A K+-selective,three-state channel from fragmented sarcoplasmic reticulum of frog leg muscle |
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Authors: | Pedro P. Labarca Christopher Miller |
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Affiliation: | (1) Graduate Program in Biophysics and Graduate Department of Biochemistry, Brandeis University, 02254 Waltham, Massachusetts;(2) Present address: Department of Physics, University of California at San Diego, 92093 La Jolla, Calif. |
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Abstract: | Summary Sarcoplasmic reticulum (SR) vesicles from frog leg muscle were fused with a planar phospholipid bilayer by a method described previously for rabbit SR. As a result of the fusion, K+-selective conduction channels are inserted into the bilayer. Unlike the two-state rabbit channel, the frog channel displays three states: a nonconducting (closed) state and two conducting states and . In 0.1m K+ the single-channel conductances are 50 and 150 pS for and , respectively. The probabilities of appearearance of the three states are voltage-dependent, and transitions between the closed and states proceed through the state. Both open states follow a quantitatively identical selectivity sequence in channel conductance: K+>NH4+>Rb+>Na+>Li+>Cs+. Both open states are blocked by Cs+ asymmetrically in a voltage-dependent manner. The zero-voltage dissociation constant for blocking is the same for both open states, but the voltage-dependences of the Cs+ block for the two states differ in a way suggesting that the Cs+ blocking site is located more deeply inside the membrane in the than in the state. |
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Keywords: | Sarcoplasmic reticulum K-channel, planar bilayer ion selectivity Cs-block excitation-contraction coupling |
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