Acetylcholine-receptor-mediated ion flux in electroplax membrane preparations.

The kinetics of acetylcholine-receptor-mediated sodium efflux from electroplax microsacs of electrophorus electricus has been analyzed. This led to the discovery that only a small fraction of the observed efflux is affected by chemical effectors such as carbamylcholine. Experimental conditions were chosen so that the receptor-mediated flux could be analyzed without the measurements' being obscured by efflux from the nonexcitable microsacs. Near equilibrium the efflux follows a single exponential decay. The apparent first order rate constant for sodium-22 efflux was determined as a function of effector concentration and is considerably higher than previously estimated. The process does not show cooperativity under the experimental conditions, in agreement with the binding isotherms of effectors and the same membrane preparation. The presence of potassium ions inhibits the receptor-mediated sodium flux. It is suggested that interaction of inorganic ions with the receptor may play an important role in the cooperative effects observed in electrophysiological experiments.