Abstract-Reactive oxygen species play an important role at the site of vascular injuries and arterial thromboses. We studied the mechanism mediating platelet aggregation induced by H 2 O 2 , a major cellular oxidant. Exposure to H 2 O 2 triggered platelet aggregation, but only when the platelets were stirred. Strong platelet aggregation induced by H 2 O 2 required the presence of the tyrosine phosphatase inhibitor sodium orthovanadate (NaVO 4 ) and was dependent on the participation of integrin ␣ IIb ␤ 3 (glycoprotein IIb-IIIa 2 Leukocytes and platelets have been found adjacent during acute coronary syndromes and even circulate as clusters in the blood of patients with unstable angina.3-5 Leukocytes and, in particular, neutrophils produce large amounts of reactive oxygen species (ROS), which could influence platelet function in a paracrine fashion. 6 Moreover, superoxide and hydroxyl radicals have been shown to be produced by anoxic and subsequently reoxygenated platelets.7 Previous studies on the effects of ROS and, in particular, hydrogen peroxide (H 2 O 2 ) on platelets indicated that ROS might exert a dual effect on platelets. In the presence of nitric oxide (NO), H 2 O 2 seems to potentiate the inhibitory effect of NO on agonist-mediated platelet activation. 8 In contrast, in the presence of arachidonate, H 2 O 2 enhances aggregation induced by phospholipid-derived arachidonate.9,10 To further characterize the effect of H 2 O 2 on platelet reactivity, we sought to characterize the effect of ROS and, in particular, of H 2 O 2 on platelet signaling pathways.Integrin ␣ IIb ␤ 3 , the classic platelet fibrinogen receptor, can exist in resting or activated states, and the latter can occur with or without ligand engagement. Activation corresponds to a structural change, whereby the affinity of ␣ IIb ␤ 3 for fibrinogen and other adhesive molecules in solution is markedly increased.11 Moreover, integrin ␣ IIb ␤ 3 can be found in 2 different compartments relative to extracellular ligands: exposed (to extracellular ligands) or cryptic (not accessible to extracellular ligands).12 Receptor molecules can be recruited from cryptic storage to the platelet surface or removed from the surface through a process of endocytosis.12-14 The molecular reactions that lead to receptor activation have been studied: ␤ 3 seems to play the role of a regulatory subunit of the receptor, whereas ␣ IIb appears to mediate receptor specificity. 15,16 There is good evidence that the activation process utilizes ATP 15 and involves interaction of the short cytoplasmic domain of ␤ 3 with a regulatory factor. 15,17,18 H 2 O 2 can induce a strong tyrosine kinase response in smooth muscle cells, especially in cells whose tyrosine phosphatases are concurrently inhibited with orthovanadate.

Background.—The PlA2 polymorphism of GPIIIa has been associated with unstable coronary syndromes in some studies, but the association has remained debated. None of the previous studies have focused on families at high risk. Risk factors tend to cluster within kindreds with high prevalence of premature coronary heart disease (CHD). Therefore, a heightened prevalence of the PlA2 polymorphism among siblings of patients with CHD would support the hypothesis that PlA2 is linked, directly or indirectly, to CHD. Objectives.—To measure the prevalence of the PlA2 polymorphism among siblings of patients with CHD before the age of 60 years and to seek an association between the PlA2 polymorphism and established atherosclerotic and thrombogenic risk factors. Methods.—From January 1994 to April 1996, we genotyped 116 asymptomatic siblings (60 Caucasians, 56 Afro-Caribbeans) of patients with CHD manifestations before the age of 60 years for the PlA polymorphism (also called HPA-1). A control cohort was used for comparison, consisting of individuals that were matched for race and geographic area but were free of CHD (n = 268, 168 Caucasians and 100 Afro-Caribbeans). In addition, we have characterized the sibling cohort for other atherogenic and thrombogenic risk factors. Results.—The prevalence of PlA2-positive individuals (PlA2[+], PlA1/A2 heterozygotes plus PlA2/A2 homozygotes) in the sibling cohort was high: 41.4%. When analyzed separately, the prevalence of PlA2(+) siblings was 53.3% among Caucasians and 28.6% among Afro-Caribbeans. There was no association between PlA2 and other established atherogenic or thrombogenic risk factors. Interestingly, the clustering of other risk factors was lesser among PlA2(+) siblings than their PlA1 counterparts. Conclusions.—This study supports the hypothesis that the prevalence of PlA2(+) individuals is high in kindreds with premature CHD. Hence, like the established risk factors that tend to cluster in families with premature CHD and contribute strongly to the accelerated atherosclerotic process affecting these individuals, the PlA2 polymorphism of GPIIIa may represent an inherited risk that promotes the thromboembolic complications of CHD. That these asymptomatic PlA2(+) siblings had overall less established risk factors than their PlA1 counterparts might represent an explanation for why they remained asymptomatic despite their PlA2 positivity.