INHIBITION OF RAT PLATELET-AGGREGATION BY MYCALOLIDE-B, A NOVEL INHIBITOR OF ACTIN POLYMERIZATION WITH A DIFFERENT MECHANISM OF ACTION FROMCYTOCHALASIN-D

In vitro effects of mycalolide-B (MB), isolated from marine sponge, we re investigated with regard to the activation of rat platelets. Collag en-induced platelet aggregation in platelet-rich plasma (PRP) was slig htly but significantly potentiated by lower concentrations of MB (0.3 and 1 mu M) but was inhibited by higher concentrations (3 and 10 mu M) ADP-induced platelet aggregation in PRP was also significantly preven ted by MB (1-10 mu M). Potentiation of ADP-induced aggregation by MB ( 0.3 mu M) was hardly observed. G-actin contents, determined by DNase I inhibition assay, were increased in resting washed platelets incubate d with MB (3 mu M). In contrast, cytochalasin-D (CD) at 3 mu M slightl y reduced G-actin contents in resting platelets. After platelet aggreg ation with collagen (3 mu g/ml) or ADP (10 mu M), G-actin contents in platelets were reduced, indicating de novo actin polymerization. MB (3 mu M) and CD (3 mu M) abolished both ADP (10 mu M)- and collagen (3 m u g/ml)-induced platelet aggregation and actin polymerization in washe d platelets. MB (1-10 mu M) had no effects on intracellular Ca2+ conce ntrations in ADP (10 mu M)-stimulated platelets. [I-125]-fibrinogen bi nding to activated platelets with ADP (10 mu M) was inhibited by MB (0 .3-3 mu M) in a concentration-dependent manner. Thrombin-induced plate let-fibrin clot retraction was inhibited by MB (1 and 10 mu M). These results suggest that MB inhibits platelet activation by interfering wi th actin polymerization through a different mechanism of action from C D. MB may be a useful tool for studying the role of actin polymerizati on in various cells.