Orai1 and Orai2 mediate store-operated calcium entry that regulates HL60 cell migration and FAK phosphorylation.

Store-operated Ca2+ entry (SOCE) is a major mechanism for the regulation of intracellular Ca2+ homeostasis and cellular function. Emerging evidence has revealed that altered expression and function of the molecular determinants of SOCE play a critical role in the development or maintenance of several cancer hallmarks, including enhanced proliferation and migration. Here we show that, in the acute myeloid leukemia cell line HL60, Orai2 is highly expressed at the transcript level, followed by the expression of Orai1. Using fluorescence Ca2+ imaging we found that Orai2 silencing significantly attenuated thapsigargin-induced SOCE, as well as knockdown of Orai1, while silencing the expression of both channels almost completely reduced SOCE, thus suggesting that SOCE in these cells is strongly dependent on Orai1 and Orai2. On the other hand, the expression of TRPC1, TRPC3 and TRPC6 is almost absent at the transcript and protein level. Bromodeoxyuridine cell proliferation assay revealed that Orai1 and Orai2 expression silencing significantly reduced HL60 cell proliferation. Furthermore, knockdown of Orai1 and Orai2 significantly attenuated the ability of HL60 to migrate in vitro as determined by transwell migration assay, probably due to the impairment of FAK tyrosine phosphorylation. These findings provide evidence for a role for Orai1 and Orai2, in SOCE and migration in the human HL60 promyeloblastic cell line. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Relationship between calcium mobilization and platelet α- and δ-granule secretion. A role for TRPC6 in thrombin-evoked δ-granule exocytosis.

Changes in cytosolic Ca(2+) concentration ([Ca(2+)]c) regulate granule secretion in different cell types. Thrombin activates PAR1 and PAR4 receptors and promotes release of Ca(2+) from distinct intracellular stores, which, in turn, activates store-operated Ca(2+) entry (SOCE). A crucial step during platelet function is the release of physiological agonists stored in secretory granules to the extracellular compartment during activation. We aim to study the role of Ca(2+) mobilization from the extracellular compartment or from different intracellular stores in platelet granule secretion. By using flow cytometry, we have found that α- and δ-granules are secreted in thrombin-stimulated platelets in the absence of extracellular Ca(2+), and in a concentration-dependent manner. Our findings show that thrombin-stimulated granule secretion depends on Ca(2+) mobilization from intracellular stores. Analysis of the kinetics of granule secretion reveals that platelet stimulation with thrombin results in rapid release of α-granules which precedes the secretion of δ-granules. Incubation of platelets with a specific antibody, which recognizes the extracellular amino acid sequence 573-586 of TRPC6, inhibited thrombin-evoked δ-granule exocytosis. Our results indicate that the mechanisms underlying thrombin-induced α- and δ-granule secretion show differences in dependency on Ca(2+) mobilization.

Identification of apoptotic bodies in equine semen.

Apoptosis in the testis is required to ensure an efficient spermatogenesis. However, sometimes, defective germ cells that are marked for elimination during this process escape elimination in the testes, giving rise to ejaculates with increased percentages of abnormal and apoptotic spermatozoa and a high percentage of apoptotic bodies. Apoptosis markers in the ejaculate have been associated with low fertility, either in animals or humans. Therefore, the goal of this study was to investigate whether fresh equine semen contains apoptotic bodies [initially named Merocyanine 540 (M540) bodies] and to study the relationship between the quantity of these bodies and cell concentration, the volume of ejaculate, viability and motility. Moreover, we also studied whether the presence apoptotic bodies in fresh semen was related to the resistance of the stallion spermatozoa to being incubated at 37 °C or being frozen and thawed. Fresh equine semen was stained with fluorescent dyes such as M540 and Annexin-V. Active Caspase 3 was studied in fresh semen through Western blotting and immunofluorescence with a specific antibody. Sperm kinematics was assessed in fresh, incubated and thawed samples using computer-assisted semen analysis, and viability was evaluated with the LIVE/DEAD Sperm Viability Kit. Overall, our results demonstrate for the first time the presence of apoptotic bodies in equine semen. The quantity of apoptotic bodies was highly variable among stallions and was positively correlated with Caspase 3 activity in fresh samples and negatively correlated with the viability and motility of stallion spermatozoa after the cryopreservation process.

