Platelet-bound Fibrinogen Research Articles

Platelets and the Lectin Pathway of Complement Activation in Patients with Systemic Lupus Erythematosus or Antiphospholipid Syndrome.

Background Patients with systemic lupus erythematosus (SLE) have an increased risk of thrombosis even when they do not have antiphospholipid syndrome (APS). Interactions between complement activation and activated platelets have been suggested in SLE and APS and could play a role in the increased thrombosis risk. Objectives To explore factors potentially related to the prothrombotic pathophysiology in patients with SLE, primary APS, and healthy controls, by investigating lectin pathway proteins (LPPs), complement activation, platelet aggregation, and platelet activation. Methods This cross-sectional cohort study included 20 SLE patients, 17 primary APS, and 39 healthy controls. Flow cytometry and light transmission aggregometry were used to assess platelet activation and aggregation. Using time-resolved immunofluorometric assays, the plasma concentrations of 11 LPPs and C3dg, reflecting complement activation, were measured. Results H-ficolin plasma concentrations were higher in SLE and APS patients than in controls ( p = 0.01 and p = 0.03). M-ficolin was lower in SLE than in APS ( p = 0.01) and controls ( p = 0.03). MAp19 was higher in APS patients than in SLE patients ( p = 0.01) and controls ( p < 0.001). In APS patients, MASP-2 and C3dg correlated negatively with platelet activation. Platelet-bound fibrinogen after agonist stimulation and C3dg concentrations correlated negatively with platelet activation. Conclusion We observed significant differences between SLE and APS patients regarding complement proteins and platelet activation. Particularly the negative correlations between MASP-2 and C3dg with platelet activation only observed in APS patients suggest that interactions between complement activation and platelets differ in SLE and APS.

Polydiacetylene Liposome Microarray toward Facile Measurement of Platelet Activation in Whole Blood.

The necessity of a simple measurement of platelet activation has been increasing in clinical medicine to regulate the proper dose of the antiplatelet drugs for patients having clinical outcomes in acute situations such as angina pectoris, stroke, or peripheral vascular disease or procedures involving angioplasty or coronary thrombolysis. We developed a self-signaling polydiacetylene (PDA) liposome microarray to detect activated platelets from whole blood samples in a single step. A specific antibody, 9F9 antibody, to platelet-bound fibrinogen was selected and conjugated to the PDA liposome microarray to quantify the fibrinogen-bound platelets. The developed PDA liposome-9F9 microarray generated an intense fluorescence signal when activated platelets in whole blood were introduced and also successfully distinguished the reduced platelet activation in the presence of Tirofiban, a model antiplatelet drug. The results of this single-step benchtop assay incorporates simple, sensitive, and rapid attributes that can detect the extent of platelet activation prior to needed clinical procedures.

Multiple Myeloma Treatment Is Associated with Enhanced Platelet Reactivity

Multiple myeloma (MM) is associated with elevated levels of thrombotic disease. Poor mobility due to bone disease and older age are contributory factors but a satisfactory molecular or cellular explanation for this has not been established. Furthermore, the treatment of newly-diagnosed or relapsed MM patients with immunomodulatory drugs increases thrombosis risk. In this study, we therefore sought to determine whether platelet function and/or haemostasis is altered during the clinical progression from the benign monoclonal gammopathy of underdetermined significance (MGUS) to the smouldering myeloma (SM, untreated) state, and to establish the impact of lenalidomide therapy in MM patients on platelet reactivity.Patients (MGUS (N=10), SM (N=10) MM (N=4), healthy volunteer/control (N=10)) were recruited at the myeloma clinics at the Royal Berkshire and Oxford University Hospital Trusts. Single blood samples were obtained from controls, MGUS and smoldering myeloma participants. For patients undergoing therapy (lenalidomide, dexamethasone and anti-coagulant or anti-platelet therapy), samples were taken at predose Cy1, Cy1 D14, Cy1 D28 (a week free from lenalidomide administration), and Cy4 D14. All blood samples were taken less than 12 hours from Lenalidomide dosing, and analysis performed within 4 hours. High throughput platelet function analysis was performed on platelet-rich plasma and washed platelets to measure platelet aggregation, secretion and fibrinogen binding in response collagen, collagen-related peptide (a collagen receptor GPVI-selective ligand), ADP, thrombin receptor activatory peptide (TRAP), the thromboxane mimetic U46619 and epinephrine, and full concentration-response profiles obtained for each. The levels of key platelet receptors including GPVI, CD61, CD41b, CD36 and CD31, were also measured by flow cytometry and remained unaltered between groups. In comparison with control samples, platelet functional responses (maximal response, maximal gain and EC50) from MGUS and smoldering myeloma patients were unaltered (EC50: ADP 1µM, collagen-related peptide (CRP-XL) 0.1µg/ml, U46619 1µM, TRAP 1.6µM, epinephrine 1.3µM). Similarly circulating levels of platelet-bound fibrinogen and P-selectin exposure were unaltered (control: 709±194 AU, MGUS: 581±129 AU, SM: 655±148 AU 9 (Mean±SD)). In MM patients during therapy, the levels of these platelet responses (aggregation, fibrinogen binding and P-selectin exposure - a marker for a-granule secretion) were increased 2-4 fold. In most patients this was notable at week 2 and had declined approximately 50% by week 4 of cycle 1 but remained above predose levels, and remained elevated to a similar level in drug cycle 4.The study of further patients (including Bortezomib treated patients as controls) will be required in order to validate these findings which suggest that (1) baseline platelet function is not substantially compromised in MM and SM, (2) lenalidomide and other immunomodulatory drugs may be able to modulate platelets in patients causing increased platelet reactivity, and (3) effects noted are lenalidomide-dependent and not related to inflammatory state secondary to disease response. The effects of exposure of platelets to lenalidomide in vitro in the presence or absence of dexamethasone and a proteasome inhibitor are being tested to explore potential direct effects of the drugs on circulating platelets. Results from these experiments will also be presented. DisclosuresGibbins:Celgene Corporation: Research Funding. Rana:Celgene Corporation: Research Funding. Khan:Celgene Corporation: Research Funding. Shapiro:Freeline Therapeutics Ltd: Consultancy. Grech:Novartis: Other: sponsorship for scientific meetings. Ramasamy:Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding.

