A perfusion chamber developed to investigate thrombus formation and shear profiles in flowing native human blood at the apex of well-defined stenoses

In Vitro Flow Chamber Design for the Study of Endothelial Cell (Patho)Physiology

In the native vasculature, flowing blood produces a frictional force on vessel walls that affects endothelial cell function and phenotype. In the arterial system, the vasculature's local geometry directly influences variations in flow profiles and shear stress magnitudes. Straight arterial sections with pulsatile shear stress have been shown to promote an athero-protective endothelial phenotype. Conversely, areas with more complex geometry, such as arterial bifurcations and branch points with disturbed flow patterns and lower, oscillatory shear stress, typically lead to endothelial dysfunction and the pathogenesis of cardiovascular diseases. Many studies have investigated the regulation of endothelial responses to various shear stress environments. Importantly, the accurate in vitro simulation of in vivo hemodynamics is critical to the deeper understanding of mechanotransduction through the proper design and use of flow chamber devices. In this review, we describe several flow chamber apparatuses and their fluid mechanics design parameters, including parallel-plate flow chambers, cone-and-plate devices, and microfluidic devices. In addition, chamber-specific design criteria and relevant equations are defined in detail for the accurate simulation of shear stress environments to study endothelial cell responses.

Global thrombosis test for assessing thrombotic status and efficacy of antithrombotic diet and other conditions

Because of the high mortality from myocardial infarction and stroke, there is a great demand for finding novel methods of diagnosis, prevention and treatment of these diseases. Most of the current tests measure important determinants of thrombosis such as platelet function, coagulation and fibrinolysis in isolation; therefore, a global test measuring the actual thrombotic status would be more useful in clinical conditions. We obtained considerable experience by using the global thrombosis test, which determines the actual thrombotic status by taking into account the measured platelet reactivity, coagulation and fibrinolytic activities. In animal experiments, we found significant correlation between the ex vivo global thrombosis test measurements and the in vivo thrombotic status. The published evidence for the benefit of an antithrombotic diet with regular physical exercise is also described.

There is a great concern in the general population how to detect the risk of thrombotic events and prevent the high mortality from stroke, myocardial infarction, sudden death and cancer-associated thrombosis. Our experience on antithrombotic fruits and vegetables intake and regular exercise assessed by the global thrombosis test suggested a potentially unique way of preventing these life-threatening diseases. In addition, global thrombosis testing may offer some benefit in detecting risk of thrombotic of forthcoming thrombotic events in cancer and COVID-19 virus-infected patients.

Are all wines made from various grape varieties beneficial in the prevention of myocardial infarction and stroke?

Aim:

Epidemiologic studies support the assumption (French paradox hypothesis) that drinking red wine is beneficial in the prevention of cardiovascular diseases. Our recent works however cast doubt on such claim. Earlier we have shown that the antithrombotic activity of various fruits and vegetables mainly depends on their varieties. For this reason, several varieties of red and white grapes were tested for antithrombotic effect in animal experiments.

Results:

Antithrombotic effect of 45 red and white grape varieties were assessed in the present study. Out of the 45, one red grape variety showed antithrombotic effect, while the majority of red and white grape varieties enhanced thrombosis.

Conclusion:

Most red and white grape varieties enhanced thrombotic activity of blood.

Red wine is widely believed to prevent heart attack and stroke as claimed by a French Paradox hypothesis. Antithrombotic effect of 45 grape varieties was measured in the present study. Only very few red and white varieties inhibited the experimentally induced thrombosis while the majority of the tested varieties enhanced thrombotic activity of blood. Thus, our findings challenge the prevailing claims of the French Paradox.

Influence of microvascular sutures on shear strain rate in realistic pulsatile flow

Arterial thrombus formation is directly related to the mechanical shear experienced by platelets within flow. High shear strain rates (SSRs) and large shear gradients cause platelet activation, aggregation and production of thrombus. This study, for the first time, investigates the influence of pulsatile flow on local haemodynamics within sutured microarterial anastomoses. We measured physiological arterial waveform velocities experimentally using Doppler ultrasound velocimetry, and a representative example was applied to a realistic sutured microarterial geometry. Computational geometries were created using measurements taken from sutured chicken femoral arteries. Arterial SSRs were predicted using computational fluid dynamics (CFD) software, to indicate the potential for platelet activation, deposition and thrombus formation. Predictions of steady and sinusoidal inputs were compared to analyse whether the addition of physiological pulse characteristics affects local intravascular flow characteristics. Simulations were designed to evaluate flow in pristine and hand-sutured microarterial anastomoses, each with a steady-state and sinusoidal pulse component. The presence of sutures increased SSR_max in the anastomotic region by factors of 2.1 and 2.3 in steady-state and pulsatile flows respectively, when compared to a pristine vessel. SSR values seen in these simulations are analogous to the presence of moderate arterial stenosis. Steady-state simulations, driven by a constant inflow velocity equal to the peak systolic velocity (PSV) of the measured pulsatile flow, underestimated SSRs by ∼ 9% in pristine, and ∼ 19% in sutured vessels compared with a realistic pulse. Sinusoidal flows, with equivalent frequency and amplitude to a measured arterial waveform, represent a slight improvement on steady-state simulations, but still SSRs are underestimated by 1-2%. We recommend using a measured arterial waveform, of the form presented here, for simulating pulsatile flows in vessels of this nature. Under realistic pulsatile flow, shear gradients across microvascular sutures are high, of the order ∼ 7.9 × 10⁶ m^-1 s^-1, which may also be associated with activation of platelets and formation of aggregates.

