PROTHROMBIN CONVERSION UNDER FLOW CONDITIONS BY PROTHROMBINASE ASSEMBLED ON ADHERENT PLATELETS

Prothrombin activation by prothrombinase was investigated on platelets adhered onto a fibrinogen-coared coverslip mounted in a flow chamber. Once bound to the fibrinogen, platelets gradually changed their morph ology: they developed pseudopods, spread over the surface and finally transformed into balloon-shaped cells. This last morphologic change re quired the presence of calcium and was accompanied by the exposure of procoagulant phospholipid at the outer membrane as detected by the cap ability of the platelets to bind fluorescein-labelled annexin V. Proth rombinase complexes were allowed to assemble on these adhered platelet s by perfusion with factor Xa and varying concentrations of factor Va and prothrombin. The steady-state rate of thrombin formation during co ntinuous now increased with the prothrombin concentration but not with the factor Va between 0.05 and 0.5 nM. Once prothrombinase was assemb led, factor Xa could be omitted from the perfusion mixture without aff ecting the steady state rate of thrombin production. Our study demonst rates the efficient ability of the procoagulant surface of adherent pl atelet to support the assembly of stable prothrombinase complexes. Thr ombin production was limited by the rate of supply of prothrombin tow ards the catalytic surface.