Calcium signaling in human platelets

A live cell micro-imaging technique to examine platelet calcium signaling dynamics under blood flow

The platelet is a specialized adhesive cell that plays a key role in thrombus formation under both physiological and pathological blood flow conditions. Platelet adhesion and activation are dynamic processes associated with rapid morphological and functional changes, with the earliest signaling events occurring over a subsecond time-scale. The relatively small size of platelets combined with the dynamic nature of platelet adhesion under blood flow means that the investigation of platelet signaling events requires techniques with both high spatial discrimination and rapid temporal resolution. Unraveling the complex signaling processes governing platelet adhesive function under conditions of hemodynamic shear stress has been a longstanding goal in platelet research and has been greatly influenced by the development and application of microimaging-based techniques. Advances in the area of epi-fluorescence and confocal-based platelet calcium (Ca(2+)) imaging have facilitated the in vitro and in vivo elucidation of the early signaling events regulating platelet adhesion and activation. These studies have identified distinct Ca(2+) signaling mechanisms that serve to dynamically regulate activation of the major platelet integrin α(IIb)β(3) and associated adhesion and aggregation processes under flow. This chapter describes in detail a ratiometric calcium imaging protocol and associated troubleshooting procedures developed in our laboratory to examine live platelet Ca(2+) signaling dynamics. This technique provides a method for high-resolution imaging of the Ca(2+) dynamics underpinning platelet adhesion and thrombus formation under conditions of pathophysiological shear stress.

Synthesis and evaluation of platelet aggregation inhibitory activity of some 3-phenyl-pyrroloquinazolinones

A series of 3-phenyl-2,7-dihydro-1H-pyrrolo[3,2-f]quinazolin-1-one derivatives (3-PPyQZ) was synthesized starting from 5-amino-indoles, via condensation with N-ethoxycarbonylthiobenzamides followed by thermal cyclization. On the basis of their structural analogy with reported anti-thrombin pyrroloquinazolines, the derivatives were first tested for their capacity to inhibit platelet aggregation. Some of them had in vitro inhibitory effects on collagen and thrombin-induced aggregation in the micromolar range, and much higher inhibition than that shown by some phenyl-pyrroloquinolinones. Experiments to determine the mechanism of action of the most potent inhibitor (compound 18) indicated that it acts in at least two sites: one preceding the agonist-induced increase of cytosolic [Ca(2+)], and one following this step of the platelet activation cascade. The compound also inhibited thrombin-evoked protein-Tyr-phosphorylation. Although it is premature to draw definitive conclusions, the present results indicate that 3-PPyQZ structure, with the quite potent inhibitor of platelet aggregation compound 18, might constitute a starting point for the synthesis of potential anti-thrombosis agents.

Effects of Salivae Miltiorrhizae Liguspyragine Hydrochloride and Glucose Injection on the levels of main platelet thrombin receptors in chronic haemodialysis patients

OBJECTIVE

To investigate the effects of Salvia Miltiorrhiza Liguspyragine Hydrochloride and Glucose Injection (, SLGI) on the expression of platelet membrane receptors proteinase-activated receptor-1 (PAR1) and proteinase-activated receptor-4 (PAR4) in end-stage renal disease (ESRD) patients on chronic haemodialysis (HD).

METHODS

Eighty-six ESRD patients on HD (treated group) were treated with SLGI, 7 days as one therapeutic course, for two successive courses. The previous therapies were unchanged. Flow cytometry was used to assess the expression of platelet PAR1 and PAR4 in the patients, and turbidity method was used to determine the platelet maximum aggregation rate (MAR). Meanwhile, renal function was measured. The final data were compared with those before treatment and with those in the normal control group (54 healthy subjects).

RESULTS

Compared with the normal control group, the expressions of PAR1 and PAR4 and platelet MAR in ESRD patients on HD was significantly higher before treatment (P=0.001, P=0.006, and P=0.008); after treatment with SLGI, the above indices in patients were remarkably decreased (P=0.036 and P=0.046), except PAR4 (P=0.067), but still higher than those in the normal control group, however, it was not statistically significant.

CONCLUSIONS

(1) The overexpression of PAR1 and PAR4 might lead to increased platelet aggregation and this could be one of the reasons for the thrombotic events in ESRD patients on HD. (2) SLGI was able to down-regulate the expression of PAR1 in ESRD patients on HD, improve platelet function, and regulate platelet activation.

Alterations in platelet function and cell-derived microvesicles in recently menopausal women: relationship to metabolic syndrome and atherogenic risk

A woman's risk for metabolic syndrome (MS) increases at menopause, with an associated increase in risk for cardiovascular disease. We hypothesized that early menopause-related changes in platelet activity and concentrations of microvesicles derived from activated blood and vascular cells provide a mechanistic link to the early atherothrombotic process. Thus, platelet functions and cellular origin of blood-borne microvesicles in recently menopausal women (n = 118) enrolled in the Kronos Early Estrogen Prevention Study were correlated with components of MS and noninvasive measures of cardiovascular disease [carotid artery intima medial thickness (CIMT), coronary artery calcium (CAC) score, and endothelial reactive hyperemic index (RHI)]. Specific to individual components of the MS pentad, platelet number increased with increasing waist circumference, and platelet secretion of ATP and expression of P-selectin decreased with increasing blood glucose (p = 0.005) and blood pressure (p < 0.05), respectively. Waist circumference and systolic blood pressure were independently associated with monocyte- and endothelium-derived microvesicles (p < 0.05). Platelet-derived and total procoagulant phosphatidylserine-positive microvesicles, and systolic blood pressure correlated with CIMT (p < 0.05), but not with CAC or RHI. In summary, among recently menopausal women, specific platelet functions and concentrations of circulating activated cell membrane-derived procoagulant microvesicles change with individual components of MS. These cellular changes may explain in part how menopause contributes to MS and, eventually, to cardiovascular disease.

