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 collagen, arachidonic acid, platelet-activating factor, ADP, or thrombin. The potency of inhibiting collagen-induced platelet aggregation is in the 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 potassium and norepinephrine-induced contractions, while beta,beta-dimethylacrylshikonin and teracrylshikonin potentiated the phasic contraction caused by nor epinephrine.