Proteinase-activated receptor 4 (PAR4): activation and inhibition of rat platelet aggregation by PAR4-derived peptides

We studied the actions of receptor-activating peptide analogues (PAR4APs), modeled on the proteolytically-revealed tethered ligand sequence of murine proteinase-activated receptor-4 (PAR4), in a rat platelet aggregation assay . The PAR4APs GYPGKF-NH2 (GY-NH2) and AYPGKF-NH2 (AY-NH2) were able to caus e aggregation with EC50 values of about 40 muM and 15 muM, respectively. Th e reverse human PAR4 sequence (VQGPYG-NH2, YG-NH2) and the PAR1AP SFLLR-NH2 , did not cause aggregation. In contrast, trans-cinnamoyl-YPGKF-NH2 (tcY-NH 2) did not cause aggregation but blocked aggregation caused by GY-NH2, AY-N H2, and thrombin without affecting ADP-mediated aggregation. We conclude th at in contrast to the PAR1AP, the PAR4APs GY-NH2 and AY-NH2 activate rat pl atelets via a PAR4-related receptor and that peptide analogues modeled on t he PAR4 tethered activating sequence can serve as useful agonist and antago nist probes for assessing the consequence of activating PAR4 either by PAR4 APs or thrombin in rat tissue preparations.