Schematic representation of the sequential signaling events induced by the interaction of platelets with immobilized VWF under high shear stress. On the left, a platelet is shown during the initial tethering to the A1 domain of immobilized VWF mediated by the GP Ib-IX-V complex. An α/β [Ca⁺⁺]_i elevation is elicited as a consequence of this interaction and leads to the release of ADP from intracellular storage granules. Src family kinases may be involved at this stage,30,32 and cAMP/cGMP levels modulate this and other downstream responses.8 Subsequent events are shown in the platelet on the right. The released ADP binds to the G_q-coupled P2Y₁ receptor, which leads to PLC activation and enhances Ca⁺⁺ release from internal stores during α/β oscillations. At this stage, a first level of localized α_IIbβ₃ activation is reached that supports a more prolonged platelet adhesion mediated by the interaction with the RGD sequence in the VWF C1 domain. Initial PI 3-K activation may enhance this response. Subsequently, further PI 3-K activation and possibly the involvement of Src family kinases contribute to a more generalized α_IIbβ₃ activation that permits soluble ligand binding (exemplified here by fibrinogen and VWF) and supports the formation of platelet-platelet aggregates. This second level of α_IIbβ₃ activation is concurrent with or subsequent to a type γ [Ca⁺⁺]_i elevation dependent on a transmembrane ion flux. The second ADP receptor, P2Y₁₂, supports the formation of larger platelet aggregates through mechanisms that occur after the measured Ca⁺⁺ oscillations. The thromboxane A₂ pathway inhibited by aspirin appears to have a very limited role in the successive stages of platelet adhesion, activation, and aggregation induced by the interaction with immobilized VWF. IP₃ indicates inositol-1,4,5-trisphosphate; Src, Src family tyrosine kinases; PLC, phospholipase C; PKC, protein kinase C; PI 3-K, phosphatidylinositol 3-kinase.