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PubMed1985

Synergistic effects of platelet-activating factor and other platelet agonists in human platelet aggregation and release: the role of ADP and products of the cyclooxygenase pathway.

The synergistic effects of platelet-activating factor (PAF) with ADP, collagen, thrombin, A23187, adrenaline, sodium arachidonate and ristocetin in human platelet aggregation and beta-thromboglobulin (beta-TG) release were investigated in citrated platelet-rich plasma (PRP). Synergism in both aggregation and release was present with all agonists except ristocetin. Upon oral intake of aspirin (ASA) the PAF-induced irreversible aggregation as well as the synergistic irreversible aggregation became reversible. Both prior to and after ASA ingestion ADP removal by creatine phosphate/creatine phosphokinase (CP/CPK) resulted in a reduced, reversible platelet aggregation induced by PAF alone or in combination with the other agonists. The ADP-removal and ASA-ingestion also strongly inhibited the beta-TG release. The synergistic aggregation and release were also inhibited by ASA and indomethacin in vitro as well as by the competitive ADP-inhibitor ATP. It is concluded that not only the activation of human platelets by low doses of PAF itself, but also the synergism of PAF and other platelet agonists is highly dependent upon ADP and products of the cyclooxygenase pathway.

Adenosine Diphosphate↗
PubMed2003

A new platelet parameter, the mean platelet component, can demonstrate abnormal platelet function and structure in myelodysplasia.

The mean platelet component (MPC) is a new platelet parameter generated by the Bayer ADVIA 120 full blood count analyser as part of the routine complete blood count (CBC) test cycle. We report a case of myelodysplasia with bleeding complications and abnormal template bleeding time in whom low mean platelet component parameters were associated with partial platelet granule deficiency, demonstrated by transmission electron microscopy. We suggest that the mean platelet component is an inexpensive and rapid test to screen for platelet dysfunction related to ultrastructural abnormalities in myelodysplasia.

Blood Platelet Disorders↗
PubMed1961

Effects of bacterial endotoxin on rabbit platelets. I. Platelet aggregation and release of platelet factors in vitro.

Incubation of platelet-rich rabbit plasma with E. coli endotoxin at 37 degrees C results in platelet aggregation and transfer of platelet 5-hydroxytryptamine to plasma. Release of 5HT is influenced by dose of endotoxin, type of anticoagulant, and temperature of incubation. A heat-labile plasma factor is necessary for the platelet-endotoxin interaction. Additional studies have shown that incubation of endotoxin with platelet-rich rabbit plasma also results in release of platelet phospholipid and bactericidins active against B. subtilis.

Animals↗
PubMed1979

A platelet defect in a patient with eosinophilic leukaemia: absent ristocetin-induced platelet aggregation associated with a reduced platelet sialic acid content.

Platelets from a patient with eosinophilic leukaemia were not aggregated by ristocetin. The defect was not corrected by normal human plasma and was due to a platelet abnormality. The patient's platelets also showed a diminished sensitivity to aggregation by bovine factor VIIIVWF. The defect was not associated with a prolonged bleeding time. No abnormalities were detected in ADP, collagen or thrombin-induced platelet aggregation. Biochemical studies showed that the platelets were deficient in sialic acid. This deficiency was associated with a reduced staining for glycoprotein I following SDS-polyacrylamide gel electrophoresis. The results suggest an acquired platelet surface abnormality.

Adenosine Diphosphate↗
PubMed1981

Platelets in myeloproliferative disorders. III: Glycoprotein profile in relation to platelet function and platelet density.

Membrane glycoproteins (GP) are implicated in platelet functions. In myeloproliferative diseases (MD), some of these functions are known to be perturbed. 16 patients with various MD were investigated for platelet functions (retention to glass beads, epinephrine- and ristocetin-induced aggregation), platelet density distribution and PAS-staining glycoprotein profile on SDS-polyacrylamide-gel-electrophoresis. An abnormal GP pattern (moderate reduction of GP (Ib + Is) and GP IIb with corresponding increase in GP IIIb) was demonstrated but no relation to platelet dysfunction or density distribution was observed. No differences between the various types of MD were noticed although the group of polycythaemia vera was the less perturbed for platelet function, platelet density and also GP profile.