NP-313, 2-acetylamino-3-chloro-1,4-naphthoquinone, a novel antithrombotic agent with dual inhibition of thromboxane A(2) synthesis and calcium entry

BACKGROUND AND PURPOSE

1,4-Naphthoquinones exhibit antiplatelet activity both in vivo and in vitro. In the present study, we investigated the antiplatelet effect of a novel naphthoquinone derivative NP-313, 2-acetylamino-3-chloro-1,4-naphthoquinone and its mechanism of action.

EXPERIMENTAL APPROACH

We measured platelet aggregation, Ca(2+) mobilization, thromboxane B2 formation and P-selectin expression and examined several enzymatic activities. Furthermore, we used the irradiated mesenteric venules in fluorescein sodium-treated mice to monitor the antithrombotic effect of NP-313 in vivo.

KEY RESULTS

NP-313 concentration-dependently inhibited human platelet aggregation induced by collagen, arachidonic acid, thapsigargin, thrombin and A23187. NP-313 also inhibited P-selectin expression, thromboxane B(2) formation and [Ca(2+) ](i) elevation in platelets stimulated by thrombin and collagen. NP-313 at 10 µM inhibited cyclooxygenase, thromboxane A(2) synthase, and protein kinase Cα, whereas it did not affect phospholipase A(2) or phospholipase C activity. In the presence of indomethacin and an adenosine 5-diphosphate scavenger, NP-313 concentration-dependently inhibited thrombin- and A23187-induced [Ca(2+)](i) increase through its inhibitory effects on Ca(2+) influx, rather than blocking Ca(2+) release from intracellular stores. NP-313 also inhibited thapsigargin-mediated Ca(2+) influx through store-operated calcium channel but had no effect on Ca(2+) influx through store-independent calcium channel evoked by the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol. Nevertheless, it had little effect on cyclic AMP and cyclic GMP levels. Also, intravenously administered NP-313 dose-dependently inhibited the thrombus occlusion of the irradiated mesenteric vessels of fluorescein-pretreated mice.

CONCLUSIONS AND IMPLICATIONS

Taken together, these results indicate that NP-313 exerts its antithrombotic activity through dual inhibition of thromboxane A(2) synthesis and Ca(2+) influx through SOCC.

Zinc deprivation impairs growth factor-stimulated calcium influx into murine 3T3 cells associated with decreased cell proliferation

Zinc plays a critical role in growth, a process that depends primarily on cell proliferation. Murine fibroblasts, Swiss 3T3 cells, were used to explore the hypothesis that a critical role of zinc in cell proliferation relates to its function in calcium influx. Cells were deprived of zinc by an impermeant chelator, diethylenetriaminepentaacetate (0.6 mmol/L), and low-calcium status was achieved by using a low- (<5 μmol/L) calcium medium. Cells were stimulated by a composite of growth factors (GF): platelet-derived GF, insulin-like GF-I, and epidermal GF. GF stimulation of cell proliferation was assessed by the incorporation of tritiated thymidine and calcium influx by the increase in fluorescence of cells loaded with Fluo-4. Proliferation was dependent on both zinc and calcium and they interacted in this process. GF stimulated an immediate sharp increase in intracellular calcium, indicative of internal calcium release, which peaked within 1 min and decreased to an elevated plateau, a pattern typical of a store-operated calcium channel. The sustained calcium influx of zinc-deprived cells was markedly lower than that of supplemented cells. Verapamil, a calcium channel blocker, also depressed both cell proliferation and calcium influx. In summary, zinc deficiency impaired GF-stimulated calcium influx into murine fibroblasts in association with decreased cell proliferation.

Reversible activation of cellular factor XIII by calcium

Factor XIII (FXIII) is a pro-transglutaminase found in the plasma as well as intracellularly in platelets and macrophages. Plasma FXIII is activated by thrombin cleavage (FXIIIa*) and acts in the final stages of blood coagulation cascade. In contrast, the function and activation of cellular FXIII are less characterized. Cellular FXIII relies on a conformational activation of the protein. The nonproteolytic activation of FXIII to FXIIIa° induced by Ca(2+) alone is well known, but up until now it has been discussed under which conditions the process can be induced and whether it can be reversed. Here, we study the nature of the Ca(2+)-induced FXIII activation. Previously used methods to evaluate FXIII activity detect both FXIIIa* and FXIIIa° because they rely on occurrence of enzyme activity or on active site Cys-314 solvent accessibility. Therefore, an analytical HPLC method was developed that separates zymogen recombinant FXIII (rFXIII) from rFXIIIa°. The data demonstrate that nonproteolytic activation and deactivation are highly dependent on Ca(2+) concentration, buffer, and salt components. Moreover, it is established that Ca(2+) activation of rFXIII is fully reversible, and only 2-5 mm CaCl(2) is sufficient to retain full rFXIIIa° activity. However, below 2 mm CaCl(2) the rFXIIIa° molecule deactivates. The deactivated molecule can subsequently undergo a new activation round. Furthermore, it is demonstrated that thermal stress of freeze-dried rFXIII can induce a new predisposed form that activates faster than nonstressed rFXIII.

Manganese enhancement in non-CNS organs

Manganese-enhanced magnetic resonance imaging (MEMRI) is a novel imaging technique capable of monitoring calcium influx, in vivo. Manganese (Mn2+) ions, similar to calcium ions (Ca2+), are taken up by activated cells where their paramagnetic properties afford signal enhancement in T(1)-weighted MRI methodologies. In this study we have assessed Mn2+ distribution in mice using magnetization-prepared rapid gradient echo (MP-RAGE) based MRI, by measuring changes in T(1)-effective relaxation times (T(1)-eff), effective R(1)-relaxation rates (R(1)-eff) and signal intensity (SI) profiles over time. The manganese concentration in the tissue was also determined using inductively coupled plasma atomic emission spectrometry (ICP-AES). Our results show a strong positive correlation between infused dose of MnCl2 and the tissue manganese concentration. Furthermore, we demonstrate a linear relationship between R(1)-eff and tissue manganese concentration and tissue-specific Mn2+ distribution in murine tissues following dose-dependent Mn2+ administration. This data provides an optimized MnCl2 dose regimen for an MP-RAGE based sequence protocol for specific target organs and presents a potential 3D MRI technique for in vivo imaging of Ca2+ entry during Ca2+-dependent processes in a wide range of tissues.

