Platelet Microaggregate Formation Research Articles

Fibrinogen-Mediated Platelet Microaggregate Formation in Stored Whole Blood

Introduction: Whole blood is regaining popularity in resuscitation after hemorrhage. The platelet storage lesion that has been observed in apheresis platelets also exists in whole blood, where platelet count and aggregation response decline rapidly after collection and during storage in the blood bank. Platelet microaggregates form in unfiltered whole blood, resulting in relatively large but unstable thrombus formation in a flow model over collagen. We hypothesized that the presence of fibrinogen activates platelets within the whole blood unit and predisposes them toward premature aggregate formation. Methods: Fresh whole blood (WB) was collected in citrate tubes (4.3%, 10.9 mM) from healthy donors (n=4) according to an institutionally approved standard operating procedure. Blood was centrifuged at 200-g to collect the platelet-rich plasma (PRP). The remaining red blood cell (RBC) fraction was centrifuged at 2000-g to pellet RBCs which were washed with HEPES-buffered saline and repelleted; platelet-poor plasma (PPP) was saved. The PRP fraction was centrifuged with 1 µM prostacyclin at 700-g, and the platelet pellet was washed with Tyrode's buffer, repelleted, and resuspended in either fibrinogen-deficient plasma (Fg-DP) or PPP. Platelets were combined with washed RBCs to generate a reassembled whole blood sample containing 150,000 platelets/µl and a hematocrit of 40%. Samples were added to minibags and stored at 4 °C for 14 days. On days of testing (0, 3, 7, 14), 1 ml aliquots were stained with 1 µM calcein-AM for 30 min at 37 °C and run on the BioFlux 1000 microfluidics platform with arterial flow rates generating a shear rate of 980 s-1 over a collagen surface. Fluorescence microscopy images were captured and quantified over 10 min to observe platelet adhesion to the surface. Complete blood count (CBC) was measured on each aliquot. In a follow-up study, unmanipulated, stored whole blood was passed through a standard transfusion filter (200 µm pores) to determine the effects of removing microaggregates prior to microfluidics flow testing at each time point. Results: CBC results are in Figure 1, illustrating the decline of platelet count over storage as previously observed. Figure 2 shows representative images captured at the end of the 10 min flow period. In samples containing normal plasma, thrombus formation is impaired by day 3 of storage with aggregate structures appearing immediately in the microscopy images. These aggregates attach and release periodically over the course of flow, an effect that persisted through day 14. In samples reconstructed with Fg-DP, no microaggregates were observed during storage, although the density and size of thrombi diminished over time. In filtered whole blood samples, thrombus formation on days 3 and 7 more closely resembled the Day 0 fresh samples, but aggregates appeared on day 14 despite filtration. Discussion: WB contains all of the elements required for resuscitation after hemorrhage, however the platelet storage lesion may affect its function. The interaction of platelets and fibrinogen in a relatively static environment results in activation and aggregation of platelets. In apheresis units, studies have shown that using a platelet additive solution to dilute fibrinogen results in better platelet performance for longer periods of storage. Further study is required to determine the clinical significance of aggregate formation and whether mitigation strategies should be explored. Disclosures No relevant conflicts of interest to declare.

Platelet microaggregation in sepsis examined by quartz crystal microbalance with dissipation technology

Antiplatelet therapies remain an area of potential interest for the treatment of sepsis; however, studies of platelet aggregation in sepsis have yielded conflicting results. We examined platelet aggregation patterns in patients with septic shock using quartz crystal microbalance with dissipation technology, a microfluidic device capable of measuring platelet microaggregate formation under flow conditions. Platelet aggregation was increased in the washed platelet samples of septic patients. Conversely, these same platelets aggregated less than healthy controls when examined in their plasma.

A primary role for kinin B1 receptor in inflammation, organ damage, and lethal thrombosis in a rat model of septic shock in diabetes.