Use of a Cyclooxygenase-2 Inhibitor Does Not Inhibit Platelet Activation or Growth Factor Release From Platelet-Rich Plasma

Background: It remains unestablished whether use of cyclooxygenase (COX)–2 inhibitors impairs platelet activation and anabolic growth factor release from platelets in platelet-rich plasma (PRP). Purpose: The purpose of this study was to assess the effects of a COX-2 inhibitor on platelet activation and anabolic growth factor release from canine PRP when using a clinically applicable PRP activator and to determine whether a 3-day washout would be sufficient to abrogate any COX-2 inhibitor-related impairment on platelet function. Study Design: Controlled laboratory study. Methods: Ten healthy dogs underwent blood collection and PRP preparation. Dogs were then administered a COX-2 inhibitor for 7 days, after which PRP preparation was repeated. The COX-2 inhibitor was continued for 4 more days and PRP preparation performed a third time, 3 days after discontinuation of the COX-2 inhibitor. Immediately after PRP preparation, the PRP was divided into 4 aliquots: 2 unactivated and 2 activated using human γ-thrombin (HGT). One activated and 1 unactivated sample were assessed using flow cytometry for platelet expression of CD62P and platelet-bound fibrinogen using the canine activated platelet-1 (CAP1) antibody. The 2 remaining samples were centrifuged and the supernatant assayed for transforming growth factor–β1 (TGF-β1), platelet-derived growth factor–BB (PDGF-BB), and thromboxane B2 (TXB2) concentrations. Differences in platelet activation and TGF-β1, PDGF-BB, and TXB2 concentrations over the 3 study weeks were evaluated using a 1-way repeated-measures ANOVA, and comparisons between activated and unactivated samples within a study week were assessed with paired t tests. Results: There were no statistically significant (P > .05) effects of the COX-2 inhibitor on percentage of platelets positive for CD62P or CAP1 or on concentrations of TGF-β1, PDGF-BB, or TXB2. All unactivated samples had low levels of activation or growth factor concentrations and significantly (P < .05) greater activation and growth factor concentrations in HGT-activated samples. Conclusion: This COX-2 inhibitor did not impair platelet activation, growth factor release, or TXB2 production in this canine PRP when using HGT as an activator. Studies are warranted to determine whether COX-2 inhibitors affect platelet activation and growth factor release from human PRPs. Clinical Relevance: These results suggest that there is no need to withhold a COX-2 inhibitor before PRP preparation, particularly if thrombin is going to be used to activate the PRP. This is clinically relevant information because many patients who are candidates for PRP therapy for treatment of musculoskeletal injury are also using COX-2 inhibitors.

Influence of Cellular Composition and Exogenous Activation on Growth Factor and Cytokine Concentrations in Canine Platelet-Rich Plasmas.

