Add time:08/28/2019         Source:sciencedirect.com

The effects of the myotonic agent, 9-anthroic acid (ANCA), and the acetylcholinesterase (AChE) inhibitor, soman, on the isolated phrenic nerve-diaphragm preparation of the rat have been studied. ANCA induced after-contractions which followed the twitches evoked by either direct or indirect stimulation. In the intermittently stimulated muscle the height of the after-contractions decreased rather rapidly and in an oscillatory fashion. The maximum height of each after-contraction was attained after the twitch had reached its peak. AChE inhibition changed the shape of these after-contractions in the indirectly, but not in the directly stimulated diaphragm. After AChE inhibition, the after-contractions decreased more slowly and in a non-oscillatory manner. Similar phenomena were observed in vivo in the gastrocnemius-soleus muscles.The effects of ANCA in the AChE-inhibited and in the non-inhibited diaphragm could be mimicked by incubation in low chloride media. Addition of ouabain to the non-inhibited diaphragm treated with ANCA, caused an immediate and striking enhancement followed by a rapid loss of the after-contractions, whereas the twitches remained fairly constant. In the AChE-inhibited diaphragm treated with ANCA, ouabain caused no increase but only a rapid and complete decay of the after-contractions with a decrease of the twitches.It is suggested that after inhibition of the AChE in the ANCA-treated muscle fibre, the site of initiation of the repetitive action potentials which cause the after-contractions shifts from the transverse tubular system to the motor end-plate. Moreover, it is suggested that the sodium pump is involved in the gradual decay of the myotonic action of ANCA.

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