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PubMed2000

Involvement of platelets in experimental mouse trypanosomiasis: evidence of mouse platelet cytotoxicity against Trypanosoma equiperdum.

Platelets play an important role in the human response to parasites. Trypanosoma equiperdum, a parasite that has the horse as its natural host, is able to induce infection in mice and thus it may represent a simple model for studying the role of platelets in the development of a parasitosis. Although several aspects of the murine response to T. equiperdum infection have been clarified, the precise mechanism of killing of the parasite is still unclear. We have studied the involvement of blood platelets in experimental murine infection with T. equiperdum. Infected mice show a progressive decrease of the number of circulating platelets. The production of thromboxane A2 (TxA2) by platelets stimulated with collagen decreases progressively with the progression of T. equiperdum infection, compatible with in vivo platelet activation or with a possible antagonistic effect by trypanosomes on the production of TxA2. Finally, mouse platelets exert in vitro a direct parasitocidal activity on T. equiperdum at ratios >/=20:1. Complement fractions do not enhance platelet trypanocidal activity, whereas IgM fractions do, at least in short-term coincubation experiments. Our data show that platelets are involved in experimental murine T. equiperdum infection and confirm that platelet parasitocidal activity is a generalized phenomenon in mammals.

Analysis of Variance↗
PubMed1994

Multiple platelet surface receptors mediate platelet adhesion to surfaces coated with plasma proteins.

The interactions between platelets and plasma proteins previously shown to adhere to biomaterials were evaluated, using monoclonal antibodies (mAbs) against specific platelet surface glycoprotein (GP) receptors. Purified 51Cr-labeled human platelets in plasma-free medium were incubated with each of the following antibodies: mAb 10E5 [anti-GP IIb/IIIa; fibrinogen, von Willebrand factor (vWF), and fibronectin receptor]; mAb 6D1 (anti-GP Ib-IX; vWF receptor); mAb IV.3 (anti-Fc gamma RII; IgG receptor); polyclonal antiserum A108 or mAb BIIG4 (anti-GP Ic-IIa; fibronectin receptor). Antibody-treated platelets were added to microtiter wells coated with fibronectin, fibrinogen, vWF, IgG, vitronectin, albumin, or platelet-poor plasma (PPP). 51Cr-labeled platelet adhesion to matrix proteins was expressed as a percentage of that measured on PPP-coated surface. Platelets adhered to fibrinogen, fibronectin, vWF, or IgG immobilized on polystyrene. Limited binding to either vitronectin or albumin was detected. Binding to fibrinogen and IgG was blocked by mAb 10E5. Binding to IgG was also blocked by mAb IV.3. Binding to fibronectin, reduced in the presence of mAb 10E5, mAb BIIG4, or the polyclonal antiserum A108 alone, was further reduced by combined 10E5 and BIIG4 or 10E5 and A108. Neither mAb 10E5 nor 6D1 alone blocked adhesion to vWF; however, the combination of 10E5 and 6D1 significantly reduced platelet adhesion to this matrix. Finally, platelet adhesion to the plasma-coated surface was reduced by mAbs 10E5 and BIIG4. These results indicate that multiple adhesion receptors can mediate platelet adhesion to matrix proteins immobilized on surfaces.

Antibodies, Monoclonal↗
PubMed1994

Study of platelet-associated immunoglobulins of IgG, IgM, IgA, and IgE classes and platelet kinetics in 33 patients with idiopathic thrombocytopenic purpura.

The role of platelet-associated immunoglobulins (PAIg) of four different immunoglobulin classes--IgM, IgG, IgA, and IgE--and their relation to platelet count and platelet kinetics was studied in 33 patients with idiopathic thrombocytopenic purpura (ITP). During the course of 1 year, repeated determinations of PAIg were made. The results indicate that PAIgG, PAIgM, and PAIgA are present in all ITP patients, and that autoantibodies of all three Ig classes show highly significant correlations to the platelet counts (p < 0.0001). Double logarithmic negative correlations have been found between PAIgG and platelet count (r = -0.71), PAIgM and platelet count (r = -0.84), and PAIgA and platelet count (r = -0.79). Statistical analyses using partial correlation and multiple regression methods showed that PAIgM is predominantly related to the platelet count, whereas PAIgG and PAIgA are only of secondary importance. Accordingly, a relation of PAIgM (and PAIgA) to increased liver destruction of platelets was found in kinetic studies using 111indium-labeled platelets. Taken together, these results suggest a predominant role of PAIgM in the pathogenesis of ITP.

