INFLUENCE OF d-TUBOCURARINE, DECAMETHONIUM AND SUCCINYLOHOLINE ON REPETITIVELY EVOKED END-PLATE POTENTIALS

¹ Department of Physiology and Pharmacology, School of Veterinary Medicine, University of Georgis, Athens, Georgia

The mode and site of the depressant action of d-tubocurarine (d-TC). decamethonium (C₁₀) and succinylcholine (SCh) on the end-plate potential (EPP) amplitude of magnesium-blocked frog sartorius nerve-muscle preparations were investigated. Trains of 110 intracellularly recorded EPP's evoked by repetitive indirect stimulation (40/sec) were analyzed to determine quantal unit size (i.e., potential change per quantal unit) and quantum content before and after exposure of the preparation to blocking agents for 30 to 60 min. Since quantal unit size is believed to be determined postjunctionally and since quantum content is believed to be determined prejunctionally based on the quantal theory of neuromuscular transmission, drug effects at each of these sites could be evaluated simultaneously with this technique. An increase in EPP amplitude or facilitation occurred during repetitive stimulation regardless of whether drugs were present. d-TC, C₁₀ and SCh did not appear to alter the time course of facilitation. Quantal unit size and mean EPP amplitude were similarly reduced in magnitude by d-TC (0.7-1.1 µM), C₁₀ (72-14 µM) and SCh (5.0 µM). Quantum content was not significantly altered by these drugs. These results indicate that the depressant action of d-TC, C₁₀ and SCh on EPP amplitude was due to a reduction of the potential change produced by a quantal unit and are consistent with a postjunctional site of action for these drugs.