Mobilization of Ca2+ by thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone in permeabilized insulin-secreting RINm5F cells: evidence for separate uptake and release compartments in inositol 1,4,5-trisphosphate-sensitive Ca2+ pool

Calcium signalling in platelets and other cells

Reviewed are new concepts and models of Ca(2+) signalling originating from work with various animal cells, as well as the applicability of these models to the signalling systems used by blood platelets. The following processes and mechanisms are discussed: Ca(2+) oscillations and waves; Ca(2+) -induced Ca(2+) release; involvement of InsP(3)-receptors and quanta1 release of Ca(2+); different pathways of phospholipase C activation; heterogeneity in the intracellular Ca(2+) stores; store-and receptor-regulated Ca(2+) entry. Additionally, some typical aspects of Ca(2+) signalling in platelets are reviewed: involvement of protein serine/threonine and tyrosine kinases in the regulation of signal transduction; possible functions of platelet glycoproteins; and the importance of Ca(2+) for the exocytotic and procoagulant responses.

Natural forms of vitamin E and 13'-carboxychromanol, a long-chain vitamin E metabolite, inhibit leukotriene generation from stimulated neutrophils by blocking calcium influx and suppressing 5-lipoxygenase activity, respectively

Leukotrienes generated by 5-lipoxygenase (5-LOX)-catalyzed reaction are key regulators of inflammation. In ionophore-stimulated (A23187; 1-2.5 μM) human blood neutrophils or differentiated HL-60 cells, vitamin E forms differentially inhibited leukotriene B(4) (LTB(4)) with an IC(50) of 5-20 μM for γ-tocopherol, δ-tocopherol (δT), and γ-tocotrienol, but a much higher IC(50) for α-tocopherol. 13'-Carboxychromanol, a long-chain metabolite of δT, suppressed neutrophil- and HL-60 cell-generated LTB(4) with an IC(50) of 4-7 μM and potently inhibited human recombinant 5-LOX activity with an IC(50) of 0.5-1 μM. In contrast, vitamin E forms had no effect on human 5-LOX activity but impaired ionophore-induced intracellular calcium increase and calcium influx as well as the subsequent signaling including ERK1/2 phosphorylation and 5-LOX translocation from cytosol to the nucleus, a key event for 5-LOX activation. Further investigation showed that δT suppressed cytosolic Ca(2+) increase and/or LTB(4) formation triggered by ionophores, sphingosine 1-phosphate, and lysophosphatidic acid but not by fMLP or thapsigargin, whereas 13'-carboxychromanol decreased cellular production of LTB(4) regardless of different stimuli, consistent with its strong inhibition of the 5-LOX activity. These observations suggest that δT does not likely affect fMLP receptor-mediated signaling or store depletion-induced calcium entry. Instead, we found that δT prevented ionophore-caused cytoplasmic membrane disruption, which may account for its blocking of calcium influx. These activities by vitamin E forms and long-chain carboxychromanol provide potential molecular bases for the differential anti-inflammatory effects of vitamin E forms in vivo.

Regulation of two insulin granule populations within the reserve pool by distinct calcium sources

Insulin granule trafficking is a key step of glucose-stimulated insulin secretion from pancreatic beta cells. Using quantitative live cell imaging, we examined insulin granule movements within the reserve pool upon secretory stimulation in betaTC3 cells. For this study, we developed a custom image analysis program that permitted automatic tracking of the individual motions of over 20,000 granules. This analysis of a large sample size enabled us to study micro-populations of granules that were not quantifiable in previous studies. While over 90% of the granules depend on Ca2+ efflux from the endoplasmic reticulum for their mobilization, a small and fast-moving population of granules responds to extracellular Ca2+ influx after depolarization of the plasma membrane. We show that this differential regulation of the two granule populations is consistent with localized Ca2+ signals, and that the cytoskeletal network is involved in both types of granule movement. The fast-moving granules are correlated temporally and spatially to the replacement of the secreted insulin granules, which supports the hypothesis that these granules are responsible for replenishing the readily releasable pool. Our study provides a model by which glucose and other secretory stimuli can regulate the readily releasable pool through the same mechanisms that regulate insulin secretion.

Sarco-endoplasmic reticulum Ca2+ ATPase (SERCA) inhibitors identify a novel calcium pool in the central nervous system

Ca2+ uptake into the endoplasmic reticulum (ER) is mediated by Ca2+ ATPase isoforms, which are all selectively inhibited by nanomolar concentrations of thapsigargin. Using ATP/Mg2+-dependent 45Ca2+ transport in rat brain microsomes, tissue sections, and permeabilized cells, as well as Ca2+ imaging in living cells we distinguish two ER Ca2+ pools in the rat CNS. Nanomolar levels of thapsigargin blocked one component of brain microsomal 45Ca2+ transport, which we designate as the thapsigargin-sensitive pool (TG-S). The remaining component was only inhibited by micromolar thapsigargin, and thus designated as thapsigargin resistant (TG-R). Ca2+ ATPase and [32P]phosphoenzyme assays also distinguished activities with differential sensitivities to thapsigargin. The TG-R Ca2+ uptake displayed unique anion permeabilities, was inhibited by vanadate, but was unaffected by sulfhydryl reduction. Ca2+ sequestered into the TG-R pool could not be released by inositol-1,4,5-trisphosphate, caffeine, or cyclic ADP-ribose. The TG-R Ca2+ pool had a unique anatomical distribution in the brain, with selective enrichment in brainstem and spinal cord structures. Cell lines that expressed high levels of the TG-R pool required micromolar concentrations of thapsigargin to effectively raise cytoplasmic Ca2+ levels. TG-R Ca2+ accumulation represents a distinct Ca2+ buffering pool in specific CNS regions with unique pharmacological sensitivities and anatomical distributions.