Functional divergence of platelet protein kinase C (PKC) isoforms in thrombus formation on collagen

Arterial thrombosis, a major cause of myocardial infarction and stroke, is initiated by activation of blood platelets by subendothelial collagen. The protein kinase C (PKC) family centrally regulates platelet activation, and it is becoming clear that the individual PKC isoforms play distinct roles, some of which oppose each other. Here, for the first time, we address all four of the major platelet-expressed PKC isoforms, determining their comparative roles in regulating platelet adhesion to collagen and their subsequent activation under physiological flow conditions. Using mouse gene knock-out and pharmacological approaches in human platelets, we show that collagen-dependent alpha-granule secretion and thrombus formation are mediated by the conventional PKC isoforms, PKCalpha and PKCbeta, whereas the novel isoform, PKC, negatively regulates these events. PKCdelta also negatively regulates thrombus formation but not alpha-granule secretion. In addition, we demonstrate for the first time that individual PKC isoforms differentially regulate platelet calcium signaling and exposure of phosphatidylserine under flow. Although platelet deficient in PKCalpha or PKCbeta showed reduced calcium signaling and phosphatidylserine exposure, these responses were enhanced in the absence of PKC. In summary therefore, this direct comparison between individual subtypes of PKC, by standardized methodology under flow conditions, reveals that the four major PKCs expressed in platelets play distinct non-redundant roles, where conventional PKCs promote and novel PKCs inhibit thrombus formation on collagen.

Lipid rafts modulate the activation but not the maintenance of store-operated Ca(2+) entry

Different studies have reported that proteins involved in Ca(2+) entry are localized in discrete plasma membrane domains known as lipid rafts, which have been suggested to support store-operated Ca(2+) entry by facilitating STIM1 clustering in endoplasmic reticulum-plasma membrane junctions as well as the interaction of STIM1 with TRPC1. Here we report that treatment of HEK293 cells with thapsigargin (TG) results in the activation of Ca(2+) entry with two components, an early, La(3+)-sensitive, component and a late component that shows both La(3+)-sensitive and -insensitive constituents. Preincubation with methyl-beta-cyclodextrin (MbetaCD) prevented TG-induced activation of Ca(2+) entry but, in contrast, enhanced this process after its activation. Addition of MbetaCD after store depletion did not modify the La(3+)-sensitive store-operated divalent cation entry but increased La(3+)-insensitive non-capacitative Ca(2+) entry. Cell stimulation with TG results in a transient increase in Orai1 co-immunoprecipitation with STIM1, TRPC1 and TRPC6. TG-induced association of these proteins was significantly attenuated by preincubation for 30 min with MbetaCD, without altering surface expression of Orai1 or TRPCs. In contrast, the association of Orai1 with STIM1 or TRPC1 was unaffected when MbetaCD was added after store depletion with TG. Addition of MbetaCD to TG-treated cells promoted dissociation between Orai1 and TRPC6, as well as non-capacitative Ca(2+) entry. TRPC6 expression silencing indicates that MbetaCD-enhanced non-capacitative Ca(2+) entry was mediated by TRPC6. In conclusion, lipid raft domains are necessary for the activation but not the maintenance of SOCE probably due to the support of the formation of Ca(2+) signalling complexes involving Orai1, TRPCs and STIM1.

Diverse effects of ethanol on Ca2+ entry and subsequent aggregation of platelets

Although alcohol is known to inhibit platelet aggregation, and transplasmalemmal Ca(2+) entry is profoundly involved in platelet aggregation, there has been limited knowledge about the relationship between alcohol and Ca(2+) entry. The purpose of this study was to determine whether and how ethanol in vitro affects Ca(2+) entry through different pathways and the subsequent aggregation of platelets. Thapsigargin, 1-oleoyl-2-acetyl-sn-glycerol (OAG), and thrombin were used to stimulate human platelets. Ca(2)(+) entry and the subsequent aggregatory responses of platelets were measured by spectrofluorometry using fura-2/AM as an indicator and the light transmission method, respectively. Thapsigargin-induced Ca(2+) entry and the following platelet aggregation were significantly inhibited by ethanol at concentrations of 0.5% or more. OAG-induced Ca(2+) entry was significantly augmented by ethanol at concentrations of 0.5% or more, whereas platelet aggregation by OAG was significantly inhibited by ethanol at concentrations of 0.5 % or more. Thrombin-induced Ca(2+) entry was not significantly affected by ethanol up to 2%, whereas platelet aggregation by thrombin was markedly inhibited by ethanol at concentrations of 0.5% or more. Thrombin-induced Ca(2+) entry in the presence of SKF-96365 was augmented by pretreatment with ethanol. Ethanol in vitro showed diverse effects on the different Ca(2+) entry pathways of platelets, whereas aggregatory responses induced by activation of the different Ca(2+) entry pathways of platelets were all inhibited by ethanol. These results suggest that ethanol inhibits platelet aggregation mainly via a mechanism(s) other than transplasmalemmal Ca(2+) entry.

Platelet adhesion and intracellular calcium levels in antigen-challenged rats

There is considerable evidence that platelet activation occurs in allergic airways diseases. In this study we aimed to investigate platelet adhesion to immobilized fibrinogen and intracellular calcium levels in a rat model of allergic inflammation. Male Wistar rats were challenged with ovalbumin (OVA). At 30 min to 24h after OVA-challenge, assays of platelet adhesion to immobilized fibrinogen and intracellular calcium levels using fura 2-AM loaded platelets were performed. The serum levels of IgE were approximately 5-fold greater in OVA-sensitized rats. A marked eosinophil influx in bronchoalveolar lavage (BAL) fluid of OVA-challenged rats at 24h after OVA-challenge was also seen. OVA-challenge resulted in a marked thrombocytopenia, as observed within 12h after OVA-challenge. The agonists ADP (0.5-50 microM) and thrombin (30-100 mU/ml) concentration-dependently increased platelet adhesion to immobilized fibrinogen. At an early time after OVA-challenge (30 min), platelets exhibited greater platelet adhesion compared with the non-sensitized group, whereas at a late time (24h) they exhibited lower platelet adhesion to both agonists. Moreover, at 30 min after OVA-challenge, intracellular calcium levels to ADP (20 microM) and thrombin (100 mU/ml)-activated platelets were greater compared with non-challenged rats. As opposed, at 24h after OVA challenge, a lower intracellular calcium level to ADP- and thrombin-activated platelets was observed. In conclusion, OVA-challenge in rats promotes a biphasic response in platelet adhesion consisting of an increased adhesion and intracellular calcium levels at an early phase (30 min), which progress to a reduction in adhesion and intracellular calcium levels at a late time (24h) after antigen challenge.

