REACTIVATING AND CHOLINOLYTIC ACTION OF TRIMEDOXIME BROMIDE AT THE NEUROMUSC~ JUNCTION OF WARM-BLOODED ANIMALS R. A. Giniatullin, I. A. Shabunova, E. N. Nikol'skii, and ~. A. Bukharaeva UDC 612.815:616-003.725 The reactivating and cholinolytic action of trimedoxime bromide was evaluated during experiments on rat soleus and diaphragm muscles accoridng to the amplitude and time course of miniature end-plate potentials and currents (MEPP and MEPC respectively). This agent reactivates acetylcholinesterase (ACHE) phosphorylation. The effects of trimedoxime bromide at concentrations of 5-i0-6-5~i0 -~ M following AChE inhibition on the amplitude and duration of MEPP arises from complex inter- action between the reactivating and cholinolytic effects. A separate evaluation of the reactivating effect (once the reactivating agent had been removed) revealed that this action increases throughout the entire range of trimedoxime bromide concentration: complete reactivation Of AChE phosphorylation was observed under the action of 2-5.10 -~ M trimedoxime bromide. Examination of the cholinolytic effect in isolation (with voltage-clamping at the muscle and intact ACHE) showed that blockade of open end-plate ionic channels underlies this effect. Reduction in ~PC amplitude together with retarded (but still exponential) decay of signals were distinguishing features of this blockade, confirming that trimedoxime bromide acts as a "very fast blocker". INTRODUCTION Trimedoxime bromide is counted as one of the most effective agents used for treating intoxication by organic phosphorus compounds which inhibit acetylcholinesterase (ACHE) [2]. Trimedoxime bromide is also used widely in neurophysiological experimentation for investi- gating the role of AChE in synaptic transmission [i, 4]. The mechanism of trimedoxime bro- mide antitoxic action at cholinergic synapses is believed to consist of two main components: reactivation of phosphorylated AChE [2] and cholinolytic action [2, 5, i0]. The ability of reactivating agents to restore this enzymatic activity has been studied using mainly bio- chemical and histochemical techniques [7, 11] but the extent of reactivation of "functional" AChE at the synapse cannot be evaluated in this way. It has been established [3, 9] that the state of synaptic AChE may be assessed from the amplitude and time course of synaptic signals. We thus set out to assess the ability of trimedoxime bromide to reactivate phos- phorylated AChE at the rat neuromuscular junction on the basis of electrophysiological factors. Secondly, we aimed to study the mechanism underlying the cholinolytic action of trime- doxime bromide. This, according to current views, may consist not just of competing for the active cholinoreceptor center (a curare-type action) [6], but also of blocking the ionto- phoretic portion of the receptor--channel complex (or channel-blocking action) [8, 12, 13]. METHODS Experiments were conducted on preparations of soleus and diaphragm muscle isolated from rats. The muscle was placed in a 1.5 cm ~ bath with Ringer--Kreb's solution of the following composition flowing through (mM): NaCI 137.0, KCI 5.0, CaCI2 2.0, MgCI2 1.0, NaHC03 ii.0, NaH2PO~ 1.0, and glucose ii.0. The solution, previously gassed for 1 h with carbagen (95% oxygen, 5% C02), had a pH of 7.3. Investigations were carried out at a temperature of 21.0 • 0.1~ Experiments began 40 min after placing the muscle in an experimental bath. The muscle was kept for 30 min ih a solution containing 1.10 -5 M armine followed by 30 min wash-out of this inhibitor to produce irreversible blockade of AChE activity. Findings from S. V. Kurashov Medical Institute, Ministry of Public Health of the RSFSR, Kazan' Translated from Neirofiziologiya, Vol. 20, No. 3, pp. 351-357, May-June, 1988. Original article submitted April 28, 1987. 256 0090-2977/88/2003-0256512.50 9 1988 Plenum Publishing Corporation