Evaluation of unfractionated heparin and recombinant hirudin on survival in a sustained ovine endotoxin shock model

OBJECTIVE: To compare and evaluate the potential benefit of two different anticoagulation regimens during endotoxemia in an ovine model. DESIGN: Animal prospective randomized and controlled study following preliminary dose-range study conforming with the Guide for the Care and Use of Laboratory Animals as promulgated by the Council of the American Physiologic Society and reviewed by the Ethical Committee for Animal Research of our institution. SETTING: Laboratories of anesthesiological investigations and hemostasis, primary care university medical center. SUBJECTS: Twenty-two adult sheep of either sex, weighing 29-42 kg (mean 35.8 kg), surgically instrumented for chronic studies and randomly allocated to receive three different treatment groups: unfractionated heparin (40 units.kg.hr; n = 7), recombinant hirudin (500 units.kg.hr; n = 7), or saline (nonanticoagulated controls; n = 8). INTERVENTIONS: Ovine model of severe endotoxin shock induced by a continuous intravenous endotoxin infusion over 72 hrs (10 ng.kg.min ). MEASUREMENTS AND MAIN RESULTS: Endotoxin infusion alone produced a progressive hypotensive, hyperdynamic shock state with right ventricle failure leading to the death of all animals of the control group within 44 hrs (median, 12 hrs). Heparin profoundly improved survival rate in this model, whereas effective anticoagulation with hirudin did not significantly increase the survival rate compared with controls. Animals in the control group died from disseminated intravascular coagulation, metabolic acidosis, and hemorrhagic pulmonary edema, whereas sheep treated with hirudin died from severe pulmonary edema (hypoxemia, increased alveoloarterial oxygen gradient associated with an increased wet/dry lung weight ratio) in the absence of disseminated intravascular coagulation. CONCLUSIONS: We conclude that systemic anticoagulation with the thrombin inhibitor hirudin is without benefit in this sheep model of lethal endotoxemia, whereas anticoagulation with heparin affords not only effective prevention of endotoxin-induced coagulation disorders but also global protection with prevention of respiratory decompensation and thus markedly improves survival rate in this situation.     

Diagnosis of abdominal mural aortic thrombus following discovery of common femoral artery and vein thrombosis by point-of-care ultrasound

Acute limb ischemia (ALI) is a limb-threatening and life-threatening disease process. Mural aortic thrombosis (MAT) is a rare cause of ALI. While there is limited evidence on the use of bedside ultrasound for the detection of ALI or MAT, duplex ultrasound remains the standard in the diagnosis and ultimate medical decision-making in patients with acute and chronic limb ischemia. Point-of-care ultrasound may be used in the evaluation of patients with signs and symptoms of this disease entity. This is a case of a 79-year-old female with a complicated medical history, who presented with a pulseless right leg and abdominal tenderness. The patient quickly decompensated requiring intubation for airway protection. A post-intubation arterial blood gas (ABG) was unsuccessfully attempted in the right femoral artery, prompting an ultrasound-guided ABG. On B-mode ultrasound evaluation, echogenic material was visualized in the right common femoral artery without evidence of Doppler flow signal. Additionally, a partially obstructing echogenic material was also noted at the femoro-saphenous vein junction with only partial compressibility by compression sonography. A computed tomography angiography of the aorta was performed indicating extensive infrarenal aortic thrombosis. The patient expired despite the relatively prompt diagnosis, highlighting the importance of early identification of acute arterial occlusion.     

Standard heparin, low molecular weight heparin, low molecular weight heparinoid, and recombinant hirudin differ in their ability to inhibit transduction by recombinant adeno-associated virus type 2 vectors.

