effects of picotamide on human platelet aggregation, the release reaction and thromboxane B2 production

We studied the effects of picotamide (N,N′ bis 3 picolyl-4-methoxy-isophthalamide) on human platelet aggregation, the release reaction and the production of thromboxane B₂ (TxB₂) induced by several platelet agonists. The effects of picotamide were compared to those of acetylsalicylic acid (ASA). Picotamide (0.5 mmol/1) inhibited platelet aggregation, the release of ATP and TxB₂ production induced by ADP, arachidonic acid (AA), collagen or the prostaglandin endoperoxide (PE) analogue U46619. ASA (0.5 mmol/1) did not affect platelet aggregation and the release of ATP induced by U46619. Picotamide and ASA inhibited the AA-induced platelet TxB₂ production both under stirring and non-stirring conditions, whereas the pure thromboxane A₂ receptor antagonist BM13177 (0.5 mmol/1) was inhibitory only under stirring conditions. Since under non-stirring conditions platelet aggregation does not occur, picotamide directly inhibits TxB₂ production, whereas BM13177 inhibits the potentiation of TxB₂ production due to TxA₂/PE-dependent platelet aggregation. Malondialdehyde (MDA) production by unstirred platelets stimulated with AA was not significantly inhibited by picotamide. In conclusion, picotamide inhibits the TxA2/PE-dependent platelet responses to agonists by a double mechanism: (i), TxA₂/PE antagonism; (ii) inhibition of thromboxane synthase.     

Mechanisms of vascular graft thrombosis: Role of altered canine platelet sensitivity to thromboxane

Using the standard turbidimetric method of platelet aggregation and quantitation of platelet secretion with ¹⁴C-Serotonin, we have examined the responsiveness of the platelets of mongrel dogs to arachidonic acid (AA), and the thromboxane agonist U46619 in the presence and absence of a subthreshold concentration of epinephrine. In response to stimulation with 750 μM AA, the platelets of 18 dogs produced irreversible aggregation (Group I), the platelets of 22 dogs showed, at most, reversible aggregation (Group II), while the platelets of 8 dogs demonstrated no aggregatory response (Group III). In the presence of AA and a subthreshold concentration of epinephrine (0.5 μM), the platelets of all three groups demonstrated enhanced aggregatory and secretory responses although the extent of ¹⁴C-Serotonin secretion differed significantly between all three groups. These differences in platelet aggregation correlate with the deposition of platelets onto synthetic vascular grafts and the maintenance of graft patency. When stimulated with 0.5 μM U46619 and a subthreshold concentration of epinephrine, the platelets of 97% Group I dogs and 75% of Group II dogs exhibited irreversible aggregation, while the platelets of all Group III dogs showed only reversible aggregation. In addition, significant differences in the extent of ¹⁴C-Serotonin secretion to this combination of agonists were observed between groups. Further examination of the specific effects of U46619 on canine platelets revealed that although the aggregatory and secretory responses to U46619 vary between the different canine platelet populations, the threshold concentration of U46619 required to produce platelet shape change is identical among all groups. Quantitation of the stable metabolite of AA produced via the cyclooxygenase pathway, thromboxane B₂(TxB₂), revealed no significant differences in the production of TxB₂ by the platelets of these different populations upon stimulation with AA. Our results suggest that the mechanisms underlying the differences in responsiveness of canine platelets to AA, are likely due to differences in sensitivity of canine platelets to TxA₂, and may be localized to the mechanism responsible for mediating platelet aggregation and secretion in response to TxA₂.     

Inhibition of cyclooxygenase-independent platelet aggregation by sodium salicylate

The effect of acetylsalicylic acid (ASA) on platelet aggregation (PA) and thromboxane A₂ (TxA₂) formation was investigated in vitro and ex vivo after 1 g or 300 mg ASA administration to healthy subjects. 50–100 μM ASA inhibited PA by single aggregating agent such as platelet aggregating factor (PAF) or epinephrine and reduced to 5% of control platelet TxB₂ formation, but did not influence PA by epinephrine plus PAF. The latter was inhibited by increasing ASA concentration. In samples incubated with 100 μM ASA and stimulated with epinephrine plus PAF, PA could be inhibited by the addition of 100–300 μM sodium salicylate. After 300 mg-1 g ASA administration to healthy subjects, the inhibition of PA by epinephrine plus PAF was more marked by highest doses of ASA. This study suggests that aspirin inhibits PA with a cy clooxygenase-independent mechanism; this effect is mediated, at least in vitro, by salicylic acid.     