Diabetes mellitus and septic shock increase the incidence of mortality by thrombosis. Although kinin B1 receptor (B1R) is involved in both pathologies, its role in platelet function and thrombosis remains unknown. This study investigates the expression, the inflammatory, and pro-thrombotic effects of B1R in a model of septic shock in diabetic rats. Sprague-Dawley rats were made diabetic with streptozotocin (STZ) (65 mg/kg, i.p.). Four days later, control and STZ-diabetic rats were injected with lipopolysaccharide (LPS) (2 mg/kg, i.p.) or the vehicle. B1R antagonist (SSR240612, 10 mg/kg by gavage) was given either acutely (12 and 24 h prior to endpoint analysis) or daily for up to 7 days. Moreover, a 7-day treatment was given either with cyclooxygenase (COX)-2 inhibitor (niflumic acid, 5 mg/kg, i.p.), non-selective COX-1 and COX-2 inhibitor (indomethacin, 10 mg/kg, i.p.), non-selective nitric oxide synthase (NOS) inhibitor (L-NAME, 50 mg/kg by gavage), iNOS inhibitor (1400W, 5 mg/kg, i.p.), or heparin (100 IU/kg, s.c.). The following endpoints were measured: edema and vascular permeability (Evans blue dye), B1R expression (qRT-PCR, western blot, flow cytometry), aggregation in platelet-rich plasma (optical aggregometry), and organ damage (histology). Rats treated with STZ, LPS, and STZ plus LPS showed significant increases in edema and vascular permeability (heart, kidney, lung, and liver) and increased expression of B1R in heart and kidney (mRNA) and platelets (protein). Lethal septic shock induced by LPS was enhanced in STZ-diabetic rats and was associated with lung and kidney damage, including platelet micro-aggregate formation. SSR240612 prevented all these abnormalities as well as STZ-induced hyperglycemia and LPS-induced hyperthermia. Similarly to SSR240612, blockade of iNOS and COX-2 improved survival. Data provide the first evidence that kinin B1R plays a primary role in lethal thrombosis in a rat model of septic shock in diabetes. Pharmacological rescue was made possible with B1R antagonism or by inhibition of iNOS and COX-2, which may act as downstream mechanisms.

Glycoprotein Ibα and FcγRIIa play key roles in platelet activation by the colonizing bacterium, Streptococcus oralis

Infective endocarditis (IE) is characterized by thrombus formation on a cardiac valve. The oral bacterium, Streptococcus oralis, is recognized for its ability to colonize damaged heart valves and is frequently isolated from patients with IE. Platelet interaction with S.oralis leads to the development of a thrombotic vegetation on heart valves, which results in valvular incompetence and congestive heart failure. To investigate the mechanism through which platelets become activated upon binding S.oralis. Platelet interactions with immobilized bacteria under shear conditions were assessed using a parallel flow chamber. S.oralis-inducible platelet reactivity was determined using light transmission aggregometry. Dense granule secretion was measured by luminometry using a luciferin/luciferase assay. Using shear rates that mimic physiological conditions, we demonstrated that S.oralis was able to support platelet adhesion under venous (50-200s(-1) ) and arterial shear conditions (800s(-1) ). Platelets rolled along immobilized S.oralis through an interaction with GPIbα. Following rolling, platelet microaggregate formation was observed on immobilized S.oralis. Aggregate formation was dependent on S.oralis binding IgG, which cross-links to platelet FcγRIIa. This interaction led to phosphorylation of the ITAM domain on FcγRIIa, resulting in dense granule secretion, amplification through the ADP receptor and activation of RAP1, culminating in platelet microaggregate formation. These results suggest a model of interaction between S.oralis and platelets that leads to the formation of a stable septic vegetation on damaged heart valves.

Correlation Between Albuminuria and Spontaneous Platelet Microaggregate Formation in Type 2 Diabetic Patients

OBJECTIVEAlbuminuria in type 2 diabetic patients is a risk factor for cardiovascular disease. We investigated the correlation between albuminuria and spontaneous microaggregation of platelets (SMAP) formed under shear stress.RESEARCH DESIGN AND METHODSThe study subjects were 401 type 2 diabetic individuals (252 with normoalbuminuria and 149 with albuminuria) who were examined for SMAP under conditions of shear stress only (no agonist stimulation) and the reversibility of platelet microaggregation after stimulation with 1 μmol/l ADP, measured by a laser light-cattering method. Active glycoprotein IIb/IIIa (GPIIb/IIIa) and P-selectin expression levels on platelets as an index of platelet activation were measured by whole-blood flow cytometry.RESULTSSMAP formation was noted in 53% of diabetic patients. All patients with SMAP showed an irreversible pattern of platelet microaggregates by a low dose of ADP. SMAP was observed in 75% of diabetic subjects with albuminuria and in 39% of those with normoalbuminuria. Multivariate logistic regression analysis identified urinary albumin excretion rate and brachial-ankle pulse-wave velocity as independent factors associated with SMAP. The degree of SMAP correlated with active GPIIb/IIIa (γ = 0.59, P < 0.001) and P-selectin (γ = 0.55, P < 0.001) expression levels. These early-activated platelet profiles were significantly inhibited in albuminuric patients with aspirin intake, although the effect was incomplete.CONCLUSIONSOur study demonstrated an independent association between albuminuria and early changes in activated platelet profiles of type 2 diabetic patients. Further follow-up and intervention studies are needed to establish whether the inhibition of SMAP affects the course of cardiovascular disease in type 2 diabetic patients.

