Antiplatelet agents in tissue factor-induced blood coagulation

Salicylamide Enhances Melanin Synthesis in B16F1 Melanoma Cells

Salicylamide, a non-steroidal anti-inflammatory drug (NSAID), is used as an analgesic and antipyretic agent. We have previously shown that several NSAIDs have anti-melanogenic properties in B16F1 melanoma cells. In contrast, we have found that salicylamide enhances melanin contents in B16F1 melanoma cells; however, the underlying mechanism is not known. Therefore, we investigated the mechanism through which salicylamide stimulates melanogenesis. Interestingly, salicylamide enhanced diphenolase activity in a cell-free assay. Western blotting and real-time RT-PCR revealed that salicylamide increased tyrosinase expression via transcriptional activation of the Mitf gene. Together, our results indicate that salicylamide could be used as an anti-hypopigmentation agent for skin and/or hair.

Fibrinolysis in Platelet Thrombi

The extent and duration of occlusive thrombus formation following an arterial atherothrombotic plaque disruption may be determined by the effectiveness of endogenous fibrinolysis. The determinants of endogenous fibrinolysis are the subject of much research, and it is now broadly accepted that clot composition as well as the environment in which the thrombus was formed play a significant role. Thrombi with a high platelet content demonstrate significant resistance to fibrinolysis, and this may be attributable to an augmented ability for thrombin generation and the release of fibrinolysis inhibitors, resulting in a fibrin-dense, stable thrombus. Additional platelet activators may augment thrombin generation further, and in the case of coronary stenosis, high shear has been shown to strengthen the attachment of the thrombus to the vessel wall. Neutrophil extracellular traps contribute to fibrinolysis resistance. Additionally, platelet-mediated clot retraction, release of Factor XIII and resultant crosslinking with fibrinolysis inhibitors impart structural stability to the thrombus against dislodgment by flow. Further work is needed in this rapidly evolving field, and efforts to mimic the pathophysiological environment in vitro are essential to further elucidate the mechanism of fibrinolysis resistance and in providing models to assess the effects of pharmacotherapy.

Hemoperfusion leads to impairment in hemostasis and coagulation process in patients with acute pesticide intoxication

Hemoperfusion (HP) is one of the important treatment modalities in extracorporeal therapy for patients with acute intoxication. Its use has declined during the past 20 years despite its efficacy, because of its side effects, especially an increased risk of bleeding. Mechanisms of hemostasis impairment have not been clearly elucidated and studies demonstrating the mechanism are lacking. It is not clear which step of the hemostatic process is impaired during HP, and whether it leads to an increased risk of bleeding. We performed both in vivo and in vitro studies to elucidate the mechanism of impairment in the hemostatic process. In patients with acute pesticide intoxication who underwent HP, the platelet count decreased rapidly during the first 30 minutes from 242.4 ± 57.7 × 10³/μL to 184.8 ± 49.6 × 10³/μL, then gradually decreased even lower to 145.4 ± 61.2 × 10³/μL over time (p < 0.001). As markers of platelet activation, platelet distribution width increased continuously during HP from 41.98 ± 9.28% to 47.69 ± 11.18% (p < 0.05), however, mean platelet volume did not show significant change. In scanning electron microscopy, activated platelets adhered to modified charcoal were observed, and delayed closure time after HP in PFA-100 test suggested platelet dysfunction occurred during HP. To confirm these conflicting results, changes of glycoprotein expression on the platelet surface were evaluated when platelets were exposed to modified charcoal in vitro. Platelet expression of CD61, fibrinogen receptor, significantly decreased from 95.2 ± 0.9% to 73.9 ± 1.6%, while those expressing CD42b, von Willebrand factor receptor, did not show significant change. However, platelet expression of CD49b, collagen receptor, significantly increased from 24.6 ± 0.7% to 51.9 ± 2.3%. Thrombin-antithrombin complex, a marker for thrombin generation, appeared to decrease, however, it was not statistically significant. Fibrin degradation products and d-dimers, markers for fibrinolysis, increased significantly during HP. Taken together, our data suggests that hemoperfusion leads to impairment of platelet aggregation with incomplete platelet activation, which was associated with reduced thrombin generation, accompanied by increased fibrinolysis.

Indomethacin inhibits melanogenesis via down-regulation of Mitf mRNA transcription

Non-steroidal anti-inflammatory drugs (NSAIDs) exhibit several divergent biological effects. In this study, we investigated the effect of indomethacin on melanin synthesis using B16F1 melanoma cells. Indomethacin inhibited α-melanocyte stimulating hormone (α-MSH)-enhanced melanin synthesis in a dose-dependent manner. Western blotting analysis revealed that indomethacin significantly suppressed tyrosinase and Mitf protein levels. In a luciferase reporter assay, we found that indomethacin reduced tyrosinase promoter activity. Moreover, real-time RT-PCR analysis showed that indomethacin lowered mRNA levels of melanogenic genes, including Mitf. Together, our findings indicate that indomethacin inhibits melanogenesis via the suppression of Mitf transcription.

2-Ethoxybenzamide stimulates melanin synthesis in B16F1 melanoma cells via the CREB signaling pathway

Non-steroidal anti-inflammatory drugs are frequently used for the treatment of inflammation, pain, and fever. In this study, we found that 2-ethoxybenzamide (ETZ) significantly enhanced melanin synthesis in B16F1 melanoma cells, and also induced melanosome formation. Therefore, we investigated the mechanism by which ETZ up-regulated melanin synthesis. Western blot analysis demonstrated that ETZ increased melanogenic protein levels, except that for TRP-2. Moreover, semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) and real-time RT-PCR analyses showed that ETZ enhanced the mRNA levels of melanogenic genes, including microphthalmia-associated transcription factor and melanocortin 1 receptor. We also observed phosphorylation of cAMP response element-binding protein (CREB) following ETZ treatment. However, ETZ did not affect intracellular cAMP levels. ERK was also activated by ETZ treatment, and melanin content was enhanced upon treatment with the specific ERK inhibitor PD98059. Together, our results indicate that ETZ induces melanin synthesis via CREB phosphorylation.

Abciximab Does Not Inhibit the Increase of Thrombin Generation Produced in Platelet-Rich Plasma In Vitro by Sodium Arachidonate or Tissue Factor

Aspirin and platelet membrane glycoprotein (GP) IIb/IIIa blockers are currently used for acute coronary events, and in percutaneous coronary intervention for preventing further coronary outcomes, because they inhibit platelet function. Aspirin also inhibits thrombin generation (TG) in platelet-rich plasma (PRP) activated by sodium arachidonate (AA). The effect of the platelet membrane GP IIb-IIIa (integrin αIIbβ3) blocker abciximab on thrombin generation was studied in vitro using PRP. Thirty healthy volunteers taking no medication, and 28 volunteers who had taken aspirin (160 mg/day for 3-4 days), were included in the protocol. Control or in vivo aspirinated PRP, stimulated or not by AA or tissue factor (TF), was investigated for the inhibitory effect of abciximab pre-incubated for 3 minutes. AA and TF added in vitro activated non-aspirinated PRP: lag-time (LT) and time to peak (TTP) were significantly shortened. Peak TG (PTG) and endogenous thrombin potential (ETG) were increased by AA but not TF; thus, AA seems to be more efficient than TF for TG in this system. Abciximab added in vitro to non-activated, non-aspirinated PRP had no effect on LT, TTP, or ETP, but caused a decrease in PTG that was not statistically significant. Abciximab (3 or 4 μg/mL) added in vitro to AA or TF-activated, non-aspirinated PRP produced no effect on TG, although in aspirinated platelets both LT and time to peak were prolonged. AA as well as TF added in vitro to PRP or in vivo aspirinated PRP increased TG, although AA seems to be more efficient in our assay system. Abciximab, which affects nonaspirinated, nonactivated PRP weakly, has no effect on AA or TF in activated control PRP or in vivo aspirinated PRP.

