MOLECULAR-BASIS FOR ADP-INDUCED PLATELET ACTIVATION I - EVIDENCE FOR 3 DISTINCT ADP RECEPTORS ON HUMAN PLATELETS

Acting through cell surface receptors, ADP activates platelets resulti ng in shape change, aggregation, thromboxane A(2) production, and rele ase of granule contents, ADP also causes a number of intracellular eve nts including inhibition of adenylyl cyclase, mobilization of calcium from intracellular stores, and rapid calcium influx in platelets, Howe ver, the receptors that transduce these events remain unidentified and their molecular mechanisms of action have not been elucidated. The re ceptor responsible for the actions of ADP On platelets has been design ated the P2T receptor, In this study we have used ARL 66096, a potent antagonist of ADP-induced platelet aggregation, and a P2X ionotropic r eceptor agonist, alpha,beta-methylene adenosine 5'-triphosphate, to di stinguish the ADP-induced intracellular events, ARL 66096 blocked ADP- induced inhibition of adenylyl cyclase, but did not affect ADP-mediate d intracellular calcium increases or shape change, Both ADP and 2-meth ylthio-ADP caused a 3-fold increase in the level of inositol 1,4,5-tri sphosphate over control levels which peaked in a similar fashion to th e Ca2+ transient. The increase in inositol 1,3,4-trisphosphate was of similar magnitude to that of inositol 1,4,5-trisphosphate. alpha,beta- Methylene adenosine 5'-triphosphate did not cause an increase in eithe r of the inositol trisphosphates. These results clearly demonstrate th e presence of two distinct platelet ADP receptors in addition to the P 2X receptor: one coupled to adenylyl cyclase and the other coupled to mobilization of calcium from intracellular stores through inositol tri sphosphates.