Autoantibodies↗
PubMed1985

Disaggregation and reaggregation of 'irreversibly' aggregated platelets: a method for more complete evaluation of anti-platelet drugs.

Anti-platelet drugs are generally screened by evaluating their ability to influence initiation and development of irreversible aggregation of platelets. However, to fully characterize the inhibitory effects of an agent, it is essential to determine if it can also disperse irreversibly aggregated cells, whether drug-treated, dispersed platelets are refractory, sensitive to some but not all aggregating agents, as sensitive to all agents as before initial exposure, hypersensitive or whether they can be restored to a sensitive state by modulation of the platelet membrane. We have developed an in vitro system for evaluating the influence of a variety of agents on disaggregation and reaggregation of aggregated blood platelets. Results demonstrate that some agents which inhibit aggregation can also cause disaggregation, while others cannot. The ability to disaggregate is often selective, revealing a dependence on the nature of the agent causing aggregation, and the time after irreversible aggregation that the inhibitor is added. Agents that elevate intracellular cyclic adenosine monophosphate levels were potent inducers of platelet dissociation in aggregates caused by adenosine diphosphate. On the other hand, antimalarials, calmodulin complexing agents and phospholipase inhibitors caused dissociation of platelet aggregates, irrespective of the agonist used. By and large, the dissociated cells were refractory to the action of agonists. Restoration of the sensitivity of disaggregated platelets by treatment with epinephrine demonstrate an ability of inhibitor treated, refractory platelets to recover full functional capacity almost immediately. Thus, careful study of the effects of inhibitors on disaggregation, recovery and reaggregation may reveal features critical to the selection of anti-platelet drugs for clinical utilization.

Adenosine Diphosphate↗
PubMed1983

Metabolism of platelet-activating factor by blood platelets and plasma.

High performance liquid chromatography in combination with a radioactivity detector was used to study the metabolism of platelet-activating factor (1-0-alkyl-2-acetyl-sn-glycero-3-phosphocholine) by washed platelets, platelet-free plasma and platelet-rich plasma obtained from rabbits and humans. Degradation of platelet-activating factor to its 2-lyso derivative was observed in rabbit and human plasma. This degradation of platelet-activating factor in plasma was completely inhibited by diisopropylfluorophosphate and was partially inhibited by ethylenediamine tetraacetic acid. Washed platelets metabolized platelet-activating factor not only to the 2-lyso compound but also, by reacylation of this lyso intermediate, to an analogue of platelet-activating factor probably containing a long-chain acyl group at the sn-2 position. These transformations occurred, but to a lesser extent, in platelet-rich plasma.

Animals↗
PubMed1993

Inhibition by cardiolipins of platelet-activating factor-induced rabbit platelet activation.

Evidence is presented that cardiolipin, a naturally occurring phospholipid, inhibits the aggregatory effect of platelet-activating factor (paf) on rabbit platelets in vitro. Bovine heart cardiolipin was shown to inhibit the aggregation of washed rabbit platelets induced by 1 x 10(-10) M and 2 x 10(-10) M paf with IC50 values (doses for half-maximal inhibition) of 8.4 +/- 0.8 x 10(-7) M and 2.6 +/- 0.6 x 10(-6) M, respectively. Phosphonocardiolipin was also able to inhibit platelet aggregation induced by 1 x 10(-10) M paf with an IC50 value of 3 +/- 1 x 10(-7) M. Both compounds, in concentrations up to 1 x 10(-5) M, were unable to aggregate washed rabbit platelets and failed to inhibit the aggregation induced by 0.9 and 1.8 microM adenosine diphosphate or 0.2-1.0 microM arachidonic acid. By contrast, the acetylated derivative of cardiolipin exerted an aggregatory effect on aspirin-treated rabbit platelets in the presence of creatine phosphate/creatine phosphokinase. This aggregation was inhibited by the specific paf antagonists BN 52021 and WEB 2086. Also, platelets treated with acetyl-cardiolipin were insensitive to the aggregatory effect of paf. Phosphatidic acid, phosphatidylglycerol, bis(dipalmitoylglycero)phosphate and their phosphono analogues were totally inactive. Similar data were obtained when platelet-rich plasma was used instead of washed rabbit platelets. Our results support the hypothesis that the effect of cardiolipin is mediated through specific paf receptors that act on the rabbit platelet membrane.