Blood Platelets↗
PubMed2005

Clopidogrel loading with eptifibatide to arrest the reactivity of platelets: results of the Clopidogrel Loading With Eptifibatide to Arrest the Reactivity of Platelets (CLEAR PLATELETS) study.

BACKGROUND: Pretreatment is not the most common strategy practiced for clopidogrel administration in elective coronary stenting. Moreover, limited information is available on the antiplatelet pharmacodynamics of a 300-mg versus a 600-mg clopidogrel loading dose, and the comparative effect of eptifibatide with these regimens is unknown. METHODS AND RESULTS: Patients undergoing elective stenting (n=120) were enrolled in a 2x2 factorial study (300 mg clopidogrel with or without eptifibatide; 600 mg clopidogrel with or without eptifibatide) (Clopidogrel Loading With Eptifibatide to Arrest the Reactivity of Platelets [CLEAR PLATELETS] Study). Clopidogrel was administered immediately after stenting. Aggregometry and flow cytometry were used to assess platelet reactivity. Eptifibatide added a > or =2-fold increase in platelet inhibition to 600 mg clopidogrel alone at 3, 8, and 18 to 24 hours after stenting as measured by 5 micromol/L ADP-induced aggregation (P<0.001). Without eptifibatide, 600 mg clopidogrel produced better inhibition than 300 mg clopidogrel at all time points (P<0.001). Glycoprotein IIb/IIIa (GPIIb/IIIa) blockade was associated with lower cardiac marker release. Active GPIIb/IIIa expression was inhibited most in the groups treated with eptifibatide (P<0.05). CONCLUSIONS: In elective stenting without clopidogrel pretreatment, use of a GPIIb/IIIa inhibitor produces superior platelet inhibition and lower myocardial necrosis compared with high-dose (600 mg) or standard-dose (300 mg) clopidogrel loading alone. In the absence of a GPIIb/IIIa inhibitor, 600 mg clopidogrel provides better platelet inhibition than the standard 300-mg dose. These results require confirmation in a large-scale clinical trial.

Adult↗
PubMed1987

Activity of platelet-derived growth factor (PDGF) in platelet concentrates and cryopreserved platelets determined by PDGF bioassay.

The bioassay of platelet-derived growth factor (PDGF) in platelets was established using the rat fibroblast cell line 3Y1. 3Y1 cells (5 X 10(3)/well) were cultured in 24-well microculture plates with RPMI 1640 medium using 1% of PDGF-free serum and 5% of platelet extracts for 4 days. Cell proliferation was measured by the increase of DNA content. This assay made it possible to measure the biological PDGF activity. PDGF activity of platelet concentrates kept the same level as that of fresh platelet samples during preservation for up to 120 h at 22 degrees C; cryopreserved platelets also retained PDGF activity well.

Biological Assay↗
PubMed1990

Functional validation of platelet-activating factor receptor sites characterized biochemically by a specific and reproducible [3H]platelet-activating factor binding in human platelets.

In human platelet membranes, [3H]platelet-activating factor(PAF)-C18 binding sites exhibited high affinity (Kd 0.074 +/- 0.005 nM, n = 28 healthy volunteers), saturability, elevated stereoselectivity, marked pharmacological specificity and small intersubject variability. The maximal binding capacity was 215 +/- 12 fmol/mg protein. Saturation of [3H]PAF binding was obtained with 0.3 nM ligand, and its isotherm was compatible with a single class of binding sites. The stereoselectivity for [3H]PAF was clearly indicated by the low displacing potency of enantio-PAF-C16 (the synthetic enantiomer of PAF) that was 5000-fold less potent than PAF. Specific [3H]PAF binding attained 65% with 0.1 nM ligand and was displaced fully not only by cold PAF but also by RP 59227 (Ki = 6.2 +/- 1.3 nM, n = 7), a novel, potent and specific PAF receptor antagonist in a pure enantiomeric form and several other antagonists such as CV-6209, WEB 2086, L-652,731 and BN 52021. Various classical pharmacological agents did not interfere with the [3H]PAF binding. In intact platelets, [3H]PAF binding shared the same properties as those just described for membrane preparations. A functional role for these binding sites was suggested by the high correlation (r = 0.94, P less than .001) between the Ki values for several known PAF antagonists determined in [3H]PAF binding and the IC50 values obtained against PAF-induced aggregation in whole platelets. Thus, the present [3H]PAF binding in human platelet membranes may be a useful pharmacological tool to study possible changes in [3H]PAF binding parameters induced by pathological states for which PAF may be directly or indirectly responsible.