Prevention of thrombotic disorders by antithrombotic diet and exercise: evidence by using global thrombosis tests

Prevention of thrombotic disorders has priority over treatment. There are only two pathologically relevant tests which are suitable for measuring the overall thrombotic status both in experimental conditions and in humans. The Global Thrombosis Test (GTT) and the Global Parallel-Plate Thrombosis Test can detect the pathologically relevant global thrombotic status. These tests have been successfully used for monitoring the effect of antithrombotic drugs and for developing novel antithrombotic agents. By using GTT, varieties of fruits, vegetables, and regular physical exercise have been tested for the effect on global thrombotic status. This review discusses the published evidence for the benefit of diet of selected fruit and vegetable varieties and doing regular physical exercise on improving thrombotic status. Future clinical trials monitored by GTT or Global Parallel-Plate Thrombosis Test could decide on the effectiveness of an experimentally proven antithrombotic diet with regular physical exercise in the prevention of thrombotic diseases.

Flow chamber and microfluidic approaches for measuring thrombus formation in genetic bleeding disorders

Platelet adhesion and aggregation, coagulation, fibrin formation, and fibrinolysis are regulated by the forces and flows imposed by blood at the site of a vascular injury. Flow chambers designed to observe these events are an indispensable part of doing hemostasis and thrombosis research, especially with human blood. Microfluidic methods have provided the flexibility to design flow chambers with complex geometries and features that more closely mimic the anatomy and physiology of blood vessels. Additionally, microfluidic systems with integrated optics and/or pressure sensors and on-board signal processing could transform what have been primarily research tools into clinical assays. Here, we describe a historical review of how flow-based approaches have informed biophysical mechanisms in genetic bleeding disorders, challenges and potential solutions for developing models of bleeding in vitro, and outstanding issues that need to be addressed prior to their use in clinical settings.

Prediction of Thrombus Growth: Effect of Stenosis and Reynolds Number

Shear stresses play a major role in platelet-substrate interactions and thrombus formation and growth in blood flow, where under both pathological and physiological conditions platelet adhesion and accumulation occur. In this study, a shear-dependent continuum model for platelet activation, adhesion and aggregation is presented. The model was first verified under three different shear conditions and at two heparin levels. Three-dimensional simulations were then carried out to evaluate the performance of the model for severely damaged (stripped) aortas with mild and severe stenosis degrees in laminar flow regime. For these cases, linear shear-dependent functions were developed for platelet-surface and platelet-platelet adhesion rates. It was confirmed that the platelet adhesion rate is not only a function of Reynolds number (or wall shear rate) but also the stenosis severity of the vessel. General correlations for adhesion rates of platelets as functions of stenosis and Reynolds number were obtained based on these cases. Finally using the new platelet adhesion rates, the model was applied to different experimental systems and shown to agree well with measured platelet deposition.

The impact of blood shear rate on arterial thrombus formation

The shear rate and corresponding shear stress have impacts on arterial thrombus formation. In particular, the effects of increasing concentration of platelets at the vessel wall and activation of platelets at this site increase the growth and stability of the thrombi which may result in a fatal narrowing of the arterial lumen. The efficacy of many antithrombotic agents is shear dependent as well. It is apparent that there is a need for a point-of-care device to rapidly monitor the risk for arterial thrombosis and to optimize antithrombotic therapy in vitro. The present review focuses on the essential role of shear rate on arterial thrombus formation in native human blood drawn directly from an antecubital vein.