Ryanodine receptors of pancreatic beta-cells mediate a distinct context-dependent signal for insulin secretion

The ryanodine (RY) receptors in beta-cells amplify signals by Ca2+-induced Ca2+ release (CICR). The role of CICR in insulin secretion remains unclear in spite of the fact that caffeine is known to stimulate secretion. This effect of caffeine is attributed solely to the inhibition of cAMP-phosphodiesterases (cAMP-PDEs). We demonstrate that stimulation of insulin secretion by caffeine is due to a sensitization of the RY receptors. The dose-response relationship of caffeine-induced inhibition of cAMP-PDEs was not correlated with the stimulation of insulin secretion. Sensitization of the RY receptors stimulated insulin secretion in a context-dependent manner, that is, only in the presence of a high concentration of glucose. This effect of caffeine depended on an increase in [Ca2+]i. Confocal images of beta-cells demonstrated an increase in [Ca2+]i induced by caffeine but not by forskolin. 9-Methyl-7-bromoeudistomin D (MBED), which sensitizes RY receptors, did not inhibit cAMP-PDEs, but it stimulated secretion in a glucose-dependent manner. The stimulation of secretion by caffeine and MBED involved both the first and the second phases of secretion. We conclude that the RY receptors of beta-cells mediate a distinct glucose-dependent signal for insulin secretion and may be a target for developing drugs that will stimulate insulin secretion only in a glucose-dependent manner.

Mechanisms and physiological significance of the cholinergic control of pancreatic beta-cell function

Acetylcholine (ACh), the major parasympathetic neurotransmitter, is released by intrapancreatic nerve endings during the preabsorptive and absorptive phases of feeding. In beta-cells, ACh binds to muscarinic M(3) receptors and exerts complex effects, which culminate in an increase of glucose (nutrient)-induced insulin secretion. Activation of PLC generates diacylglycerol. Activation of PLA(2) produces arachidonic acid and lysophosphatidylcholine. These phospholipid-derived messengers, particularly diacylglycerol, activate PKC, thereby increasing the efficiency of free cytosolic Ca(2+) concentration ([Ca(2+)](c)) on exocytosis of insulin granules. IP3, also produced by PLC, causes a rapid elevation of [Ca(2+)](c) by mobilizing Ca(2+) from the endoplasmic reticulum; the resulting fall in Ca(2+) in the organelle produces a small capacitative Ca(2+) entry. ACh also depolarizes the plasma membrane of beta-cells by a Na(+)- dependent mechanism. When the plasma membrane is already depolarized by secretagogues such as glucose, this additional depolarization induces a sustained increase in [Ca(2+)](c). Surprisingly, ACh can also inhibit voltage-dependent Ca(2+) channels and stimulate Ca(2+) efflux when [Ca(2+)](c) is elevated. However, under physiological conditions, the net effect of ACh on [Ca(2+)](c) is always positive. The insulinotropic effect of ACh results from two mechanisms: one involves a rise in [Ca(2+)](c) and the other involves a marked, PKC-mediated increase in the efficiency of Ca(2+) on exocytosis. The paper also discusses the mechanisms explaining the glucose dependence of the effects of ACh on insulin release.

Ca2+-induced Ca2+ release from the endoplasmic reticulum amplifies the Ca2+ signal mediated by activation of voltage-gated L-type Ca2+ channels in pancreatic beta-cells

Stimulus-secretion coupling in pancreatic beta-cells involves membrane depolarization and Ca(2+) entry through voltage-gated L-type Ca(2+) channels, which is one determinant of increases in the cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)). We investigated how the endoplasmic reticulum (ER)-associated Ca(2+) apparatus further modifies this Ca(2+) signal. When fura-2-loaded mouse beta-cells were depolarized by KCl in the presence of 3 mm glucose, [Ca(2+)](i) increased to a peak in two phases. The second phase of the [Ca(2+)](i) increase was abolished when ER Ca(2+) stores were depleted by thapsigargin. The steady-state [Ca(2+)](i) measured at 300 s of depolarization was higher in control cells compared with cells in which the ER Ca(2+) pools were depleted. The amount of Ca(2+) presented to the cytoplasm during depolarization as estimated from the integral of the increment in [Ca(2+)](i) over time (integralDelta[Ca(2+)](i).dt) was approximately 30% higher compared with that in the Ca(2+) pool-depleted cells. neo-thapsigargin, an inactive analog, did not affect [Ca(2+)](i) response. Using Sr(2+) in the extracellular medium and exploiting the differences in the fluorescence properties of Ca(2+)- and Sr(2+)-bound fluo-3, we found that the incoming Sr(2+) triggered Ca(2+) release from the ER. Depolarization-induced [Ca(2+)](i) response was not altered by, an inhibitor of phosphatidylinositol-specific phospholipase C, suggesting that stimulation of the enzyme by Ca(2+) is not essential for amplification of Ca(2+) signaling. [Ca(2+)](i) response was enhanced when cells were depolarized in the presence of 3 mm glucose, forskolin, and caffeine, suggesting involvement of ryanodine receptors in the amplification process. Pretreatment with ryanodine (100 microm) diminished the second phase of the depolarization-induced increase in [Ca(2+)](i). We conclude that Ca(2+) entry through L-type voltage-gated Ca(2+) channels triggers Ca(2+) release from the ER and that such a process amplifies depolarization-induced Ca(2+) signaling in beta-cells.