The purposes of this study were to (1) evaluate correlations among platelet, leukocyte, growth factor, and cytokine concentrations in canine platelet-rich plasmas (PRPs) produced from five different canine PRP-concentrating systems and (2) compare the effects of different activation protocols on platelet activation and growth factor release from one of these PRPs. PRP was made using blood from 15 dogs and each of 5 different PRP systems in a cross-over design. Complete blood counts were performed to quantify platelet and leukocyte concentrations. PRPs were activated, or not, according to manufacturer instructions, and transforming growth factor-β1 (TGF-β1), platelet-derived growth factor-BB (PDGF-BB), vascular endothelial growth factor, and tumor necrosis factor-alpha (TNF-α) were quantified. Differences among platelet, leukocyte, and growth factor concentration were compared among the different systems. Correlations between platelet and anabolic growth factor concentrations were assessed. Subsequently, PRP was made from 12 additional dogs using one of the devices. Each PRP was divided into three aliquots that were activated with calcium chloride (CaCl2), human γ-thrombin (HGT), or not activated. Expression of CD62P and platelet-bound fibrinogen (CAP1) was quantified for each activation group. Concentrations of TGF-β1, PDGF-BB, and TNF-α were also quantified for each activation group and a fourth group that was frozen/thawed. Differences among activation groups were assessed by a Friedman test. There were statistically significant differences among the PRPs made with difference devices with regard to platelet, leukocyte, TGF-β1, and PDGF-BB concentrations (p < 0.0001). There were weak to moderate correlations (R2 = 0.07-0.58) between platelet and anabolic growth factor concentrations but it appeared that activation had a greater effect on growth factor concentration than did cellular composition. Intentional platelet activation significantly increased CD62P and CAP1 expression as well as TGF-β1 and PDGF-BB concentrations in the one PRP in which all activation methods were assessed. Activation with HGT resulted in the greatest platelet activation, and CaCl2 and freeze/thaw elicited moderate increases in either growth factor release or CD62P and CAP1 expression. There are positive correlations between platelet and anabolic growth factor concentrations in canine PRPs. However, intentional platelet activation has a greater effect on growth factor delivery than platelet concentration. Thrombin provides more robust activation than CaCl2.

The nitric oxide donor pentaerythritol tetranitrate reduces platelet activation in congestive heart failure.

BackgroundPlatelet activation associated with endothelial dysfunction and impaired endogenous platelet inhibition is part of the cardiovascular phenotype of congestive heart failure (CHF) and contributes to the increased risk for thromboembolic complications. Pentaerythritol tetranitrate (PETN) has been shown to release nitric oxide without development of nitrate tolerance. We investigated the effect of chronic PETN treatment on platelet activation and aggregation in an experimental CHF model.Methods and ResultsChronic ischemic heart failure was induced in male Wistar rats by coronary artery ligation. Starting 7 days thereafter, rats were randomised to placebo or PETN (80 mg/kg twice daily). After 9 weeks, activation of circulating platelets was determined measuring platelet bound fibrinogen, which requires activated glycoprotein IIb/IIIa on the platelet surface. Binding was quantified by flow-cytometry using a FITC-labelled anti-fibrinogen antibody. Platelet-bound fibrinogen was significantly increased in CHF-Placebo (mean fluorescence intensity: Sham 88±4, CHF-Placebo 104±6, p<0.05) and reduced following treatment with PETN (89±7, p<0.05 vs. CHF-Placebo). Maximal and final ADP-induced aggregation was significantly enhanced in CHF-Placebo vs. Sham-operated animals and normalized / decreased following chronic PETN treatment. Moreover, platelet adhesion was significantly reduced (number of adherent platelets: control: 85.6±5.5, PETN: 40±3.3; p<0.001) and VASP phosphorylation significantly enhanced following in vitro PETN treatment.ConclusionChronic NO supplementation using PETN reduces platelet activation in CHF rats. Thus, PETN may constitute a useful approach to prevent thromboembolic complications in CHF.

The direct factor Xa inhibitor Rivaroxaban reduces platelet activation in congestive heart failure

BackgroundPlatelet activation in congestive heart failure (CHF) contributes to an increased risk for thromboembolic complications. Rivaroxaban, the first oral direct FXa inhibitor is approved in Europe for prevention and treatment of venous thrombosis, pulmonary embolism, and prevention of thromboembolic events in atrial fibrillation. As heart failure is an important risk factor for thromboembolism and increased platelet activation is common in heart failure, we investigated the potential effect of Rivaroxaban treatment on platelets in an experimental CHF model. Methods and resultsChronic myocardial infarction was induced in male Wistar rats by coronary ligation. Rats were randomized to placebo or Rivaroxaban (3 and 10mg/kg once daily). After 10 weeks platelet activation was assessed. Platelet-bound fibrinogen, detected by flow-cytometry, was significantly increased in CHF-Placebo (p<0.05) and reduced following treatment with Rivaroxaban (p<0.05 vs. CHF-Placebo). ADP-induced aggregation was significantly enhanced in CHF-Placebo vs. sham-operated animals (p<0.05) and normalized following chronic FXa inhibition (p<0.05 vs. CHF-Placebo). In separate in vitro experiments, attenuated platelet aggregation was present after incubating whole blood directly with Rivaroxaban but absent when the experiment was performed in platelet-rich plasma only. Thus, a direct effect on platelets could be excluded. ConclusionChronic direct factor Xa inhibition using Rivaroxaban reduces platelet activation in CHF rats by attenuating the secondary phase of ADP-induced platelet aggregation. Thus, Rivaroxaban may constitute a useful approach to prevent thromboembolic complications and reduce platelet activation in CHF at the same time.

