M_r 6,400 aurin tricarboxylic acid directly activates platelets

ATA is a novel anticoagulant polymetic anionic aromatic compound that inhibits von Willebrand factor binding to platelet glycoprotein Ib and thereby prevents ristocetin- and shear stress-induced platelet aggregation. To investigate its mechanism of action, ATA fractions of homogeneous M_r have been prepared by size exclusion chromatography. ATA fractions of M_r ≥ 2,500 are most effective at inhibiting vWF-mediated platelet aggregation, and ATA of M_r = 2,500 also inhibits thrombin-induced platelet activation. Paradoxical results were observed in studies of ATA with M_r = 6,400. This fraction of ATA stimulates aggregation of washed platelets or platelet-rich plasma. The dose/response of aggregation shows a bell-shaped curve with maximal aggregation at approximately 2 μg/ml. Platelet aggregation is associated with phosphoinositide turnover and protein kinase C- and calcium-dependent protein phosphorylation. Platelet signalling responses to ATA are inhibited by platelet pretreatment with PGI₂ or dibutyryl-cyclic AMP, but are unaffected by inhibiting platelet cyclooxygenase with aspirin. These results suggest that M_r 6,400 ATA directly activates platelet phospholipase C to initiate platelet aggregation. This effect, unique to M_r 6,400 ATA, could potentially mitigate ATA's beneficial anti-thrombotic effect on vWF-mediated platelet responses, and should be considered when analyzing results of experiments that utilize unfractionated ATA.