Efecto del cizallamiento y del difosfato de adenosina sobre la activación plaquetaria y la formación de microagregados en controles sanos. Valoración mediante citometría de flujo

Introduction Several studies show that high shear rate cause platelet activation. But the behaviour of these activated platelets by shear and subsequent stimulus by, for example, ADP is not well established. This paper investigates an in vitro model of blood flow conditions in non-stenotic arteries. Material and methods Platelet activation is studied by flow cytometry. The shear is induced in a cone-plate viscometer at 230 s −1 that mimics the blood flow conditions in healthy arteries. CD62 and GpIIb/IIIa in their active form are selected as platelet activation markers, as well as for the formation of platelet microaggregates (MAP). The percentage of spontaneously activated platelets and the number of MAP are determined. The percentage of activated platelets and MAP after stimulating with ADP is then evaluated. In parallel, the same procedure is followed, but after previously subjecting blood samples to a shear for 5 min. In these sheared samples the percentage of activated platelets and MAP also are measured before and after stimulating with ADP. Results Shearing, as well as ADP, increases the percentage of activated platelets in whole blood. When the platelets are subsequently subjected to ADP, the percentage of activated platelets is significantly lower than when the ADP directly stimulates platelets without shearing. Conclusions Platelets subjected to shearing become refractory when they are subsequently activated by action of a physiological agonist such as ADP. However the platelets that are not sheared respond appropriately to this agonist. This refractory response in vitro may represent a cellular defence mechanism to prevent a greater degree of in vivo activation-aggregation when platelets are faced with an agonist in areas where the shear rate increases in the blood flow.

Utilidad de la citometría de flujo para valorar el efecto de la atorvastatina sobre la función plaquetaria y la movilización del Ca 2+ en pacientes hiperlipémicos

Introduction Thrombotic events are common in hyperlipemic patients in which platelet activation play a decisive role. The presence of activated platelets, as described in a previous paper, is a major risk factor for thrombosis in patients with a high plasma cholesterol and it is therefore of great interest to have a lipid lowering drug that, in addition to normalising plasma lipids, also decreases this activation of circulating platelets. The main aims of this study are to assess whether the increased platelet activation in hypercholesterolemic patients is accompanied by changes in cytoplasmic free calcium mobilisation in platelets and the formation of platelet microaggregates, and whether these changes are normalised by treatment with atorvastatin. Material and methods To achieve these aims 30 hypercholesterolemia patients and 40 normolipemic controls were recruited. The percentage of CD62 positive platelets, the number of spontaneously formed platelet microaggregates (MAP/5000 platelets) and the free calcium mobilization in platelets were evaluated in citrated blood . An EPICS-XL flow cytometer and the antibodies and fluorochromes CD61.PE, CD62.FITC and FLUO3-AM were used for this study. The patients were evaluated before and after two months of atorvastatin (20 mg/day) treatment. Results The results show that atorvastatin treatment significantly decreases the plasma concentration of total and LDL-Cholesterol and triglycerides. In parallel a decrease was noted in the percentage of activated circulating CD62 positive platelets (2.41±1.55% to 1.45±1.06%, P<0.05) and also in the number of spontaneously formed MAP (390±43 to 322±48, P<0.05). These changes were accompanied by the normalization in the free calcium mobilization in platelets. Conclusions As a conclusion it is noted that the flow cytometry of whole blood is a useful technique for evaluating aspects such as those raised in this study. Hypercholesterolemia patients showed more spontaneous platelet activation, which was accompanied by the formation of platelet microaggregates and changes in the cytoplasmic free calcium mobilization, which were normalised by treatment with atorvastatin, which has not been previously reported in the literature.

