MR 6,400 AURIN TRICARBOXYLIC-ACID DIRECTLY ACTIVATES PLATELETS

ATA is a novel anticoagulant polymeric anionic aromatic compound that inhibits von Willebrand factor binding to platelet glycoprotein Ib and thereby prevents ristocetin- and shear stress-induced platelet aggreg ation. To investigate its mechanism of action, ATA fractions of homoge neous M(r) have been prepared by size exclusion chromatography. ATA fr actions of M(r) greater-than-or-equal-to 2,500 are most effective at i nhibiting vWF-mediated platelet aggregation, and ATA of M(r) = 2,500 a lso inhibits thrombin-induced platelet activation. Paradoxical results were observed in studies of ATA with M(r) = 6,400. This fraction of A TA stimulates aggregation of washed platelets or platelet-rich-plasma. The dose/response of aggregation shows a bell-shaped curve with maxim al aggregation at approximately 2 mug/ml. Platelet aggregation is asso ciated with phosphoinositide turnover and protein kinase C- and calciu m-dependent protein phosphorylation. Platelet signalling responses to ATA are inhibited by platelet pretreatment with PGI2 or dibutyryl-cycl ic 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 beneficia l anti-thrombotic effect on vWF-mediated platelet responses, and shoul d be considered when analyzing results of experiments that utilize unf ractionated ATA.