Cytosolic calcium in platelet activation

Calcium‐dependent antimicrobials: Nature‐inspired materials and designs

Bacterial infection remains a major complication answering for the failures of various implantable medical devices. Tremendous extraordinary advances have been published in the design and synthesis of antimicrobial materials addressing this issue; however, the clinical translation has largely been blocked due to the challenge of balancing the efficacy and safety of these materials. Here, calcium's biochemical features, natural roles in pathogens and the immune systems, and advanced uses in infection medications are illuminated, showing calcium is a promising target for developing implantable devices with less infection tendency. The paper gives a historical overview of biomedical uses of calcium and summarizes calcium's merits in coordination, hydration, ionization, and stereochemistry for acting as a structural former or trigger in biological systems. It focuses on the involvement of calcium in pathogens' integrity, motility, and metabolism maintenance, outlining the potential antimicrobial targets for calcium. It addresses calcium's uses in the immune systems that the authors can learn from for antimicrobial synthesis. Additionally, the advances in calcium's uses in infection medications are highlighted to sketch the future directions for developing implantable antimicrobial materials. In conclusion, calcium is at the nexus of antimicrobial defense, and future works on taking advantage of calcium in antimicrobial developments are promising in clinical translation.

Using extracellular calcium concentration and electric pulse conditions to tune platelet-rich plasma growth factor release and clotting

Platelet-rich plasma (PRP) is an emerging autologous biologic method for wound healing. Clinicians apply PRP either topically (where it is activated ex-vivo before treatment by adding an external agent to trigger clotting and the release of growth factors that facilitate wound healing) or through injection (where it is activated in vivo at the injury site with no prior activation before injection). Because topical PRP activation typically utilizes bovine thrombin, which has significant potential side effects and high costs, recent studies have assessed the efficacy of combining extracellular calcium (EC) and electric pulses (EPs) to activate PRP. The potential to apply this novel technique to PRP both topically and internally via injection raises the question about the ability to tune the clotting time and growth factor release for a given application. While previous studies have assessed the impact of applying EPs of various durations either directly (conductive coupling) or indirectly (capacitive coupling) to PRP containing EC, no studies have assessed the tunability of this activation based on modifying EP parameters, EP delivery method (conductive or capacitive coupling), and the EC concentration. We hypothesize that tuning these parameters will modify intracellular calcium uptake to permit the control of growth factor release and clotting time, which are critical for optimizing PRP for either topical or internal clinical applications. A pilot study for a single donor demonstrates the potential for tunability as a function of the intensity of membrane manipulation and calcium concentration, which facilitate the increase of cytosolic calcium. This motivates future studies assessing EC and EP optimization and in vivo studies to determine the overall efficacy of this tunability for wound healing.

Phosphorylation of CalDAG-GEFI by protein kinase A prevents Rap1b activation

BACKGROUND

Signaling via protein kinase A (PKA) and protein kinase G (PKG) is critical for maintaining platelets in the resting state. Both kinases down-regulate the activity of the small GTPase Rap1b, a critical signaling switch for integrin activation and platelet aggregation. However, the mechanism of Rap1b regulation by PKA and PKG is largely unknown.

OBJECTIVE

To identify the PKA phosphorylation sites in calcium and diacylglycerol-regulated guanine nucleotide exchange factor I (CalDAG-GEFI), the main GEF for Rap1b in platelets, and the effect of CalDAG-GEFI phosphorylation in Rap1b activation.

METHODS

The phosphorylation sites in CalDAG-GEFI were identified by radio-active phosphate incorporation assay and mass spectrometry. Phospho-antibody was developed to detect CalDAG-GEFI phosphorylation in Western blots. Rap1b activation was detected by Rap1-GTP pull-down assay.

RESULTS

S587 was identified as the major PKA phosphorylation site in CalDAG-GEFI, while S116/117 was weakly phosphorylated. Phosphorylation of S587 correlated with the inhibitory effect of PKA on Rap1b activation in platelets. In HEK293 cells, expression of a phospho-mimetic mutant of CalDAG-GEFI (S587D) abolished agonist-induced Rap1b activation. Mutation of S587 to alanine partially reversed the inhibitory effect of PKA signaling on Rap1b activation, while mutation of S116, S117 and S587 to alanine completely abolished the inhibitory effect of PKA on Rap1b activation.

CONCLUSION

Our study strongly suggests that phosphorylation of CalDAG-GEFI is a critical mechanism by which PKA controls Rap1b-dependent platelet aggregation.

