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PubMed1995

Relationship between platelet density and platelet aggregation, ATP release, and cytosolic-free calcium mobilization in rabbits.

AIM: To study the relationship of platelet density and platelet functions, including aggregation, ATP release, and intracellular calcium mobilization. METHODS: Platelet density was evaluated by discontinuous gradients of Percoll and cytosolic-free calcium concentration in single platelets was measured by fluorescence and imaging. RESULTS: Rabbit platelets were assorted into 3 subpopulations using discontinuous gradients of Percoll: high density (HD, > 1.062, 20%-30%), intermediate density (ID, around 1.057, 40%-50%) and, low density (LD, < 1.051, 10%-15%). The sizes of platelets in these subpopulations were widely different (HD: 725 +/- 76; ID: 443 +/- 47; and LD: 307 +/- 46) and well correlated with the density (r = 0.988, P < 0.01). Thrombin 0.5 U-induced aggregations in the 3 subpopulations were 78 +/- 8%, 69 +/- 4%, and 62 +/- 3%, respectively and a difference was found between HD and LD (P < 0.05). The amounts of ATP released during aggregation were 4.7 +/- 0.9, 3.4 +/- 1.0, and 2.6 +/- 0.6 mumol/4 x 10(5) platelets in HD, ID, and LD groups, respectively. Thrombin 150 U-induced [Ca2+]i mobilization in HD (990 +/- 130 nmol L-1) was different (P < 0.01) from those in ID (410 +/- 80 nmol L-1) and LD (400 +/- 40 nmol L-1). Serotonin (5-HT) 3 mumol L-1 also caused [Ca2+]i mobilization in single-washed platelet subpopulations and results were similar to those of thrombin although it did not induce any detectable aggregation or release reaction. CONCLUSION: Platelet functions and intracellular calcium concentraotion were increased with platelet density in rabbits.

Adenosine Triphosphate↗
PubMed1994

Effects of magnesium on platelet aggregation and adhesion. Magnesium modulates surface expression of glycoproteins on platelets in vitro and ex vivo.

Magnesium deficiency and its association with platelet hyperreactivity has been well recognised in a variety of diseases including myocardial infarction, preeclampsia, and diabetes. In order to investigate potential effects of intravenous Mg2+ supplementation, platelet function was studied by measurements of in vitro bleeding time (BT) and of fibrinogen (Fg)-mediated aggregation of washed platelets. In addition, the effect of Mg2+ on platelet adhesion onto immobilised Fg, on Fg binding to activated platelets, and on surface expression of GMP-140 or GP53 was evaluated. Mg2+ (4 mM) prolonged in vitro BT by 30% and inhibited Fg-mediated aggregation significantly, independent of the agonist used to initiate platelet aggregation (ADP, collagen, epinephrine, thrombin, phorbol ester). Adhesion of resting platelets to immobilised Fg was reduced by 50% in the presence of 2 mM Mg2+. Moreover, Mg2+ reduced Fg binding to ADP- or collagen-stimulated platelets as well as surface expression of GMP-140 with an IC50 of approximately 3 mM. Intravenous administration of Mg2+ to healthy volunteers inhibited both ADP-induced platelet aggregation (p < 0.05) by 40% and binding of Fg or surface expression of GMP-140 by 30% (p < 0.05). Thus, pharmacological concentrations of Mg2+ effectively inhibit platelet function in vitro and ex vivo.

Bleeding Time↗
PubMed1995

Axial dependence of collagen-induced thrombus formation in flowing non-anticoagulated human blood. Anti-platelet drugs impair thrombus growth and increase platelet-collagen adhesion.

