Different G protein-coupled signaling pathways are involved in α granule release from human platelets

Alpha granule release plays an important role in propagating a hemostatic response upon platelet activation. We evaluated the ability of various agonists to cause a granule release in platelets. Alpha granule release was measured by determining P-selectin surface expression in aspirin-treated washed platelets. ADP-induced P-selectin expression was inhibited both by MRS 2179 (a P2Y1 selective antagonist) and AR-C69931MX (a P2Y12 selective antagonist), suggesting a role for both Galpha(q) and Galpha(i) pathways in ADP-mediated alpha granule release. Consistent with these observations, the combination of serotonin (a Got, pathway stimulator) and epinephrine (a Galpha(z) pathway stimulator) also caused alpha granule release. Furthermore, U46619-induced P-selectin expression was unaffected by MRS 2179 but was dramatically inhibited by AR-C69931, indicating a dominant role for P2Y12 in U46619-mediated alpha granule release. Additionally, the Galpha(12/13)-stimulating peptide YFLLRNP potentiated of. granule secretion in combination with either ADP or serotonin/epinephrine costimulation but was unable to induce secretion by itself. Finally, costimulation of the G(alphai) and Galpha(12/13) pathways resulted in a significant close-dependent increase in of, granule release. We conclude that ADP-induced alpha granule release in aspirin-treated platelets occurs through costimulation of Galpha(q) and Galpha(i) signaling pathways. The P2Y12 receptor plays an important role in thromboxane A(2)-rnediated alpha granule release, and furthermore activation of Galpha(12/13) and Galpha(q) signaling pathway can cause alpha granule release.