THE ANTI-AGGREGATING PEPTIDE KRDS IMPAIRS ALPHA-GRANULE RELEASE, WHEREAS RGDS DOES NOT

The effects on platelet activation of two different tetrapeptides, KRD S present on human lactotransferrin and RGDS present on adhesive prote ins such as human fibrinogen a-chain, were compared by a combination o f morphological and functional techniques. Ultrastructural observation s of alpha-thrombin stimulated platelets (0.05 U/ml), show strong plat elet aggregation and full alpha-granule release. In the presence of RG DS (0.1-1 mM) aggregation was impaired but secretion was not blocked a nd platelets had released their alpha-granule contents. Platelets appe ared uniformly degranulated with a dense central meshwork of microfila ments. In the presence of KRDS (0.5-1 mM), the platelets were activate d with shape change and pseudopod formation. Aggregation was also impa ired, but to a lesser extent since RGDS is active at a concentration a s low as 0.1 mM, and, in contrast to RGDS, secretion was severely redu ced. Electron microscopy showed that numerous alpha-granules were stil l scattered in the cytoplasmic matrix or often gathered in the centre of the platelet, but the majority of the open canalicular system ciste rnae remained clear. An immunoelectron microscopic study using immunog old and monospecific antibodies directed against fibrinogen and the al pha-granule membrane protein P-selectin (GMP 140) was performed. In th e presence of RGDS, fibrinogen was released and P-selectin was translo cated to the platelet surface; in contrast, in the presence of KRDS, f ibrinogen remained localized in the alpha-granule, and the P-seIectin associated with the alpha-granule. These observations were accompanied by same functional results: thrombin-induced platelet aggregation was inhibited by both peptides, and in contrast to RGDS, secretion was se verely reduced in the presence of KRDS: serotonin release from dense g ranule was reduced by 73% compared to the control. These results show that these two tetrapeptides, in spite of same structural similarities , act differently in impairing platelet function. KRDS interfering wit h both the dense and alpha-granule release reaction may be a useful to ol for a better understanding of the platelet secretion mechanism.