Phosphatidylserine surface expression and integrin αIIbβ3 activity on thrombin/convulxin stimulated platelets/particles of different sizes

Abstract

Platelets stimulated by a combination of thrombin/convulxin have been shown to develop two to three populations characterized by different phosphatidylserine (PS) surface expression and integrin αIIbβ3 activity. To determine how these markers are distributed on the surface of platelets/particles, we studied Annexin V and PAC-1 binding to platelets/particles of different sizes by flow cytometry analysis and evaluated influences of calpain and caspase inhibitors on thrombin/convulxin-activated platelets. Analysed platelets/particles were divided by their sizes, according to the standard size beads, into seven populations from 0.37 to 4.8 μm. PAC-1 binding/μm2 was almost equal in platelets/particles ranging from 1.2 to 4.8 μm and was significantly lower on smaller-sized particles sizes (0.37-0.7 μm). PS surface exposure/μm2 was high in the particles of 0.37-1.2 μm and very low in platelets (2.6-4.8 μm). Upon thrombin/convulxin stimulation caspase inhibitors prevented microparticle (MP) formation, while a calpain inhibitor stimulated MP formation. It was also shown that stimulated platelets are heterogeneous not only in their ability to activate αIIbβ3 integrin complex and expose PS on their surface, but also in the distribution of activation markers, which strongly depends on platelet/particle size and that platelets/particles of different sizes provide different responses to the same stimulus. © 2008 The Authors.