Shape and Cytoplasmic Filaments in Control and Lidocaine-treated Human Platelets

Abstract

In order to examine the internal structure of platelets we used a negative stain technique following rapid lysis of gel-filtered platelets on electron microscope grids. Platelets activated by the preparative procedure contained an internal network of thin (7-8 nm) filaments in the body of the cells, continuous with parallel filaments in the microspikes. The filaments bound HMM-S1 to form arrowhead structures. The tight bundles of thin filaments in the microspikes possessed a striking axial periodicity similar to the periodicity seen in actin paracrystals containing muscle regulatory proteins. Nearly complete microtubule circlets were sometimes present in or adjacent to lysing cells with formed microspikes; they appeared as continuous loops. Lidocaine (30 mM) reversibly suppressed and reversed microspike formation in 70% or more of platelet populations even after adenosine diphosphate addition. Treated platelets were spherical, not discoid, lacked the thin-filament network and microtubules and contained an amorphous or granular internal cytoplasm. Bundles of thick 0.3-μm long filaments, similar to myosin aggregates, formed in vitro from purified platelet myosin were seen only in treated stored cells, and not in controls. We conclude that lidocaine suppression of microspikes is correlated with disassembly of thin-filament bundles, that reformation of microspikes involves reassembly of filaments into bundles, and that microtubules are not an obligatory part of microspikes.