The membrane of the mammalian spermatozoa: much more than an inert envelope.

Sperm plasma membrane is a very important structure that functions to protect sperm against extracellular injuries and to respond to physiological challenges. It plays a crucial role during sperm capacitation, in sperm-egg interaction and, finally, in fertilization. Concerning sperm technology, possibly the most important factors causing damage in mammalian spermatozoa membranes are initiated by the osmotic stress generated by dehydration of the cells during freezing and thawing. These changes are rapidly derived to the plasma and organelle membranes that gradually experiment loss of membrane architecture, causing unbalanced production of reactive oxygen species and increased lipid peroxidation. Other procedures such as sperm sorting or liquid storage of sperm also induce harmful changes in the integrity of the membrane. The specific composition of lipids of the sperm membranes may provide clues for understanding the mechanisms behind the differences found in the response to stress in different species. In the present review, we deal with the composition, architecture and organization of the sperm plasma membrane, emphasizing the factors that can affect membrane integrity. The intracellular signalling pathways related with membrane reorganization during capacitation and acrosome reaction are also reviewed.

TMEM97 facilitates the activation of SOCE by downregulating the association of cholesterol to Orai1 in MDA-MB-231 cells.

The expression of TMEM97, a regulator of cholesterol transport, has been reported to be enhanced in some tumour cells. We have recently shown that TMEM97 is involved in the proliferation of the breast cancer cell line MDA-MB-231, probably through changes in store-operated calcium entry (SOCE). By using silencing and overexpression of TMEM97 in MDA-MB-231 cells (two manoeuvres that either reduce or increase the calcium influx, respectively), we show enhanced cholesterol uptake in these cells as compared to the non-tumoral breast cell line, MCF10A. The enhanced cholesterol uptake in MDA-MB-231 cells was inhibited by silencing TMEM97, while overexpression of this protein increased cholesterol uptake in MCF10A cells and, therefore, indicating that this protein plays a role in the enhanced cholesterol uptake in MDA-MB-231 cancer cell line. TMEM97 silencing and overexpression resulted in an increase and decrease in the association of cholesterol to the SOCE calcium channel Orai1, respectively. Interestingly, silencing of TMEM97 in MDA-MB-231 cells significantly reduced the co-localization of Orai1 with the SOCE regulatory protein STIM1. Finally, neither silencing nor overexpression of TMEM97 altered SOCE in MDA-MB-231 cells transfected with the cholesterol insensible mutant of Orai1(Y80E). Our results reveal a novel regulatory mechanism of SOCE that relies on TMEM97 activity that courses through the reduction of the cholesterol content in the plasma membrane, and subsequently, by impairing its interaction with Orai1.

STIM1, Orai1 and hTRPC1 are important for thrombin- and ADP-induced aggregation in human platelets.

Ca(2+) entry, particularly store-operated Ca(2+) entry (SOCE), has been reported to be crucial for a variety of cellular functions. SOCE is a mechanism regulated by the Ca(2+) content of the stores, where the intraluminal Ca(2+) sensor STromal Interaction Molecule 1 (STIM1) has been reported to communicate the filling state of the intracellular Ca(2+) stores to the store-operated Ca(2+)-permeable channels in the plasma membrane, likely involving Orai1 and TRPC proteins, such as TRPC1. Here we have investigated the role of Orai1, STIM1 and TRPC1 in platelet aggregation, an event that occurs during the process of thrombosis and hemostasis. Electrotransjection of cells with anti-STIM1 (25-139) antibody, directed towards the Ca(2+)-binding motif, significantly reduced thrombin-induced aggregation and prevented ADP-evoked response. Extracellular application of the anti-STIM1 antibody, in order to block the function of plasma membrane-located STIM1, reduced thrombin- and ADP-stimulated platelet aggregation to a lesser extent. Introduction of an anti-Orai1 (288-301) antibody, which binds the STIM1-binding site located in the Orai1 C-terminus, or extracellular application of anti-hTRPC1 (557-571) antibody to impair hTRPC1 channel function, significantly reduced thrombin- and ADP-induced platelet aggregation. These findings suggest a role of STIM1, Orai1 and hTRPC1 in thrombin- and ADP-induced platelet aggregation probably through the regulation of Ca(2+) entry, which might become targets for the development of therapeutic strategies to treat platelet hyperactivity and thrombosis disorders.

