Changes in the composition of the platelet cytoskeleton in response to ADP: effects of MK-852 and ARL 66096.

Platelet activation by adenosine diphosphate (ADP) results in an alteration in the composition of the cytoskeleton. Here we have determined the effects of MK-852 and ARL 66096 on the cytoskeletal changes that occur. MK-852 is a GPIIb/IIIa antagonist that inhibits aggregation by interfering with fibrinogen binding ARL 66096 is a P2T antagonist that selectively inhibits ADP-induced aggregation. Neither agent inhibits the shape change response. Experiments were performed in hirudinized platelet-rich plasma. Platelet activation led to a significant and sustained increase in the cytoskeletal content of actin binding protein (ABP), myosin, alpha-actinin, a 66K protein and actin, and a significant decrease in a 31K protein. In the presence of MK-852 there was no increase in ABP or the 66K protein and no decrease in the 31K protein. The increase in myosin and alpha-actinin became reversible but there was still incorporation of actin into the cytoskeleton. In the presence of ARL 66096 there was no increase in ABP or the 66K protein and no decrease in the 31K protein. ARL 66096 also prevented incorporation of alpha-actinin and actin. As with MK-852, myosin incorporation became reversible. The results suggest that (1) myosin is incorporated into the cytoskeleton transiently during shape change, (2) ADP interaction with the ADP aggregation receptor (but not that for shape change) is associated with alpha-actinin and actin incorporation into the cytoskeleton, and (3) further changes that occur are consequent to fibrinogen binding and platelet aggregation.