Inhibition of human platelet function in vitro and ex vivo by acetaminophen

The effects of acetaminophen (APAP) in vitro, or ex vivo following APAP ingestion, on human platelet aggregation, ¹⁴C-5HT secretion, and thromboxane B₂ (TxB₂) formation were assessed. APAP added in vitro to citrated platelet-rich plasma (PRP) inhibited aggregation, secretion, and TxB₂ formation induced by collagen, epinephrine, arachidonate, and the ionophore A23187, but had no effect on the responses induced by the endoperoxide analog U44069. Arachidonate-induced responses were inhibited by lower concentrations of APAP than were the responses to the other agonists. In PRP obtained 1 hour after ingestion of 650 mg or 1000 mg APAP, arachidonate-induced TxB₂ formation was inhibited by 40–99% in five subjects tested, whereas inhibition of collagen- or epinephrine-induced TxB₂ formation was less consistent. Aggregation and secretion responses were not altered by APAP ingestion m 4 of the 5 subjects, but were inhibited in the remaining subject, who had the highest plasma APAP levels. In contrast to aspirin and indomethacin, APAP-induced inhibition of collagen-stimulated TxB₂ formation could be partially overcome with increasing collagen concentrations. No such partial correction occurred with epinephrine, however. In washed platelet suspensions labeled with ³H-arachidonate, both APAP and aspirin inhibited the formation of labeled PGD₂ and PGE₂, as well as TxB₂. These results suggest that APAP acts in human platelets as a reversible inhibitor of cyclo-oxygenase, as found previously in other tissues, and that recent APAP ingestion can, on occasion, produce inhibition of platelet functional responses measured in vitro.     

Platelet aggregation following exposure of phospholipase-treated platelet rich plasma to hydrogen peroxide

Hydrogen peroxide at micromolar concentrations (250–500 μM) can induce platelet aggregation of phospholipase-treated PRP. This effect, which occurs independently of the release of ADP, is blocked by aspirin, furosemide, catalase and 2-mercaptoethanol. PRP preincubated with H₂O₂ for 2–5 min. does not respond to Phl-H₂O₂ or collagen. This inhibitory effect is abolished with longer preincubations. In the presence of ADP or epinephrine, H₂O₂ enhances aggregation, if added to PRP with the inducers, and decreases the platelet response to the inducers, if preincubated with PRP for 2 min. The data suggest that micromolar concentrations of H₂O₂, which could be generated at sites of platelet plug formation by granulocytes, could influence the processes of hemostasis and thrombosis.     

Effect of submaximal and incremental upper extremity exercise on platelet function and the role of blood shear stress

Introduction: Platelets are involved in the pathogenesis of atherosclerosis. Although physical exercise is recommended to prevent atherosclerosis, the effect of exercise on platelet function and the underlying mechanisms of these effects are not completely understood. Accordingly, we aimed to examine the effect of different intensities acute arm exercises on platelet function. In addition, we evaluated the effect of lipid peroxidation and fluid shear rate on platelet response. Materials and methods: Twenty four healthy sedentary male volunteers aged 18–24 years performed submaximal and incremental exercises by upper extremity ergometer. The shear rate in the right artery was measured by Power Doppler Ultrasound (US) at rest and immediately after exercise. Pre and postexercise maximum intensities of ADP and collagen-induced platelet aggregation were measured using the impedance technique. Bioluminescent detection of thrombin-induced platelet ATP release and measurement of thromboxane B₂ (TxB₂) levels (as a marker of thromboxane A₂ (TxA₂) formation) by enzyme-linked immunoassay were performed before and after exercise. Results and conclusion: Shear rate increased after both submaximal and incremental exercise. Collagen-induced platelet aggregation increased after submaximal exercise, while ADP-induced aggregation and thromboxane B₂ levels did not alter with this protocol. Incremental exercise caused increased collagen and ADP-induced platelet aggregation and thromboxane B₂ levels. Neither of the protocols altered platelet ATP release. It was shown that acute upper extremity exercise increased platelet aggregation, without an increase in platelet release. Collagen-induced signalling pathways were more sensitive than those induced by ADP. The increase in thromboxane B₂ after incremental exercise implied increase in thromboxane A₂ formation and lipid peroxidation. Despite a significant correlation between platelet aggregation and thromboxane B₂ levels at rest, we found no clear-cut relationship between thromboxane A₂ formation, blood shear rate and platelet response to exercise.     