Geometric design of microfluidic chambers: platelet adhesion versus accumulation

Arterial, platelet-rich thrombosis depends on shear rates and integrin binding to either a collagen surface or to the growing thrombus, which are mechanistically different. In general, small microfluidic test sections may favor platelet-surface adhesion without testing for the primary mode of intra-arterial thrombosis, i.e. platelet-platelet bonding and accumulation. In the present report, the ratio of platelet-platelet to platelet-surface interactions, R, and the percentage of platelet-platelet interactions, P, are estimated using an analytical approach for circular and rectangular test sections. Results show that the test section geometry strongly affects both R and P, with test section height in low-aspect ratio channels or diameter greater than 90 μm dominated by platelet-platelet interactions (R >10). Increasing rectangular test section aspect ratio decreases the required height. R increases linearly while P approaches 100 % asymptotically with increasing channel dimension. Analysis of platelet shape shows that the assumption of spherical platelets has a small effect on R compared to discoid platelets adhering flat against test section wall. However, an increase in average platelet volume resulted in a large decrease in R. Nonetheless, Monte Carlo simulations of a typical distribution of human platelet sizes show intrasubject variation in platelet size has only a 10 % net effect on R. Finally, experiments of thrombus formation show that platelet-surface lag times and platelet-platelet accumulation are similar for rectangular microfluidic test sections and round test sections when R >10. The findings show that the size of a microfluidic test section should be carefully considered in studies of cell-cell accumulation versus cell-surface adhesion.

Computational Modeling of Thrombotic Microparticle Deposition in Nonparallel Flow Regimes

Thrombotic microparticles (MPs) released from cells and platelets in response to various stimuli are present in elevated numbers in various disease states that increase the risk for thrombotic events. In order to understand how particles of this size may localize in nonparallel flow regimes and increase thrombotic risk, a computational analysis of flow and MP deposition was performed for 3 deg of stenosis at moderate Reynolds number (20 < Re < 80) and for recirculation zones at low Reynolds (∼1) number. The results indicate that MP deposition results primarily from impaction and not by diffusive flux.

Platelet transport rates and binding kinetics at high shear over a thrombus

Thrombus formation over a ruptured atherosclerotic plaque cap can occlude an artery with fatal consequences. We describe a computational model of platelet transport and binding to interpret rate-limiting steps seen in experimental thrombus formation over a collagen-coated stenosis. The model is used to compute shear rates in stenoses with growing boundaries. In the model, moving erythrocytes influence platelet transport based on shear-dependent enhanced diffusivity and a nonuniform platelet distribution. Adhesion is modeled as platelet-platelet binding kinetics. The results indicate that observed thrombus growth rates are limited by platelet transport to the wall for shear rates up to 6000 s(-1). Above 7000 s(-1), the thrombus growth rate is likely limited by binding kinetics (10(-4) m/s). Thrombus growth computed from these rate-limiting steps match the thrombus location and occlusion times for experimental conditions if a lag time for platelet activation is included. Using fitted parameters, the model is then used to predict thrombus size and shape at a higher Reynolds number flow consistent with coronary artery disease.

Microfluidic thrombosis under multiple shear rates and antiplatelet therapy doses

The mainstay of treatment for thrombosis, the formation of occlusive platelet aggregates that often lead to heart attack and stroke, is antiplatelet therapy. Antiplatelet therapy dosing and resistance are poorly understood, leading to potential incorrect and ineffective dosing. Shear rate is also suspected to play a major role in thrombosis, but instrumentation to measure its influence has been limited by flow conditions, agonist use, and non-systematic and/or non-quantitative studies. In this work we measured occlusion times and thrombus detachment for a range of initial shear rates (500, 1500, 4000, and 10000 s(-1)) and therapy concentrations (0-2.4 µM for eptifibatide, 0-2 mM for acetyl-salicylic acid (ASA), 3.5-40 Units/L for heparin) using a microfluidic device. We also measured complete blood counts (CBC) and platelet activity using whole blood impedance aggregometry. Effects of shear rate and dose were analyzed using general linear models, logistic regressions, and Cox proportional hazards models. Shear rates have significant effects on thrombosis/dose-response curves for all tested therapies. ASA has little effect on high shear occlusion times, even at very high doses (up to 20 times the recommended dose). Under ASA therapy, thrombi formed at high shear rates were 4 times more prone to detachment compared to those formed under control conditions. Eptifibatide reduced occlusion when controlling for shear rate and its efficacy increased with dose concentration. In contrast, the hazard of occlusion from ASA was several orders of magnitude higher than that of eptifibatide. Our results show similar dose efficacy to our low shear measurements using whole blood aggregometry. This quantitative and statistically validated study of the effects of a wide range of shear rate and antiplatelet therapy doses on occlusive thrombosis contributes to more accurate understanding of thrombosis and to models for optimizing patient treatment.

