Thrombin binding to platelets and their activation in plasma

Coated platelets introduce significant delay in onset of peak thrombin production: Theoretical predictions

Platelets play a crucial role in the initiation, progress, termination as well as regulation of blood coagulation. Recent studies have confirmed that not all but only a small percentage of thrombin-activated platelets ("coated" platelets) exhibit procoagulant properties (namely the expression of phosphatidylserine binding sites) required for the acceleration and progress of coagulation. A mechanistic model is developed for in vitro coagulation whose key features are distinct equations for coated platelets, thrombin dose-dependence for coated platelets, and competitive binding of coagulation factors to platelet membrane. Model predictions show significant delay in the onset of peak Va production, and peak thrombin production when dose-dependence is incorporated instead of a fixed theoretical maximum percentage of coated platelets. Further, peak thrombin concentration is significantly overestimated when either fractional presence of coated platelets is ignored (by 299.4%) or when dose-dependence on thrombin is ignored (by 24.7%).

Influence of Different Anticoagulant Agents on Fibrinopeptide A Generation

The purpose of this study was to determine the in vitro effects of different anticoagulant drugs on fibrinopeptide A (FPA) generation inhibition and to identify whether there is any correlation between FPA generation, Hemochron ACT, global clotting as says, and chromogenic assays. Unfractionated heparin is a conventionally used anticoagulant. New anticoagulant drugs such as low molecular weight heparins (LMWHs), pentasaccharide, and antithrombin drugs are now approved for various indications. Anti-Xa drugs are in various phases of clinical development. The influence of different anticoagulant agents has been studied on fibrinopeptide A generation, Hemochron celite ACT, global clotting assays, and chromogenic anti-Xa and anti-Ila assays. Different LMWHs (Clivarin, Dalteparin, Enoxaparin, and Tinzaparin), anti-Xa agents (Pentasaccharide, DX-9065a and unfractionated heparin), and anti-Ila agents (PEG-Hirudin, Hirudin, Efegatran and Argatroban) were studied. The blood from healthy volunteers (n=4) was drawn for each drug. Imuclone FPA enzyme-linked immunosorbent kit assay, Hemochron celite ACT assay, global clotting assays (PT, APTT, Heptest-HI, thrombin time), and Loyola chromogenic anti-Xa and anti-Ila assays were studied. Pentasaccharide demonstrated minimal effects on the whole blood clotting time such as ACT and on inhibition of FPA generation (IC50 > 25 /g/mL). DX-9065a exhibited a significant prolongation of ACT and marked inhibition of FPA generation (IC50 = 4.12 Ag/mL). Unfractionated heparin showed a marked inhibition of FPA generation (IC,, = 5.16 Aog/mL). Pentasaccharide, DX-9065a and UFH showed a marked correlation between ACT and inhibition of FPA generation. LMWHs demonstrated concentrationdependent inhibition of FPA generation. LMWHs studied showed good correlation between FPA generation inhibition and ACT test. Similar correlation was seen between FPA generation inhibition and the APTT, anti Xa (heptest-HI assay) and anti-IIa activity. Anti-IIa drugs demonstrated concentration-dependent inhibition of FPA generation. Their FPA generation inhibition potency is correlated with the ACT assay. A strong correlation between Hemochron ACT and FPA generation inhibition was observed. Based on this significant correlation, the FPA generation inhibition can be predicted by point-of-care ACT assay.

Antithrombotic triple therapy and coagulation activation at the site of thrombus formation: a randomized trial in healthy subjects

BACKGROUND

Patients with acute coronary syndrome and concomitant atrial fibrillation may require antithrombotic triple therapy but clinical evidence of safety and efficacy is poor. We have therefore studied the combination of different antithrombotic medicines for coagulation activation in an in vivo model in the skin microvasculature.

