INHIBITION OF PLATELET-AGGREGATION BY SHIKONIN DERIVATIVES ISOLATED FROM ARNEBIA-EUCHROMA

Four antiplatelet components were isolated from Arnebia euchroma. They inhibited the aggregation of washed rabbit platelets caused by collag en, arachidonic acid, platelet-activating factor, ADP, or thrombin. Th e potency of inhibiting collagen-induced platelet aggregation is in th e following order: acetylshikonin (IC50 = 2.1 muM), teracrylshikonin ( 2.8 muM), beta,beta-dimethylacrylshikonin (4.2 muM), and shikonin (10. 7 muM). In rat aorta, acetylshikonin and shikonin inhibited high potas sium and norepinephrine-induced contractions, while beta,beta-dimethyl acrylshikonin and teracrylshikonin potentiated the phasic contraction caused by nor epinephrine.