Glucose regulation of free Ca(2+) in the endoplasmic reticulum of mouse pancreatic beta cells

Free Ca(2+) was measured in organelles of individual mouse pancreatic beta cells loaded with the low affinity indicator furaptra. After removal of cytoplasmic indicator by controlled digitonin permeabilization the organelle Ca(2+) was located essentially in the endoplasmic reticulum (ER), >90% being sensitive to inhibition of sarco(endo)plasmic reticulum Ca(2+)-ATPases. The Ca(2+) accumulation in the ER of intact beta cells depended in a hyperbolic fashion on the glucose concentration with half-maximal and maximal filling at 5.5 and >20 mM, respectively. Also elevation of cytoplasmic Ca(2+) by K(+) depolarization significantly enhanced the Ca(2+) accumulation. In permeabilized beta cells 1-3 mM ATP caused rapid Ca(2+) filling of the ER reaching almost 500 microM. At 50 nM, Ca(2+) ER became half-maximally filled at 45 microM ATP, whereas only 3.5 microM ATP was required at 200 nM Ca(2+). Inositol 1,4,5-trisphosphate induced a rapid release of about 65% of the ER Ca(2+), and its precursor phosphatidylinositol 4,5-bisphosphate was found to slowly mobilize 75% by another mechanism. It is concluded that glucose is an efficient stimulator of Ca(2+) uptake in the ER of pancreatic beta cells both by increasing ATP and cytoplasmic Ca(2+). Because physiological concentrations of cytoplasmic ATP are in the mM range, Ca(2+) sequestration can be anticipated to be modulated by factors reducing its ATP sensitivity.

Influence of calcium on proliferation and phenotype alteration of cardiomyocyte in vitro

An accelerated weight gain is noted in the heart of Ca-deficient, hypertensive chick embryos maintained in a shell-less culture in vitro. We previously observed that the Ca handling property of cardiomyocytes isolated from the shell-less embryo is altered, i.e., faster Ca uptake, suggesting a requirement for adequate Ca supply and/or proper Ca handling in embryonic cardiac development. In this study, we have examined the function of Ca on cardiomyocytes by analyzing the effects of 1) various Ca concentration in the culture medium (NCa, 1.8 mmol/ L; HCa, 2.8 mmol/L; LCa, 0.9 mmol/L), and 2) various modulators of Ca handling on cell proliferation and phenotype regulation in chick embryonic cardiomyocytes. The analytical parameters included cell number, DNA content, expression of cell cycle-specific and cardiomyocyte-specific proteins, and creatine phosphokinase (CPK) and lactate dehydrogenase (LDH) enzyme activities. Cell number and total DNA were significantly larger (P < 0.01) in LCa cultures compared with those in NCa. The level of LDH was elevated (P < 0.01), but that of CPK was lowered in LCa. Expression of the G1-S-specific protein PCNA was raised, but that of the contractile proteins myosin and tropomyosin was substantially suppressed in LCa; in HCa, the cells did not proliferate as well, whereas the level of contractile proteins was higher. Thapsigargin, a sarcoplasmic reticulum (SR)-specific, Ca-ATPase inhibitor, simulated the effects of LCa by enhancing cell proliferation and lowering the expression of tropomyosin. These results suggest that culturing in low Ca concentration and inhibition of SR Ca pumping enhance myocardial cell proliferation and suppress sarcomeric protein expression, perhaps by inducing cellular de-differentiation. The in vitro effects of medium Ca concentration and Ca handling modulators on cardiomyocytes also suggest that the in vivo cardiomegaly of the SL embryos is a direct result of Ca-deficiency, and that Ca is important in the phenotype regulation of cardiomyocytes.

Control of the calcium concentration involved in acetylcholine release and its facilitation: an additional role for synaptic vesicles?

2,5-Diterbutyl-1,4-benzohydroquinone, a specific blocker of Ca2+-ATPase pumps, increased acetylcholine release from an identified synapse of Aplysia, as well as from Torpedo and mouse caudate nucleus synaptosomes. Because 2,5-diterbutyl-1,4-benzohydroquinone does not change the presynaptic Ca2+ influx, the enhancement of acetylcholine release could be due to an accumulation of Ca2+ in the terminal. This possibility was further checked by studying the effects of 2,5-diterbutyl-1,4-benzohydroquinone on twin pulse facilitation, classically attributed to residual Ca2+. While preventing the fast sequestration of Ca2+ by presynaptic organelles, 2,5-diterbutyl-1,4-benzohydroquinone magnified both twin pulse facilitation observed under low extracellular Ca2+ concentration and twin pulse dysfacilitation observed under high extracellular Ca2+ concentration. Thus, it is concluded that 2,5-diterbutyl-1,4-benzohydroquinone, by preventing Ca2+ buffering near transmitter release sites, modulates acetylcholine release. As 2,5-diterbutyl-1,4-benzohydroquinone was also shown to decrease by 50% the uptake of 45Ca2+ by isolated synaptic vesicles, we propose that synaptic vesicles can control the presynaptic Ca2+ concentration triggering the release of neurotransmitter.

A tool coming of age: thapsigargin as an inhibitor of sarco-endoplasmic reticulum Ca(2+)-ATPases

Thapsigargin is the most widely used inhibitor of the ubiquitous sarco-endoplasmic reticulum Ca(2+)-ATPases in mammalian cells. Over the past ten years, this guaianolide compound of plant origin has become a popular tool in a host of studies directed at elucidating the mechanisms of intracellular Ca2+ signalling. Its remarkable potency and selectivity have been instrumental in widening our view of the function of intracellular Ca2+ stores to include such key aspects as store-operated Ca2+ entry or the involvement of the stores in protein synthesis or cell growth. In this article Marek Treiman, Casper Caspersen and Søren Brøgger Christensen review the key pharmacological features of thapsigargin action; they also discuss some of the ways in which its unique properties have shown to be important for obtaining new insights into the biology of Ca2+ stores, and how these properties might encompass a therapeutic potential. In parallel, attention is drawn to some of the limitations and pitfalls encountered when working with thapsigargin.

