[Study of neuromuscular transmission with evoked electromyography--10. Comparison of the responses during partial blockade by bungarotoxin and non-depolarizing relaxants].

Effects of alpha-bungarotoxin (alpha BuTX) on muscle compound action potentials (CAPs) which were elicited from gastrocnemius muscle by sciatic nerve stimulation in cats were studied, and the results were compared with those of non-depolarizing relaxants including d-tubocurarine, pancuronium and vecuronium. The amplitude of CAP by the second member of the paired stimuli (test response) was compared with that evoked by the first component (conditioning response). The interval between the two components of the paired stimuli (the pair interval) was increased stepwise from 7 to 1,000 msec and a curve (recovery curve, RC) was obtained by relating the changes in pair interval to the difference in amplitude of the test and conditioning responses. The following results were obtained. (1) A progressive and irreversible neuromuscular block was observed after intravenous administration of alpha BuTX. During the progress of the blockade, RC of CAPs showed a pattern which was characterized by profound potentiations of test responses at shorter intervals of paired stimuli, followed by gradual recovery to similar amplitude with conditioning responses at 500 msec or over of pair interval. (2) With non-depolarizing relaxants, RC changed to the pattern of slight potentiation at a very short interval of stimuli, followed by a notable depression at longer intervals. (3) The mechanism of the development of these difference of RCs between alpha BuTX and the relaxants was 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 these relaxants, and the inhibition was more prominent with d-tubocurarine than with other two relaxants.