Atherosclerotic geometries exacerbate pathological thrombus formation poststenosis in a von Willebrand factor-dependent manner

Rupture of a vulnerable atherosclerotic plaque causes thrombus formation and precipitates cardiovascular diseases. In addition to the thrombogenic content of a plaque, also the hemodynamic microenvironment plays a major role in thrombus formation. How the altered hemodynamics around a plaque promote pathological thrombus formation is not well understood. In this study, we provide evidence that plaque geometries result in fluid mechanical conditions that promote platelet aggregation and thrombus formation by increased accumulation and activity of von Willebrand factor (vWF) at poststenotic sites. Resonant-scanning multiphoton microscopy revealed that in vivo arterial stenosis of a damaged carotid artery markedly increased platelet aggregate formation in the stenotic outlet region. Complementary in vitro studies using microfluidic stenotic chambers, designed to mimic the flow conditions in a stenotic artery, showed enhanced platelet aggregation in the stenotic outlet region at 60-80% channel occlusion over a range of input wall shear rates. The poststenotic thrombus formation was critically dependent on bloodborne vWF and autocrine platelet stimulation. In stenotic chambers containing endothelial cells, flow provoked increased endothelial vWF secretion in the stenotic outlet region, contributing to exacerbated platelet aggregation. Taken together, this study identifies a role for the shear-sensitive protein vWF in transducing hemodynamic forces that are present around a stenosis to a prothrombogenic microenvironment resulting in spatially confined and exacerbated platelet aggregation in the stenosis outlet region. The developed stenotic microfluidic chamber offers a realistic platform for in vitro evaluation of shear-dependent thrombus formation in the setting of atherosclerosis.

Potential fluid mechanic pathways of platelet activation

Platelet activation is a precursor for blood clotting, which plays leading roles in many vascular complications and causes of death. Platelets can be activated by chemical or mechanical stimuli. Mechanically, platelet activation has been shown to be a function of elevated shear stress and exposure time. These contributions can be combined by considering the cumulative stress or strain on a platelet as it is transported. Here, we develop a framework for computing a hemodynamic-based activation potential that is derived from a Lagrangian integral of strain rate magnitude. We demonstrate that such a measure is generally maximized along, and near to, distinguished material surfaces in the flow. The connections between activation potential and these structures are illustrated through stenotic flow computations. We uncover two distinct structures that may explain observed thrombus formation at the apex and downstream of stenoses. More broadly, these findings suggest fundamental relationships may exist between potential fluid mechanic pathways for mechanical platelet activation and the mechanisms governing their transport.

Blocking αvβ3 integrin by a recombinant RGD disintegrin impairs VEGF signaling in endothelial cells

Vascular endothelial growth factor (VEGF) and αvβ3 integrin are key molecules that actively participate in tumor angiogenesis and metastasis. Some integrin-blocking molecules are currently under clinical trials for cancer and metastasis treatment. However, the mechanism of action of such inhibitors is not completely understood. We have previously demonstrated the anti-angiogenic and anti-metastatic properties of DisBa-01, a recombinant His-tag RGD-disintegrin from Bothrops alternatus snake venom in some experimental models. DisBa-01 blocks αvβ3 integrin binding to vitronectin and inhibits integrin-mediated downstream signaling cascades and cell migration. Here we add some new information on the mechanism of action of DisBa-01 in the tumor microenvironment. DisBa-01 supports the adhesion of fibroblasts and MDA-MB-231 breast cancer cells but it inhibits the adhesion of these cells to type I collagen under flow in high shear conditions, as a simulation of the blood stream. DisBa-01 does not affect the release of VEGF by fibroblasts or breast cancer cells but it strongly decreases the expression of VEGF mRNA and of its receptors, vascular endothelial growth factor receptors 1 and 2 (VEGFR1 and VEGFR2) in endothelial cells. DisBa-01 at nanomolar concentrations also modulates metalloprotease 2 (MMP-2) and 9 (MMP-9) activity, the latter being decreased in fibroblasts and increased in MDA-MB-231 cells. In conclusion, these results demonstrate that αvβ3 integrin inhibitors may induce distinct effects in the cells of the tumor microenvironment, resulting in blockade of angiogenesis by impairing of VEGF signaling and in inhibition of tumor cell motility.

Characterization of collagen thin films for von Willebrand factor binding and platelet adhesion