Mechanisms regulating prostaglandin F2 alpha secretion from the bovine endometrium

The mechanism that regulates luteolytic PGF2 alpha secretion as stimulated by oxytocin is thought to involve induction of the inositol (1,4,5)-trisphosphate-diacylglycerol second messenger system, which mobilizes intracellular calcium and activates protein kinase C. In Experiment 1, endometrial explants taken from heifers on d 18.5 to 19.5 postestrus had increased PGF2 alpha secretion after treatment with 1 microM calcium ionophore A23187 to increase intracellular calcium, 100 nM phorbol 12-myristate 13-acetate to activate protein kinase C, and 100 nM oxytocin. The stimulatory effects of oxytocin and calcium ionophore A23187 plus phorbol 12-myristate 13-acetate did not differ from each other. In Experiment 2, endometrial explants taken from cows on d 18.5 to 19.5 postestrus had increased PGF2 alpha secretion after treatment with 0.2 and 2 microM thapsigargin to mobilize intracellular calcium that was sensitive to inositol (1,4,5)-trisphosphate. Secretion of PGF2 alpha was also increased by 100 nM oxytocin and was influenced by the interaction of thapsigargin and oxytocin such that 100 nM oxytocin did not further increase the secretion of PGF2 alpha in the presence of 2 microM thapsigargin. In Experiment 3, 100 nM oxytocin stimulated greater production of inositol trisphosphate and total inositol phosphates in the endometrium of cyclic cows than in the endometrium of pregnant cows on d 16.5 to 17.0 postestrus, although luteolysis was not yet initiated in the cyclic cows. These results are consistent with the hypothesis that the activation of the inositol (1,4,5)-trisphosphate-diacylglycerol second messenger system by oxytocin is involved in the stimulation of PGF2 alpha secretion from the endometrium during late diestrus in cows.

Endoplasmic reticulum Ca2+ is important for the proteolytic processing and intracellular transport of proinsulin in the pancreatic beta-cell

The role of intracellular Ca2+ in the proteolytic processing and intracellular transport of secretory granule proproteins was investigated by pulse-chase radiolabelling of isolated rat islets of Langerhans. The conversion of proinsulin was inhibited by depletion of medium Ca2+ with EGTA and by blocking the transport of Ca2+ into cells with the Ca2+-channel antagonists verapamil, nifedipine and NiCl2. Proinsulin conversion was also reduced by the endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin, indicating that the process requires transport of Ca2+ into the endoplasmic reticulum. This was supported by the finding that proinsulin processing was inhibited when Ca2+ was depleted before or during pulse-labelling, but not after transport of the protein to post-endoplasmic-reticulum compartments. Similarly, the inhibition of proinsulin processing was reversed by re-introduction of medium Ca2+ around the time of radiolabelling, but not after 15 min of chase incubation. Ca2+ depletion also decreased proteolytic maturation of the prohormone convertases PC1, PC2 and carboxypeptidase H. Secretion experiments suggested that the rate and extent of proinsulin transport into secretory granules were inhibited marginally by Ca2+ depletion, whereas those of the convertases were markedly impeded. Inhibition of proinsulin conversion by Ca2+ depletion was thus not simply related to the Ca2+-dependencies of mature PC1 and PC2, but also to a requirement for endoplasmic reticulum Ca2+ in proteolytic maturation of the convertases and in their transfer to secretory granules. The results also suggest that the Ca2+ required for prohormone processing in the granules enters the secretory pathway via the endoplasmic reticulum.

Thiol oxidation by 2,2'-dithiodipyridine causes a reversible increase in cytoplasmic free Ca2+ concentration in pancreatic beta-cells. Role for inositol 1,4,5-trisphosphate-sensitive Ca2+ stores

2,2'-Dithiodipyridine (2,2'-DTDP), a reactive disulphide that mobilizes Ca2+ from ryanodine-sensitive Ca2+ stores in muscle, induced a biphasic increase in cytoplasmic free Ca2+ concentration ([Ca2+]i) in pancreatic beta-cells loaded with fura 2. This increase consisted of an early transient followed by a second, slower, rise. The [Ca2+]i transient was dependent on extracellular Ca2+ and disappeared on treatment with nimodipine. The reactive disulphide caused plasma membrane depolarization, as studied by the perforated-patch configuration of the patch-clamp technique. Hence membrane depolarization and opening of the L-type voltage-gated Ca2+ channels were responsible for the first transient in [Ca2+]i. The second slower increase in [Ca2+]i was prolonged but readily reversed by the disulphide-reducing agent 1,4-dithiothreitol. This increase in [Ca2+]i was not decreased by nimodipine or by omission of extracellular Ca2+, but was eliminated when the Ins(1,4,5)P3-sensitive Ca2+ pool was first depleted by carbachol. Ryanodine or its beta-alanyl analogue did not release Ca2+ from intracellular stores, and a high concentration of ryanodine did not inhibit Ca2+ release by 2,2'-DTDP. The disulphide compound suppressed glucose metabolism and decreased the mitochondrial inner-membrane potential. We conclude that thiol oxidation by 2,2'-DTDP affects Ca2+ homeostasis in beta-cells by multiple mechanisms. However, unlike the situation in muscle, in beta-cells 2,2'-DTDP releases Ca2+ from intracellular pools by mechanisms that do not involve activation of ryanodine receptors. Instead, in these cells the Ins(1,4,5)P3-sensitive intracellular Ca2+ store comprises an alternative target for the Ca(2+)-mobilizing action of the reactive disulphide compound.

