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ECPB 2013, 61(1): 7–13
Experimental physiology and biochemistry

Influence of thyroxine on neuromuscular transmission condition in skeletal muscle of white rats

TRUSH V.
Abstract

During experiments on 4–5 month old white rats-females by electrophysiological method in situ we studied the dynamics of functional changes of neuromuscular transmission in a forward tibial muscle in a process of chronic daily injection of thyroxine in a dose similar to physiological (10 mkg/kg). In acute experiment on animals under narcosis electromyography method was used to define the latent period of the caused excitement of the muscle (M-response) and its amplitude before muscle’s work (initial values) and after rhythmic work in a range of frequencies from 8 to 100 imp/s with external loading in 20 g. The electric response of the muscle was caused by electric irritation of a small-tibial nerve by superthreshold impulses (voltage 500 mV) of duration of 0,15 ms and of frequency of 4 imp/s.

It has been established, that at the initial stages of thyroxine’s injection (after 10–30 daily injections) the shortening of the latent period of caused muscle’s excitement is observed comparing with the control (p<0,05), that under conditions of electric stimulation of the nerve, that innervates the muscle, testifies in favor of simplification of synaptic transmission. At the same time, in the process of further injection of hormone (after 40–60 its injections) the latent period of M-response of the muscle comes back to the initial level, that testifies about adaptation of the neuromuscular system to long injection of thyroid hormones in a constant dose.

Long work of the muscle of the animals, which were receiving thyroxine, doesn’t affect the latent period of the caused excitement of the muscle, like control rats, that testifies in a favor of absence of synaptic fatigue after long work of the muscle. Therefore, chronic injection of thyroxine in a dose similar to physiological, doesn’t affect stability of the synapse to fatigue’s development and reliability of a neuromuscular transmission. At the initial stages of thyroxine injection (later 10–30 daily injections) is observed the increase of initial amplitude of M-response of the muscle (p<0,05 concerning initial levels), that testifies in favor of initial facilitating action of thyroid hormones on the condition of synaptic transmission and reflects the increase of excitability of muscular fibers. At the same time, throughout the entire period of thyroxine injection in the organism (from 10 days to 2nd months) considerable decrease of M-response’s amplitude after long work of the muscle is observed comparing with the initial level (p<0,05), that is not characteristic for control animals and testifies in favor of fatigue development in muscular fibers.

Thus, chronic injection of thyroxine in a dose similar to physiological, at the initial stages (in the first month of injection) is accompanied with shortening of the latent period of the M-response of the forward tibial muscle and by increasing its initial (before muscle work) amplitude, that reflects simplification of synaptic transmission and increase of excitability of muscular fibers. At the same time, throughout the entire period of thyroxine’s injection (from 10 days to 2nd months) the considerable decrease of amplitude of M-response of the muscle after its long work is observed in comparison with the initial level, which is not characteristic for control animals and testifies in favor of fatigue development in muscular fibers and increased their fatigue.

Keywords: thyroxine, skeletal muscle, a neuromuscular transmission, the caused excitement of a muscle

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