HT after TG demonstrated that the TG-sensitive compartment partly overlapped the 5-HT-sensitive stores. Pre-treatment of TSMCs with TG significantly inhibited the increase in [Ca2+]i induced by 5-HT in a time-dependent manner. Similar results were obtained with two other Ca2+ -ATPase inhibitors, cyclopiazonic acid and 2,5-di-t-butylhydroquinone. Although these inhibitors had no effect on phosphoinositide hydrolysis, Ca2+ -influx was stimulated by these agents. These results suggest that depletion of the agonist-sensitive Ca2+ stores is sufficient for activation of Ca2+ influx. Some characteristics of the Ca2+ -influx activated by depletion of internal Ca2+ stores were compared with those of the agonist-activated pathway. 5-HT-stimulated Ca2+ influx was inhibited by La3+, membrane depolarisation, and the novel Ca2+ -influx blocker 1-¿beta-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenethyl¿-1H-imidazole hydrochloride (SKF96365). Likewise, activation of Ca2+ influx by TG also was blocked by La3+, membrane depolarisation, and SKF96365. These results suggest that (1) in the absence of PI hydrolysis, depletion of the agonist-sensitive internal Ca2+ stores in TSMCs is sufficient for activation of Ca2+ influx, and (2) the agonist-activated Ca2+ influx pathway and the influx pathway activated by depletion of the inositol 1,4,5-trisphosphate-sensitive Ca2+ pool are indistinguishable.

Ca2+-Dependent Production and Release of IL-8 in Human Neutrophils

IL-8, a potent neutrophil chemoattractant that is elevated about 200-fold in exudative neutrophils isolated from localized inflammatory sites in vivo, is thought to play a major role in recruitment of neutrophils to inflammatory sites. Incubation of peripheral blood neutrophils with thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-sequestering-ATPase, causes a dose-dependent induction of IL-8 synthesis that continues for up to 8 h. Cycloheximide inhibits the thapsigargin-induced IL-8 production, suggesting the induction of protein synthesis de novo. In addition, Northern blot analysis of mRNA isolated from neutrophils indicates that thapsigargin treatment increases IL-8 mRNA in a time- and dose-dependent manner. Thapsigargin also induces a biphasic rise in the intracellular Ca2+ concentration, [Ca2+]i, which is composed of an initial (within 15 s) EGTA-insensitive elevation in [Ca2+]i, followed by a delayed (2-min) EGTA-sensitive component. Addition of EGTA before thapsigargin inhibited the induction of IL-8 production. Experiments in which EGTA was added at various times after thapsigargin treatment indicated that a sustained Ca2+ influx was required for maximum IL-8 production. Ascomycin and cyclosporin A, inhibitors of the Ca2+-dependent phosphatase, calcineurin, also inhibited thapsigargin-induced IL-8 production. Thus, in neutrophils, a prolonged increase in [Ca2+]i stimulates IL-8 transcription and synthesis, possibly through a calcineurin-dependent pathway.

ATP stimulation of Ca2+ -dependent plasminogen release from cultured microglia

1. ATP (10-100 microM), but not glutamate (100 microM), stimulated the release of plasminogen from microglia in a concentration-dependent manner during a 10 min stimulation. However, neither ATP (100 microM) nor glutamate (100 microM) stimulated the release of NO. A one hour pretreatment with BAPTA-AM (200 microM), which is metabolized in the cytosol to BAPTA (an intracellular Ca2+ chelator), completely inhibited the plasminogen release evoked by ATP (100 microM). The Ca2+ ionophore A23187 induced plasminogen release in a concentration-dependent manner (0.3 microM to 10 microM). 2. ATP induced a transient increase in the intracellular calcium concentration ([Ca2+]i) in a concentration-dependent manner which was very similar to the ATP-evoked plasminogen release, whereas glutamate (100 microM) had no effect on [Ca2+]i (70 out of 70 cells) in microglial cells. A second application of ATP (100 microM) stimulated an increase in [Ca2+]i similar to that of the first application (21 out of 21 cells). 3. The ATP-evoked increase in [Ca2+]i was totally dependent on extracellular Ca2+, 2-Methylthio ATP was active (7 out of 7 cells), but alpha,beta-methylene ATP was inactive (7 out of 7 cells) at inducing an increase in [Ca2+]i. Suramin (100 microM) was shown not to inhibit the ATP-evoked increase in [Ca2+]i (20 out of 20 cells). 2'- and 3'-O-(4-Benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP), a selective agonist of P2X7 receptors, evoked a long-lasting increase in [Ca2+]i even at 1 microM, a concentration at which ATP did not evoke the increase. One hour pretreatment with adenosine 5'-triphosphate-2', 3'-dialdehyde (oxidized ATP, 100 microM), a selective antagonist of P2X7 receptors, blocked the increase in [Ca2+]i induced by ATP (10 and 100 microM). 4. These data suggest that ATP may transit information from neurones to microglia, resulting in an increase in [Ca2+]i via the ionotropic P2X7 receptor which stimulates the release of plasminogen from the microglia.

Regulation of capacitative Ca2+ influx in human neutrophil granulocytes. Alterations in chronic granulomatous disease

Ca2+ entry through the capacitative (store-regulated) pathway was shown to be inhibited in neutrophil granulocytes by the protein kinase C activator phorbol 12-myristate 13-acetate and the chemoattractant N-formyl-methionyl-leucyl-phenylalanine (fMLP) by a hitherto unknown mechanism. Measuring both Ca2+ and Mn2+ entry into store-depleted cells we show in the present study that inhibition of the capacitative pathway is absent in various forms of chronic granulomatous disease. To establish the possible relationship between inhibition of the capacitative pathway and ability of O-2 production and consequent membrane depolarization, gradual changes of the membrane potential were evoked in neutrophils of healthy individuals. This was accomplished by pharmacological manipulation of the membrane potential and by variations of the concentration and type of the stimulant. Close relationship was observed between membrane depolarization and inhibition of Mn2+ entry through the capacitative transport route. Our results provide an explanation for the inhibitory action of fMLP and phorbol 12-myristate 13-acetate on capacitative cation influx and reveal that upon physiological stimulation, Ca2+ entry into neutrophils is restricted by the depolarization accompanying O-2 production.