METHODS AND RESULTS

Platelet activation (β-thromboglobulin [β-TG]) and thrombin generation (prothrombin fragment 1 + 2 [F1+2 ], thrombin-antithrombin complex [TAT]) were studied in an open-label, randomized, parallel group trial in 60 healthy male subjects (n = 20 per group) who received ticagrelor and acetylsalicylic acid (ASA) in combination with dabigatran (150 mg bid), rivaroxaban (20 mg od) or phenprocoumon (INR 2.0-3.0). Coagulation biomarkers in shed blood were assessed at 3 h after monotherapy with the medicines under study, at 3 h after triple therapy dosing and at steady state trough conditions. Single doses of ticagrelor, dabigatran or rivaroxaban caused comparable decreases in shed blood β-TG and were more pronounced than phenprocoumon at an INR of 2.0-3.0. In contrast, thrombin generation was more affected by rivaroxaban and phenprocoumon than by dabigatran. During triple therapy a similarly sustained inhibition of platelet activation and thrombin generation with a maximum decrease of β-TG, F1+2 and TAT at 3 h post-dosing was noted, which remained below pre-dose levels at trough steady state.

CONCLUSION

A triple therapy at steady state with ticagrelor plus ASA in combination with dabigatran or rivaroxaban is as effective as a combination with phenprocoumon for platelet activation and thrombin generation in vivo.

A simplified mathematical model for thrombin generation

A new phenomenological mathematical model based directly on laboratory data for thrombin generation and having a patient-specific character is described. A set of the solved equations for cell-based models of blood coagulation that can reproduce the temporal evolution of thrombin generation is proposed; such equations are appropriate for use in computational fluid dynamic (CFD) simulations. The initial values for the reaction rates are either taken from already existing model or experimental data, or they can obtained from simple reasoning under certain assumptions; it is shown that coefficients can be adjusted in order to fit a range of different thrombin generation curves as derived from thrombin generation assays. The behaviour of the model for different platelet concentration seems to be in good agreement with reported experimental data. It is shown that the reduced set of equations used represents to a good approximation a low-order model of the detailed mechanism and thus it can represent a cost-effective and-case specific mathematical model of coagulation reactions up to thrombin generation.

Coordinate activation of human platelet protease-activated receptor-1 and -4 in response to subnanomolar alpha-thrombin

We previously demonstrated that human platelets activated with SFLLRN release PAR-1 activation peptide, PAR-1-(1-41), even in the presence of hirudin. This observation suggests that during their activation, platelets generate a protease that activates PAR-1. In this study, PAR-1 and -4 activation peptides were detected 10 s after <or=1.0 nm alpha-thrombin, 10 microm SFLLRN, or 100 microm AYPGKF were added to platelets. When SFLLRN or AYGPKF were added to platelets, generation of PAR-1 and -4 activation peptides was complete at 10 s. Generation of both PAR-1 and -4 activation peptides in response to 1 nm alpha-thrombin was significantly inhibited by affinity-purified anti-PAR-1-(35-62) IgY, anti-PAR-4-(34-54) IgY, and by the specific PAR-1 antagonist BMS 200261, but not by the PAR-4 antagonist YD3. Effective inhibition of platelet aggregation in response to 1.0 nm alpha-thrombin occurred only in the presence of both anti-PAR span antibodies. We conclude that platelet activation initiated with <or=1.0 nm alpha-thrombin proceeds via simultaneous PAR-1 and -4 activation. Inhibiting the activation of either PAR inhibits activation of the other. Both PAR-1 and -4 activation must be inhibited to prevent platelet activation subsequent to thrombin binding to platelets. The more efficient generation of PAR activation peptides by platelets activated with SFLLRN or AYGPKF, compared with alpha-thrombin, indicates that a platelet-derived serine protease that is inactivated by soybean trypsin inhibitor propagates PAR-1 and -4 activation.

Thrombolytic activity of BB-10153, a thrombin-activatable plasminogen

BACKGROUND

BB-10153 is an engineered variant of human plasminogen, modified to be activated to plasmin by thrombin. Thrombin-activatable plasminogen was designed as a novel thrombolytic agent which would persist in the blood as a prodrug and be activated to plasmin only at fresh or forming thrombi by the thrombin that is tightly localized there. We previously described the construction of several thrombin-activatable plasminogens and their in vitro clot lysis activity.

OBJECTIVES AND METHODS

The current study was an examination of the thrombolytic properties of BB-10153 in vivo; comparison was made with tissue-type plasminogen activator (t-PA) in a femoral artery copper coil thrombosis model in the anesthetized dog and rabbit. Heparin was not coadministered so that the fundamental activity of the agents could be compared.