Dual effects of acetylsalicylic acid on ERK signaling and Mitf transcription lead to inhibition of melanogenesis

Acetylsalicylic acid (ASA) is widely used as an analgesic/antipyretic drug. It exhibits a wide range of biological effects, including preventative effects against heart attack and stroke, and the induction of apoptosis in various cancer cells. We previously found that ASA inhibits melanogenesis in B16 melanoma cells. However, the mechanisms of how ASA down-regulates melanin synthesis remain unclear. Here, we investigated the effect of ASA on melanogenic pathways, such as extracellular signal-regulated kinase (ERK) and microphthalmia-associated transcription factor (Mitf) transcription. ASA significantly inhibited melanin synthesis in a dose-dependent manner without oxidative stress and cell death. Semi-quantitative reverse transcription-polymerase chain reaction analysis showed that the inhibitory effect of ASA might be due to the inhibition of Mitf gene transcription. Interestingly, ASA also induced ERK phosphorylation. Additionally, treatment with PD98059, a specific ERK phosphorylation inhibitor, abolished the anti-melanogenic effect of ASA. These results suggest that the depigmenting effect of ASA results from down-regulation of Mitf, which is induced by both the induction of ERK phosphorylation and the inhibition of Mitf transcription.

Effects of anticoagulation on markers of activation of clotting following major orthopedic surgery

INTRODUCTION

This study examines makers of activation of clotting following three chemoprophylactic regimens used for prevention of postoperative venous thromboembolic disease (TED) following high-risk surgery for TED.

METHODS

Patients having elective primary knee or hip replacement surgery received variable dose warfarin (target international normalized ratios 2.0-2.5), 1 mg warfarin daily starting 7 days preoperatively or aspirin 325 mg daily starting on the day of surgery. Twelve patients in each group were treated for 28 ± 2 days. Thrombin-antithrombin (T-AT) and prothrombin fragment F1 + 2 were measured at baseline and postoperative days 3 and 28 ± 2.

RESULTS

Thrombin-antithrombin and F1 + 2 on postoperative day 3 were equal for the study groups. By days 28 ± 2, variable dose warfarin therapy group suppressed production of F1 + 2 (P = 0.002) with no difference in the T-AT accumulation. F1 + 2 for other patients overlapped the normal range.

CONCLUSION

The signals of activated clotting following surgery did not differentiate the three regimens on postoperative day 3. Variable dose warfarin was associated with suppression of F1 + 2 after 1 month of therapy, with no effect on accumulation of T-AT. Fixed low-dose warfarin started 7 days prior to surgery and aspirin are not inferior on postoperative day 3, but appear to be inferior over a longer treatment.

Why does aspirin decrease the risk of venous thromboembolism? On old and novel antithrombotic effects of acetyl salicylic acid

It is well established that aspirin, an irreversible inhibitor of platelet cyclooxygenase activity, is effective in secondary prevention of arterial thromboembolic events. The pooled results of the recent randomized, multicenter WARFASA and ASPIRE aspirin trials showed a 32% reduction in the rate of recurrence of venous thromboembolism (VTE) in patients receiving aspirin following VTE. These clinical data support evidence that platelets contribute to the initiation and progression of venous thrombosis and aspirin inhibits thrombin formation and thrombin-mediated coagulant reactions. In addition to the known acetylation of serine 529 residue in platelet cyclooxygenase-1, the postulated mechanisms of aspirin-induced antithrombotic actions also involve the acetylation of other proteins in blood coagulation, including fibrinogen, resulting in more efficient fibrinolysis. This review summarizes current knowledge on the aspirin-induced antithrombotic effects that potentially explain clinical studies showing reduced rates of VTE events in aspirin-treated subjects.

The effect of corn trypsin inhibitor and inhibiting antibodies for FXIa and FXIIa on coagulation of plasma and whole blood

BACKGROUND

Corn trypsin inhibitor (CTI), an inhibitor of FXIIa, is used to prevent plasma coagulation by contact activation, to specifically investigate tissue factor (TF)-initiated coagulation.

OBJECTIVE

In the present work the specificity of CTI for factor (F) XIIa is questioned.

METHODS AND RESULTS

In the commercial available plasma coagulation assays CTI was found to double activated partial thromboplastin time (APTT) at a plasma concentration of 7.3 ± 1.5 μm CTI (assay concentration 2.4 μm). No effect was found on the prothrombin time (PT) when high TF concentrations were used. Also, with specific antibodies for FXIIa and for FXIa only APTT was found to be extended but not PT. With specific enzyme assays using chromogenic substrates CTI was shown to be a strong inhibitor of FXIIa and a competitive inhibitor of FXIa with Ki  = 8.1 ± 0.3 μm, without effect on the coagulation factors FVIIa, FIXa, FXa and thrombin. In thrombin generation and coagulation (free oscillation rheometry, FOR) assays, initiated with low TF concentrations, no effect of CTI (plasma concentrations of 4.4 and 13.6 μm CTI, 25 resp. 100 mg L(-1) in blood) was found with ≥ 1 pm TF. At ≤ 0.1 pm TF in the FOR whole blood assay the coagulation time (CT) concentration dependently increased while the plasma CT became longer than the observation time.

CONCLUSION

To avoid inhibition of FXIa and the thrombin feedback loop we recommend that for coagulation assays the concentration of CTI in blood should be below 20 mg L(-1) (1.6 μm) and in plasma below 3 μm.

Antiplatelet agents can promote two-peaked thrombin generation in platelet rich plasma: mechanism and possible applications

BACKGROUND

Thrombin generation assay is a convenient and widely used method for analysis of the blood coagulation system status. Thrombin generation curve (TGC) is usually bell-shaped with a single peak, but there are exceptions. In particular, TGC in platelet-rich plasma (PRP) can sometimes have two peaks.

OBJECTIVE

We sought to understand the mechanism underlying the occurrence of two peaks in the PRP thrombin generation curve.

METHODS

Tissue factor-induced thrombin generation in PRP and platelet-poor plasma (PPP) was monitored using continuous measurement of the hydrolysis rate of the thrombin-specific fluorogenic substrate Z-Gly-Gly-Arg-AMC. Expression of phosphatidylserine (PS) and CD62P on the surface of activated platelets was measured by flow cytometry using corresponding fluorescently labeled markers.

