High-speed platelet adhesion under conditions of rapid flow

The recognition of exposed collagen by circulating platelets is an initial step in the formation of the hemostatic plug or a thrombus after vascular injury. Theoretical calculations of the speed of platelet function required for effective hemostasis have suggested very short reaction times. However, it is not known how fast platelets can adhere to collagen under arterial flow conditions or which membrane proteins are involved. We have used a continuous-flow, microaffinity column linked to a resistive-particle counter to detect platelet adhesion. Adhesion of human platelets to native type I collagen was extremely rapid, with exponential half-times as short as 240 ms, and was nearly complete by 2 s. This RGD-independent process was not associated with platelet aggregation or secretion. The monoclonal antibody 6F1 directed against the glycoprotein Ia/IIa complex inhibited adhesion, suggesting that this complex plays an important role in the initial phases of platelet-collagen interaction under flow conditions. In addition, divalent cations were required for adhesion, as indicated by inhibition with EDTA in plasma and the dependence on Mg2+ for washed platelets.