Store-independent Orai1-mediated Ca2+ entry and cancer.

Ca2+ channels play an important role in the development of different types of cancer, and considerable progress has been made to understand the pathophysiological mechanisms underlying the role of Ca2+ influx in the development of different cancer hallmarks. Orai1 is among the most ubiquitous and multifunctional Ca2+ channels. Orai1 mediates the highly Ca2+-selective Ca2+ release-activated current (ICRAC) and participates in the less Ca2+-selective store-operated current (ISOC), along with STIM1 or STIM1 and TRPC1, respectively. Furthermore, Orai1 contributes to a variety of store-independent Ca2+ influx mechanisms, including the arachidonate-regulated Ca2+ current, together with Orai3 and the plasma membrane resident pool of STIM1, as well as the constitutive Ca2+ influx processes activated by the secretory pathway Ca2+-ATPase-2 (SPCA2) or supported by physical and functional interaction with the small conductance Ca2+-activated K+ channel 3 (SK3) or the voltage-dependent Kv10.1 channel. This review summarizes the current knowledge concerning the store-independent mechanisms of Ca2+ influx activation through Orai1 channels and their role in the development of different cancer features.

(-)­Oleocanthal inhibits proliferation and migration by modulating Ca2+ entry through TRPC6 in breast cancer cells.

Triple negative breast cancer is an aggressive type of cancer that does not respond to hormonal therapy and current therapeutic strategies are accompanied by side effects due to cytotoxic actions on normal tissues. Therefore, there is a need for the identification of anti-cancer compounds with negligible effects on non-tumoral cells. Here we show that (-)­oleocanthal (OLCT), a phenolic compound isolated from olive oil, selectively impairs MDA-MB-231 cell proliferation and viability without affecting the ability of non-tumoral MCF10A cells to proliferate or their viability. Similarly, OLCT selectively impairs the ability of MDA-MB-231 cells to migrate while the ability of MCF10A to migrate was unaffected. The effect of OLCT was not exclusive for triple negative breast cancer cells as we found that OLCT also attenuate cell viability and proliferation of MCF7 cells. Our results indicate that OLCT is unable to induce Ca2+ mobilization in non-tumoral cells. By contrast, OLCT induces Ca2+ entry in MCF7 and MDA-MB-231 cells, which is impaired by TRPC6 expression silencing. We have found that MDA-MB-231 and MCF7 cells overexpress the channel TRPC6 as compared to non-tumoral MCF10A and treatment with OLCT for 24-72 h downregulates TRPC6 expression in MDA-MB-231 cells. These findings indicate that OLCT impairs the ability of breast cancer cells to proliferate and migrate via downregulation of TRPC6 channel expression while having no effect on the biology of non-tumoral breast cells.

Protein complex immunological separation assay (ProCISA): a technique for investigating single protein properties.

Analysis of the posttranslational modification of proteins, such as phosphorylation, might yield misleading results due to the presence of other proteins with similar electrophoretic properties that coimmunoprecipitate with the target protein. The aim of the present work was to develop a reliable, easy and economical technique to completely isolate a protein from its complex. Here we present a new assay developed to fully isolate proteins from macromolecular complexes that consists of an initial SDS/PAGE (under reducing conditions), which isolates the target protein, followed by transfer of the proteins to a buffer, from which the target protein is recaptured by conventional immunoprecipitation. This technique, that we have termed "Protein Complex Immunological Separation Assay" (ProCISA), successfully separated proteins of different sizes, such as pp60Src and the IP3 receptor (IP3R), from their complexes. We show that ProCISA allows the investigation of the tyrosine phosphorylation state of isolated proteins. This technique could also be used to study other posttranslational modifications without risk of misleading results resulting from contamination with other proteins of similar electrophoretic mobility which complex with the protein of interest.

