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Extracellular Ca 2+ modulates ADP‐evoked aggregation through altered agonist degradation: implications for conditions used to study P2Y receptor activation
Author(s) -
Jones Sarah,
Evans Richard J.,
MahautSmith Martyn P.
Publication year - 2011
Publication title -
british journal of haematology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.907
H-Index - 186
eISSN - 1365-2141
pISSN - 0007-1048
DOI - 10.1111/j.1365-2141.2010.08499.x
Subject(s) - apyrase , chemistry , p2y receptor , agonist , extracellular , adenosine diphosphate , biophysics , receptor , thromboxane , p2 receptor , platelet , thromboxane a2 , biochemistry , platelet activation , purinergic receptor , pharmacology , medicine , biology , platelet aggregation
Summary 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+ ] o 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+ ] o ‐dependence of platelet activation. Reducing [Ca 2+ ] o 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+ ] o ‐dependence. Prevention of ADP degradation abolished the differences between low and physiological [Ca 2+ ] o 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+ ] o . As reported previously, thromboxane A 2 generation was enhanced at low [Ca 2+ ] o , 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.