Ligand specificity and ticlopidine effects distinguish three human platelet ADP receptors

Defective platelet aggregation and increased resistance to thrombosis in purinergic P2Y1 receptor–null mice

“…Moreover, whereas the calcium response induced by ADP was abolished in the knock-out mice (Figure 3b), ADP still inhibited the PGE 1 -stimulated accumulation of cAMP in intact platelets, this effect being blocked by AR-C69931MX (Figure 3c). These results unambiguously demonstrate the existence of the putative P2cyc (13) (or P2T AC [14] or P2Y AC [15,16]) platelet ADP receptor, coupled to the inhibition of adenylyl cyclase, which is distinct from and independent of the P2Y 1 receptor.…”

“…To date, antithrombotic drugs selectively inhibiting ADP-induced platelet activation act on the P2cyc receptor without affecting the P2Y 1 receptor activation. These drugs are the thienopyridine compounds ticlopidine and clopidogrel or the ATP analogues termed AR-C66096, AR-C67085, and AR-C69931MX (13)(14)(15)(16)(17). This study demonstrates that the P2Y 1 receptor is crucial under circumstances where collagen and ADP are the principal agents involved in platelet activation.…”

“…In platelets, the P2Y 1 receptor has been shown to be necessary for ADP-induced aggregation (9)(10)(11), whereas a separate P2 receptor coupled to inhibition of adenylyl cyclase is thought to complete and to amplify the response (11,14,16,18). However, doubts still remained as to whether the P2Y 1 receptor might be coupled to adenylyl cyclase inhibition, at least in native cells (35).…”

Thrombin stimulation of p38 MAP kinase in human platelets is mediated by ADP and thromboxane A2 and inhibited by cGMP/cGMP-dependent protein kinase

“…Aggregation was carried out using a Biodata PAP-4 aggregometer (Alpha Laboratories, Eastleigh, United Kingdom) with platelet-rich plasma, and calcium transients were determined with the fluorescence indicator Fura-2, as previously described. 4,11 …”

“…2,3 ADP is released from platelet-dense granules upon activation by agonists such as thrombin and collagen, and binds to purinergic receptors (P2Y 1 , P2Y 12 , P2X 1 ) to reinforce platelet aggregation and thrombus formation. 2,[4][5][6] Recently, new mechanisms for agonist-induced platelet activation and secretion were proposed to involve sequential activation of cGMP-dependent protein kinase (PKG)/p38/integrin or Akt/ eNOS/sGC/cGMP/PKG secretion, respectively, 7-9 whereas others indicate an essential role of Src/ERK-mediated TxA 2 generation for fibrinogen receptor activation in human platelets. 10 We 11,12 and others 10,13 were unable to reproduce the reported ERK activation by PKG.…”