Role of proton gradients and vacuola H(+)-ATPases in the refilling of intracellular calcium stores in exocrine cells.

Numerous hormones and neurotransmitters activate cells by increasing cytosolic calcium concentration ([Ca(2+)](i)), a key regulatory factor for many cellular processes. A pivotal feature of these Ca(2+) signals is the release of Ca(2+) from intracellular stores, which is followed by activation of extracellular calcium influx, allowing refilling of the stores by SERCA pumps associated with the endoplasmic reticulum. Although the mechanisms of calcium release and calcium influx have been extensively studied, the biology of the Ca(2+) stores is poorly understood. The presence of heterogeneous calcium pools in cells has been previously reported [1] [2] [3]. Although recent technical improvements have confirmed this heterogeneity [4], knowledge about the mechanisms underlying Ca(2+) transport within the stores is very scarce and rather speculative. A recent study in polarized exocrine cells [5] has revealed the existence of Ca(2+) tunneling from basolateral stores to luminal pools, where Ca(2+) is initially released upon cell activation. Here, we present evidence that, during stimulation, Ca(2+) transported into basolateral stores by SERCA pumps is conveyed toward the luminal pools driven by proton gradients generated by vacuolar H(+)-ATPases. This finding unveils a new aspect of the machinery of Ca(2+) stores.

Thrombin induces apoptotic events through the generation of reactive oxygen species in human platelets.

BACKGROUND: Thrombin is a major physiological platelet agonist that activates a number of cell functions including aggregation. Platelet stimulation with thrombin has been shown to result in the development of apoptotic events, including activation of caspases-3 and -9, cytochrome c release and phosphatidylserine (PS) exposure; however, the mechanism underlying the activation of apoptosis remains unclear. OBJECTIVES: In the present study, we aim to investigate whether endogenously generated reactive oxygen species upon thrombin stimulation is required for the activation of apoptosis in human platelets. METHODS: Changes in the mitochondrial membrane potential were registered using the dye JC-1; caspase-3 and -9 activity was determined from the cleavage of their respective specific fluorogenic substrates; PS externalization was estimated using annexin V-fluorescein isothicyanate and cytochrome c release was detected by Western blotting in samples from the mitochondrial and cytosolic fractions. RESULTS: Treatment of platelets with thrombin stimulates mitochondrial membrane potential depolarization and endogenous generation of H(2)O(2) . Platelet exposure to exogenous H(2)O(2) results in cytochrome c release and activation of caspases-9. In addition, H(2)O(2) induces the activation of caspase-3 and PS exposure by a mechanism dependent on cytochrome c release and caspase-9 activation. Finally, thrombin-evoked development of apoptotic events was impaired by treatment with catalase. CONCLUSION: Our results indicate that thrombin-induced apoptosis is likely mediated by endogenous generation of H(2)O(2) in human platelets.

Modulation of CCK-8-evoked intracellular Ca2+ waves by hydrogen peroxide in mouse pancreatic acinar cells.

In the present study we have employed single cell imaging analysis to monitor the propagation of cholecystokinin-evoked Ca(2+) waves in mouse pancreatic acinar cells. Stimulation of cells with 1 nM CCK-8 led to an initial Ca(2+) release at the luminal cell pole and subsequent spreading of the Ca(2+) signal towards the basolateral membrane in the form of a Ca(2+) wave. Inhibition of sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA) activity by 1 microM thapsigargin, preincubation in the presence of 100 microM H(2)O(2) or inhibition of PKC with either 5 microM Ro31-8220 or 3 microM GF-109203-X all led to a faster propagation of CCK-8-induced Ca(2+) signals. The propagation of CCK-8-evoked Ca(2+) signals was slowed down by activation of PKC with 1 microM PMA, and preincubation of cells in the presence of H(2)O(2) counteracted the effect of PKC inhibition. The protonophore FCCP (100 nM) and the inhibitor of the mitochondrial Ca(2+)-uniporter Ru360 (10 microM) led to an increase in the propagation rate of CCK-8-evoked Ca(2+) waves. Finally, depolymerisation of actin cytoskeleton with cytochalasin D (10 microM) led to a faster propagation of CCK-8-evoked Ca(2+) signals. Stabilization of actin cytoskeleton with jasplakinolide (10 microM) did not induce significant changes on CCK-8-evoked Ca(2+) waves. Preincubation of cells in the presence of H(2)O(2) counteracted the effect of cytochalasin D on CCK-8-evoked Ca(2+) wave propagation. Our results suggest that spreading of cytosolic Ca(2+) waves evoked by CCK-8 can be modulated by low levels of oxidants acting on multiple Ca(2+)-handling mechanisms.

