Comparison of the effects of mercuric chloride and methylmercuric chloride on the isolated phrenic nerve and on the phrenic nerve-diaphragm preparation from the rat.

This study compared the effects of HgCl(2) and CH(3)HgCl on the compound action potential of the isolated rat phrenic nerve and the twitch contractions of rat phrenic nerve-diaphragm preparations during direct and indirect stimulation. Both HgCl(2) and CH(3)HgCl, at a concentration of 3.7 x 10(-5)m, inhibited the twitches during indirect stimulation. CH(3)HgCl was slightly more potent than HgCl(2). The inhibition might be explained by a simultaneous inhibition of the phrenic nerve. CH(3)HgCl caused an additional curare-like inhibition at the neuromuscular junction. HgCl(2) potentiated the indirectly elicited twitches before the inhibitory effect ensued. The twitch potentiation was inhibited by low temperature (20 degrees C), high Ca(2+) (10.8 mm) and d-tubocurarine (7.3 x 10(-7)m). It was probably caused by multiple excitation of some of the nerve terminals. Both agents were less efficient at inhibiting the preparations during direct stimulation than during indirect stimulation. However, during direct stimulation, HgCl(2) was a considerably more potent inhibitor than CH(3)HgCl. Both mercury compounds caused a slow increase in the baseline tension. An additional rapid initial phase with HgCl(2) could be partly inhibited by dantrolene (1.5 x 10(-5)m), high Ca(2+) (10.8 x 10(-3)m) or low temperature (20 degrees C), indicating that it was a depolarization contracture. The slow phase was not affected by dantrolene, and may be characterized as a rigor. The effects of HgCl(2) and CH(3)HgCl were irreversible on washing, but they could be partly antagonized by washing plus exposure to dithiothreitol, to keep SH groups in the reduced state.