Von Willebrand factor (VWF) binding and platelet adhesion to subendothelial collagens are initial events in thrombus formation at sites of vascular injury. These events are often studied in vitro using flow assays designed to mimic vascular hemodynamics. Flow assays commonly employ collagen-functionalized substrates, but a lack of standardized methods of surface ligation limits their widespread use as a clinical diagnostic. Here, we report the use of collagen thin films (CTF) in flow assays. Thin films were grown on hydrophobic substrates from type I collagen solutions of increasing concentration (10, 100, and 1000 μg/mL). We found that the corresponding increase in fiber surface area determined the amount of VWF binding and platelet adhesion. The association rate constant (k(a)) of plasma VWF binding at a wall shear stress of 45 dyn/cm(2) was 0.3 × 10(5), 1.8 × 10(5), and 1.6 × 10(5) M(-1) s(-1) for CTF grown from 10, 100, and 1000 μg/mL solutions, respectively. We observed a 5-fold increase in VWF binding capacity with each 10-fold increase in collagen solution concentration. The association rates of Ser1731Thr and His1786Asp VWF mutants with collagen binding deficiencies were 9% and 22%, respectively, of wild-type rates. Using microfluidic devices for blood flow assays, we observed that CTF supported platelet adhesion at a wall shear rate of 1000 s(-1). CTF grown from 10 and 100 μg/mL solutions had variable levels of platelet surface coverage between multiple normal donors. However, CTF substrates grown from 1000 μg/mL solutions had reproducible surface coverage levels (74 ± 17%) between normal donors, and there was significantly diminished surface coverage from two type 1 von Willebrand disease patients (8.0% and 24%). These results demonstrate that collagen thin films are homogeneous and reproducible substrates that can measure dysfunctions in VWF binding and platelet adhesion under flow in a clinical microfluidic assay format.

Thrombin flux and wall shear rate regulate fibrin fiber deposition state during polymerization under flow

Thrombin is released as a soluble enzyme from the surface of platelets and tissue-factor-bearing cells to trigger fibrin polymerization during thrombosis under flow conditions. Although isotropic fibrin polymerization under static conditions involves protofibril extension and lateral aggregation leading to a gel, factors regulating fiber growth are poorly quantified under hemodynamic flow due to the difficulty of setting thrombin fluxes. A membrane microfluidic device allowed combined control of both thrombin wall flux (10(-13) to 10(-11) nmol/mum(2) s) and the wall shear rate (10-100 s(-1)) of a flowing fibrinogen solution. At a thrombin flux of 10(-12) nmol/mum(2) s, both fibrin deposition and fiber thickness decreased as the wall shear rate increased from 10 to 100 s(-1). Direct measurement and transport-reaction simulations at 12 different thrombin flux-wall shear rate conditions demonstrated that two dimensionless numbers, the Peclet number (Pe) and the Damkohler number (Da), defined a state diagram to predict fibrin morphology. For Da < 10, we only observed thin films at all Pe. For 10 < Da < 900, we observed either mat fibers or gels, depending on the Pe. For Da > 900 and Pe < 100, we observed three-dimensional gels. These results indicate that increases in wall shear rate quench first lateral aggregation and then protofibril extension.

Validation of the human tissue factor/FVIIa complex as an antithrombotic target and the discovery of a synthetic peptide

This review focuses on the validation of the principal initiator of human coagulation, the tissue factor (TF)/coagulation factor (F)VIIa complex, as an antithrombotic target, as well as on the discovery of a cyclic pentapeptide (PN7051), which dose-dependently inhibits TF/FVIIa-induced coagulation and thrombus formation. Target validation and studies of antithrombotic efficacy were performed with a human thrombosis model employing non-anticoagulated blood from severe homozygous FVII-deficient patients and healthy individuals at blood-flow conditions mimicking those in healthy and diseased vessels. Additional validation included an anti-TF monoclonal antibody, recombinant TF pathway inhibitor, recombinant inactivated-active site FVIIa and all-trans retinoic acid. Structural and biological characterization of PN7051 and other peptides from the same FVII domain indicate that PN7051 interferes with an essential interaction between the epidermal growth factor domain-2-like and the catalytic domains of FVIIa. A peptidomimetics approach is suggested to further improve the antithrombotic potency of PN7051.

Thrombus formation on apex of arterial stenoses: the need for a fluid high shear stenosis diagnostic device

This review is focused upon the studies of thrombus formation in human non-anticoagulated blood on an apex of an eccentric stenosis positioned in the blood flow channel of a parallel-plate perfusion chamber. Thrombus formation in blood from healthy individuals and patients with various bleeding disorders, as well as the effects of a diet supplement and pharmacological interventions, are discussed in view of thrombus-forming mechanisms under these complex blood-flow conditions. Hallmarks of this significantly enhanced thrombus formation are the apparent dependence on thrombin generation, shear-induced platelet activation, induction of platelet procoagulant activity and pronounced platelet microparticle formation that parallel the growth of these fibrin-rich thrombi. The development of miniature models of these blood-flow devices for diagnostic purposes is suggested for the assessment and monitoring of the efficacy of antithrombotic regimens in blood from patients with atherosclerotic disease in parallel with assessments of platelet microparticle formation, shear-induced platelet activation and platelet procoagulant activity.

Blood flow devices in medical research and clinical testing in humans: are we approaching personalized medicine?