Muscarinic receptor stimulation modulates the effect of halothane on Mn2+ influx in airway smooth muscle

Prior studies suggest that the mechanism of action by which halothane relaxes airway smooth muscle depends on the contractile state of the cell. We hypothesized that halothane would inhibit the influx of Ca2+ into canine airway smooth muscle cells during submaximal, but not maximal, muscarinic stimulation. This hypothesis was tested by using the rate of quenching of fura 2 fluorescence by Mn2+ in strips of canine tracheal smooth muscle as an index of Ca2+ influx. Acetylcholine (ACh) produced a dose-dependent increase in Mn2+ influx. Halothane (0.64 +/- 0.05 microM) significantly decreased Mn2+ influx and intracellular Ca2+ concentration when added to strips stimulated with a submaximal concentration of ACh (0.3 microM) but had no effect on Mn2+ influx or intracellular Ca2+ concentration during maximal stimulation with ACh (100 microM). Similar results were observed when the strips were treated with verapamil. These results demonstrate that anesthetic effects on Ca2+ homeostasis in intact canine tracheal smooth muscle cells may be critically modulated by receptor-linked mechanisms.

Inhibitory effects of N-acetylcysteine on superoxide anion generation in human polymorphonuclear leukocytes

It has been suggested that reactive oxygen species released by activated polymorphonuclear leukocytes (PMN) in man is one mechanism of tissue injury. Therapeutic action aimed at increasing antioxidant defence mechanisms is still a clinical challenge. This study examines the activity of N-acetylcysteine, a known antioxidant, in the protection of PMN exposed in-vitro to the chemoattractant peptide fMet-Leu-Phe (FMLP), the protein kinase C activator phorbol myristate acetate or the lipid peroxidation promoter t-butyl hydroperoxide. FMLP (3-300 nM) and phorbol myristate acetate (160 pm-160 nM) induced concentration-related superoxide anion generation. Pre-treatment with N-acetylcysteine (33-333 microM) resulted in concentration-related inhibition of superoxide production induced by FMLP (30 nM) or phorbol myristate acetate (16 nM);-log IC50 values were 3.97 +/- 0.07 and 3.91 +/- 0.10, respectively. Changes in intracellular calcium ion concentration ([Ca2+]i) induced by FMLP (30 nM) were studied in fura-2-loaded human PMN. FMLP produced a transient calcium response, i.e. a peak followed by decay to a residual value above baseline. N-Acetylcysteine (333 microM) did not affect either basal [Ca2+]i values or changes in [Ca2+]i values after treatment with FMLP. Activation by phorbol myristate acetate caused a reduction in glutathione levels from 5.94 +/- 0.86 (control) to 1.84 +/- 0.51 nmol/3 x 10(6) cells (P < 0.05 compared with control). Pre-treatment with N-acetylcysteine (333 microM) fully reversed the reduction in glutathione levels induced by phorbol myristate acetate (4.83 +/- 0.68 nmol/3 x 10(6) cells; P > 0.05 compared with control). Exposure to t-butyl hydroperoxide (0.5 mM, 30 min) markedly increased malondialdehyde levels (from 0.03 +/- 0.02 to 0.73 +/- 0.07 nmol/10(6) cells), and index of lipid peroxidation. Malondialdehyde levels were significantly reduced in PMN treated with N-acetylcysteine (333 microM; 0.55 +/- 0.04 nmol/10(6) cells; P < 0.05 compared with untreated cells exposed to t-butyl hydroperoxide). In conclusion, N-acetylcysteine reduces superoxide generation in response to FMLP and phorbol myristate acetate and partially protects against lipid peroxidation in PMN from man. The protection afforded by N-acetylcysteine is not related to alteration of the intracellular calcium signal but might be effected by replenishment of the intracellular glutathione levels.

Delayed autoregulation of the Ca2+ signals resulting from capacitative Ca2+ entry in bovine pulmonary artery endothelial cells

1. In calf pulmonary artery endothelial (CPAE) cells loaded with fura-2, the effects of ATP on Ca2+ entry were mediated entirely by the ability of P2U purinoceptors to stimulate InsP3 formation, empty intracellular Ca2+ stores and thereby activate capacitative Ca2+ entry. 2. Restoration of extracellular Ca2+ to cells with empty intracellular stores evoked transient increases in cytosolic [Ca2+] ([Ca2+]i) which then declined to an elevated plateau. These overshoots in [Ca2+]i were not a consequence of store refilling nor of desensitization of the capacitative pathway. Similar responses were recorded from cells in which Ca2+ uptake into mitochondria had been inhibited by microinjection of Ruthenium Red. The amplitudes of the capacitative Ca2+ signals decreased at lower extracellular [Ca2+], but [Ca2+]i invariably overshot before slowly declining to an elevated plateau. Even modest increases in [Ca2+]i therefore caused a delayed attenuation of the Ca2+ signal evoked by capacitative Ca2+ entry. 3. Modest pre-elevation of [Ca2+]i inhibited the ability of subsequent capacitative Ca2+ entry to further increase [Ca2+]i. The onset of the inhibition was slow (half-time (t1/2), approximately 100 s) and more tightly correlated with the preceding peak [Ca2+]i than with the [Ca2+]i immediately preceding Ca2+ entry. Recovery was also slow and complete only after [Ca2+]i had returned to its basal level for 320 +/- 3 s. 4. In thapsigargin-treated cells loaded with mag-fura-2, the peak [Ca2+]i that followed restoration of extracellular Ca2+ was accompanied by an abrupt approximately 2.5-fold decrease in the rate of Mn2+ entry, which then continued indefinitely at the reduced rate, demonstrating a rapid partial inactivation of the capacitative pathway. 5. The half-time for Ca2+ removal from the cytosol was significantly slower during the rising (t 1/2 = 22 +/- 2.5 s) than during the falling (t 1/2 = 7.1 +/- 0.7 s) phase of the Ca2+ overshoot evoked by addition of extracellular Ca2+ to thapsigargin-treated cells. 6. We conclude that an increase in [Ca2+]i rapidly inhibits the capacitative pathway and more slowly activates mechanisms that remove Ca2+ from the cytosol. Reversal of either or both of these regulatory mechanisms can occur only a considerable time after [Ca2+]i has been completely restored to its resting level. These mechanisms are likely to protect cells from excessive increases in [Ca2+]i and contribute to oscillatory changes in [Ca2+]i.

