Mobility of GPIIb-IIIa receptors within membranes of surface- and suspension-activated platelets does not depend on assembly and contraction of cytoplasmic actin

Investigations using fibrinogen coupled to colloidal gold (Fgn/Au) have shown that glycoprotein IIb-IIIa (GPIIb-IIIa) receptors are mobile and undergo centripetal reorganization on spread platelets following surface activation. The assembly of cytoplasmic actin and its constriction into an inner filamentous zone have been proposed as the mechanism driving the Fgn/Au receptor complexes across the plasma membrane. The present study has used cytochalasin B (CB), an agent known to inhibit actin assembly and cause breakdown of newly formed actin filaments to test that hypothesis. At a concentration which inhibited pseudopod formation and spreading, CB did not block movement of Fgn/Au receptor complexes toward platelet centers or into the open canalicular system (OCS). Channels of the OCS filled with Fgn/Au receptor complexes were evident in over 90% of the CB-treated, surface-activated platelets. The findings do not support the concept that assembly of cytoplasmic actin and its contraction move receptor-ligand complexes on the platelet plasma membrane.