Involvement of calmodulin in platelet reaction

Penothiazines, known as selective inhibitors of calmodulin, completely inhibited platelet aggregation and secretion induced by ADP, collagen, epinephrine, thrombin or calcium ionophore. They also completely inhibited aggregation induced by exogenous arachidonate (AA) or a mixture of thromboxane A₂ and prostaglandin endoperoxides (TxA₂/PG G₂,H₂). Also, in the presence of these calmodulin inhibitors, the release of AA from platelet phospholipids (PL) was dosedependently inhibited in stimulated platelets. These observations suggest that in platelet reaction, calmodulin is involved in at least two different steps of the reaction: activation of phospholipases and contraction of platelet actomyosin after the formation of TxA₂.     

Effects of trifluoperazine on platelet activation

Previous reports of the inhibitory effects of trifluoperazine on platelet responses to different aggregating agents have been conflicting, and the mechanism of action remains unclear. We have found that aggregation by minimum concentrations of collagen and arachidonic acid, and second phase aggregation by minimum concentrations of ADP, thrombin, epinephrine and the calcium ionophore A23187 were inhibited by 40–60μM trifluoperazine. The first phase of aggregation by a minimum concentration of epinephrine was completely inhibited by 100μM trifluoperazine, and the first phase of aggregation induced by ADP, thrombin or A23187 was decreased by 300μM trifluoperazine. The platelet shape change caused by collagen, but by no other aggregating agent examined, was inhibited by 300μM trifluoperazine. Secretion of ³H-5 hydroxytryptamine by minimum concentrations of ADP, collagen, epinephrine and arachidonic acid was completely suppressed by 50μM trifluoperazine. Secretion by thrombin and A23187 was incompletely inhibited by 300μM trifluoperazine. Thromboxane B₂ formation caused by all aggregating agents, except epinephrine, was incompletely suppressed by 50μM trifluoperazine, and 300μM trifluoperazine only caused complete inhibition of thromboxane B₂ formation by ADP, collagen and epinephrine. The phorbol ester, TPA, which mimics diacylglycerol by activating protein kinase C, caused aggregation and secretion. Aggregation, but not secretion, by low concentrations of TPA was inhibited by concentrations of trifluoperazine as low as 50μM. However, aggregation by a combination of TPA and A23187 was only inhibited by concentrations of trifluoperazine in excess of 100 μM. Secretion by TPA was inhibited by concentrations of trifluoperazine in excess of 200μM. Our findings suggest that low concentrations of trifluoperazine inhibit platelet activation by inhibiting phospholipase A₂, and that higher concentrations inhibit platelet responses by interfering with protein kinase C.     

Prostacyclin potentiates 13-azaprostanoic acid-induced platelet deaggregation

The specific thromboxane A₂/prostaglandin H₂ (TXA₂/PGH₂) antagonist 13-azaprostanoic acid (13-APA) reverses platelet aggregation stimulated by TXA₂/PGH₂ and the prostaglandin endoperoxide analog U46619. The present report demonstrates that the deaggregatory properties of 13-APA are potentiated by prostacyclin (PGI₂). Human platelet-rich plasma was aggregated with U46619. Deaggregation was induced 2 min subsequent to the addition of the aggregating agent. Concentrations of 13-APA or PGI₂ which induced 20 percent deaggregation were determined. Simultaneous addition of half of these concentrations resulted in 60 percent deaggregation, demonstrating that the observed response was supraadditive. Measurement of cyclic adenosine 3′:5′ monophosphate (cAMP) in resting or deaggregating platelets demonstrated that 13-APA itself did not stimulate cAMP production nor did 13-APA facilitate PGI₂-induced increases in cAMP. In separate studies PGI₂ and 13-APA were added to PRP prior to the induction of aggregation by U46619. Under these conditions, additive inhibition of aggregation was observed. Thus, it is clear that the pharmacological interaction between PGI₂ and 13-APA depends upon the relative state of platelet activation.     

