Factors Influencing the Response of Human Blood Platelets to Analogues of ADP Which May Act as Partial Agonists at the ADP Receptor
Authors: EGAN Christopher M. , FISHER Antony P. , SCRUTTON Michael C. 
Published: March 2005

The prior addition of non‐aggregating concentrations of the divalent cation ionophore, A‐23187, causes human platelets to aggregate in response to a subsequent addition of the 2′,3′‐dialdehyde and 2′,3′‐dialcohol derivatives of ADP (oADP and orADP). Previous studies [Pearce et al. (1978) Eur. J. Biochem. 88, 543–555] have shown that these derivatives act as partial agonists at the platelet ADP receptor inducing only the transition from discoid to globular morphology (‘shape change’). A secretion response is also observed on addition of a low concentration of ionophore A‐23187 prior to orADP. These responses are not observed if ionophore A‐23187 is added prior to the 2′,3′‐dialdehyde and 2′,3′‐dialcohol derivatives of ATP (oATP and orATP) and are markedly inhibited by prior addition of the ADP antagonist, adenosine 5′‐[β,γ‐methylene]triphosphate.
The aggregation response to oADP in the presence of ionophore A‐23187 is reduced but not eliminated by addition of 3 mM EGTA when studies are performed in heparinised platelet‐rich plasma. Additions of 3 mM EGTA in citrated platelet‐rich plasma, or of 4 mM EDTA in either system completely inhibits this response. Inhibitors which are reported to elevate the intracellular concentration of adenosine 3′:5′‐monophosphate (cyclic AMP) or to prevent Ca2+ movement also inhibit the aggregation response to oADP which is observed in the presence of ionophore A‐23187.
Prior addition of inhibitors of adenylate cyclase fails to cause an aggregation response to subsequent addition of oADP or orADP. Certain of these inhibitors enhance and prolong the shape change response to oADP or orADP but only at concentrations an order of magnitude in excess of those required to antagonise inhibition by agents such as prostaglandin E1, which act by increasing the concentration of cyclic AMP.
The concentration of prostaglandin E1, adenosine or papaverine required to inhibit shape change induced by oADP is one to two orders of magnitude lower than that required to inhibit shape change induced by ADP.
Prior addition of oADP decreases the lag phase in the response of human platelets to arachidonate while also increasing the concentration required to observe half‐maximal response, and causing a decrease in the extent of that response. Prior addition of oATP also diminishes the extent of this response and increases the concentration of arachidonate required but has no effect on the lag phase.
The data suggest that oADP and orADP are capable only of acting as partial agonists at the ADP receptor because of a defective ability to increase cytosolic Ca2+ concentration. This defect is rectified by the presence of low concentrations of ionophore A‐23187, which promotes mobilisation of Ca2+ from an intracellular store. The results do not appear consistent with the thesis that a decrease in platelet cyclic AMP is an initiating event in aggregation induced by ADP, but do support a model which implicates cyclic AMP in depletion of cytosolic Ca2+.