αIIbβ3 is required for heparin-induced potentiation of human platelets. (A) Fibrinogen receptor antagonists inhibit platelet spreading on immobilized heparin. Washed human platelets were added to heparin-coated glass slides in the presence of either a control IgG Fab fragment (left panel) or glycoprotein IIb-IIIa receptor antagonists, 10 μg/mL abciximab (middle panel) or 6.67 μg/mL eptifibatide (left panel). Note that platelet spreading 45 minutes later was abolished by either fiban. (B) Eptifibatide blocks heparin-induced platelet potentiation in solution. Washed platelets were stirred at 1000 rpm at 37°C alone or in the presence of heparin with or without 6.67 μg/mL eptifibatide, lysed, and subjected to immunoblot analysis as described in the legend to Figure 1. (C) Functional αIIbβ3 complexes are required for heparin-induced platelet potentiation. Platelets from a normal person or from a patient with variant Glanzmann thrombasthenia whose platelets express 100% levels of αIIbβ3 lacking most of the C-terminus of the β3 cytoplasmic domain were stirred in the presence or absence of UFH. Note that the increase in Akt phosphorylation normally induced by heparin exposure does not take place in platelets expressing signaling-defective αIIbβ3.