Effects of wortmannin upon the late stages of the secretory pathway of AtT-20 cells.

Heterotrimeric GTP-binding (G) proteins, termed Ge, have a role in the late stages of the adrenocorticotrophin (ACTH) secretory pathway in the mouse AtT-20/D16-16 anterior pituitary tumour cell line. The wortmannin sensitivity of Ge-controlled mechanisms in AtT-20 cells was investigated to provide information on the possible mechanisms linking Ge with secretion. Permeabilised cells exposed to calcium ions (10(-9) to 10(-3) M), guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) (10(-8) to 10(-4) M) and mastoparan (10(-8) to 10(-5) M) demonstrated a significant and concentration-dependent stimulation of ACTH secretion from non-stimulated levels for all three agents. Coincubation with wortmannin (10(-5) M) significantly inhibited both calcium-independent and -stimulated secretion. The effect of wortmannin was concentration-dependent being maximal at 10(-6) M. The study shows that wortmannin inhibits both calcium-independent and -stimulated secretion from permeabilised AtT-20 cells indicating a role for phosphatidylinositol-3 kinase in determining the size of the readily releasable pool of ACTH and/or in mediating calcium/Ge-evoked secretion from this pool.