Identification of a functional prostanoid-like receptor in the protozoan parasite, Trypanosoma cruzi

Trypanosoma cruzi infection in humans and experimental animals causes Chagas disease which is often accompanied by myocarditis, cardiomyopathy, and vasculopathy. T. cruzi-derived thromboxane A2 (TXA2) modulates vasculopathy and other pathophysiological features of Chagasic cardiomyopathy. Here, we provide evidence that epimastigotes, trypomastigotes, and amastigotes of T. cruzi (Brazil and Tulahuen strains) express a biologically active prostanoid receptor (PR) that is responsive to TXA2 mimetics, e.g. IBOP. This putative receptor, TcPR, is mainly localized in the flagellar membrane of the parasites and shows a similar glycosylation pattern to that of bona fide thromboxane prostanoid (TP) receptors obtained from human platelets. Furthermore, TXA2-PR signal transduction activates T. cruzi-specific MAPK pathways. While mammalian TP is a G-protein coupled receptor (GPCR); T. cruzi genome sequencing has not demonstrated any confirmed GPCRs in these parasites. Based on this genome sequencing it is likely that TcPR is unique in these protists with no counterpart in mammals. TXA2 is a potent vasoconstrictor which contributes to the pathogenesis of Chagasic cardiovascular disease. It may, however, also control parasite differentiation and proliferation in the infected host allowing the infection to progress to a chronic state.

Anti-platelet activity of diacetylated obovatol through regulating cyclooxygenase and lipoxygenase activities

Obovatol has been reported biological activities such as muscle relaxative, anti-gastric ulcer, anti-allergic and anti-bacterial activities. The present study was undertaken to investigate the effect of diacetylated obovatol, an obovatol derivative, on rabbit platelet aggregation, and their possible molecular mechanisms. Effects of diacetylated obovatol on platelet activation including aggregation and serotonin secretion were examined. In addition, we investigated the effect of diacetylated obovatol on archidonic acid and metabolites liberation and intracellular calcium mobilization. Diacetylated obovatol concentration-dependently inhibited the washed rabbit platelet aggregation induced by collagen and arachidonic acid, suggesting that diacetylated obovatol may selectively inhibits collagen- and arachidonic acid-mediated signal transduction. In accordance with these results, diacetylated obovatol showed a concentration-dependent decrease in cytosolic Ca(2+) mobilization and serotonin secretion. However, diacetylated obovatol did not inhibit arachidonic acid liberation; on the other hand, diacetylated obovatol inhibited the formation of arachidonic acid metabolites such as thromboxane A(2), prostaglandin D(2) and 12-HETE through interfering with cyclooxygenase (COX)-1 and lipoxygenase (LOX) activities. The results demonstrated that diacetylated obovatol has antiplatelet activities through inhibition of COX-1 and LOX activities.

JJK694, a synthesized obovatol derivative, inhibits platelet activation by suppressing cyclooxygenase and lipoxygenase activities

Obovatol has various biological activities, including anti-proliferative, neurotrophic, anti-fibrillogenic, anti-platelet, anti-fungal and anti-inflammatory activities. In this study, we investigated the effects of JJK694, a synthesized obovatol derivative, on rabbit platelet activation and its molecular mechanisms. JJK694 significantly inhibited washed rabbit platelet aggregation and serotonin secretion induced by collagen and arachidonic acid, but had little effect on thrombin- or U46619-induced aggregation. These results suggest that JJK694 selectively inhibits collagen- and arachidonic acid-mediated signaling. JJK694 also showed a concentration-dependent decrease in cytosolic Ca(2+) mobilization, but it had no effect on arachidonic acid liberation. On the other hand, it significantly inhibited the formation of arachidonic acid metabolites, including thromboxane A(2) (TXA(2)), prostaglandin D(2), and 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE), by suppression of cyclooxygenase (COX)-1 and lipoxygenase (LOX) activities. These results indicate that JJK694 hasanti-platelet activities through inhibition of arachidonic acid metabolite production by suppression of COX-1 and LOX activities.

Historical perspective and future directions in platelet research

Platelets are a remarkable mammalian adaptation that are required for human survival by virtue of their ability to prevent and arrest bleeding. Ironically, however, in the past century, the platelets' hemostatic activity became maladaptive for the increasingly large percentage of individuals who develop age-dependent progressive atherosclerosis. As a result, platelets also make a major contribution to ischemic thrombotic vascular disease, the leading cause of death worldwide. In this brief review, I provide historical descriptions of a highly selected group of topics to provide a framework for understanding our current knowledge and the trends that are likely to continue into the future of platelet research. For convenience, I separate the eras of platelet research into the "Descriptive Period" extending from ~1880-1960 and the "Mechanistic Period" encompassing the past ~50 years since 1960. We currently are reaching yet another inflection point, as there is a major shift from a focus on traditional biochemistry and cell and molecular biology to an era of single molecule biophysics, single cell biology, single cell molecular biology, structural biology, computational simulations, and the high-throughput, data-dense techniques collectively named with the "omics postfix". Given the progress made in understanding, diagnosing, and treating many rare and common platelet disorders during the past 50 years, I think it appropriate to consider it a Golden Age of Platelet Research and to recognize all of the investigators who have made important contributions to this remarkable achievement..

