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Fragments of rat anterior pituitary glands incubated in vitro and challenged with either of two ACTH secretagogues were used to investigate the extent to which the acute, biphasic, feedback-like inhibitory effects on hormone secretion exerted by the synthetic glucocorticoid dexamethasone were related to alterations in second messenger responses. Addition of dexamethasone was shown to cause both an immediate inhibition (fast inhibition) of the release of ACTH-like immunoreactivity induced by arginine vasopressin (AVP) or corticotrophin-releasing factor (CRF-41), and also an inhibition that occurred after removal of the steroid and was maximal 90 min after its introduction (early delayed inhibition).

The accumulation of adenosine 3',5'-monophosphate (cAMP) in the tissue was enhanced in a dose-related manner by CRF-41, as was that of phosphate esters of inositol (inositol phosphates) by AVP. The dose–response curve for the effect of CRF-41 on cAMP production was markedly shifted to the right by dexamethasone acting in the time-domain of fast inhibition (i.e. the response was attenuated, but not abolished). Application of the steroid during the same time-period reduced significantly the inositol phosphate response induced by the higher concentration of AVP tested (3000 mmol/l), but had no effect on the action of a lower concentration (30 mmol/l).

In contrast, the cAMP and inositol phosphate dose–response curves to CRF-41 and AVP respectively were unaffected by the glucocorticoid when it was applied at the time which generated early delayed inhibition of ACTH release.

It is concluded that part of the fast inhibitory action of dexamethasone on ACTH secretion in vitro (but not its delayed effects) involves a rapid alteration of second messenger responses to secretagogues, but the precise mechanism underlying this process remains to be established.

Journal of Endocrinology (1989) 122, 545–551

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