Vol. 100, No. 1,1981 May 15, 1981 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Pages 86-90 ACETYLCHOLINE RECEPTOR INACTIVATION IN TORPEDO CALIFORNICA ELECTROPLAX ME}~RANE VESICLES. DETECTION OF TWO PROCESSES IN THE MILLISECOND AND SECOND TIME REGIONS. Jeffery W. Walker and Mark G. McNamee Department of Biochemistry and Biophysics University of California, Davis, CA 95616 and Elena Pasquale, Derek J. Cash and George P. Hess 270 Clark Hail, Cornell University Ithaca, NY 14853 Received March 16,1981 SUMMARY: The rates of inactivations of the acetylcholine receptor-controlled ion flux were measured with membrane vesicles prepared from the electric organ of Torpedo californica. A flow quench technique with a time resolution of 2 msec was used. The vesicles were preincubated with carbamyleholine for various periods of time and then the rate of 86Rb+ influx was measured. The influx rate decreased progressively with the length of preincubation time. This decrease (inactivation) in influx rate occurred in two different time zones. A fast inactivation process with a half-time of ~300 msec resulted in at least a 60-fold decrease of ion flux rate. The remaining ion flux activity decreased to an undetectable level in a slow inactivation process with a half- time of 6-7 seconds. Previously, only one inactivation process has been observed by ion flux measurements with receptor-rich vesicles from Electrophorus electrieus or T__orpedo species. INTRODUCTION The inactivation (desensitization) of the acetylcholine receptor due to exposure to acetylcholine or carbamylcholine (agonists) was first observed by Katz and Thesleff(1) in electrophysiological experiments with muscle cells. In their experiments the electrical signal due to agonists first increased and then returned to the original value, even with the agonist concentration remaining constant. Later, receptor inactivation was observed in measurements of acetyl- choline receptor-controlled ion flux in membrane vesicles prepared from the ABBREVIATIONS TPS, Torpedo physiological saline; PMSF, Phenylmethylsulfonylfluoride; NEM, N-ethyl maleimide; Carb, carbamylcholine. 0006-291X/81/090086-05501.00/0 Copyright © 1981 by Academic Press, Inc. All rights of reproduction in any forrn reserved. 86