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Activation of Torpedo acetylcholine receptors expressed in mouse fibroblasts. Single channel current kinetics reveal distinct agonist binding affinities.
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MedLine Citation:
PMID:  1698917     Owner:  NLM     Status:  MEDLINE    
The experiments described examine single channel currents recorded through Torpedo acetylcholine receptor channels stably expressed by a mouse fibroblast cell line. Closed-duration histograms were constructed from currents elicited by 0.5-300 microM acetylcholine (ACh). The concentration dependence of closed durations is well described by a four-state linear scheme with the addition of open-channel block by ACh. Analysis of closed durations measured at low concentrations gives estimates of the rate of opening of doubly liganded receptors, beta, the rate of dissociation of ACh from doubly liganded receptors, k-2, and the rate of channel closing, alpha. The rate of ACh dissociation from singly liganded receptors, k-1, is then deduced from closed-duration histograms obtained at intermediate ACh concentrations. With k-1, k-2 and beta determined, the rates of ACh association, k+1 and k+2, are estimated from fitting closed-duration histograms obtained over a range of high ACh concentrations. A complete set of rate constants is presented for three experimental conditions: (a) Ca2(+)-free extracellular solution containing 1 mM free Mg2+ at 22 degrees C, (b) Ca2(+)-free solution at 12 degrees C, and (c) extracellular Ca2+ and Mg2+, both at 0.5 mM, at 22 degrees C. For all three conditions the dissociation constant for the first agonist binding site is approximately 100-fold lower than that for the second site. The different affinities are due primarily to different dissociation rates. Both the association and dissociation rates depend strongly on temperature. At 22 degrees C ACh associates at diffusion-limited rates, whereas at 12 degrees C association is 30- to 60-fold slower. Also slowed at 12 degrees C are beta (4-fold), k-2 (3-fold), k-1 (25-fold), and alpha (15-fold). In contrast to the activation rate constants, those for ACh-induced block decrease only twofold between 22 and 12 degrees C. Changing from a Ca2(+)-free to a Ca2(+)-containing extracellular solution does not affect k+1 and k+2, but increases beta (twofold) and decreases k-2, k-1, and alpha (all twofold). Spectral analysis of single channel currents supports the parameter estimates obtained from fitting the open- and closed-duration histograms, and improves resolution of brief channel blockages produced by ACh.
S M Sine; T Claudio; F J Sigworth
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Publication Detail:
Type:  Journal Article; Research Support, U.S. Gov't, P.H.S.    
Journal Detail:
Title:  The Journal of general physiology     Volume:  96     ISSN:  0022-1295     ISO Abbreviation:  J. Gen. Physiol.     Publication Date:  1990 Aug 
Date Detail:
Created Date:  1990-11-15     Completed Date:  1990-11-15     Revised Date:  2008-11-20    
Medline Journal Info:
Nlm Unique ID:  2985110R     Medline TA:  J Gen Physiol     Country:  UNITED STATES    
Other Details:
Languages:  eng     Pagination:  395-437     Citation Subset:  IM    
Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06510.
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MeSH Terms
Acetylcholine / metabolism,  pharmacology
Cell Line
Cells, Cultured
Dose-Response Relationship, Drug
Fibroblasts / drug effects,  physiology,  ultrastructure*
Ion Channels / drug effects,  physiology
Receptors, Cholinergic / genetics,  metabolism,  physiology*
Torpedo / metabolism*
Grant Support
Reg. No./Substance:
0/Ion Channels; 0/Receptors, Cholinergic; 51-84-3/Acetylcholine

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine

Full Text
Journal Information
Journal ID (nlm-ta): J Gen Physiol
ISSN: 0022-1295
ISSN: 1540-7748
Publisher: The Rockefeller University Press
Article Information
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Print publication date: Day: 1 Month: 8 Year: 1990
Volume: 96 Issue: 2
First Page: 395 Last Page: 437
ID: 2228994
Publisher Id: 91011330
PubMed Id: 1698917

Activation of Torpedo acetylcholine receptors expressed in mouse fibroblasts. Single channel current kinetics reveal distinct agonist binding affinities

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