Recombinant adeno-associated virus type 2 (rAAV) is a promising vector for in vivo gene therapy. Transduction by rAAV requires binding to heparan sulfate proteoglycan on the cell surface, and heparin can block this binding. Because heparin is administered to most patients undergoing cardiovascular gene transfer in order to prevent thrombotic events, it is important to identify anticoagulants which do not interfere with rAAV transduction. Therefore, we examined the influence of different anticoagulants on rAAV transduction in vitro. rAAV transduction was inhibited by 40.5 +/- 7.9% at heparin concentrations of 0.1 U/ml, and by 81.7 +/- 3.6% at 1.0 U/ml. The low molecular weight (LMW) heparin tinzaparin inhibited rAAV transduction by 20.2 +/- 3.8% at 0.1 U/ml and 37.1 +/- 1.8% at 1.0 U/ml. The inhibitory effect was significantly weaker compared with heparin at 1.0 U/ml, (P < 0.01). The LMW heparinoid danaparoid inhibited rAAV transduction by 8.8 +/- 3.5% at 0.1 U/ml (P < 0.01 compared with heparin). In contrast, recombinant hirudin did not interfere at all with rAAV transduction. In summary, the results demonstrate that inhibition of rAAV transduction by heparin occurs rapidly and at therapeutically used concentrations. LMW heparinoids and above all recombinant hirudin might be alternatives for heparin when vascular gene transfer with rAAV requires transient anticoagulation.     

Acute antithrombotic effects of ximelagatran, an oral direct thrombin inhibitor, and r-hirudin in a human ex vivo model of arterial thrombosis

Summary. Background: Thrombin plays a major role in thrombus formation through activation of platelets and conversion of fibrinogen to fibrin. Objectives: To investigate the antithrombotic effects of the oral direct thrombin inhibitor (DTI) ximelagatran and the parenteral DTI r‐hirudin in humans. Subjects and methods: Healthy male volunteers randomized into four parallel groups each with 15 subjects received either ximelagatran (20, 40 or 80 mg orally) or r‐hirudin (0.4 mg kg^?1 intravenous bolus + infusion of 0.15 mg kg^?1 h^?1 for 2 h and 0.075 mg kg^?1 h^?1 for 3 h). Antithrombotic effects were assessed as changes in total thrombus area (TTA) and total fibrin area (TFA) from baseline, using the Badimon perfusion chamber model at baseline and 2 h and 5 h after drug administration. Results: Two hours postdosing, ximelagatran showed antithrombotic effects at both high and low shear rates (TTA% of mean baseline value ± SEM was 76 ± 13% and 71 ± 17% [both P < 0.05] for the 20‐mg dose, 85 ± 11% [P > 0.05] and 62 ± 15% [P < 0.05] for the 40‐mg dose and 60 ± 11% and 26 ± 7% [both P < 0.05] for the 80‐mg dose, respectively). r‐Hirudin also showed a significant antithrombotic effect at high and low shear rates (76 ± 11% [P = 0.05] and 57 ± 17% [P < 0.05] of baseline values, 2 h postdosing, respectively). The inhibitory effects on TFA were similar to those on TTA. Conclusions: The oral DTI ximelagatran shows antithrombotic effects under both high and low shear conditions. The antithrombotic effect of 40–80 mg ximelagatran appeared comparable to that of parenterally administered r‐hirudin, which has been previously demonstrated to be clinically effective in acute coronary syndromes.     

Repeated doses of oral and subcutaneous heparins have similar antithrombotic effects in a rat carotid arterial model of thrombosis

Although heparins are usually injected intravenously or subcutaneously, antithrombotic activity is observed in rat models following single oral heparin doses. Since repetitive dosing is usually needed for thromboprophylaxis, study objectives were to determine whether repetitive oral heparin prevented arterial thrombosis and to compare effectiveness to subcutaneous administration. Wistar rats were given subcutaneous or oral unfractionated heparin ([UFH] 1 mg/kg per 48 h), low-molecular-weight heparin ([LMWH] tinzaparin, 0.1 mg/kg per 12 h), or saline for 30 days. On the last day, thrombosis was initiated by placing 30% FeCl(3)-soaked filter paper on the distal carotid. Subsequent flow measurements, for a 60-minute period, included recorded time of initial thrombus formation (time till thrombus begins [TTB]), and time until carotid occlusion (time till occlusion [TTO]). The formed thrombus was dried and weighed. The activated partial thromboplastin time (aPTT), anti-factor Xa, and antithrombin activity were determined from the plasma. Both oral and subcutaneous heparins significantly increased TTB and TTO. Time of initial thrombus formations were 12.6 ± 1.1, 21.2 ± 2.2, 25.3 ± 3.9, 21.7 ± 3.1, and 21.3 ± 1.7 minutes and TTOs were 29.3 ± 3.6, 54.8 ± 4.0, 60.0 ± 0.3, 56.7 ± 3.3, and 58.3 ± 1.7 minutes (mean ± SEM) for control, subcutaneous UFH, oral UFH, subcutaneous LMWH, and oral LMWH, respectively. Thrombus weight was 2.52 ± 0.29 g in control and was reduced to 43%, 23%, 33%, and 28% of control weight for subcutaneous UFH, oral UFH, subcutaneous LMWH, and oral LMWH, respectively. Thrombus weight was significantly less for oral compared to subcutaneous UFH. The aPTT for oral UFH, and anti-factor Xa activity in the LMWH-treated groups were significantly greater than control (two-tailed t tests). These findings confirm that orally administered heparins are absorbed. Repeated treatment with oral heparin showed similar antithrombotic activity compared to subcutaneous heparin. Oral heparin use for arterial thromboprophylaxis should be further investigated.     

