Glucocorticoids rapidly inhibit oxytocin-stimulated adrenocorticotropin release from rat anterior pituitary cells, without modifying intracellular calcium transients.

Glucocorticoid hormones suppress the secretion of ACTH evoked by secretagogues such as CRF and arginine vasopressin. In this study, we investigated the effects of glucocorticoids on ACTH release induced by oxytocin (OT) and on intracellular free calcium ion levels in corticotropes prepared from the adenohypophyses of female Wistar rats. Pulsatile additions of physiological concentration of OT (10 nM) to superfused anterior pituitary cells caused pulsatile ACTH release about 4-fold above basal secretion with similar peak amounts of ACTH during subsequent OT pulses. Exposure of the cells to corticosterone (100 nM) or to a selective glucocorticoid receptor agonist RU 28362 (100 nM) for 30 min suppressed OT-stimulated but not basal ACTH release by approximately 60%. Inhibition gradually disappeared during subsequent pulses of OT in the absence of corticosterone. Pretreatment with the selective antagonist RU 38486 (1 microM) completely blocked the inhibitory effect of corticosterone on OT-induced ACTH secretion. Changes in free cytosolic calcium levels in single cultured pituitary cells were measured using the calcium indicator Fura-2. OT caused calcium transients in corticotropes, which were identified by immunocytochemistry. They responded in a similar manner to a second OT stimulus when preincubated for 30 min with corticosterone (1 microM) or with RU 28362 (1 microM). Our data indicate that glucocorticoids, via glucocorticoid receptors, rapidly inhibit OT-stimulated ACTH secretion by corticotropes without affecting intracellular calcium transients due to OT. Therefore, we conclude that rapid inhibition of ACTH release by glucocorticoids interferes with cellular signal transduction beyond the step of calcium mobilization.