Effects of prolactin on platelet activation and blood clotting

Increased levels of prolactin often coincide with an increased risk for thromboembolic events, but it is unclear whether a direct causal relation exists. Our aim was to examine the effect of prolactin on platelet function. In addition to using recombinant prolactin for experiments in vitro, we analyzed platelet function by flow cytometry in a group of 13 females with hyperprolactinaemia and 18 healthy female controls. Platelet activation was measured by P-selectin expression and by the amount of platelet-bound fibrinogen after stimulation with adenosine di phosphate (ADP), collagen-related peptide and the protease activated receptor (thrombin receptor) (PAR)-activating peptides PAR4-AP and PAR1-AP. Free oscillation rheometry was used to measure clotting time in whole blood. No significant effect on platelet activation or clotting time could be seen in in vitro experiments by adding recombinant prolactin. However, significantly lower P-selectin expression was found in the hyperprolactinemic group when platelets were activated by ADP (5 and 10 μM) or PAR4-AP. The expression of fibrinogen did not differ between the two groups for any of the activators used. For all samples, inverse significant correlations between P-selectin expression and prolactin concentration were found for both 5 μM ADP (r = − 0.61, p < 0.01), 10 μM ADP (r = − 0.62, p < 0.001) and PAR4-AP (r = − 0.69, p < 0.001). Thrombin cleavage of recombinant prolactin resulting in a 16 kDa C-terminal fragment did not alter the P-selectin expression upon activation. We found an indirect inhibitory effect of prolactin on platelets in hyperprolactinemic patients, suggesting that prolactin might have a protective role in thromboembolic disease.

Intracoronary administration of abciximab via an intracoronary perfusion catheter in patients with a thrombotic coronary occlusion—a single center experience

Aims: At concentrations superior to those achieved with the standard intravenous dose for coronary procedures, abciximab has an active dethrombotic effect by displacing platelet-bound fibrinogen. This analysis investigates whether administration of abciximab by local intracoronary infusion through the ClearWay (CX) RX perfusion catheter improves coronary blood flow (TIMI flow) by reducing thrombus burden. Methods and Results: This retrospective study included 68 patients who presented with an acute coronary syndrome due to an intracoronary thrombus between May 2009 and April 2011. The primary endpoint was defined as improvement in Thrombolysis In Myocardial Infarction (TIMI) flow after intracoronary application of abciximab via the ClearWay (CW) RX perfusion catheter. The population (mean age 58±11 years) consisted of 54 patients (79%) with an ST-elevation myocardial infarction and 14 patients (21%) with a non-ST-elevation myocardial infarction. The balloon-diameter of the perfusion catheter was 1 mm in 25 (37%), 1.5 mm in 23 (34%), 2.0 mm in 14 (20%), and 3.0 mm in 6 patients (9%), respectively. Successful positioning of the balloon within the thrombus was not possible in 4 patients (6%). After infusion of abciximab using the perfusion catheter TIMI flow improved by one grade in 17 patients (25%), by two grades in 9 patients (13%), and by three grades in 7 patients (10%), TIMI flow remained unchanged in 21 patients (31%), and even worsened by one grade in 4 patient (6%) (χ test, Pb.001). The procedure was complicated by an air embolization in 5 patients (7%). Air embolization occurred using a 2 mm balloon (three patients) or a 3 mm balloon (two patients), but not using a 1 mm or 1.5 mm balloon (χ, Pb.003). After the use of the perfusion catheter in these patients TIMI flow improved by one degree in 2 of these patients, was unchanged in 2 of these patients and worsened by one grade in one patient. After additional treatment with thrombectomy (54 patients, 79%), initial balloon dilatation (46 patients, 68%), direct stenting (22 patients, 32%), and stent implantation (64 patients, 94%), the final TIMI flow was TIMI 3 in 57 patients (84%), TIMI 2 in 4 patients (6%), TIMI 1 in 6 patients (9%), and TIMI 0 in 1 patient (1%). Conclusion: The intracoronary infusion of abciximab using the ClearWay (CX) RX perfusion catheter helps to improve myocardial perfusion in patients with acute coronary syndrome due to an intracoronary thrombus. The use of perfusion catheters with a balloon ≥2 mm can be associated with air embolism.

