title: Selective enhancement of an A type potassium current by dexamethasone in a corticotroph cell line.
abstract: 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.
authors.affiliation: Department of Pharmacology, University of Edinburgh, UK.
authors.forename: A J
authors.initials: AJ
authors.lastname: Pennington
authors.forename: J S
authors.initials: JS
authors.lastname: Kelly
authors.forename: F A
authors.initials: FA
authors.lastname: Antoni
mesh_terms: D000324:Adrenocorticotropic Hormone / Q000378:metabolism*; D000818:Animals; D002460:Cell Line; D003346:Corticotropin-Releasing Hormone / Q000494:pharmacology; D003907:Dexamethasone / Q000494:pharmacology*; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D051379:Mice; D018408:Patch-Clamp Techniques; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D011691:Puromycin / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D015221:Potassium Channels; D013779:Tetrodotoxin; D011691:Puromycin; D003907:Dexamethasone; D000324:Adrenocorticotropic Hormone; D003346:Corticotropin-Releasing Hormone