Platelet thrombus formation on collagen fibrils is most pronounced at the upstream end of the surface, and it gradually decreases along the axis in parallel with the direction of the blood flow. This phenomenon, known as axial dependent platelet thrombus formation, is explained by the balance of the platelet supply to the surface and the consumption of platelets by growing thrombi. In the present study we have affected this balance by (A) inhibiting the growth of platelet thrombi by aspirin (ASA) or clopidogrel, and thus increasing the platelet concentration at the surface, and by (B) utilising blood from cigarette smokers, who have enhanced thrombus formation immediately after smoking, and thus decreasing the platelet concentration at the surface. Thrombus formation in non-anticoagulated blood was triggered by collagen fibrils positioned in a parallel-plate perfusion chamber at a wall shear rate of 2600 s-1 which is characteristic for moderately stenosed arteries. Morphometrical assessment of thrombus formation was performed at axial positions of 1 and 13 mm downstream to the blood flow inlet at the collagen surface. Platelet-collagen adhesion and thrombus volume in blood from nonsmokers were decreased at the downstream location by 39% (p < or = 0.0001) and by 60% (p < or = 0.0001), respectively. However, increasing the platelet concentration at the surface by partially inhibiting the thrombus growth by ASA or clopidogrel, reduced substantially the axial decrease in platelet adhesion and thrombus volume.(ABSTRACT TRUNCATED AT 250 WORDS)

Aspirin↗
PubMed1995

Sphingosine-1-phosphate: a platelet-activating sphingolipid released from agonist-stimulated human platelets.

Sphingosine-1-phosphate (Sph-1-P) is the initial product of catabolism of sphingosine by sphingosine kinase and is cleaved by Sph-1-P lyase to a fatty aldehyde and ethanolamine phosphate. This phosphorylated sphingoid base is not only an intermediary catabolite, but also a bioactive lipid with important functions, including stimulation of cell proliferation in Swiss 3T3 fibroblasts and inhibition of tumor cell motility. In the present study, we examined functional roles of Sph-1-P in human platelets. Sph-1-P induced platelet shape change and aggregation reactions, although it failed to elicit secretion. Sphingosine, ceramide, sphingomyelin, and N,N-dimethylsphingosine did not mimic the positive effects of Sph-1-P on platelets. Subthreshold concentrations of Sph-1-P and weak platelet agonists such as adenosine diphosphate (ADP) and epinephrine synergistically elicited aggregation, which may be important for efficient amplification of platelet activation. Sph-1-P induced intracellular Ca2+ mobilization and the dose-response for Ca2+ release correlated closely with the concentration required for induction of shape change. On addition of [3H]sphingosine to intact platelets, the label was rapidly converted to Sph-1-P, and subsequently to ceramide and sphingomyelin. Interestingly, the Sph-1-P formed was specifically released into medium on stimulation of platelets with physiologic agonists. The amount of Sph-1-P in platelets, as measured by its conversion into radiolabeled N-acetyl-Sph-1-P, was 1.4 nmol/10(9) cells and was about four times higher than the mass of Sph present. When compared by mole percent Sph-1-P/phospholipid, the value for platelets is over 10 times higher than that for neutrophils. Our results suggest that Sph-1-P, rapidly converted from sphingosine, abundantly stored in platelets, and released on the cell activation, may play a physiologic role in thrombosis, hemostasis, and the natural wound-healing processes.

Adenosine Diphosphate↗
PubMed1996

Platelets from bleeding Simmental cattle have a long delay in both ADP-activated expression of GpIIB-IIIA receptors and fibrinogen-dependent platelet aggregation.

We have previously reported that platelets from bleeding Simmental cattle do not aggregate in vitro in response to ADP, collagen and calcium ionophore A23187, though calcium mobilization and myosin light chain phosphorylation do occur. The aggregation abnormality, measured by aggregometry, was ascribed to abnormal cytoskeletal expression, with the maximal numbers of activated GpIIb-IIIa receptors per platelet no different from that seen in normal bovine platelets activated with ADP. We have therefore sought to compare the kinetics of micro-aggregation with the rate of expression of GpIIb-IIIa receptors required for mediating fibrinogen (Fg)-dependent platelet aggregation, to provide a more direct molecular explanation for the aggregation abnormality. We compared aggregation kinetics of ADP-activated platelets using both aggregometry and particle counting to monitor micro-aggregation. Fibrinogen receptor expression was monitored with FITC-labelled human Fg and with the reporting antibody for activated GpIIb-IIIa, FITC-PAC1, using flow cytometry. The affected platelets show a marked delay in onset of microaggregation for ADP-activated platelets stirred with human Fg, paralleded by an unusual delay in activated GpIIb-IIIa receptor expression (DARE) for otherwise competent Fg binding. The on-rates for Fg binding to platelets maximally pre-activated with PMA are identical for normal and affected platelets, whether comparing the binding of human or bovine Fg. The unique DARE syndrome explains the observed delay in aggregation of platelets from affected Simmental cattle and predicts the bleeding problems due to delayed binding of adhesive proteins.

