FLOW CYTOMETRIC ANALYSIS OF SURFACE-MEMBRANE PROTEINS ON ACTIVATED PLATELETS AND PLATELET-DERIVED MICROPARTICLES FROM HEALTHY AND THROMBASTHENIC INDIVIDUALS

We used flow cytometry to investigate surface membrane protein express ion by platelets and platelet-derived microparticles from normal indiv iduals and a patient with Glanzmann's thrombasthenia. Microparticles w ere detected by both forward scatter and side scatter using FACScan. T he binding of coagulation factors on microparticles was investigated b y using monoclonal anti-Factor IX (IXa) and anti-Factor X (Xa) antibod ies. Furthermore, the procoagulant activity of microparticles was meas ured with a chromogenic substrate (S-2222) using a microtiter enzyme-l inked immunosorbent assay. Both types of platelets showed similar rele ase of microparticles. Microparticles released from platelets after ac tivation with the calcium ionophore A23187 did not bind factors IXa an d Xa, but when purified factors Va and Xa were added to the incubation buffer, factor Xa binding increased markedly in both normal and throm basthenic platelets. Both normal and thrombasthenic platelets showed a similar time-dependent release of microparticles when activated with A23187. However, the binding of an antibody to granule membrane protei n-140 also increased time-dependently in normal microparticles, but wa s little increased in thrombasthenic microparticles. These findings su ggest that glycoprotein IIb/IIIa does not participate in the expressio n of prothrombinase activity on the surface of activated platelets and microparticles, whereas this glycoprotein appears to have an importan t role in the movement of granule membrane protein-140 from platelets to microparticles.