Acetylsalicylic acid enhances purinergic receptor-mediated outward currents in rat megakaryocytes

Purinergic receptor activation increases cytosolic Ca2+concentration in a fluctuating fashion, triggering oscillatory outward Ca2+-activated K+currents in rat megakaryocytes (MKs). Whole cell and nystatin-perforated patch-clamp techniques were used to analyze changes in ionic conductance in MK with acetylsalicylic acid (ASA), a cyclooxygenase-1 inhibitor and antithrombotic agent. MKs are a model for platelet reactivity, particularly in ASA treatment failure (ASA resistance). Freshly isolated MKs were incubated 30 min in the absence or presence of 1 mM ASA. Using a K+-rich internal solution, we recorded outward currents in response to 10 μM ATP, 10 μM ADP, and 5 μM 2-methyl-thio-ADP (2MeSADP) in the voltage-clamp mode. Agonist-induced currents decreased in amplitude over time, but this decline was attenuated by ASA in both continuous and repeated agonist challenge, indicating increased MK reactivity with ASA treatment. In separate experiments, heterologous desensitization was observed when MKs were stimulated with ADP after exposure to a thromboxane receptor agonist (U46619), indicating cross talk between thromboxane and purinergic pathways. Different cells, treated with ASA or MRS2179 (P2Y1 receptor antagonist), were stimulated with 2MeSADP. The dose-response curve was shifted to the left in both cases, suggesting increased MK reactivity. ASA also caused an increased interval between currents (delay). ASA attenuated desensitization of purinergic receptors and increased delay, again suggesting cross talk between purinergic and thromboxane pathways. These findings may be relevant to ASA resistance, because individual variations in sensitivity to the multiple effects of ASA on signaling pathways could result in insensitivity to its antiplatelet effects in some patients.

Cordycepin (3‘-deoxyadenosine) inhibits human platelet aggregation induced by U46619, a TXA2 analogue

Cordycepin (3′-deoxyadenosine), which comes from Cordyceps militaris, the Chinese medicinal fungal genus Cordyceps, is known to have anti-tumour activity. In this study, we investigated the novel effect of cordycepin on human platelet aggregation that was induced by U46619, a thromboxane A2 (TXA2) analogue. TXA2 is an aggregation-inducing autacoidal molecule that is produced in various agonist-activated platelets. Cordycepin completely inhibited U46619-induced platelet aggregation and simultaneously reduced cytosolic free Ca2+ ([Ca2+]i), which was increased by U46619 (5 μM) up to 66%. Furthermore, the U46619-stimulated phosphorylation of Ca2+-dependent proteins (20 kDa of a myosin light chain and 47 kDa of pleckstrin) was strongly inhibited by cordycepin. These results suggest that cordycepin may have a beneficial effect on autacoidal TXA2-mediated thrombotic diseases by inhibiting TXA2-induced platelet aggregation via suppression of the Ca2+ level.

Inhibitory mechanisms of dihydroginsenoside Rg3 in platelet aggregation: Critical roles of ERK2 and cAMP

Ginsenoside Rg3, a single ginseng saponin, is known to be a major anti-platelet component of protopanaxadiol that is isolated from Korean red ginseng. In this study, we investigated whether dihydroginsenoside Rg3, a stable chemical derivative of ginsenoside Rg3, also demonstrated anti-platelet activity. Dihydroginsenoside Rg3 inhibited thrombin-induced platelet aggregation in a concentration-dependent manner with an IC50 (concentration producing 50% inhibition) of 18.8 ± 0.4 μM. Ginsenoside Rg3 inhibited platelet aggregation which was induced by thrombin (0.1 U mL−1) with an IC50 of 40.2 ± 0.9 μM. We next determined whether dihydroginsenoside Rg3 affected different types of ligand-induced platelet aggregation. We found that dihydroginsenoside Rg3 inhibited collagen-induced platelet aggregation with an IC50 of 20.0 ± 0.9 μM. To elucidate the inhibitory mechanism of dihydroginsenoside Rg3 on aggregation, we analysed its downstream signalling pathway. It was interesting to note that dihydroginsenoside Rg3 elevated cyclic AMP production in resting platelets, but did not affect cyclic GMP production. In addition, we found that dihydroginsenoside Rg3 potently suppressed phosphorylation of extracellular signal-regulated kinase 2 (ERK2), which was stimulated by collagen (2.5 μg mL−1), but not of p38 mitogen-activated protein kinase. Taken together, our results indicate that dihydroginsenoside Rg3 potently inhibited platelet aggregation via the modulation of downstream signalling components such as cAMP and ERK2.

Store-operated Ca2+ entry is sensitive to the extracellular Ca2+ concentration through plasma membrane STIM1

Store-operated Ca(2+) entry (SOCE) is a major mechanism for Ca(2+) influx in platelets and other cells activated by a reduction in Ca(2+) concentration in the intracellular stores. SOCE has been reported to be regulated by extracellular Ca(2+), although the underlying mechanism remains unclear. Here we have examined the involvement of plasma membrane-located STIM1 (PM-STIM1) in the regulation of SOCE by extracellular Ca(2+). Treatment of platelets with the SERCA inhibitor thapsigargin (TG) induced Mn(2+) entry, which was inhibited by extracellular Ca(2+) in a concentration-dependent manner. Incubation of platelets with a specific antibody, which recognizes the extracellular amino acid sequence 25-139 of PM-STIM1 that contains the Ca(2+)-binding domain, prevented the inactivation of Ca(2+) entry induced by extracellular Ca(2+). TG induced translocation of STIM1 to the plasma membrane (PM), an event that was found to be Ca(2+)-dependent. In addition, TG stimulated association of PM-STIM1 with Orai1, an event that was not prevented by stabilization of the membrane cytoskeleton using jasplakinolide. These findings suggest that PM-STIM1 is important for the inactivation of SOCE by extracellular Ca(2+), an event that is likely to be mediated by interaction with Orai1.