Adenosine Diphosphate↗
PubMed1998

Glycoprotein VI is a major collagen receptor for platelet activation: it recognizes the platelet-activating quaternary structure of collagen, whereas CD36, glycoprotein IIb/IIIa, and von Willebrand factor do not.

Simple collagen-related peptides (CRPs) containing a repeat Gly-Pro-Hyp sequence are highly potent platelet agonists. Like collagen, they must exhibit tertiary (triple-helical) and quaternary (polymeric) structure to activate platelets. Platelet signaling events induced by the peptides are the same as most of those induced by collagen. The peptides do not recognize the alpha 2 beta 1 integrin. To identify the signaling receptor involved, we have evaluated the response to the CRP, Gly-Lys-Hyp(Gly-Pro-Hyp)10-Gly-Lys-Hyp-Gly of platelets with defined functional deficiencies. These studies exclude a primary recognition role for CD36, von Willebrand factor (vWF), or glycoprotein (GP) IIb/IIIa. Thus, both CD36 and vWF-deficient platelets exhibited normal aggregation, normal fibrinogen binding, and normal expression of CD62 and CD63, measured by flow cytometry, in response to the peptide, and there was normal expression of CD62 and CD63 on thrombasthenic platelets. In contrast, GPVI-deficient platelets were totally unresponsive to the peptide, indicating that this receptor recognizes the Gly-Pro-Hyp sequence in collagen. GPVI-deficient platelets showed some fibrinogen binding in response to collagen but failed to aggregate and to express CD62 and CD63. Collagen, but not CRP-XL, contains binding sites for alpha 2 beta 1. Therefore, it is possible that collagen still induces some signaling via alpha 2 beta 1, leading to activation of GPIIb/IIIa. Our findings are consistent with a two-site, two-step model of collagen interaction with platelets involving recognition of specific sequences in collagen by an adhesive receptor such as alpha 2 beta 1 to arrest platelets under flow and subsequent recognition of another specific collagen sequence by an activatory receptor, namely GPVI.

Binding Sites↗
PubMed1998

Platelets in thrombotic disorders: quantitative and qualitative platelet disorders predisposing to arterial thrombosis.

Endogenous thrombopoietin (TPO) stimulates platelet production in nonhuman primates by inducing dose-dependent megakaryocyte development from early marrow hematopoietic progenitors and subsequent proliferation and endoreduplication. In nonhuman primates, recombinant human TPO, nonpegylated or pegylated rHu megakaryocyte growth and development factor produce log-linear responses in peak peripheral platelet counts (or peripheral platelet mass turnover) and marrow megakaryocyte volume, ploidy, number, and mass. Mpl ligands can support normal peripheral platelet concentrations after myelosuppressive chemotherapy in baboons, and correct the thrombocytopenia in human immunodeficiency virus-infected chimpanzees. Whereas mpl ligands do not induce platelet aggregation in vitro, mpl ligands enhance aggregatory responsiveness of platelets to physiologic agonists both in vitro and transiently ex vivo after treatment with mpl ligands in nonhuman primates. However, platelet recruitment into forming thrombus is not augmented by these agents when evaluated in quantitative rabbit or baboon models of platelet-dependent thrombus formation, except for the effect of platelet concentration, per se. These findings indicate that appropriate dosing of mpl ligands prevents thrombocytopenia without increasing the risk of platelet-dependent thrombo-occlusive complications.

Animals↗
PubMed2006

Effect of platelet antigen polymorphism on platelet inhibition by aspirin, clopidogrel, or their combination.

