Myotonia in the rat diaphragm preparation caused by the sulfhydryl inhibiting para-substituted mercuribenzoates.

The sulfhydryl (SH) inhibiting para-substituted mercuribenzoates like pOHMB caused a myotonia which appeared as a smooth myotonic profile during recording of the response to twitch stimulation (0.1/s) of isolated rat diaphragm preparations. The maximum myotonic tension varied from about 10% to 200% of the twitch tension at pOHMB addition, and the myotonic repetitive action potential activity varied from a few to more than one hundred action potentials in different cells of the same preparation. The myotonia did not appear after pretreatment with the SH-reducing agent dithiothreitol, and both dithiothreitol and N-ethyl-maleimide inhibited the motonia. The myotonia increased with temperature, appearing at about 32 degrees C. Increased twitch frequency and tetanic stimulation decreased the myotonia. No change of threshold was observed in myotonic preparations. The myotonia was depressed in K+-free solution, and it was blocked by K+ concentrations exceeding 1.5 X normal. The myotonia was reduced when the NaCl was replaced by sucrose or choline chloride. In Ca2+-free solution the time to maximal myotonic tension and the variability of the maximal myotonia were reduced. CA2+ concentrations above normal inhibited the myotonia. No myotonia was observed in the slow twitch soleus muscle. pOHMB also caused a twitch depression during indirect and direct stimulation. The depression was observed in soleus muscle and in the diaphragm below 30 degrees C. The depression was thus independent of the myotonia.