Organization of Ca2+ stores in myeloid cells: association of SERCA2b and the type-1 inositol-1,4,5-trisphosphate receptor

In this study, we have analysed the relationship between Ca2+ pumps and Ins(1,4,5)P3-sensitive Ca2+ channels in myeloid cells. To study whether sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA)-type Ca(2+)-ATPases are responsible for Ca2+ uptake into Ins(1,4,5)P3-sensitive Ca2+ stores, we used the three structurally unrelated inhibitors thapsigargin, 2,5-di-t-butylhydroquinone and cyclopiazonic acid. In HL-60 cells, all three compounds precluded formation of the phosphorylated intermediate of SERCA-type Ca(2+)-ATPases. They also decreased, in parallel, ATP-dependent Ca2+ accumulation and the amount of Ins(1,4,5)P3-releasable Ca2+. Immunoblotting with subtype-directed antibodies demonstrated that HL-60 cells contain the Ca2+ pump SERCA2 (subtype b), and the Ca(2+)-release-channel type-1 Ins(1,4,5)P3 receptor. In subcellular fractionation studies, SERCA2 and type-1 Ins(1,4,5)P3 receptor co-purified. Immunofluorescence studies demonstrated that both type-1 Ins(1,4,5)P3 receptor and SERCA2 were evenly distributed throughout the cell in moving neutrophils. During phagocytosis both proteins translocated to the periphagosomal space. Taken together, our results suggest that in myeloid cells (i) SERCA-type Ca(2+)-ATPases function as Ca2+ pumps of Ins(1,4,5)P3-sensitive Ca2+ stores, and (ii) SERCA2 and type-1 Ins(1,4,5)P3 receptor reside either in the same or two tightly associated subcellular compartments.

Evidence for vacuolar-type proton pumps in nonmitochondrial and inositol 1,4,5-trisphosphate-sensitive calcium stores of insulin-secreting cells

This study examines whether acidic, vacuolar-type, proton-pump-carrying organelles of insulin-secreting cells (clonal endocrine pancreatic cell line INS-1) function as rapidly exchanging, inositol 1,4,5-trisphosphate-sensitive calcium stores. Calcium uptake into calcium stores will be modulated by the proton concentration within the stores, since calcium pumps in general appear to mediate a countertransport of calcium with protons. We therefore tested for sensitivity of calcium sequestration by nonmitochondrial stores (inhibition of mitochondrial calcium uptake by 2 microM ruthenium red) in saponin-permeabilized cells to proton-conducting ionophores and proton pump inhibition, using this as a marker for involvement of acidic organelles. Calcium sequestration was partially inhibited by the protonophores nigericin (10-50 microM) and carbonylcyanide m-chlorophenylhydrazone (CCCP; 20-50 microM), as well as by inclusion of 30 mM NH4Cl. Bafilomycin A1, a potent and selective inhibitor of vacuolar-type proton pumps, alone (1 - 500 nM) had no effect on calcium sequestration. however, it induced an inhibitory effect in the presence of nigericin or CCCP, even at low concentrations (5 microM) of these ionophores, lacking itself an inhibitory action on calcium sequestration. Bafilomycin A1 then was already maximally active at a concentration as low as 10 nM. Corres ponding to inhibition of total nonmitochondrial calcium sequestration, filling of inositol 1,4,5-trisphosphate-sensitive stores was decreased or even abolished by the protonophores alone or the protonophores combined with bafilomycin A1. We conclude that vacuolar-type proton pumps are present in at least a part of nonmitochondrial and inositol 1,4,5-trisphosphate-sensitive calcium stores in INS-1 cells. This assigns these stores to organelles such as secretory granules, the trans Golgi network, or endosomes. Luminal acidity of these stores will stimulate calcium sequestration by providing more protons for countertransport of calcium by calcium pumps. High concentrations of protonophores may be required for inhibitory effects because otherwise the proton pumps may be able to compensate sufficiently for ionophore-mediated proton loss. The lack of effect of bafilomycin A1 without protonophores may be due to a sufficient luminal buffering capacity or to preceding inhibition of the pump by an inside-positive transmembrane potential.

The effect of 2,5-di-(tert-butyl)-1,4-hydroquinone on force responses and the contractile apparatus in mechanically skinned muscle fibres of the rat and toad