Adult↗
PubMed1979

Comparison of platelet count and platelet protein methods for determination of platelet MAO activity.

Platelet monoamine oxidase (MAO) activity in 10 normal volunteers was studied as a function of platelet protein or electronically-determined platelet counts. Comparisons of the two methods were made for samples assayed on the same day as well as one week later. The MAO activities resulting from both methods were significantly correlated and reproducible but the results of the platelet count method were, in most instances, slightly but significantly more reliable than the platelet protein method. The relevance of these results to the controversy concerning platelet MAO activity in schizophrenia is discussed.

Blood Cell Count↗
PubMed1979

Thrombocytopenic episodes in patients with well-functioning renal allografts. Inverse relationship between platelet count and platelet size pointing to intermittent platelet destruction.

20 out of 50 patients with well-funcioning renal allograft displayed at least one platelet count below 110 X 10(9)/1 (mean -2 SD of controls). For estimation of platelet production during thrombocytopenic episodes, the percentage of large platelets in the peripheral blood was determined which revealed an inverse relationship (p less than 0.01) to the platelet count, indicating that these thrombocytopenias were due to increased platelet consumption. Immunosuppressive treatment as well as rejection processes could be excluded as major pathogenetic factors whereas anti-platelet autoantibodies may contribute to this phenomenon.

Autoantibodies↗
PubMed1987

ADP-induced platelet shape change and mobilization of cytoplasmic ionized calcium are mediated by distinct binding sites on platelets: 5'-p-fluorosulfonylbenzoyladenosine is a weak platelet agonist.

Platelet stimulation with ADP results in several responses, including shape change, increase in cytoplasmic ionized calcium concentration [Ca2+]i, an inhibition of adenylate cyclase. 5'-p-Fluorosulphonyl benzoyladenosine (FSBA), which covalently labels an ADP binding site on platelets, blocks platelet shape change but not the inhibition of cyclic AMP levels by ADP, whereas p-chloromercuribenzenesulfonate (pCMBS), a nonpenetrating thiol reagent, has the opposite effects. We examined the effect of FSBA and pCMBS on ADP-induced increase in [Ca2+]i using platelets loaded with fluorescent Ca2+ indicators quin2 and fura-2. FSBA (50 to 200 mumol/L) induced a dose-dependent rise in [Ca2+]i, indicating that it is a weak platelet agonist. Under conditions of covalent labeling of the ADP binding sites, FSBA (50 to 100 mumol/L) did not inhibit the ADP-induced increase in [Ca2+]i or its inhibition of adenylate cyclase, whereas pCMBS (up to 1 mmol/L) abolished both these responses but not shape change. These findings suggest that ADP-induced Ca2+ mobilization and inhibition of adenylate cyclase are mediated by platelet binding sites distinct from those mediating shape change.

4-Chloromercuribenzenesulfonate↗
PubMed1991

The effect of inhibitors of platelet aggregation on the metabolism of platelet-activating factor (PAF) in washed rabbit platelets.

The rabbit platelet metabolizes platelet-activating factor (PAF) intracellularly. PAF is deacetylated to produce lysoPAF which, in turn, can be acylated to produce 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine (alkylacyl GPC). Some PAF receptor antagonists have been shown to inhibit this metabolic conversion. In the present study we examined whether the PAF receptor antagonists SRI 63-441 and WEB 2086 would inhibit the metabolism of PAF by intact rabbit platelets. In addition, we examined whether iloprost, a stable analogue of prostaglandin I2 (PGI2), and a potent inhibitor of platelet activation induced by a range of agonists, would also inhibit PAF metabolism. We found that SRI 63-441 and WEB 2086 caused an almost complete inhibition of the conversion of PAF to alkylacyl GPC. Iloprost caused up to a 50% inhibition of PAF metabolism compared to antagonist-free controls. Iloprost (and PGI2) is thought to inhibit platelet response by elevation of cAMP, while receptor antagonists act by blocking PAF binding to its receptor. Since iloprost caused partial inhibition of PAF metabolism, the results of this study suggest that inhibition of PAF metabolism does not occur solely due to competitive inhibition of PAF binding to its receptor.

