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 microM), teracrylshikonin (2.8 microM), beta, beta-dimethylacrylshikonin (4.2 microM), and shikonin (10.7 microM). 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 norepinephrine.