Quantitative venographic assessment of deep vein thrombosis in the evaluation of streptokinase and heparin therapy.

A technique of quantitative venography has been developed in which values are assigned to the deep veins of the calf, knee, thigh, and pelvis, based upon the calculated volume and degree of occlusion of these venous segments. A maximum score of 40 units reflects complete thrombosis of all segments. This technique has been applied to a randomized, single-blind study of streptokinase versus heparin treatment. Each group of 12 patients had similar mean inital venographic scores; follow-up venograms were performed 5 days after the start of therapy. Streptokinase patients with high initial scores (larger than 20) showed a mean improvement of 12.1 units, while those with low initial scores(less than 20) were essentially unchanged. Heparin patients with high scores had a minimal mean improvement of 1.1 units, but those with low scores had a significant mean extension of thrombosis of 8.6 units. Patients with symptoms for less than 7 days showed greated mean improvement (12.7 units) with streptokinase that those with a longer duration of symptoms (2.0 units); heparin patients in these subgroups showed a mean worsening of 7.5 units and no change, respectively. Extrinsic venous obstruction by tumor did not prevent an excellent response to streptokinase. No single test of coagulation of fibriolysis was a reliable indicator of the degree of venographic response to lytic therapy. Pyrexia and hemorrhagic complications occurred in over one-half of the streptokinase patients; one had an anaphylactic reaction, and one died of intracerebral hemorrhage during therapy. The data suggest that lytic therapy is best restricted to the patient with acute extensive thrombosis. Also, continuous infusions of heparin according to current guidelines may be inadequate to prevent thrombus growth in some patients.     

Successful treatment of a catheter-related right atrial thrombosis with recombinant tissue plasminogen activator and heparin

Deep venous thrombosis is a possible complication of indwelling central venous catheters (CVC), with an incidence as high as 61%. We report a case of successful thrombolysis of a CVC-related right atrial thrombus in a pediatric cancer patient with recombinant human tissue plasminogen activator (0.1 mg/kg per h for 12 h) and heparin (10 IU/kg per h for 24 h) administered for 6 days. Daily echocardiographic examination showed progressive lysis of the thrombus. The thrombolytic treatment was associated with mild oozing from the venipuncture sites, but no major bleeding was noted; moreover, thrombin, thromboplastin time and fibrinogen were normal or only minimally altered. Anticoagulant therapy, with or without CVC removal, is the treatment of choice for uncomplicated CVC-related thrombosis. Fibrinolytic therapy may be indicated in some cases at risk of pulmonary embolism or to avoid open heart surgery. Recombinant human tissue plasminogen activator is increasingly used for thrombolytic treatment of organ and limb thrombosis, but experience with it in the pediatric hematology-oncology setting is still limited. This report showed that administering recombinant human tissue plasminogen activator in a pediatric cancer patient prior to hematopoietic stem cell transplantation was effective and safe under strict biochemical and instrumental monitoring. Further studies are needed to determine the best antithrombotic treatment for CVC-related thrombosis, and also the dosage of the medication selected and the duration of treatment.     

阿加曲班与低分子肝素在连续性血液净化治疗抗凝中的对比研究

目的:探讨阿加曲班在连续性血液净化(CBP)治疗抗凝中的疗效及安全性.方法:将30例多器官功能障碍综合征(MODS)患者分成实验组和对照组,每组各15例,在行CBP治疗时,实验组采用阿加曲班注射液抗凝,对照组采用低分子肝素钙抗凝,CBP治疗中监测患者活化部分凝血活酶时间(APTT)、血管路动脉压、静脉压变化;观察管路、滤器凝血情况;观察患者有无组织器官出血等不良反应.结果:阿加曲班组治疗后4 h及8 h静脉端APTT明显增高,动脉端APTT也增高,但不如静脉端明显,而且治疗后恢复接近治疗破前水平.低分子肝素组滤器及或管路凝血发生率26.7%;阿加曲班组滤器及或管路凝血发生率6.7%.阿加曲班组滤器及或管路凝血或器官出血发生率有下降趋势,但无统计学差异(P>0.05).结论:本研究已观察到阿加曲班的体外抗凝效果有优于低分子肝素的趋势,而很少并发出血,可应用于伴有出血倾向的MODS患者行CBP治疗时的抗凝.     

