Role of the 807 C/T polymorphism of the alpha2 gene in platelet GP Ia collagen receptor expression and function--effect in thromboembolic diseases.

The variability of the platelet GP Ia/IIa density has been associated with the 807 C/T polymorphism (Phe 224) of the GP Ia gene in American Caucasian population. We have investigated the genotype and allelic frequencies of this polymorphism in Spanish Caucasians. The T allele was found in 35% of the 284 blood donors analyzed. We confirmed in 159 healthy subjects a significant association between the 807 C/T polymorphism and the platelet GP Ia density. The T allele correlated with high number of GP Ia molecules on platelet surface. In addition, we observed a similar association of this polymorphism with the expression of this protein in other blood cell types. The platelet responsiveness to collagen was determined by "in vitro" analysis of the platelet activation and aggregation response. We found no significant differences in these functional platelet parameters according to the 807 C/T genotype. Finally, results from 3 case/control studies involving 302 consecutive patients (101 with coronary heart disease, 104 with cerebrovascular disease and 97 with deep venous thrombosis) determined that the 807 C/T polymorphism of the GP Ia gene does not represent a risk factor for arterial or venous thrombosis.     

Different role of platelet glycoprotein GP Ia/IIa in platelet contact and activation induced by type I and type III collagens

The role of glycoprotein Ia/IIa was studied during platelet contact and aggregation induced by type I and type III collagen. The anti-glycoprotein Ia/IIa (6F1) antibody inhibited type I collagen-induced aggregation but did not inhibit the first contact between platelets and collagen. In contrast, it was without effect either on type III collagen-induced contact or platelet interaction with the subendothelium in a static assay. Platelet aggregation induced by type III collagen was only slightly slowed down by 6F1 but pp72 spleen tyrosine kinase phosphorylation was not modified even at concentrations of 6F1 that completely blocked platelet activation induced by type I collagen. Our results indicate that glycoprotein Ia/IIa is not a primary binding site for type I or type III collagen on the platelet membrane. This receptor is more specifically involved in type I collagen-induced platelet spreading and aggregation.     

Glycoprotein Ia/IIa-mediated activation-dependent platelet adhesion to collagen.

In the presence of anti-glycoprotein (GP) IIb/IIIa antibody at the concentration which completely inhibit platelet aggregation, ADP (0.5-3 microM) increased platelet adhesion to collagen in a concentration-dependent manner under static conditions when platelet-rich plasma (PRP) was used for the assay instead of washed platelets. This was also supported by the results of scanning electron microscopic analyses. The ADP-induced platelet adhesion to collagen was inhibited by PGI2 or beraprost, a stable analogue of PGI2, in a concentration-dependent manner (1-10 ng/ml). These findings suggested the presence of activation-dependent platelet adhesion to collagen. ADP-induced platelet adhesion to collagen was almost completely inhibited by anti-GPIa/IIa and anti-GPIIa antibodies. In the present study, we provide the first direct evidence that the activation-dependent platelet adhesion to collagen is induced by ADP and that GPIa/IIa also plays an important role in the mechanisms of this adhesion.     

807C/T polymorphism of platelet glycoprotein Ia gene is associated with cerebral hemorrhage in a Chinese population

Platelet glycoprotein (GP) mediated the role of platelet in coagulation. Platelet GP Ia 807C/T is the only GP polymorphism associated with the expression levels of GP Ia/IIa (the platelet collagen receptor). Recently, the GP Ia 807C/T polymorphism has been reported to have no association with cerebral hemorrhage (CH) in two studies pertained to Caucasian populations. The purpose of this study is to evaluate the association between platelet GP Ia 807C/T polymorphism and CH in a Han Chinese population. We performed genotype analysis for platelet GP Ia 807C/T polymorphism in a case-control study involving 195 patients with CH and 116 age- and sex-matched controls. In contrast to previous reports, we found that the frequencies of GP Ia 807C/T T allele, CT and TT genotype were much higher in CH patients than in controls (33.9% vs. 22.8%, p = 0.004; 45.5% and 11.1% vs. 40.4% and 2.6%, p = 0.022). Logistic regression analysis revealed that the presence of GP Ia 807C/T C allele and CC genotype were both associated with a decreased risk of CH compared with T allele, CT and TT genotypes, respectively (adjusted odds ratio [OR] = 0.565, 95% CI: 0.384–0.887, p = 0.005; adjusted OR = 0.172, 95% CI: 0.043–0.639, p = 0.009; adjusted OR = 0.254, 95% CI: 0.085–0.961, p = 0.041, respectively). These findings indicated that platelet GP Ia 807C/T polymorphism could be a protective factor of CH in the Chinese population.     