OBJECTIVES: We studied the modifier effect of platelet antigen polymorphism (PlA2) on platelet inhibition by acetylsalicylic acid (ASA, i.e., aspirin), clopidogrel, or their combination in patients with coronary heart disease. BACKGROUND: Clopidogrel, when administered with ASA, was shown to significantly improve the outcome of patients with acute coronary syndromes compared with patients receiving only ASA. We have shown previously that the effect of ASA on platelets is modified by the glycoprotein IIIa single nucleotide polymorphism PlA2. Hence, an important pharmacogenetic question remains whether the antiplatelet effect of clopidogrel is uniform for all patients or, like acetylsalicylic acid, more selective. METHODS: Thirty PlA1/A1 and 30 PlA1/A2 patients were assigned randomly to ASA 325 mg/day, clopidogrel 75 mg/day, or both. After 10 days, platelet function was studied. RESULTS: Clopidogrel provided stronger platelet inhibition than ASA with adenosine diphosphate as the agonist, and combination therapy resulted in greater inhibition than either inhibitor used alone (p < 0.0001). The use of ASA resulted in greater inhibition compared with clopidogrel with epinephrine (p < 0.0001) and collagen as agonists (p < 0.0001). With collagen as the agonist, platelets from PlA1/A2 donors were markedly and significantly less inhibited by ASA (p = 0.005). In contrast, with clopidogrel, no significant difference could be detected between inhibition of Pl(A1/A1) and Pl(A1/A2) platelets. CONCLUSIONS: The combination of ASA and clopidogrel appears superior to either agent alone in inhibiting platelet function. Pl(A2) functions as an important modifier for platelet responsiveness to ASA but not to clopidogrel. These findings could have significant impact on the future design of pharmacogenetic antithrombotic strategies for patients with coronary heart disease.

Adenosine Diphosphate↗
PubMed1994

Dynamics of platelet glycoprotein IIb-IIIa receptor expression and fibrinogen binding. II. Quantal activation parallels platelet capture in stir-associated microaggregation.

There is broad agreement that platelet aggregation is generally dependent on fibrinogen (Fg) binding to the glycoprotein (GP) IIb-IIIa receptor expressed on the activated platelet surface. We therefore compared rates and extents of aggregation and of fibrinogen receptor expression and specific Fg binding to activated platelets, as a function of ADP concentration. Human citrated platelet-rich plasma (diluted 10-fold) was stirred with adenosine diphosphate (ADP) for 10 s or 2 min to measure rates and extent of aggregation, respectively, determined from the decrease in the total number of particles. The number of fibrinogen receptors and bound Fg were measured from mean fluorescence values obtained with FITC-labeled IgM monoclonal antibody PAC1 and the IgG monoclonal antibody, 9F9, respectively, using flow cytometry as presented in part I (Frojmovic et al., 1994). Because flow cytometric and aggregation measurements were routinely determined at room temperature and 37 degrees C, respectively, we also compared and found temperature-independent initial rates of aggregation. The fraction of platelets with fluorescence values above one critical threshold value, corresponding to maximally "activated" platelets (P*), increased with increasing activator concentration and correlated linearly with the fraction of platelets recruited into aggregates for ADP (r > 0.9). Aggregation was not rate-limited by fibrinogen receptor expression or by Fg binding. It appears that each platelet expresses its maximal Fg receptors at a critical ADP concentration, i.e., occupancy of ADP receptors. This, in turn, leads to rapid Fg occupancy and capture of such "quantally activated" platelets into aggregates.

Adenosine Diphosphate↗
PubMed1996

Inhibition of PAF-induced platelet aggregation by WEB 2086 'in-vitro', an antagonist to the receptor for platelet-activating factor, in bovine.

The sensitivity of bovine platelet aggregation in response to PAF stimulation and the ability of WEB 2086 (a thieno-triazolodiazepine) to inhibit response to PAF-induced platelet aggregation were investigated in the blood from five healthy male Belgian Blue calves. The recorded response to PAF showed a plateau which was dependent on the PAF concentration. Platelet aggregation induced by PAF consists of two mechanisms: reversible and irreversible aggregations which are accompanied by the release of platelet granule contents. Reversible aggregation occurred above (2 . 10(-9) mol/l) PAF, and irreversible aggregation occurred above (2 . 10(-7) mol/l) PAF. Addition of WEB 2086 to bovine platelets in vitro induced a rightward shift in the dose-response curve to PAF. WEB 2086 inhibited PAF-induced aggregation in a competitive reversible manner (pA2 = 7.61). The results of our study show that PAF induces platelet aggregation in platelet-rich plasma (PRP) and that addition of WEB 2086 to bovine platelets in vitro inhibits PAF-induced Platelet Aggregation.

