Summary
We tested different types of clot for their ability to provoke procoagulant activity
in platelets: normal clots from platelet poor plasma (des AABB- or fibrin II clots),
similar clots in which the adsorbed thrombin has been inhibited by hirudin, and clots
obtained by the action of two snake venom enzymes that release only fibrinopeptide
A (des AA- or fibrin I clots). Analogous clots from fibrinogen solutions were also
tested.
In platelet rich plasma (PRP), where platelet coagulant phospholipids (PCP) are rate
limiting for thrombin generation, the addition of any type of clot enhances the generation
of thrombin thus it induces the appearance of PCP. Clots containing active adsorbed
thrombin are the most potent ones in this respect. Lactate dehydrogenase (LDH) levels
do not increase in the course of the thrombin generation so the platelets are not
damaged in the process. Non-centrifugable PCP could be demonstrated to appear during
the process, so the production of procoagulant phospholipid microparticles must be
part of the mechanism. Membrane transbilayer phosphatidyl serine movement (flip-flop)
can not be demonstrated in PRP as the activated platelets are caught in the emerging
clot.
In order to demonstrate flip-flop, we tried to investigate the influence of clots
on washed platelets. However, contrary to platelets in a plasma milieu, isolated platelets
are damaged by fibrin clots, especially in the presence of thrombin, as can be judged
from the appearance of LDH
We conclude that, in PRP, clots induce the appearance of PCP from platelets by vesiculation,
possibly accompanied by flip-flop and that thrombin accelerates the process but is
not an absolute requirement