Prostacyclin release from the coronary vascular wall by vasoactive substances

Which vasoactive substances that are synthesized in vivo could induce the release of a sufficient amount of prostacyclin (PGI₂) to inhibit platelet aggregation from the vascular wall was investigated in the isolated dog heart perfused by a modified method of Langendorff. Infusion of 5 μM bradykinin or 25 u/ml crude thrombin into the heart for 30 sec resulted in the transient appearance of inhibitory activity of platelet aggregation. The inhibitory activity was stable at alkaline pH but unstable at acidic pH and thermolabile. The appearance of the inhibitory activity was prevented by treatment of the coronary vessel with 30 μM indomethacin or 1 mM tranylcypromine. These results indicated that the inhibitory activity was caused by PGI2. When 25 μM acetylcholine, 25 μM noradrenaline, 25 μM isoproterenol, 10 μM adenosine triphosphate (ATP 5 μM adenosine, 1 μM angiotensin II, 25 μM histamine or 1 μM serotonin was infused for 30 sec, no inhibitory activity of platele aggregation was observed. Bradykinin (5 × 10⁻⁹ 5 × 10⁻⁶ M) and purified thrombin (1 × 10⁻⁹ 1 × 10⁻⁷ M) induced a dose-dependent release of PGI₂ which was assayed using a radioimmunoassay for 6-keto-prostaglandin F₁ (6-keto-PGF₁).     

Endothelin and platelet function

The effects of newly discovered vasoconstrictor peptide endothelin was studied on human, rabbit and canine platelet function. Endothelin (0.01 nM - lμM) did not promote platelet aggregation. In human platelets, endothelin (0.1 μM) did not significantly affect aggregation responses to ADP, collagen, epinephrine, arachidonic acid, PGH₂ or thrombin. Endothelin did not promote the mobilization of intracellular calcium in Fura2 loaded human platelets. In rabbit and canine platelets endothelin produced signficant potentiation of platelet aggregation mediated by low concentrations of ADP. Aggregation responses to higher concentration of ADP (5 μM) were unaffected by endothelin. These data reveal that under certain circumstances endothelin may potentiate rabbit and canine platelet aggregation responses to ADP, however endothelin does not produce direct effects on human platelet function.     

Cooperative effect of GNB3 825C>T and GPIIIa PI(A) polymorphisms in enhanced platelet aggregation

Introduction: Platelet aggregation contributes to various thrombembolic disorders. Environmental factors affect platelet aggregability but only partially explain the interindividual variability in aggregation. While the platelet glycoprotein IIb/IIIa is involved in the pathogenesis of acute coronary syndromes whereas most platelet activating stimuli act via G Protein coupled receptors we investigated whether the 825C>T polymorphism of the gene GNB3 encoding the G protein β3 subunit together with the platelet glycoprotein (GP) IIIa Pl(A) polymorphism are predictive of platelet aggregability on stimulation with various agonists acting via GPCRs. Materials and methods: Platelet aggregation was measured by turbidometry in 150 non-smoking individuals aged 18–40 years at a density of 2×10⁵ platelets/μl with various agonists according to the method of Born. Genotypes of the GNB3 825C>T and glycoprotein IIb/IIIa PI(A) polymorphisms were determined using Pyrosequencing technology and restriction analysis. All functional studies were completed within 3 h. The data were analysed by Student's t-test for paired data. Results: Low concentrations of agonists resulted in enhanced platelet aggregation in subjects with the GNB3 CC-genotype compared to carriers of a 825T-allele. This effect was further enhanced in carriers of the GPIIIa Pl(A2) allele (2 μM ADP: 42% vs. 19%, p=0.017; 1 μM U-46619: 51% vs. 30%, p=0.03; 5 μM epinephrine: 69% vs. 53%, p=0.025). No significant pattern of aggregation was observed on stratification by GPIIIa genotypes alone. Conclusions: Our findings indicate that two genetic markers contribute synergistically to increased platelet aggregation. This will help to identify patients at increased risk for thrombosis.     

Inhibition of platelet aggregation by some flavonoids.

The inhibitory effects of five flavonoids on the aggregation and secretion of platelets were studied. These flavonoids inhibited markedly platelet aggregation and ATP release of rabbit platelets induced by arachidonic acid or collagen, and slightly those by platelet-activating factor. ADP-induced platelet aggregation was also suppressed by myricetin, fisetin and quercetin. The IC50 on arachidonic acid-induced platelet aggregation was: fisetin, 22 microM; kaempferol, 20 microM; quercetin, 13 microM; morin, 150 microM less than IC50 less than 300 microM. The thromboxane B2 formations were also inhibited by flavonoids in platelets challenged with arachidonic acid. Fisetin, kaempferol, morin and quercetin antagonized the aggregation of washed platelets induced by U46619, a thromboxane A2/prostaglandin endoperoxides mimetic receptor agonist. In human platelet-rich plasma, quercetin prevented the secondary aggregation and blocked ATP release from platelets induced by epinephrine or ADP. These results demonstrate that the major antiplatelet effect of flavonoids tested may be due to both the inhibition of thromboxane formation and thromboxane receptor antagonism.     

