Surface expression and functional characterization of alpha-granule factorV in human platelets: effects of ionophore A23187, thrombin, collagen, andconvulxin

Factor V (FV) present in platelet alpha-granules has a significant but inco mpletely understood role in hemostasis. This report demonstrates that a fra ction of platelets express very high levels of surface-bound, alpha-granule FV on simultaneous activation with 2 agonists, thrombin and convulxin, an activator of the collagen receptor glycoprotein VI, This subpopulation of a ctivated platelets represents 30.7% +/- 4.7% of the total population and is referred to as convulxin and thrombin-induced-FV (COAT-FV) platelets. COAT -FV platelets are also observed on activation with thrombin plus collagen t ypes I, V, or VI, but not with type iii, No single agonist examined was abl e to produce COAT-FV platelets, although ionophore A23187 in conjunction wi th either thrombin or convulxin did generate this population. COAT-FV plate lets bound annexin-V, indicating exposure of aminophospholipids and were en riched in young platelets as identified by the binding of thiazole orange. The functional significance of COAT-FV platelets was investigated by demons trating that factor Xa preferentially bound to COAT-FV platelets, that COAT -FV platelets had more FV activity than either thrombin or A23187-activated platelets, and that COAT-FV platelets were capable of generating more prot hrombinase activity than any other physiologic agonist examined. Microparti cle production by dual stimulation with thrombin and convulxin was less tha n that observed with A23187, indicating that microparticles were not respon sible for all the activities observed. These data demonstrate a new procoag ulant component produced from dual stimulation of platelets with thrombin a nd collagen. COAT-FV platelets may explain the unique role of or-granule FV and the hemostatic effectiveness of young platelets. (C) 2000 by The Ameri can Society of Hematology.