RESULTS

The addition of the P(2)Y(12) receptor antagonist MeS-AMP (160 µM), 83 nM prostaglandin E(1) (PGE(1)), or 1.6% DMSO to PRP caused the appearance of two peaks in the TGC. The PS exposure after thrombin activation on washed platelets in a suspension supplemented with DMSO, PGE(1) or MeS-AMP was delayed, which could indicate mechanism of the second peak formation. Supplementation of PRP with 1.6% DMSO plus 830 nM PGE(1) mediated the disappearance of the second peak and decreased the amplitude of the first peak. Increasing the platelet concentration in the PRP promoted the consolidation of the two peaks into one.

CONCLUSIONS

Procoagulant tenase and prothrombinase complexes in PRP assemble on phospholipid surfaces containing PS of two types--plasma lipoproteins and the surface of activated platelets. Thrombin generation in the PRP can be two-peaked. The second peak appears in the presence of platelet antagonists as a result of delayed PS expression on platelets, which leads to delayed assembly of the membrane-dependent procoagulant complexes and a second wave of thrombin generation.

Measuring the mechanical properties of blood clots formed via the tissue factor pathway of coagulation

Thrombelastography (TEG) is a method that is used to conduct global assays that monitor fibrin formation and fibrinolysis and platelet aggregation in whole blood. The purpose of this study was to use a well-characterized tissue factor (Tf) reagent and contact pathway inhibitor (corn trypsin inhibitor, CTI) to develop a reproducible thrombelastography assay. In this study, blood was collected from 5 male subjects (three times). Clot formation was initiated in whole blood with 5 pM Tf in the presence of CTI, and fibrinolysis was induced by adding tissue plasminogen activator (tPA). Changes in viscoelasticity were then monitored by TEG. In quality control assays, our Tf reagent, when used at 5 pM, induced coagulation in whole blood in 3.93 ± 0.23 min and in plasma in 5.12 ± 0.23 min (n=3). In TEG assays, tPA significantly decreased clot strength (maximum amplitude, MA) in all individuals but had no effect on clot time (R time). The intraassay variability (CVa<10%) for R time, angle, and MA suggests that these parameters reliably describe the dynamics of fibrin formation and degradation in whole blood. Our Tf reagent reproducibly induces coagulation, making it an ideal tool to quantify the processes that contribute to mechanical clot strength in whole blood.

The inhibitory effect of non-steroidal anti-inflammatory drugs (NSAIDs) on the monophenolase and diphenolase activities of mushroom tyrosinase

In the present work, we investigated the effect of non-steroidal anti-inflammatory drugs (NSAIDs) on the monophenolase and diphenolase activity of mushroom tyrosinase. The results showed that diflunisal and indomethacin inhibited both monophenolase and diphenolase activity. For monophenolase activity, the lag time was extended in the presence of diflunisal. In the presence of indomethacin, the lag time did not change. IC₅₀ values of monophenolase activity were estimated to be 0.112 mM (diflunisal) and 1.78 mM (indomethacin). Kinetic studies of monophenolase activity revealed that both diflunisal and indomethacin were non-competitive inhibitors. For diphenolase activity, IC₅₀ values were estimated to be 0.197 mM (diflunisal) and 0.509 mM (indomethacin). Diflunisal and indomethacin were also found to be non-competitive diphenolase inhibitors.

Thrombin generation and bleeding in haemophilia A

Haemophilia A displays phenotypic heterogeneity with respect to clinical severity. The aim of this study was to determine if tissue factor (TF)-initiated thrombin generation profiles in whole blood in the presence of corn trypsin inhibitor (CTI) are predictive of bleeding risk in haemophilia A. We studied factor(F) VIII deficient individuals (11 mild, 4 moderate and 12 severe) with a well-characterized 5-year bleeding history that included haemarthrosis, soft tissue haematoma and annual FVIII concentrate usage. This clinical information was used to generate a bleeding score. The bleeding scores (range 0-32) were separated into three groups (bleeding score groupings: 0, 0 and < or = 9.6, >9.6), with the higher bleeding tendency having a higher score. Whole blood collected by phlebotomy and contact pathway suppressed by 100 microg mL(-1) CTI was stimulated to react by the addition of 5 pM TF. Reactions were quenched at 20 min by inhibitors. Thrombin generation, determined by enzyme-linked immunosorbent assay for thrombin-antithrombin was evaluated in terms of clot time (CT), maximum level (MaxL) and maximum rate (MaxR) and compared to the bleeding score. Data are shown as the mean+/-SD. MaxL was significantly different (P < 0.001) between the groups: 504 +/- 114, 315 +/- 117 and 194 +/- 91 nM; with higher thrombin concentrations in the groups with lower bleeding scores. MaxR was higher in the groups with a lower bleeding score; 97 +/- 51, 86 +/- 60 and 39 +/- 16 nM min(-1) (P = 0.09). No significant difference was detected in CT among the groups, 5.6 +/- 1.3, 4.7 +/- 0.7 and 5.6 +/- 1.3 min. Our empirical study in CTI-inhibited whole blood shows that the MaxL of thrombin generation appears to correlate with the bleeding phenotype of haemophilia A.

Down-regulation of tyrosinase expression by acetylsalicylic acid in murine B16 melanoma

Acetylsalicylic acid (aspirin; ASA) is widely used as an analgesic/antipyretic drug. ASA exhibits a wide range of biological effects, including preventative effects against heart attack, stroke, and the development of some types of cancer. However, the effects of ASA on melanogenesis are not well known. Therefore, we investigated the effect of ASA on melanin production using B16 murine melanoma cells and demonstrated a new biological effect of ASA. In the presence of alpha-melanocyte stimulating hormone (alpha-MSH), B16 melanoma cells are stimulated to enhance melanin synthesis. ASA (2 mM) inhibited alpha-MSH-enhanced melanin synthesis in melanoma more strongly than other well-known anti-melanogenic agents such as arbutin (2 mM) and kojic acid (200 microM). Interestingly, ASA did not inhibit the catalytic activity of mushroom tyrosinase (concentration range 0.5-4.0 mM). To clarify the target of ASA action in melanogenesis, we performed Western blotting for tyrosinase, which is a key melanogenic enzyme. ASA inhibited tyrosinase expression in a dose-dependent manner. Therefore, the depigmenting effect of ASA might be due to inhibition of tyrosinase expression or enhancement of tyrosinase degradation. This study suggests that ASA is a candidate anti-melanogenic agent and it might be effective in hyperpigmentation disorders.

Effects of plasma membrane Ca(2+) -ATPase tyrosine phosphorylation on human platelet function

BACKGROUND

The plasma membrane Ca(2+)-ATPase (PMCA) plays an essential role in maintaining low intracellular Ca(2+) ([Ca(2+)](i)) in resting platelets. Earlier studies demonstrated that platelet activation by thrombin results in tyrosine phosphorylation of PMCA, which inhibits pump activity.

OBJECTIVES

The objective was to determine the functional consequences of PMCA tyrosine phosphorylation.