Platelet function and Isoprostane biology. Should isoprostanes be the newest member of the orphan-ligand family?

While there have been many reports investigating the biological activity and signaling mechanisms of isoprostanes, their role in biology, particularly in platelets, appears to still be underestimated. Moreover, whether these lipids have their own receptors is still debated, despite multiple reports that discrete receptors for isporpstanes do exist on platelets, vascular tissues, amongst others. This paper provides a review of the important literature of isoprostanes and provides reasoning that isoprostanes should be classified as orphan ligands until their receptor(s) is/are identified.

Signaling through G(alpha)13 switch region I is essential for protease-activated receptor 1-mediated human platelet shape change, aggregation, and secretion

This study investigated the involvement of Galpha(13) switch region I (SRI) in protease-activated receptor 1 (PAR1)-mediated platelet function and signaling. To this end, myristoylated peptides representing the Galpha(13) SRI (Myr-G(13)SRI(pep)) and its random counterpart were evaluated for their effects on PAR1 activation. Initial studies demonstrated that Myr-G(13)SRI(pep) and Myr-G(13)SRI(Random-pep) were equally taken up by human platelets and did not interfere with PAR1-ligand interaction. Subsequent experiments revealed that Myr-G(13)SRI(pep) specifically bound to platelet RhoA guanine nucleotide exchange factor (p115RhoGEF) and blocked PAR1-mediated RhoA activation in platelets and human embryonic kidney cells. These results suggest a direct interaction of Galpha(13) SRI with p115RhoGEF and a mechanism for Myr-G(13)SRI(pep) inhibition of RhoA activation. Platelet function studies demonstrated that Myr-G(13)SRI(pep) specifically inhibited PAR1-stimulated shape change, aggregation, and secretion in a dose-dependent manner but did not inhibit platelet activation induced by either ADP or A23187. It was also found that Myr-G(13)SRI(pep) inhibited low dose, but not high dose, thrombin-induced aggregation. Additional experiments showed that PAR1-mediated calcium mobilization was partially blocked by Myr-G(13)SRI(pep) but not by the Rho kinase inhibitor Y-27632. Finally, Myr-G(13)SRI(pep) effectively inhibited PAR1-induced stress fiber formation and cell contraction in endothelial cells. Collectively, these results suggest the following: 1) interaction of Galpha(13) SRI with p115RhoGEF is required for G(13)-mediated RhoA activation in platelets; 2) signaling through the G(13) pathway is critical for PAR1-mediated human platelet functional changes and low dose thrombin-induced aggregation; and 3) G(13) signaling elicits calcium mobilization in human platelets through a Rho kinase-independent mechanism.

[Thrombocytopathy and blood complications in uremia]

Bleeding diathesis and thrombotic tendencies are characteristic findings in patients with end-stage renal disease. The pathogenesis of uremic bleeding tendency is related to multiple dysfunctions of the platelets. The platelet numbers may be reduced slightly, while platelet turnover is increased. The reduced adhesion of platelets to the vascular subendothelial wall is due to reduction of GPIb and altered conformational changes of GPIIb/IIIa receptors. Alterations of platelet adhesion and aggregation are caused by uremic toxins, increased platelet production of NO, PGI(2), calcium and cAMP as well as renal anemia. Correction of uremic bleeding is caused by treatment of renal anemia with recombinant human erythropoietin or darbepoetin alpha, adequate dialysis, desmopressin, cryoprecipitate, tranexamic acid, or conjugated estrogens. Thrombotic complications in uremia are caused by increased platelet aggregation and hypercoagulability. Erythrocyte-platelet-aggregates, leukocyte-platelet-aggregates and platelet microparticles are found in higher percentage in uremic patients as compared to healthy individuals. The increased expression of platelet phosphatidylserine initiates phagocytosis and coagulation. Therapy with antiplatelet drugs does not reduce vascular access thrombosis but increases bleeding complications in endstage renal disease patients. Heparin-induced thrombocytopenia (HIT type II) may develop in 0-12 % of hemodialysis patients. HIT antibody positive uremic patients mostly develop only mild thrombocytopenia and only very few thrombotic complications. Substitution of heparin by hirudin, danaparoid or regional citrate anticoagulation should be decided based on each single case.

