A direct inhibitory action of prostaglandins upon ACTH secretion at the late stages of the secretory pathway of AtT-20 cells.

1. The mouse AtT-20/D16-16 anterior pituitary tumour cell line was used as a model system for the study of the effects of prostaglandins upon the late stages of the adrenocorticotrophin (ACTH) secretory pathway. 2. Calcium (1 nM - 100 microM), guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S) (1 - 100 microM) and mastoparan (1 and 10 microM) all stimulated ACTH secretion from permeabilized AtT-20 cells in a concentration-dependent manner. GTP-gamma-S and mastoparan stimulated ACTH secretion from permeabilized cells in the absence of calcium. Co-incubation with prostaglandins E(1) and E(2) (PGE(1), PGE(2)) (10 microM) but not prostaglandin F(2 alpha) (PGF(2 alpha)) (10 microM) significantly inhibited calcium-, GTP-gamma-S and mastoparan-evoked secretion by 30 - 50%. 3. The effects of PGE(1) and PGE(2) upon GTP-gamma-S (100 microM)-, calcium (10 microM)- and mastoparan (10 microM)-evoked secretion were concentration-dependent. PGE(1) significantly inhibited GTP-gamma-S- and calcium-evoked secretion at concentrations of PGE(1) above 1 microM but mastoparan-evoked secretion only at the highest concentration of PGE(1) investigated (10 microM). PGE(2) was much more potent than PGE(1) and significantly inhibited GTP-gamma-S- and calcium-evoked secretion at 10 nM and above and mastoparan-evoked secretion above 1 microM. 4. The inhibitory effects of PGE(1) and PGE(2) upon calcium-, GTP-gamma-S- and mastoparan-stimulated ACTH secretion from permeabilized cells were pertussis toxin (PTX) sensitive. 5. In intact cells PGE(1), PGE(2) and PGF(2 alpha) (1 nM - 10 microM) acting singly had little or no effect upon ACTH secretion. However, only PGE(2) (1 nM - 10 microM) significantly inhibited corticotrophin-releasing factor-41 (CRF-41) (100 nM)-evoked secretion in a concentration dependent manner. 6. The present study finds that prostaglandins of the E series exert an inhibitory action, via a pertussis toxin-sensitive GTP-binding (G)-protein, in the late stages of the ACTH secretory pathway distal to the G-exocytosis (Ge)/calcium point of control.