RESULTS

BB-10153, administered as an intravenous bolus of 5 mg kg(-1) in the dog and 10 mg kg(-1) in the rabbit, produced a comparable incidence of reperfusion to 3 mg kg(-1) t-PA. Reocclusion at these doses occurred in 4/4 dogs and 5/7 rabbits treated with t-PA and in 2/6 dogs and 0/10 rabbits treated with BB-10153. There was no reocclusion in three dogs dosed with 10 mg kg(-1) BB-10153. BB-10153 did not affect plasma alpha2-antiplasmin levels or the bleeding time, whereas 3 mg kg(-1) t-PA caused marked depletion of alpha2-antiplasmin and fibrinogen and increased the bleeding time. The plasma half-life of BB-10153 was 3-4 h.

CONCLUSIONS

The long half-life and thrombus-selective thrombolytic activity of BB-10153 might allow it to overcome the bleeding and reocclusion shortfalls in the performance of current thrombolytics.

Bivalirudin for Endovascular Intervention in Acute Ischemic Stroke: Case Report

OBJECTIVE AND IMPORTANCE

Intra-arterial thrombolysis has been demonstrated to improve recanalization and outcomes among patients with acute ischemic stroke. However, thrombolytic agents have limited effectiveness and are associated with a significant risk of bleeding. Bivalirudin is a direct thrombin inhibitor that has been demonstrated in the cardiology literature to have a more favorable efficacy and bleeding profile than other antithrombotic medications. We report the use of bivalirudin during endovascular treatment of acute stroke, when hemorrhagic complications are not uncommon.

CLINICAL PRESENTATION

A 71-year-old woman with atrial fibrillation presented with right hemiparesis and aphasia and was found to have a National Institutes of Health Stroke Scale score of 10. Computed tomographic scans revealed no evidence of intracranial hemorrhage, aneurysm, or ischemic stroke. Cerebral angiography revealed thromboembolic occlusion of the superior division of the left middle cerebral artery.

INTERVENTION

For anticoagulation, a loading dose of bivalirudin was intravenously administered before the interventional procedure, followed by continuous infusion. Attempts to remove the clot with an endovascular snare failed to induce recanalization of the vessel. Bivalirudin was then administered intra-arterially. Immediate postprocedural angiography demonstrated restoration of flow in the left middle cerebral artery. Repeat computed tomographic scans demonstrated no intracranial hemorrhage. The patient's hemiparesis and aphasia were nearly resolved and her National Institutes of Health Stroke Scale score was 2 at the time of her discharge 5 days later.

CONCLUSION

To our knowledge, this is the first report of the use of bivalirudin for treatment of acute ischemic stroke. Bivalirudin may be a useful agent for intravenous anticoagulation and intra-arterial thrombolysis in this setting.

Protease activated receptors 1 and 4 govern the responses of human platelets to thrombin

Studies carried out in the past 12 years have established that activation of protease-activated receptor (PAR)-1 and PAR-4 by thrombin essentially drives human platelet activation. Thrombin is the most potent physiologic agonist of platelets. PAR-1 and PAR-4 are found on human platelets and are half of the family of the four known 7-transmembrane receptors that are activated by a single proteolytic cleavage within the amino terminal extracellular domain of these receptors. This review will consider the direct and indirect evidence that apparently support the idea that PAR-1 and PAR-4 activation by thrombin drives platelet activation

Engineering a bioactive matrix by modifications of calcium sulfate

The goal of this study was to define the conditions for the fabrication of a bioactive matrix that induces and supports cell proliferation and tissue regeneration. The proposed hypothesis was that a composite graft could be engineered by the absorption of platelet-rich plasma (PRP) onto calcium sulfate (CS). Evaluation of the biological activity of the engineered grafts was based on osteoblast proliferation studies and scanning electron microscopy (SEM) analyses. Graft samples were created in a standard size and shape so that the surface available for attachment and cell proliferation was always identical. Proliferation data were expressed as counts per minute per group and differences among groups were statistically analyzed by analysis of variance followed by the Scheffé test (alpha = 0.1). SEM analysis showed that the combination of CS and PRP presents a preserved crystalline structure well integrated by organic matrix. This combination showed the highest cell proliferation levels (p < 0.001). Further evaluations demonstrated that PRP is activated when combined with CS. When tested as a possible carrier for biologically active molecules such as platelet-derived growth factor (PDGF), CS showed increased cell proliferation (p < 0.001). SEM revealed adherent osteoblasts with broad flattened edges on CS-PRP. This study proposes CS as an efficient carrier for PRP or PDGF and supports the use of these combinations as bioactive matrices in clinical or laboratory applications.

