Increased aggregation response of platelets in patients with inflammatory bowel disease

Background Platelets play an important role in hemostatic and inflammatory responses. To evaluate any potential enhancement of platelet activity in patients with inflammatory bowel disease (IBD), we measured the platelet aggregation responses to various stimuli. Methods Twenty-two healthy controls, 24 patients with ulcerative colitis (UC) and 25 patients with Crohn's Disease (CD) were studied. The aggregation responses induced by three agonists (epinephrine, collagen, and ADP) were measured by an 8-channel aggregometer. The platelet-derived microparticles (PDMP) levels were measured by an enzyme-linked immunosorbent assay. Results Twenty-one out of the 22 healthy controls did not respond to epinephrine (0.1 μg/ml), collagen (0.2 μg/ml), or ADP (1.0 μM). Eight out of the 12 active UC patients were sensitive to all agonists, and 4 patients showed increased sensitivity to epinephrine/collagen or epinephrine/ADP. Three out of the 12 inactive UC patients were normal, but 9 of these patients showed increased sensitivity, mainly to epinephrine. Ten out of the 12 active CD patients were sensitive to all agonists, and 2 active CD patients were sensitive to epinephrine/collagen or epinephrine/ADP. Eight out of the 13 inactive CD patients were sensitive to two or all agonists. Even after remission, almost all of the UC and CD patients showed some increased sensitivity to the agonists. The platelet number and the plasma PDMP levels were significantly higher in the active IBD patients than in the control group. Conclusions Platelet aggregation responses are enhanced in IBD, even in inactive-phase patients. This increased sensitivity of the platelets may play an important role in the pathophysiology of IBD.     

Platelet dysfunction in nutritional vitamin b(12) deficiency

Platelet aggregation to four different agonists was measured in platelet-rich plasma from 17 patients with nutritional vitamin B(12) deficiency; 10 of these patients were also iron deficient. Impaired aggregation to ADP, collagen, epinephrine and ristocetin was found for 4, 8, 10 and 6 patients, respectively. Bleeding manifestations were seen in 5 patients, one belonging to the vitamin B(12) deficiency group and 4 to the combined deficiency group. Three of these 5 patients had normal platelet aggregation. These results show that impaired platelet aggregation is common in nutritional vitamin B(12) deficiency but that this impairment does not reliably correlate with a clinical bleeding tendency.     

Mechanisms involved in the antiplatelet activity of Escherichia coli lipopolysaccharide in human platelets

In this study, Escherichia coli lipopolysaccharide (LPS) dose-dependently (100–300 μg/ml) and time-dependently (10–60 min) inhibited platelet aggregation in human platelets stimulated by agonists. LPS also dose-dependently inhibited the phosphoinositide breakdown and the intracellular Ca⁺² mobilization in human platelets stimulated by collagen. LPS (300 μg/ml) also significantly inhibited the thromboxane A₂formation stimulated by collagen in human platelets. Moreover, LPS (100–300 μg/ml) dose-dependently decreased the fluorescence of platelet membranes tagged with diphenylhexatrience. In addition, LPS (200 and 300 μg/ml) significantly increased the formation of cyclic GMP but not cyclic AMP in platelets. LPS (200 μg/ml) also significantly increased the production of nitrate within a 30 min incubation period. Rapid phosphorylation of a platelet protein of M _r 47 000, a marker of protein kinase C activation, was triggered by phorbol-12-13-dibutyrate (PDBu, 50 n M ). This phosphorylation was markedly inhibited by LPS (200 μg/ml) within a 30 min incubation period.
These results indicate that the antiplatelet activity of LPS may be involved in two important pathways. (1) LPS may induce conformational changes in the platelet membrane, leading to change in the activity of phospholipase C. (2) LPS also activated the formation of nitric oxide (NO)/cyclic GMP in human platelets, resulting in inhibition of platelet aggregation. Therefore, LPS-mediated alteration of platelet function may contribute to bleeding diathesis in septicaemic and endotoxaemic patients.     