Animals↗
PubMed2004

Does clopidogrel increase the degree of platelet inhibition when a platelet glycoprotein IIb/IIIa inhibitor has been given? Insights from an optical platelet aggregometry study.

INTRODUCTION: While the CURE trial demonstrated the benefits of clopidogrel in acute coronary syndromes, patients receiving glycoprotein IIb/IIIa antagonists were excluded. Given the frequent coadministration of these two medications, we sought to examine their interaction and their combined effect on platelet inhibition. METHODS: Ten patients admitted to the hospital with stable or unstable angina underwent phlebotomy prior to, three hours and six hours after administration of a standard oral loading dose of clopidogrel. The samples were then treated in vitro with incremental concentrations of tirofiban (0, 10, 20, 40, 60, and 80 ng/mL), and optical platelet aggregometry was performed utilizing ADP and TRAP as agonists. We analyzed the combined effects of these agents using a mixed effects model with time and tirofiban concentration as fixed effects, and subject and timing of phlebotomy as random effects. RESULTS: There was no evidence of additional inhibition of platelet aggregation due to clopidogrel regardless of the concentration of tirofiban or the study agonist (ADP 20 muM or iso-TRAP). Specifically, there was no difference in the tirofiban dose-response curves with either platelet agonist for any of the three time points (before, and three and six hours after, clopidogrel administration). DISCUSSION: There is no evidence that the combination of clopidogrel and tirofiban achieves greater inhibition of platelet aggregation than tirofiban alone.

Aged↗
PubMed1985

Inhibition of platelet-activating-factor-induced human platelet activation by prostaglandin D2. Differential sensitivity of platelet transduction processes and functional responses to inhibition by cyclic AMP.

It has been proposed that cyclic AMP inhibits platelet reactivity: by preventing agonist-induced phosphoinositide hydrolysis and the resultant formation of 1,2-diacylglycerol and elevation of cytosolic free Ca2+ concentration [( Ca2+]i); by promoting Ca2+ sequestration and/or extrusion; and by suppressing reactions stimulated by (1,2-diacylglycerol-dependent) protein kinase C and/or Ca2+-calmodulin-dependent protein kinase. We used the adenylate cyclase stimulant prostaglandin D2 to compare the sensitivity to cyclic AMP of the transduction processes (phosphoinositide hydrolysis and elevation of [Ca2+]i) and functional responses (shape change, aggregation and ATP secretion) that are initiated after agonist-receptor combination on human platelets. Prostaglandin D2 elicited a concentration-dependent elevation of platelet cyclic AMP content and inhibited platelet-activating-factor(PAF)-induced ATP secretion [I50 (concn. causing 50% inhibition) approximately 2 nM], aggregation (I50 approximately 3 nM), shape change (I50 approximately 30 nM), elevation of [Ca2+]i (I50 approximately 30 nM) and phosphoinositide hydrolysis (I50 approximately 10 nM). A 2-fold increase in cyclic AMP content resulted in abolition of PAF-induced aggregation and ATP secretion, whereas maximal inhibition of shape change, phosphoinositide hydrolysis and elevation of [Ca2+]i required a greater than 10-fold elevation of the cyclic AMP content. This differential sensitivity of the various responses to inhibition by cyclic AMP suggests that the mechanisms underlying PAF-induced aggregation and ATP secretion differ from those underlying shape change. Thus a major component of the cyclic AMP-dependent inhibition of PAF-induced platelet aggregation and ATP secretion is mediated by suppression of certain components of the activation process that occur distal to the formation of DAG or elevation of [Ca2+]i.

