Heparin-induced potentiation of murine platelets is mediated by integrin αIIbβ3. (A) Platelets from β3 knockout mice cannot spread on immobilized heparin. Washed mouse platelets from wild-type (WT) and αIIbβ3-deficient (β3^−/−) mice were added to glass slides that had been coated with 1% BSA, 100 μg/mL fibrinogen (Fg), or 10 U/mL heparin (Hep) in the presence of 2μM ADP to activate the integrin. Note that the failure of αIIbβ3-deficient platelets to spread on heparin is nearly identical to their well-characterized inability to spread on immobilized fibrinogen. (B) Heparin-induced phosphorylation of Akt in murine platelets requires αIIbβ3. Platelets from WT and integrin-β3-deficient mice were placed in an aggregometer cuvette in the presence of the indicated concentrations of UFH, allowed to stir at 1000 rpm for 2 minutes at 37°C, lysed in SDS sample buffer, and subjected to immunoblot analysis using antibodies specific for Akt antigen or phosphoserine₄₇₃ Akt. Note that heparin failed to induce dose-dependent Akt phosphorylation in αIIbβ3-deficient platelets.