THAPSIGARGIN AMPLIFIES THE PLATELET PROCOAGULANT RESPONSE CAUSED BY THROMBIN

The platelet procoagulant response involves an increase in surface-exp osed phosphatidylserine, which allows binding and assembly of enzyme c omplexes of the coagulation pathway resulting in acceleration of the c lotting process. This response essentially requires the presence of ex tracellular Ca2+, and varies in extent with the type of agonist used. In the present paper we demonstrate that the moderate procoagulant res ponse of human platelets caused by thrombin is strongly amplified by t he presence of thapsigargin, an inhibitor of the microsomal Ca2+-ATPas e. Thapsigargin, like thrombin, has only a weak effect on procoagulant activity. The large increase in procoagulant activity observed with t he combined action of these two agonists is associated with increased shedding of microvesicles from the platelet plasma membrane as well as with inhibition of transport of a fluorescent-labeled analog of phosp hatidylserine from the outer to the inner leaflet of the plasma membra ne by the aminophospholipid translocase. The latter two observations s upport current concepts regarding the mechanism of development of proc oagulant activity. Although the synergistic effect of thapsigargin on thrombin-induced procoagulant activity is at least in part due to the high levels of intracellular [Ca2+] evoked by these agonists, the data clearly indicate that a rise of the intracellular [Ca2+] is insuffici ent to completely explain this response. The present findings suggest that additional factors control expression of procoagulant activity up on stimulation of platelets by thrombin.