In this study, we investigated the effect of the Ca2+ pump inhibitor, 2,5-di-(tert-butyl)-1,4-hydroquinone on the function of the contractile apparatus, Ca2+ uptake, the permeability of the sarcoplasmic reticulum to Ca2+ and excitation-contraction coupling, in mechanically skinned muscle fibres of the rat and toad. 2,5-di-(tert-butyl)-1,4-hydroquinone had no significant effect on the maximum force and Ca2+ sensitivity of the contractile apparatus in rat and toad fibres at concentrations of 20 and 5 microM respectively. In rat fibres, 2,5-di-(tert-butyl)-1,4-hydroquinone was found to inhibit sarcoplasmic reticulum Ca2+ loading in a dose dependent manner, with a half maximal effect at 2 microM. In toad fibres, 5 microM 2,5-di-(tert-butyl)-1,4-hydroquinone completely blocked sarcoplasmic reticulum Ca2+ loading. Exposure to 5 mM BAPTA revealed a small resting sarcoplasmic reticulum Ca2+ leak in unstimulated rat fibres. This Ca2+ leak was not significantly affected by the presence of 20 microM 2,5-di-(tert-butyl)-1,4-hydroquinone, suggesting that 2,5-di-(tert-butyl)-1,4-hydroquinone does not substantially block or activate the sarcoplasmic reticulum Ca2+ release channels. Depolarisation-induced force responses elicited in rat and toad skinned fibres were not significantly affected by 0.5 microM 2,5-di-(tert-butyl)-1, 4-hydroquinone. In the rat fibres, 5 and 20 microM 2,5-di-(tert-butyl)-1,4-hydroquinone greatly increased the peak and duration of initial depolarisation-induced force responses, while subsequent responses were reduced. 2,5-di-(tert-butyl)-1,4-hydroquinone did not affect excitation contraction coupling, as depolarisation-induced force responses similar to initial controls could be elicited after 2,5-di-(tert-butyl)-1,4-hydroquinone exposure, provided that the initial Ca2+ release in 2,5-di-(tert-butyl)-1,4-hydroquinone was chelated with 0.5 mM EGTA (to prevent Ca(2+)-dependent damage) and the sarcoplasmic reticulum was reloaded with Ca2+. In the toad fibres, 5 microM 2,5-di-(tert-butyl)-1, 4-hydroquinone had a similar effect on depolarisation-induced force responses to that observed at 20 microM 2,5-di-(tert-butyl)-1, 4-hydroquinone in rat fibres. This study shows that 2,5-di-(tert-butyl)-1,4-hydroquinone specifically and reversibly inhibits the sarcoplasmic reticulum Ca2+ pump in skeletal muscle and therefore, 2,5-di-(tert-butyl)-1,4-hydroquinone could be a valuable tool for investigating the role of the sarcoplasmic reticulum in Ca2+ homeostasis in skeletal muscle.

Characterisation of endoplasmic reticulum and plasma membrane Ca(2+)-ATPases in pancreatic beta-cells and in islets of Langerhans

We have investigated the plasma membrane (PMCA) and endoplasmic reticulum (SERCA) Ca(2+)-ATPases involved in active transport of Ca2+ in pancreatic beta-cell lines (MIN6, HIT T15, RINm5F) and in islets of Langerhans. Under selective membrane phosphorylation conditions (at low ATP concentration, in the presence of Ca2+ and La3+ and in the absence of Mg2+ at 4 degrees C) the only labelled proteins are the phosphoenzyme intermediates of the Ca(2+)-ATPases. Under these conditions, beta-cell membranes incorporated 32P from [gamma-32P]ATP into two proteins with molecular mass on acidic SDS-polyacrylamide gels of around 115 and 150 kDa. The 150 kDa band was identified as PMCA (i) by reaction with a monoclonal anti-human erythrocyte plasma membrane Ca(2+)-ATPase antibody; (ii) by its typical tryptic cleavage pattern which generated an 80 kDa band; (iii) by lack of inhibition of its autophosphorylation by SERCA-specific inhibitors. The 115 kDa band was identified as SERCA (i) by reaction with a polyclonal anti-rat fast skeletal muscle Ca(2+)-ATPase antibody; (ii) by the concentration-dependent inhibition of its autophosphorylation by thapsigargin and 2,5-di(t-butyl)-1,4-benzohydroquinone (tBHQ), which are specific inhibitors of SERCA. The 115 kDa band was further characterised as the SERCA-2b isoform by reaction with a polyclonal rabbit antibody against the 12 C-terminal amino acids of SERCA-2b.

Effects of caffeine on cytoplasmic free Ca2+ concentration in pancreatic beta-cells are mediated by interaction with ATP-sensitive K+ channels and L-type voltage-gated Ca2+ channels but not the ryanodine receptor

In the pancreatic beta-cell, an increase in the cytoplasmic free Ca2+ concentration ([Ca2+]i) by caffeine is believed to indicate mobilization of Ca2+ from intracellular stores, through activation of a ryanodine receptor-like channel. It is not known whether other mechanisms, as well, underlie caffeine-induced changes in [Ca2+]i. We studied the effects of caffeine on [Ca2+]i by using dual-wavelength excitation microfluorimetry in fura-2-loaded beta-cells. In the presence of a non-stimulatory concentration of glucose, caffeine (10-50 mM) consistently increased [Ca2+]i. The effect was completely blocked by omission of extracellular Ca2+ and by blockers of the L-type voltage-gated Ca2+ channel, such as D-600 or nifedipine. Depletion of agonist-sensitive intracellular Ca2+ pools by thapsigargin did not inhibit the stimulatory effect of caffeine on [Ca2+]i. Moreover, this effect of caffeine was not due to an increase in cyclic AMP, since forskolin and 3-isobutyl-1-methylxanthine (IBMX) failed to raise [Ca2+]i in unstimulated beta-cells. In beta-cells, glucose and sulphonylureas increase [Ca2+]i by causing closure of ATP-sensitive K+ channels (KATP channels). Caffeine also caused inhibition of KATP channel activity, as measured in excised inside-out patches. Accordingly, caffeine (> 10 mM) induced insulin release from beta-cells in the presence of a non-stimulatory concentration of glucose (3 mM). Hence, membrane depolarization and opening of voltage-gated L-type Ca2+ channels were the underlying mechanisms whereby the xanthine drug increased [Ca2+]i and induced insulin release. Paradoxically, in glucose-stimulated beta-cells, caffeine (> 10 mM) lowered [Ca2+]i. This effect was due to the fact that caffeine reduced depolarization-induced whole-cell Ca2+ current through the L-type voltage-gated Ca2+ channel in a dose-dependent manner. Lower concentrations of caffeine (2.5-5.0 mM), when added after glucose-stimulated increase in [Ca2+]i, induced fast oscillations in [Ca2+]i. The latter effect was likely to be attributable to the cyclic AMP-elevating action of caffeine, leading to phosphorylation of voltage-gated Ca2+ channels. Hence, in beta-cells, caffeine-induced changes in [Ca2+]i are not due to any interaction with intracellular Ca2+ pools. In these cells, a direct interference with KATP channel- and L-type voltage-gated Ca(2+)-channel activity is the underlying mechanism by which caffeine increases or decreases [Ca2+]i.