Animals↗
PubMed2001

Fc-receptor dependent platelet aggregation induced by monoclonal antibodies against platelet glycoprotein Ib or von Willebrand factor.

In this paper we describe two pathways leading to platelet activation by crosslinking glycoprotein (GP) Ibalpha to the platelet Fc-receptor (FcgammaRII). First the monoclonal antibody (MoAb) 9C8, raised against human platelet GPIbalpha, dose-dependently induced platelet aggregation of citrate-anticoagulated platelet-rich plasma, an effect that can be inhibited by several activation inhibitors. The FcgammaRII-inhibitory MoAb IV.3 was able to prevent the aggregatory effects of MoAb 9C8, indicating that crosslinking of the antigen GPIbalpha to the FcgammaII-receptor is necessary for the activating effect. Secondly we observed a synergistic activating effect of two anti-von Willebrand factor (vWF) MoAbs IC1E7 and B724, both known to enhance vWF binding to GPIbalpha in the presence of shear or ristocetin. When these antibodies are added together to PRP, platelet aggregation is induced without further need for an additional modulator. This effect can be blocked by either MoAb IV.3 or an inhibitory anti-GPIb MoAb, indicating that again the platelet activation results from signaling through FcgammaRII crosslinked to vWF bound to GPIbalpha. In addition, both the anti-GPIb MoAb 9C8, or the two anti-vWF MoAbs 1C1E7 and B724 induce genuine platelet activation, as evidenced by the secretion of ATP and protein tyrosine phosphorylation. These findings with both anti-GPIb and anti-vWF MoAbs add further proof to recent reports demonstrating an interaction between the platelet receptors GPIb and FcgammaRII, suggesting a role for the FcgammaII-receptor in GPIb-related signaling.

Adenosine Triphosphate↗
PubMed1998

In vitro efficacy of platelet glycoprotein IIb/IIIa antagonist in blocking platelet function in plasma of patients with heparin-induced thrombocytopenia.

Heparin-induced thrombocytopenia (HIT) is an important complication following administration of heparin. Platelet activation and aggregation induced by heparin/platelet factor 4/immunoglobulin complexes are thought to be the underlying mechanism for this condition, so it was hypothesized that abciximab (a humanized murine monoclonal antibody directed against the glycoprotein IIb/IIIa receptor) would prevent heparin-induced platelet aggregation and activation in plasma from patients with HIT. Platelet aggregation was tested in vitro with platelet-poor plasma (obtained from 23 patients with HIT), platelet-rich plasma (from normal donors with known reactivity), heparin (0.5 U/ml), and ascending doses of abciximab (0.07-0.56 microg/ml). The ability of abciximab to prevent platelet activation was also evaluated using flow cytometry (P selectin expression, mepacrine release, microparticle formation) and platelet factor 4 immunoassay. In vitro, abciximab inhibited heparin-induced platelet aggregation in a dose-dependent fashion (IC50 0.103 microg/ml) and inhibited microparticle formation, the expression of P-selectin, release of mepacrine and platelet factor 4. These findings suggest that abciximab may be useful in treatment of patients with HIT and warrants further clinical evaluation.

Abciximab↗
PubMed2002

Mechanisms for increment of platelet associated IgG and platelet surface IgG and their implications in immune thrombocytopenia associated with chronic viral liver disease.

In addition to hypersplenism, immunological destruction of platelets by elevated platelet associated IgG (PAIgG) and platelet surface IgG (PSIgG) has been proposed as a causative factor for thrombocytopenia in chronic liver disease (CLD), although the implication of PAIgG may be debatable since recent investigations on idiopathic thrombocytopenic purpura disclosed the fact that PAIgG largely relates to the intra-platelet IgG in alpha-granules and not to PSIgG. Further, with regard to the elevated PSIgG of CLD, characterization as to whether it mainly represents anti-platelet glycoprotein (GP) antibodies or IgG contained in the immune complex has not been elucidated. Thirty-seven patients with chronic viral liver disease (CVLD); 31 hepatitis C and 6 hepatitis B were included in this study. First we monitored the changes in levels of PAIgG, alpha-granule IgG, PSIgG and mean platelet volume (MPV) during the course of partial splenic arterial embolization (PSE). The elevated level of PAIgG decreased after PSE, paralleling that of alpha-granule IgG, while PSIgG showed no change; MPV decreased reciprocally with the increase of platelet count. These results indicate that the increment of PAIgG in CVLD may be caused by accelerated destruction of platelets; this generally evokes hyperproduction of large-sized thrombocytes, which have an increased capability to uptake circulating IgG. To characterize PSIgG, we then tested CVLD patients for antiplatelet GP antibodies and found only a 5.4% positivity. It was also found that circulating immune complex levels in CVLD patients were clearly elevated, correlating with the levels of PSIgG. Thus, it was surmised that immune complexes bound to the platelet surface, and not platelet specific GP antibodies, may be playing a crucial role in platelet destruction of CVLD, possibly through phagocytosis by macrophages.

