Protection by plasma proteins against condensed tannin-mediated platelet activation

Condensed tannins isolated from cotton bracts are potent platelet agonists. They promote the secretion of 5-hydroxytryptamine from washed bovine platelets with an EC₅₀ of 35 μg/ml. However, when platelet-rich plasma was used in lieu of washed platelets, the potency of tannin was decreased over 50-fold. Reconstitution experiments demonstrated that some component in the plasma was responsible for the diminished potency of tannin. Fractionation of platelet-poor plasma suggested that albumin may be the inhibitory plasma component. Confirmation of this hypothesis was obtained from the finding that purified serum albumin at physiologic concentrations inhibited the tannin-mediated release of platelet 5-HT by more than 70%. These data suggest that any activation of platelets which may occur in vivo by the tannins present in inhaled cotton dust would require higher tannin concentrations than would have been predicted from our previous studies using washed platelets.