Presynaptic Effect of the Aziridinium Ion of Acetylcholine Mustard (Methyl-2-acetoxyethyl-2′-chloroethylamine) on the Phrenic Nerve – Rat Diaphragm Preparation

Abstract

The rat diaphragm has been used to investigate the neuromuscular blocking action of acetylcholine mustard which yields a potent nicotinic agonist, an aziridinium ion, in aqueous medium. Evidence was obtained that the acetylcholine mustard aziridinium ion impaired neuromuscular activity when the phrenic nerve was stimulated and that the ion did not directly inhibit muscle contraction. Impairment of neuromuscular activity was characterized by a latent period and depended both on the concentration of aziridinium ion and the frequency of stimulation of the phrenic nerve. Elevated concentrations of Ca²⁺ and choline changed the response of the rat diaphragm to the aziridinium ion, the former increasing the rate of development of neuromuscular block and the latter protecting against neuromuscular block. These results indicated that the aziridinium ion may act either at the site of choline uptake or have an effect on acetylcholine synthesis in the nerve ending and that impairment of neuromuscular transmission in the rat diaphragm involved the availability of acetylcholine. Similar results were obtained with acetylcholine mustard aziridinium ion subjected to alkaline hydrolysis. This substance is thought to be choline mustard aziridinium ion. Although difficult to prove with the rat diaphragm it is possible that acetylcholinesterase of this preparation could hydrolyze acetylcholine mustard aziridinium ion at the neurotransmitter site and the resultant choline mustard aziridinium ion would interfere with the uptake of choline and eventually prevent neuromuscular transmission. This hemicholinium-like hypothesis for the mechanism of action of choline mustard aziridinium ion is compatible with reported data for toxicity of acetylcholine mustard aziridinium ion in the mouse.