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 turnout 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 muM - 100 muM), guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S) (1-100 muM) and mastoparan (1 and 10 muM) 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 muM) but not prostaglandin F-2alpha (PGF(2alpha)) (10 muM) 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 muM)- calcium (10 muM)- and mastoparan (10 muM)-evoked secretion were concentration-dependent. PGE(1) significantly inhibited GTP-gamma-S- and calcium-evoked secretion at concentrations of PGE(1) above 1 muM but mastoparan-evoked secretion only at the highest concentration of PGE1 investigated (10 muM). PGE(2) was much more potent than PGE1 and significantly inhibited GTP-gamma-S- and calcium-evoked secretion at 10 nM and above and mastoparan-evoked secretion above 1 muM.

4 The inhibitory effects of PGE1 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(2alpha) (1 nM - 10 muM) acting singly had little or no effect upon ACTH secretion. However, only PGE(2) (1 nM - 10 muM) 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.