METHODS

A decapeptide including the tyrosine phosphorylation site of PMCA and a scrambled version were synthesized and introduced into human platelets using saponin. Fura-2 calcium monitoring and aggregometry were used to characterize the effects of inhibition of tyrosine phosphorylation.

RESULTS

Western blot analysis of immunoprecipitates showed that introduction of the inhibitory peptide decreased tyrosine phosphorylation of PMCA by nearly 60% in saponin-permeabilized, thrombin-treated platelets as compared with the scrambled control peptide. Concomitant with inhibition of PMCA tyrosine phosphorylation was a significant decrease in [Ca(2+)](i) during thrombin-mediated platelet activation. The functional consequence of reduced PMCA tyrosine phosphorylation and decreased [Ca(2+)](i) was a significant delay in the onset of thrombin-mediated platelet aggregation.

CONCLUSIONS

The results demonstrate that PMCA tyrosine phosphorylation regulates [Ca(2+)](i) during platelet activation, which affects downstream events in the activation process. Moreover, PMCA tyrosine phosphorylation and resultant inhibition of PMCA activity produces a positive feedback loop mechanism by enhancing the increase in [Ca(2+)](i) accompanying platelet activation.

Antithrombotic properties of aspirin and resistance to aspirin: beyond strictly antiplatelet actions

Aspirin is effective in the prevention of cardiovascular events in high-risk patients. The primary established effect of aspirin on hemostasis is to impair platelet aggregation via inhibition of platelet thromboxane A(2) synthesis, thus reducing thrombus formation on the surface of the damaged arterial wall. Growing evidence also indicates that aspirin exerts additional antithrombotic effects, which appear to some extent unrelated to platelet thromboxane A(2) production. Aspirin can reduce thrombin generation with the subsequent attenuation of thrombin-mediated coagulant reactions such as factor XIII activation. Aspirin also acetylates lysine residues in fibrinogen resulting in increased fibrin clot permeability and enhanced clot lysis as well as directly promoting fibrinolysis with high-dose aspirin. The variable effectiveness of aspirin in terms of clinical outcomes and laboratory findings, which has been termed aspirin resistance, may be related to these additional antithrombotic effects that are altered when associated with common genetic polymorphisms such as the Leu33Pro beta(3)-integrin or Val34Leu factor XIII mutations. However, the clinical relevance of these observations is still unclear. Elucidation of the actual impacts of aspirin other than antiaggregation effects could be important in view of the widespread use of this drug in the prevention of thrombotic manifestations of atherosclerosis.

Qualitative thrombelastographic detection of tissue factor in human plasma

BACKGROUND

Tissue factor (TF) is the principal in vivo initiator of coagulation, with normal circulating TF concentrations reported to be approximately 23-158 pg/mL. However, patients with atherosclerosis or cancer have been reported to have TF concentrations ranging between 800 and 9000 pg/mL. Of interest, thrombelastographic (TEG)-based measures of clot initiation and propagation have demonstrated hypercoagulability in such patients at risk for thromboembolic events. Thus, our goal in the present investigation was to establish a concentration-response relationship of the effect of TF on TEG variables, and determine specificity of TF-mediated events with a monoclonal TF antibody.

METHODS

Thrombelastography was performed on normal human plasma exposed to 0, 500, 1000, or 2000 pg/mL TF. Additional experiments with plasma exposed to 0 or 750 pg/mL TF in the presence or absence of a monoclonal TF antibody (1:360 dilution, 10 min incubation) were also performed. Clot initiation time (R) and the speed of clot propagation (MRTG, maximum rate of thrombus generation) were determined.

RESULTS

The addition of TF to normal plasma resulted in a significant, concentration-dependent decrease in R and increase MRTG values. The addition of TF antibody to samples with TF significantly increased R and decreased MRTG values compared to samples with TF addition.

CONCLUSIONS

In conclusion, changes in TEG variables in conjunction with use of a TF antibody can detect pathological concentrations of TF in human plasma in vitro. Further investigation is warranted to determine if TEG(R)-based monitoring could assist in the detection and prevention of TF-initiated thromboembolic events.

The Resuscitative Fluid You Choose May Potentiate Bleeding

BACKGROUND

Trauma is the leading cause of death in the younger population in the United States, frequently from the development of hemorrhagic shock. Controversy exists over the type of volume resuscitation for restoring hemodynamic stability that should be used in hemorrhagic shock. Little is known about how various resuscitative paradigms affect the coagulation cascade, which is essential to controlling hemorrhagic shock.

METHODS

We studied the effect of various resuscitative formulas on blood coagulation using a new model of whole blood in a controlled setting with corn trypsin inhibitor and a 5-pM stimulus of tissue factor. We investigated thrombin generation, fibrin formation, and platelet activation with four diluents: 0.9% NaCl (NS), lactated Ringer's solution (LR), 6% hydroxyethyl starch (HES), and 3% NaCl (HS), each from 0% to 75% blood dilution. Thrombin generation was measured periodically during a time course of 20 minutes in its complex with antithrombin III. Platelet activation and fibrinopeptide A (FPA) release were monitored in serum at a 20-minute time point. Fibrin clots were collected and weighed.

RESULTS

The coagulation markers (thrombin generation, platelet activation, and FPA release) were significantly different by dilution (p < 0.001 in all) and diluent by dilution (p < 0.001 in all). Thrombin generation, platelet activation, and FPA release decreased the least with the diluents NS and LR. LR caused the least amount of variation in thrombin generation over the dilution course. HS produced the most dramatic change in all of the markers; no coagulation was seen between 30% to 75% dilution (p < 0.05). HES produced greater decreases in thrombin generation and FPA release than NS and LR. Fibrin clot mass decreased with a 10% to 20% dilution for NS and LR, whereas stable fibrin mass did not decrease with the diluents HES and HS at 10% to 20% dilutions. At >30% dilutions, HS produced no stable clots and HES dramatically decreased clot formation by 61% and maintained this level.

CONCLUSIONS

LR and NS had the least effect on thrombin generation, clot formation, and platelet activation at various concentrations compared with HES and HS. This observational data suggests that volume expanders such as HES and HS may be detrimental in treatment of hemorrhagic shock.

Greater inhibitory effects of bivalirudin compared with unfractionated heparin plus eptifibitide on thrombin-induced platelet activation

OBJECTIVES

The effects of antithrombotic agents on the activation of platelets by thrombin were determined in blood from patients (n=12) with symptomatic coronary artery disease.

METHODS

Blood obtained immediately before cardiac catheterization was incubated in vitro with therapeutic concentrations of unfractionated heparin (1.2 and 2.0 U/ml) alone and in combination with eptifibatide (unfractionated heparin 1.2 U/ml+eptifibatide, 1.7 microg/ml) or bivalirudin (8 and 14 microg/ml). An activated clotting time was determined. Platelet activation was induced with thrombin (0, 5, 15, and 40 U/ml) and assessed with the use of flow cytometry. Fibrin polymerization was prevented by the peptide Gly-Pro-Arg-Pro. The activation of glycoprotein IIb-IIIa and surface expression of P-selectin were identified with antibodies (PAC-1 and anti-CD62).