The autophagy-related protein LC3 is processed in stallion spermatozoa during short-and long-term storage and the related stressful conditions.

Use of cooled and frozen semen is becoming increasingly prevalent in the equine industry. However, these procedures cause harmful effects in the sperm cell resulting in reduced cell lifespan and fertility rates. Apoptosis and necrosis-related events are increased during semen cryopreservation. However, a third type of cell death, named autophagy, has not been studied during equine semen storage. Light chain (LC)3 protein is a key component of the autophagy pathway. Under autophagy activation, LC3-I is lipidated and converted to LC3-II. The ratio of LC3-II/LC3-I is widely used as a marker of autophagy activation. The main objective of this study was to investigate whether LC3 is processed during cooling, freezing and the stressful conditions associated with these technologies. A secondary objective was to determine if LC3 processing can be modulated and if that may improve the quality of cryopreserved semen. LC3 processing was studied by Western blot with a specific antibody that recognized both LC3-I and LC3-II. Viability was assessed by flow cytometry. Modulation of LC3-I to LC3-II was studied with known autophagy activators (STF-62247 and rapamycin) or inhibitors (chloroquine and 3-MA) used in somatic cells. The results showed that conversion of LC3-I to LC3-II increased significantly during cooling at 4°C, freezing/thawing and each of the stressful conditions tested (UV radiation, oxidative stress, osmotic stress and changes in temperature). STF-62247 and rapamycin increased the LC3-II/LC3-I ratio and decreased the viability of equine sperm, whereas chloroquine and 3-MA inhibited LC3 processing and maintained the percentage of viable cells after 2 h of incubation at 37°C. Finally, refrigeration at 4°C for 96 h and freezing at -196°C in the presence of chloroquine and 3-MA resulted in higher percentages of viable cells. In conclusion, results showed that an 'autophagy-like' mechanism may be involved in the regulation of sperm viability during equine semen cryopreservation. Modulation of autophagy during these reproductive technologies may result in an improvement of semen quality and therefore in higher fertility rates.

Autophagy-related proteins are functionally active in human spermatozoa and may be involved in the regulation of cell survival and motility.

Macroautophagy (hereafter autophagy) is an evolutionarily highly conserved cellular process that participates in the maintenance of intracellular homeostasis through the degradation of most long-lived proteins and entire organelles. Autophagy participates in some reproductive events; however, there are not reports regarding the role of autophagy in the regulation of sperm physiology. Hence, the aim of this study was to investigate whether autophagy-related proteins are present and functionally active in human spermatozoa. Proteins related to autophagy/mitophagy process (LC3, Atg5, Atg16, Beclin 1, p62, m-TOR, AMPKα 1/2, and PINK1) were present in human spermatozoa. LC3 colocalized with p62 in the middle piece of the spermatozoa. Autophagy activation induced a significant increase in motility and a decrease in PINK1, TOM20 expression and caspase 3/7 activation. In contrast, autophagy inhibition resulted in decreased motility, viability, ATP and intracellular calcium concentration whereas PINK1, TOM20 expression, AMPK phosphorylation and caspase 3/7 activation were significantly increased. In conclusion our results show that autophagy related proteins and upstream regulators are present and functional in human spermatozoa. Modification of mitochondrial proteins expression after autophagy activation/inhibition may be indicating that a specialized form of autophagy named mitophagy may be regulating sperm function such as motility and viability and may be cooperating with apoptosis.

Are tyrosine phosphorylation of p125(FAK) and paxillin or the small GTP binding protein, rho, needed for CCK-stimulated pancreatic amylase secretion?

