Dexamethasone-mediated inhibition of calcium transients and ACTH release in a pituitary cell line (AtT-20)

In the corticotroph-like murine pituitary tumor cell line, AtT-20, adrenoco rticotropic hormone release is triggered by corticotropin-releasing hormone and is attenuated by the synthetic adrenal steroid dexamethasone. The prec ise mechanisms by which dexamethasone inhibits secretion are under investig ation We examined whether dexamethasone can modulate release via regulation of calcium homeostasis. More specifically, we have evaluated the effects o f dexamethasone on calcium current, intracellular calcium concentration, an d adrenocorticotropic hormone release. Using perforated patch-clamp and cal cium imaging with fura PE3/AM, we found that dexamethasone decreases calciu m current and intracellular calcium levels. The inhibition of current by de xamethasone is not, however, altered by the calcium channel antagonists nif edipine (L-type) or omega-agatoxin TVA (P/Q-type), despite the presence of these calcium channel subtypes in AtT-20 cells and the exclusive coupling o f adrenocorticotropic hormone release to the L-type channel in these cells. We also evaluated the temporal relationship between dexamethasone-mediated inhibition of secretion and calcium influx. Whereas a prolonged (2 h) incu bation with dexamethasone inhibits corticotropin-induced release by similar to 40%, a rapid (10 min) incubation (a time interval sufficient for dexame thasone-mediated inhibition of calcium transients) does not inhibit release . These data suggest, therefore, that dexamethasone does, indeed, modulate calcium homeostasis in AtT-20 cells, but that this effect is not responsibl e for its inhibition of secretion. (C) 1999 Published by Elsevier Science I nc. All rights reserved.