Ca2+ oscillations and intercellular Ca2+ waves in ATP-stimulated articular chondrocytes

Cytosolic Ca2+ oscillations are known to occur in many cell types stimulated with agonists linked to the phosphoinositide signaling pathway. Trains of repetitive short-lasting Ca2+ spikes could be induced in articular chondrocytes by extracellular ATP, an agonist potently effective in stimulating cartilage resorption. The mechanism of these Ca2+ oscillations was studied by computerized video imaging on primary cultures of articular chondrocytes. Few cycles of oscillatory activity could be evoked in the absence of extracellular Ca2+, while, for oscillations to be sustained, Ca2+ influx was required. Thapsigargin irreversibly blocked Ca2+ oscillations, thus demonstrating the crucial involvement of intracellular stores in triggering the rhythmic activity. Apart from activating intracellular Ca2+ release, extracellular ATP also induced a noncapacitive Ca2+ influx in these cells. This ATP-mediated influx modulates both the oscillation frequency and intracellular stores refilling. In monolayers of confluent cells, Ca2+ oscillations spread from cell to cell in the form of intercellular waves. Propagating waves could also be observed in the absence of extracellular Ca2+, demonstrating that Ca2+ itself is not required for signal coordination. These results demonstrate that complex spatiotemporal pathways of Ca2+ oscillations and intercellular Ca2+ waves could be activated in articular chondrocytes during degenerative diseases.

Role of calcium during lipopolysaccharide stimulation of neutrophils

This study investigated the role of intracellular calcium concentration ([Ca]i) as a possible intermediate in the lipopolysaccharide (LPS) second messenger pathway for the activation of neutrophils (polymorphonuclear leukocytes [PMNs]). Isolated PMNs were loaded with the calcium-sensitive fluorescent dye fura-2. The PMNs were stimulated with either LPS or the positive control formyl-Met-Leu-Phe (fMLP). As expected, PMN exposure to fMLP increased [Ca]i. However, LPS stimulation did not induce any detectable changes. Depletion of intracellular Ca stores with thapsigargin, or extracellular Ca with EGTA, significantly inhibited the upregulation of the CD11b/CD18 integrin in response to fMLP but not LPS. We conclude that [Ca]i is not an early intermediate in the second-messenger pathway for the activation of PMNs by LPS.

Depletion of calcium stores by thapsigargin induces membrane depolarization by cation entry in human neutrophils

The ability of various cations to change the electrical potential of the plasma membrane was examined in human neutrophils by the use of the fluorescent cationic dye 3,3'-dipropylthiadicarbocyanine. When the cells were suspended in 140 mM KCl, the fluorescence was high, indicating depolarized neutrophils. Suspension in 145 mM N-methyl-D-glucamine chloride (NMG), replacing sodium and potassium chloride, resulted in hyperpolarized neutrophils. After depletion of the intracellular calcium stores of the NMG-suspended cells with thapsigargin and EDTA or EGTA, the addition of cations depolarized the neutrophils, suggesting the existence of pathways for cation entry. Besides Na+ and K+, several divalent cations were effective in the sequence: Ca2+ > Mn2+ > Ba2+ > Cd2+ > Mg2+ > Co2+ > Zn2+ > Ni2+. Pretreatment of the neutrophils with 0.5 or 1 mM CaCl2, resulting in loading of calcium stores, reduced the ability of some of the cations to depolarize the NMG-suspended cells. From the depolarizing effects of the cations it is concluded that the entries of Ca2+, Mg2+, Mn2+, Ba2+, probably Co2+, to some extent Na+ and K+, but hardly Cd2+, Zn2+, or Ni2+, are regulated by the filling state of the intracellular calcium stores in human neutrophils. The store-regulated entry pathway may contribute to the control of the membrane potential and become active when the neutrophils are stimulated.

The rotavirus nonstructural glycoprotein NSP4 mobilizes Ca2+ from the endoplasmic reticulum

