Cytochalasins, a class of fungal metabolites, affect a wide variety of motile functions of eukaryotic cells1-3. Recently, several laboratories have shown that cytochalasins inhibit actin polymerization in vitro, presumably by binding with high affinity to growing ends of actin nuclei and filaments (F-actin), and preventing addition of monomers (G-actin) to these sites4-8. Cytochalasins have also been reported, in certain conditions, to induce depolymerization of actin filaments in vitro6. However, correlations of these effects of cytochalasins on actin polymerization in vivo have been limited to electron microscopic studies. Recently, Morris and Tannenbaum reported that there was no net depolymerization of actin in spreading fibroblasts treated with cytochalasin D, despite the well known morpho-logical effects of the drug on these cells9. Here, we describe results indicating that cytochalasin D can inhibit the rapid polymerization of actin in human platelets after thrombin stimulation and induce rapid depolymerization of filamentous actin in Stimulated platelets. Both of these effects correlate with observed changes in platelet shape.
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