Adenosine Triphosphate↗
PubMed1996

Evaluation of platelet function using the in vitro bleeding time and corrected count increment of transfused platelets. Comparison between platelet concentrates derived from pooled buffy coates and apheresis.

The functional capacity of transfused platelets was evaluated with in vitro bleeding time (IVBT) and corrected count increment (CCI) in order to compare platelet concentrates (PCs) derived from pooled buffy coats (BC-PCs) with PCs collected by apheresis (A-PCs). The suspension medium in the BC-PCs was 30% CPD plasma and 70% of an additive solution (containing sodium and potassium chloride, sodium citrate and phosphate, mannitol), and in the A-PCs the medium was 100% CPD plasma. IVBT was evaluated using a Thrombostat 4000/2. BC-PC and A-PC were transfused 57 and 41 times, respectively to 36 patients with chemotherapy-induced thrombocytopenia. PCs transfused within 2 days of donation were considered fresh, and those transfused within 3-5 days were considered stored. IVBT was determined before, as well as 10-30 min and 24 h after transfusion; CCI was determined 10-30 min and 24 h after transfusion. The median pretransfusion IVBT value was 486 s. It was measurable in 21 of 98 (21%) of the transfusions, i.e. below the cutoff limit of 486 s. Ten to 30 min after transfusion, the IVBT showed a measurable reduction in 90% of the transfusions with fresh BC-PCs, 92% of those with fresh a-PCs, 63% of those with stored BC-PCs and 79% of those with stored A-PCs. After 24 h, the corresponding values were 63% for fresh BC-PCs, 50% for fresh A-PCs, 26% for stored BC-PCs and 38% for stored A-PCs. The median value of CCI 10-30 min after transfusion was 20 for fresh BC-PCs, 17 for fresh A-PCs, 16 for stored BC-PCs and 14 for stored A-PCs. The difference in IVBT between fresh and stored BC-PCs was significant (p = 0.032), unlike that between fresh and stored A-PC. After 24 h the corresponding values were 7 for fresh BC-PCs, 4 for fresh A-PCs, 4 for stored BC-PCs and 3 for stored A-PCs. When all transfusions with fresh PCs (BC-PCs + A-PCs) were compared with all transfusions with stored PCs, a statistical difference was demonstrated in both CCI (p = 0.027) and IVBT (p = 0.043). Spearman's rank correlation coefficient (rs) was -0.41 between CCI and IVBT < 486 s 10-30 min after transfusion, and -0.55 between the posttransfusion platelet count and IVBT, indicating a relatively poor correlation between CCI and IVBT, and a slightly better correlation between platelet count and IVBT. In conclusion, BC-PCs showed a slightly higher CCI and a better response in IVBT than A-PCs. No statistical difference was demonstrated between BC-PCs and A-PCs transfused within 2 days after donation, with respect to function and recovery in vivo. BC-PCs stored for 3 days or more showed about the same CCI and IVBT as stored A-PC but significantly lower CCI and higher IVBT than fresh BC-PCs. This may indicate that the preparation and/or storage conditions were not optimal. IVBT seems to be a useful possibility to test the in vivo behavior of transfused platelets.

Adolescent↗
PubMed2004

Tea polyphenols inhibit acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine acetyltransferase (a key enzyme in platelet-activating factor biosynthesis) and platelet-activating factor-induced platelet aggregation.