Animals↗
PubMed1987

Decrease in agonist affinity for human platelet thromboxane A2/prostaglandin H2 receptors induced by a platelet-derived supernatant.

Platelets possess membrane receptors which mediate the aggregatory response to thromboxane A2 (TXA2) and prostaglandin H2 (PGH2). It has been observed recently that the affinities for a series of TXA2/PGH2 mimetics are decreased in crude human platelet membranes and solubilized membranes compared to intact washed platelets. The present study investigated the notion that platelets contain a substance that is released during platelet lysis that reduces the affinity of the TXA2/PGH2 receptor for agonists. The displacement of 9,11-dimethylmethano-11,12-methano-16-(3-iodo-4-hydroxyphenyl)-13, 14-dihydro-13 - aza-15 alpha beta-omega-tetranor-TXA2 ([125I]PTA-OH), a TXA2/PGH2 receptor antagonist, from its binding site in intact washed platelets by TXA2/PGH2 mimetics and antagonists was characterized in the presence or absence of the supernatant (50,000 g) obtained from sonicated platelets. In the presence of the supernatant, there was a significant (P less than 0.025) increase in the IC50 values for the TXA2/PGH2 mimetics U46619, SQ26655, and ONO11113. The increase in the IC50 for U46619 induced by the supernatant was abolished by either boiling or treating the supernatant with trypsin. The supernatant did not affect the Kd or Bmax of [125I]PTA-OH or the IC50 of the TXA2/PGH2 antagonist, SQ29548. Pretreatment of the platelets with the supernatant resulted in a significant (P less than 0.02) reduction in the aggregation response induced by U46619. Gel filtration (Sephacryl S200) of the supernatant revealed a fraction (molecular weight approximately 100,000 daltons) which significantly increased the IC50 for U46619 to displace [125I]PTA-OH from its binding site. Thus, human platelets appear to possess a protein(s) that is released into the supernatant upon sonication and inhibits the binding of TXA2/PGH2 agonists but not antagonists to their receptor. This protein may play a role in the regulation of platelet responses to the aggregatory stimuli TXA2/PGH2.

Blood Platelets↗
PubMed1994

Inhibitory effect of high-density lipoprotein on platelet function is mediated by increase in nitric oxide synthase activity in platelets.

Although high-density lipoprotein (HDL) has been found to decrease platelet function per se, little is known regarding the mechanism of its platelet inhibitory effect. In this study, we confirmed the inhibitory effect of HDL on platelet aggregation and 14C-serotonin release in thrombin-activated washed human platelets. The inhibition of platelet function was associated with an increase in nitric oxide synthase activity, measured as the conversion of 3H-L-arginine to 3H-L-citrulline as well as nitrite release in the platelet supernates. The inhibition of platelet function by HDL was reversed by preincubation of washed platelets with an inhibitor of nitric oxide synthase, NG-nitro-L-arginine methyl ester (L-NAME), and potentiated by co-incubation with the precursor of nitric oxide, L-arginine. These observations suggest that HDL decreases platelet function by increasing nitric oxide synthase activity in human platelets.

Amino Acid Oxidoreductases↗
PubMed1984

Interaction of wheat germ agglutinin with human platelets: a model for studying platelet response.

Binding of wheat germ agglutinin (WGA) to human platelets and their activation were studied. The binding of fluoresceinyl-substituted WGA (F-WGA) was saturable, specific, reversible and cooperative. The apparent association constant (Ka) was 2.4 X 10(6) M-1. Activation of platelets was measured by change in light transmission (LT) (aggregation) and by assays of 14C-serotonin and beta-thromboglobulin (secretion). The maximum platelet activation was obtained with 15 micrograms/ml WGA. At this concentration only 17% of all available binding sites were occupied. Increasing the WGA concentration diminished the change in LT and the secretion from alpha granules but not from dense bodies. The addition of EDTA partially reduced the platelet aggregation without any effect on the secretion. Remaining LT change was insensitive to all metabolic inhibitors tested (CP/CPK, arginine, indomethacin, PGE1, chlorpromazine, colchicine and cytochalasin B). The secretion measured in the presence of EDTA was only reduced by preincubation with PGE1 and CPZ. Succinyl-WGA which only binds glucoconjugates containing GlcNAc did not induce any platelet activation, whereas limulin, which binds to glucoconjugates containing NeuAc did induce platelet agglutination and secretion. These results indicate that: (1) the platelet aggregation and secretion induced by WGA occur with only partial occupancy of the membrane binding sites by the lectin; (2) the platelet LT change is due in part to the agglutinating properties of the lectin bound to the platelet surface, and in another part to the aggregation reaction mediated by a platelet constituent secreted from the storage sites in response to the binding of the lectin.

