Apyrase and inhibitors of Src, Rac-1, or Rho kinase decrease platelet spreading on fibrinogen. Platelets were allowed to adhere to fibrinogen in the presence of a Src-kinase inhibitor (20 μM PP2), a Rac-1 inhibitor (100 μM NSC23766), or a Rho kinase inhibitor (5 μM H1152) for 1 hour. (A-B) Adherent platelets were fixed, permeabilized, and stained with fluorescently labeled phalloidin for F-actin. The Src (A) and Rac-1 (B) inhibitor and to a lesser extent the Rho kinase inhibitor (B) led to impaired spreading of platelets adhering to both low- and high-density fibrinogen. (C) Analysis of FAK immunoprecipitated from platelets adherent in the presence of Rac-1 and Rho kinase inhibitors revealed decreased phosphorylation of FAK in platelets adherent to high- but not to low-density fibrinogen. (D-E) Presence of apyrase led to less extensive spreading in platelets on both high- and low-density fibrinogen, but to decrease in FAK phosphorylation in platelets on high-density fibrinogen only. Noncontiguous lanes from a single blot are shown. Results shown are representative of 3 independent experiments.