GPIIB-IIIA ANTAGONISTS CAUSE RAPID DISAGGREGATION OF PLATELETS PRETREATED WITH CYTOCHALASIN-D - EVIDENCE THAT THE STABILITY OF PLATELET AGGREGATES DEPENDS ON NORMAL CYTOSKELETAL ASSEMBLY

Platelet activation is accompanied by changes in the composition of th e platelet cytoskeleton with rapid incorporation and displacement of c ertain proteins. Here we have inhibited cytoskeletal assembly by pretr eating platelets with cytochalasin D (CyD) and investigated the effect on the stability of the aggregates that form, The experiments were pe rformed in both citrated and hirudinized platelet-rich plasma (PRP) an d aggregation was induced by adenosine diphosphate (ADP), collagen, th e TXA(2)-mimetic U46619 and adrenaline. Platelets in the aggregates th at formed, underwent rapid disaggregation on addition of EDTA or a GpI Ib-IIIa antagonist such as MK-852 and GR144053F, all of which are agen ts that interfere with the ability of fibrinogen to interact with GpII b-IIIa. This was the case irrespective of the aggregating agent used a nd occurred in both citrated and hirudinized PRP, In contrast, the rat e of disaggregation brought about by some other agents, iloprost and A RL 66096, appeared to be unaffected by CyD, Information was also obtai ned on the effects of CyD on the cytoskeletal changes brought about by ADP and the effects on the cytoskeleton of subsequent addition of MK- 852, The results show that CyD retards the incorporation of certain pr oteins (actin, myosin, alpha-actinin, actin binding protein and a 66 K protein) into the cytoskeleton and that subsequent addition of MK-852 results in rapid displacement of some of these with re-incorporation of a 31 K protein. The results suggest that the early changes in the c ytoskeleton following platelet activation contribute to the stability of the aggregates that form, and that interference with these early ch anges results in aggregates that are easily disassembled by agents tha t interfere with GpIIb-IIIa-fibrinogen complex formation.