Summary
The platelet procoagulant response involves an increase in surface-exposed phosphatidylserine,
which allows binding and assembly of enzyme complexes of the coagulation pathway resulting
in acceleration of the clotting process. This response essentially requires the presence
of extracellular Ca2+, and varies in extent with the type of agonist used. In the present paper we demonstrate
that the moderate procoagulant response of human platelets caused by thrombin is strongly
amplified by the presence of thapsigargin, an inhibitor of the microsomal Ca2+-ATPase. Thapsigargin, like thrombin, has only a weak effect on procoagulant activity.
The large increase in procoagulant activity observed with the 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 phosphatidylserine from the outer to the inner leaflet of the plasma membrane by
the aminophospholipid translocase. The latter two observations support current concepts
regarding the mechanism of development of procoagulant 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 insufficient to completely explain this response. The present findings suggest
that additional factors control expression of procoagulant activity upon stimulation
of platelets by thrombin.