Combination of aspirin and metoclopramide produces a synergistic antithrombotic effect in a canine model of coronary artery thrombosis

Summary— We compared the antithrombotic properties of low doses of aspirin (0.03, 0.1 mg kg⁻¹ intravenously [iv]) and metoclopramide (0.1, 0.3 mg kg⁻¹ iv) alone or in combination. The animal model chosen for this study involved the generation of cyclic flow variations (CFV) in the circumflex coronary artery of anaesthetized dogs as a result of a critical coronary stenosis associated with a controlled arterial lesion at the site of stenosis. Subsequent regular CFV represent sequential thrombus formation and embolization in the damaged vessel. Neither aspirin nor metoclopramide alone demonstrated antithrombotic properties at the doses tested. However, the combination of aspirin 0.1 mg kg⁻¹ iv and metoclopramide 0.3 mg kg⁻¹ iv produced a significant antithrombotic effect, reducing the frequency of large CFV from 6.7 ± 0.5 to 0.8 ± 0.4 cycles h⁻¹ ( P < 0.01) and increasing minimum mean coronary blood flow from 5.0 ± 1.1 to 23.7 ± 2.6 mL min⁻¹ ( P < 0.01). This result apparently reflects an antithrombotic synergism between aspirin and metoclopramide since the effects of the combination were greater than the combined effects of the individual treatments. The antithrombotic influence of metoclopramide could be due to its 5HT₂-antagonist or α₂-antagonist properties, both of which would inhibit platelet aggregation. This demonstration of a synergistic antithrombotic action of the combination of aspirin and metoclopramide is of interest since these two agents are often combined in clinical use. Its therapeutic relevance, however, remains to be established.     

The antithrombotic effects of warfarin and heparin following infusions of tissue thromboplastin in rabbits: clinical implications.

An animal model for the production of stasis thrombi was employed to obtain the data reported in this study. Rabbits treated with warfarin (1.5 mg/kg/day) exhibited a maximal increase in prothrombin time and decreases in factor VII, factor X, and prothrombin within 48 hr with no additional changes occurring after 10 days of drug administration. In contrast, Xa inhibitory activity was unchanged after 48 hr of warfarin treatment but was significantly increased by the tenth day. When thrombosis was induced by infusions of 60 micrograms of tissue thromboplastin, the warfarin regimen produced an antithrombotic effect by the sixth hour, which increased to significance by day 2 and was further significantly increased by day 10. These three stages correspond to the initial depletion of the vitamin K-dependent clotting factors, the maximal depletion of these proteins, and the maximal increase in Xa inhibitory activity, respectively. Thus these experiments separate the antithrombotic potential of warfarin into two components: an early effect related to the decrease in factor VII and a delayed augmentation of Xa inhibitory acticity. Intravenous heparin alone (5 U/kg) did not protect against infusions of 60 micrograms of tissue thromboplastin but did provide an antithrombotic effect against 45 micrograms of the same infusate. Higher doses of heparin, however, did protect against infusion of 60 micrograms of tissue thromboplastin. After 48 hr of warfarin treatment, 5 U/kg heparin increased protection against 60 micrograms of tissue thromboplastin to a degree equivalent to that provided after 10 days of warfarin therapy alone.     