Alprostadil↗
PubMed1987

Platelet aggregation induced by ether-linked phospholipids. 1. Inhibitory actions of bovine serum albumin and structural analogues of platelet activating factor.

Ether-linked lysophosphatidic acid was able to induce aggregation of platelets from various animals. Feline platelets were the most sensitive followed in decreasing order by human, bovine and rabbit platelets. ONO-6240 and CV-3988, which are antagonists of platelet activating factor, did not inhibit aggregation of feline platelets induced by ether-linked lysophosphatidic acid, indicating that the aggregation induced by the lysophosphatidic acid did not involve receptors for platelet activating factor. An addition of bovine serum albumin caused dose-dependent inhibitions on the aggregation of platelets by both ether-linked lysophosphatidic acid and platelet activating factor. Moreover, 0.5% bovine serum albumin reversed platelet aggregation by the active phospholipid before this aggregation reached a maximum. These results suggest that the lysophosphatidic acid exerts its stimulatory action extracellularly, probably by interacting with specific sites on the surface of platelet.

Animals↗
PubMed1988

Platelet-activating factor binding and metabolism during human platelet aggregation.

Platelet-activating factor (PAF) binding and metabolism were quantified during human platelet aggregation induced by [3H]PAF. Platelet aggregation was maximal within 45 to 75 sec. PAF binding was maximal within 15 to 60 sec and then remained stable for at least 20 min. Total binding during aggregation elicited by 1.2 pmol [3H]PAF was 13.4% +/- 1.0% (0.16 +/- 0.01 pmol/8 x 10(7) platelets) (mean +/- SEM) and specific, receptor binding was 4.3% +/- 0.3% (0.05 +/- 0.003 pmol/8 x 10(7) platelets). Specific binding was fully reversible at the time of maximal aggregation. Scatchard analysis of PAF binding at the time of maximal platelet aggregation revealed 124 +/- 56 receptors per platelet with a dissociation constant of 0.56 +/- 0.04 nM. Binding of 52 +/- 23 PAF molecules per platelet elicited maximal platelet aggregation (mean +/- SD). Less than 2% of the bound PAF was metabolized at the time of maximal aggregation and the principal metabolite was the inactive product lysoPAF. Increased metabolism to alkyl-acyl-glycero-3-phosphocholine was observed at later times. These studies indicate that PAF receptor internalization and PAF metabolism are not essential to human platelet activation by PAF.

Binding, Competitive↗
PubMed1989

Congenital changes of platelet functions in stroke-prone SHR: aggregability of gel-filtered platelets, PRP and whole blood, and effects of hypotensive treatment.

Platelet aggregation in whole blood, platelet rich plasma, and gel-filtered platelets were markedly attenuated in SHRSP compared with those in age-matched normotensive WKY. The result was consistent with the previous report of washed platelets. Despite prevention of high blood pressure, a long duration of hypotensive treatment only slightly improved aggregability of washed platelets but did not restore it to the range of age-matched WKY platelets. Blood pressure, heart ratios and thrombin-induced washed platelet aggregation were examined in SHRSP, WKY, and the cross (F1: WKY x SHRSP). The higher blood pressure and heart ratios the lower platelet aggregability was observed in the three strains, and there was no overlapping distribution of these values. F1 progeny exhibited intermediate values in blood pressure, heart ratio and platelet aggregability between the parental values. These results suggested that hypofunctions of SHRSP platelet were not secondary changes due to high blood pressure, but primary changes which are genetically linked to high blood pressure.