We previously reported that expression of rotavirus nonstructural glycoprotein NSP4 is responsible for an increase in cytosolic free Ca2+ concentration ([Ca2+]i) in Spodoptera frugiperda (Sf9) insect cells (P. Tian, Y. Hu, W. P. Schilling, D. A. Lindsay, J. Eiden, and M. K. Estes, J. Virol. 68:251-257, 1994). The purpose of the present study was to determine the mechanism by which NSP4 causes an increase in [Ca2+]i by measuring the permeability of the cytoplasmic and endoplasmic reticulum (ER) membranes in recombinant-baculovirus-infected Sf9 cells. No obvious change in plasmalemma permeability to divalent cations was observed in cells expressing NSP4 compared with that in cells expressing another rotaviral glycoprotein (VP7) when the influx of Ba2+, a Ca2+ surrogate, was monitored. The basal Ca2+ permeability of the internal Ca2+ store was evaluated by measuring the release of Ca2+ induced by ionomycin, a Ca2+ ionophore, or thapsigargin, an inhibitor of the ER Ca(2+)-ATPase pump, following suspension of the cells in Ca(2+)-free extracellular buffer. Releasable Ca2+ decreased with time to a greater extent in cells expressing NSP4 compared with that in cells expressing VP7, suggesting that NSP4 increases the basal Ca2+ permeability of the ER membrane. To determine the possible mechanism by which NSP4 increases ER permeability, purified NSP4 protein or a 22-amino-acid synthetic peptide consisting of residues 114 to 135 (NSP4(114-135) was added exogenously to noninfected Sf9 cells during measurement of [Ca2+]i. Both NSP4 and the NSP4(114-135 peptide produced a time-dependent increase in [Ca2+]i that was attenuated by prior inhibition of phospholipase C with U-73122. Pretreatment of the cells with thapsigargin completely blocked the increase in [Ca2+]i produced by NSP4(114-135, but the peptide only partially reduced the change in [Ca2+]i produced by thapsigargin. No changes in [Ca2+]i were seen in cells treated with control peptides. These results suggest that (i) exogenous NSP4 increases [Ca2+]i through the activation of phospholipase C, (ii) Ca2+ release by exogenous NSP4 is from a store that is a subset of the thapsigargin-sensitive compartment, and (iii) amino acid residues 114 to 135 of NSP4 are sufficient for this activity. In contrast to exogenous NSP4, the mechanism by which endogenously expressed NSP4 increases [Ca2+]1 appears to be unrelated to phospholipase C, since no effect of U-73122 was seen on the elevated [Ca2+]1 in cells expressing NSP4 and exogenously applied NSP4(114-135) caused a further increase in [Ca2+]1 in cells expressing NSP4 protein.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+ increase and Ca(2+)-influx in human tracheal smooth muscle cells: role of Ca2+ pools controlled by sarco-endoplasmic reticulum Ca(2+)-ATPase 2 isoform

1. The contribution of sarco-endoplasmic reticulum Ca(2+)-ATPases (SERCA)-regulated Ca2+ stores to the increase in intracellular free calcium ([Ca2+]i) induced by bradykinin (BK) was investigated in fura-2 loaded human tracheal smooth muscle cells (TSMC). For this purpose, we used thapsigargin, a selective inhibitor of Ca(2+)-ATPases of intracellular organelles. 2. Thapsigargin (10(-9) to 10(-6) M) induced a dose-dependent increase in [Ca2+]i in the presence of external Ca2+ with an EC50 value of 7.33 +/- 1.26 nM. In Ca(2+)-free conditions, the addition of Ca2+ (1.25 mM) caused an increase in [Ca2+]i which was directly proportional to the pre-incubation time of the cells with thapsigargin. Net increases of 60 +/- 9, 150 +/- 22 and 210 +/- 27 nM were obtained after 1, 3 and 5 min, respectively. 3. In the presence of extracellular Ca2+, BK induced a typical biphasic increase in [Ca2+]i with a fast transient phase and a sustained phase. The sustained component was reversed by addition of a bradykinin B2-receptor antagonist (Hoe 140, 10(-6) M) to the buffer as well as by deprivation of Ca2+. The transient phase induced by BK, histamine and carbachol was inhibited in a time-dependent way by preincubation of the cells with thapsigargin. 4. Comparative western blotting of human TSMC membranes using anti-SERCA2 isoform-specific antibodies clearly showed the greater expression of the 100-kDa SERCA2-b isoform compared with the SERCA2-a isoform. 5. Our data show that thapsigargin-sensitive Ca2+ stores contribute significantly to the activation of human TSMC which suggests a role for these stores in the subsequent induction of Ca2+ influx. These stores appear to be controlled by the Ca2+-ATPases (SERCA2-b isoform) which could also participate in the regulation of Ca2+ influx through the plasma membrane.

T cell receptor-mediated Ca2+ signaling: release and influx are independent events linked to different Ca2+ entry pathways in the plasma membrane

In this study, we showed that cross-linking CD3 molecules on the T cell surface resulted in Ca2+ release from the intracellular stores followed by a sustained Ca2+ influx. Inhibition of release with TMB-8 did not block the influx. However, inhibition of phospholipase C activity suppressed both Ca2+ release and influx. Once activated, the influx pathway remained open in the absence of further hydrolysis of PIP2. Thapsigargin, a microsomal Ca(2+)-ATPase inhibitor, stimulated Ca2+ entry into the cells by a mechanism other than emptying Ca2+ stores. In addition, Ca2+ entry into the Ca(2+)-depleted cells was stimulated by low basal level of cytosolic Ca2+, not by the emptying of intracellular Ca2+ stores. Both the Ca2+ release and influx were dependent on high and low concentrations of extracellular Ca2+. At low concentrations, Mn2+ entered the cell through the Ca2+ influx pathway and quenched the sustained phase of fluorescence; whereas, at higher Mn2+ concentration both the transient and the sustained phases of fluorescence were quenched. Moreover, Ca2+ release was inhibited by low concentrations of Ni2+, La3+, and EGTA, while Ca2+ influx was inhibited by high concentrations. Thus, in T cells Ca2+ influx occurs independently of IP3-dependent Ca2+ release. However, some other PIP2 hydrolysis-dependent event was involved in prolonged activation of Ca2+ influx. Extracellular Ca2+ influenced Ca2+ release and influx through the action of two plasma membrane Ca2+ entry pathways with different pharmacological and biochemical properties.