A potential role for extracellular nitric oxide generation in cGMP-independent inhibition of human platelet aggregation: biochemical and pharmacological considerations

1. Nitric oxide (NO) is a potent inhibitor of platelet activation, that inhibits the agonist-induced increase in cytosolic Ca2+ concentration through both cGMP-dependent and independent pathways. However, the NO-related (NOx) species responsible for cGMP-independent signalling in platelets is unclear. We tested the hypothesis that extracellular NO, but not NO+ or peroxynitrite, generated in the extracellular compartment is responsible for cGMP-independent inhibition of platelet activation via inhibition of Ca2+ signalling. 2. Concentration-response curves for diethylamine diazeniumdiolate (DEA/NO; a spontaneous NO generator), S-nitroso-N-valerylpenicillamine (SNVP; an S-nitrosothiol) and 3-morpholinosydnonomine (SIN-1; a peroxynitrite generator) were generated in platelet-rich plasma (PRP) and washed platelets (WP) in the presence and absence of a supramaximal concentration of the soluble guanylate cyclase inhibitor, ODQ (20 microM). All three NOx donors displayed cGMP-independent inhibition of platelet aggregation in PRP, but only DEA/NO exhibited cGMP-independent inhibition of aggregation in WP. 3. Analysis of NO generation using an isolated NO-electrode revealed that cGMP-independent effects coincided with the generation of substantial levels of extracellular NO (>40 nM) from the NOx donors. 4. Reconstitution of WP with plasma factors indicated that the copper-containing plasma protein, caeruloplasmin (CP), catalysed the release of NO from SNVP, while Cu/Zn superoxide dismutase (SOD) unmasked NO generated from SIN-1. The increased generation of extracellular NO correlated with a switch to cGMP-independent effects with both NOx donors. 5. Analysis of Fura-2 loaded WP revealed that only DEA/NO inhibited Ca2+ signalling in platelets via a cGMP-independent mechanism. However, preincubation of SNVP and SIN-1 with CP and SOD, respectively, induced cGMP-independent inhibition of intraplatelet Ca2+ trafficking by the NOx donors. 6. Taken together, our data suggest that extracellular NO (>40 nM) is required for cGMP-independent inhibition of platelet activation. Plasma constituents may play an important pharmacological role in activating cGMP-independent signalling by S-nitrosothiols or peroxynitrite generators.

Effects of diltiazem on platelet activation and cytosolic calcium during percutaneous transluminal coronary angioplasty

AIMS

To evaluate effects of diltiazem on platelet hyper-reactivity in situations associated with endothelial injury and their possible relationship to cytosolic calcium concentration.

METHODS

Blood samples were collected at seven time points from 35 patients undergoing percutaneous transluminal coronary angioplasty (PTCA) who received combined diltiazem and aspirin/ticlopidine therapy or aspirin/ticlopidine therapy alone. Platelet expression of glycoprotein IIb/IIIa and P-selectin, production of thromboxane B(2), and cytosolic calcium concentration were measured, respectively, by whole blood flow cytometry, radioimmunoassay, and fluorospectrophotometry. The effects of diltiazem of different concentrations on expression of glycoprotein IIb/IIIa and P-selectin were also studied in vitro in blood samples from patients with chronic stable angina.

RESULTS

Of the two treatments, aspirin/ticlopidine therapy did not prevent an acute increase of expression of glycoprotein IIb/IIIa and P-selectin and plasma thromboxane B(2) five minutes and 10 minutes after first inflation and 10 minutes after PTCA, whereas combined diltiazem and aspirin/ticlopidine therapy had a significant inhibitory effect. In the group receiving aspirin/ticlopidine therapy, there was a short term increase of platelet [Ca(2+)](i) immediately after PTCA which was significantly reduced by diltiazem treatment. Expression of glycoprotein IIb/IIIa and P-selectin was significantly inhibited in vitro by diltiazem in the concentration of 200 ng/ml or higher, but not 50 ng/ml.

CONCLUSIONS

Combined diltiazem and aspirin/ticlopidine therapy significantly inhibited platelet activation that continued in the presence of conventional aspirin/ticlopidine treatment. Antiplatelet effects of diltiazem were probably a consequence of reduction of platelet [Ca(2+)](i) and may only be achieved in higher than therapeutic concentrations.