This review focuses on studies of blood flow devices employed in man to unravel the mechanisms of bleeding and thrombotic disorders, and on the characterization of novel experimental antithrombotic entities and drug candidates in biopharmaceutical research and development. Clinical studies with drug candidates and new therapeutic strategies have also been performed, and the predictability of these experimental approaches to clinical situations is excellent. Based on the solid validation of these flow devices, miniature flow devices employing nonanticoagulated blood drawn directly from an antecubital vein should be developed for diagnostic purposes. It is anticipated that such a diagnostic flow device could develop into a personalized medicine approach.

In vitro flow evaluation of recombinant tissue factor pathway inhibitor immobilized on collagen impregnated Dacron

Recombinant tissue factor pathway inhibitor (rTFPI) was immobilized on collagen impregnated (CI) knitted Dacron surfaces and its resistance to fibrin deposition evaluated following exposure to nonanticoagulated whole blood. Recombinant TFPI readily adsorbed to the CI Dacron surface and maintained its inhibitory activity. Under static conditions, rTFPI treated CI Dacron showed little fibrin deposition when compared with untreated surfaces. Treated samples exposed to flowing native blood at wall shear rates of 100 or 200 sec(-1) also demonstrated reduced fibrin deposition (up to 56%) compared with untreated samples. To assess the relative roles of the contact and tissue factor pathways in fibrin formation on artificial grafts, flow studies were performed with whole blood containing corn trypsin inhibitor, a potent inhibitor of FXIIa and contact activation. Corn trypsin inhibitor reduced fibrin deposition on untreated CI Dacron by 40%. Immobilized rTFPI alone, or corn trypsin inhibitor in combination with immobilized rTFPI, reduced fibrin deposition by 58% and 61%, respectively. These data suggest that immobilized rTFPI slows fibrin deposition on the vascular graft material by inhibiting both the contact pathway and blood borne tissue factor procoagulant activity arising from either the alternatively spliced form of tissue factor or from tissue factor containing microparticles.

Antiplatelet therapy: in search of the 'magic bullet'

The central importance of platelets in the development of arterial thrombosis and cardiovascular disease is well established. No other single cell type is responsible for as much morbidity and mortality as the platelet and, as a consequence, it represents a major target for therapeutic intervention. The growing awareness of the importance of platelets is reflected in the increasing number of patients receiving antiplatelet therapy, a trend that is likely to continue in the future. There are, however, significant drawbacks with existing therapies, including issues related to limited efficacy and safety. The discovery of a 'magic bullet' that selectively targets pathological thrombus formation without undermining haemostasis remains elusive, although recent progress in unravelling the molecular events regulating thrombosis has provided promising new avenues to solve this long-standing problem.

Signaling events underlying thrombus formation

Recent in vivo studies have highlighted the dynamic and complex nature of platelet thrombus growth and the requirement for multiple adhesive receptor-ligand interactions in this process. In particular, the importance of von Willebrand factor (VWF) in promoting both primary adhesion and aggregation under high shear conditions is now well established. In general, the efficiency with which platelets adhere and aggregate at sites of vessel wall injury is dependent on the synergistic action of various adhesive and soluble agonist receptors, with the contribution of each of the individual receptors dependent on the prevailing blood flow conditions. In this review, we will discuss the major platelet adhesive interactions regulating platelet thrombus formation under high shear, with specific focus on the VWF (GPIb and integrin alphaIIbbeta3) and collagen receptors (GPVI and integrin alpha2beta1). We will also discuss the signaling mechanisms utilized by these receptors to induce platelet activation with specific emphasis on the role of cytosolic calcium flux in regulating platelet adhesion dynamics. The role of soluble agonists in promoting thrombus growth will be highlighted and a model to explain the synergistic requirement for adhesive and soluble stimuli for efficient platelet aggregation will be discussed.

Methods and models to evaluate shear-dependent and surface reactivity-dependent antithrombotic efficacy

The purpose of the present communication is to evaluate the importance of blood flow and surface reactivity for measurement of antithrombotic drug activity or efficacy in selected model systems of thrombus formation. Such information is essential for proper evaluation of antithrombotic drug profiles. The continuous development of flow-dependent thrombosis models for in vitro (anticoagulated blood) and ex vivo (native blood) studies and their application in in vivo animal models from the early 1970s and onwards are briefly considered. Central to this process was the development of various types of perfusion chambers in which a thrombogenic surface is exposed to flowing blood. Such perfusion chambers have been inserted into arteriovenous (AV) shunts in baboon, pig, dog, and rabbit. These approaches have allowed reproducible testing of traditional and novel experimental antithrombotic drugs, and studies on novel drug strategies under well-defined shear conditions and surface reactivity. Shear-dependent antithrombotic efficacy in these models is observed with anticoagulants such as unfractionated heparin, low-molecular weight heparins, or selective inhibitors of thrombin, Factor Xa, or Factor VIIa. However, the degree of shear dependency depends on the nature of the thrombogenic surface, e.g., the inhibition is more pronounced on a tissue factor (TF)-rich surface than on a collagen-rich surface, particularly at venous or low arterial shear. Platelet antagonists such as the COX-1 inhibitor aspirin, inhibitors of thromboxane A2 (TxA2) synthetase, the TxA2 platelet receptor, and of von Willebrand factor (vWf) are shear dependent also, being more efficient at high arterial shear. In contrast, the platelet ADP antagonist clopidogrel, or antagonists to the active platelet membrane glycoprotein IIb-IIIa complex (GPIIb-IIIa) are shear independent. At extremely high arterial shear, which activates platelets and elicit aggregates of circulating platelets, aspirin looses its antithrombotic effect, whereas ADP and GPIIb-IIIa antagonists still interrupt thrombus formation. In general, results obtained with these models mimic and predict antithrombotic efficacy in man when comparison is possible. Information on antithrombotic efficacy in flow devices with various thrombogenic surfaces is now sufficiently available to suggest recommendations for experimental conditions, particularly with regard to blood flow and reactive surfaces.

