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 alter ation in the composition of the cytoskeleton. Here we have determined the effects of MK-852 and ARL 66096 on the cytoskeletal changes that o ccur. MK-852 is a GPIIb/IIIa antagonist that inhibits aggregation by i nterfering with fibrinogen binding. ART,66096 is a P-2T antagonist tha t 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 sust ained increase in the cytoskeletal content of actin binding protein (A BP), 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 inc rease in ABP or the 66K protein and no decrease in the 31K protein. Th e increase in myosin and alpha-actinin became reversible but there was still incorporation of actin into the cytoskeleton. In the presence o f ARL 66096 there was no increase in ABP or the 66K protein and no dec rease: in the 31K protein. ARL 66096 also prevented incorporation of a lpha-actinin and actin. As with MK-852, myosin incorporation became re versible. The results suggest that (1) myosin is incorporated into the cytoskeleton transiently during shape change, (2) ADP interaction wit h the ADP aggregation receptor (but not that for shape change) is asso ciated with alpha-actinin and actin incorporation into the cytoskeleto n, and (3) further changes that occur are consequent to fibrinogen bin ding and platelet aggregation.