Influence of intracellular chelating agents on formation of spike-like pseudopods by human platelets

Recent reports have suggested that spike-like pseudopods develop on platelets loaded with intracellular chelating agents during glass activation, and proposed that the extensions are caused by an unusual organization of newly assembled actin filaments. The present study has reexamined this hypothesis. Platelets loaded with one of the chelating agents, Quin II or BAPTA, seldom formed spike-like pseudopods when exposed to glass at 37 C for 30-60 min. However, when chilled and rewarmed the Quin II-or BAPTA-loaded platelets readily developed one or several spike-like extensions after a 30-60-min exposure to glass or on shaking in suspension. Thin section and negative-stain electron microscopy demonstrated that the major constituents of spike-like pseudopods were microtubules. Unusual coils of actin filaments were not observed. The observation was confirmed by immunofluorescence microscopy employing an anti-tubulin antibody and fluorescein-conjugated antimouse IgG. Cytochalasin B, an agent that inhibits new actin filament formation had virtually no effect on spike formation by chilled-rewarmed, Quin II- or BAPTA-loaded cells, whereas prior exposure to vincristine, an agent that dis-assembles microtubules and prevents their reformation, blocked spike development. Taxol, a drug that stabilizes microtubules and prevents their disassembly by cold or vincristine, prevented spike formation. Results indicate that microtubule assembly is the major cause of spike-like pseudo-pod formation, and the increased assembly may be due to binding of free cytoplasmic calcium by intracellular chelating agents.