Characterization of PGE2 inhibition of corticotropin releasing factor-mediated ACTH release

The role of prostaglandin E2 (PGE2) on the mechanism of corticotropin releasing factor (CRF) induced adrenocorticotropin (ACTH) release was studied in primary rat pituitary cell culture. The continuous incubation of pituitary cells with PGE2 inhibited CRF-stimulated ACTH with an ED50 of 1.2 X 10(-9) M PGE2. PGE2, however, did not alter the spontaneous release of ACTH. PGE (10(-8) M) significantly decreased 10(-10) M, 10(-9) M, and 10(-8) M CRF-mediated ACTH release by 42%, 47%, and 31% of total CRF stimulated ACTH release. Time course experiments demonstrated a PGE2-induced inhibition by 20 min of CRF incubation which continued for 3 h. After a 2-h incubation with PGE2, the wash-out of PGE2 from the culture medium just prior to the addition of CRF eliminated the inhibitory activity of PGE2. PGE2 decreased the amount of CRF-stimulated ACTH from cells incubated with cycloheximide (P less than 0.01). The inhibitory activity of PGE2 (10(-8) M) on CRF-stimulated ACTH was unaltered by the addition of 3 mM or 7 mM CaCl2 to the standard culture media (1.6 mM CaCl2). The inhibition of CRF-induced ACTH release by maximal inhibitory concentrations of PGE2 (10(-7) M) and cortisol (5 X 10(-7) M) were not additive. In conclusion, PGE2 may play an important role in modulating pituitary ACTH release. Its inhibitory activity occurs by 20 min of CRF incubation, is in part independent of protein synthesis, requires the continued presence of PGE2, is not reversed with CaCl2, and is not additive with the inhibitory activity of cortisol.