Defective platelet aggregation and increased platelet turnover in patients with myelofibrosis and other myeloproliferative diseases

In 9 patients with myeloproliferative diseases (MPD) (6 with myelofibrosis, MF, 1 with Ph1 positive chronic granulocytic leukaemia, CGL, 1 with primary eosinophilia, PE, 1 with pre-leukaemia syndrome, preL) collagen, epinephrine, and ADP-induced aggregation, N-ethylmaleimide-induced malondialdehyde (MDA) production, beta-thromboglobulin (beta-TG) plasma levels, and platelet turnover were studied. Collagen-induced aggregation was found to be normal in 7 patients, absent in 1, and reduced in 1. In all but 3 patients, aggregation with ADP was markedly reduced. Epinephrine-induced aggregation was decreased in 7 patients. No difference was found between mean MDA production in MPD (3.21 +/- 0.50 nmol/10(9) PLTs) and in control group of 21 normal subjects (3.04 +/- 0.26 nmol/10(9) PLTs). Mean beta-TG levels were significantly higher (P less than 0.01) in MPD patients (165.00 +/- 28.29 ng/ml) than in healthy controls (81.76 +/- 14.63 ng/ml). Mean platelet production half-time was significantly shorter in MPD (2.48 +/- 0.24 d) than in the control group (3.43 +/- 0.17 d), after adjustment for age by covariance analysis (P less than 0.005). Our data do not indicate an abnormal prostaglandin synthesis and are consistent with the hypothesis that a disseminated intravascular platelet aggregation might take place in MPD patients.     

同型半胱氨酸作用于正常个体血小板凝聚反应的体外研究

目的　研究人体血清或血浆同型半胱氨酸 (HCY)的升高与血小板高凝状态的关系。方法　全血或浓缩血小板与不同浓度的 HCY于 37℃ (用于血小板凝聚检测 )或 2 2～ 2 5℃ [用于血小板纤维原结合及 P-选择素 (P- selectin)表达的检测 ]作用 15 min,后应用血小板凝聚仪和流式细胞仪分别检测 HCY对血小板活性的作用。结果　 HCY在浓度 30 μm ol/ L 时能增加一磷酸腺苷 (ADP)和胶原诱导的全血和浓缩血小板凝聚反应 [(3.0± 0 .8) Ω/ min vs(5 .0± 2 .0 ) Ω/ m in,P<0 .0 5 ,n=9和 (8.5± 1.5 ) Ω/ min vs(11± 2 .5 ) Ω/ min,P<0 .0 5 ,n=6 ],但在浓度 10 0 0μm ol/ L时却抑制 ADP和胶原诱导的全血和浓缩血小板凝聚反应 [(7.0± 4 .0 )Ω vs (3.6± 2 .6 )Ω和 (6 .8± 2 .2 )Ω vs (4.1± 3.3)Ω ,P<0 .0 5 ,n=9],血小板凝聚性的改变并不伴随血小板纤维原结合及 P- selectin表达的变化。结论　体外实验表明 HCY能协同促进已激活的血小板凝聚反应 ,可能是 HCY与体内血栓形成有关的机理之一     

Modulation of platelet aggregation responses by leukocytapheresis therapy in patients with active ulcerative colitis

Background Recent studies suggest that platelet activation plays an important role in the pathophysiology of inflammatory bowel disease. In this study, we evaluated the effects of leukocytapheresis (LCAP) on platelet functions in patients with ulcerative colitis (UC). Methods Thirteen patients with active UC (five women and eight men) were treated with LCAP therapy. Platelet-rich plasma (PRP) was prepared, and platelet aggregation in response to agonist solution (epinephrine, collagen, and ADP) was measured with a platelet aggregometer. Platelet-derived microparticle (PDMP) plasma levels were determined by enzyme-linked immunosorbent assay. Results Nine patients responded to LCAP therapy, but no clinical responses were observed in four patients. The aggregation response to 0.1 μg/ml epinephrine was enhanced in all patients. In all responders, enhanced epinephrine aggregation was normalized after the LCAP session. However, in the four nonresponders, enhanced epinephrine aggregation was maintained after the LCAP session. In responders, the mean maximum aggregation induced by 0.1 μg/ml epinephrine was 76.8 ± 5.0% before and 15.4 ± 3.8% after LCAP, respectively (P < 0.05). Increased aggregation responses to both 0.2 μg/ml collagen and 1.0 μM ADP were observed, and LCAP also normalized these enhanced responses. LCAP significantly reduced circulating PDMP levels (56.8 ± 28.3 U/ml before and 46.3 ± 30.4 U/ml after LCAP, P < 0.05). Conclusions LCAP reduced enhanced platelet aggregation responses in active UC patients. Because platelets play an important role in inflammatory and immune responses, therapeutic effects of LCAP may be partially mediated by reduction of increased platelet aggregation activities.     

