Summary
Platelet activation results in shape change, aggregation, generation of thromboxane
A2, and release of granule contents. We have recently demonstrated that secreted ADP
is essential for thromboxane A2-induced platelet aggregation (J. Biol. Chem. 274: 29108-29114, 1999). The aim of
this study was to investigate the role of secreted ADP interacting at P2 receptor
subtypes in platelet secretion. Platelet secretion induced by the thromboxane A2 mimetic U46619 was unaffected by adenosine-3’phosphate-5’-phosphate, a P2Y1 receptor
selective antagonist. However, AR-C66096, a selective antagonist of the P2T
AC
receptor, inhibited U46619-induced platelet secretion, indicating an important role
for Gi signaling in platelet secretion. Selective activation of either the P2T
AC
receptor or the α2A adrenergic receptor did not cause platelet secretion, but potentiated U46619-induced
platelet secretion. SC57101, a fibrinogen receptor antagonist, failed to inhibit platelet
secretion, demonstrating that outside-in signaling was not required for platelet secretion.
Since Gi signaling results in reduction of basal cAMP levels through inhibition of adenylyl
cyclase, we investigated whether this is the signaling event that potentiates platelet
secretion. SQ22536 or dideoxyadenosine, inhibitors of adenylyl cyclase, failed to
potentiate U46619-induced primary platelet secretion, indicating that reduction in
cAMP levels does not directly contribute to platelet secretion. Wortmannin, a selective
inhibitor of PI-3 kinase, minimally inhibited U46619-induced platelet secretion when
it was solely mediated by Gq, but dramatically ablated the potentiation of Gi signaling. We conclude that signaling through the P2TAC receptor by secreted ADP causes positive feedback on platelet secretion through a
PI-3 kinase pathway.
Keywords
Platelet activation - ADP - P2T
AC
receptor - epinephrine