Platelet phosphatidylserine exposure and procoagulant
activity in clotting whole blood - different effects of collagen, TRAP and
calcium ionophore A23187.
Ramstrom S, Ranby M, Lindahl TL.
Department of Biomedicine and Surgery, Division of Clinical Chemistry,
University Hospital, SE-581 85 Linkoping, Sweden.
We have studied the effects of different platelet agonists on phosphatidylserine
(PS) exposure and clotting times in blood without anticoagulants. Similar
reductions in clotting time were obtained for collagen, TRAP-6 or calcium
ionophore A23187 (50 micro mol/L), in spite of huge differences in PS expression
[6.7 +/- 2.4%, 2.3 +/- 0.5% and 99.9 +/- 0.1%, respectively (mean +/- SD, n =
5)]. Furthermore, the clotting times were much longer for samples with A23187
exposing the same amounts of PS as samples with collagen or TRAP-6. Annexin V
reversed the clotting time reduction, but could not prevent coagulation.
Addition of phospholipid vesicles containing 20% PS neither affected the
clotting times nor induced clotting in recalcified, platelet-free plasma. We
conclude that platelet PS exposure is necessary, but not sufficient, for the
coagulation amplification observed when platelets are stimulated via
physiological receptors in a whole blood environment.
