[Study of neuromuscular transmission with evoked electromyography--11. Comparison between blocks by magnesium and by...

Effects of magnesium (Mg) on muscle compound action potentials (CAPs) which were elicited from gastrocunemius muscle by sciatic nerve stimulation in cats were studied, and the results were compared with those of alpha-bungarotoxin (alpha BuTX) and of non-depolarizing relaxants which had been reported previously. Recovery curves (RCs) of CAPs and train-of-four ratios (TOFR) as parameters of neuromuscular blockade were estimated during blocks by Mg as well as the methods described in the previous reports. The following results were obtained. 1) Neuromuscular blocks which were dependent on serum [Mg2+] were observed in accordance with cumulative dosages of MgSO4 solutions. 2) Recovery curves of CAPs showed a pattern which was characterized by extreme potentiations of test responses at shorter intervals of paired stimuli, followed by a slight depression at longer intervals than 500 msec. This pattern of RC was remarkably different from those after alpha BuTX and relaxants. 3) The mechanisms to produce these differences of RCs were discussed, and it is concluded that the notable depression of RCs derived from muscle relaxants may be caused by inhibitory effect on nerve terminals of relaxants which have different mechanism from that by Mg.