The effects of arm cranking exercise and training on platelet aggregation in male spinal cord individuals

Platelet aggregation at rest and in responses to exercise and training were compared between spinal cord injured (SCI) individuals (N=5) and able-bodied subjects (N=7). All participants performed arm cranking exercise at 60-65% VO(2peak) for 30 min. Venous blood samples were obtained before and after sub-maximal exercise and measured for platelet aggregation using ADP and collagen. To assess the effects of arm cranking training, platelet aggregation was re-measured in all subjects at rest and in response to the sub-maximal arm cranking exercise after 12 weeks of individually supervised training programme. Before training, the resting mean values of platelet aggregation induced by ADP and collagen were not different (P>0.05) between SCI and able-bodied. However the SCI individuals, but not the able-bodied subjects, exhibited a significantly (P<0.05) higher maximal platelet aggregation induced by ADP and collagen following sub-maximal arm cranking exercise. Although VO(2peak) after training was significantly increased (P<0.05) in both groups, the resting mean values of platelet aggregation induced with ADP and collagen were not significantly different (P>0.05) from those observed before training and were not different (P>0.05) between SCI and able-bodied. Post-training, the SCI individuals, but not able-bodied individuals, exhibited a significant decrease (P<0.05) in platelet aggregation following sub-maximal arm cranking exercise and this occurred with both ADP and collagen. These results suggest that SCI individuals, but not normal subjects increase their platelet aggregation following sub-maximal arm cranking exercise. Furthermore, arm cranking training in SCI individuals, appears to diminish the percentage of platelet aggregation ex vivo.     

A monoclonal antibody to the human platelet glycoprotein IIb/IIIa complex induces platelet activation

The platelet glycoprotein (GP) IIb/IIIa complex functions as the receptor for fibrinogen on activated platelets. The effects of two anti-GPIIb/IIIa monoclonal antibodies on platelet function were studied. These antibodies, 6C9 and C17, recognized different epitopes, which were exclusively present on the undissociated GPIIb/IIIa complex. Whereas C17 inhibited the binding of fibrinogen to platelets and platelet aggregation induced by adenosine diphosphate (ADP) or collagen, 6C9 caused irreversible aggregation of platelets, both in the presence and absence of extracellular fibrinogen. When incubated with unstirred (non-aggregating) platelets, 6C9 induced release of alpha and dense granule-constituents as well as binding of 125I-fibrinogen to platelets. The latter was evidently mediated in part by platelet-derived ADP, since it was inhibited to a large extent by apyrase, the ADP-hydrolyzing enzyme. F(ab')2 fragments of 6C9 did not induce platelet-release reactions but caused (slow) aggregation of platelets in the presence of extracellular fibrinogen. These results indicate that binding of an antibody to a specific site on the platelet GPIIb/IIIa complex may cause fibrinogen-mediated aggregation. The Fc part of the platelet-bound antibody appears to be involved in the induction of platelet release.     

Differential inhibitory effects of platelet glycoprotein IIb/IIIa antagonists on aggregation induced by procoagulant agonists

