α‐granule secretion from α‐toxin permeabilized, MgATP‐exposed platelets is induced independently by H and Ca2

In order to better understand granule release from platelets, we developed an α-toxin permeabilized platelet model to study α-granule secretion. Secretion of α-granules was analyzed by flow cytometry using P-selectin as a marker for α-granule release. P-selectin surface expression occurred when platelets were permeabilized in the presence of Ca2+. Responsiveness to Ca2+ was lost 30 min after permeabilization but could be reconstituted with MgATP. α-toxin–permeabilized, MgATP-exposed platelets also degranulated within a pH range of 5.4–5.9 without exposure to and independent of Ca2+. ATP, GTP, CTP, UTP, and ITP supported Ca2+-induced α-granule secretion, while H+-induced α-granule secretion occurred only with ATP and GTP. Both Ca2+- and H+-induced α-granule secretion required ATP hydrolysis. Kinase inhibitors blocked both Ca2+- and H+-induced secretion. These data suggest that α-granule secretion in this permeabilized platelet system shares many characteristics with granule secretion studied in other permeabilized cell models. Furthermore, these results show that H+ can trigger α-granule release independent of Ca2+. J. Cell. Physiol. 179:1–10, 1999. © 1999 Wiley-Liss, Inc.