Effects of recombinant human megakaryocyte growth and development factor on platelet activation

The c-Mpl receptor for thrombopoietin and its recombinant related protein, the megakaryocyte growth and development factor (MGDF), is also expressed on circulating platelets. In the present study we evaluated the effect of MGDF on platelet aggregation in platelet-rich plasma (PRP) and in whole blood. The results obtained indicate that MGDF by itself did not affect platelet aggregation. However, when added before other agonists such as adenosine diphosphates (ADP), epinephrine (EPI), and thrombin (THR), it rendered platelets more sensitive. This "priming" effect of MGDF was dependent on the dose and on the time of platelet preincubation, and it occurred both in PRP and in whole-blood platelet aggregation. MGDF also "primed" the release of adenosine triphosphates and the production of thromboxane B2 by platelets stimulated with ADP, EPI, and THR. When added 15 seconds after the preincubation of platelets with subthreshold concentrations of ADP, EPI, and THR, MGDF exhibited a synergism with these agonists. Moreover, we observed a "priming" effect of MGDF on the phosphorylation of p-42 mitogen-activated protein kinase promoted by ADP, EPI, and THR. These observations suggest that thrombopoietin may play a physiologic role in modulating the response of platelets to several stimuli and thereby their hemostatic potential.     

Attenuated platelet sensitivity to collagen in patients with neurofibromatosis type 1

Haemostatis has not previously been studied in patients with neurofibromatosis 1 (NF-1), despite case reports of an association with von Willebrand disease and reported excessive bleeding in those undergoing surgery for neurofibromas. Platelets from NF-1 patients ( n = 28) were tested for aggregation and ATP release with agonists including ADP, arachidonic acid, thrombin and collagen. Mepacrine staining of platelets and three different assays for von Willebrand factor (VWF) were also performed.
In response to collagen as the platelet agonist, tested at both 2 and 1 μg/ml, NF-1 patients had an attenuated rate of aggregation ( P < 0.007), aggregation lag phase ( P < 0.005) and ATP release ( P < 0.045), as well as requiring higher collagen concentrations to attain threshold aggregation response ( P = 0.041). Normal platelets resuspended in selected NF-1 plasma exhibited significantly reduced platelet aggregation and release compared to controls, which was not corrected by mixing 1:1 with normal plasma. Collagen binding activity was reduced in NF-1 patients compared with controls (127% v 161%, P = 0.05).
As a group, patients with NF-1 display defective platelet function characterized by in vitro evidence of impaired responsiveness to collagen. It is suggested that a plasma factor, present in a significant proportion of NF-1 patients, may interfere with the ability of collagen to interact with other proteins such as von Willebrand factor and the platelet collagen receptor.     

Platelet aggregation impacts thrombin generation assessed by calibrated automated thrombography

A calibrated automated thrombogram (CAT) is performed usually with human platelet-free plasma (PFP) but may be more relevant with platelet-rich plasma (PRP). In this case, platelets are not stimulated by subendothelial molecules like collagen. Our aim was to assess the consequence of strong (collagen) or weak (ADP) induction of platelet release and aggregation on thrombin generation. Platelet aggregation in PRP was triggered with 10 µg/mL collagen or 10 µM ADP using a lumi-aggregometer. Thrombin generation curves were monitored by CAT in different conditions: PRP, PRP with activated platelets (actPRP), aggregated PRP (agPRP), aggregated platelets resuspended in autologous PFP (resPRP), PFP and PFP obtained after aggregation (agPFP). We found a 3-fold shortening of the lag time and time to peak and a marked increase in velocity and thrombin peak without changes in endogenous thrombin potential (ETP) in agPRP with both agonists compared with PRP. The same holds true in agPFP but with a marked increase in ETP compared with PFP. Similar changes in the kinetics of thrombin generation were observed with actPRP-collagen and to a lesser extent in resPRP-collagen compared with PRP. By contrast, there were no modifications of the thrombin generation curves in actPRP-ADP. Alpha-2-macroglobin-thrombin complexes were unchanged in the different PRP conditions but were increased in PFP prepared from agPFP compared to control PFP. Platelet aggregation during activation by agonists other than thrombin did not increase thrombin generation but accelerated its kinetics mainly via platelet content release and platelet-derived extracellular vesicules formation. In diseases characterized by altered platelet granule content or release as well as altered platelet activation, a platelet aggregation step prior to CAT analysis may be clinically relevant to improve laboratory estimation of the bleeding/thrombotic balance.     

