EFFECTS OF CELL ADHESION PEPTIDE, Arg-Gly-Asp-Ser (RGDS), ON RESPONSES OF WASHED PLATELETS FROM HUMANS, RABBITS AND RATS

Abstract

Fibrinogen (Fbg) is a cofactor in the aggregation of human platelets, and washed platelets do not aggregate to a significant extent in response to ADP unless Fbg is added to the suspension; however, exogenous Fbg is not required for ADP-induced aggregation of washed platelets from rabbits or rats. Since, with human platelets, Arg-Gly-Asp-Ser (RGDS) inhibits aggregation and the binding of 125I-Fbg to ADP-stimulated platelets, its effects on the responses of rabbit and rat platelets were studied in an attempt to elucidate the differences in Fbg requirements of platelets from the three species. Aggregation and Fbg binding were studied using washed platelets suspended in Tyrode solution containing albumin, apyrase and 2 mM Ca2+. 50 μM RGDS caused over 80% inhibition of the aggregation of human platelets stimulated with 9 yM ADP in the presence of 0.2 yM Fbg, but only 3-9% inhibition of the ADP-induced aggregation of rabbit or rat platelets regardless of whether exogenous Fbg was added. 50 yM RGDS also inhibited the aggregation of human platelets stimulated with thrombin (0.9 U/mL), but produced no more than 3% inhibition with rabbit or rat platelets. The binding of 125I-Fbg to ADP-stimulated human platelets was inhibited by 80-90% by 30 yM RGDS, but even at 50 μM, RGDS inhibited Fbg binding to rabbit or rat platelets by only 15-27%. The differences were due to the species of platelets, since, with both human and rabbit platelets, human Fbg could be replaced by rabbit Fbg without significantly changing the results. RGDS, added to human platelets that had been aggregated with thrombin, did not cause deaggregation, but did partially inhibit aggregation when added within 1 min; this inhibitory effect was less than when RGDS was added before thrombin, and decreased progressively as the length of time before the addition of RGDS was increased. These observations indicate a difference in aggregation mechanism between human platelets and those from rabbits and rats, and are consistent with a Fbg-independent component to the aggregation of rabbit and rat platelets.