Prostaglandins as inhibitors of human platelet aggregation

The potencies of prostaglandins (PG) I2, PGD2 and PGE1 as inhibitors of human platelet aggregation induced by threshold concentrations of four aggregating agents were determined in platelet-rich plasma from normal individuals who had not ingested aspirin. The order of activity against ADP, adrenaline and collagen was always PGI2 greater than PGD2 greater than PGE1. However, PGD2 and PGE1 were almost equipotent with PGI2 when tested against arachidonic acid (AA). The threshold inhibitory effects of PGD2, PGE1 and PGI2 could be over come by increasing the concentrations of the aggregating agents AA, collagen or ADP. Adrenaline was found to be different from the other aggregating agents. It could overcome inhibition of platelet aggregation by PGD2 but could not overcome inhibition by PGI2 or PGE1. These facts support the hypothesis that platelet receptors for PGI2 and PGE1 are similar to each other and different from the receptor(s) for PGD2. PRP obtained from normal subjects after the ingestion of aspirin exhibited only one wave of aggregation in response to ADP, adrenaline or collagen, PGI2, PGD2 and PGE1 were all powerful inhibitors of this single wave of aggregation. The inhibitory activity of all three prostaglandins at threshold concentrations was overcome by increasing the concentration of ADP or collagen but not by increasing the concentration of adrenaline.