Low-density platelet populations demonstrate low in vivo activity in sporadic Alzheimer disease

Platelets contain a substantial quantity of amyloid-precursor protein (APP) and β-amyloid. However, despite the large importance of APP and β-amyloid to dementia, little is known about platelets in sporadic Alzheimer dementia (AD). Furthermore, platelet heterogeneity influences human pathology and has been described to affect the progression of AD. This study investigated AD platelets with respect to density diversity and in vivo activity associated with density sub-fractions. We included 39 AD patients and used, as controls, 22 elderly individuals without apparent memory disorder. A continuous Percoll™ gradient covering the density span 1.04–1.09 kg/l provided the basis to divide platelets of whole blood into density fractions (n = 16). All platelet populations were evaluated accordingly. Platelet counts were determined electronically. A flow-cytometer was put to use to measure surface-bound fibrinogen as a measure of platelet in vivo activity. Samples obtained from patients diagnosed with sporadic AD contained platelets (fractions numbers 4–16) that circulated with significantly less surface-bound fibrinogen, i.e., their platelet activation in vivo was reduced, compared with controls. In particular, highly significant differences (p < 0.001) were obtained for the six less dense platelet populations (fractions numbers 11–16) when comparing sporadic AD with controls. In contrast, the densest AD platelets in fractions numbers 1–3 did not differ significantly from control cells with respect to in vivo platelet-bound fibrinogen. It is concluded that sporadic AD is characterized by lower density platelet populations that, while circulating, exhibited reduced activation. The clinical significance of this finding is unclear but these results suggest the importance of platelet heterogeneity in dementia as a topic for further investigation.

Flow cytometry analysis of porcine platelets: Optimized methods for best results

Animal models are essential tools for the in vivo evaluation of pharmacological modulation of platelet function and the mechanisms underlying thrombosis. In particular, pigs are being increasingly used in cardiovascular and platelet research. One standard method for the investigation of platelet function under experimental conditions is flow cytometry. However, this approach is limited by a shortage of feasible antibodies and a lack of incubation protocols with regard to porcine platelets. This study aimed to establish a method for the investigation of porcine platelets in flow cytometry. Platelets from pigs and human donors were stained with various commercially available specific antibodies against platelet receptors CD41a, CD42bα, CD62P, activated CD41/CD61, and platelet-bound fibrinogen. Staining procedures were performed in undiluted or diluted whole blood (WB) or platelet-rich plasma (PRP). Samples were treated with PBS buffer as control or with adenosine diphosphate (ADP) to induce platelet activation. Flow cytometry was performed using standard methodology. Furthermore, platelet counts were determined and ADP-induced platelet aggregations of both species were examined to confirm that the agonist ADP reliably activates human as well as porcine platelets. Five of the investigated antibodies bound to human, but not to porcine platelets only. However, two chicken-derived antibodies directed against CD62P (09-143) and fibrinogen (09-038) as well as a monoclonal mouse anti-CD62P (KO2.5) and a polyclonal rabbit anti-fibrinogen antibody (F0111) allowed reliable detection of porcine platelet activation. Moreover, binding intensity of the 09-143 antibody was increased when incubated in porcine PRP compared to WB, whereas antibody binding of both anti-fibrinogen antibodies to porcine platelets was only observed when incubated in a WB-buffer solution. KO2.5 antibody binding was detectable employing PRP as well as the WB-buffer solution. The feasibility of our new incubation protocols was confirmed by successful investigation of platelet activation in a porcine in vivo cardiopulmonary bypass model. In conclusion, we describe a reliable method to detect the activation of porcine platelets and therefore provide a useful tool for platelet flow cytometry in porcine models. Notably, the applied incubation protocol and medium, in which platelets are suspended, have major effects on antibody-binding properties.

Platelet Density Distribution in Essential Thrombocythemia

Essential thrombocythemia (ET) is characterized by high platelet counts and a slightly increased bleeding risk. Why severe hemorrhage does not occur more frequently is not known. Variations of platelet density (kg/l) depend mainly on cell organelle content in that high-density platelets contain more α and dense granules. This study compares ET patients (n = 2) and healthy volunteers (n = 2) with respect to platelet density subpopulations. A linear Percoll™ gradient containing prostaglandin E₁ was employed to separate platelets according to density. The platelet population was subsequently divided by density into 16 or 17 subpopulations. Determination of platelet counts was carried out. In each density fraction, platelet in vivo activity, i.e. platelet-bound fibrinogen, was measured using a flow cytometer. To further characterize platelet subpopulations, we determined intracellular concentrations of CD40 ligand (CD40L) and P-selectin in all fractions. Patients and controls demonstrated similar density distributions, i.e. 1 density peak. High-density platelets had more surface-bound fibrinogen in conjunction with signs of platelet release reactions, i.e. with few exceptions they contained less CD40L and P-selectin. Peak density platelets showed less surface-bound fibrinogen. These platelets contained less CD40L and P-selectin than nearby denser populations. The light platelets had more surface-bound fibrinogen than peak platelets together with elevated concentrations of CD40L. In ET, the malignant platelet production could exist together with platelets originating from normal megakaryocytes. It is also possible that clonal megakaryocytes produce platelets covering the entire density span. The ‘normal’ density distribution offers a tenable explanation as to why serious bleedings do not occur more frequently.