Disaggregation Following Agonist-Induced Platelet Activation in Patients on Dual Antiplatelet Therapy

Disaggregation as the difference between maximal and final platelet aggregation by light transmission aggregometry indicates the stability of platelet aggregates. We evaluated the extent of disaggregation after platelet stimulation with adenosine diphosphate (ADP), arachidonic acid (AA), collagen, epinephrine, and thrombin receptor-activating peptide (TRAP)-6 in 323 patients on dual antiplatelet therapy with daily aspirin and clopidogrel (group 1), prasugrel (group 2), or ticagrelor (group 3) therapy. All patients in group 1 underwent elective angioplasty and stenting, whereas all patients included in groups 2 and 3 suffered from acute coronary syndromes (STEMI or NSTEMI) and underwent urgent PCI. Significant differences between maximal and final platelet aggregation were observed with all agonists throughout the groups (all p<0.001). Disaggregation was highest using AA (clopidogrel 36.5%; prasugrel/ticagrelor 100%) and ADP (clopidogrel 21.7%; prasugrel/ticagrelor 100%). In contrast, low disaggregation was observed after platelet stimulation with collagen and TRAP-6 in clopidogrel-treated patients, and after platelet stimulation with collagen and epinephrine in prasugrel- and ticagrelor-treated patients. In conclusion, pathways of platelet activation that are not inhibited by standard antiplatelet therapy allow persisting platelet aggregation and may at least in part be responsible for adverse ischemic events.     

Evaluation of an automated light transmission aggregometry

Light transmission aggregometry (LTA) is the “gold standard” for platelet function assessment, but it is time-consuming and labor intensive. Recently, an automated platelet aggregation method has been developed on a routine coagulation analyzer (Sysmex CS-2100i). In this study, the performances of CS-2100i including repeatability, correlation with a reference aggregometer (Chrono-log Model 700), and the threshold limitation of platelet counts in platelet-rich plasma (PRP) were evaluated for clinical use. The agonists were adenosine diphosphate (ADP), arachidonic acid, collagen, epinephrine, and ristocetin. The platelet concentration of PRP was adjusted with platelet-poor plasma (PPP) and physiological saline (PS). The CS-2100i showed an excellent repeatability and a strong correlation with the Chrono-log 700 in performing platelet aggregation, and its threshold limitation of platelet counts in PRP was 80 × 10⁹/L. PPP had an inhibitory impact on platelet aggregation induced by ADP, arachidonic acid, collagen or epinephrine; while PS had an inhibitory impact on ristocetin-induced aggregation. PS should be used to adjust PRP for ADP-, arachidonic acid-, collagen-, or epinephrine-induced aggregation; while PPP was recommended for ristocetin-induced aggregation. The CS-2100i showed an excellent repeatability, a strong correlation with Chrono-log 700, a lower platelet count requirement, a shorter turnaround time for samples, the advantage of being a walk-away technology, and the ability to perform a highly standardized platelet function assessment.     