Receptor occupancy regulates Ca2+ entry and intracellular Ca2+ redistribution in activated human platelets

Fura-2-loaded human platelets were used to study Ca2+ release from intracellular compartments, as well as Ca2+ influx from the extracellular space. We investigated the response towards the endoperoxide/thromboxane-receptor agonist. U46619, and the inhibitor of the endoplasmic-reticulum Ca(2+)-ATPase, thapsigargin. U46619 dose-dependently depleted intracellular Ca2+ stores, followed by active sequestration of released Ca2+. Ca2+ influx induced by U46619 largely relies on receptor occupancy. Removing the thromboxane analogue from its receptor by using the endoperoxide/thromboxane-receptor antagonist BM 13177 largely blunted U46619-mediated Ca2+ influx. The Ca(2+)-ATPase inhibitor thapsigargin evoked a gradual rise in intracellular Ca2+, which was potentiated by a preceding activation of platelets with the receptor agonist U46619. This agonist-sensitizing effect also depends on receptor occupancy. Removing U46619 from its receptor by addition of the endoperoxide/thromboxane-receptor antagonist BM13177 suppressed the sensitizing effect completely. Furthermore, interrupting downstream receptor signalling events by raising intracellular levels of cyclic nucleotides (cyclic AMP, cyclic GMP) again suppressed the U46619-sensitizing effect on thapsigargin-induced Ca2+ release. This study indicates that the process of Ca2+ release followed by resequestration in response to a platelet agonist by its own is not sufficient to produce the sensitizing effect. Rather, a continuously occupied receptor triggering sustained downstream signalling events seems to be required for sensitization. The presence of a receptor agonist may induce an increased cycling of Ca2+ between the agonist-responsive and the thapsigargin-dischargeable compartment, leading to faster and more intense accumulation of Ca2+ in the cytosolic compartment after inhibition of the Ca(2+) ATPase. Suggestively, receptor occupancy increases the Ca(2+)-releasing potency of thapsigargin by coupling the thapsigargin-sensitive Ca(2+)-storing compartments with an agonist-responsive compartment that exhibits a high leakage rate in stimulated platelets.

Thapsigargin selectively stimulates synthesis of phosphatidylglycerol in N1E-115 neuroblastoma cells and phosphatidylinositol in C6 glioma cells

Phospholipid metabolism was studied in N1E-115 neuroblastoma and C6 glioma cells exposed to thapsigargin, a selective inhibitor of endoplasmic reticulum Ca(2+)-ATPase that raises the cytosolic free Ca2+ concentration [Ca2+]i. Thapsigargin caused only a transient increase of [Ca2+]i (< 1 min) in N1E-115 cells similar in magnitude and duration to agonist-induced calcium release mediated by inositol trisphosphate. Sustained elevation of [Ca2+]i due to influx of extracellular calcium, as occurs in most other cell lines including C6 cells, did not occur in N1E-115 cells. Increased uptake of inorganic phosphate (Pi) associated calcium influx was observed in C6 but not in N1E-115 cells. Thapsigargin affected phospholipid synthesis in both cell lines, most likely by inhibiting phosphatidic acid phosphohydrolase as indicated by diversion of [3H]oleic acid incorporation from triacylglycerol to phospholipid synthesis and stimulation of [32P]Pi incorporation into anionic phospholipids at the expense of phosphatidylcholine synthesis. The response to increased phosphatidate/phosphatidyl-CMP availability was cell specific. Thapsigargin (> 100 nM) selectively stimulated phosphatidylglycerol synthesis 20-30-fold in N1E-115 neuroblastoma cells while phosphatidylinositol synthesis was increased < 2-fold. In contrast, phosphatidylglycerol was not affected in C6 glioma cells and phosphatidylinositol synthesis was stimulated 8-fold by thapsigargin (> 1 microM). Agonist-stimulated calcium release did not increase phosphatidylglycerol synthesis in N1E-115 cells. Thapsigargin-stimulated phosphatidylglycerol synthesis and agonist-stimulated phosphatidylinositol synthesis could occur at the same time. Similar results were obtained with TMB-8, an inhibitor of intracellular Ca2+ release that decreases diacylglycerol utilization by blocking choline uptake and phosphatidylcholine synthesis without affecting resting [Ca2+]i. Thus [Ca2+]i does not directly mediate the effects of thapsigargin, TMB-8 or agonist stimulation on anionic phospholipid metabolism. These additional effects may limit the use of thapsigargin to assess Ca(2+)-dependence of phospholipid metabolism associated with Ca(2+)-mediated signal transduction.

Ca2+ stores in insulin-secreting cells: lack of effect of cADP ribose

Ca2+ stores were examined in several insulin secreting cell types by measuring uptake and release of Ca2+ by permeabilised cells. In pancreatic islet cells or INS-1 cells, < 20% of the ATP-dependent, thapsigargin-sensitive Ca2+ pool could be released by saturating concentrations of inositol (1,4,5)P3 (InsP3). InsP3 released > 60% of the thapsigargin-sensitive Ca2+ pool in RINm5F cells. The total Ca2+ content of the thapsigargin-sensitive pool was similar in each of these cell types. Neither cADP ribose (cADPR; 1 microM) nor caffeine (10 mM) caused significant Ca2+ release from any of the permeabilised insulin-secreting cell preparations. ATP elicited similar increases in intracellular Ca2+ concentration ([Ca2+]i) in single, living INS-1 and RINm5F cells, and similar fold increases in InsP3 levels in cell populations. The Ca2+ ATPase inhibitor thapsigargin, added after ATP, caused smaller [Ca2+]i increases in RINm5F than in INS-1 cells. This is consistent with the presence of a smaller InsP3-sensitive Ca2+ pool in living INS-1 cells. The data indicate that InsP3 receptors are present in only a small subfraction of the Ca2+ ATPase-containing Ca2+ stores in INS-1 and pancreatic beta-cells, and that cADP ribose/caffeine-sensitive Ca(2+)-induced Ca2+ release channels may be entirely absent from this endocrine cell type.