The dopamine agonism on ADP-stimulated platelets is mediated through D2-like but not D1-like dopamine receptors

Monoamines such as serotonin and epinephrine are known to be involved in platelet activation and aggregation. Dopamine is another monoamine identified in platelets, but published data about its effect on platelets and the receptors involved are controversial. In the present study, we investigated the dopamine agonism in platelets and the receptors involved in these pathways. Platelet-rich plasma (PRP) of healthy individuals was treated with agonists (ADP, epinephrine, dopamine) and various dopamine receptor and transporter antagonists such as SCH-23390, raclopride, clozapine, methylphenidate, and cocaine. Platelet activation was investigated by flow cytometry (CD62P and CD63 surface expression), optical aggregometry, and microaggregate adhesion to collagen IV in a flow chamber system. In our study, dopamine on its own had no effect on platelet activation in the different assays. However, when used in combination with ADP (10 muM), dopamine in a range of 1 to 100 muM significantly potentiated platelet microaggregate formation and adhesion to collagen under low shear flow conditions. Specific antagonists for D2-like receptors (L-741,626, raclopride, and clozapine) completely diminished the dopamine effect at selective concentrations, but not the effect of epinephrine. Neither the D1-like receptor antagonist SCH-23390 nor dopamine transporter antagonists (methylphenidate, cocaine) showed inhibitory effects on the dopamine agonism. Thus, dopamine is an ADP-dependent platelet agonist which acts via D2-like but not D1-like receptors or adrenergic receptors. Because many psychopharmacological drugs are directed to D2-like receptors, platelet dysfunction in patients being treated with such drugs may be linked to these mechanisms.

Effects of platelet glycoprotein IIb-IIIa number and glycoprotein IIIa Leu33Pro polymorphism on platelet aggregation and sensitivity to glycoprotein IIb-IIIa antagonists

We investigated the influence of glycoprotein (GP) IIIa Leu33Pro polymorphism, platelet GP IIb-IIIa number, and plasma fibrinogen concentration on platelet aggregation and antiaggregatory action of GP IIb-IIIa antagonists. Healthy volunteers with GP IIIa Pro33(−) (Leu33Leu33, n = 20) and Pro33(+) (Leu33Pro33, n = 13, and Pro33Pro33, n = 2) genotypes were included into the study. GP IIIa Leu33Pro substitution was associated with the increase of the level and rate of platelet microaggregate formation induced by GP IIb-IIIa activating antibody CRC54 (100, 200, 400 µg/ml) against the epitope within 1–100 residues of GP IIIa N-terminal part (p from 0.001 to 0.047). No significant differences were detected between parameters of platelet aggregation induced by ADP (1.25, 2.5, 5.0, 20 µM) in GP IIIa Pro33(+) and Pro33(−) donors. GP IIb-IIIa antagonist Monafram (F(ab’)2 fragment of GP-IIb-IIIa blocking antibody CRC64) (1, 2, 3 µg/ml), but not eptifibatide (50, 100, 150 ng/ml) inhibited ADP-induced aggregation slightly less efficiently in GP IIIa Pro33(+) group (p < 0.05 at 1 and 2 µg/ml Monafram). GP IIb-IIIa number (evaluated as maximal binding of 125I-labelled antibody CRC64) varied from 40.5 to 80.8 × 103 per platelet with no significant influence of GP IIIa genotype. Consistent correlations were revealed between GP IIb-IIIa quantity and the level and rate of ADP-induced aggregation (r from 0.353 to 0.583, p from <0.001 to 0.037) as well as resistance (level of residual aggregation) to both GP IIb-IIIa antagonists (r from 0.345 to 0.602, p from <0.001 to 0.042). ADP-induced aggregation was considerably increased and efficiency of GP IIb-IIIa antagonists decreased in donors with high in comparison with low GP IIb-IIIa quantity (>60 and 40–50 × 103 per platelet respectively, p < 0.01 for most tests). No correlations were observed between all tested parameters and plasma fibrinogen concentration. Our results indicate that inter-individual variability of platelet GP IIb-IIIa number significantly affects platelet aggregation and antiaggregatory effects of GP IIb-IIIa antagonists. Contribution of this factor is higher than that of GP IIIa Leu33Pro polymorphism and variations of fibrinogen concentration.