Expression of transient receptor potential mRNA isoforms and Ca(2+) influx in differentiating human stem cells and platelets

Store-regulated Ca(2+) entry (SOCE) is an important mechanism of elevating cytosolic [Ca(2+)]i in platelets, though the Ca(2+) influx channels involved are still unclear. We screened human platelets and their precursor cells (human stem cells and megakaryocytes) for the presence of candidate influx channels, i.e., isoforms of the Trp family of proteins. Primary stem cells were cultured with thrombopoietin to allow differentiation into megakaryocytes. The undifferentiated stem cells (CD34(+)) showed mRNA expression of only a spliced variant Trp1A. Immature (CD61(+)/CD42b(low)) and mature (CD61(+)/CD42b(high)) megakaryocytes as well as platelets expressed in addition unspliced Trp1 as well as Trp4 (less abundant) and Trp6 isoforms. This unspliced isoform appeared to be specific for cells of the megakaryocyte/platelet lineage, since immature (CD14(+)/CD61(-)/CD42b(-)) and mature monocytes expressed only the Trp1A isoform. This conclusion was confirmed by the presence of Trp1A, 3, 4 and 6 transcripts in the immature megakaryocytic Dami cell line, and of Trp1, 1A, 4 and 6 transcripts in the more mature CHRF-288 cell line. The up-regulation of Trp1, 4 and 6 in the lineage from primary stem cells to mature megakaryocytes and platelets was accompanied by increased influx of extracellular Ca(2+) after pretreatment of the cells with thapsigargin or thrombin. Expression of new Trp isoforms in the differentiated cells is thus accompanied by increased SOCE.

Agonist-Induced Regulation of Myosin Phosphatase Activity in Human Platelets Through Activation of Rho-Kinase

Human platelets contained about 15 times lower amounts of Rho-kinase than Ca2+/calmodulin-dependent myosin light chain (MLC) kinase. Anti–myosin-binding subunit (MBS) antibody coimmunoprecipitated Rho-kinase of human platelets, and addition of GTPγS-RhoA stimulated phosphorylation of the 130-kD MBS of myosin phosphatase and consequently inactivated myosin phosphatase. Two kinds of selective Rho-kinase inhibitors, HA1077 and Y-27632, reduced both GTPγS-RhoA–dependent MBS phosphorylation and inactivation of the phosphatase activity. Activation of human platelets with thrombin, a stable thromboxane A2 analog STA2, epinephrine, and serotonin resulted in an increase in MBS phosphorylation, and the agonist-induced MBS phosphorylation was prevented by pretreatment with the respective receptor antagonist. HA1077 and Y-27632 inhibited MBS phosphorylation in platelets stimulated with these agonists. These compounds also blocked agonist-induced inactivation of myosin phosphatase in intact platelets. In addition, HA1077 and Y-27632 inhibited 20-kD MLC phosphorylation at Ser19 and ATP secretion of platelets stimulated with STA2, thrombin (0.05 U/mL), and simultaneous addition of serotonin and epinephrine, whereas these compounds did not affect MLC phosphorylation or ATP secretion when platelets were stimulated with more than 0.1 U/mL thrombin. Thus, activation of Rho-kinase and the resultant phosphorylation of MBS is likely to be the common pathway for platelet activation induced by various agonists. These results also suggest that Rho-kinase–mediated MLC phosphorylation contributes to a greater extent to the platelet secretion induced by relatively weak agonists.

Neonatal and maternal platelet cytosolic calcium in normotensive and hypertensive pregnancies

A prospective study investigated platelet cytosolic calcium in non-pregnant volunteers (n = 30) and samples from the umbilical veins of babies from both normotensive (n = 18) and hypertensive (n = 15) primigravidae, and their mothers. There was no significant difference between the neonatal umbilical venous platelet cytosolic calcium concentration (p[Ca2+]i) in babies born to normotensive primigravidae or to those whose pregnancies were complicated by gestational hypertension (88 x 9 (SE) 2 x 5) in normotensive primagravidae, 80 x 6 (2 x 8) in pregnancy induced hypertension without proteinuria, and 89 x 3 (3 x 2) nmol/l in pre-eclampsia. There was also no significant difference in the p[Ca2+]i from the umbilical veins of the pregnancies studied and those of non-pregnant female volunteers in the follicular phase of their menstrual cycle. This was despite a gradual and significant rise in p[Ca2+]i with increasing severity of disease in the mothers of the babies studied (119 x 9 (4 x 1) in normotensive primagravidae, 130 x 8 (7 x 3) in pregnancy induced hypertension without proteinuria, and 148 x 2 (4 x 5 ) nmol/l in pre-eclampsia). The mean maternal p[Ca2+]i in the three samples returned to concentrations comparable with those in non-pregnant subjects by 12 weeks after birth. These data demonstrate no significant difference between the mean p[Ca2+]i in non-pregnant women and those obtained from the umbilical venous blood of normotensive or hypertensive primigravidae. They suggest that the functional hypoactivity of neonatal platelets is probably not secondary to a decrease in basal p[Ca2+]i. They also suggest that the progressively raised p[Ca2+]i in normal and hypertensive pregnancies might be due to a pregnancy specific factor that does not cross the placenta,

Presence of lipocortins I and IV, but not II and VI, in human platelets

The present investigation revealed the presence of lipocortins I and IV, but not lipocortins II and VI, in human platelets. Lipocortin I was found in the Triton-soluble fraction of both resting and thrombin-activated platelets and was not covalently bound to skeletal components. Without detergents, when resting platelets were lysed and fractionated in the absence of Ca2+, lipocortin I was found only in the cytosolic fraction, whereas, in the presence of Ca2+, lipocortin I was associated only with the crude particulate and not with the membrane nor the cytosolic fractions.

