Heterotrimeric GTP-binding (G) proteins, termed Ge. have a role in the late
stages of the adrenocorticotrophin (ACTH) secretory pathway in the mouse A
tT-20/D16-16 anterior pituitary tumour cell line. The wortmannin sensitivit
y of Ge-controlled mechanisms in AtT-20 cells was investigated to provide i
nformation on the possible mechanisms linking Ge with secretion. Permeabili
sed cells exposed to calcium ions(10(-9) to 10(-3) M), guanosine 5'-O-(3-th
iotriphosphate) (GTP-gamma -S)(10(-8) to 10(-4) M) and mastoparan(10(-8) to
10(-5) M) demonstrated a significant and concentration-dependent stimulati
on of ACTH secretion from non-stimulated levels for all three agents. Coinc
ubation with wortmannin (10(-5) M) significantly inhibited both calcium-ind
ependent and -stimulated secretion. The effect of wortmannin was concentrat
ion-dependent being maximal at 10(-6) M. The study shows that wortmannin in
hibits both calcium-independent and -stimulated secretion from permeabilise
d AtT-20 cells indicating a role for phosphatidylinositol-3 kinase in deter
mining the size of the readily releasable pool of ACTH and/or in mediating
calcium/Ge-evoked secretion from this pool. (C) 2001 Elsevier Science B.V.
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