Our hypothesis is that the platelet response (shape change, aggregation, secretion and procoagulant activity) to activators may primarily involve the regulation of cytoskeletal assembly (actin polymerization and interaction with myosin, actin-binding protein, and Alpha-actinin-like protein) and the transmembrane interaction of these cytoskeletal assemblies with surface membrane proteins. Such transmembrane interaction would control the expression of aggregation, adhesion, and platelet procoagulant activity (via exposure of high affinity factor Va receptors). It is proposed to test this hypothesis by coordinately examining actin polymerization (by a DNAase I inhibition assay for monomeric actin), in situ actin-binding protein and myosin light chain phosphorylation (total platelet sample denatured and resolved on SDS-PAGE), composition on SDS-PAGE of cytoskeletal cores resistant to detergent extraction due to crosslinking by associated cytoskeletal proteins, and the association of radioiodinated surface membrane proteins with such cores. These processes will be modified in situ by specific inhibitors of platelet phosphorylation, (aspirin, PGI2, dibutyrul cAMP, calmodulin in hibitors) and cytoskeletal protein assembly, (cytochalasin B, phorbol myristate acetate). In vitro experiments with isolated actin-binding protein will address the possible role of phosphorylation in regulation of pseudopodal cytoskeletal assembly. Our long term objectives are to understand what regulates platelet cytoskeletal assembly and function, the nature of transmembrane interaction, and how such transmembrane interactions might control critical hemostatic platelet responses.