INTRODUCTION: In addition to mediating the final common pathway of aggregation, the glycoprotein (GP) IIb/IIIa receptor participates in the activation of coagulation on the platelet surface. High-affinity conformation of GP IIb/IIIa in response to collagen-induced inside-out signalling seems to be mediated by GP VI(-FcRgamma) and reinforced by release of soluble mediators. METHODS: We assessed the effects of the three currently available GP IIb/IIIa antagonists--abciximab, tirofiban and eptifibatide--on platelet aggregation induced by various procoagulant and GP VI-related agonists, i.e. collagen-related peptide (CRP), convulxin and collagen fibrils, in PPACK-anticoagulated platelet-rich plasma. RESULTS: At concentrations that equally inhibited 80% of ADP-induced maximal aggregation abciximab-inhibited GP VI-mediated platelet responses to CRP or convulxin significantly more than the low-molecular-weight antagonists (CRP: abciximab 75+/-18%, tirofiban 41+/-7% and eptifibatide 41+/-6%; convulxin: abciximab 90+/-6%, tirofiban 64+/-20%, eptifibatide 61+/-14%, p<0.01 for all). In contrast, aggregation induced by collagen was equally abolished with all antagonists under the similar conditions. During CRP- or convulxin-triggered platelet activation, inhibition of fibrin polymerisation with GPRP potentiated the antiaggregatory effects of tirofiban and eptifibatide to reach that of abciximab. GPRP as such did not affect platelet aggregation. CONCLUSIONS: GP IIb/IIIa antagonists exhibit distinct inhibition profiles in platelet aggregation, depending on fibrin polymerization and calcium. Specifically, the ability of procoagulant platelet agonists to expose pre-activated and ligand-bound GP IIb/IIIa from the internal pool seems important.     

Influence of hypercapnia and hypoxia on rabbit platelet aggregation

The influence of changes in pCO₂, pH and pO₂ on the aggregation of rabbit blood platelets was studied , with emphasis on hypercapnia, acidocis and hypoxia. Hypercapnia combined with acidosis caused a reduction in rabbit platelet aggregation, as induced by collagen, thrombin and ADP; the effect being most pronounced with collagen and smallest with ADP. Hypoxia reduced thrombin induced platelet aggregation, but had no effect on ADP and collagen induced aggregation. Synergistic activation of rabbit platelets, as induced by the addition of serotonin to platelet rich plasma together with collagen or ADP, seemed to be equally sensitive to changes in pCO₂ and pH as activation by the individual agents, and insensitive to changes in pO₂.     

Exercise-induced changes in platelet aggregation; a comparison of whole blood and platelet rich plasma techniques

Studies have been performed to assess the effect of exercise on spontaneous platelet aggregation in shaken whole blood, and on agonist-induced platelet aggregation in whole blood and platelet rich plasma (PRP). Spontaneous platelet aggregation in shaken whole blood was increased following exercise compared to pre-exercise values. The increase in spontaneous aggregation after exercise correlated inversely with the increase in white cell count in whole blood. Platelet sensitivity in whole blood to adrenaline, collagen and adenosine diphosphate (ADP) was increased following exercise. Changes in platelet sensitivity to adrenaline following exercise correlated with increases in plasma noradrenaline levels but not with changes in blood cell counts. In PRP, platelet sensitivity to ADP and to collagen was increased following exercise when the pre and post-exercise PRP platelet counts were not corrected to allow for the increase in platelet count which occurred with exercise. When the PRP platelet counts were corrected, no changes in platelet sensitivity to any agonist after exercise were observed.     

Dipyridamole inhibits platelet aggregation in whole blood

Dipyridamole possesses antithrombotic properties in the animal and in man but it does not inhibit platelet aggregation in plasma. We evaluated the effect of dipyridamole ex vivo and in vitro on platelet aggregation induced by collagen and adenosine-5'-diphosphate (ADP) in human whole blood with an impedance aggregometer. Two hundred mg dipyridamole induced a significant inhibition of both ADP- and collagen-induced aggregation in human blood samples taken 2 hr after oral drug intake. Administration of the drug for four days, 400 mg/day, further increased the antiplatelet effect. A significant negative correlation was found between collagen-induced platelet aggregation in whole blood and dipyridamole levels in plasma (p less than 0.001). A statistically significant inhibition of both collagen (p less than 0.0025) and ADP-induced (p less than 0.005) platelet aggregation was also obtained by incubating whole blood in vitro for 2 min at 37 degrees C with dipyridamole (3.9 microM). No such effects were seen in platelet-rich plasma, even after enrichment with leukocytes. Low-dose adenosine enhanced in vitro inhibition in whole blood. Our results demonstrate that dipyridamole impedes platelet aggregation in whole blood by an interaction with red blood cells, probably involving adenosine.     

