Abstract
Platelet hyperreactivity constitutes an important thrombotic risk factor; however, standardized methods for its measurement are lacking. We recently reported that aggregometry using a submaximal concentration of epinephrine (0.4 μM in citrated platelet-rich plasma) identifies individuals with in vitro platelet hyperreactivity; this hyperreactivity was reproducible on multiple occasions over long periods of time (up to 3 years). To better understand this aberrant reactivity, we studied in a larger group of subjects (n=404) the association between healthy individuals’ platelet reactivity to epinephrine and their platelet phenotype as measured by other functional and biochemical assays. Fourteen percent (n=56) of our study cohort showed a hyperreactive response (> 60% aggregation) to 0.4 μM epinephrine; the remainder showed a minimal response (see figure). Subjects with hyperreactivity to epinephrine were more likely to exhibit hyperaggregability to other agonists (ADP, arachidonic acid, collagen, collagen-related peptide and ristocetin; p<.04), increased spontaneous aggregation (p<.001), shorter PFA-100 closure times with both epinephrine (p<.001) and ADP (p<.02) cartridges and increased surface expression of P-selectin after exposure to ADP (p<.02) and to shear stress (p<.01). Subjects exhibiting platelet hyperreactivity in citrated specimens also did so in specimens anticoagulated with the thrombin inhibitor Phe-Pro-Arg-chloromethyl ketone, suggesting that the hyperreactive phenotype is independent of calcium concentration and residual plasma thrombin activity. Taken together, these data obtained using multiple assays and methods of platelet stimulation are most consistent with the existence of a hyperreactive phenotype that is not agonist specific, but is rather a "global" property of the platelet. Platelet hyperreactivity was not associated with age, race, body mass index, smoking status or presence of hypertension, but was independently associated with female gender (p=.02) and with higher fibrinogen levels (p=.002). To explore potential mechanisms responsible for platelet hyperreactivity to epinephrine, we measured expression of key surface membrane glycoproteins (GPs) and genotyped polymorphisms located on candidate genes relevant to epinephrine-mediated platelet activation. We found that hyperreactivity was strongly associated with increased expression of both quiescent and activated forms of the fibrinogen receptor GP IIb-IIIa (p<.005) and with the T allele of the C825T polymorphism on the gene (GNB3) encoding the beta-3 subunit of G proteins (p<.03), but not with the G1838A polymorphism on the gene encoding the α2A-adrenergic receptor. Aggregometry using submaximal concentrations of epinephrine thus identifies a global hyperreactive platelet phenotype and may be useful in settings where platelet hyperreactivity bears special relevance - for example, in populations of patients at risk for thrombosis. Our findings also suggest that the physiologic determinants of platelet hyperreactivity in response to different types of stimulation may share common signaling pathways, possibly involving G protein-coupled receptors and increased GP IIb-IIIa expression.
Variation in Platelet Reactivity (404 healthy subjects) Variation in Platelet Reactivity (404 healthy subjects)
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