THE ROLE OF INORGANIC IONS IN ION EXCHANGE PROCESSES AT THE CHOLINERGIC RECEPTOR OF VOLUNTARY MUSCLE

Abstract

Evidence is presented that time cholinergic receptor of voluntary muscle is normally occupied by calcium or magnesium ions and that quaternary ammonium agonists react with the receptors by ion exchange. Two carbachol ions exchange for one bivalet inorganic cation and receptors are located in pairs that can be bridged by calcium or magnesium ions. The calcium-magnesium selectivity constant at the receptor has been estimated to be 0.25 with a higher affinity for magnesium. In the case of time decamethonium ion (C1O) one end combines with a receptor and the ends of two C1O ions are needed to displace calcium or magimesium from a pair of negatively charged receptors. The other ends of the C1O ions react with acceptors where they exchange almost exclusively for calcium ions. A geimeral thermodynamically based ternary ion exchange equation has been derived to relate the concentration of agonist to that of antagonist at constant depolarization in the presence of a constant concentration of inorganic counter ion. Time quaintitative agreement between experiment and this equation and its modifications has been examined over a wide range of drug concentrations.