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, adrenocorticotropic hormone release is triggered by corticotropin-releasing hormone and is attenuated by the synthetic adrenal steroid dexamethasone. The precise mechanisms by which dexamethasone inhibits secretion are under investigation. We examined whether dexamethasone can modulate release via regulation of calcium homeostasis. More specifically, we have evaluated the effects of dexamethasone on calcium current, intracellular calcium concentration, and adrenocorticotropic hormone release. Using perforated patch-clamp and calcium imaging with fura PE3/AM, we found that dexamethasone decreases calcium current and intracellular calcium levels. The inhibition of current by dexamethasone is not, however, altered by the calcium channel antagonists nifedipine (L-type) or omega-agatoxin IVA (P/Q-type), despite the presence of these calcium channel subtypes in AtT-20 cells and the exclusive coupling of 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) incubation with dexamethasone inhibits corticotropin-induced release by approximately 40%, a rapid (10 min) incubation (a time interval sufficient for dexamethasone-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 responsible for its inhibition of secretion.