Calcium signaling in platelets

Agonist-induced elevation in cytosolic Ca2+ concentrations is essential for platelet activation in hemostasis and thrombosis. It occurs through Ca2+ release from intracellular stores and Ca2+ entry through the plasma membrane (PM). Ca2+ store release is a well-established process involving phospholipase (PL)C-mediated production of inositol-1,4,5-trisphosphate (IP3), which in turn releases Ca2+ from the intracellular stores through IP3 receptor channels. In contrast, the mechanisms controlling Ca2+ entry and the significance of this process for platelet activation have been elucidated only very recently. In platelets, as in other non-excitable cells, the major way of Ca2+ entry involves the agonist-induced release of cytosolic sequestered Ca2+ followed by Ca2+ influx through the PM, a process referred to as store-operated calcium entry (SOCE). It is now clear that stromal interaction molecule 1 (STIM1), a Ca2+ sensor molecule in intracellular stores, and the four transmembrane channel protein Orai1 are the key players in platelet SOCE. The other major Ca2+ entry mechanism is mediated by the direct receptor-operated calcium (ROC) channel, P2X1. Besides these, canonical transient receptor potential channel (TRPC) 6 mediates Ca2+ entry through the PM. This review summarizes the current knowledge of platelet Ca2+ homeostasis with a focus on the newly identified Ca2+ entry mechanisms.

Dysregulation of Ca2+ movement in platelets from patients with acute ischaemic stroke

1. Platelets play a pivotal role during acute ischaemic stroke. An increase in cytosolic Ca(2+) concentrations ([Ca(2+)](i)) triggers intracellular signal transduction, leading to platelet aggregation and thrombosis. In the present study, we examined the differences between platelets from acute ischaemic stroke patients and at-risk controls in terms of the increase in platelet [Ca(2+)](i). 2. Thirty-one patients with acute ischaemic stroke and 27 at-risk controls were enrolled in the present study. Platelet [Ca(2+)](i) was measured using the fluorescent dye fura-2 after stimulation with 100 micromol/L arachidonic acid (AA), 10 micromol/L ADP, 1 micromol/L platelet-activation factor (PAF) and 0.1 U/mL thrombin. 3. Basal [Ca(2+)](i) was higher in the stroke group compared with at-risk controls, irrespective of the presence or absence of extracellular Ca(2+). In Ca(2+)-containing medium, both PAF and ADP, but not AA and thrombin, significantly increased platelet [Ca(2+)](i) in the stroke group compared with the at-risk controls. However, in Ca(2+)-free medium, only PAF significantly increased platelet [Ca(2+)](i) in the stroke group compared with the at-risk controls. Basal [Ca(2+)](i) and PAF-induced platelet [Ca(2+)](i) increases were still higher in the stroke group at the subacute stage than in the at-risk controls. 4. The results of the present study provide direct evidence that Ca(2+) signalling in platelets from acute ischaemic stroke patients was altered in response to particular stimuli. The dysregulation of Ca(2+) movement in platelets may persist up to the subacute stage of ischaemic stroke.

Vardenafil, but not sildenafil or tadalafil, has calcium-channel blocking activity in rabbit isolated pulmonary artery and human washed platelets

BACKGROUND AND PURPOSE

Phosphodiesterase type-5 (PDE5) inhibitors constitute a novel and important therapeutic option for the treatment of pulmonary hypertension. The effects of the PDE5 inhibitors sildenafil, tadalafil and vardenafil on rabbit isolated pulmonary artery ring preparations and on intracellular Ca2+ concentration of thrombin-stimulated human platelets were investigated.

EXPERIMENTAL APPROACH

Rabbit pulmonary artery rings were mounted in 10 mL organ bath containing Krebs solution. Tissues were connected to force-displacement transducers, and changes in isometric force were recorded. Ca2+ flux in human washed platelets was measured.

KEY RESULTS

Sildenafil, tadalafil and vardenafil (0.0001-10 microM) concentration-dependently relaxed endothelium-intact and endothelium-denuded pulmonary artery rings. Endothelium denudation caused rightward shifts in the concentration-response curves to sildenafil, tadalafil and vardenafil (9-, 12- and 123-fold, respectively). Incubation with N(omega)-nitro-L-arginine methyl ester (100 microM) or ODQ (1H-[1,2,4] oxadiazolo [4,3,-a]quinoxalin-1-one) (10 microM) caused similar reductions of PDE5-induced vasorelaxations in intact rings. Sildenafil and tadalafil did not affect the phenylephrine-induced contractions, whereas vardenafil reduced the maximal responses, and shifted the phenylephrine-induced contraction curves to the right in endothelium-denuded rings (5- and 19-fold for 1 and 10 microM, respectively). Vardenafil (but neither sildenafil nor tadalafil) caused a marked rightward shift and a decrease of maximal contractile response to CaCl2. Vardenafil, but neither sildenafil nor tadalafil, significantly reduced the Ca2+ mobilization and Ca2+ influx in thrombin-stimulated washed platelets.

CONCLUSIONS AND IMPLICATIONS

Our results indicate that vardenafil, in contrast to sildenafil or tadalafil, also blocked Ca2+ fluxes, thus enhancing its vasorelaxation of the pulmonary artery.

