Association of localized Ca2+ gradients with redistribution of glycoprotein IIb-IIIa and F-actin in activated human blood platelets.

We monitored the intracellular distribution of ionized free Ca2+ concentration ([Ca2+]i) in individual human platelets by digital imaging fluorescence microscopy with fura 2 during platelet activation induced by surface contact or a soluble platelet agonist (thrombin). Contact of platelets with glass resulted in pseudopod formation and spreading, accompanied by a nonuniform rise in [Ca2+]i. The rise in [Ca2+]i was maximal during pseudopod formation. Locally elevated [Ca2+]i was frequently found in pseudopodia and at the edge and core of spread platelets. This pattern was faithfully duplicated by the local pattern of distribution of the cytoskeletal components F-actin, gelsolin, and surface glycoproteins (GP) IIb-IIIa but not by calmodulin. Platelets stimulated by thrombin also showed an inhomogeneous rise in [Ca2+]i, which was well correlated with the staining of F-actin and GPIIb-IIIa. Cytochalasin D, an inhibitor of actin polymerization, inhibited the inhomogeneous increase or redistribution of F-actin and GPIIb-IIIa but did not inhibit the rise in mean [Ca2+]i. These observations suggest that a localized change in [Ca2+]i may be associated with cytoskeletal reorganization and redistribution of GPIIb-IIIa in activated platelets.