Receptor-mediated activation of recombinant Trpl expressed in Sf9 insect cells

The Drosophila proteins, Trp and Trpl, are suggested to be cation channels responsible for depolarization of the receptor potential associated with stimulation of insect photoreceptor cells by light. Consistent with this hypothesis, we recently showed that recombinant Trpl forms Ca(2+)- and Ba(2+)-permeable non-selective cation channels when expressed in Sf9 cells using the baculovirus expression vector. As Trpl may be activated in the photoreceptor cell after stimulation of phospholipase C, we hypothesized that a similar regulation of recombinant Trpl may be observed in the Sf9 cell after activation of heterologous membrane receptors linked to Ca(2+)-signal-transduction pathways. To test this hypothesis, Ca2+ signalling was examined in Fura-2-loaded Sf9 cells infected with baculovirus containing cDNA for the M5 muscarinic receptor alone (M5 cells) or in cells co-infected with both M5 and Trpl-containing baculoviruses (M5-Trpl cells). Addition of carbachol (100 microM) to M5 cells produced an increase in cytosolic free Ca2+ concentration ([Ca2+]i) (mean +/- S.D.; n = 17) from 101 +/- 20 to 762 +/- 178 nM which declined to a sustained elevated level of 384 +/- 102 nM after 3 min. The sustained component was eliminated by removal of extracellular Ca2+ or by addition of La3+ or Gd3+ (10 microM). In M5-Trpl cells, basal [Ca2+]i increased as a function of time after infection. To evaluate the contribution of Ca2+ influx to the overall profile observed, Ba2+, a Ca2+ surrogate that is not a substrate for the Ca2+ pump, was used. The increase in basal [Ca2+]i seen in M5-Trpl cells was associated with an increase in basal Ba2+ influx. Addition of carbachol to M5-Trpl cells at 30-36 h after infection produced a large increase in [Ca2+]i to a sustained value of 677 +/- 143 nM. This change in [Ca2+]i was (1) blocked by atropine, (2) attenuated in the absence of extracellular Ca2+, and (3) relatively insensitive to La3+, but blocked by Gd3+ in the 0.1-1 mM range. In the presence of 10 microM Gd3+ to block the endogenous-receptor-mediated Ca(2+)-influx in M5-Trpl cells. In sharp contrast increase in Ba2+ influx in M5-Trpl cells. In sharp contrast, neither Ca2+ nor Ba2+ influx through Trpl was affected by thapsigargin, a selective inhibitor of the endoplasmic reticulum Ca(2+)-ATPase pump.(ABSTRACT TRUNCATED AT 400 WORDS)

Thapsigargin inhibits Ca2+ entry into human neutrophil granulocytes

The mechanism of Ca2+ entry after ligand binding to receptors on the surface of non-excitable cells is a current focus of interest. Considerable attention has been given to Ca2+ influx induced by emptying of intracellular pools. Thapsigargin, an inhibitor of microsomal Ca(2+)-ATPase, is an important tool in inducing store-regulated Ca2+ influx. In the present paper we show that, at concentrations above 500 nM, thapsigargin also has an opposite effect: it inhibits store-regulated Ca2+ influx into Fura-2-loaded human neutrophil granulocytes. As thapsigargin has been frequently applied at concentrations up to 2 microM, its inhibitory action on plasma-membrane Ca2+ fluxes deserves consideration.

Thapsigargin protects human erythrocyte Ca(2+)-ATPase from proteolysis

The effect of thapsigargin on the activation by partial proteolysis of the plasma membrane Ca(2+)-ATPase was studied in intact human erythrocyte membranes and in the purified enzyme. The enzyme was maximally activated in the absence of thapsigargin within 1 min of exposure to trypsin. However, in the presence of thapsigargin maximal activation was achieved only after 5 min trypsin digestion. Thapsigargin did not alter the pattern of proteolysis as revealed by SDS-PAGE of the tryptic fragments, although it slowed down the rate of appearance of the fragments. Thapsigargin also enhanced the activation of the enzyme by calmodulin. These findings suggest that, although thapsigargin at low concentrations has no effect on the catalytic activity of the Ca(2+)-ATPase in vitro in the absence of calmodulin, it could interfere with its regulation in vivo.

Delayed activation of plasma membrane Ca2+ pump in human neutrophils

The interplay between Ca2+ efflux mechanisms of the plasma membrane (PM) and transient changes of the cytosolic concentration of ionized calcium ([Ca2+]i) was studied in suspensions of human neutrophils loaded with the [Ca2+]i indicator, Fura-2. To reveal Ca2+ efflux through PM the interference of intracellular Ca stores was prevented by preincubating the cells in the presence of EGTA, thapsigargin, and ionomycin. Addition of econazole prevented varying entry of divalent cations regulated by the filling state of Ca stores. The preincubation seemed to empty and permeabilize virtually all Ca stores, ensuring that the monitored changes of [Ca2+]i were caused exclusively by PM Ca2+ transporters. Following preincubation, the addition of CaCl2 induced, mediated by ionomycin, a transient rise of [Ca2+]i, a spike, eventually decreasing to an intermediary [Ca2+]i level. The ATP-dependent decrease of [Ca2+]i terminating the spike was abolished by the calmodulin antagonist, N-(6-aminohexyl)-1-naphthalenesulfonamide (W-7), but not by the protein kinase C inhibitor, staurosporine, nor by Na(+)-free medium, suggesting that neither activity of protein kinase C nor Na+/Ca2+ exchange was necessary for generation of the Ca2+ spike. In conclusion, the PM Ca2+ pump was responsible for the Ca2+ spike by responding to the rapid rise of [Ca2+]i by a delayed activation, possibly involving calmodulin. This characteristic feature of the PM pump may be important for the generation of cellular [Ca2+]i spikes in general.

