Differential effects of mercurial and methylmercurial ions on the phrenic nerve and diaphragm of the mouse.

Both Hg2+ and CH3Hg+ exert a depressive action on the twitch amplitude elicited by nerve stimulations more rapidly than that by muscle stimulation. CH3Hg+ was relatively more potent than Hg2+ in the nerve depressive action but was weaker on the muscle. Hg2+ induced a rapid phase and a slow phase of contracture in the diaphragm, while CH3Hg+ induced only a slow one. The biphasic contracture induced by Hg2+ could be inhibited by repetitive washout of the diaphragm with low Ca2+ (10(-3) mM) Krebs and also by treatment with glycerol to close T-tubule, while the slow contracture induced by CH3Hg+ was little affected by glycerol treatment. None of denervation, d-tubocurarine and tetrodotoxin inhibited the contracture induced by Hg2+ and CH3Hg+, indicating that the contracture was induced by a direct action on the muscle. Hg2+, but not CH3Hg+, rapidly caused a decrease of the membrane potential, decreased the amplitude and prolonged the duration of the muscle action potential. Hg2+ also initially decreased and then increased 45Ca2+ uptake of the diaphragm, but CH3Hg+ had no effect, except a prolonged incubation at a high concentration. The action of Hg2+ could be antagonized by glutathione, and CH3Hg+ was more susceptible than Hg2+ to the antagonistic action of glutathione. Both Hg2+ and CH3Hg+ did not change the ATP content of the diaphragm. All of these findings indicate that the monovalent organic CH3Hg+, with a higher lipophilicity, possesses a higher affinity to the nerve while the divalent cationic Hg2+ exerted more effects than CH3Hg+ on the sarcolemma of the mouse diaphragm.