Studies of a possible role of tyrosine phosphorylation in the secretory process in rat pancreatic acinar cells provide conflicting conclusions. Recent studies show that tyrosine phosphorylation of the focal adhesion kinase, p125FAK and the cytoskeletal protein, paxillin, may mediate a number of cellular changes and this phosphorylation is dependent on the activation of the small GTP binding protein, p21Rho (Rho). In this work we have investigated the role of tyrosine phosphorylation of each of these proteins and of the activation of Rho in pancreatic enzyme secretion. Pretreatment with genistein, a tyrosine kinase inhibitor, decreased CCK-8-stimulated tyrosine phosphorylation of p125FAK and paxillin and CCK-8-stimulated amylase secretion by more than 60%, raising the possibility that tyrosine phosphorylation of these two proteins could be important in the ability of CCK-8 to stimulate amylase release. However, genistein did not alter the amylase release stimulated by TPA but inhibited TPA-stimulated p125FAK and paxillin tyrosine phosphorylation by 70%. Pretreatment with C3 transferase, which specifically inactivates Rho, causes a decrease in CCK-8-induced maximal amylase release by 33%. Moreover, C3 transferase pretreatment causes a 48% and a 38% decrease in the tyrosine phosphorylation of p125FAK and paxillin by CCK-8, respectively. Pretreatment with different concentrations of cytochalasin D, an actin cytoskeleton assembly inhibitor, completely inhibited CCK-8-stimulated tyrosine phosphorylation of p125FAK and paxillin without having any effect on either the potency or efficacy of CCK-8 at stimulating amylase release. Furthermore, cytochalasin D completely inhibited TPA-stimulated tyrosine phosphorylation of both proteins without affecting TPA-stimulated amylase release. These results show that tyrosine phosphorylation of p125FAK and paxillin is not required for CCK-8 stimulation of enzyme secretion. However, our results suggest Rho is involved in the CCK-8 stimulation of amylase release by a parallel pathway to its involvement in the CCK-8-stimulated tyrosine phosphorylation of p125FAK and paxillin.

A role for phosphoinositides in tyrosine phosphorylation of p125 focal adhesion kinase in rat pancreatic acini.

Previous studies have shown that different agonists increase tyrosine phosphorylation of the focal adhesion related proteins p125(FAK), p130(Cas), and paxillin in different cell types and that tyrosine phosphorylation depends on the integrity of the actin cytoskeleton. Because phosphoinositides are important for the maintenance of the cytoskeleton, the role of phosphoinositides in the tyrosine phosphorylation of these proteins in response to occupancy of m3 muscarinic and CCK(A) receptors has been investigated in pancreatic acini. Addition of carbachol or CCK-8 to pancreatic acini resulted in rapid increases in the tyrosine phosphorylation of p125(FAK), p130(Cas), and paxillin. Pretreatment of pancreatic acini with LY294002 or wortmannin resulted in a concentration-dependent inhibition of tyrosine phosphorylation of p125(FAK), p130(Cas), and paxillin stimulated by carbachol or CCK-8. Carbachol- or CCK-8-stimulated tyrosine phosphorylation of these proteins was not inhibited by rapamycin, PD 98059 or SB 203580, and thus it was dissociated from the activation of p70 S6 or MAP kinases. These results indicate that m3 muscarinic and CCK(A) receptor-mediated increase in p125(FAK), p130(Cas), and paxillin tyrosine phosphorylation in pancreatic acini depends on the ability of these cells to synthesise phosphoinositides.

Histamine and the cAMP pathway in the guinea-pig pancreas.

This study investigates the interaction between histamine and the adenylate cyclase systems involved in the secretion of amylase from the guinea-pig pancreatic lobules. Histamine increased amylase release, reaching a maximum response at 10(-5) M. Similarly, vasoactive intestinal peptide (VIP) evoked significant increase in amylase release, though not in a dose-dependent fashion. When the pancreatic lobules were incubated with histamine in combination with VIP, forskolin or 3-isobutyl-1-methylxanthine (IBMX), amylase secretion was increased as compared to histamine alone. The stimulatory effect of VIP was also increased by the presence of forskolin or IBMX. These findings suggest that in guinea-pig pancreatic lobules, VIP, forskolin and IBMX, agents involved in the cyclic adenosine monophosphate (cAMP) pathway, potentiate histamine stimulated amylase release.