Variability of platelet aggregate dispersal with glycoprotein IIb-IIIa antagonists eptifibatide and abciximab.

Utilization of glycoprotein IIb-IIIa (GPIIb-IIIa) inhibitors improves outcomes of patients with acute coronary syndromes (ACS), including those undergoing percutaneous coronary intervention (PCI). These results may be related to the ability of the inhibitors to destabilize coronary thrombi, reduce microembolization, and restore vessel patency. To evaluate in vitro the ability of GPIIb-IIIa antagonists, abciximab and eptifibatide, to promote the disaggregation of platelet-rich thrombus. Antagonist-induced disaggregation was assayed in plasma by aggregometry, as well as in whole blood by point of care and capillary perfusion systems. Fibrinogen dissociation from the platelet surface was quantified by flow cytometry. Significant disaggregation of 5 microm ADP-induced aggregates was observed after addition of either agent. The maximum extent and rate of disaggregation were significantly higher with eptifibatide than with abciximab. Both antagonists also dispersed 2 microg mL(-1) collagen-induced aggregates, again with eptifibatide having a greater effect. Eptifibatide, but not abciximab (up to 10 microg mL(-1)), was efficient at dissociating aggregates to single platelets in whole blood and dispersing aggregates that had been aged for 30 min before treatment. Eptifibatide also reduced existing thrombus burden in the perfusion model under arterial flow conditions. A key mechanism of aggregate dispersal was antagonist-induced displacement of platelet-bound fibrinogen, which was greater with eptifibatide, a competitive inhibitor of fibrinogen binding, than with the noncompetitive inhibitor, abciximab. These results suggest that drug concentration and residence time, along with thrombus extent and age, may be critical determinants in promoting timely recanalization.

The cannabinoid receptor‐1 antagonist rimonabant inhibits platelet activation and reduces pro‐inflammatory chemokines and leukocytes in Zucker rats

We investigated the effect of rimonabant on inflammation and enhanced platelet reactivity in type 2 diabetic Zucker rats, an experimental model of impaired glucose tolerance and the metabolic syndrome. Rimonabant (10 mg kg(-1) by gavage) was fed for 2 weeks to 3-month-old male obese Zucker rats as an impaired glucose tolerance model and for 10 weeks to 6-month-old male obese Zucker rats as a model of the metabolic syndrome. RANTES (Regulated upon Activation, Normal T cell Expressed, and Secreted) and MCP-1 (monocyte chemotactic protein-1) serum levels were determined by ELISA. Leukocyte populations were quantitatively assessed using a veterinary differential blood cell counter. Platelet activation was assessed by flow-cytometry, platelet aggregation, and adhesion of isolated platelets to immobilized fibrinogen. RANTES and MCP-1 serum levels were increased in obese vs lean Zucker rats and significantly reduced by long-term treatment with rimonabant, which slowed weight gain in rats with the metabolic syndrome. Neutrophils and monocytes were significantly increased in young and old obese vs lean Zucker rats and lowered by rimonabant. Platelet-bound fibrinogen was significantly enhanced in obese vs lean Zucker rats of both age, and was reduced by rimonabant. Platelets from obese rats were more sensitive to thrombin-induced aggregation and adhesion to fibrinogen, which were both attenuated by rimonabant therapy. We demonstrate positive modulation of circulating neutrophil and monocyte numbers, reduced platelet activation and lower RANTES and MCP-1 levels by rimonabant in Zucker rats. This may potentially contribute to a reduction of cardiovascular risk.

Effect of Nitric Oxide on the Cryopreservation of Platelets

To determine whether nitric oxide (NO) could inhibit activation of platelets stored in a cold or frozen state, we measured platelet P-selectin expression and platelet-bound fibrinogen in platelet-rich plasma (PRP) with S-nitrosoglutathione (GSNO) (Sigma, USA) by flow cytometry. PRP was prepared by centrifuging venous blood collected in a 3.2% sodium citrate tube from 10 healthy donors. It was aliquotted into 4 groups (no cryoprotectant, GSNO, GSNO/dimethyl sulfoxide [DMSO] [Sigma], and DMSO), and stored at room, cold and freezing temperatures for 24 hrs. We performed a flow cytometric analysis of all specimens stained with FITC-fibrinogen and PE-CD62P monoclonal antibodies (Becton Dickinson, USA). The results were compared according to the storage temperature and agonist among 4 groups. GSNO inhibited significantly the activation of frozen platelets, but not in the presence of DMSO. GSNO was also shown to preserve the aggregability of frozen platelets because in the presence of GSNO the delta percent change of P-selectin expression and fibrinogen binding of frozen platelets increased significantly irrelevant to DMSO. GSNO inhibited the activation of frozen platelets and preserved the platelet aggregability; therefore, it may be used as a protectant for platelet cryopreservation.