Blockade of the inward rectifier potassium current by the Ca(2+)-ATPase inhibitor 2',5'-di(tert-butyl)-1,4-benzohydroquinone (BHQ)

1. We have investigated the effect of 2',5'-di (tert-butyl)-1,4-benzohydroquinone (BHQ) and thapsigargin, inhibitors of the intracellular Ca(2+)-ATPase, on ionic currents in rat basophilic leukaemia (RBL-2H3) cells under whole cell voltage clamp. 2. The whole cell current was inwardly rectifying and reversed at -35 +/- 6 mV (n = 16). The conductance of the inward current increased as the concentration of extracellular K+ was raised from 2.7 to 5.4, 10.8 and 21.6 mM. BaCl2 (100 microM) reduced the current to a small linear component and shifted the reversal potential to -4 +/- 3 mV (n = 6). A concentration of 50 microM BaCl2 produced 45 +/- 10% (n = 4) blockade of the inward current. 3. BHQ and thapsigargin were examined for their effects on the inwardly rectifying current. A maximal blockade of inward current was obtained within 6 min after perfusion with 10 microM BHQ. The small current remaining after blockade with BHQ had a linear voltage-dependence and reversed direction at -6 +/- 9 mV (n = 6). Thapsigargin (up to 3 microM) was without effect on the inward rectifier. 4. In contrast to the blockade of the inward rectifier produced by BaCl2 which was predominantly on the steady state current, particularly at the very hyperpolarized holding potentials (-120 mV), blockade by BHQ was equally strong on the instantaneous as well as the steady state current. 5. Blockade of the inward rectifier by BHQ may cause depolarization of the cell which will affect Ca2+ influx during investigations with BHQ. Thapsigargin does not block the inward rectifier and will not inhibit Ca2+ influx in this way.

Ca2+ signaling in Sf9 insect cells and the functional expression of a rat brain M5 muscarinic receptor

The purpose of the present study was to examine Ca2+ signaling mechanisms in Sf9 cells and to demonstrate expression and functional linkage of a mammalian receptor to changes in cytosolic free Ca2+ concentration ([Ca2+]i). Addition of p-octopamine (50 microM to fura 2-loaded Sf9 cells produced a small transient increase in [Ca2+]i from a basal level of 58 +/- 10 to 194 +/- 7.6 (SD) nM. The response to octopamine was inhibited by both cyproheptadine and chlorpromazine and was mimicked by clonidine. In contrast, [Ca2+]i did not change in response to dopamine (50 microM), substance P (50 nM), histamine (50 microM), ATP (50 microM), acetylcholine (10 or 100 microM), carbachol (10 or 100 microM), serotonin (50 microM), epinephrine (10 microM), or bradykinin (50 nM). The Ca(2+)-adenosinetriphosphatase inhibitors thapsigargin (200 nM) and 2,5-di-tert-butylhydroquinone (BHQ; 10 microM) increased [Ca2+]i to 307 +/- 13 and 137 +/- 20 nM, respectively. In contrast to BHQ, the response to thapsigargin was attenuated by La3+ or removal of extracellular Ca2+ and increased by elevation of extracellular Ca2+. These results suggest that thapsigargin but not BHQ stimulates Ca2+ influx. The rat brain muscarinic receptor (subtype M5) was incorporated into the baculovirus by homologous recombination. Addition of carbachol (100 microM) increased [Ca2+]i from 92.7 +/- 6.4 to 480 +/- 26 nM in Sf9 cells infected with recombinant virus containing the M5 receptor cDNA. The effect of carbachol on [Ca2+]i was concentration dependent with a 50% effective concentration of approximately 30 microM and was blocked by atropine (10 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol 1,4,5-trisphosphate-, GTP-, arachidonic acid- and thapsigargin-mediated intracellular calcium movement in PANC-1 microsomes

Inositol 1,4,5-trisphosphate (IP3)-, GTP-, arachidonic acid- and thapsigargin-mediated Ca2+ release from endoplasmic reticulum (ER)-enriched microsomes was studied in a PANC-1 cell line. IP3 maximally caused an approximately 20% release of actively accumulated Ca2+. This effect was completely blocked by heparin. In the presence of 3% polyethylene glycol (PEG), GTP maximally discharged about 60% of Ca2+ from the microsomes. This effect involved a GTP hydrolytic process, not the IP3-activated Ca2+ channel. Arachidonic acid maximally released approximately 80% of Ca2+ from PANC-1 microsomes. Metabolites of arachidonic acid did not appear to be involved in arachidonic acid-mediated Ca2+ release. However, other fatty acids also induced similar releasing effects suggesting that arachidonic acid-induced Ca2+ release appeared to be non-specific. Thapsigargin was shown to inhibit Ca2+ accumulation into and induce Ca2+ release from PANC-1 microsomes. The thapsigargin-releasable Ca2+ pool included the IP3- or arachidonic acid-sensitive pool. Studies on liposomes suggested that both arachidonic acid and thapsigargin did not exert either a Ca2+ ionophore-like or a membrane detergent-like effect. The present results have provided evidence for the existence of multiple non-mitochondrial Ca2+ pools in PANC-1 cells. These Ca2+ pools could be released by various Ca2+ mediators via different mechanisms.

Characterization of receptor-mediated and store-regulated Ca2+ influx in human neutrophils

1. It is not known to what extent the emptying of intracellular Ca2+ stores participates in the mediation of chemoattractant-induced Ca2+ influx in human neutrophils. To study this question, we compared the properties of bivalent-cation influx in response to the chemoattractant N-formyl-L-methionyl-L-leucyl-L-phenyl-alanine (f-MLP) and to the microsomal Ca(2+)-ATPase inhibitor thapsigargin. 2. The influx pathway activated by f-MLP and thapsigargin had identical properties of permeation. Mn2+ influx became saturated at around 1 mM extracellular Mn2+, whereas Ca2+ influx did not become saturated up to concentrations of 10 mM. 3. The influx of the two bivalent cations, Mn2+ and Ca2+, was activated to a similar extent and with identical kinetics of activation. 4. The Mn2+ influx activated by f-MLP and thapsigargin was blocked, with identical dose-inhibition curves, by four imidazole analogues. 5. The same relationship between the emptying of Ca2+ stores and bivalent-cation influx was observed for f-MLP and thapsigargin, with a half-maximal activation of the influx at 40% emptying of intracellular stores. 6. In conclusion, neutrophils possess a single type of Ca(2+)-influx pathway that is activated by receptor agonists and by store depletion. Receptor agonists activate this influx pathway to a large extent, if not completely, through the depletion of intracellular Ca2+ stores.