RESULTS

The activated clotting time was 258+/-13 s with 1.2 U/ml unfractionated heparin alone, 396+/-8 s with unfractionated heparin+eptifibatide, and 348+/-9 s with 8 microg/ml bivalirudin. The binding of PAC-1 (reflecting the potential to aggregate) was decreased most effectively by bivalirudin (percentage of platelets binding PAC-1 in response to 15 U/ml thrombin-unfractionated heparin=54+/-7%, unfractionated heparin+eptifibatide=12+/-4%, bivalirudin=1+/-0.3% of platelets, P<0.05 for bivalirudin compared with unfractionated heparin or unfractionated heparin+eptifibatide and unfractionated heparin+eptifibatide compared with unfractionated heparin). Bivalirudin prevented thrombin-induced surface expression of P-selectin more effectively than unfractionated heparin alone or unfractionated heparin+eptifibatide (percentage of platelets expressing P-selectin in response to 15 U/ml thrombin-unfractionated heparin alone=74+/-5%, unfractionated heparin+eptifibatide=53+/-7%, bivalirudin=1+/-0.3%, P<0.05 for bivalirudin compared with unfractionated heparin or unfractionated heparin+eptifibatide).

CONCLUSION

Comparison between pharmacologic concentrations shown to be therapeutic demonstrated that bivalirudin inhibited thrombin-induced activation of platelets to a greater extent than unfractionated heparin alone or in combination with eptifibatide.

Models of blood coagulation

Our research aims to provide quantitatively transparent, biologically realistic descriptions of the processes involved in hemostasis which will permit predictions of the behavior of the coagulation system in normal and pathologic states. We use four models of coagulation: (1) numerical approximations of the tissue factor (Tf) pathway of thrombin generation based upon mechanism and dynamics; (2) Tf activation of the "blood coagulation proteome" from isolated cells and proteins; (3) Tf activated contact pathway inhibited whole blood in vitro; and (4) blood shed from standardized microvascular wounds in vivo. The results from these models are integrated in interactive assessments aimed at achieving convergence of biochemical rigor and biological authenticity. Microvascular injury is the most biologically secure but least accessible to mechanistic study. Numerical models while quantitatively transparent are biologically limited. By the integrated analyses of all four models, we establish observations which require inclusion or discovery of new parameters to achieve mechanistically interpretable biological reality. Discoveries made in this fashion have included thrombin's role in the initiation phase, TFPI/ATIII/APC synergy interactions, rfVIIa in fVII deficiency, the roles of fVIII and fIX in the Tf reaction, and the cleavage of fIX by fXa membrane. Ideally, our results will provide descriptions which predict the behavior of the biological blood coagulation system under normal and pathologic conditions.

The effects of bivalirudin compared with those of unfractionated heparin plus eptifibatide on inflammation and thrombin generation and activity during coronary intervention

OBJECTIVE

To characterize effects of bivalirudin compared with unfractionated heparin plus eptifibatide on inflammation, and thrombin generation and activity after percutaneous coronary intervention.

METHODS

We measured the concentration in blood of fibrinopeptide A, prothrombin fragment 1+2, soluble CD40 ligand, interleukin 1 receptor antagonist, interleukin 6, and high sensitivity C-reactive protein in 63 patients treated with aspirin and clopidogrel and undergoing elective percutaneous coronary intervention, who were randomized to treatment with either bivalirudin (n=34) or unfractionated heparin plus eptifibatide (n=29).

RESULTS

Neither generation nor activity of thrombin increased 10 min after percutaneous coronary intervention in patients randomized to bivalirudin or unfractionated heparin plus eptifibatide. However, prothrombin fragment 1+2 increased modestly and comparably in both groups after 1 day. Inflammation, reflected by concentrations of interleukin 6 and high sensitivity C-reactive protein in blood, increased similarly 1 day after percutaneous coronary intervention in patients treated with either regimen. In a subset of patients (n=12 in each group) from whom blood was obtained 30 days after percutaneous coronary intervention, the concentration of high sensitivity C-reactive protein was lower in those who had been treated with bivalirudin (by 3.5 mg/l, P=0.002).

CONCLUSION

The early effects on inflammation and thrombin generation and activity are similar after treatment with bivalirudin alone compared with unfractionated heparin plus eptifibatide in patients treated with aspirin and clopidogrel who are undergoing percutaneous coronary intervention for symptoms of stable angina. The decreased concentration of high sensitivity C-reactive protein seen 30 days after percutaneous coronary intervention in those treated with bivalirudin is consistent with greater attenuation of inflammation that may have contributed to the trend toward reduced mortality 1 year later in those treated with bivalirudin in REPLACE-2.

Thrombin generation in vascular tissue

BACKGROUND

Classically, it is thought that the vast majority of thrombin is generated on the surface of platelets, however, thrombotic events occur in patients despite treatment with potent antiplatelet agents.

METHODS AND RESULTS

In freshly harvested left internal mammary artery (IMA) sections, addition of CaCl2 and platelet-poor plasma (PPP) were sufficient to stimulate a profound burst of thrombin and this effect was inhibited by antitissue factor antibodies. Ultracentrifugation of PPP to remove platelet microparticles had no effect on thrombin generation. Both the extrinsic and factor VIII-dependent pathways were necessary for IMA-supported thrombin generation as PPP derived from individuals deficient in factors V, VII, VIII or X did not support thrombin production. Small amounts of thrombin were generated utilizing factor IX (FIX)-deficient plasma, however, thrombin was not generated by aorta from FIX-deficient mice when FIX-deficient plasma was used. The addition of non-lipidated tissue factor (0.6 pM) and CaCl2 to actively proliferating cultured human aortic smooth muscle cells (SMC) resulted in a pronounced burst of thrombin generation occurring between 3 and 15 min after treatment. In the absence of tissue factor, thrombin was generated but at a slower rate and with a peak value 26% of that observed in the presence of tissue factor.

CONCLUSION

Significant thrombin generation can occur on vascular tissue in the absence of platelets or platelet microparticles and on the surface of non-apoptotic SMC.

Spatial propagation and localization of blood coagulation are regulated by intrinsic and protein C pathways, respectively

Blood coagulation in vivo is a spatially nonuniform, multistage process: coagulation factors from plasma bind to tissue factor (TF)-expressing cells, become activated, dissociate, and diffuse into plasma to form enzymatic complexes on the membranes of activated platelets. We studied spatial regulation of coagulation using two approaches: 1), an in vitro experimental model of clot formation in a thin layer of plasma activated by a monolayer of TF-expressing cells; and 2), a computer simulation model. Clotting in factor VIII- and factor XI-deficient plasmas was initiated normally, but further clot elongation was impaired in factor VIII- and, at later stages, in factor XI-deficient plasma. The data indicated that clot elongation was regulated by factor Xa formation by intrinsic tenase, whereas factor IXa was formed by extrinsic tenase on activating cells and diffused into plasma, thus sustaining clot growth. Far from the activating cells, additional factor IXa was produced by factor XIa. Exogenously added TF had no effect on the clot growth rate, suggesting that plasma TF does not contribute significantly to the clot propagation process in a reaction-diffusion system without flow. Addition of thrombomodulin at 3-100 nM caused dose-dependent termination of clot elongation with a final clot size of 2-0.2 mm. These results identify roles of specific coagulation pathways at different stages of spatial clot formation (initiation, elongation, and termination) and provide a possible basis for their therapeutic targeting.

