Apparent cooperative effects in acetylcholine receptor-mediated ion flux electroplax membrane preparations. |
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Authors: | G P Hess J P Andrews G E Struve |
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Institution: | Section of Biochemistry 270 Clark Hall Cornell University Ithaca, N.Y. 14853 U.S.A. |
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Abstract: | The kinetics of acetylcholine receptor-mediated flux of 22sodium ions from microsacs has been measured in the presence of activators (carbamylcholine and decamethonium) and an inhibitor (d-tubocurarine) of neural transmission. The dependence of the first-order rate constant, kobs, for 22sodium ion efflux on either decamethonium or carbamylcholine concentration does not exhibit cooperativity. The apparent cooperativity observed by Kasai and Changeux in dose-response curves for 22sodium flux from the same preparation is adequately accounted for by the contribution which efflux from non-excitable microsacs, the main component of the preparation, makes to the measurements. d-Tubocurarine was found to be a non-competitive inhibitor of decamethonium-activated 22sodium efflux. The results of the kinetic measurements are in agreement with equilibrium measurements of the interaction of decamethonium with the same microsac preparation, i.e. adherence to a classic Langmuir binding isotherm and separate binding sites for activators and inhibitors of neural activity. The results indicate a direct relationship between ligand binding and receptor-mediated ion flux. How these two processes contribute to electrophysiological measurements is not apparent. |
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Keywords: | To whom reprint requests and correspondence should be addressed |
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