Selective enhancement of an A type potassium current by dexamethasone in a corticotroph cell line

Perforated patch recording was used to examine the effect of the synthetic steroid dexamethasone on the whole cell potassium (K+) current, in the mouse corticotroph tumour cell line AtT20/D16-16. In 15 out of 52 control cells (29%) there was a rapidly-activating, rapidly-inactivating K+ current of the A type, the amplitude of which was strongly dependent on the holding potential in use prior to its activation by depolarising voltage pulses, and which was blocked by 1 mM 4-aminopyridine (4-AP, n = 5). The effect of dexamethasone (100 nM, 2 h, 37 degrees C) was that the A current increased in prevalence (24 out of 31 cells, 77%), lost its dependence on holding potential (over the range studied), and as a result became significantly larger than in controls, for certain voltage steps (peak A current density was 18.5 +/- 2.4 pA/pF (n = 12) for control cells and 26.3 +/- 3.9 pA/pF (n = 18) for dexamethasone treated cells, for a step to +30 mV from -60 mV, values are mean +/- SEM). All cells exhibited a slowly-activating, sustained K+ current, which was unaffected by changes in the holding potential, unaffected by 4-AP and consisted of at least 3 components: one blocked by 30 mM tetraethylammonium(TEA) or 100 nM charybdotoxin (CTX); a second blocked by 100 nM apamin; and a third not blocked by TEA, CTX, apamin, clofilium (100 nM) or niflumic acid (0.1 mM). Dexamethasone produced no change in the slowly-activating, sustained current nor in any of its individual components. The effect of dexamethasone on the A current was completely blocked by 0.1 mM puromycin, a protein synthesis blocker, while puromycin alone did not affect the size or frequency of the A current, nor alter the slowly-activating, sustained current. Secretion studies using 4-AP confirmed that the A current has a role in stimulated adrenocorticotrophic hormone (ACTH) secretion. In summary, AtT20 cells contain at least four types of K+ current: an A current and 3 currents contributing to the slowly-activating current. Selective enhancement of the A current by dexamethasone, shown here to require synthesis of new protein, is one of the mechanisms whereby glucocorticoids exert inhibitory control on ACTH secretion.