title: Ionic selectivity, saturation, and block in a K+-selective channel from sarcoplasmic reticulum.
abstract: The open-channel conductance properties of a voltage-gated channel from sarcoplasmic reticulum were studied in planar phospholipid membranes. The channel is ideally selective for K+ over Cl- and for K+ over Ca++. In symmetrical 1 M solutions, the single-channel conductance (in pmho) falls in the order: K+ (214) > NH4+ (157) > Rb+ (125) > Na+ (72) > La+ (8.1) > Cs+ (< 3). In neutral bilayers, the channel conductance saturates with ion activity according to a rectangular hyperbolic relation, with half-saturation activities of 54 mM for K+ and 34 mM for Na+. Under symmetrical salt conditions, the K+:Na+ channel conductance ratio increases with salt activity, but the permeability ratio, measured by single-channel bi-ionic potentials, is constant between 20 mM and 2.5 M salt; the permeability ratio is equal to the conductance ratio in the limit of low-salt concentration. The channel conductance varies < 5% in the voltage range -100 to +70 mV. The maximum conductance varies K+ and Na+ is only weakly temperature dependent (delta H++ = 4.6 and 5.3 kcal/mol, respectively), but that of Li+ varies strongly with temperature (delta H++ = 13 kcal/mol). The channel's K+ conductance is blocked asymmetrically by Cs+, and this block is competitive with K+. The results are consistent with an Eyring-type barriers as it permeates the channel. The data conform to Lüger's (1973. Biochem. Biophys. Acta. 311:423-441) predictions for a "pure" single-ion channel.
mesh_terms: D000818:Animals; D002586:Cesium / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism; D066298:In Vitro Techniques; D007473:Ion Channels / Q000502:physiology*; D008051:Lipid Bilayers / Q000378:metabolism*; D008094:Lithium / Q000378:metabolism; D008433:Mathematics; D008564:Membrane Potentials; D011188:Potassium / Q000378:metabolism*; D011817:Rabbits; D012413:Rubidium / Q000378:metabolism; D012519:Sarcoplasmic Reticulum / Q000502:physiology*; D012964:Sodium / Q000378:metabolism
chemical_list: D002712:Chlorides; D007473:Ion Channels; D008051:Lipid Bilayers; D002586:Cesium; D008094:Lithium; D012964:Sodium; D012413:Rubidium; D011188:Potassium