The mechanism of anti-platelet activity of davallialactone: involvement of intracellular calcium ions, extracellular signal-regulated kinase 2 and p38 mitogen-activated protein kinase

This study was designed to investigate the effect of davallialactone, which was isolated from the mushroom Inonotus xeranticus, on platelet aggregation induced by collagen, thrombin and ADP. We found that davallialactone dose-dependently inhibited platelet aggregation that was stimulated either by collagen (2.5 microg/ml), a potent ligand of integrin alpha2beta1 and glycoprotein VI, or by thrombin (0.1U/ml), a potent agonist of the protease-activated receptors (PARs) PAR1 and PAR3. In addition, davallialactone inhibited platelet aggregation induced by ADP, an agonist of P2Y receptor. To understand the mechanism of anti-platelet activity, we determined whether davallialactone affected the downstream signaling in collagen-activated platelets. Using the fura-2/AM fluorometric assay, we found that davallialactone dose-dependently inhibited intracellular calcium concentration levels ([Ca2+]i). Moreover, davallialactone inhibited the phosphorylation of extracellular signal-regulated protein kinase (ERK)-2 and p38 mitogen-activated protein kinase (MAPK), in a dose-dependent manner. The tyrosine phosphorylation of 60 and 85kDa proteins, which were activated by collagen, were differentially inhibited by davallialactone. Taken together, these data suggest that davallialactone may have potential anti-platelet aggregation activity via suppression of intracellular downstream signaling pathways.

Functional relevance of the de novo coupling between hTRPC1 and type II IP3 receptor in store-operated Ca2+ entry in human platelets

Store-operated Ca2+ entry (SOCE), a major mechanism for Ca2+ entry in non-excitable cells, is regulated by the filling state of the intracellular Ca2+ stores. We have previously reported that a de novo conformational coupling between the type II IP3 receptor (IP3RII) and hTRPC1 channel occurs after depletion of the intracellular Ca2+ stores in human platelets, which might be involved in the activation of SOCE in these cells. Here we present for the first time direct evidence for the functional relevance of the coupling between hTRPC1 and IP3RII in SOCE in human platelets. Our data suggest that at least two pathways may contribute to SOCE in these cells. An early component, insensitive to cytochalasin D (Cyt D), is followed by a late component which is sensitive to Cyt D. Introduction of a peptide corresponding to IP3RII(317-334) (IP3BD-peptide(317-334)) in the cells by electrotransjection impairs the coupling between hTRPC1 and IP3RII but not the interaction between hTRPC1 and STIM1 induced by store depletion. Coimmunoprecipitation experiments indicated that endogenously expressed hTRPC1 interacts with the IP3BD-peptide(317-334). Electrotransjection of cells with IP3BD-peptide(317-334), significantly attenuated the late stage of Ca2+ and Mn2+ entry induced by 10 nM thapsigargin (TG) or 20 microM 2,5-di-(tert-butyl)-1,4-hydroquinone (TBHQ), providing evidence for a functional role of the de novo coupling between hTRPC1 and IP3RII in the activation of SOCE in human platelets.

Reassociation and translocation of glycoprotein IIB-IIIA in EDTA-treated human platelets

Platelet membrane glycoproteins IIb and IIIa form a calcium-dependent heterodimer that plays a key role in platelet adhesion and aggregation. The present objective was to measure the dissociation and reassociation of GPIIb-IIIa by flow cytometric analysis of platelets labelled with mAbs specific for the glycoprotein complex or each monomer. In agreement with previous studies, EDTA chelation of extracellular calcium, [Ca2+]o, dissociated the heterodimer in a time and temperature dependent manner. Agonist stimulation of EDTA-treated platelets induced subunits to reassociate with the following order of potency: thrombin > collagen > ADP. Two-fold increases in GPIIb-IIIa and GPIIb indicate that thrombin caused reassociation of surface subunits and concurrent translocation of complexes from intracellular pools. The latter was partially inhibited by cytochalasin B thus indicating that a subpopulation of GPIIb-IIIa required cytoskeletal remodelling for translocation. Surface GPIIIa as reported by anti-CD61 declined more and upregulated less compared with GPIIb-IIIa or GPIIb. Results suggest that EDTA incubation might have altered the conformation of this epitope and decreased mAb binding. Collagen induced GPIIb-IIIa reassociation but not translocation of cryptic complexes. BAPTA suppression of rises in cytosolic calcium concentration or low [Ca2+]o inhibited GPIIb-IIIa reassociation, thus indicating that this reaction was driven by signal transduction. Thrombin and collagen induced a comparable level of aggregation of EDTA-treated platelets despite a 3-fold difference in cell surface GPIIb-IIIa. It is concluded that the effects of EDTA on GPIIb-IIIa dissociation and loss of adhesive functions are largely but not completely reversible.

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.

Thrombin-initiated platelet activation in vivo is vWF independent during thrombus formation in a laser injury model

Adhesion of platelets to an injured vessel wall and platelet activation are critical events in the formation of a thrombus. Of the agonists involved in platelet activation, thrombin, collagen, and vWF are known to induce in vitro calcium mobilization in platelets. Using a calcium-sensitive fluorochrome and digital multichannel intravital microscopy to image unstimulated and stimulated platelets, calcium mobilization was monitored as a reporter of platelet activation (as distinct from platelet accumulation) during thrombus formation in live mice. In the absence of vWF, platelet activation was normal, but platelet adherence and aggregation were attenuated during thrombus formation following laser-induced injury in the cremaster muscle microcirculation. In WT mice treated with lepirudin, platelet activation was blocked, and platelet adherence and aggregation were inhibited. The kinetics of platelet activation and platelet accumulation were similar in FcRgamma(-/-) mice lacking glycoprotein VI (GPVI), GPVI-depleted mice, and WT mice. Our results indicate that the tissue factor-mediated pathway of thrombin generation, but not the collagen-induced GPVI-mediated pathway, is the major pathway leading to platelet activation after laser-induced injury under the conditions employed. In the tissue factor-mediated pathway, vWF plays a role in platelet accumulation during thrombus formation but is not required for platelet activation in vivo.