The Glycoprotein VI-Phospholipase Cγ2 Signaling Pathway Controls Thrombus Formation Induced by Collagen and Tissue Factor In Vitro and In Vivo

Objective— Both collagen and tissue factor can be initiating factors in thrombus formation. We investigated the signaling pathway of collagen-induced platelet activation in interaction with tissue factor–triggered coagulation during the thrombus-forming process.

Methods and Results— In murine blood flowing over collagen, platelet exposure of phosphatidylserine and procoagulant activity, but not adhesion, completely relied on each of the following signaling modules: glycoprotein VI (GPVI), FcR γ-chain, Src kinases, adaptor protein LAT, and phospholipase Cγ2 (PLCγ2). On flow in the presence of tissue factor, these signaling components were essential for platelet aggregation and greatly enhanced fibrin clot formation. Collagen-stimulated thrombin generation relied on the presence and activity of GPVI, FcR γ-chain, Src kinase, LAT, and PLCγ2. The physiological importance of this GPVI pathway was shown in a FeCl 3 -induced in vivo murine thrombosis model. In both venules and arterioles, signaling through GPVI, FcR γ-chain, and Src kinases enhanced the formation of phosphatidylserine-exposing and fibrin-rich thrombi.

Conclusions— The GPVI-PLCγ2 activation pathway regulates collagen-dependent coagulation in venous and arterial thrombus formation.

Thrombin generation profiles in deep venous thrombosis

BACKGROUND

Reliable markers and methods to predict risk for thrombosis are essential to clinical management.

OBJECTIVE

Using an integrated approach that defines an individual's comprehensive coagulation phenotype might prove valuable in identifying individuals at risk for experiencing a thrombotic event.

METHODS

Using a numerical simulation model, we generated tissue factor (TF) initiated thrombin curves using coagulation factor levels from the Leiden Thrombophilia Study population and evaluated thrombotic risk, by sex, age, smoking, alcohol consumption, body mass index (BMI) and oral contraceptive (OC) use. We quantitated the initiation, propagation and termination phases of each individuals' comprehensive TF-initiated thrombin generation curve by the parameters: time to 10 nm of thrombin, maximum time, level and rate (MaxR) of thrombin generated and total thrombin.

RESULTS

The greatest risk association was obtained using MaxR; with a 2.6-fold increased risk at MaxR exceeding the 90th percentile. The odds ratio (OR) for MaxR was 3.9 in men, 2.1 in women, and 2.9 in women on OCs. The association of risk with thrombin generation did not differ by age (OR:2.8 OR:2.5), BMI (OR:2.9 OR:2.3) or alcohol use. In both numerical simulations and empirical synthetic plasma, OC use created extreme shifts in thrombin generation in both control women and women with a prior thrombosis, with a larger shift in thrombin generation in control women. This suggests an interaction of OC use with underlying prothrombotic abnormalities.

CONCLUSIONS

Thrombin generation based upon the individual's blood composition is associated with the risk for thrombosis and may be useful as a predictive marker for evaluating thrombosis on an individual basis.

The plasma hemostatic proteome: thrombin generation in healthy individuals

BACKGROUND AND OBJECTIVES

The range of plasma concentrations of hemostatic analytes in the population is wide. In this study these components of blood coagulation phenotype are integrated in an attempt to predict clinical risk.

METHODS

We modeled tissue factor (TF)-induced thrombin generation in the control population (N = 473) from the Leiden Thrombophilia Study utilizing a numerical simulation model. Hypothetical thrombin generation curves were established by modeling pro- and anticoagulant factor levels for each individual. These curves were evaluated using parameters which describe the initiation, propagation and termination phases of thrombin generation, i.e. time to 10 nm thrombin (approximate clot time), total thrombin and the maximum rates and levels of thrombin generated.

RESULTS AND CONCLUSIONS

The time to 10 nm thrombin varied over a 3-fold range (2.9-9.5 min), maximum levels varied over a approximately 4-fold range (200-800 nm), maximum rates varied approximately 4.8-fold (90-435 nm min(-1)) and total thrombin varied approximately 4.5-fold (39-177 microm s(-1)) within this control population. Thrombin generation curves, defined by the clotting factor concentrations, were distinguished by sex, age, alcohol consumption, body mass index (BMI) and oral contraceptive (OC) use (OC > sex > BMI > age). Our results show that the capacity for thrombin generation in response to a TF challenge may represent a method to identify an individual's propensity for developing thrombosis.

Platelet Collagen Receptors and Coagulation. A Characteristic Platelet Response as Possible Target for Antithrombotic Treatment

Collagen is a unique agonist of platelets, because it acts as an immobilized ligand that only causes platelet activation after stable adhesion. This review addresses the present understanding of how platelet interaction with collagen supports the process of thrombin generation and coagulation. Only some of the collagen-adhered platelets, that is, those showing profound changes in shape and shedding microparticles (resembling apoptotic cells), appear to contribute to the procoagulant activity of platelets. The main signaling receptor for collagen, glycoprotein VI, plays a key role in the platelet procoagulant response during thrombus formation; this is a reason why new anti-glycoprotein-VI antibodies are promising antithrombotic tools.

Inherited bleeding disorder due to familial type 2 platelet cyclo-oxygenase deficiency

Inherited platelet cyclo-oxygenase (COX) deficiency is a rare bleeding disorder. We report here the first case of familial type 2 platelet COX deficiency responsible for a moderate bleeding phenotype. The propositus was admitted in the emergency department for major epistaxis following treatment with aspirin. Epinephrine closure time is very sensitive to drugs which inhibit COX but the test was normal in patients with inherited COX deficiency. This clinical and biological data suggest that the anti-platelet effect of aspirin may be dependent on mechanisms other than the inhibition of COX. Thrombin generation test confirmed mild bleeding phenotype in patients with COX deficiency as they had normal thrombin generating capacity.

Elective percutaneous coronary intervention using broad-spectrum antiplatelet therapy (eptifibatide, clopidogrel, and aspirin) alone, without scheduled unfractionated heparin or other antithrombin therapy

BACKGROUND

Adjunctive pharmacotherapy during percutaneous coronary intervention (PCI) has historically consisted of a regimen of antiplatelet agents accompanied by an antithrombin agent, typically unfractionated heparin. Paradoxically, unfractionated heparin may activate platelets, induce other pro-thrombotic activities, increase bleeding complications, and cause thrombocytopenia. To optimize patient care and avoid the potential risks of unfractionated heparin in patients undergoing elective PCI, one of the authors began to use adjunctive pharmacotherapy consisting of broad-spectrum antiplatelet therapy alone, without scheduled unfractionated heparin or other antithrombin therapy.