Characterization of an antiglycoprotein Ib monoclonal antibody that specifically inhibits platelet-thrombin interaction

Platelet glycoprotein Ib (GPIb) acts as a high-affinity thrombin binding site and as a receptor for von Willebrand Factor (vWF). A new anti-GPIb monoclonal antibody (mAB) VM16d was produced that specifically inhibited platelet-thrombin but not platelet-vWF interaction. The epitope for VM16d was located within the 45 kDa N-terminal region of the a-chain of GPIb. VM16d inhibited platelet aggregation induced by low dose thrombin (0.05 U/ml) but did not affect platelet aggregation induced by ristocetin, bovine vWF, ADP or collagen. The same inhibitory effects on thrombin-induced platelet aggregation were observed with the whole IgG molecule of VM16d and its F(ab′)₂ and F(ab′) fragments. VM16d also inhibited ¹⁴C-serotonin secretion induced by low dose thrombin and binding of ¹²⁵I-thrombin but not ristocetin-dependent binding of ¹²⁵I-vWF to platelets. These data indicate that the high-affinity thrombin binding site is located on the N-terminal 45 kDa domain of GPIb and that it is topographically separated from the vWF binding site.     

Effect of phorbol 12-myristate 13-acetate on collagen-induced signal transduction in rabbit platelet

Investigations were made on the inhibitory effect of phorbol 12-myristate 13-acetate (PMA), a powerful activator on protein kinase C, on collagen-induced signal transduction in washed rabbit platelets. Upon activation of the platelets with a low-dose of collagen (5 μg/ml), which was suppressed by 10 μM indomethacin, pretreatment of the platelets with 2 nM PMA caused prolongation of lag phase (2 min) before the onsets of the aggregation and ATP secretion as compared with PMA-untreated platelets (30 sec). Under this condition, appearance of the cell responses including the phosphatidic acid formation, thromboxane (Tx) generation and Ca²⁺-influx was similarly retarded for 2–3 min, whereas arachidonic acid liberation from the membrane phospholipids was not significantly affected by the PMA pretreatment. After such lag phase, every response appeared rapidly and reached almost the control value (without PMA). Upon activation of the same platelets with a high-dose of collagen (50 μg/ml), which was only half suppressible by indomethacin, PMA in the presence of indomethacin almost completely suppressed the phosphatidic acid formation as well as the aggregation and ATP secretion. Thus, our results suggest that collagen-platelet interaction may elicit direct activation of phospholipase A2 and C, and that the latter enzyme activation may be regulated by a negative effect of protein kinase C. However, the phospholipase A2 activation may be regulated by a mechanism independent of such effect. In PMA-pretreated platelets in response to a low-dose of collagen, the prolonged lag phase for aggregation appears to be due to impaired conversion of liberated arachidonic acid to TxA₂.     

Discrimination between platelet prostaglandin receptors with a specific antagonist of bisenoic prostaglandins

Platelet aggregation and secretion induced by PGG₂ and by analogues of PGH₂ and PGE₂ were inhibited by NO164. Inhibition was apparently competitive (K_i 20 μM). Responses to ADP, vasopressin and arachidonic acid were unaffected. Inhibition of ADP-induced platelet aggregation by PGD₂ and PGE₂ (but not by PGE₁) was antagonised by NO164.     

The in vivo effect in humans of pyridoxal-5′-phosphate on platelet function and blood coagulation

