ADP-dependence of platelet activation induced by a thrombin receptor agonist

The synthetic peptide SFLLRNPNDKYEPF (Thrombin Receptor Agonist: TRA) has recently been shown to mimic the new amino-terminus created by cleavage of the thrombin receptor on platelets and therefore to act as a receptor agonist. In the present work, this peptide proved a potent aggregating agent for human platelets with an ED50 of 3 microM. The ADP removing enzyme system creatine phosphate/creatine phosphokinase (CP/CPK) and the ADP receptor antagonist ATP alpha S strongly inhibited platelet aggregation in response to low doses of TRA, indicating that TRA-induced platelet aggregation, like thrombin-induced aggregation is an ADP mediated event. CP/CPK had however no effect on aggregation in response to the agonist peptide at higher concentrations. Similar results were obtained with regard to the influence of TRA and thrombin on platelet adenylyl cyclase activity, while both agents induced nucleotide release from platelets in a dose-dependent manner. These results confirm that the aggregating effect of alpha-thrombin on human platelets is closely linked to its esterolytic activity at the receptor level and show that this aggregation is an ADP mediated event.