Reversal of Dexamethasone Inhibition of Adrenocorticotropin Release in a Mouse Pituitary Tumor Cell Line Either by Growing Cells in the Absence of Dexamethasone or by Addition of Hypothalamic Extract*

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Abstract
Release of ACTH by a mouse pituitary tumor cell line (AtT-20/D-16v) is inhibited by 10-8-10-6 M 9.alpha.-fluoro-16.alpha.-methyl-11.beta.,17.alpha.,21-trihydroxy-1,4-pregnadiene-3,20-dione (dexamethasone). Dexamethasone does not inhibit cell growth in this concentration range. Cortisol and corticosterone are almost as potent as dexamethasone in inhibiting ACTH release, whereas 17.beta.-estradiol and testosterone have no effect. In rapidly growing cultures of tumor cells removal of dexamethasone leads to complete reversal of the inhibitory effect of the steroid on ACTH accumulation in culture medium within 4-5 days (3.5-4 generation times). The extent of reversal of the dexamethasone effect in slowly growing cultures (generation time 96-150 h) and rapidly growing cultures (24-30 h) is proportional to the amount of growth that takes place in the absence of dexamethasone. Addition of hypothalamic extract to dexamethasone-treated cultures and untreated cultures stimulates the release of ACTH 4- to 8-fold. The response occurs within 15 min after addition of the extract and is dependent on the dose of extract.