缺氧诱导因子1在创伤性深静脉兔模型血栓形成及消退过程中的表达（英文）

背景：深静脉血栓形成及消退过程中的分子机制极其复杂,目前已有研究表明,缺氧与损伤因素参与了深静脉血栓形成及消退过程。目的：观察静脉壁中缺氧诱导因子1在创伤性深静脉血栓模型兔血栓形成及消退过程中的表达变化。方法：将日本大耳白兔采用钳夹双侧股静脉＋石膏固定双下肢建立兔创伤性深静脉血栓模型。PCR及ELISA检测股静脉组织中缺氧诱导因子1mRNA、蛋白在兔创伤性深静脉血栓建模后表达的变化。结果与结论：模型兔创伤后24h,经B超监测,血栓形成率为58%;血栓栓子随造模后时间延长逐渐缩小机化,创伤后第5天开始经B超监测模型组血栓形成的血管腔有断续血流信号。模型兔股静脉组织中缺氧诱导因子1在造模后1,3,5d表达逐渐升高（P〈0.05或P〈0.01）,此后7-14d表达逐渐下降（P〈0.05或P〈0.01）。结果证实,在创伤性深静脉血栓形成过程,缺氧诱导因子1表达上调,在血栓形成后的溶解及机化再通过程中,缺氧诱导因子1表达下调。     

Antithrombotic effects of FK419, a novel nonpeptide platelet GPIIb/IIIa antagonist, in a guinea pig photochemically induced middle cerebral artery thrombosis model: comparison with ozagrel and argatroban

Platelet activation and subsequent aggregation play a key role in the pathogenesis of ischemic brain damage. Recent studies revealed that enhanced platelet activation is also observed after ischemia, suggesting that secondary thrombus formation might participate in the development of cerebral infarction. The binding of platelet glycoprotein GPIIb/IIIa (integrin alpha(IIb)beta3) to fibrinogen is the final common pathway in platelet aggregation. Therefore, GPIIb/IIIa antagonists might be useful in acute ischemic stroke as well as in the secondary prevention of ischemic stroke. In the present study, we evaluated the effect of three compounds, FK419 ((S)-2-acetylamino-3-[(R)-[1-[3-(piperidin-4-yl) propionyl] piperidin-3-ylcarbonyl] amino] propionic acid trihydrate), a novel nonpeptide GPIIb/IIIa antagonist, ozagrel, a selective thromboxane A(2) synthase inhibitor, and argatroban, a thrombin inhibitor, on middle cerebral artery (MCA) patency and ischemic brain damage using photochemically induced MCA thrombosis model in guinea pigs. FK419, ozagrel, or argatroban was administered 5 min after the termination of photoirradiation. FK419 dose-dependently improved MCA patency by decreasing the total occlusion time, time to continuous reperfusion, and the number of cyclic flow reductions, at doses that inhibited ADP-induced platelet aggregation ex vivo. In contrast, ozagrel only improved total occlusion time, and argatroban showed no improvement in MCA patency. FK419 also reduced ischemic brain damage in a dose-dependent fashion, whereas ozagrel and argatroban did not. Finally, FK419 ameliorated neurological deficits, whereas ozagrel and argatroban did not. These results indicate that FK419, a GPIIb/IIIa antagonist, ameliorates ischemic brain damage by improving MCA patency after occlusion and that FK419 is a promising candidate for the treatment of acute ischemic stroke.     

Decreased prothrombotic effects of pegylated recombinant human megakaryocyte growth and development factor in thrombocytopenic state in a rat thrombosis model

Previous in vitro studies demonstrated that thrombopoietin (TPO) acts on platelets to activate a variety of intracellular signaling pathways and to enhance platelet sensitivity to multiple agonists. Little is known, however, about whether TPO exerts prothrombotic effects in vivo. The aim of this study was to examine the effects of pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF), a pegylated N-terminal domain of human TPO, in a rat model of venous thrombosis. A microthrombus was photochemically induced on the vessel wall of a mesenteric venule, but the vessel was not occluded by it. A single intravenous injection of PEG-rHuMGDF (3 microg kg(-1)) after the thrombus generation into normal rats enhanced the thrombus size, resulting in transient thrombotic occlusion in the majority of rats. Stimulatory effects on thrombus growth were also observed following administration of glycosylated recombinant human full-length TPO (6 microg kg(-1)). In rats rendered thrombocytopenic by total body irradiation, however, PEG-rHuMGDF, even at 300 microg kg(-1), did not induce a significant increase in thrombus size or thrombotic occlusion. Platelets from thrombocytopenic rats had decreased surface levels of c-Mpl and decreased sensitivity to PEG-rHuMGDF in an in vitro aggregation response. Thus, decreased prothrombotic effects of PEG-rHuMGDF in thrombocytopenic rats might be the result not only of low platelet counts but also of decreased platelet reactivity to PEG-rHuMGDF. These results indicate that PEG-rHuMGDF has little effect on venous thrombus formation in thrombocytopenic states associated with high endogenous TPO levels.     

