GLUCOCORTICOID NEGATIVE FEEDBACK IN SHEEP CORTICOTROPHS - A COMPARISON WITH ATT-20 CORTICOTROPH TUMOR-CELLS

Early glucocorticoid feedback in sheep anterior pituitary (AP) cells was compared and contrasted with that in mouse pituitary tumor AtT-20 cells. Dexamethasone (DEX) inhibited corticotropin-releasing hormone (CRH)-stimulated adrenocorticotropin (ACTH) release in a concentration- and time-dependent manner with similar potency amongst cell types. This inhibition was mediated through type II glucocorticoid receptors and required the synthesis of new protein. However, stimulation of protein kinase C with phorbol 12-myristate 13-acetate (PMA) resulted in greater ACTH release and greater inhibition by DEX in sheep AP cells. In contrast to sheep AP cells, AtT-20 cells were insensitive to glucocorticoids when secretion was stimulated by KCl depolarization or the voltage-dependent calcium channel agonist, maitotoxin (MTX). In both cell types, CRH-, KCl-, and MTX-stimulated ACTH release was inhibited by the calcium channel blocker, nifedipine (NIF). Whereas NIF also inhibited PMA-induced ACTH secretion in AtT-20 cells, it did not in sheep AP cells. These data demonstrate that early glucocorticoid feedback is operative in sheep corticotrophs and that AtT-20 cells appear to serve as an appropriate mechanistic model for aspects of negative feedback when the CRH-protein kinase A pathway is activated but may not be appropriate when ACTH secretion is activated via other intracellular signaling pathways.