Agonist-generated inside-out signals enable the platelet integrin ␣ IIb ␤ 3 to bind soluble ligands such as fibrinogen. We found that inhibiting actin polymerization in unstimulated platelets with cytochalasin D or latrunculin A mimics the effects of platelet agonists by inducing fibrinogen binding to ␣ IIb ␤ 3 . By contrast, stabilizing actin filaments with jasplakinolide prevented cytochalasin D-, latrunculin A-, and ADP-induced fibrinogen binding. Cytochalasin D-and latrunculin A-induced

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... inogen was inhibited by ADP scavengers, suggesting that subthreshold concentrations of ADP provided the stimulus for the actin filament turnover required to see cytochalasin D and latrunculin A effects. Gelsolin, which severs actin filaments, is activated by calcium, whereas the actin disassembly factor cofilin is inhibited by serine phosphorylation. Consistent with a role for these factors in regulating ␣ IIb ␤ 3 function, cytochalasin D-and latrunculin A-induced fibrinogen binding was inhibited by the intracellular calcium chelators 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester and EGTA acetoxymethyl ester and the Ser/Thr phosphatase inhibitors okadaic acid and calyculin A. Our results suggest that the actin cytoskeleton in unstimulated platelets constrains ␣ IIb ␤ 3 in a low affinity state. We propose that agonist-stimulated increases in platelet cytosolic calcium initiate actin filament turnover. Increased actin filament turnover then relieves cytoskeletal constraints on ␣ IIb ␤ 3 , allowing it to assume the high affinity conformation required for soluble ligand binding.