Platelets activated by collagen through the immunoreceptor tyrosine-based activation motif in the Fc receptor γ-chain play a pivotal role in the development of myocardial ischemia-reperfusion injury

Platelet activation and the formation of platelet microaggregates in coronary vessels play pivotal roles in myocardial ischemia and reperfusion injury. The Fc receptor γ-chain (FcR γ) is coexpressed with glycoprotein (GP) VI, forming a platelet collagen receptor, and the activation of platelets by collagen is closely coupled with tyrosine phosphorylation of the FcRγ. To examine the functional significance of platelet FcR γ/GPVI complex in the early phase of myocardial ischemia and reperfusion injury in mice, we performed coronary occlusion and reperfusion experiments using wild type mice and FcRγ-deficient (FcRγ –/–) mice that lack GPVI. The infarct size was significantly smaller in FcRγ –/– mice subjected to occlusion and reperfusion of the coronary artery than in control FcR γ +/+ mice. Twenty-four hours after the reperfusion, electron microscopy of the injured tissue showed substantially more platelet aggregation and occlusive platelet microthrombi in the capillaries of the damaged areas of the wild type mice than in those of the FcR γ –/– mice. Platelet Syk was scarcely activated in the FcR γ –/– mice after myocardial ischemia and reperfusion, but significantly activated in the FcR γ +/+ mice. CD11b expression on neutrophils was elevated after myocardial ischemia and reperfusion in both mouse groups, whereas myeloperoxidase activity in the injured areas was significantly lower in the FcRγ –/– mice than in the FcRγ +/+ mice. These results suggest that the collagen-induced activation of platelets through the FcR γ plays a pivotal role in the extension of myocardial ischemia-reperfusion injury. FcRγ and GPVI may be important therapeutic targets for myocardial ischemia-reperfusion injury.

Upregulation of GP IIb/IIIa receptors during platelet activation: Influence on efficacy of receptor blockade

Introduction GP IIb/IIIa inhibitor doses high enough to inhibit platelet macroaggregation may not completely prevent formation of microaggregates. Platelets contain an internal pool of GP IIb/IIIa receptors which externalizes with activation and supports microaggregation. This study assesses microaggregation in the presence of the two GP IIb/IIIa inhibitors abciximab and tirofiban. Methods Citrated whole blood was preincubated with abciximab (5 μg/ml), tirofiban (50 ng/ml), or saline as control and activated with TRAP (5 μM) or ADP (2 μM) at 37 °C under constant stirring. Microaggregate formation and receptor expression were determined with flow cytometry. Results Within few seconds after TRAP-activation the platelet count dropped from 266,000/μl to 20,000/μl, the number of microaggregates increased from 3700/μl to 10,100/μl and the mean number of GP IIb/IIIa receptors increased from 53,000 to 65,000/platelet. With TRAP+abciximab the platelet count dropped from 259,000 to 113,000/μl, microaggregates increased from 2500 to 9300/μl, GP IIb/IIIa receptors from 56,000 to 77,000/platelet. Platelet microaggregate formation was reversible. With TRAP and tirofiban platelet count dropped to only 190,000/μl, there was no increase in platelet microaggregates, receptors increased to 66,000/platelet. Platelet activation with ADP gave similar results. Conclusions During the early phase of activation additional GP IIb/IIIa receptors externalize to the platelet surface. Abciximab does not block these new receptors sufficiently to prevent microaggregate formation. However, the number of unblocked receptors is not high enough to maintain a stable aggregate, microaggregation is reversible. With tirofiban there is no microaggregate formation, possibly because the inhibitor rapidly binds to newly externalized receptors.

The Cytoplasmic Tail of GPIbß Modulates Signalling Induced by GPIb/VWF Interactions under High Shear Conditions.

The platelet glycoprotein (GP) Ib/IX/V receptor complex controls crucial steps in haemostasis and thrombosis by mediating adhesion to von Willebrand factor (VWF). Nevertheless, the downstream consequences of VWF binding to GPIb remain unclear. The GPIb/IX/V complex consists of four polypeptides GPIbα, GPIbβ, GPIX and GPV. We have shown that a palmitylated (Pal-) peptide based on the sequence of the GPIbβ cytoplasmic domain between R151 and A161 (Pal-RRLRARARARA abbreviated to RARA) increases the velocity of platelets rolling on VWF, and inhibits VWF-dependent adhesion and aggregation.The peptide RARA is delivered into platelets and does not cause any reduction in the surface expression of GPIb-IX-V. We examined the effect of RARA and control peptides (Pal-RAAAARARARA and Pal-RRLRADADADA) on GPIb/IX/V-mediated tyrosine kinase signalling induced by VWF/GPIb interactions under shear conditions using a cone and plate viscometer. Whole platelet lysate phosphotyrosine immunoblots were characterised in the presence and absence of RARA and the control peptides before and after shearing at 120 dynes/cm2 for 5 mins. We then further investigated the effect of RARA on PI-3 kinase signalling, the tyrosine kinase Src and calcium influx under the conditions described above.The platelet permeable peptide RARA alone specifically caused protein tyrosine phosphorylation under resting conditions and caused platelet microaggregate formation with a 35% reduction in single platelets as assessed by flow cytometry. In addition RARA significantly inhibited shear-induced aggregation. When tyrosine phosphorylation was examined in platelets sheared for 5 minutes at 120 dynes/cm2, we observed RARA failed to inhibit tyrosine phosphorylation despite inhibiting shear-induced aggregation.RARA caused Src kinase activation as determined by site-specific phosphorylation of Y416 on Src. Furthermore the Src inhibitor PP1 inhibited RARA induced tyrosine phosphorylation. RARA caused a Src-dependent increase in intracellular [Ca2+] and inhibited any further increase in intracellular calcium induced by thrombin. Pretreatment of platelets with the PI-3 kinase inhibitor wortmannin demonstrated that tyrosine phosphorylation induced by RARA was not PI-3 kinase dependent.These results indicate that a platelet permeable peptide corresponding to specific residues of the cytoplasmic tail of GPIbβ directly stimulates Src, and suggest that the mechanism of its inhibitory effect on VWF-mediated platelet adhesion and aggregation involves the tyrosine kinase-mediated desensitisation of a GPIb/IX/V-triggered signalling pathway. This study demonstrates a novel role for the GPIbβ cytoplasmic tail in mediating Src-dependent shear induced signalling through GPIb/VWF.