Dilation-Dependent Activation of Platelets and Prothrombin in Human Thoracic Ascending Aortic Aneurysm

The purpose of this study was to investigate whether thoracic ascending aortic aneurysm (TAAA) induces platelet activation as mural thrombus participates in aortic dilatation in abdominal aortic aneurysm and TAAA are associated with rheological factors favoring coagulation activation. We studied the relation between coagulation activation and aortic diameter in Marfan patients (MFS) with various aortic diameters (n=52). We then studied patients presenting large aneurysms associated with bicuspid aortic valve (BAV) and degenerative form. Lastly, we used immunochemistry and biochemistry to investigate prothrombin/thrombin retention within the aortic wall. Microparticles, sGPV, tissue factor, and TAT complexes were increased in plasma from MFS with large aneurysms (>or=45 mm) compared to MFS with limited aortic dilatation (<45 mm). Similar elevations were observed in all patients with large aortic aneurysms, regardless of the etiology, the site of maximal aortic dilation, associated valvulopathy, risk factors, or treatments. P-selectin and platelet-bound fibrinogen were also increased, demonstrating platelet activation in large aneurysms. Significant increase in sCD146 plasma concentration suggested alteration of endothelium. Platelet activation occurs in patients with large aneurysms of the ascending aorta, is dependent on aortic dilation, and is associated with thrombin generation, part of which appears to be retained in mucoid degeneration areas.

Arginyl-glycyl-aspartyl (RGD) epitope of human apolipoprotein (a) inhibits platelet aggregation by antagonizing the IIb subunit of the fibrinogen (GPIIb/IIIa) receptor

An unknown epitope of apolipoprotein (a) antagonizes fibrinogen binding to agonist-stimulated platelet's fibrinogen (GPIIb/IIIa) receptor yielding lipoprotein (a) mediated decreased platelet aggregation. The purpose of this study was to test the hypothesis that human apolipoprotein (a)'s single arginyl-glycyl-aspartyl (RGD) epitope, unique to apolipoprotein (a) in lipoprotein (a) binds to the RGD binding motif on the IIb subunit of the GPIIb/IIIa receptor thus reducing platelet-bound fibrinogen and consequently decreasing agonist-stimulated platelet aggregation. Platelets ( N = 30 subjects) were prepared from fresh plasma, washed three times in Tyrode's buffer and stimulated using 10 μM ADP or 2 μg/ml collagen. Lipoprotein (a) was isolated from plasma using lectin affinity chromatography followed by ultracentrifugation. The peptide RGDS inhibited 125I-labelled lipoprotein (a) binding to autologous platelets with IC-50's of 25.1 ± 2.2 (mean ± SEM) and 15.4 ± 1.3 μM for collagen- and ADP-stimulation respectively. Further, RGDS reduced platelet binding of 125I-labelled fibrinogen IC-50's of 35.5 ± 3.2 (mean ± SEM) and 20.7 ± 2.2 μM for collagen- and ADP-stimulation respectively. The monoclonal antibody PAC-1, uniquely directed at the RGD binding motif on the IIb subunit on collagen- and ADP-stimulated platelets, inhibited binding of 125I-labelled lipoprotein (a) with IC-50's of 6.4 ± 0.7 and 2.5 ± 2.2 μg/10 8 platelets for collagen- and ADP-stimulation respectively. Additionally, PAC-1 reduced platelet bound of 125I-labelled fibrinogen with IC-50's of 9.0 ± 1.4 and 4.1 ± 2.2 μg/10 8 platelets for collagen- and ADP-stimulation respectively. In a dose-related fashion, a polyclonal antibody, specific for the RGD epitope on apolipoprotein (a), restored platelet aggregation to control levels, inhibited 125I-labelled lipoprotein (a) binding, and increased 125I-labelled fibrinogen by displacing lipoprotein (a) from the GPIIb/IIIa receptor. Thus a never before demonstrated aspect of the mechanism of lipoprotein (a)'s suggested novel role as an endogenous regulator of fibrinogen binding to collagen- and ADP-stimulated platelets has been shown. In conclusion, lipoprotein (a), via apolipoprotein (a)'s RGD epitope, binds to the RGD binding motif on the IIb protein of the GPIIb/IIIa receptor consequently reducing platelet-bound fibrinogen which results in decreased platelet aggregation.

Do platelet apoptosis, activation, aggregation, lipid peroxidation and platelet–leukocyte aggregate formation occur simultaneously in hyperlipidemia?

