Effect of Corticotropin-Releasing Factor, Dexamethasone and Actinomycin D on the Release of ACTH from Rat Pituitariesin Vivoandin Vitro

Abstract

The interaction of corticotropinreleasing factor (CRF) preparations, dexamethasone and actinomycin D (Act D) on ACTH release from the rat pituitary was studied in vivo and in vitro. In in vivo studies, ACTH release, as indicated by plasma corticosterone levels, following iv injection of the crude rat pituitary stalk and median eminence (SME) extracts or a purified porcine hypothalamic CRF, was considerably suppressed by pretreatment of the rat with 400 μg/100 g body wt of dexamethasone ip. One hundred μg Act D/100 g body wt injected ip before, but not after, dexamethasone prevented the dexamethasone suppression of the action of these CRF preparations. In in vitro experiments, rat pituitaries released a considerable amount of ACTH into the incubation medium when 0.4–0.8 rat SME was added to the medium. When the pituitaries were preincubated with 2–20 μg/ml of dexamethasone for 2 hr, the amount of ACTH released by SME was greatly reduced. Preincubation with Act D, 0.5–2 μg/ml, also decreased SME-induced ACT...