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.     

Asialo von Willebrand factor enhances platelet adhesion to vessel subendothelium

Native von Willebrand factor (N-vWF) binds to platelets activated by thrombin, ADP or ristocetin. Asialo vWF (As-vWF) induces platelet aggregation in absence of platelet activators. N-vWF mediates platelet adhesion to vessel subendothelium at high shear rates. We have investigated the role of As-vWF in supporting platelet deposition to rabbit vessel subendothelium at a shear rate of 2,000 sec-1, using the Baumgartner perfusion system. We have studied the effects of the addition of As-vWF (from 2 to 12 micrograms/ml) to perfusates consisting of washed red blood cells, 4% human albumin and washed platelets. Our results show a significant increase in platelet deposition on subendothelium (p less than 0.01) in perfusions to which As-vWF had been added. Blockage of the platelet glycoproteins Ib and IIb/IIIa (GPIb and GPIIb/IIIa) by specific monoclonal antibodies (LJIb1 and LJCP8, respectively) resulted in a decrease of platelet deposition in both types of perfusates prepared with N-vWF and As-vWF. Our results indicate that As-vWF enhances platelet deposition to vessel subendothelium under flow conditions. Furthermore, they suggest that this effect is mediated by the binding of As-vWF to platelet membrane receptors, which in turn, promote platelet spreading and adhesion to the subendothelium.     

Glycosaminoglycan inhibition of collagen induced platelet aggregation

Chondroitin 6-sulfate (Ch6-S), a glycosaminoglycan (GAG), has been shown to inhibit collagen fibrillogenesis and collagen induced platelet aggregation. Complexation of soluble microfibrillar collagen with Ch6-S at low pH followed by saline dialysis results in the stabilization of 300Å wide microfibrillar aggregates with no banding in the electron microscope. These structures, which may be intermediates in collagen fibrillogenesis, do not aggregate platelets or cause serotonin release, whereas fibrils formed from uncomplexed microfibrillar collagen induce platelet aggregation and serotonin release. Neutral complexation of microfibrillar collagen with Ch6-S does not inhibit fibril formation or platelet aggregation. Soluble Ch6-S does not interfere with platelet aggregation in response to soluble microfibrillar collagen, indicating that Ch6-S does not block sites on the platelet membrane or on collagen fibrils which may be specifically involved in the collagen-platelet interaction. These results imply that GAG complexes with collagen may be suitable as blood compatible materials.     

The influence of glutathione and other thiols on human platelet aggregation

The platelet membrane contains sulfhydryl groups which are essential for normal platelet function. Reduced glutathione (GSH) and other thiols such as cysteine and 6-mercaptopurine were found to inhibit human platelet aggregation induced by adenosine diphosphate (ADP), collagen and arachidonic acid. The inhibition of ADP-induced aggregation by GSH (IC₅₀=0.61 ± 0.05 mM) was greater than that by cysteine (IC₅₀=13 ± 1 mM) or 6-mercaptopurine (IC₅₀=5.4 ± 0.2 mM). Two other thiols, dithiothreitol and beta-mercaptoethanol were found to cause platelet aggregation instead of inhibition. The interaction of GSH with the ADP receptor was noncompetitive in nature.     

Inhibition of platelet aggregation in whole blood by adrenoceptor antagonists

It has recently become possible to study platelet aggregation in whole blood which may more closely resemble the in-vivo situation as the platelets are left in their natural milieu with red and white cells present which themselves can influence aggregation. The effects of 4 adrenoceptor antagonists on platelet aggregation in whole blood were studied in-vitro using the Clay-Adams Ultra Flo 100 whole blood platelet counter. Labetalol, pindolol and propranolol inhibited aggregation to 0.5 microgram/ml collagen in a dose dependent manner, and were synergistic with prostacyclin in inhibiting collagen induced aggregation. These 3 drugs also promoted reversal of aggregation induced by 10 microM ADP, but only inhibited 0.5 mM arachidonic acid induced aggregation at high drug concentrations. Atenolol had no effect on either collagen, ADP or arachidonic acid induced aggregation. The anti-platelet effect of these drugs may be of value in the treatment of vascular disease.     