Cytoplasmic Ca2+ determines the rate of Ca2+ entry into Mardin-Darby canine kidney-focus (MDCK-F) cells

Transformed Mardin-Darby canine kidney-focus (MDCK-F) cells exhibit spontaneous Ca2+ oscillations from an inositol 1,4,5-triphosphate-sensitive cytoplasmic Ca2+ store. In this study, Ca2+ entry from the extracellular space and its role in generation of oscillations were investigated by means of Ca2+ video imaging and the Fura-2/Mn2+ quenching technique. Oscillations were dependent on extracellular Ca2+ concentration and were inhibited by extracellularly applied La3+, Co2+ and Ni2+. Depolarization of the cell membrane with high K+ concentrations and the L-type Ca2+ channel blocker nifedipine had no effect on oscillations, indicating the lack of involvement of voltage-gated Ca2+ channels. Mn2+ quenching experiments disclosed significant Ca2+ influx into MDCK-F cells. The rate of this influx was constant between Ca2+ spikes, but markedly increased during the spontaneous Ca2+ spikes. Similar transient increases in Ca2+ entry could be mimicked by agents triggering intracellular Ca2+ release such as bradykinin and thapsigargin. We conclude that the plasma membrane of MDCK-F cells exhibits a marked voltage-independent Ca2+ permeability permitting Ca2+ entry into the cytoplasm. The rate of Ca2+ entry which determines the frequency of oscillations is most likely to be regulated by the cytoplasmic Ca2+ concentration.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Vectorial Ca2+ flux from the extracellular space to the endoplasmic reticulum via a restricted cytoplasmic compartment regulates inositol 1,4,5-trisphosphate-stimulated Ca2+ release from internal stores in vascular endothelial cells

Depletion of the Ins(1,4,5)P3-sensitive intracellular Ca2+ store of vascular endothelial cells after selective inhibition of the endoplasmic-reticulum (ER) Ca2+ pump by thapsigargin or 2,5-di-t-butylhydroquinone (BHQ) increases Ca2+ influx from the extracellular space in the absence of phosphoinositide hydrolysis. One model to account for these results suggests a close association between the internal store and the plasmalemma, allowing for the vectorial movement of Ca2+ from the extracellular space to the ER. Furthermore, recent evidence suggests that Ins(1,4,5)P3-induced Ca2+ release from intracellular stores is regulated by the free cytosolic Ca2+ concentration ([Ca2+]i). Thus agonist-induced Ca2+ entry may directly regulate Ca2+ release from internal stores. To test these hypotheses, we examined the effect of 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole (SKF 96365), an inhibitor of Ca2+ influx, on unidirectional 45Ca2+ efflux (i.e. retrograde radioisotope flux via the influx pathway) and on [Ca2+]i as measured by fura-2. Bradykinin produced a transient increase in [Ca2+]i, reflecting release of Ca2+ from internal stores, and a sustained increase indicative of Ca2+ influx. In the absence of agonist, 45Ca2+ efflux was slow and monoexponential with time. Addition of BK dramatically increased 45Ca2+ efflux; 50-60% of the 45Ca2+ associated with the cell monolayer was released within 2 min after addition of bradykinin. Both the bradykinin-induced change in [Ca2+]i and the stimulation of 45Ca2+ efflux was completely blocked by loading the cells with the Ca2+ chelator BAPTA. At a supermaximal concentration of bradykinin (50 nM), SKF 96365 (50 microM) inhibited the rise in [Ca2+]i attributed to influx without affecting release from internal stores. At a threshold concentration of bradykinin (2 nM), SKF 96365 blocked influx, but stimulated Ca2+ release from internal stores, as indicated by increases in both the transient component of the fura-2 response and 45Ca2+ efflux. Thapsigargin (200 nM) and BHQ (10 microM) produced an increase in 45Ca2+ efflux that was completely blocked by SKF 96365 or by cytosolic loading with BAPTA. These results suggest the existence of a restricted sub-plasmalemmal space that is defined by an area of surface membrane which contains the Ca(2+)-influx pathway but is devoid of Ca2+ pumps, and by a section of ER that is rich in thapsigargin-sensitive Ca(2+)-pump units.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for agonist-induced export of intracellular Ca2+ in epithelial cells

There is increasing evidence that some agonists not only induce intracellular Ca2+ increases, due to store release and transmembranous influx, but also that they stimulate Ca2+ efflux. We have investigated the agonist-stimulated response on the intracellular Ca2+ activity ([Ca2+]i) in the presence of thapsigargin (10(-8) mol/l, TG) in HT29 and CFPAC-1 cells. For CFPAC-1 the agonists ATP (10(-7)-10(-3) mol/l, n = 9), carbachol (10(-6)-10(-3) mol/l, n = 5) and neurotensin (10(-10)-10(-7) mol/l, n = 6) all induced a concentration-dependent decrease in [Ca2+]i in the presence of TG. Similar results were obtained with HT29 cells. This decrease of [Ca2+]i could be caused by a reduced Ca2+ influx, either due to a reduced driving force for Ca2+ in the presence of depolarizing agonists or due to agonist-regulated decrease in Ca2+ permeability. Using the fura-2 Mn2+ quenching technique we demonstrated that ATP did not slow the TG-induced Mn2+ quench. This indicates that the agonist-induced [Ca2+]i decrease in the presence of TG was not due to a reduced influx of Ca2+ into the cell, but rather due to stimulation of Ca2+ export. We used the cell attached nystatin patch clamp technique in CFPAC-1 cells to examine whether, in the presence of TG, the above agonists still led to the previously described electrical changes. The cells had a mean membrane voltage of -49 +/- 3.6 mV (n = 9). Within the first 3 min ATP was still able to induce a depolarization which could be attributed to an increase in Cl- conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation by calcium alone of chloride secretion in T84 epithelial cells