Vitamin B₆ has an antithrombotic effect. This, based on the results of in vitro studies, has been attributed to an antiplatelet effect. We assessed the in vivo effect of vitamin B₆ by measuring the effect of long-term administration of vitamin B₆ on platelet function and blood coagulation. Vitamin B₆ (pyridoxine hydrochloride), 100mg twice daily p.o. for fifteen days, was administered to 10 healthy volunteers. The bleeding time was measured before the first dose and 15 days after. A baseline value, the acute effect, chronic effect, and the acute-on-chronic effect of vitamin B₆ was estimated by measuring platelet function. The following tests were performed: platelet aggregation induced by collagen, ADP and epinephrine; thromboxane A₂ (TxA₂)-production and prostacyclin inhibition of ADP-induced aggregation. The effects on the coagulation system were monitored by measuring: the prothrombin time, activated partial thromboplastin time and levels of coagulation factor. Vitamin B₆ significantly prolonged the bleeding time from 4.1 ± 1.1 minutes to 6.8 ± 1.0 minutes (p = 0.0063). Aggregation of platelets with collagen was slightly but not significantly inhibited. Platelet aggregation induced with the agonists ADP or epinephrine was significantly inhibited by vitamin B₆, and the platelets tended to aggregate at a slightly decreased rate. The mean TxA₂-production was slightly, but not significantly, decreased. Vitamin B₆ had no effect on the sensitivity of platelets to prostacyclin, or on the coagulation system. Our results indicate that the antithrombotic effects of vitamin B₆ is limited to inhibition of platelet function; there was no measurable influence on coagulation. The results of this in vivo study are however such that clinical trials are warranted to further assess the efficacy of vitamin B₆ as an antiplatelet drug.     

Some properties of the human platelet vasopressin receptor

Aggregation of human platelets by vasopressin is potently inhibited by 1-[βmercapto-(β,β′-cyclopentamethylene propionic acid)]-L-arginine vasopressin, a selective vasopressor (V₁) antagonist. 1-Desamino-8-D-arginine-vasopressin, a selective anti-diuretic (V₂) agonist failed to induce aggregation and acted as a weak antagonist. Vasopressin analogues which lacked the N-terminal amino group or which contained an uncharged amino acid residue at position 8 acted as partial agonists for the human platelet. The response to such partial agonists could be enhanced by increasing the cytosolic Ca²⁺ concentration but not by altering the level of cyclic-3′,5′-AMP. These observations provide further evidence indicating that the platelet vasopressin receptor is of the V₁ sub-type.     

Platelet aggregation studies in whole human blood

Since the development of the photometric aggregometer, platelet aggregation studies are routinely performed on platelet-rich plasma (PRP). We have studied platelet aggregation in fresh citrated whole human blood using the recently developed Ultra Flo 100 Whole Blood Platelet Counter. Aggregation of platelets in whole blood was induced with adenosine 5′-diphosphate (ADP; 0.25–10μM), collagen (0.25–1.0μg/ml) and thrombin (0.05–0.2U/ml). Platelet aggregation induced by ADP and thrombin was maximum within 1 min, and that of collagen within 3 mins. Aggregation responses to low concentration of ADP and thrombin were rapidly reversible, whereas responses to collagen and high concentrations of ADP and thrombin were virtually irreversible. The aggregation of platelets was indicated by a fall in platelet count; confirmed by scanning electron micrographs which revealed the presence of large aggregates of platelets, and was prevented when blood was treated with EDTA as anticoagulant. The present technique appears to be rapid, sensitive and reliable; and allows direct measurement of platelet aggregation and disaggregation in whole blood in vitro and ex-vivo.     

Enhanced platelet aggregation with TRAP-6 and collagen in platelet aggregometry in patients with venous thromboembolism

The role of platelet hyperaggregability as a possible risk factor for venous thromboembolism is not well defined. Some authors described enhanced maximal platelet aggregation in platelet aggregometry as a contributing factor for arterial and venous thrombosis. This syndrome has been termed “sticky-platelet syndrome” (SPS). The diagnosis of SPS is based on the demonstration of platelet hyperaggregability in aggregometry after stimulation with epinephrine (EPI) and/or adenosine diphosphate (ADP).

We investigated platelet hyperaggregability in platelet-rich plasma (PRP) of patients (n=34) with unexplained venous thromboembolism in comparison to healthy individuals (n=53). For analysis, platelet aggregometry was performed and the influence of epinephrine, adenosine diphosphate, collagen (Coll) and thrombin receptor-activated peptide (TRAP-6) as agonist were determined. Compared to the control group, patients with venous thromboembolism showed an enhanced maximal platelet aggregation with low concentrations of TRAP-6 (2 μM) and collagen (0.05 μM). In contrast, we could not detect an increased platelet aggregation with EPI or ADP.

Our results indicate that platelet hyperaggregability may represent an independent risk factor in patients with otherwise unexplained venous thromboembolism. In our study, low concentrations of TRAP-6 and collagen are superior to EPI and ADP to define platelet hyperreactivity in platelet aggregometry.