Quantitation of platelet aggregation and microaggregate formation in whole blood by flow cytometry

Platelet aggregation and microaggregate formation were measured in samples of stirred whole blood by flow cytometry. Blood samples were stirred in a multi-sample agitator with ADP, fixed and labelled with a platelet-specific CD42a-FITC fluorescent antibody. The blood was then diluted and applied directly to a flow cytometer. Platelets were identified using a gating procedure based on their expression of CD42a and then quantified. Aggregation was monitored as a fall in the number of single platelets. Both reversible and irreversible aggregation responses to ADP were determined and these were found to correlate directly with aggregation responses determined using a well-established single platelet counting technique using the Ultra-Flo 100 Whole Blood Platelet Counter. We found from flow cytometry that ADP-induced aggregation was coupled with a transient formation of platelet microaggregates over the initial 60 s following ADP addition. Assessment of single platelet loss by flow cytometry was found to be a reliable way of monitoring aggregation responses and provided new information on rapid microaggregate formation in ADP-stimulated blood.

Differential effect of the GPIIb/IIIa antagonist orbofiban on human platelet aggregate formation in vitro

Platelet microaggregates have been demonstrated in the systemic circulation of patients with atherosclerotic cardiovascular diseases. Glycoprotein IIb/IIIa antagonists have been reported to play roles in platelet activation, which may be associated with pro-thrombotic events. We report the effect of the orally active GPIIb/IIIa antagonist, orbofiban, on human platelet microaggregate formation in vitro. Laser light scatter (LSS) technology was used to monitor small, medium and large platelet aggregates formed in platelet-rich plasma in response to ADP or thrombin receptor activating peptide (TRAP)-6. ADP at 0.5 μM induced only small platelet aggregates that were prevented by orbofiban in a concentration-dependent manner. Likewise, orbofiban dissolved small aggregates after their formation. In marked contrast, in the presence of strong agonist stimulation (20 μM ADP or 3 μM TRAP-6) which generated primarily large platelet aggregates, orbofiban blocked the large aggregates while significantly augmenting small aggregates by three- to sixfold. The data suggest that GPIIb/IIIa antagonists do not induce platelet microaggregation directly but may augment small platelet microaggregate formation indirectly at conditions of strong stimuli. The percentage of the microaggregate population expressing P-selectin remained the same in the presence of orbofiban, indicating that these small microaggregates remain activated, although the mean intensity of expression was diminished, possibly reflecting the reduced size of the particles or density of P-selectin molecules. In summary, an increase in small platelet microaggregates might have contributed to pro-thrombotic events in orbofiban-treated patients.

