Vasoactive intestinal peptide (VIP) stimulates both adenosine 3',5'-cyclic monophosphate (cAMP) accumulation and prolactin release in normal rat pituitary cells in culture. cAMP accumulation is significant (P < 0.01) at VIP concentrations as low as 1 nM and reaches a maximum with 0.1 ,IM. Addition of dexamethasone as early as 15 min before VIP inhibits VIP stimulation of both cAMP production and PRL secretion. The rapid inhibition is dose-dependent: it appears at doses as low as 0.01 pM and is complete at 1 pM dexamethasone. Increasing concentrations of dexamethasone induce a noncompetitive type of inhibition, as shown by the decrease in Vm.,. with no change in the apparent K.for VIP. Cycloheximide (1 mM) counteracts the inhibitory effect of dexamethasone on VIP-induced cAMP production, which suggests the involvement ofa rapid protein synthesis mechanism. Ru-26988, a specific glucocorticoid devoid of any mineralocorticoid activity and which does not bind to intracellular transcortin-like component, also produces an inhibition of VIP-induced cAMP accumulation. Corticosterone also inhibits VIP-induced cAMP production but at concentrations higher than those ofdexamethasone. In contrast, aldosterone, progesterone, estradiol, and testosterone have no effect. These results demonstrate that, in normal rat pituitary cells in culture, glucocorticoids at physiological concentrations rapidly inhibit the cAMP production and prolactin release induced by VIP by acting through specific glucocorticoid receptors.Vasoactive intestinal peptide (VIP) (9,10).In the light of both in vivo and in vitro experiments (6, 11), we suggested that corticosteroids could be involved in the regulation of VIP action on the adenohypophysis. We had previously shown that addition of dexamethasone (Dex) for 2 days to cultures of enriched pituitary rat lactotrophs completely abolished the stimulatory effect ofVIP on PRL secretion (6). Therefore, we have investigated (a) the involvement of the cAMP system in the biological effect of VIP on PRL release, (b) the kinetics and the stoichiometry of the interaction between VIP and corticosteroids, and (c) the mechanism by which Dex inhibits PRL release at the pituitary level.In the present communication, we present evidence that, in normal rat pituitary cells in culture, Dex at concentrations as low as 1 pM rapidly inhibits both the cAMP accumulation and the PRL release induced by VIP, by acting through specific glucocorticoid receptors. MATERIALS AND METHODSIsolation and Cell Culture. Male Wistar rats (45-60 days old) were used. Fifteen to 20 anterior pituitaries were minced in small pieces and dispersed enzymatically by incubation in 15-20 ml of minimal essential medium with Earle's salts containing 0.1% trypsin (1:250) (GIBCO) and 0.1% fatty acid-free bovine serum albumin (Sigma) for 2 hr at 370C (12) at pH 7.5 in a silicone-treated Bellco spinner flask. Cells were filtered though 100-,um-mesh nylon to remove DNA of broken cells and centrifuged at 400 g for 5 min; the pellet was resuspended, w...

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