Vanadate inhibits the calcium extrusion in rat pancreatic acinar cells.

Our objective was to evaluate the role of vanadate on calcium extrusion in Fura-2-loaded rat pancreatic acinar cells by digital microscopic fluorimetry and spectrofluorimetry. In the absence of extracellular calcium, perfusion of pancreatic acinar cells with 1 nM CCK-8 and 1 mM vanadate did not significantly affect the typical transient calcium spike induced by CCK-8, but the plateau phase of calcium in response to CCK-8 remained elevated. In addition, vanadate was able to inhibit calcium efflux evoked by CCK-8 when we determined directly calcium transport across plasma membrane using Calcium Green-5N hexapotassium salt (cell impermeant form) in cell populations. The effect of vanadate on calcium extrusion was strongly blocked by the sulfhydryl-reducing agent dithiothreitol (DTT). The present results demonstrate that vanadate is able to irreversibly inhibit the calcium extrusion. This effect of vanadate can be blocked using DTT, indicating that its action is probably mediated by oxidation of sulfhydryl groups of Ca2+-ATPases.

Phenylarsine oxide evokes intracellular calcium increases and amylase secretion in isolated rat pancreatic acinar cells.

The effects of the thiol reagent, phenylarsine oxide (PAO, 10(-5)-10(-3) M ), a membrane-permeable trivalent arsenical compound that specifically complexes vicinal sulfhydryl groups of proteins to form stable ring structures, were studied by monitoring intracellular free calcium concentration ([Ca2+]i) and amylase secretion in collagenase dispersed rat pancreatic acinar cells. PAO increased [Ca2+]i by mobilizing calcium from intracellular stores, since this increase was observed in the absence of extracellular calcium. PAO also prevented the CCK-8-induced signal of [Ca2+]i and inhibited the oscillatory pattern initiated by aluminium fluoride (AlF-4). In addition to the effects of PAO on calcium mobilization, it caused a significant increase in amylase secretion and reduced the secretory response to either CCK-8 or AlF-4. The effects of PAO on both [Ca2+]i and amylase release were reversed by the sulfhydryl reducing agent, dithiothreitol (2 mM). Pretreatment of acinar cells with high concentration of ryanodine (50 microM) reduced the PAO-evoked calcium release. However, PAO was still able to release a small fraction of Ca2+ from acinar cells in which agonist-releasable Ca2+ pools had been previously depleted by thapsigargin (0.5 microM) and ryanodine receptors were blocked by 50 microM ryanodine. We conclude that, in pancreatic acinar cells, PAO mainly releases Ca2+ from the ryanodine-sensitive calcium pool and consequently induces amylase secretion. These effects are likely to be due to the oxidizing effects of this compound.

Collaborative effect of SERCA and PMCA in cytosolic calcium homeostasis in human platelets.

Intracellular free Ca2+ concentration ([Ca2+]c) is finely regulated by several mechanisms that either increase or reduce [Ca2+]c. Two different Ca2+ pumps have been described so far as the main mechanisms for Ca2+ removal from the cytosol, either by its sequestration into the stores, mediated by the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) or by Ca2+ extrusion to the extracellular medium, by the plasma membrane Ca2+-ATPase (PMCA). We have used inhibitors of these pumps to analyze their Ca2+ clearance efficacy in human platelets stimulated by the physiological agonist thrombin. Results demonstrate that, after platelet stimulation with thrombin, activation of SERCA precedes that of PMCA, although the ability of PMCA to remove Ca2+ from the cytosol last longer than that of SERCA. The efficacy of SERCA and PMCA removing Ca2+ from the cytosol is reduced when the concentration of thrombin increases. This phenomenon correlates with the greater increase in [Ca2+]c induced by higher concentrations of thrombin, which further confirms that SERCA and PMCA activities are regulated by [Ca2+]c.