Measurement of platelet aggregation during antiplatelet therapy in ischemic stroke

Aspirin, ticlopidine and clopidogrel are used as a pharmacological means to efficiently decrease the number of reoccurrence of ischemic stroke (100-325 mg/d). This antiplatelet treatment could prevent the secondary stroke by approximately 22%. Laboratory effective platelet inhibition for the clinician, and methods for routine screening evaluation for the laboratory were studied. (1) For the standardisation of platelet aggregation technology blood samples of 150 healthy persons were studied in 5 centres. CARAT TX computerised optical aggregometer was used for measuring with collagen (2 microg/ml), epinephrine (10 microM), arachidonic acid 0.5 mM and ADP 5 microM as inductors. (2) Laboratory tests were compared in each centres performed in platelet-rich plasma of ischemic cardiovascular and stroke patients (n=823) taking 100-325 mg aspirin/d. (3) Blood samples of 555 ischemic stroke patients treated with aspirin (100-325 mg/d), 96 patients treated with ticlopidine (500 mg/d), and 67 patients treated with clopidogrel (75 mg/d) were evaluated, respectively.(1) The mean of maximal aggregation (%) - 2SD of untreated controls (n=150) were detected for collagen with 64%, epinephrine 59% and ADP 62%. (2) In 823 aspirin treated patients were found similar inhibition in different centres with same methods for standardisation. The mean inhibition level was in case of collagen 38%, epinephrine 37% and ADP 61%. (3) The distribution of ineffective platelet inhibition was detected in 17% of aspirin group (collagen and epinephrine), 4% of ticlopidine and 18% of clopidogrel group with ADP, respectively. Our findings were in the stroke cohort: effective inhibition levels: 36% in aspirin group, 73% in ticlopidine and 25% treated with clopidogrel. Platelet aggregation tests could help to find the optimal, and "custom taylored" dose of antiaggregating drugs in the secondary prevention of ischemic stroke.     

Phorbol esters sensitize platelets to activation by physiological agonists

Phorbol esters such as phorbol 12, 13-dibutyrate (PdBu; 40 to 200 nmol/L) or 12-O-tetradecanoyl phorbol 13-acetate (20 to 80 nmol/L) added to aspirinized platelet-rich plasma (PRP) 5 to 15 seconds prior to various platelet stimuli (epinephrine, ADP, prostaglandin endoperoxide analog U44069, collagen, PAF, or vasopressin) potentiate the rate and extent of aggregation and ATP secretion induced by those agonists. Platelet aggregation, but not secretion, is potentiated at low concentrations of agonists; platelet secretion is potentiated at higher concentrations of the platelet stimuli. Potentiation of platelet responses was also observed when the preincubation time with PdBu was extended to 12 minutes and also occurred in washed platelets. The potentiating effect of phorbol esters is not mediated by formation of arachidonate metabolites or by released ADP. The sensitizing effect of PdBu on platelet aggregation induced by epinephrine is unique, since in contrast to the other platelet stimuli it is also found at maximal concentrations of epinephrine and does not diminish with prolonged preincubation of platelets with PdBu. Activation of protein kinase C ranges from 20% to 80% over control after 1 to 10 minutes of platelet pretreatment with PdBu but dramatically increases after subsequent addition of a stimulus such as vasopressin. In contrast, agonist- induced myosin light chain phosphorylation is reduced after platelet pretreatment with PdBu. The results indicate that protein kinase C activation enhances platelet aggregation and dense granule secretion triggered by physiologic stimuli, although it desensitizes agonist- induced myosin light chain phosphorylation.     

Platelet aggregation and P-selectin levels during exercise treadmill test in patients with ischaemic heart disease

INTRODUCTION: Coronary artery disease (CAD) is associated with higher platelet activation sometimes despite aspirin use. There are conflicting data concerning platelet activation course during physical exercise in patients on aspirin with CAD. AIM: To assess platelet activation pattern during physical exercise in patients with CAD. METHODS: The study included 35 patients (20 men, 15 women) aged 64.7+/-10 years with CAD (CCS II) on aspirin treatment (75 mg daily) and a control group of 10 healthy subjects adjusted for age and gender. Treadmill testing was performed using the Bruce protocol. Platelet aggregation was measured with optical aggregation with the agonists ADP (10 microM), collagen (2 microg/ml) and arachidonic acid (0.5 mg/ml) before and at peak exercise; P-selectin platelet and soluble expression (basal and after stimulation with thrombin) was assessed with cytofluorometry before, at peak exercise and 1 hour after. RESULTS: There were no differences in collagen and ADP aggregation between patients and the control group. There was a significant increase of ADP aggregation at peak exercise in the control group (p <0.05). There was a positive correlation between platelet aggregation before exercise and at peak exercise with ADP (r=+0.86) and with collagen (r=+0.61). There was no difference in soluble P-selectin concentration between patients and the control group. Platelet P-selectin expression without stimulation with thrombin 1 hour after exercise was significantly higher in patients than in the control group (p <0.05). CONCLUSIONS: 1. Physical exercise does not intensify platelet aggregation in patients with CAD on 75 mg aspirin daily. 2. Despite taking aspirin, platelet activation measured with the expression of platelet P-selectin increases and there is further intensification during exercise testing. 3. The concentration of soluble P-selectin in patients with CAD does not reflect the expression of platelet P-selectin.     