Experimental Study of Fibrin/Fibrin-Specific Molecular Interactions Using a Sphere/Plane Adhesion Model

Fibrin, the biopolymer produced in the final step of the coagulation cascade, is involved in the resistance of arterial thrombi to fragmentation under shear flow. However, the nature and strength of specific interactions between fibrin monomers are unknown. Thus, the shear-induced detachment of spherical monodispersed fibrin-coated latex particles in adhesive contact with a plane fibrin-coated glass surface has been experimentally studied, using an especially designed shear stress flow chamber. A complete series of experiments for measuring the shear stress necessary to release individual particles under various conditions (various number of fibrin layers involved in the adhesive contact, absence or presence of plasmin, the main physiological fibrinolytic enzyme) has been performed. The nonspecific DLVO interactions have been shown to be negligible compared to the interactions between fibrin monomers. A simple adhesion model based on the balance of forces and torque on particles, assuming an elastic behavior of the fibrin polymer bonds, to analyze the experimental data in terms of elastic force at rupture of an elementary intermonomeric fibrin bond has been used. The results suggested that this force (of order 400 pN) is an intrinsic quantity, independent of the number of fibrin layers involved in the adhesive contact. Copyright 2001 Academic Press.

Shear-induced platelet activation and platelet microparticle formation in native human blood

Shear-induced platelet activation and platelet microparticle formation are triggered in native human blood by high arterial shear or by a sudden increase in shear as introduced by a stenosis with potential consequences for collagen-induced platelet thrombus formation. Blood was drawn from healthy volunteers and directly perfused ex vivo over various well-defined eccentric stenoses. Shear-induced platelet activation was determined by using flow cytometry to assess: 1) GPIIb-IIIa activation by fluorescein isothiocyanate (FITC)-labeled Mab PAC-1; and 2) translocation of membrane aminophospholipids (procoagulant activity) by FITC-labeled Annexin V. Microparticle formation was measured by flow cytometry and FITC-labeled Mab Y2/51 directed against GPIIIa. Significant platelet activation and platelet microparticle formation were elicited when the wall shear rate reached 10,500 sec-1 for a period of 0.075 sec. Prolonged exposure to or a rapid increase in shear further enhanced activation and microparticle formation. Shear-induced platelet activation was associated with significantly increased collagen-induced platelet thrombus formation that was insensitive to aspirin ingestion. Exposure of native blood to very high shear thus activates platelets to express GPIIb-IIIa, renders the platelet membrane procoagulant and stimulates microparticle formation. These responses are associated with enhanced collagen-induced thrombus formation by prostaglandin-independent mechanisms.

Mechanical factors affecting hemostasis and thrombosis

Both physical and chemical factors can influence the activity of platelets and coagulation factors responsible for the formation of thrombotic and hemostatic masses in the vicinity of an injured vessel wall. Studies performed in controlled shear devices (viscometers) have indicated that physical factors alone can induce platelet aggregation, even in the absence of exogenous chemical factors. The physical considerations which appear to be important for the local activation of hemostatic/thrombotic mechanisms appear to be related to the magnitude of the shear rate/stress, the duration of the applied physical force and the local geometry. Blood flow alone has multiple influences on platelet and coagulative mechanisms. It has been well established that at physiologically encountered shear conditions, increases in the local shear rate enhance the attachment of platelets to the vessel wall and the growth of platelet aggregates on adherent platelets. In contrast, increases in local shear conditions inhibit the production of fibrin formation on surfaces where tissue factor (TF) is exposed. At levels of shear rate/stress high as compared to normal physiological conditions, but comparable to those observed at the apex of severely stenosed vessels, platelet aggregate formation is dependent on the duration of the exposure time. Considerable advances in our understanding of flow-related mechanisms have evolved from the use of well-defined perfusion chambers employing parallel flow streamlines. However, processes leading to hemostasis and thrombosis generally occur in more complicated flow situations where flow streamlines are not parallel and in which abnormally high, as well as abnormally low, shear rates and shear stress levels may be encountered in close proximity to each other.

