The inhibitory mechanism of 6 traditional Chinese medicines on rabbit platelet aggregation in vitro, and the suppressive effect of oral administration of Toki-syakuyakusan on hyper-aggregability of the platelet from rabbit fed high cholesterol diet for 2 months, were investigated. Collagen-induced aggregation was inhibited by Keisi-bukuryogan, Kami-syoyo-san, Dai-saiko-to, Toki-syakuyaku-san, Hatimi-zio-gan and Syo-saiko-to in their lower concentrations than those inhibiting arachidonic acid- and thrombin-induced aggregation. These traditional Chinese medicines inhibited the release of [
3H] arachidonic acid from membrane phospholipids by phospholipase A
2, in [
3H] arachidonic acid-labelled platelets under stimulation with collagen and thrombin in the concentration ranges that inhibited each aggregation. In their higher concentrations to inhibit arachidonic acid-induced aggregation, they suppressed the conversion of arachidonic acid to thromboxane A
2 by about 50%. However, they had no effect on diacylglycerol formation induced by thrombin. The oral administration of Toki-syakuyaku-san depressed the increased aggregability of platelets from rabbit fed high cholesterol diet by 20-40% at the period of 1-2 months of feeding, without affecting plasma and platelet cholesterol level. These results indicate that the traditional Chinese medicines used here have an inhibitory effect on platelet phospholipase A
2 activation, rather than on cyclooxygenase, and therefore inhibit platelet activation in vitro and ex vivo.
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