Thrombin induces IL-10 production in microglia as a negative feedback regulator of TNF-alpha release

Interleukin-10 (IL-10), an immunosuppressive cytokine, is produced by monocyte/macrophage lineage cells, T cells, and B cells in the immune system. Here, we show that thrombin induces IL-10 expression in cultured rat microglia. Thrombin treatment increases IL-10 mRNA expression after 3 h and IL-10 release into the culture medium 12 h after thrombin treatment. Neutralizing antibodies against IL-10 significantly enhanced TNF-alpha release from thrombin-treated microglia. IL-10 release was suppressed by an inhibitor of p38 MAPK, SB203580 but not by an inhibitor of ERK pathway, PD98059, whereas both SB203580 and PD98059 inhibited TNF-alpha release. These results suggest that thrombin induces IL-10 and TNF-alpha expression through different signaling mechanisms, and that IL-10 inhibits TNF-alpha release as a negative feedback regulation.

Low preoperative antithrombin activity causes reduced response to heparin in adult but not in infant cardiac-surgical patients

We evaluated the interaction of preoperative antithrombin (AT) activity and intraoperative response to heparin in cardiac surgery. Heparin anticoagulation is essential during cardiopulmonary bypass (CPB). Heparin itself has no anticoagulant properties, however it causes a conformational change of the physiologic plasma inhibitor AT that converts this slow-acting serine protease inhibitor into a fast acting one. Thus, adequate AT activity is a prerequisite for sufficient heparin anticoagulation. AT activity is reduced by long-term heparin therapy. This prospective, observational study investigated 1516 consecutive cardiac-surgical patients (1304 patients >1 yr (Group A) and 212 patients < or = 1 yr (Group I)). AT activity was measured the day before surgery by a chromogenic substrate assay. The celite-activated activated clotting time (ACT) was used to guide intraoperative heparin administration. Heparin sensitivity was calculated and the postoperative blood loss and perioperative blood requirement was recorded. Infant patients had significantly less preoperative AT activity compared with older patients: 84 (33)% vs 97 (17)%, median (interquartile range) (P < 0. 05). The subgroup of patients aged <1 mo (n = 64) demonstrated a preoperative AT activity of 56 (27)% as compared with 90 (23)% in infant patients between one month and one year (n = 148). In adult patients, preoperative AT activity depended predominantly on preoperative heparin treatment: 62% of the patients with an AT activity <80% were pretreated with heparin. Five minutes after heparin but before CPB the ACT was 587 (334) s in Group A patients with AT activity > or = 80%, and 516 (232) in patients with AT activity < or = 80% (P < 0.05). The target ACT of 480 s was achieved in 70% of patients with normal AT activity in Group A compared with only 54% of patients with AT activity <80% (P < 0.05). In Group A patients with decreased AT activity, 18% demonstrated an inadequate ACT response-defined as ACT <400 s-to the first bolus injection of heparin. In Group I, preoperative AT activity did not influence the ACT response (ACT 5 min after heparin: 846 [447] s in patients with AT activity > or = 80% vs 1000 [364] s in patients with decreased AT activity). The heparin sensitivity was 2.4 (1.1) s/unit heparin/kg compared with 1.5 (0.8) s/unit heparin/KG in group A (P < 0.05). These results suggest that preoperative diminished AT activity causes reduced response to heparin in adult but not in infant patients. Infant patients demonstrate a higher heparin sensitivity despite lower preoperative AT activity. Measurement of preoperative AT activity identifies adult patients at risk of reduced sensitivity to heparin.

IMPLICATIONS

In patients less than one year of age, low antithrombin (AT) activity is caused by the immature coagulation system. Despite low AT activity, these young patients demonstrate a normal or increased response to heparin anticoagulation before cardiopulmonary bypass (CPB). In contrast, in patients older than one year of age and adult patients decreased preoperative AT activity is mainly caused by preoperative heparin therapy and causes insufficient response to heparin anticoagulation with a standard heparin dosage. Measurement of preoperative AT activity identifies patients at risk of inadequate anticoagulation during CPB.

