Glucocorticoids stimulate the activity of large-conductance Ca 2+-activated K + channels in pituitary GH 3 and AtT-20 cells via a non-genomic mechanism

Abstract

The effects of glucocorticoids on ion currents were investigated in pituitary GH 3 and AtT-20 cells. In whole-cell configuration, dexamethasone, a synthetic glucocorticoid, reversibly increased the density of Ca 2+-activated K + current ( I K(Ca)) with an EC 50 value of 21 ± 5 μM. Dexamethasone-induced increase in I K(Ca) density was suppressed by paxilline (1 μM), yet not by glibenclamide (10 μM), pandinotoxin −Kα (1 μM) or mifepristone (10 μM). Paxilline is a blocker of large-conductance Ca 2+-activated K + (BK Ca) channels, while glibenclamide and pandinotoxin-Kα are blockers of ATP-sensitive and A-type K + channels, respectively. Mifepristone can block cytosolic glucocorticoid receptors. In inside-out configuration, the application of dexamethasone (30 μM) into the intracellular surface caused no change in single-channel conductance; however, it did increase BK Ca-channel activity. Its effect was associated with a negative shift of the activation curve. However, no Ca 2+-sensitiviy of these channels was altered by dexamethasone. Dexamethasone-stimulated channel activity involves an increase in mean open time and a decrease in mean closed time. Under current-clamp configuration, dexamethasone decreased the firing frequency of action potentials. In pituitary AtT-20 cells, dexamethasone (30 μM) also increased BK Ca-channel activity. Dexamethasone-mediated stimulation of I K(Ca) presented here that is likely pharmacological, seems to be not linked to a genomic mechanism. The non-genomic, channel-stimulating properties of dexamethasone may partly contribute to the underlying mechanisms by which glucocorticoids affect neuroendocrine function.