Acetylcholine receptor: evidence for a voltage-dependent regulatory site for acetylcholine. Chemical kinetic measurements in membrane vesicles using a voltage clamp

Abstract

Acetylcholine receptor mediated ion translocation in membrane vesicles prepared from the Electrophorus electricus electroplax was investigated under voltage clamp conditions by using a quench-flow technique that allows the translocation to be measured in the millisecond to second time region. Two rate coefficients were measured over a 500-fold concentration range of acetylcholine, at a transmembrane voltage, Vm, of -45 mV, at pH 7.0, 1 degrees C. JA is the rate coefficient for ion translocation by the active state of the receptor in the absence of inactivation (desensitization), and alpha is the rate coefficient for the inactivation of the receptor by acetylcholine. (1) The values of JA and alpha increase with increasing acetylcholine concentration up to 300 microM. At higher concentrations, a concentration-dependent decrease in the ion flux rate was observed without a concomitant change in the inactivation rate. This inhibitory effect has not been reported previously and was not observed with acetylcholine or carbamoylcholine in the absence of a transmembrane voltage. (2) The value of the maximum influx rate coefficient, 26 s-1, is approximately twice that observed at 0 mV [JA(max) = 15 s-1]. This is consistent with previous interpretations that related JA(max) values to the channel-opening equilibrium constant, 1/phi, and with the relation of 1/phi to the mean lifetime of the open receptor channel in muscle cells, which is dependent on Vm. (3) The maximum observed inactivation rate coefficient [alpha(max) = 8.5 s-1] is somewhat larger than that observed at 0 mV [alpha(max) = 5 s-1].(ABSTRACT TRUNCATED AT 250 WORDS)