Effects of cytochalasin E and latrunculin A on platelet ultrastructure and F-actin formation. Platelets were incubated in the presence of vehicle (A-C), 4 μM cytochalasin E (D-F), or 4 μM latrunculin A (G-I) for 20 minutes. Actin-disrupting agents had little effect on the morphology of resting platelets. (B) Platelets treated with SFLLRN for 10 minutes demonstrated pseudopod formation. A subpopulation demonstrated granule centralization but most showed only marked degranulation. Pretreatment with (E) cytochalasin E or (H) latrunculin A inhibited pseudopod formation. The OCS becomes expanded and enlarged under these conditions. (C) Platelets treated with PMA for 20 minutes demonstrated degranulation and pseudopod formation. Pretreatment with (F) cytochalasin E or (I) latrunculin A prevented formation of pseudopodia and resulted in enlargement and expansion of the OCS. (J) Platelets were incubated in the presence of vehicle (▪), 4 μM cytochalasin E (□), or 4 μM latrunculin A (▦) for 20 minutes. Platelets were then stimulated with buffer alone, SFLLRN for 10 minutes, or PMA for 20 minutes. Platelets were subsequently fixed and stained with 10 μM FITC-phalloidin in the presence of 0.1% Triton X-100. Fluorescence was then quantified by flow cytometry. Data are expressed as percentage of change in FITC fluorescence compared with samples exposed to buffer alone. Magnification of panels A, D, F, and I is 22 000 ×; B and E, 18 000 ×; C, 26 500 ×; G, 17 000 ×; and H, 15 000 ×.