Altered calcium signalling in platelets from bile-duct-ligated rats

In the present study, we have analysed the mechanisms of Ca2+ entry and release in platelets obtained from BDL (bile-duct-ligated) rats, 11–13 days and 4 weeks after surgery. Platelets were washed and loaded with fura-2, and [Ca2+]i (cytosolic Ca2+ concentration) was determined in cell suspensions by means of fluorescence spectroscopy. Basal [Ca2+]i was similar in platelets from BDL rats compared with those from their respective controls, both in the absence and presence of extracellular Ca2+. Platelet stimulation with thrombin in the absence and presence of extracellular Ca2+ induced a rapid rise in [Ca2+]i that was of greater magnitude in platelets from BDL rats than in controls. Ca2+ storage was significantly elevated in platelets from BDL rats, as well as the activity of SERCA (sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase). Capacitative Ca2+ entry, as evaluated by inhibition of SERCA with thapsigargin, was also altered in platelets from BDL rats, having lower rates of Ca2+ entry. In conclusion, chronic BDL alters intracellular Ca2+ homoeostasis in platelets, such that an enhanced Ca2+ release is evoked by thrombin, which may be due to an increased amount of Ca2+ stored in the intracellular organelles and secondary to an enhanced activity of SERCA. These alterations are already evident before cirrhosis has completely developed and occurs during the cholestasis phase.

Dynamics of Calcium Fluxes in Nonexcitable Cells: Mathematical Modeling

Mathematical models simulating the dynamics of calcium redistribution (elicited by experimental interference with the pathways of calcium fluxes) in cellular compartments have been developed, based on a minimal scheme of the pathways of calcium fluxes in nonexcitable cells suspended in calcium-free medium. The models are consistent with available experimental data. All parameters are quantitatively related to the intrinsic properties of calcium adenosine triphosphatases (ATPases) and cellular membranes; there is no interdependence between the parameters. The models can be used as the basis for quantitative analysis and interpretation of experimental data. The activities of plasma membrane and sarcoendoplasmic reticulum calcium ATPases (PMCA and SERCAs) are governed by different mechanisms. PMCA is likely to undergo transitions from inactive to active to "dormant" (not identical to the initial) and back to inactive states, the mean duration of the cycle lasting for minutes or longer. The sequence of the transitions is initiated, presumably, by an increase in cytosolic calcium concentration. The transition of PMCA from inactive to active (at least at low rates of increase in cytosolic calcium concentration) is likely to be slower than that from active to dormant. SERCA, presumably, transits from inactive to active state in response to increases in calcium leakage from calcium stores. Whereas PMCA extrudes excess calcium (a definite quantity of it) in a short pulse, SERCA retakes calcium back into the stores permanently at a high rate. The models presented here may be the best means for the moment to quantitatively relate the dynamics of calcium fluxes in nonexcitable cells with known or putative properties of the mechanisms underlying activation of calcium ATPases.

Inhibition of PAR4 Signaling Mediates Ethanol-Induced Attenuation of Platelet Function In Vitro

BACKGROUND

Reduction in coronary heart disease morbidity in response to moderate consumption of alcoholic beverages may be partly mediated by ethanol-induced inhibition of platelet function. However, the precise mechanisms by which ethanol modulates platelet activation induced by thrombin, which plays a central role in hemostasis, remain unclear. The goal of this study was to investigate ethanol-induced changes in platelet function and clarify the underlying mechanisms including PAR1 and PAR4 activity and [Ca2+]i dynamics in vitro.

METHODS

Platelet aggregation, increase in intracellular calcium ([Ca2+]i), and release of platelet factor 4 and beta-thromboglobulin induced by alpha-thrombin, PAR1-agonist peptide (AP), or PAR4-AP were assessed in the presence or absence of ethanol.

RESULTS

Ethanol exposure inhibited low-dose thrombin (0.5 nM)-induced aggregation but not an increase in [Ca2+]i. In contrast, ethanol had no effect on high-dose thrombin (10 nM)-induced aggregation or the [Ca2+]i increase. Ethanol did not significantly inhibit thrombin-induced release of platelet factor 4 and beta-thromboglobulin. Ethanol reduced PAR1-AP-induced aggregation, but did not affect the spike form of [Ca2+]i increase. In contrast, ethanol inhibited the increase in [Ca2+]i as well as the aggregation in response to PAR4-AP and resulted in delayed [Ca2+]i peak time. Furthermore, ethanol inhibited both PAR1-AP- and PAR4-AP-induced platelet factor 4 and beta-thromboglobulin release.

CONCLUSIONS

These data suggest that ethanol inhibits platelet aggregation via inhibition of PAR4 signaling and subsequent inhibition of Ca2+ influx and granule release. This phenomenon may contribute to the reduction in coronary heart disease morbidity in response to consumption of alcoholic beverages.

Development and validation of a platelet calcium flux assay using a fluorescent imaging plate reader

Calcium signaling in platelets is an important physiological response to various aggregation stimuli. Loading platelets with various fluorescent dyes and measuring the change in calcium concentration using a spectrofluorometer has been the traditional approach to studying calcium signaling. This method suffers from the need for large platelet samples and a decrease in total fluorescence signal with time due to photobleaching. Therefore, it is rarely used to measure the quantitative effect of an agonist or antagonist on calcium signaling. Adaptation of these measurements to a fluorescent imaging plate reader (FLIPR) format allows the sample size to be reduced by 5- to 10-fold, and the microplate format allows a significant increase in throughput. Addition of the agonists to all wells simultaneously serves to normalize the total response. This article describes the first use of a FLIPR to study the calcium flux in human platelets. The IC(50) values showed a linear correlation with the K(i) for receptor binding in washed platelets. The generality of the methodology was shown by measuring EC(50) values for agonists and IC(50) values for antagonists of the platelet G protein-coupled receptor P2Y(1) and for the ion channel P2X(1).

Mechanisms of antiplatelet activity of PC-09, a newly synthesized pyridazinone derivative

In this study, we examined whether PC-09, a new pyridazinone derivative, has antiplatelet activity in vitro and further investigated the possible mechanisms involved. Pretreatment with PC-09 resulted in an inhibition on rabbit platelet aggregation and ATP release induced by arachidonic acid, collagen or thrombin, with the IC(50) values of 5.4 to 76.8 muM. The thromboxane B(2) formation caused by collagen or thrombin was markedly inhibited by PC-09, but there was no alteration in that caused by arachidonic acid. The rise of platelet intracellular calcium level stimulated by aggregation agonists and collagen-induced platelet membrane surface glycoprotein IIb/IIIa expression was also reduced by PC-09. In addition, PC-09 itself significantly increased the cyclic AMP level through inhibiting cyclic AMP phosphodiesterase activity. These findings demonstrate that PC-09 is an inhibitor of platelet aggregation, which may be associated with mechanisms including inhibition of thromboxane A(2) formation, intracellular calcium mobilization and platelet surface GPIIb/IIIa expression accompanied by increasing cyclic AMP level.