Thrombopoietin measurement in thrombocytosis: dysregulation and lack of feedback inhibition in essential thrombocythaemia

Essential thrombocythaemia (ET), a myeloproliferative disorder (MPD) manifested by excessive platelet production, lacks a specific diagnostic test to facilitate differentiation from other thrombocytoses. We studied thrombopoietin (TPO) levels in 41 patients with thrombocytosis: 25 ET patients, eight with other MPD, and eight with reactive thrombocytosis. Mean age and platelet counts for these groups were comparable. TPO levels for 96 healthy individuals provided a reference range for normal. The majority of ET patients (19/25 or 76%) had normal TPO levels. No patient with ET had a TPO level below 75 pg/ml, compared with 57% of healthy donors and 8/16 (50%) patients with other thrombocytoses ( P  < 0.05). TPO levels in ET are not appropriately down-regulated, as occurs with cytokines relevant to other MPD. In thrombocytosis, a TPO level <75 pg/ml indicates that ET is unlikely.     

Interaction between platelets and lupus anticoagulant

10 consecutive patients fulfilled the diagnostic criteria for lupus anticoagulant. 4 had concomitant systemic lupus erythematosus, 1 Waldenstrom's disease and 5 had no apparent underlying disease. Only the case with Waldenstrom's disease presented a bleeding tendency, with bleeding time greater than 20 min; the others had a history of thrombotic complications. A defect of platelet aggregation induced by ADP, epinephrine, collagen and arachidonic acid was documented in the Waldenstrom's disease case whose lupus anticoagulant was an IgM. In the others, lupus anticoagulant, identified as IgG immunoglobulins, produced no aggregation abnormalities. However, beta-thromboglobulin levels in platelets, plasma and urine were consistent with a pattern of platelet activation in all cases. IgG immunoglobulins separated from sera of 6 patients showed lupus anticoagulant activity, with no effects on platelet aggregation of normal platelet-rich plasma, but they induced secretion of beta-thromboglobulin from normal platelets.     

Heterogeneous defects of platelet secretion and responses to weak agonists in patients with bleeding disorders

Eleven patients with mild bleeding disorders had as a common abnormality, impaired platelet aggregation and secretion with low concentrations (0.5-1.0 micrograms/ml) of collagen and, in most cases, an absence of second phase aggregation with epinephrine. Platelet granule contents were normal, ruling out storage pool deficiency. To characterize further the platelet abnormalities, we measured aggregation, 14C-5HT secretion, and TxB2 formation induced by a variety of platelet agonists. In eight of the 11 patients we observed decreased initial rates as well as extents of aggregation with one or more weak agonists (ADP, epinephrine, thromboxane A2 and the endoperoxide analogue U44069), i.e. agonists which induced secretion only as a result of aggregation, but normal responses to strong agonists such as arachidonate and high (10 micrograms/ml) concentrations of collagen, which can induce secretion in the presence or absence of aggregation. In all of these patients, TxB2 formation with arachidonate and all concentrations of collagen was normal. The platelet defects in these eight patients have been designated as weak agonist response defects (WARDs). In contrast, the initial aggregation responses to all weak agonists were normal in the three other patients, while secretion and TxB2 formation induced by strong agonists were impaired. Thus, in contrast to the eight patients above, the platelet defects in these three patients were characteristic of defects in the secretion response per se. The results obtained in the 11 patients studied indicate that these types of platelet disorders, previously referred to as primary secretion defects, include defects in the initial platelet responses which precede secretion (WARD) as well as defects in the secretory mechanism per se. Both groups of defects appear to be heterogeneous in nature.     