The benefit-to-risk profile of melagatran is superior to that of hirudin in a rabbit arterial thrombosis prevention and bleeding model

Summary.  Although hirudin is better than heparin at preventing recurrent ischemia in patients with unstable angina, hirudin produces more bleeding. The purpose of this study was to use a rabbit arterial thrombosis prevention and ear bleeding model to determine whether for equivalent efficacy, melagatran, a synthetic direct thrombin inhibitor, is safer than hirudin. A combination of balloon injury and stasis was used to induce thrombosis in the distal aorta, and patency and blood flow were continuously monitored with ultrasonic flow probes. Rabbits were randomized to melagatran (in total doses of 78–313 nmol kg⁻¹), hirudin (in total doses of 18–107 nmol kg⁻¹), or saline over 90 min. To assess safety, blood loss from standardized ear incisions was measured. Both melagatran and hirudin produced dose-dependent increases in patency and blood flow. At doses that maintained the highest levels of patency, however, melagatran produced 2–3-fold less bleeding than hirudin. Thus, at maximally effective doses, melagatran causes less bleeding than hirudin in this model. These findings raise the possibility that some direct thrombin inhibitors are safer than others.     

High vs. low doses of low-molecular-weight heparin for the treatment of superficial vein thrombosis of the legs: a double-blind, randomized trial

In contrast with extensive information on the management of deep vein thrombosis of the lower extremities, little is known on the most appropriate treatment of the superficial vein thrombosis (SVT). In a multicenter, prospective, controlled, double-blind, double-dummy clinical trial, 164 consecutive patients with acute SVT of the great saphenous vein were randomized to receive the s.c. administration of either fixed prophylactic doses (2850 a-Xa IU) or body-weight adjusted therapeutic doses of nadroparin once daily for 1 month. The main study outcome was to compare the rate of asymptomatic and symptomatic extension of SVT and/or venous thromboembolic (VTE) complications during a 3-month follow-up period. Of the 81 patients randomized to the prophylactic doses, seven [8.6%; 95% confidence interval (CI), 3.5-17.0] developed SVT progression or VTE complications as compared with six of the 83 (7.2%; 95% CI, 2.8-15.1) allocated to the treatment group (absolute difference, 1.4; 96% CI, -6.9 to 9.7; P = 0.74). No patient in either group developed major bleeding. Our findings suggest that therapeutic doses of low-molecular-weight heparin, administered for 1 month in patients with SVT of the greater saphenous vein do not improve results obtained by prophylactic doses, administered for the same period, over a 3-month follow-up period.     

Distinct antithrombotic consequences of platelet glycoprotein Ibα and VI deficiency in a mouse model of arterial thrombosis

BACKGROUND: Collagen and von Willebrand factor (VWF) are considered essential to initiate platelet deposition at sites of vascular injury, but their respective roles remain to be elucidated. METHODS: We used a model of carotid artery thrombosis induced by a ferric chloride injury to compare the time to first occlusion and occlusion rate at 25 min postinjury in mice lacking the collagen receptor, glycoprotein (GP) VI, or the ligand-binding domain of the VWF receptor, GP Ibalpha. RESULTS: In normal mice used as controls (n = 12), a complete obstruction of blood flow developed within 8.05 +/- 0.47 min (mean +/- SEM), and the occlusion rate was 100%. The results were variable in 26 GP VI(-/-) mice. The artery never occluded in eight mice, but the time to first occlusion in the remaining 18 (8.36 +/- 0.27 min) was not different from normal (P = 0.556). Nonetheless, the occlusion rate was 42%, because in seven mice the occluded artery reopened and stayed patent at 25 min. In contrast, the artery never occluded in 12 mice lacking GP Ibalpha. In ex vivo perfusion experiments, GP VI(-/-) platelets failed to form thrombi onto collagen type I fibrils, but formed thrombi of normal size when exposed to endothelial or fibroblast extracellular matrix. CONCLUSIONS: Absence of GP Ibalpha function has a more profound antithrombotic effect in vivo than absence of the GP VI-dependent pathway of collagen-induced adhesion/activation. Components of the extracellular matrix may elicit a thrombogenic response in the absence of GP VI but not GP Ibalpha.