An investigation on the role of vacuolar-type proton pumps and luminal acidity in calcium sequestration by nonmitochondrial and inositol-1,4,5-trisphosphate-sensitive intracellular calcium stores in clonal insulin-secreting cells

To test whether in RINm5F rat insulinoma cells luminal acidity and the activity of a vacuolar-type proton pump are involved in calcium sequestration by intracellular calcium stores sensitive to inositol 1,4,5-trisphosphate (InsP3) we examined the effects of various proton-conducting ionophores and ammonium chloride, and of bafilomycin, a specific inhibitor of vacuolar proton pumps, on this parameter. Bafilomycin in concentrations up to 1 microM did not affect calcium sequestration by nonmitochondrial, InsP3-sensitive stores at all; 50 microM carbonylcyanide m-chlorophenylhydrazone, 50 microM monensin and 30 mM NH4Cl, which are diverse ways to dissipate transmembrane pH gradients, did not inhibit calcium sequestration. This argues against signficant involvement of internal acidity and vacuolar proton pumps in calcium sequestration by InsP3-sensitive stores in RINm5F cells. The proton-potassium-exchanging ionophore nigericin (20-100 microM), however, inhibited calcium sequestration by nonmitochondrial and InsP3-sensitive stores. This effect was dependent on the presence of potassium and could be reversed by inclusion of carbonylcyanide m-chlorophenylhydrazone or acetate in the incubation medium. Thus, the inhibitory effect of nigericin appears to be based on proton extrusion coupled to potassium influx across the membrane of calcium stores in RINm5F cells, creating an internal alkalinization of these stores. The effect of nigericin implies the continuous maintenance of an outside-to-inside potassium concentration gradient by nonmitochondrial calcium stores in RINm5F cells. This feature will be of potential interest in the identification of InsP3-sensitive calcium-storing organelles.

Ionomycin enhances Ca2+ influx by stimulating store-regulated cation entry and not by a direct action at the plasma membrane

In fura-2-loaded ECV304 cells ionomycin elicited a saturable biphasic change in intracellular Ca2+ concentration ([Ca2+]i), where the initial phase represented mobilization of intracellular stores and the sustained component represented Ca2+ influx. To examine whether ionomycin could stimulate influx via a store-dependent mechanism. Mn2+ entry was monitored by the quenching of fura-2 fluorescence: influx was enhanced even after ionomycin wash-out, provided that internal stores were not refilled with Ca2+. Moreover, the maximal rate of histamine-stimulated Mn2+ entry was unaffected by ionomycin, suggesting a common route of entry. The Ca(2+)-entry blocker SK&F 96365 inhibited both the ionomycin-induced Mn2+ entry and the sustained [Ca2+]i response to the ionophore (leaving the initial peak [Ca2+]i response unaffected). In other experiments, although addition of ionomycin further increased the plateau phase induced by 100 microM histamine, the increase was completely abolished by pretreatment with the store Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA). Furthermore, in store-depleted cells, re-addition of 1 mM extracellular Ca2+ (in the presence of CPA plus histamine) led to a rapid rise in [Ca2+]i, dependent on Ca2+ influx, with kinetics that were not enhanced by ionomycin. These data suggest that ionomycin acts primarily at the level of the internal Ca2+ stores, so that, at the concentrations used here (< or = 1 microM), it increases Ca2+ (and Mn2+) influx via activation of endogenous entry pathways and not by plasmalemmal translocation.

Simultaneous oscillations of cytoplasmic free Ca2+ concentration and Ins(1,4,5)P3 concentration in mouse pancreatic beta-cells

Changes in the cytoplasmic free Ca2+ concentration ([Ca2+]i) in stimulated cells are often oscillatory, but the mechanisms that drive these oscillations are still a matter of controversy: different models of the generation of these [Ca2+]i oscillations make different assumptions as to whether oscillations in Ins(1,4,5)P3 concentration are necessary for this process. We have looked for changes in inositol polyphosphate levels that might occur in suspensions of murine pancreatic beta-cells when these cells are induced to display synchronized oscillations in [Ca2+]i by the sequential addition of glucose, an alpha 2-adrenergic stimulus and extracellular Ca2+. The intracellular level of Ins(1,4,5)P3 oscillated in a manner approximately in synchrony with changes in [Ca2+]i. Oscillations in the levels of Ins(1,4,5)P3 metabolites [Ins(1,3,4)P3 and inositol bisphosphates] were slightly delayed relative to the Ins(1,4,5)P3 oscillations, and the concentration of Ins(1,3,4,5,6)P5 remained approximately constant during the [Ca2+]i oscillations. These results demonstrate that [Ins(1,4,5)P3] and [Ca2+]i oscillate in synchrony in at least one type of cell. Whether such oscillations in intracellular [Ins(1,4,5)P3] provide a primary driving force for [Ca2+]i oscillations either in beta-cells or in other stimulated cells remains to be determined. Even if they do not, the [Ins(1,4,5)P3] oscillations will at least provide an amplifying influence on the [Ca2+]i changes.