Blood flow and antithrombotic drug effects

This paper reviews the importance of blood flow phenomena in models of experimental thrombosis used for measuring antithrombotic drug efficacy. The characteristics of these systems and their application for studies with human blood and in animal models are considered. Central to these investigations has been the development of various types of perfusion chambers in which a thrombogenic test surface is exposed to flowing blood under well-defined conditions of blood flow and device geometry. Such perfusion chambers, which have been used in vitro, ex vivo, and in vivo by insertion into arteriovenous shunts in various animal species, have allowed reproducible testing of both conventional and experimental agents. Shear-dependent antithrombotic effects have been observed with anticoagulants such as heparin and with selective inhibitors of thrombin, factor Xa, and factor VIIa. However, the degree of shear dependency depends on the chemical composition of the thrombogenic surface; for example, anticoagulant effects may be more pronounced on a tissue factor-rich surface than on a collagen-rich surface, particularly at venous or low arterial shear rates. Platelet inhibitors such as aspirin, thromboxane antagonists, or inhibitors of von Willebrand factor platelet interactions are also shear dependent, being more efficient at high shear rates. In contrast, inhibitors of adenosine diphosphate-dependent platelet reactions or antagonists of the platelet membrane glycoprotein IIb/IIIa complex are shear rate independent. At very high shear rates characteristic of severely stenotic arteries, aspirin loses its direct antithrombotic effect, whereas adenosine diphosphate pathway inhibitors and GP IIb/IIIa antagonists are still beneficial. In general, results obtained with many of these models have predicted antithrombotic efficacy in human beings when comparisons were possible. Therefore shear-dependent models of experimental thrombosis are routinely used in the evaluation of antithrombotic pharmacologic agents, both preclinically and clinically.

Effects of curvature and stenosis-like narrowing on wall shear stress in a coronary artery model with phasic flow

To gain insight into the details of intracoronary flow we have used computational fluid dynamic techniques to determine the velocity and wall shear stress distributions in both steady- and phasic-flow models of a curved coronary artery with several degrees of stenosis. The steady-flow Reynolds number was 500 and the peak phasic flow Reynolds number was 700. Without stenosis and at 25% (area) stenosis wall shear stress and velocities are higher at the outer wall than the inner wall but retain the same direction as the superimposed flow. At higher stenoses laminar flow separation occurs and the inner wall is exposed to shear stresses that vary widely, both temporally and spatially.

Role of platelets in restenosis after percutaneous coronary revascularization

The role of platelets in the process of restenosis after percutaneous coronary intervention is not fully understood. After vascular injury there is extensive platelet activation, adhesion, aggregation and secretion. Through the liberation of growth factors, such as platelet-derived growth factor, and surface expression of cell adhesion molecules, such as the glycoprotein IIb/IIIa integrin, platelets appear to be a pivotal mediator of the vascular injury response. Experimental models have demonstrated that profound, prolonged thrombocytopenia, or blockade of the IIb/IIIa receptor, may reduce neointimal hyperplasia after arterial balloon injury. However, multiple clinical trials testing conventional or new platelet agents have not yielded any salutary effects. The recent finding that abciximab, a monoclonal antibody fragment directed against IIb/IIIa, reduced clinical restenosis after coronary angioplasty by 26% in patients raises questions about the mechanism of benefit. The alpha v beta 3 vitronectin receptor is responsible for binding endothelial cells to platelets, and it also has a key role in modulating smooth muscle cell migration. It is possible that the antibody fragment exerts its effect on restenosis by means of alpha v beta 3, because abciximab fully cross-reacts to this integrin owing to the shared beta 3 subunit. To date, the other platelet glycoprotein IIb/IIIa inhibitors, including Integrelin, Tirofiban, Lamifiban and Xemilofiban, are specific in binding to this particular integrin. Considerable further study is necessary to unravel the effects of platelets on the restenosis process.

Review on the multifactorial aspects of bioincompatibility in CPB

Over the last few years, a great emphasis has been placed on the bioincompatibility of foreign, nonphysiological materials used during CPB. It is only recently that bioincompatibility has objectively been recognized to be multifactorial, with the dominant implicated sources being cardiotomy blood suction, shear stresses or flow dynamics and, to a lesser degree, the blood/artificial surface interactions, the heparin/protamine complex and the various forms of emboli or debris. In summary, in CPB, the reduction of postoperative complications, such as whole body inflammatory reactions, bleeding and organ dysfunctions, can only be impacted if the primary, dominant sources of bioincompatibility are reduced or eliminated.