The parasitic hematophagous worm Haemonchus contortus inhibits human platelet aggregation and adhesion: partial purification of a platelet inhibitor

Blood sucking parasites elaborate mechanisms to counteract the hemostatic system of their victim. Haemonchus contortus worms use several mechanisms directed against the normal platelet hemostatic function. Platelet adhesion onto collagen and fibrinogen, and the ristocetin-mediated interaction of von Willebrand Factor with glycoprotein (GP) Ib were inhibited by the protein extract of adult worms. Also platelet aggregation induced by collagen, thrombin, ADP, ristocetin or A23187 was inhibited. Although we obtained evidence for interference with fibrinogen binding to GPIIb/IIIa, the strongest inhibition was seen when the agonists collagen or thrombin were used. A small multisubunit inhibitor of collagen-induced platelet aggregation was partially purified using anion exchange chromatography, gel filtration and RP-HPLC. The inhibitor has a pI between 4 and 6.5, elutes with a molecular weight of 23,800 Da after gel filtration, and is part of the elaborate broad-spectrum antiplatelet activity that results in the potent synergistic anti-hemostatic cocktail produced by H. contortus.     

Increased platelet sensitivity among individuals with aspirin resistance - platelet aggregation to submaximal concentration of arachidonic acid predicts response to antiplatelet therapy

Aspirin 'resistance' (AR) is a phenomenon of uncertain etiology describing decreased platelet inhibition by aspirin. We studied whether (i) platelets in AR demonstrate increased basal sensitivity to a lower degree of stimulation and (ii) platelet aggregation with submaximal stimulation could predict responses to aspirin. Serum thromboxane B(2) (TxB(2)) levels and platelet aggregation with light transmission aggregometry (LTA) were measured at baseline and 24 hours after 325 mg aspirin administration in 58 healthy subjects. AR was defined as the upper sixth of LTA (> or = 12%) to 1.5 mM AA. Baseline platelet aggregation with submaximal concentrations of agonists [ADP 2 microM, arachidonic acid (AA) 0.75 mM, collagen 0.375 and 0.5 microg/ml] was greater in AR subjects compared with non-AR subjects, but not with higher concentrations (ADP 5 microM and 20 microM, AA 1.5 mM and collagen 1 microg/ml). Post-aspirin platelet aggregation was elevated in AR subjects with both submaximal and maximal stimulation. Baseline and post-aspirin serum TxB(2) were higher in AR subjects and decreased further with ex-vivo COX-1 inhibition, suggesting incompletely suppressed COX-1 activity. Pre-aspirin platelet aggregation to 0.75 AA demonstrated a dichotomous response with 29/58 subjects having aggregation < or = 15% and 29/58 subjects having aggregation > or = 75%. In the high aggregation group 28% had AR compared to 6% in the non-AR group (p = 0.04). In conclusion, platelets in AR subjects demonstrate increased basal sensitivity to submaximal stimulation, which could predict responses to antiplatelet therapy.     

Comparison of turbidimetric aggregation and in vitro bleeding time (PFA-100) for monitoring the platelet inhibitory profile of antiplatelet agents in patients undergoing stent implantation