Journal Article↗
PubMed1997

PMA induces platelet activation of specific antigens (CD62/CD63) in GpIIb-IIIa deficient platelets from Glanzmann's thrombasthenia.

Glanzmann's thrombasthenia (GT) is a hereditary platelet disorder resulting from a quantitative or qualitative deficiency of the major platelet membrane complex GPIIb-IIIa (CD41) required for platelet aggregation. We investigated by flow cytometry, the expression of CD41, fibrinogen, and of two platelet activation-related antigens, CD62 and CD63, (i) before and after activation of platelets by PMA, and (ii) on the surface and within the cytoplasm of resting platelets, after permeabilization by saponin. Platelets from a series of normal subjects and from nine members of two GT families, were reacted with FITC-conjugated antibodies and analyzed on a flow cytometer. Fluorescence intensities measured on normal and GT platelets were quantified by using calibrated beads. Results showed lack of both GPIIb-IIIa and fibrinogen, on the platelet surface and also within the cytoplasm in five of these GT patients, whereas GPIIb-IIIa and fibrinogen remained normal in the four other cases. However, CD62 and CD63 antigenic levels were found within normal range for all members of these families, after PMA stimulation and also after platelet permeabilization. This work therefore showed that the lack of CD41 in GT, which causes strong disturbance of platelet aggregation, may not be associated with the deficiency of activation pathways.

Journal Article↗
PubMed1998

Activation of leukocytes in whole blood samples by N-formyl-methionyl-leucyl-phenylalanine (FMLP) enhances platelet aggregability but not platelet P-selectin exposure and adhesion to leukocytes.

Adhesion of platelets to neutrophils and monocytes is believed to play an important role in intercellular communication. Evidence has been provided that such heterotypic cell-cell contacts via adhesion molecules may be directly involved in intercellular signal transduction as well as facilitate the action of soluble signal transmitters, e.g. cathepsin G, PAF or nitric oxide. With respect to platelet activation, stimulatory and inhibitory effects of leukocytes have been reported, and the results obtained seem to be influenced by the experimental conditions. We investigated the effect of leukocyte stimulation on platelet behaviour in samples of human citrated whole blood. Adding the chemotactic peptide FM LP, which stimulates neutrophils and monocytes but not lymphocytes and platelets, to stirred whole blood samples resulted in a significant enhancement ( P < 0.01) of spontaneous as well as ADP-induced platelet aggregation (25 vs 33% and 66 vs 69% , respectively). In contrast stirring-induced as well as ADP-induced increase of P-selectin exposure (33 and 107% , respectively) was not affected by FMLP. In unstirred whole blood samples, about 10 to 20% of neutrophils and monocytes had bound platelets to their surfaces, and the number of these heterotypic conjugates was enhanced about twofold during spontaneous platelet aggregation. Addition of FMLP significantly reduced the stirring-induced formation of platelet-neutrophil conjugates but not of platelet-monocyte conjugates. These results indicate that neutrophil and/or monocyte activation in whole blood may enhance platelet aggregation, but not secretion (CD62P exposure) and formation of heterotypic platelet-leukocyte conjugates.

Journal Article↗
PubMed1998

Spreading of platelets: a morphological marker for platelet reactivity in peripheral arterial occlusive disease.

