Cytochalasin D and E: Effects on Fibrinogen Receptor Movement and Cytoskeletal Reorganization in Fully Spread, Surface-Activated Platelets: A Correlative Light and Electron Microscopic Investigation

This study investigates the involvement of actin microfila-ments in fibrinogen receptor redistribution and cytoskeletal reorganization that takes place in fully spread, surface-activated platelets. Colloidal gold-labeled fibrinogen (Fgn-Au label) in conjunction with video-enhanced differential interference contrast light microscopy (VDIC) was used to identify fibrinogen binding sites, glycoprotein IIb/IIIa (GPIIb/IIIa), on fully spread platelets. Platelets were treated with cytochala-sins D and E (5 x 10-5 mol/L to 5 x 10–8 mol/L) for 10 minutes, before or after incubation with Fgn-Au label. Results observed with VDIC were subsequently confirmed by high-voltage transmission and low voltage-high resolution scanning electron microscopic examination of the specimens. Preincubation of activated platelets with cytochalasin D or E (5 x 10–5 and 5 x 10-6 mol/L) inhibited fibrinogen receptor redistribution and abolished cytoskeletal reorgani- zation in fully spread platelets. After surface-activated platelets were incubated with Fgn-Au label, treatment with the above concentrations of cytochalasin D or E disrupted cy-toskeletal reorganization and caused random movement of previously redistributed receptor-ligand complexes. Incubation of platelets with cytochalasin E 5 x 10-6 mol/L prevented platelet activation and spreading. Thus, actin filaments appear necessary for platelet spreading from the discoid to the fully spread stage. The ligand-triggered, cy-toskeletally directed movement of fibrinogen receptors in fully spread platelets appears to be dependent on the presence of intact, polymerized actin. This movement is distinct from the cytochalasin-insensitive accumulation of GPIIb/IIIa-ligand in the channels of the open canalicular system. © 1992 by The American Society of Hematology. 0006–4971 /92/ 7901-0013$3.00/0