Human platelet thrombin receptors. Roles in platelet activation

Platelets are essential participants in hemostasis and thrombosis. Platelets normally circulate in blood as discoid resting cells that become critical constituents of hemostatic plugs or arterial thrombi only after specific receptors on platelet membranes interact with their ligands (agonists) to initiate the reactions that lead to platelet activation. The well-characterized events associated with platelet activation include activation of membrane receptors, shape change, granular secretion, cytoskeletal reassembly, platelet cohesion, and aggregation. The plasma protease alpha-thrombin is the most potent physiologic platelet agonist; this enzyme has other key roles in hemostasis, in the genesis of arterial thrombi, and in embryonic development, inflammation, wound healing, and cell proliferation.

In vivo thrombin generation and activity during and after intravenous infusion of heparin or recombinant hirudin in patients with unstable angina pectoris

In patients with unstable angina, intravenous heparin reduces thrombin activity but does not influence thrombin generation. Recombinant hirudin, a direct thrombin inhibitor, may be more effective in inhibiting both thrombin generation and activity. We measured the plasma levels of prothrombin fragment 1+2 (a marker of thrombin generation) and fibrinopeptide A (a marker of thrombin activity) in 67 patients with unstable angina enrolled in the GUSTO (Global Use of Strategies to Open Occluded Coronary Arteries) IIb trial who were receiving either recombinant hirudin (31 patients) or heparin (36 patients). Blood samples were obtained at baseline (before any treatment), after 3 to 5 days of study drug infusion (immediately before discontinuation), and 1 month later. In the patients receiving recombinant hirudin, the prothrombin fragment 1+2 levels measured immediately before drug discontinuation were significantly lower than at baseline (P:=0.0014), whereas they had not changed in the patients receiving heparin; at this time point, the difference between patients receiving hirudin and those receiving heparin was statistically significant (P:=0.032). One month later, the prothrombin fragment 1+2 levels in both groups were similarly persistently high and did not differ from baseline. Fibrinopeptide A plasma levels at the end of infusion were significantly lower than at baseline in both treatment groups (P:=0. 0005 for hirudin and P:=0.042 for heparin) and remained lower after 1 month (P:=0.0001 for both hirudin and heparin). The fibrinopeptide A plasma levels were not different between patients treated with hirudin versus heparin at baseline, at the end of infusion, and after 1 month. Thus, in patients with unstable angina, in vivo thrombin generation and activity are reduced during intravenous infusion of recombinant hirudin. However, the inhibition of thrombin generation is not sustained, and after 1 month, the majority of patients have biochemical signs of increased thrombin generation.

Inhibition of platelet-dependent prothrombinase activity and thrombin generation by glycoprotein IIb/IIIa receptor-directed antagonists: potential contributing mechanism of benefit in acute coronary syndromes

The glycoprotein (GP) IIb/IIIa receptor antagonists used widely in the medical treatment of acute coronary syndromes and during percutaneous coronary interventions, prevent fibrinogen cross-linking and platelet aggregation, critical initiating steps in arterial thrombosis. Their anticoagulant properties, particularly when administered conjunctively with heparin preparations, are less well-characterized. In a series of in vitro studies, increasing concentrations of abciximab, tirofiban, and eptifibatide either alone or in combination with unfractionated heparin (UFH) or fractionated heparin (enoxaparin) were added to washed platelets suspended in Tyrode's buffer. Following platelet activation and prothrombinase assembly, thrombin generation was determined by enzyme-linked immunosorbent assay (ELISA). There was a concentration-dependent reduction in platelet-dependent thrombin generation with each of the GPIIb/IIIa receptor antagonists. The combination of tirofiban and UFH yielded percent, absolute and relative reductions (compared with tirofiban alone) of 48.0%, 16.9%, and 35.2%, respectively. The corresponding values for eptifibatide and abciximab were 38.0%, 13.5%, 35.5%, and 55.1%, 3.8%, 8.4%, respectively. Thrombin generation was decreased by an additional 2 to 3% (absolute reduction) with high concentrations of enoxaparin in combination with either eptifibatide or abciximab. Platelet GPIIb/IIIa receptor antagonists, beyond their ability to prevent fibrinogen-mediated aggregation, inhibit platelet-dependent prothrombinase activity and thrombin generation in a concentration-dependent manner. Heparin facilitates the existing anticoagulant properties, supporting combination therapy in clinical practice. The potential added benefit of fractionated heparin over UFH will require further investigation.