Formation of platelet microaggregates correlates with adverse clinical outcome in patients with coronary artery disease

Platelet activation plays a pivotal role in the pathogenesis of acute coronary syndromes. Laser-light scattering in a platelet aggregometer was used to evaluate aggregate size and number quantitatively. Small platelet aggregates ultimately develop into medium and then large platelet aggregates. Thus the measurement of small platelet aggregates is important in the evaluation of thrombus formation. We examined the relationship between small platelet aggregates and the occurrence of subsequent cardiovascular events. We followed-up 204 patients (149 men and 55 women, mean age 68 +/- 9 years) with coronary artery disease (CAD) for 48 months. Blood sampling to determine platelet aggregation was performed on the day of hospital discharge. The degree of small platelet aggregates [relative risk 4.34, 95% confidence interval (1.62-11.7), p = 0.004] and low left-ventricular ejection fraction [relative risk 2.88, 95% confidence interval (1.23-6.73), p = 0.015] were independent predictors of the occurrence of cardiovascular events in multivariate Cox hazard analysis. In Kaplan-Meier analysis, the degree of small platelet aggregates correlated with the probability of cardiovascular event occurrence. In patients with CAD, an increase in small platelet aggregates is closely correlated with the future occurrence of cardiovascular events

Thromboagglutination by Anticardiolipin Antibody Complex in the Antiphospholipid Syndrome: A Possible Mechanism of Immune-Mediated Thrombosis

The tendency for patients with antiphospholipid syndrome (APLS) to thromboembolism and the criteria for its detection are well established, whereas the mechanism of this thrombophilic syndrome is still obscure. Using immunofluorescent techniques and a microscopic spontaneous platelet aggregation test (mSPAT), we have previously demonstrated platelet binding by antibody followed by aggregation in patients with lupus anticoagulant. In the present study, we investigated 18 anticardiolipin antibody-positive (ACLA pos.) patients, comprised of 12 primary APLS and 6 secondary APLS lupus patients. We demonstrated direct platelet microaggregate formation in 16/18 cases, whereas this finding was not present in 20 ACLA-negative (ACLA neg.) subjects ( P<.001). Indirect testing revealed 10/12 cases of platelet microaggregation and none in the ACLA neg. subjects. In addition, immunofluorescent studies showed that platelet antibody complex binding occurred in 8/12 cases tested directly and in 11/12 cases in indirect testing, as compared to 1 out of 20 binding in ACLA neg. subjects ( P<.001). Antibody complex binding was inhibited in two cases by prior extraction of phospholipids. Platelet aggregation could be demonstrated in two ACLA neg. individuals by the addition of exogenous ACLA to platelets in EDTA–plasma. We therefore propose a mechanism for immune-mediated thrombosis in APLS in which calcium-independent platelet aggregation, or thromboagglutination, is initiated by an ACLA–phospholipid complex present in the patients' plasma. Following the antibody complex-induced platelet agglutination, thrombosis proceeds by release, recruitment, and fibrin formation. The mechanism should have important implications in the diagnosis, management, and monitoring of APLS.

Transient adhesion refractoriness of circulating platelets under shear stress: the role of partial activation and microaggregate formation by suboptimal ADP concentration.

Exposure of whole blood (WB) to subendothelial extracellular matrix (ECM) under shear stress in the cone and plate(let) analyser (CPA) results in platelet adhesion, followed by release reaction and aggregation of circulating platelets on the adherent platelets. The properties of circulating non-adhered platelets in the CPA was studied by exposure of WB to ECM at a high shear rate (1300/s) for 2 min (1st run), followed by transfer of the suspension to a new ECM-coated well for a second run (2nd run) under similar conditions. The results of the 2nd run demonstrated transient adhesion refractoriness associated with platelet microaggregate formation in the suspension. The adhesion refractoriness was dependent on platelet activation during the 1st run and was prevented by addition of apyrase (ADP scavenger) or ADP receptor inhibitor, suggesting a role for ADP in mediating this response. Furthermore, exposure of WB samples to suboptimal concentrations of ADP (0.4-1 micromol/l) or a thrombin receptor activating peptide (TRAP) (5 micromol/l) for 2 min resulted in a similar transient platelet adhesion refractoriness to ECM under flow conditions. The transient platelet refractoriness and microaggregate formation induced by ADP was associated with a transient reduction in glycoprotein (GP)Ib, increased P-selectin expression and increased fibrinogen binding by circulating platelets. These data suggest a role for platelet agonists at suboptimal concentrations in modulating platelet function and limiting the expansion of the thrombus.