BACKGROUND: Tea extracts have antiallergic and anti-inflammatory actions in rats and mice. However the mechanism through which tea polyphenols act in vivo are still incompletely understood. We found inhibitory effects of black tea extracts on an fMLP-induced aggregating response in a rabbit platelet-polymorphonuclear leukocyte (PMN) system. METHOD: To elucidate whether 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine (PAF) production in PMNs and/or PAF-stimulated platelet activation were inhibited, the effects of tea polyphenols were investigated on the enzyme activity of acetyl-CoA:1-alkyl-sn-glycero-3-phosphocholine acetyltransferase (EC 2.3.1.67), PAF biosynthesis in A23187-activated rabbit PMNs, and rabbit platelet aggregation. By comparing the inhibitory effects of 31 galloyl esters and gallic acid, the structure-inhibitory activity relationship was characterized. RESULTS: Theaflavin and its galloyl esters and pentagalloyl glucose were found to be potent inhibitors of the acetyltransferase (IC(50) = 28-58 microM) and the PAF biosynthesis as well as (-)-epicatechin-3-O-gallate (IC(50) = 72 +/- 13 microM) and (-)-epigallocatechin-3-O-gallate (IC(50) = 46 +/- 6 microM). On the other hand, flavan-3-ols without galloyl group at C-3 and gallic acid did not show significant enzyme inhibition. In addition, theaflavin and its galloyl esters (IC(50) = 32-77 microM) and geranyl gallate, farnesyl gallate and geranylgeranyl gallate (IC(50) = 6.4-7.6 microM) were found to be potent inhibitors of PAF- and TPA-induced rabbit platelet aggregation but not A23187-induced aggregation. CONCLUSIONS: Theaflavin and its galloyl esters in black tea extract, and isoprenyl gallates were potent inhibitors of PAF synthesis and platelet aggregation and these activities may be relevant to the claimed therapeutic effects of tea extracts.

Acetyltransferases↗
PubMed2004

Effects of a snake venom metalloproteinase, triflamp, on platelet aggregation, platelet-neutrophil and neutrophil-neutrophil interactions: involvement of platelet GPIbalpha and neutrophil PSGL-1.

The biologically active components from Viperidae venoms specifically affect cell-matrix interactions, and have been utilized for developing anti-adhesive therapy as anti-thrombotic and anti-angiogenic agents. Utilizing platelet aggregometry coupled with flow cytometry, we found that a metalloproteinase isolated from Trimeresurus flavoviridis, termed triflamp, inhibited heterotypic adhesion between platelets and neutrophils in whole blood samples. Triflamp is a monomeric glycoprotein with a molecular weight of approximately 28 kDa. Triflamp has a N-terminal amino acid sequence homologous to other venom metalloproteinases isolated from T. flavoviridis. The enzymatic activity of triflamp was inhibited by EDTA and phenanthroline but not by PMSF. Moreover, triflamp is a pure alpha-fibrinogenase. Studies aimed at determining the nature of triflamp in affecting platelets or neutrophils revealed a selective inhibitory activity to glycoprotein (GP) Ibalpha-dependent platelet aggregation and PSGL-1-dependent neutrophil homotypic aggregation, indicating that its effects are rather specific. As judged by Western blotting, GPIbalpha on platelets and PSGL-1 on neutrophils are the substrates of triflamp. In conclusion, we suggest the novel role of venom metalloproteinase from Viperidae affecting the blood cell-cell interactions, thus offering a potential approach for further exploration of anti-inflammatory agents.

Animals↗
PubMed1989

Abolition of in vivo platelet thrombus formation in primates with monoclonal antibodies to the platelet GPIIb/IIIa receptor. Correlation with bleeding time, platelet aggregation, and blockade of GPIIb/IIIa receptors.

We studied the dose-response effects of the F(ab')2 fragments of murine monoclonal antibodies to the platelet GPIIb/IIIa receptor (7E3 and 10E5) on in vivo platelet thrombus formation in a well-characterized monkey model in which the carotid artery is stenosed and thrombus formation is provoked and augmented by intimal damage and the infusion of subaggregating doses of epinephrine. Both antibodies abolished thrombus formation with a mean dose of -0.2 mg/kg. Ex vivo platelet aggregation was not always abolished at doses that abolished thrombus formation; similarly, bleeding times were only moderately prolonged (9.1 +/- 1.4 minutes) at these doses. Increasing the dose above that required to abolish thrombus formation consistently produced abolition of ex vivo platelet aggregation, marked prolongation of the bleeding time (14.2 +/- 1.5 minutes), and nearly quantitative blockade of GPIIb/IIIa receptors. We conclude that in a significant percentage of animals, the extent of GPIIb/IIIa blockade required to prevent vasoocclusive thrombus formation in this model is less than that required for abolition of platelet aggregation, and that the preservation of only a minority of functional GPIIb/IIIa receptors might be adequate to maintain a nearly normal bleeding time.

Animals↗