ABSTRACT
1. Four structural systems are involved in the process of platelet activation that leads to aggregation: 1) the membrane system, i.e., the cytoplasmic membrane, the dense tubular structure and the open canalicular structure; 2) alpha and dense granules; 3) the peripheral microtubular coils; 4) the microfibrillar meshwork of actin-myosin bundles. 2. We added four compounds which modify cell ultrastructure to normal platelet-rich plasma to analyze the behavior of the structural systems of platelet activation: vinblastine (100 micrograms/ml) and cimetidine (100 micrograms/ml) that act on the membrane system, ticlopidine (200 micrograms/ml) and colchicine (100 micrograms/ml) that affect primarily the microtubular structure, cytochalasin B (30 micrograms/ml) and phorbol myristate acetate (100 ng/ml) that act upon the granular system, and cytochalasin D (30 micrograms/ml) and concanavalin A (50 micrograms/ml) that influence the microfibrillar structure. Platelet aggregation was stimulated by epinephrine or thrombin. 3. Cimetidine and ticlopidine prevented aggregation. However, neither substance affected the microtubular structure. Colchicine and cytochalasin B only partially impaired aggregation, because pieces of microtubules remained in the presence of these substances. The other substances did not present anti-aggregant activity and did not preserve the microtubules. 4. We infer that the disappearance of the microtubules is necessary to produce aggregation. When they remain intact no aggregation is produced, even though the other structural systems are activated.