Platelet-surface contact is the first step in thrombus formation. Platelet spreading makes this initial contact irreversible. On the other hand plasma lipids and fibrinogen have been described to activate platelets or promote adhesion. We therefore investigated whether platelet spreading under stagnation-point flow conditions correlated with plasma concentrations of cardiovascular risk factors such as fibrinogen and high density lipoprotein (HDL)-cholesterol. Platelet rich plasma (PRP) from patients with peripheral arterial occlusive disease and healthy controls was examined by means of the Stagnation-Point Flow Adhesio- Aggregometer (SPAA). The SPAA comprises a microscopic setup with a flow chamber that permits direct observation and quantitation of platelet deposition onto standardized surfaces. After the flow experiments the deposited platelets were analyzed morphometrically for the degree of spreading expressed as inverse circularity (1/C). 1/C was correlated over 2 X 2 tables of fibrinogen combined with plasma levels of HDL-cholesterol, each of which was divided into a low and high value group. The patient and control group differed significantly with regard to 1/C, i.e. patient platelets demonstrated more adhesive platelets with a more extensive degree of spreading. 1/C was inversely correlated with HDL-cholesterol and showed significant differences between the patient and the control group. Increased 1/C values were found when associated with high fibrinogen levels and simultaneously with low HDL-cholesterol concentrations. Platelet spreading shows a correlation with increased levels of independent plasmatic risk factors for thrombosis in PAOD patients. Obtained during stagnation-point flow, spreading seems to be a morphological marker for platelet hyperreactivity.

Journal Article↗
PubMed1998

Role of GPIIb-IIIa in platelet-monocyte and platelet-neutrophil conjugate formation in whole blood.

Platelets in stirred whole blood can be induced to form aggregates and also to form heterotypic platelet-monocyte (P/M) and platelet-neutrophil (P/N) conjugates. Here we have investigated the effects of three GPIIb-IIIa antagonists (GR144053F, MK-852 and Reopro, a CD62P-blocking antibody, GA6, and EDTA on the conjugate formation that occurs on stirring whole blood and in response to adding ADP and PAF. We have confirmed the identities of the conjugates by light microscopy after cell sorting. Platelet aggregation was measured by platelet counting. Monocytes, neutrophils, P/M and P/N were detected and quantitated using immunofluorescence and flow cytometry. Stirring whole blood resulted in both platelet aggregation and formation of P/M but not P/N. Adding ADP or PAF to whole blood caused rapid platelet aggregation and generation of both P/M and P/N. All of the GPIIb-IIIa antagonists studied had similar effects: inhibition of stirring-induced platelet aggregation and P/M formation, and inhibition of ADP-induced platelet aggregation and P/N formation. In contrast, they accelerated ADP induced-P/M conjugate formation and PAF-induced formation of both P/M and P/N. Both EDTA and GA6 completely inhibited P/M and P/N, which is commensurate with CD62P being involved in platelet-leucocyte conjugate formation. The results of these investigations suggest that GPIIb-IIIa has a dual role in determining the interaction between platelets and leukocytes.

Journal Article↗
PubMed1997

Platelets mediate tumor cell adhesion to the subendothelium under flow conditions: involvement of platelet GPIIb-IIIa and tumor cell alpha(v) integrins.

The aim of our study was to explore the role of platelets and their specific integrin receptors in mediating the interaction of 4 human tumor cell lines (3 melanoma and 1 carcinoma) with the extracellular matrix (ECM) under static and arterial flow conditions. Under static conditions, all 4 cell lines adhered to the ECM. The adhesion capacity of all 4 cell lines was virtually abolished by application of flow during incubation with the ECM. Under static conditions, tumor cell adhesion was not affected by adding platelets to the cell suspension and was slightly reduced by pre-coating the ECM with platelets prior to the addition of tumor cells. In contrast, under flow conditions, platelets significantly increased tumor cell adhesion to the ECM, the enhancing effect being more pronounced when platelets were pre-incubated with the ECM prior to the addition of tumor cells than when incubated simultaneously with the cells. Platelet-mediated tumor cell adhesion under flow was markedly inhibited by blockade of the platelet GPIIb-IIIa or of the tumor cell alpha(v) integrins. Platelets of a Glanzmann thrombastenia (GT) patient were unable to support tumor cell adhesion to the ECM under flow. Our results suggest that the interaction of tumor cells with subendothelium-bound platelets under flow conditions is mediated by platelet GPIIb-IIIa and by tumor cell alpha(v) integrins independently of the nature of the beta subunit.

Animals↗