Effects of P2Y 1 and P2Y 12 receptor antagonists on platelet aggregation induced by different agonists in human whole blood

ADP plays a major role in the amplification of platelet aggregation induced by other platelet agonists. ADP initiates platelet activation via the P2Y 1 receptor and amplifies platelet activation via the P2Y 12 receptor. Using the selective P2Y 1 receptor antagonist A2P5P and the selective P2Y 12 receptor antagonist AR-C69931MX, we assessed the relative contributions of P2Y 1 receptor and P2Y 12 receptor activation to platelet aggregation in hirudin-anticoagulated whole blood induced by PAF, 5HT, epinephrine, TRAP, streptokinase, U46619 and collagen. The effects of aspirin were assessed concurrently. A2P5P and AR-C69931MX variably inhibited aggregation induced by most of the agonists studied, whereas aspirin only inhibited aggregation induced by streptokinase and collagen. In some experiments, A2P5P and AR-C69931MX yielded additive inhibition of aggregation. All three antagonists interacted synergistically to inhibit collagen-induced aggregation. These studies demonstrate that P2Y 1 receptor activation plays a significant role in amplifying aggregation induced by agonists other than ADP, in addition to the established roles of P2Y 12 receptor activation and thromboxane A 2 synthesis.     

Testing agonist-induced platelet aggregation by the Impact-R [Cone and plate(let) analyzer (CPA)]

The Impact-R [Cone and plate(let) analyzer (CPA)] is useful to assess platelet adhesion in different diseases and to monitor antiplatelet therapy. The purpose of the present study was to adapt this system to test agonist-induced platelet aggregation. Blood samples were tested by light transmission platelet aggregometry (LTA), Impact-R regular test and Impact-R agonist-response test. In the latter, samples were pre-incubated for 1 min with an agonist leading to platelet activation, micro-aggregates formation and reduced adhesion. Impact-R regular test of ten healthy volunteers demonstrated platelet adhesion (surface coverage, SC) of 11.2 ± 2.6% while LTA induced by ADP, ristocetin, epinephrine, collagen and arachidonic acid (AA) yielded maximal aggregation (81% to 93%). In the Impact-R agonist-response test, SC was reduced to 2.2 ± 1.0%, 1.2 ± 0.9%, 2.3 ± 1.0%, 2.2 ± 0.8% and 2.4 ± 0.4%, respectively. Prostaglandin E₁ treatment weakened SC reduction in response to ADP and epinephrine (SC of 8.8 ± 1.8% and 9.5 ± 2.0%, respectively). Inhibition of P2Y₁₂ receptor with 2MeSAMP resulted in a dose-dependent decrease in maximal aggregation in the ADP-induced test, which inversely correlated to SC in the Impact-R ADP-response test. The Impact-R agonist-response tests detected aggregation defects in patients with storage pool disease, severe von Willebrand disease and epinephrine response deficiency, and may be useful to assess the effect of different agonists on platelet aggregation.     

Partial inhibition of platelet aggregation by nebulized pentamidine in severe haemophiliacs

The antiparasite agent pentamidine has been shown to inhibit human platelet aggregation in vitro at concentrations that (potentially) may be attained in patient plasma after the administration of the drug by nebulizer. We measured platelet aggregation in platelet-rich plasma (PRP) before and after the administration of 300 mg nebulized pentamidine to 10 HIV-positive patients with severe haemophilia on prophylaxis against Pneumocystis carinii pneumonia. All patients had normal platelet counts. PAF-acether, U46619, collagen and ADP at different concentrations were used as agonists. Platelet aggregation was lower in PRP samples taken at the end of pentamidine administration and 1 h thereafter than in samples taken at the same time points in control experiments (without the administration of pentamidine). The inhibition of platelet aggregation was mild and tended to be overcome by higher concentrations of platelet agonists. The bleeding time was prolonged from 5 to 15 min in one patient but did not change in the remaining nine patients. In conclusion, this controlled study shows that nebulized pentamidine inhibits platelet aggregation in HIV-positive haemophiliacs without significantly affecting their bleeding times. Although this mild inhibitory effect may not be clinically relevant in haemophiliacs with normal platelet counts despite their defect in intrinsic coagulation, patients with HIV-related thrombocytopenia should be monitored to detect any excessive prolongation of their bleeding times after nebulized pentamidine.