Interaction of prostaglandins with adenosine diphosphate induced increase in cytosolic free calcium in human platelets

The interaction of prostaglandins with changes in cytosolic Ca2+ concentration ([Ca2+]) and aggregation of human platelets induced by adenosine diphosphate (ADP) were investigated. Cytosolic [Ca2+] was measured with the fluorescent dye Quin2. Addition of ADP (0.25-2.5 mumol l-1) to platelet suspensions produced a dose dependent increase in cytosolic [Ca2+] from a basal level of 51 +/- 1 nmol l-1 to maximum levels exceeding 1 mumol l-1 and induced platelet aggregation. Chelation of extracellular calcium with 100 mumol l-1 EGTA markedly reduced the increase in cytosolic [Ca2+] induced by 0.25 mumol l-1 ADP, while pretreatment with the calcium entry blocker verapamil was without effect. Stimulation of cyclic AMP with prostaglandins (PGD2, PGE1, PGE2, PGI2, but not PGF2 alpha) and forskolin, or incubation with dibutyryl-cAMP, inhibited the rise in cytosolic [Ca2+] and platelet aggregation following ADP. We conclude that prostaglandins inhibit the increase in cytosolic [Ca2+] and aggregation of human platelets induced by ADP, probably by stimulation of cyclic AMP generation, thereby opposing the mechanism by which ADP increases cytosolic [Ca2+] and subsequently induces platelet aggregation.

Effects of the antithrombitic agent PCA 4230 on agonist-induced Ca2+ entry and Ca2+ release in human platelets

We have studied the effects of the antithrombitic agent PCA 4230 on the entry of Mn2+, used here as a Ca2+ surrogate for Ca2+ channels, and on the release of Ca2+ from the intracellular stores in stimulated human platelets loaded with fura-2. PCA 4230 prevented receptor-operated calcium entry activated by thrombin, ADP and collagen with no modification of the Ca2+ release from the intracellular stores. PCA 4230 also inhibited cytochrome P-450-mediated O-dealkylase activity with the same concentration-dependence as the thrombin-induced Mn2+ entry. These results suggest that the inhibitory effects of PCA 4230 on Ca2+ influx may be due to its interaction with cytochrome P-450, which has been proposed recently to be involved in the activation of receptor-operated Ca2+ channels. In addition, PCA 4230 inhibited both PAF-induced Ca2+ entry and Ca2+ release, behaving as a PAF-antagonist. All these effects contribute to explain the antithrombitic action of PCA 4230.

Comparison of the role of protein kinase C in platelet functional responses induced by three different mechanisms, PAF, ionomycin and arachidonic acid

The role of protein kinase C (PKC) in modulating platelet activation has been examined in platelets pre-incubated with either the PKC activator 12-O-tetradecanoylphorbol 13-acetate (TPA) or the non-specific protein kinase inhibitor, staurosporine. In order to determine where in the signal transduction pathway PKC is exerting its effect platelets were activated either with a receptor-operated stimulus platelet activating factor (PAF) or by direct elevation of [Ca2+]i (ionomycin) or with arachidonic acid which is converted into thromboxane B2 (TxB2). In PAF-stimulated platelets activation of PKC inhibited both [Ca2+]i elevation and TxB2 generation but had no effect on 5-hydroxytryptamine (5-HT) release whilst staurosporine increased the duration of [Ca2+]i elevation and potentiated TxB2 generation but inhibited 5-HT release. In ionomycin-stimulated platelets modulation of PKC had no effect on [Ca2+]i elevation but in contrast to PAF-stimulated platelets PKC activation caused potentiation of TxB2 generation and 5-HT release whilst inhibition of PKC caused inhibition of TxB2 generation and 5-HT release. Modulation of PKC did not affect arachidonic acid-induced TxB2 generation. These findings suggest that in receptor activated platelets endogenously activated PKC is exerting a negative feedback role, however, when [Ca2+]i elevation is not modified by PKC activation or inhibition (such as in ionomycin stimulated platelets) the relationship between the state of PKC activation and subsequent platelet functional responses corresponds more closely. The findings from this study suggest a different relationship between PKC and TxB2 generation than between PKC and dense granule release in PAF-stimulated platelets.