Early caspase-3 activation independent of apoptosis is required for cellular function

A number of pro-apoptotic stimuli induce the activation of caspase-9, an initiator protease that activates executioner caspases, such as caspase-3, leading to the development of programmed cell death. Here we demonstrate that cell (platelets and pancreatic acinar cells) stimulation with agonists induces a bimodal activation of caspase-3. The early caspase-3 activation occurs within 1 min of stimulation and is independent on caspase-9 or mitochondrial cytochrome c release suggesting that is a non-apoptotic event. The ability of agonists to induce early activation of caspase-3 is similar to that observed for other physiological processes. Activation of caspase-3 by physiological concentrations of cellular agonists, including thrombin or CCK-8, is independent of rises in cytosolic calcium concentration but requires PKC activation, and is necessary for agonist-induced activation of the tyrosine kinases Btk and pp60src and for several cellular functions, including store-operated calcium entry, platelet aggregation, or pancreatic secretion. Thus, early activation of caspase-3 seems to be a non-apoptotic event required for cellular function.

Cytoplasmic free calcium mobilization in platelets, expression of P-selectin, phosphatidylserine, and microparticle formation, measured by whole blood flow cytometry, in hypertensive patients. Effect of doxazosin GITS

The effects of doxazosin on expression of CD62 (P-selectin) and phosphatidylserine on platelet membrane and platelet calcium flux were studied in 50 uncomplicated essential hypertensive patients (World Health Organization stages 1-2) and 80 normotensive control subjects, matched for age, sex, and cardiovascular risk factors. Hypertensive patients showed greater in vivo platelet activation at baseline than control patients (percentage of CD62-positive platelets, 4.1+/-2.2% versus 2.4+/-1.5%, p<0.001; percentage of phosphatidylserine-positive platelets, 0.8+/-0.5% versus 0.5+/-0.3%, p<0.001). Increased platelet activation was associated with significant changes in the mobilization of free intraplatelet calcium, evaluated by a whole blood flow cytometric kinetic method. With this method, an arbitrary Ca(2+) mobilization index was defined as the ratio of cytoplasmic free calcium before activation with thrombin to the slope of the calcium removal rate following the action of the agonist. This index was significantly higher in untreated hypertensive patients than in normotensive controls (0.12+/-0.06 versus 0.05+/-0.08, p<0.001). Treatment of hypertensive patients with doxazosin gastrointestinal therapeutic system (4 mg/day as a single dose) for 2 months normalized both platelet activation and Ca(2+) mobilization. Changes in the expression of CD62 and phosphatidylserine in the platelet membrane after treatment with doxazosin gastrointestinal therapeutic system may be related to normalization of the kinetics of cytoplasmic free Ca(2+). Normalization of platelet activation may represent an additional beneficial effect to the known antihypertensive action of doxazosin gastrointestinal therapeutic system.

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.

Agonist-induced calcium response in single human platelets assayed in a microfluidic device

To facilitate drug discovery directed toward platelet-specific targets, we developed a platelet isolation and fluorophore-loading method that yields functionally responsive platelets in which we were able to detect agonist-induced calcium flux using a microfluidics-based screening platform. The platelet preparation protocol was designed to minimize preparation-induced platelet activation and to optimize signal strength. Measurement of platelet activation, as monitored by ratiometric determination of agonist-induced calcium flux in fluor-loaded human platelets, was optimized in a macrosample cuvette format in preparation for detection in a microfluidic chip-based assay. For the microfluidic device used in these studies, a cell density of 1 to 2 x 10(6) platelets per milliliter and a nominal flow rate of 5 to 10 nl per second provided optimal event resolution of 5 to 20 platelets traversing the detection volume per unit time. Platelets responded in a dose-dependent manner to adenosine diphosphate and protease-activating peptide (PAR) 1 thrombin receptor-activating peptide (TRAP). The work presented here constitutes proof-of-principle experiments demonstrating the enabling application of a microfluidic device to conduct high-throughput signaling studies and drug discovery screening against human platelet targets.

Effects of DK-002, a synthesized (6aS,cis)-9,10-Dimethoxy-7,11b-dihydro-indeno[2,1-c]chromene-3,6a-diol, on platelet activity

In the present study, the mechanism of antiplatelet activity of DK-002, a synthesized (6aS,cis)-9,10-Dimethoxy-7,11b-dihydro-indeno[2,1-c]chromene-3,6a-diol, was investigated. DK-002 inhibited the thrombin, collagen, and ADP-induced rat platelet aggregation in a concentration-dependent manner, with IC50 values of 120, 27, and 47 microM, respectively. DK-002 also inhibited thrombin-induced dense granule secretion, thromboxane A2 synthesis, and [Ca2+]i elevation in platelets. DK-002 did not show any significant effect on ADP-induced inhibition of cyclic AMP elevation by prostaglandin E1, but DK-002 was confirmed to inhibit ADP-induced [Ca2+]i elevation and shape change. DK-002 inhibited 4-bromo-A23187-induced [Ca2+]i elevation in the presence of creatine phosphate/creatine phosphokinase (CP/CPK, a ADP scavenging system) and indomethacin (a specific inhibitor of cyclooxygenase). DK-002 also inhibited Ca2+ mobilization in thrombin- or 4-bromo-A23187-stimulated platelets through its inhibitory effects on both Ca2+ release from intracellular stores and Ca2+ influx, in the presence of CP/CPK and indomethacin. Taken together, the present study shows that DK-002 has inhibitory effects on stimulation of platelets, and suggests that its antiplatelet activity might be related to the inhibitory mechanism on Ca2+ mobilization in stimulated platelets.