Animals↗
PubMed1992

Preferred use of primary radiolabeled anti-platelet monoclonal antibodies. Comparison of immunoblotting methods for the analysis of functional domains on human platelets.

Several methodologies used for the identification and characterization of platelet receptors for antiplatelet monoclonal antibodies are compared. Two antiplatelet monoclonal antibodies, are investigated due to their potent effects on human platelet function. A platelet-activating monoclonal antibody, called M.Ab.F11, is able to induce platelet aggregation and granular secretion. A platelet-inhibitory monoclonal antibody, named G10, strongly blocks the platelet aggregation and granular secretion induced by M.Ab.F11, as well as by physiological agonists. In order to identify the specific antigens recognized by these monoclonal antibodies, we tested a number of immunostaining methods. Comparison of various procedures revealed that a high degree of nonspecific interactions with platelet proteins occurred when the commonly used secondary reagents, protein A and radiolabeled or enzyme-conjugated secondary antibodies interacted with the platelet proteins either in the presence or absence of primary monoclonal antibodies. On the other hand, we observed a high degree of specificity and selectivity when only radiolabeled anti-platelet monoclonal antibodies were used as single reagents. It is established that M.Ab.F11 interacts with the platelet membrane proteins of 32 and 35 Kd, and M.Ab.G10 recognizes 100 Kd protein, which corresponds to GPIIIa molecule.

Antibodies, Monoclonal↗
PubMed1992

In vivo experiments indicate that relatively high platelet deposition in von Willebrand's disease 'Vicenza' is caused by normal platelet-VWF levels rather than by high VWF-multimers in plasma.

Von Willebrand's disease (vWD) 'Vicenza' is characterized by low plasma von Willebrand Factor antigen (vWF:Ag) and very low levels of Ristocetin Cofactor activity (RiCof). The hemorrhagic tendency in vWD 'Vicenza' is, however, mild and bleeding times in this rare vWD-subtype are only slightly prolonged (1). Larger than normal multimers of plasma-vWF and normal levels of platelet-vWF have both been suggested to compensate the defects that are normally present in vWD. To elucidate the mechanisms involved, whole blood of four patients with vWD 'Vicenza' was circulated through a rectangular perfusion chamber (2) with fibrillar collagen as adhesive surface. Under these conditions, both plasma and platelet vWF participate to platelet adhesion. Compared to perfusion results with blood of normal donors, platelet adhesion of 'Vicenza' patients was decreased. However, the Vicenza defect was less than was observed in parallel experiments with blood of vWD type 1 subtype platelet-low patients and blood of a severe vWD patient. Adhesion was not increased in perfusions with only plasma of the 'Vicenza' vWD-patients. Equal vWF:Ag concentrations of normal multimeric composition were just as effective as the high multimeric 'Vicenza' vWF. Therefore, the abnormal plasma-vWF in 'Vicenza' patients does not cause the relatively high adhesion obtained with whole blood. In contrast, platelets of vWD 'Vicenza' patients resuspended in human albumine solution (HAS) showed far better adhesion than (vWF-poor) platelets of a patient with severe vWD. The values were at least comparable and tended to be higher than those obtained with normal platelets or with platelets of patients with vWD type I subtype platelet normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Biopolymers↗
PubMed1986

Influence of type IIa, IIb and IV-hyperlipoproteinemia on platelet prostaglandin sensitivity, plasma thromboxane B2 and platelet half-life.

It is generally accepted that in hyperlipoproteinemia (HLP) the vascular prostacyclin formation is diminished. We wondered, whether HLP is also associated with changes in platelet sensitivity to antiaggregatory prostaglandins. Therefore, we examined the platelet sensitivity to the prostaglandins PGI2 and PGD2 as well as plasma thromboxane B2 (TXB2)-levels in 24 patients with HLP type IIa, IIb and IV. We found a marked decrease of platelet sensitivity to PGI2 in all the patients examined, which was more pronounced in type IIb than in types IIa and IV. Platelet sensitivity to PGD2 showed no difference in the hyperlipemic patients. Plasma TXB2-levels were significantly increased in comparison to a control group, the changes being most pronounced in patients with type IV HLP. Platelet half-life was significantly shortened in the HLP-patients. This in-vivo platelet function parameter was found to be reduced in patients with type IIa HLP to the greatest extent. Our findings suggest that platelet deposition in HLP is promoted not only by diminished vascular PGI2-formation, but also by decreased sensitivity of the platelets to antiaggregatory prostaglandins. The high TXB2-levels and the shortened platelet half-life reflect the in-vivo activated platelet population in these patients.