The present study compared classical ADP-induced platelet aggregation vs. PFA-100 closure times using collagen/ADP membrane cartridges to monitor the degree of platelet-inhibiting effect of three drug regimens: ticlopidin, abciximab/ticlopidin and loading dose clopidogrel, each on top of aspirin (acetylsalicylic acid, ASA) during and after elective stent placement (intervention) in a total of 31 patients with acute coronary syndrome. Ticlopidin was started directly after stent implantation, abciximab was started before coronary intervention and given intravenously for 12 h, and a clopidogrel loading dose was given before intervention. The 10 patients treated with ticlopidin (500 mg daily) showed no significant prolongation of PFA closure times and a slight increase of ADP-induced platelet aggregation shortly after intervention. In 11 patients treated with abciximab/ticlopidin, the PFA closure times were significantly prolonged, and ADP-induced platelet aggregation was reduced by more than 80% during the 12-h abciximab infusion after intervention. The 10 patients pretreated with loading dose clopidogrel (450 mg followed by 75 mg daily) showed an intermediate but significant prolongation of PFA closure times and reduction of ADP-induced platelet aggregation at levels between the ticlopidin/aspirin- and the abciximab/ticlopidin/aspirin-treated groups. At 20 h after intervention, a similar degree of PFA closure time prolongation and inhibition of ADP-induced aggregation was observed in the abciximab/ticlopidin/aspirin- and the clopidogrel/aspirin-treated patient groups. Both measurement of PFA-100 closure times and inhibition of ADP-induced platelet aggregation showed a similar degree of platelet inhibition, but had rather broad SD ranges, which limit their precision for the follow-up of individual patients. In conclusion, abciximab on top of ticlopidin/aspirin showed a stronger antiplatelet effect for only less than 20 h, as compared to loading dose clopidogrel/aspirin in acute coronary syndrome patients undergoing stent implantation. Whether such a short-term superiority of abciximab, as compared to loading dose clopidogrel, translates into an overall clinical benefit of thombotic and bleeding complications remains to be established in a randomized clinical trial.     

Insulin enhances platelet activation in vitro.

Diabetes mellitus is associated with an increased risk of atherothrombotic complications. There is accumulating evidence of platelet hyperreactivity in diabetes, which may be of importance in the pathogenesis of diabetic vascular complications. Platelets possess insulin receptors, but their physiological relevance is not clear, and data on insulin effects on platelet function in the literature are less than consistent. We therefore investigated the influence of insulin on platelet activation in vitro. Fasting blood samples were collected in 20 healthy males, using citrate or hirudin as anticoagulants. Platelet activation was measured as platelet P-selectin expression and fibrinogen binding using whole blood flow cytometry in unstimulated and adenosine diphosphate (ADP) stimulated samples, incubated with 0-10000 microU/ml insulin for 20 min. The effect of insulin (30 or 300 microU/ml, incubated for 3 min) on platelet aggregation was studied using Born aggregometry in platelet-rich plasma (PRP). Insulin enhanced platelet fibrinogen binding more than P-selectin expression in unstimulated and ADP stimulated samples (P<.001 by analysis of variance [ANOVA]; n=20). Insulin (30 or 300 microU/ml) also enhanced ADP-induced platelet aggregation in PRP (P<.01 or P<.001; n=14). The platelet activating effect of insulin was verified in hirudinized samples (n=12), indicating that it was not dependent on unphysiologically low extracellular calcium concentrations. Thus, insulin enhances platelet activation in vitro, independently of extracellular calcium concentrations. Beneficial effects of insulin treatment on platelet function in vivo are probably related to improved metabolic control, rather than to direct platelet stabilizing effects.     

Superoxide dismutase cooperates with prostacyclin to inhibit platelet aggregation: a comparative study in washed platelets and platelet rich plasma.

The role of superoxide anions (O2-) in human platelet aggregation in Krebs' buffer or plasma was investigated. In indomethacin (10 microM)-treated washed platelets superoxide dismutase (SOD; 60 U/ml) or ferricytochrome c (FCC; 70 microM) inhibited platelet aggregation by thrombin but not that by collagen or ADP. In addition, in indomethacin (10 microM)-treated washed platelets, SOD significantly potentiated the anti-aggregatory activity of prostacyclin (PGI2) or iloprost when thrombin but not collagen was used as the aggregating agent. In platelet rich plasma, SOD (60 U/ml) did not inhibit platelet aggregation nor did it potentiate the anti-aggregatory activity of iloprost when ADP, collagen or thrombin were used as aggregating agents. Thus, O2- participate in the aggregatory activity of thrombin but not collagen or ADP and PGI2 or iloprost, by reducing the sensitivity of platelets to thrombin, co-operate with SOD to inhibit thrombin-induced platelet aggregation. The interpretation of the use of SOD in experiments involving endothelium-derived relaxing factor (NO) is discussed.