Rates and equilibria at the acetylcholine receptor of Electrophorus electroplaques: A study of neurally evoked postsynaptic currents and of voltage-jump relaxations

Abstract

Kinetic measurements are employed to reconstruct the steady-state activation of acetylcholine [ACh] receptor channels in Electrophorus electropiaques. Neurally evoked postsynaptic currents (PSCs) decay exponentially; at 15°C the rate constant, α, equals 1.2 ms-1 at 0 mV and decreases e-fold for every 86 mV as the membrane voltage is made more negative. Voltage-jump relaxations have been measured with bath-applied ACh, decamethonium, carbachol. or suberyldicholine. We interpret the reciprocal relaxation time 1/τ as the sum of the rate constant a for channel closing and a first-order rate constant for channel opening. Where measurable, the opening rate increases linearly with [agonist] and does not vary with voltage. The voltage sensitivity of small steady-state conductances (e-fold for 86 mV) equals that of the closing rate α. confirming that the opening rate has litde or no additional voltage sensitivity. Exposure to α-bungarotoxin irreversibly decreases the agonist-induced conductance but does not affect the relaxation kinetics. Tubocurarine reversibly reduces both the conductance and the opening rate. In the simultaneous presence of two agonist species, voltage-jump relaxations have at least two exponential components. The data are fit by a model in which (a) the channel opens as the receptor binds the second in a sequence of two agonist molecules, with a forward rate constant to 107 to 2 × 108 M-1 s-1; and (b) the channel then closes as either agonist molecule dissociates, with a voltage-dependent rate constant of 102 to 3 × 103 s-1. © 1977, Rockefeller University Press., All rights reserved.