Abnormal platelet and lymphocyte calcium handling in prehypertensive rats

We have reported that the basal and stimulated cytosolic free calcium concentrations [( Ca2+]i) are elevated in platelets isolated from 12-14-week-old spontaneously hypertensive rats (SHR) as compared with normotensive Wistar-Kyoto (WKY) rats. To determine whether altered cell calcium metabolism precedes the development of overt hypertension, we measured [Ca2+]i under resting and stimulated conditions in blood platelets and thymic lymphocytes isolated from 4-week-old prehypertensive SHR and WKY rats. Blood pressure was similar in both groups (SHR 95 +/- 8 versus WKY rats 92 +/- 7 mm Hg). Basal [Ca2+]i in platelets was higher in SHR than WKY rats (63.4 +/- 3.9 versus 54.8 +/- 3.1 nM, p less than 0.003). Also the [Ca2+]i response to thrombin was greater in SHR than WKY rats in both the presence and absence of extracellular calcium. For lymphocytes, although no difference was detected in basal [Ca2+]i, the concanavalin A-induced peak [Ca2+]i was higher for SHR than WKY rats in both calcium-containing and calcium-free media. These results suggest that agonist-stimulated calcium influx and calcium discharge from intracellular stores are enhanced in both platelets and lymphocytes of 4-week-old SHR. We conclude that abnormalities in calcium metabolism in two different cell types precede the development of overt hypertension in the SHR.

Ebselen affects calcium homeostasis in human platelets

Ebselen (PZ 51, 2-phenyl-1,2-benzoisoselenazol-3-(2H)-one) is a selenoorganic compound with anti-inflammatory properties. Its pharmacological action is thought to originate from its peroxidase activity which could lower the peroxide tonus required for cyclooxygenase and lipoxygenase activations. From experiments with aspirin-treated human platelets we now present evidence that ebselen also affects intracellular calcium homeostasis by inhibiting the agonist-triggered increase in intracellular calcium. Using Mn2+ entry to quench the fura-2 fluorescence after cell stimulation, we could exclude an interaction of ebselen with receptor-operated calcium channels and therefore an inhibition of extracellular calcium influx. It became evident from whole cell experiments and by using isolated platelet microsomal vesicles that ebselen inhibits the inositol 1,4,5-trisphosphate (IP3) induced calcium release. Besides this inhibitory effect of ebselen on the calcium release higher concentrations of the compound (greater than or equal to 5 microM) induced a calcium release from our microsomal vesicles which also could be reversed by dithiothreitol. An activation of inflammatory cells is usually associated with increased cytosolic calcium concentrations. An inhibition of such calcium movements by ebselen may account for an up to now unidentified anti-inflammatory mechanism of ebselen action which is linked to a direct effect of this compound rather than to its peroxidase-like activity.

Cytosolic ionized calcium in human platelets: the influence of collagen and a novel antiplatelet agent

Two phases of calcium mobilization were observed when aequorin-loaded human platelets, suspended in a nominally calcium-free medium containing 0.1 mM EGTA, were stimulated with collagen. The first phase coincided with platelet shape change, and the second phase corresponded to aggregation. On the other hand, only one [Ca2+]i peak was found in systems containing 1.0 mM Ca, or 1.0 or 2.0 mM EGTA. A novel antiplatelet compound alpha,alpha'-bis [3-(N,N-diethylcarbamoyl)piperidino]-p-xylene dihydrobromide, inhibited both [Ca2+]i peaks. It is suggested that inhibition of the mobilization of intraplatelet calcium stores as well as the blocking of transmembrane calcium flux may be responsible for the platelet aggregation-inhibitory action of this compound.

Increase in the cytosolic concentration of calcium in platelets of diabetics type II

The intracellular concentration of calcium (Cai) was measured in platelets of healthy control subjects and diabetics type II. Cai was elevated in platelets of diabetics type II under basal conditions, after stimulation by collagen, in the presence and absence of calcium in the incubation medium. The increase in Cai after stimulation by collagen was inhibited by nitrendipine in both kinds of platelets, however Cai remained elevated in platelets of diabetics. Our data and findings of others may be understood as indications of disturbances in the handling of Cai in diabetes.

Elevated platelet intracellular calcium concentration in bipolar depression

Baseline and thrombin-stimulated free intracellular calcium concentrations in blood platelets were significantly higher in untreated depressed bipolar patients than in untreated unipolar depressed patients or controls. Platelet intracellular calcium ion concentrations in euthymic-treated bipolar patients were equivalent to control values, suggesting but not proving a state-dependent change in intracellular calcium ion dynamics in bipolar depression. Unipolar and some subsets of bipolar patients appear not to exhibit this change.

