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

Abstract

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 μM), guanosine-5′-O-(3-thiotriphosphate) (GTP-γ-S) (1-100 μM) and mastoparan (1 and 10 μM) all stimulated ACTH secretion from permeabilized AtT-20 cells in a concentration-dependent manner, GTP-γ-S and mastoparan stimulated ACTH secretion from permeabilized cells in the absence of calcium. Co-incubation with prostaglandins E1 and E2 (PGE1, PGE2) (10 μM) but not prostaglandin F2α (PGF2α) (10 μM) significantly inhibited calcium-, GTP-γ-S and mastoparan-evoked secretion by 30-50%. 3. The effects of PGE1 and PGE2 upon GTP-γ-S (100 μM)-, calcium (10 μM)- and mastoparan (10 μM)-evoked secretion were concentration-dependent. PGE1 significantly inhibited GTP-γ-S- and calcium-evoked secretion at concentrations of PGE1 above 1 μM but mastoparan-evoked secretion only at the highest concentration of PGE1 investigated (10 μM). PGE2 was much more potent than PGE1 and significantly inhibited GTP-γ-S- and calcium-evoked secretion at 10 nM and above and mastoparan-evoked secretion above 1 μM. 4. The inhibitory effects of PGE1 and PGE2 upon calcium-, GTP-γ-S- and mastoparan-stimulated ACTH secretion from permeabilized cells were pertussis toxin (PTX) sensitive. 5. In intact cells PGE1, PGE2 and PGF2α (1 nM-10 μM) acting singly had little or no effect upon ACTH secretion. However, only PGE2 (1 nM-10 μM) 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.