Blockade of platelet alpha2B-adrenergic receptors in patients with coronary artery disease: a novel antiaggregant mechanism

Abstract

Purpose: Platelets play a vital role in hemostasis and thrombosis. Catecholamines have a profound effect on platelet aggregation and atherothrombosis but the exact mechanism involved is insufficiently understood. In this report, we demonstrate the existence and role of alpha2B–adrenergic receptors (α2B–ARs) in platelets from patients with Coronary Artery Disease (CAD) compared to normal individuals. Methods: Sixteen healthy individuals and 7 patients with CAD who were under dual antiplatelet therapy with clopidogrel 75mg and acetylsalicylic acid 100mg were included. Blood samples were obtained from which platelets were isolated. The presence of α2B-ARs in platelets was proven by Western blot analysis and real time PCR. In order to investigate their function, we performed light transmittance aggregometry by examining the inhibitory effects of specific α2B-AR antibodies. Results: Pretreatment of human platelets with agents that selectively block α2B-ARs showed a substantial inhibition in platelet aggregation that had been induced by adenosine diphosphate (ADP), by epinephrine and by arachidonic acid. The percent aggregation decreased from 81.5±1.7% to 35.8±5% for ADP with α2B-Abs, from 72.2±1.9% to 25.5±4.3% for epinephrine with α2B-Abs, and from 87±2.1% to 47.9±6.2% for arachidonic acid with α2B-Abs, p<0.05 for all. Patients with CAD showed a significantly reduced expression of the receptor in the platelets (149.6±100.8 in normals versus 32.1±17.7, p = 0.015). Notably, pretreatment with α2B –Abs resulted in a significant further reduction of platelet aggregation. The ADP-treated plasma showed a significant reduction in platelet aggregation after pre-treatment with α2B-Abs (42±5.8% for the control group versus 31.7±2.2% for the group with α2B-Abs pretreatment, p< 0.001). When arachidonic acid was used to induce aggregation, pre-treatment with α2B -Abs reduced platelet aggregation from 41.2±7.3% to 28.2±7.5%, p= 0.006). Similarly, the presence of α2B-Abs decreased platelet aggregation induced by epinephrine from 44.4±22.1% to 19.1±7.2% (p = 0.01). Conclusions: Our results reveal that contrary to previous knowledge, the α2B–AR subtype does exist in platelets and is an important regulator of aggregation. Inhibition of α2B-ARs in platelets may offer a novel therapeutic opportunity in the prevention of atherothrombotic events. Interestingly, even in patients with coronary artery disease who were receiving dual antiplatelet therapy with aspirin and clopidogrel, the inhibition of α2B-ARs had an additional antiaggregant effect.