Platelet GPIIb/IIIa receptor blockade reduces infarct size in a canine model of ischemia-reperfusion

OBJECTIVESWe studied the effects of N-acetyl-cys-asn-(5,5-dimethyl-4-thiazolidine-carbonyl)-4-amino-methyl-phe-gly-asp-cys, monoacetate (MK-0852) (platelet GPIIb/IIIa receptor blocker) on peak reactive hyperemia, distribution of blood flow, regional contractile function and infarct size in a canine model of acute ischemia-reperfusion injury.BACKGROUNDPlatelet activation and formation of platelet microaggregates in coronary vessels could contribute to ischemia-induced myocyte injury. Inhibition of platelet aggregation could reduce ischemia-reperfusion injury.METHODSThree groups of dogs (n = 10/group) were studied; group 1—heparin (HEP) (100 U/kg/h intravenously), group 2—MK-0852 (300 μg/kg intravenous bolus followed by 3 μg/kg/min for 3 h) and group 3—MK-0852 plus HEP. Infarct size after 60 min regional ischemia and 3 h reperfusion was evaluated by tetrazolium staining and normalized to risk area (Monastral blue dye).RESULTSInfarct size in HEP-treated controls was 32.4 ± 2.8%; in MK-0852 without or with HEP groups, infarct size was 17.4 ± 1.9% (p = 0.001) and 23.4 ± 3.0% (p = 0.04), respectively. Cardiac hemodynamics and rate-pressure product were comparable between groups. Multivariate analysis using collateral blood flow as the independent variable confirmed the cytoprotective actions of MK-0852. Postischemic peak reactive hyperemia in the infarct-related artery was depressed in all groups; during reperfusion, transmural distribution of myocardial blood flow returned to near control levels, but severe regional hypokinesia persisted.CONCLUSIONSDiminished infarct size with MK-0852 treatment suggests an additional mechanism of benefit for GPIIb/IIIa blockers beyond stabilization of a “culprit” acute coronary lesion. This cytoprotective effect was unrelated to preservation of coronary vasoreactivity (assessed by reactive hyperemia), restoration of blood flow across the myocardium or acute improvement in contractility.

Mildly Oxidized Low Density Lipoprotein Rapidly Stimulates via Activation of the Lysophosphatidic Acid Receptor Src Family and Syk Tyrosine Kinases and Ca2+ Influx in Human Platelets

In contrast to native low density lipoprotein (LDL), mildly oxidized LDL (mox-LDL) induced platelet shape change and stimulated during shape change the tyrosine phosphorylation of specific proteins including Syk; the translocation of Src, Fyn, and Syk to the cytoskeleton; and the increase of cytosolic Ca(2+) due to mainly Ca(2+) entry. The stimulation of these early signal pathways by mox-LDL was inhibited by desensitization of the lysophosphatidic acid (LPA) receptor and specific LPA receptor antagonists, was independent of the alpha(IIb)beta(3)-integrin, and was mimicked by LPA. Stimulation of tyrosine phosphorylation and Syk activation were independent of the increase of cytosolic Ca(2+) and were suppressed by genistein and two specific inhibitors of the Src family tyrosine kinases, PP1 and PD173956. In contrast to PP1 and PD 173956, genistein prevented shape change by mox-LDL. The results indicate that mox-LDL, through activation of the LPA receptor, stimulates two separate early signal pathways, (a) Src family and Syk tyrosine kinases, and (b) Ca(2+) entry. The activation of these early signaling pathways by mox-LDL probably plays a role in platelet responses subsequent to shape change. The inhibition of mox-LDL-induced platelet activation by LPA receptor antagonists or dietary isoflavonoids such as genistein could have implications in the prevention and therapy of cardiovascular diseases.

Effect of smoke inhalation injury on thromboxane levels and platelet counts

Thirteen goats with chronic lung lymph fistula suffering from smoke inhalation injury were studied for a 6-h period. All animals were found to have acute respiratory failure after injury, the arterial TXA2 (thromboxane A2) levels had increased by 5 min after injury, they peaked at 2 h, and then subsequently declined. Peripheral platelet counts decreased progressively within 2 h, then gradually recovered but were still lower than baseline values at 6 h after injury. EVLW (extravascular lung water), Q1 (lung lymph flow rate) and L/P (total lymph protein/plasma protein concentration), all increased and peaked at 2 h after injury. A negative linear correlation was observed between the TXA2 levels and the platelet counts. Furthermore, differences between the venous and arterial platelet counts were markedly increased. All these observations suggested that the increased TXA2 triggered the formation of platelet microaggregates which were then trapped in the microvasculatures of viscera such as the lung. This might be one of the causes of the decreased platelet counts in peripheral blood. Both TXA2 levels and platelet counts were closely correlated with EVLW, suggesting that TXA2 may indirectly cause the increased pulmonary microvascular permeability by promoting platelet aggregation and microaggregate formation in the pulmonary microvasculature. This may be one of the reasons for the developing pulmonary oedema in goats following smoke inhalation injury.