Characterization of the effect of the adenosine agonist cyclohexyladenosine on platelet activating factor-induced increases in [Ca2+]i in human platelets in vitro

Previous studies have shown that adenosine agonists acting at A-2 receptors inhibit platelet aggregation. Since an increase in cytosolic Ca2+ concentration (delta [Ca2+]i) is closely associated with the time frame of platelet aggregation, we have examined the effect of adenosine receptor function on induced increases of [Ca2+]i by a potent platelet activator, platelet activating factor (PAF). We loaded washed platelets with Fura-2, then induced increases in [Ca2+]i with various concentrations of PAF, and then determined EC50 values (PAF concentration at half-maximal response) and values for maximal response of delta[Ca2+]i (max-delta[Ca2+]i). The EC50 for PAF-delta[Ca2+]i was 112 +/- 37 (SD) pM and the max-delta[Ca2+]i was 284 +/- 138 (SD) nM. Our results show that PAF-delta[Ca2+]i was inhibited in a non-competitive manner by the adenosine receptor agonist cyclohexyladenosine (CHA) with an IC50 of 14.9 microM. This inhibition was partially reversed by theophylline, an adenosine receptor antagonist, with an IC50 of 19 microM. Based on the results of these studies together with evidence from other research groups that platelets do not possess A-1 receptors, our results suggest that CHA inhibited PAF-delta[Ca2+]i in platelets through an activation of A-2 receptors.

Inhibition of mural thrombus formation by novel nipecotoylpiperazine antiplatelet agents

The effectiveness of two closely related nipecotoylpiperazine derivatives, BPAT-143 and BPAT-117, as antiplatelet agents was measured by their ability to inhibit the accumulation of human blood platelets on collagen-coated (type 1) glass in a parallel plate flow chamber. Whole human blood, with fluorescently labeled platelets, was perfused through the flow chamber, and epi-fluorescent video microscopy was used to visualize the dynamics of individual platelet adhesion and thrombus formation on the collagen-coated surface. Digital image processing was used to analyze the dynamics of thrombus growth on the surfaces. The collagen-coated surface serves as a model for the damaged blood vessel wall, as collagen is a primary component of the matrix beneath endothelial cells. At a concentration of 50 microM, BPAT-117 (the considerably more hydrophobic molecule) inhibited platelet accumulation by striking 90 +/- 2% (+/- S.E.), while it took 2- to 4-fold higher concentrations of BPAT-143 to register meaningful to comparable effects (52 +/- 6% and 80 +/- 4%, respectively). This further corroborates the substantial impact of hydrophobic features within the matrix of appropriately structured molecules on their ability to alter platelet function.

Inhibition of thrombus formation in vivo by novel antiplatelet agent

The antithrombotic and antihemostatic effects of the novel antiplatelet compound, alpha, alpha'-bis[3-(N,N-diethylcarbamoyl)piperidino]-p-xylene dihydrobromide (GT-12), were investigated in a baboon model of platelet-dependent thrombosis under high flow conditions. In this model, segments of collagen-coated tubing and Dacron vascular graft were placed as thrombus-inducing extension devices in exteriorized femoral arteriovenous shunts. The deposition of 111In-labeled platelets was measured for each thrombogenic segment throughout 1 hour by using gamma camera imaging. In addition, the 125I-fibrin retained in the forming thrombus was measured. Intravenous infusion of GT-12 (100 mumol/kg, 63.3 mg/kg) over a 15-minute period before the insertion of the test segments prolonged the bleeding time from a baseline value of 4.4 +/- 0.4 min to 7.6 +/- 1.0 min (p = 0.036) and inhibited platelet aggregation ex vivo induced by adenosine diphosphate (ED50 4.7 +/- 0.9 to 10.3 +/- 2.2 microM; p less than 0.02) and collagen (ED50 2.0 +/- 0.4 to 8.0 +/- 2.4 micrograms/ml; p less than 0.05). Deposition of platelets and fibrin was decreased in concert by 30% (p less than 0.05) for vascular grafts and possibly collagen segments at the end of the 60-minute observation period. We conclude that GT-12 is antithrombotic for Dacron graft-induced thrombus formation in vivo.

Relationships between chemical structure and inhibition of epinephrine-induced human blood platelet aggregation

The effect of structural features in a series of carbamoylpiperidine and nipecotoylpiperazine congeners on epinephrine-induced aggregation of human blood platelets is examined. Epinephrine-induced primary aggregation is effectively inhibited by the nipecotoylpiperazine derivatives (culminating at an IPA50 of 11.9 microM). While among nipecotoylpiperazine as well as carbamoylpiperidine congeners there are potent inhibitors of ADP-stimulated platelet function (cresting at an IA50 of 12.4 and 11.4 microM, respectively), the carbamoylpiperidine analogs are much less active (e.g., IPA50 of 298.1), or practically inactive, in impeding epinephrine-induced primary aggregation (PA).