title: A maxi calcium-activated potassium channel from chick lens epithelium.
abstract: The apical membrane of embryonic chick lens epithelium contains at high density, a large conductance K+ channel whose open probability is increased by Ca++ at the inner surface of the membrane and by depolarization. The conductance of the channel when it is fully open in symmetrical 150 mM K+ solutions is 214 +/- 3 pS (mean +/- std. error). The current through the channel is a function of the K+ concentration. Gating (open probability) at positive transmembrane voltages increases as the internal [Ca++] is raised above 10(-7) M. The open probability decreases monotonically as the transmembrane voltage is made more negative. The channel is at least 87 times more permeable to K+ than to Na+ or Li+ and shows appreciable permeability to Rb+ and NH4+. It has at least three subconductance levels amounting to approximately 3/4, 1/2, and 1/4 the fully open unitary conductance. The occurrence of these subconductance levels is highly variable from one patch to another. The channel is blocked by physiological levels of internal Na+ but not over a physiological voltage range. This block is partially overcome by elevated external K+. This K+ channel from chick lens epithelium is blocked by a number of compounds known to block BK channels in other tissues. Here we show that decamethonium and Ba++ are effective blockers when added to the inner bathing solution at concentrations greater than .1 mM. Tetraethylammonium, Cs+, quinine, quinidine and Ba++ are all effective blockers when applied to the outer side of the channel in the .1 mM - 5 mM range. With the exception of internal Ba++, all of these compounds produce a fast flicker-type blockade. We use a one-site model to quantify the blockade caused by these flicker producing agents. The voltage dependence of the blockade by Cs+ suggests that this channel probably allows multiple occupancy.
authors: Department of Physiology and Biophysics, Mayo Foundation, Rochester, MN.
authors: J L
authors: JL
authors: Rae
authors: J
authors: J
authors: Dewey
authors: J S
authors: JS
authors: Rae
authors: K
authors: K
authors: Cooper
mesh_terms: D000818:Animals; D001464:Barium / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D002463:Cell Membrane Permeability / Q000187:drug effects; D002586:Cesium / Q000494:pharmacology; D002642:Chick Embryo; D002645:Chickens; D003650:Decamethonium Compounds / Q000494:pharmacology; D004848:Epithelium / Q000378:metabolism; D007908:Lens, Crystalline / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D011802:Quinidine / Q000494:pharmacology; D011803:Quinine / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
chemical_list: D003650:Decamethonium Compounds; D015221:Potassium Channels; D013757:Tetraethylammonium Compounds; D002586:Cesium; D001464:Barium; D012964:Sodium; D011803:Quinine; C033019:decamethonium; D011802:Quinidine; D002118:Calcium