DIFFERENTIAL-EFFECTS OF G-PROTEIN ACTIVATORS ON 5-HYDROXYTRYPTAMINE AND PLATELET-DERIVED GROWTH-FACTOR RELEASE FROM STREPTOLYSIN-O-PERMEABILIZED HUMAN PLATELETS

In this paper we have used streptolysin O (SLO)-permeabilized human pl atelets to examine the G-protein(s) that control Ca2+-independent secr etion from alpha and dense-core granules. As shown for electropermeabi lized platelets, Ca2+ alone stimulated a concentration-dependent incre ase in 5-hydroxytryptamine (5-HT) (dense-core-granule marker) and plat elet-derived growth factor (PDGF) (alpha-granule marker) release from the SLO-permeabilized cells. The EC(50) values for Ca2+-dependent 5-HT and PDGF release were 5 mu M and 10 mu M respectively. Guanosine 5'-[ gamma-thio]triphosphate (GTP[S]) (100 mu M) stimulated Ca2+-independen t release from both alpha and dense-core granules. In contrast, AlF4- had no effect on Ca2+-independent release from either alpha or dense-c ore granules. Neither GTP[S] nor AlF4- appeared to have a significant effect on Ca2+-dependent release from alpha and dense-core granules. G TP[S] can activate both heterotrimeric and low-molecular-mass G-protei ns, whereas AlF4- activates only heterotrimeric G-proteins. Our result s, therefore suggest that secretion in the human platelet is regulated by a small G-protein. Both GTP[S]- and Ca2+-dependent secretion were effected by extending the time between permeabilization with SLO and s timulation of secretion. GTP[S]-stimulated secretion from alpha and de nse-core granules decreased rapidly after permeabilization. In contras t, Ca2+-dependent 5-HT and PDGF release ran clown at a much lower rate . These observations indicate that GTP[S] and Ca2+ act through paralle l pathways to stimulate secretion from SLO-permeabilized platelets.