1. The goal of this study was to determine if an increase in cytoplasmic calcium concentration ([Ca2+]i), in the absence of additional second messengers derived from membrane phospholipid turnover, is a sufficient signal to induce chloride secretion across monolayers of the human colonic epithelial line, T84. 2. Thapsigargin was used to increase [Ca2+]i by inhibiting the endomembrane Ca(2+)-ATPase. [Ca2+]i was monitored in monolayers by fura-2 fluorescence spectroscopy, chloride secretion by measuring changes in short circuit current (Isc) in modified Ussing chambers, and inositol phosphates were measured by radio-h.p.l.c. of extracts of cells prelabelled with [3H]-inositol. 3. Thapsigargin increased [Ca2+]i and Isc in parallel, without increasing any inositol phosphates. The effect of thapsigargin on Isc was abolished by the intracellular calcium chelator, bis-(o-aminophenoxy)-ethane-N,N,N',N"-tetraacetic acid (BAPTA). 4. Increasing [Ca2+]i with thapsigargin did not prevent a subsequent calcium response to carbachol or histamine if extracellular calcium was available. In the absence of extracellular calcium, only one such release of calcium to hormonal stimulation occurred when cells were pretreated with thapsigargin, and a second response to either carbachol histamine was essentially abolished. 5. Addition of carbachol or histamine to thapsigargin-treated cells mounted in Ussing chambers caused a transient further increase in Isc followed by termination of the response, even though [Ca2+]i continued to rise. 6. We conclude that an elevation in [Ca2+]i is a sufficient signal to induce chloride secretion in T84 cells. Rather than being required to stimulate secretory responses, additional second messengers induced by hormonal secretagogues (such as inositol phosphates) may in fact serve to limit the secretory response.

Sphingosine mobilizes intracellular calcium in human neutrophils

The effect of sphingosine on the cytosolic free Ca2+ concentrations, [Ca2+]i, of human neutrophils was re-examined using Fura-2 loaded cells. We found that sphingosine induced a dose-dependent elevation of [Ca2+]i. At sphingosine concentrations > or = 10 microM, the rise in [Ca2+]i was biphasic; an initial phase increasing basal [Ca2+]i by 100% was succeeded by a second phase which raised [Ca2+]i to several microM. The enhanced signal was sustained and slowly approached the Fmax of Fura-2 over 10 min. Although cytotoxicity assays indicate that Fura-2 leakage contributed to the rise in fluorescence, EGTA, surprisingly, had no effect on the time course of this response. The explanation was that EGTA blocked Fura-2 leakage from and trypan blue uptake by neutrophils. Thus, in the presence of EGTA, biphasic increases in the fluorescent signal can be attributed mainly to release of intracellular Ca2+. Mn2+ quenching studies confirmed that sphingosine mobilized Ca2+ in two distinct phases and promoted the influx of Mn2+. Mn2+ entry, however, was not matched by substantial Ca2+ influx. Sphingosine elevation of [Ca2+]i was insensitive to pertussis toxin treatment of neutrophils and was not correlated with (1,4,5)IP3 formation. Studies with semi-permeabilized cells show that sphingosine, up to 80 microM, neither mobilized Ca2+ significantly nor inhibited active Ca2+ sequestration. Sphingosylphosphorylcholine induced a small but dose-dependent release of Ca2+. We hypothesize that a metabolite of sphingosine may release Ca2+ directly in intact neutrophils.

A patch-clamp study of the Ca2+ mobilization from internal stores in bovine aortic endothelial cells. II. Effects of thapsigargin on the cellular Ca2+ homeostasis

Evidence was provided, in the preceding paper (Thuringer & Sauvé, 1992), that the external Ca(2+)-dependent phase of the Ca2+ signals evoked by bradykinin (BK) or caffeine in bovine aortic endothelial cells (BAE), differ in their respective sensitivity to procaine. To examine whether the emptying of the InsP3-sensitive Ca2+ store is the signal for activating the agonist-evoked Ca2+ entry, we have investigated the effects of thapsigargin (TSG), a known inhibitor of the microsomal Ca(2+)-ATPase activity in a variety of cell types, via the activity of calcium-activated potassium channels [K(Ca2+) channels]. In cell-attached experiments, the external application of TSG caused a sustained or oscillatory activation of K(Ca2+) channels depending on both the cells and doses tested. The TSG-evoked channel activity could be reversibly blocked by removing extracellular Ca2+, and strongly decreased by adding 10 mM procaine to the bath medium. In Ca(2+)-free external conditions, TSG did not promote an apparent Ca2+ discharge from internal stores but prevented in a dose- and time-dependent manner the subsequent agonist-evoked channel activity related to the release of internally sequestered Ca2+. These results confirm that TSG and BK release Ca2+ from the same internal stores but with different kinetics. Because the channel response to caffeine was found to be poorly sensitive to procaine, in contrast to that evoked by BK and TSG, it may be concluded that both BK and TSG activate the same Ca2+ entry pathway. Therefore, the emptying of the InsP3-sensitive Ca2+ store is likely to be the main signal for activating the agonist-evoked Ca2+ entry in BAE cells.