METHODS

Five hundred consecutive patients undergoing scheduled, elective PCI (stent deployment, cutting balloon atherotomy, conventional balloon angioplasty, or high-speed rotational atherectomy) received adjunctive pharmacotherapy consisting of eptifibatide, clopidogrel, and aspirin.

RESULTS

The technical success rate was 100%. During the first 24 hours, there were no major adverse clinical events. Non-Q-wave myocardial infarction occurred in 1.6% of patients, major and minor bleeding complications in 0.2% and 0.6%, respectively, and thrombocytopenia in 0.6%. During the first 30 days, there was 1 (0.2%) major adverse clinical event.

CONCLUSIONS

For elective PCI, adjunctive pharmacotherapy consisting of broad-spectrum antiplatelet therapy alone, without scheduled unfractionated heparin or other antithrombin therapy, appears to be safe and may prove to be efficacious.

Combined effects of melagatran and eptifibatide on platelet aggregation inhibition but not thrombin generation inhibition

The aim of our study was to investigate the combined in vitro effects of melagatran and eptifibatide on platelet aggregation and thrombin generation under low and high coagulant challenge in tissue-factor-activated, platelet-rich plasma. Increasing amounts of melagatran dose-dependently decreased prothrombin fragment 1.2 and activated factor X values, and dose-dependently prolonged the lag phase until the onset of platelet aggregation. Eptifibatide exerted a dose-dependent anti-aggregating effect under both high and low coagulant challenge. The combination of melagatran and eptifibatide resulted in significant additive prolongation of the lag phase until the onset of platelet aggregation, which was more pronounced under low coagulant challenge. Under low, but not under high, coagulant challenge, the combination of melagatran and eptifibatide had a significant additive inhibitory effect on platelet aggregation. No additive effects on decreasing prothrombin fragment 1.2 and activated factor X values were observed with combined administration of the drugs. The present study demonstrates the additive effect of melagatran and eptifibatide on platelet aggregation inhibition and on prolongation of the lag phase until the onset of platelet aggregation.

Initiating and potentiating role of platelets in tissue factor-induced thrombin generation in the presence of plasma: subject-dependent variation in thrombogram characteristics

The hemostatic activity of plasma is determined by platelet activation and coagulation, which processes are mutually stimulatory. We studied this interaction by measuring the cleavage of fluorescent thrombin substrate in platelet-rich plasma (PRP), using the calibrated thrombogram method. In freshly isolated human plasma, thrombin formation triggered by tissue factor was fully dependent on the presence of platelets. It was abolished by annexin A5, indicating dependence on phosphatidylserine (PS) exposure at activated platelets. Comparison of plasmas from various subjects showed considerable interindividual variation in total amount of thrombin generation, regardless of whether platelets or PS-containing phospholipids were present. Integrin alphaIIbbeta3 antagonists and ADP receptor blockage, but not aspirin, decreased the rate of thrombin generation (thrombin peak level) and extended the time of onset. Platelet inhibition with cAMP-elevating agents decreased the thrombin-forming rate, but surprisingly shortened the onset time. Stimulation of platelets with agonists of Gi/q-coupled receptors and, to a larger extent, with collagen or Ca2+-ionophore increased the rate of thrombin generation and shortened its onset. In PRP from donors with low and high generation, platelet inhibitors and activators were similarly effective. Taken together, these results indicate that, in tissue factor-triggered PRP, PS exposure on activated platelets regulates both onset and rate of thrombin generation. However, coagulant activity rather than platelet activation determines the total amount of thrombin formed, i.e. the endogenous thrombin potential. Thus, kinetics of thrombin generation in PRP are controlled by platelet inhibitors and agonists, but the process is restricted in amount by the subject-dependent variation in coagulation.

Thrombin generation: phenotypic quantitation

An individual's ability to generate thrombin following tissue factor stimulus was evaluated in 13 healthy male donors in a 6-month study. Thrombin generation in whole blood collected by phlebotomy, contact pathway suppressed by the presence of 100 micro g mL-1 corn trypsin inhibitor, was initiated by the addition of 5 pm tissue factor/10 nm phospholipid. Reactions were quenched at 20 min by the addition of an ethylenediaminetetraacetic acid (EDTA), benzamidine, FPRck cocktail. Thrombin generation was determined by an ELISA for thrombin-antithrombin III (TAT) complex formation. Results showed that the levels of TAT observed varied from 245 to 775 nm. Thrombin production was consistent within each individual, CVi = 11.6%, but varied significantly within the group, CVg = 25.2%, and correlated inversely with an individual's clotting time (r = - 0.54, P = 0.07). No correlations were individually observed between TAT and C-reactive protein, antithrombin III, factors II, V, VII, VIII, IX and X, fibrinogen and prothrombin time. However, computer simulations, which integrated each individual's coagulation factor levels using the Speed Rx method (Hockin et al., J Biol Chem 2002; 277: 18322), predicted maximum active thrombin levels (ranging from calculated values of 220-500 nm) consistent with the empirically determined values. Overall, these data suggest that thrombin generated in whole blood exclusively by tissue factor stimulation can be used as an integrative phenotypic marker to determine an individual's response to a tissue factor challenge.

Platelet ADP receptors contribute to the initiation of intravascular coagulation

While the adenosine 5'-diphosphate (ADP) pathway is known to enhance thrombus formation by recruiting platelets and leukocytes to the primary layer of collagen-adhering platelets, its role for the initiation of coagulation has not been revealed. Ex vivo inhibition of the P2Y12 ADP receptor by clopidogrel administration diminished the rapid exposure of tissue factor (TF), the major initiator of coagulation, in conjugates of platelets with leukocytes established by the contact of whole blood with fibrillar collagen. Under in vitro conditions, the P2Y12 and P2Y1 ADP receptors were both found to be implicated in the exposure of TF in collagen-activated whole blood. Immunoelectron-microscopy revealed that collagen elicited the release of TF from its storage pools within the platelets. Functional activation of the intravascular TF was reduced by inhibition of the ADP receptors, partially due to the disruption of the platelet-neutrophil adhesions. Injection of collagen into the venous system of mice increased the number of thrombin-antithrombin complexes, indicative for the formation of thrombin in vivo. In P2Y1-deficient mice, the ability of collagen to enhance the generation of thrombin was impaired. In conclusion, the platelet ADP pathway supports the initiation of intravascular coagulation, which is likely to contribute to the concomitant formation of fibrin at the site of the growing thrombus.

Effects of 2 different antiplatelet regimens with abciximab or tirofiban on platelet function in patients undergoing coronary stenting

BACKGROUND

We sought to compare the antiplatelet effects of the glycoprotein IIb-IIIa receptor blockers abciximab or tirofiban, combined with an adjuvant therapy with clopidogrel and aspirin.