Newer antithrombin agents in acute coronary syndromes

Thrombin, through its procoagulant and prothrombotic actions, plays a central role in the pathogenesis of unstable angina and acute myocardial infarction. Antithrombin therapy with unfractionated heparin has several important disadvantages, such as a variable anticoagulant effect, sensitivity to platelet factor 4, an inability to inhibit clot-bound thrombin, and the potential to cause thrombocytopenia. Alternative approaches have focused on novel anticoagulants, including direct antithrombins (eg, hirudin) and low-molecular-weight heparins (eg, enoxaparin). Direct antithrombins bind tightly to thrombin without requiring the cofactor antithrombin. Low-molecular-weight heparins display enriched anti-factor Xa activity, improved bioavailability, and facilitated administration versus unfractionated heparin. Recent trials demonstrate that direct antithrombins reduce rates of death and myocardial infarction early in patients without ST elevation, but the treatment effect diminishes over time. In contrast, treatment with enoxaparin shows superiority versus unfractionated heparin, and the treatment effect is durable over time. Whether thrombolysis with adjunctive treatment with low-molecular-weight heparins will show efficacy in patients with ST-segment elevation is the subject of ongoing trials.

Preanalytical requirements for flow cytometric evaluation of platelet activation: choice of anticoagulant

Accurate assessment of in vivo or in vitro platelet activation requires optimal preanalytical conditions to prevent artefactual in vitro activation of the platelets. The choice of anticoagulant is one of the critical preanalytical conditions as anticoagulants exert different effects on the activation of platelets ex vivo. We tested the effectiveness of Diatube-H (also known as CTAD; sodium citrate, theophylline, adenosine and dipyridamole) and citrate vacutainer tubes in preventing artefactual activation of platelets and preserving functional reserve. Platelet surface expression of the CD62P (reflecting alpha granule release), CD63 (reflecting lysosomal release) and modulation of normal platelet membrane glycoproteins CD41a and CD42b, were measured in whole blood and in isolated platelets immediately after collection and at 6, 24 and 48 h after venipuncture. Samples taken into Diatube-H showed less spontaneous platelet activation than did those taken into citrate. To measure in vitro platelet functional reserve, thrombin was added as agonist to blood stored for varying periods up to 48 h. Although Diatube-H suppressed in vitro platelet activation for up to 4 h, in samples kept for 6-24 h before thrombin addition, the inhibitory effect was lost and platelets responded fully to agonist activation. Hence, Diatube-H preserved platelets and allowed for measurement of in vivo platelet activation as well as thrombin-induced in vitro platelet activation after 6-24 h, in both whole blood and isolated platelets.

Thrombin, a survival factor for cultured myoblasts

Three members of the family of protease-activated receptors (PARs), PARs-1, -3 and -4, have been identified as thrombin receptors. PAR-1 is expressed by primary myoblast cultures, and expression is repressed once myoblasts fuse to form myotubes. The current study was undertaken to investigate the hypothesis that thrombin inhibits myoblast fusion. Primary rodent myoblast cultures were deprived of serum to promote myoblast fusion and then cultured in the presence or absence of thrombin. Thrombin inhibited myoblast fusion, but another notable effect was observed; 50% of control cells were apoptotic within 24 h of serum deprivation, whereas less than 15% of thrombin-treated cells showed signs of apoptosis. Proteolysis was required for the effect of thrombin, but no other serine protease tested mimicked the action of thrombin. Neither a PAR-1- nor a PAR-4-activating peptide inhibited apoptosis or fusion, and myoblast cultures were negative for PAR-3 expression. Myoblasts exposed to thrombin for 1 h and then changed to medium without thrombin accumulated apoptosis inhibitory activity in their medium over the subsequent 20 h. Thus the protective action of thrombin appears to be effected through cleavage of an unidentified thrombin receptor, leading to secretion of a downstream apoptosis inhibitory factor. These results demonstrate that thrombin functions as a survival factor for myoblasts and is likely to play an important role in muscle development and repair.