Adult↗
PubMed1998

Human platelet activation by thrombolytic agents: effects of tissue-type plasminogen activator and urokinase on platelet surface P-selectin expression.

The mechanisms that underlie reocclusion during thrombolytic therapy have not yet been clarified. The purpose of this study was to investigate the activating effects of tissue-type plasminogen activator and urokinase and the inhibitory effects of acetylsalicylic acid by measuring platelet surface P-selectin as a marker of platelet activation. After addition of urokinase (final concentration 192 U/ml, 1920 U/ml, or 19,200 U/ml) or tissue-type plasminogen activator (final concentration 120 U/ml, 1200 U/ml, or 12,000 U/ml) to platelet-rich plasma from 12 healthy persons, platelet surface P-selectin expression was measured by means of flow cytometry with an anti-CD62 monoclonal antibody. The presence of urokinase and tissue-type plasminogen activator increased platelet surface P-selectin expression in a concentration-dependent manner. In the next step, either 160 mg/day (n = 6) or 660 mg/day (n = 6) acetylsalicylic acid was administered to the 12 healthy persons, and venous blood samples were collected after 7 days of treatment. Platelet surface P-selectin expression was measured with the method used earlier and after addition of tissue-type plasminogen activator or urokinase. Although the effect of acetylsalicylic acid at 160 mg/day on P-selectin expression was minimal, a dose of 660 mg/day suppressed platelet P-selectin expression and inhibited the platelet activating effects of tissue-type plasminogen activator and urokinase in a statistically significant way. Platelets were activated by tissue-type plasminogen activator or urokinase, and this platelet activation was suppressed with administration of acetylsalicylic acid at 660 mg/day.

Aspirin↗
PubMed2000

Comparison of platelet aggregability and P-selectin surface expression on platelets isolated by different methods.

Three methods commonly used for isolation of blood platelets from plasma were compared. Platelets were isolated by: 1) a washing method; 2) a method of metrizamide-gradient centrifugation; 3) a modified method of gel-filtration. The last method employed BSA-Sepharose gel instead of routinely used Sepharose gel saturated with BSA. BSA-Sepharose gel was prepared by covalent binding of thermally deactivated BSA to CNBr-activated Sepharose 2B. In contrast to platelets isolated by the other methods, an aggregability of the gel-filtered platelets and control platelets in plasma, both activated with ADP, were comparable. When expression of P-selectin on the surface of freshly isolated platelets was examined, the gel-filtered platelets exhibited the same extent of fluorescence signal as platelets in the citrated blood, whereas platelets isolated by the other methods exhibited twice the extent of the signal. The methods involving the centrifugation process cause a low but a significant platelet activation.

Adenosine Diphosphate↗
PubMed2001

Role of nitric oxide synthase in collagen-platelet interaction: involvement of platelet nonintegrin collagen receptor nitrotyrosylation.

Platelets possess the endothelial isoform of nitric oxide synthase (eNOS), which plays an important role in platelet function. Other laboratories, including ours, have reported that nitric oxide (NO) is released upon exposure of platelets to collagen, but the mechanism of the interaction is not yet established. The objective of this study is to examine the possible role of nonintegrin receptor nitrotyrosylation on collagen-induced platelet aggregation. Results of the study show that two platelet proteins with M(r) of 65- and 23-kDa proteins are nitrotyrosylated in a time-dependent manner after the addition of type I collagen. The M(r) 65-kDa protein is identified as the platelet receptor for type I collagen. The recombinant protein of the platelet receptor for type I collagen can also be nitrotyrosylated. The nitrotyrosylated recombinant protein loses its ability to inhibit type I collagen-induced platelet aggregation. In addition, the polyclonal anti-65 kDa immunoprecipitates eNOS suggesting that the platelet nonintegrin receptor for type I collagen is closely linked to the eNOS. These results demonstrate that the inhibitory effect of NO on collagen-induced platelet aggregation may be mediated by the nitrotyrosylation of the 65-kDa receptor.

Blood Platelets↗