Participation of Protein Kinases in Complement CSb-9-Induced Shedding of Platelet Plasma Membrane Vesicles

The formation of membrane microparticles through vesiculation of the platelet plasma membrane is known to provide catalytic surface for several enzyme complexes of the coagulation system, and to underlie the procoagulant responses elicited with platelet activation. This induced shedding of vesicles from the plasma membrane is most prominent when platelets are activated by the terminal complement proteins, C5b-9, by a Ca’+ ionophore, or by the combination of thrombin plus collagen. Although shown to require elevated [Ca‘’], the cellular events that initiate plasma membrane evagination and fusion to form the shed vesicles remain unresolved. To gain additional insight into the cellular events that regulate membrane microparticle formation, we have examined how this process is influenced by the activity of cellular protein kinases. Cytoplasmic [Ca”] of gel-filtered platelets was increased by membrane assembly of the terminal complement proteins C5b-9 in the presence of selective inhibitors of protein kinase or phosphatase reactions, and