Anticoagulant fucoidan fractions from Fucus vesiculosus induce platelet activation in vitro

Anticoagulant fucoidan fractions of different molecular weight and sulfate content were prepared and investigated for their effects on platelet function in vitro. The fucoidan fractions were incubated with human platelet rich plasma (PRP) at concentrations of 5, 10 and 50 micrograms/ml. Platelet activation was subsequently studied by a standard aggregation assay and flow cytometric determination of the activation dependent platelet-surface markers CD62p (P-selectin, GMP-140) and CD63 (GP53). All fucoidan fractions induced irreversible platelet aggregation in a dose-dependent manner. Comparing fractions of identical molecular weight (100 kDa) the low sulfate content fucoidan FF5 (S = 7.6%) exerted a significantly greater effect than the highly sulfated fucoidan FF7 (S = 10.2%) over the whole concentration range (n = 5, P < 0.05). Among fractions of identical sulfate content fucoidan-induced platelet aggregation was also found to depend on the molecular weight of the fucoidan. At concentrations of 10 and 50 micrograms/ml the high molecular weight fraction FF7/1 (150 kDa) showed a significantly greater effect than the 50 kDa fraction FF7/3 (24.8 +/- 6.7 vs. 7.0 +/- 3.5 and 54.6 +/- 13.5 vs. 15.0 +/- 9.0%, respectively; mean +/- SD, n = 5, P < 0.05). The molecular weight dependence of the fucoidan effect was also reflected by the flow cytometric data. Coincubation of FF7/1 and FF7/3 (10 micrograms/ml) with PRP increased the number of CD62p and CD63 positive platelets by 9.0 +/- 3.3 vs. 2 +/- 1.9 and 7.1 +/- 2.4 vs. 3.2 +/- 2.6% over control values, respectively (n = 5, P < 0.05). In conclusion, our results show that the low molecular weight fucoidan FF7/3 combines potent anticoagulant and fibrinolytic properties with only minor platelet activating effects and is therefore a suitable substance for further pharmacological studies.

Binding of thrombin to the G-protein-linked receptor, and not to glycoprotein Ib, precedes thrombin-mediated platelet activation

The roles of the G-protein-linked thrombin receptor and platelet glycoprotein Ib (GPIb) as alpha-thrombin-binding sites on platelets remain controversial. alpha-Thrombin has been proposed to bind to both GPIb and the hirudin-like domain of the G-protein-linked receptor (from which it cleaves the NH2-terminal extracellular domain to release a 41-mer peptide (TR-(1-41), where TR is alpha-thrombin receptor)) to initiate platelet activation. Using affinity-purified rabbit anti-human TR-(1-41) IgG and immunoblotting, we demonstrated TR-(1-41) release from platelets suspended in Tyrode's buffer containing 2 mM CaCl2 and incubated with >/=0.5 nM alpha-thrombin for 10-60 s at 37 degrees C. As quantified by enzyme-linked immunosorbent assay, 0.32-0.59 nM TR-(1-41) was released from washed platelets (5 x 10(11) platelets/liter) after their incubation with 10 nM alpha-thrombin for 10 s. Parallel binding of alpha-thrombin to and activation of the platelets were confirmed by flow cytometry. A monoclonal antibody against the hirudin-like domain of the G-protein-linked receptor abrogated alpha-thrombin binding to platelets, cleavage of TR-(1-41), and platelet activation by </=1.0 nM (but not 10 nM) alpha-thrombin. Proteolysis of platelet GPIb with Serratia marcescens protease or O-sialoglycoprotein endopeptidase had no effect on alpha-thrombin binding to platelets or their subsequent activation. In contrast, chymotrypsin, which cleaves both GPIb and the G-protein-linked receptor, abrogated alpha-thrombin binding to platelets, TR-(1-41) release, and platelet activation. Furthermore, monoclonal antibodies directed against the reported alpha-thrombin-binding site on GPIb inhibited neither alpha-thrombin binding to nor activation of the platelets. Thus, alpha-thrombin binds to and cleaves the G-protein-linked receptor when it activates platelets, and GPIb does not appear to serve as an important binding site when alpha-thrombin activates platelets.