Mechanism of the differential sensitivity in the rat adductor and abductor laryngeal muscles to a non-depolarizing neuromuscular blocker.

We have compared the neuromuscular blocking effects of tubocurarine at pre- and postsynaptic sites in the lateral cricoarytenoid muscle (LCA) (one of the adductor muscles of the vocal cords) and in the posterior cricoarytenoid muscle (PCA) (sole abductor muscle of the vocal cords) of the rat. Fine wire electrodes were inserted into both muscles and evoked compound electromyographic (EMG) responses measured by supramaximal stimulation of the recurrent laryngeal nerves. End-plate potentials (EPP), mean quantal content and carbachol sensitivity to tubocurarine in both muscles were measured using intracellular microelectrodes. Tubocurarine produced a concentration-dependent reduction in the EMG responses, EPP amplitude, mean quantal content and carbachol sensitivity. The LCA muscle was more resistant to tubocurarine than the PCA muscle in EPP amplitude, mean quantal content and carbachol sensitivity, suggesting unequal pre- and postsynaptic sensitivity for inhibition of elicited acetylcholine release, reduction in EPP amplitude and loss of evoked muscle action potentials. Examination of muscle fibre composition demonstrated that the LCA muscle contained a significantly higher fraction of slow twitch muscle fibres than PCA muscle. However, the sizes of the fibres were similar in both muscles. We conclude that the mechanism of unequal sensitivity to a non-depolarizing neuromuscular blocker in the LCA and PCA muscles may be explained by differential sensitivities at the pre- and postsynaptic sites of the neuromuscular junction.