Objectives: The circulating lipoproteins may cause some abnormalities in platelet composition and function in hypercholesterolemia. The aim of this study was to investigate whether platelet apoptosis, platelet activation, platelet aggregation, platelet–leukocyte aggregate (PLA) formation and lipid peroxidation occur simultaneously in hyperlipidemia. Design and methods: Expression of GpIIb/IIIa (CD41a), P-selectin (CD62-P), platelet-bound fibrinogen (antifibrinogen), platelet membrane phosphatidylserine (PS), platelet–monocyte aggregates (mono-PLA) and platelet–neutrophil aggregates (neut-PLA) was measured in eight hyperlipidemic and eight normal subjects using flow cytometry. ADP (10 μM) was used to activate platelets. Furthermore, ADP induced platelet aggregation responses, platelet malondialdehyde (MDA) and glutathione (GSH) levels were determined. Results: Before platelet activation, platelet CD62-P, antifibrinogen, annexin-V, mono-PLA, neut-PLA and platelet MDA levels as well as platelet aggregation responses in the hyperlipidemics were significantly higher than those in the controls ( P < 0.01, P < 0.01, P < 0.01, P < 0.001, P < 0.001, P < 0.01, P < 0.001, respectively), whereas GpIIb/IIIa expression and GSH levels were not different significantly ( P > 0.05). In the control group, CD62-P, antifibrinogen and annexin-V levels increased significantly after ADP activation ( P < 0.05, P < 0.05, P < 0.01, respectively). In hyperlipidemic subjects, annexin-V expression increased significantly after activation ( P < 0.01), whereas expression of GpIIb/IIIa, CD62-P and antifibrinogen remained unchanged ( P > 0.05). The levels of total cholesterol (T-CHO), low density lipoprotein cholesterol (LDL-C), serum fibrinogen (S-FGN) and high density lipoprotein cholesterol (HDL-C) in patients were found to be correlated with platelet CD62-P, antifibrinogen, annexin-V, mono-PLA and MDA. Conclusions: In conclusion, it seems that in hyperlipidemia, some platelets are in an activated state in circulation, and that increased lipid peroxidation, early apoptosis, platelet–leukocytes aggregate formation and platelet aggregation altogether accompany this process.

Reduced basal nitric oxide bioavailability and platelet activation in young spontaneously hypertensive rats

Objective: To investigate the role of basal nitric oxide (NO) bioavailability for platelet activation in young spontaneously hypertensive rats before onset of hypertension. Phosphorylation of the vasodilator-stimulated phosphoprotein (VASP) in platelets was used as a sensitive monitor of in vivo NO bioavailability. Methods and results: Whole blood samples were taken from 10-week-old Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). In vivo surface-expression of P-selectin and platelet-binding of fibrinogen were assessed by flow cytometry. Platelet VASP-phosphorylation at its serine 239 (Ser 239) and serine 157 (Ser 157) residues was assessed using specific antibodies to determine NO bioavailability in vivo, and compared with endothelial vasomotor function. The increment in vascular tone following inhibition of NO-synthase in slightly preconstricted aortic rings was reduced indicating less NO formation under physiological stimulation (WKY 71.1±4.1%; SHR 57.8±2.4%, P<0.05). In vivo platelet VASP-phosphorylation was significantly reduced at both phosphorylation sites in SHR (mean fluorescence for Ser 239: WKY: 15.2±0.6; SHR: 11.7±0.5, P<0.01; Ser 157: WKY: 53.0±3.0; SHR: 35.0±3.5, P<0.05). Surface-expression of P-selectin and membrane-bound fibrinogen were significantly enhanced in SHR compared with WKY (P-selectin: WKY: 23.2±3.4; SHR 58.3±7.9, P<0.001; platelet-bound fibrinogen: WKY: 8.6±0.5; SHR: 13.5±1.1, P<0.001). In vitro preincubation of platelets with the NO donor sodium nitroprusside normalized platelet surface-expression of P-selectin in SHR. Conclusion: Using VASP-phosphorylation as a sensitive monitor of in vivo NO bioavailability, these data provide evidence that reduced vascular NO formation in vivo contributes to increased platelet activation in young SHR.

Increased platelet activation in young Zucker rats with impaired glucose tolerance is improved by acarbose.

Patients with diabetes display increased platelet activation. Recent data show a markedly increased risk for cardiovascular events already in pre-diabetic individuals with impaired glucose tolerance (IGT). We investigated whether IGT is associated with platelet activation. Blood samples were collected from young lean (control) and obese Zucker rats, an established model of IGT, after single oral application of sucrose (4 g.kg-1). Platelet-bound fibrinogen and platelet surface-expression of P-selectin were assessed as indices of platelet activation using flow cytometry. In lean Zucker rats, acute sucrose application induced fibrinogen-binding and P-selectin surface-expression, which was prevented by co-administration of acarbose (10 mg.kg-1). In obese Zucker rats, platelet activation was already maximally increased under baseline conditions with no significant increase after sucrose application. Chronic treatment with acarbose (15 mg.kg-1.day-1) significantly reduced platelet activation in these animals. Acute ingestion of sucrose induces platelet activation which is prevented by acarbose. IGT is associated with marked platelet activation that can be reduced by chronic administration of acarbose. The positive modulation of platelet activation by acarbose may contribute to the reduction of cardiovascular events in patients with IGT.