STUDY DESIGN AND METHODS

Twenty patients undergoing coronary stenting were randomly assigned to receive either abciximab or tirofiban combined with aspirin and clopidogrel. Serial blood samples were taken to assess platelet aggregation, P-selectin expression, thrombin generation, and platelet-induced endothelial cell expression of MCP-1, uPAR, and ICAM-1. Results and conclusions The therapy with aspirin plus clopidogrel attenuated agonist-induced platelet aggregation and P-selectin surface exposure (P <.05 vs aspirin monotherapy). Both tirofiban and abciximab further reduced agonist-induced platelet aggregation (P <.05), and decreased thrombin generation but had no effect on platelet alpha-granule release. None of the antithrombotic strategies significantly affected platelet-induced endothelial cell activation. Since platelet adhesion/degranulation initiates an inflammatory/mitogenic response in the vascular wall, future therapeutic strategies will have to be aimed at the inhibition of platelet release reactions.

Differential involvement of the P2Y1 and P2Y12 receptors in platelet procoagulant activity

OBJECTIVE

In vivo, activated platelets contribute to the initiation of thrombin generation through the exposure of phosphatidylserine to form a procoagulant catalytic surface and through platelet-leukocyte interactions, which lead to the exposure of leukocyte tissue factor (TF). On the basis of observations that the platelet P2Y1 and P2Y12 receptors both contribute to thrombosis and thrombin formation in an in vivo model of TF-induced thromboembolism, we further characterized the role of these receptors in thrombin generation.

METHODS AND RESULTS

By using the selective P2 antagonists MRS2179 and AR-C69931MX, the P2Y12 receptor was found to be involved in thrombin-induced exposure of PS on isolated platelets and consequently in TF-induced thrombin formation in platelet-rich plasma. By contrast, the P2Y1 receptor was not involved in phosphatidylserine exposure nor in thrombin generation in platelet-rich plasma. In addition, both receptors were found to contribute to the interactions between platelets and leukocytes mediated by platelet P-selectin exposure, which result in TF exposure at the surface of leukocytes.

CONCLUSIONS

Overall, these results point to a differential involvement of the 2 platelet ADP receptors in the generation of thrombin and provide further evidence for the relevance of molecules targeting these receptors as antithrombotic agents.

Thrombin formation

The generation of the enzyme thrombin from its precursor prothrombin is the central event of the blood coagulation process, which is essential to hemostasis and the culprit in thrombosis. Thrombin is produced by a complex series of proteolytic events that are initiated when cryptic tissue factor interacts with plasma factor VIIa to initiate the complex series of events leading to the formation of the blood coagulation enzyme complexes that lead to the efficient generation of the enzyme. During these processes, thrombin contributes to both the generation of the catalysts involved in its ultimate production and to the catalysts that lead to attenuation of its production. Thrombin-catalyzed events both enhance and diminish the process of thrombin generation, which is down-regulated by stoichiometric and dynamic inhibitory processes. The combinations of intensities of activation and inhibition processes provide tight regulation of the hemostatic process, establishing reaction thresholds, essentially leading to an "on/off" switch. This review provides a brief summary of the evolution of knowledge with respect to present-day concepts of thrombin generation via the tissue factor pathway and its regulation.

Different effects of various anti-GPIIb-IIIa agents on shear-induced platelet activation and expression of procoagulant activity

Inhibitors of the platelet glycoprotein (GP)IIb-IIIa receptor (integrin alphaIIbbeta3) reduce acute thrombotic events in patients with coronary artery disease. To characterize the mechanism of action of these drugs, we evaluated the effects of different GPIIb-IIIa antagonists on shear-induced platelet aggregation, activation, and the expression of procoagulant activity. Samples of platelet-rich plasma from 16 volunteers were exposed to the shear rate of 10 800 s-1 for 6 min in an optically modified cone-plate viscometer. Abciximab, tirofiban and eptifibatide inhibited aggregation to a similar extent (mean +/- SD: 74.1 +/- 8.5%, 69.5 +/- 13.6%, 65.6 +/- 17.0%, respectively), but only abciximab inhibited significantly microparticle release associated with shear-induced platelet activation (64.4 +/- 13.6%, P = 2.2 x 10-7; tirofiban = 20.0 +/- 23.4%; eptifibatide = 23.9 +/- 17.4%). P-selectin platelet surface translocation was also strongly inhibited by abciximab, weakly by eptifibatide, but not by tirofiban. The addition of anti-alphavbeta3 to tirofiban enhanced the inhibiting effects on shear-induced P-selectin translocation and microparticle release. Shearing of platelet-rich plasma shortened the re-calcification clotting time after addition of kaolin from 106.9 +/- 14.3 to 94.2 +/- 10.7 s (mean +/- SD; P = 0.0013). This effect, which is mediated by the appearance of procoagulant phospholipids on the surface of sheared platelets and microparticles, was prevented by abciximab and by the combination of tirofiban and anti-alphavbeta3, but not by tirofiban alone or eptifibatide. The ability to inhibit shear-induced platelet activation, as evidenced by microparticle release and P-selectin surface translocation as well as the expression of procoagulant activity, differentiates the effects of anti-GPIIb-IIIa agents, which may explain the distinct antithrombotic efficacy of the agents.

What is all that thrombin for?

The hemostatic process initiated by the exposure of tissue factor to blood is a threshold limited reaction which occurs in two distinct phases. During an initiation phase, small amounts of factor (F)Xa, FIXa and thrombin are generated. The latter activates the procofactors FV and FVIII to the activated cofactors which together with their companion serine proteases form the intrinsic FX activator (FVIIIa-FIXa) and prothrombinase (FVa-FXa) which generate the bulk of FXa and thrombin during a propagation phase. The clotting process (fibrin formation) occurs at the inception of the propagation phase when only 5-10 nM thrombin has been produced. Consequently, the vast majority (greater than 95%) of thrombin is produced after clotting during the propagation phase of thrombin generation. The blood of individuals with either hemophilia A or hemophilia B has no ability to generate the intrinsic FXase, and hence is unable to support the propagation phase of the reaction. Since clot based assays conclude before the propagation phase they are not sensitive to hemophilia A and B. The inception and magnitude of the propagation phase of thrombin generation is influenced by genetic polymorphisms associated with thrombotic and hemorrhagic disease, by the natural abundance of pro- and anticoagulants in healthy individuals and by pharmacologic interventions which influence thrombotic pathology. Therefore, it is our suspicion that the performance of the entire process of thrombin generation from initiation through propagation and termination phases of the reaction are relevant with respect to both hemorrhagic and thrombotic pathology.

Inhibition of localized thrombosis in P2Y1-deficient mice and rodents treated with MRS2179, a P2Y1 receptor antagonist

Previous studies in experimental models revealed a role for the P2Y1 platelet ADP receptor in systemic vascular thromboembolism models. In the present work, we used models of localized arterial and venous thrombosis to assess the role of the P2Y1 receptor in these processes. Arterial thrombosis was induced in one mesenteric arteriole of a mouse using FeCl3, while venous thrombosis was studied in a Wessler model adapted to rats. P2Y1-deficient mice and mice treated with the P2Y1 antagonist MRS2179 displayed significantly less arterial thrombosis than their respective controls. Combination of P2Y1 deficiency with P2Y12 inhibition led to a significant additive effect. Venous thrombosis was slightly but significantly inhibited in MRS2179-treated rats. These results demonstrate a role for the P2Y1 receptor in both arterial and venous thrombosis, further establishing this receptor as a potential target for antithrombotic drugs.