Extracellular Ca(2+) modulates ADP-evoked aggregation through altered agonist degradation: implications for conditions used to study P2Y receptor activation.

ADP is considered a weak platelet agonist due to the limited aggregation responses it induces in vitro at physiological concentrations of extracellular Ca(2+) [(Ca(2+) )(o) ]. Lowering Ca(2+) paradoxically enhances ADP-evoked aggregation, an effect that has been attributed to enhanced thromboxane A(2) production. This study examined the role of ectonucleotidases in the Ca(2+) -dependence of platelet activation. Reducing Ca(2+) from millimolar to micromolar levels converted ADP (10 μmol/l)-evoked platelet aggregation from a transient to a sustained response in both platelet-rich plasma and washed suspensions. Blocking thromboxane A(2) production with aspirin had no effect on this Ca(2+) -dependence. Prevention of ADP degradation abolished the differences between low and physiological Ca(2+) resulting in a robust and sustained aggregation in both conditions. Measurements of extracellular ADP revealed reduced degradation in both plasma and apyrase-containing saline at micromolar compared to millimolar Ca(2+) . As reported previously, thromboxane A(2) generation was enhanced at low Ca(2+) , however this was independent of ectonucleotidase activity(.) P2Y receptor antagonists cangrelor and MRS2179 demonstrated the necessity of P2Y(12) receptors for sustained ADP-evoked aggregation, with a minor role for P2Y(1) . In conclusion, Ca(2+) -dependent ectonucleotidase activity is a major factor determining the extent of platelet aggregation to ADP and must be controlled for in studies of P2Y receptor activation.