Cytochalasin D inhibits actin polymerization and induces depolymerization of actin filaments formed during platelet shape change

Cytochalasins, a class of fungal metabolites, affect a wide variety of motile functions of eukaryotic cells^1-3. 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 sites^4-8. 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.