The K+ channel of sarcoplasmic reticulum. A new look at Cs+ block.
K+-selective ion channels from mammalian sarcoplasmic reticulum were inserted into planar phospholipid bilayers, and single-channel currents measured in solutions containing Cs+. Current through this channel can be observed in symmetrical solutions containing only Cs+ salts. At zero voltage, the Cs+ conductance is approximately 15-fold lower than the corresponding K+ conductance. The open channel rectifies strongly in symmetrical Cs+ solutions, and the Cs+ currents are independent of Cs+ concentration in the range 18-600 mM. Biionic (Cs+/K+) reversal potentials are only 10 mV, showing that Cs+ is nearly as permeant as K+, though much less conductive. Addition of Cs+ to symmetrical K+ solutions reduces current through the channel in a voltage-dependent way. The results can be explained by a free energy profile in which the channel's selectivity filter acts in two ways: to provide binding sites for the conducting ions and to serve as a major rate-determining structure. According to this picture, the main difference between high-conductance K+ and low-conductance Cs+ is that Cs+ binds to an asymmetrically positioned site approximately 20-fold more tightly than does K+.
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Cukierman
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Yellen
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Miller
D000818:Animals; D002586:Cesium / Q000494:pharmacology*; D004553:Electric Conductivity; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D007700:Kinetics; D008051:Lipid Bilayers; D008433:Mathematics; D008954:Models, Biological; D009132:Muscles / Q000502:physiology; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D011188:Potassium / Q000378:metabolism*; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000502:physiology*
D007473:Ion Channels; D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D002586:Cesium; D011188:Potassium