Drug actions at mammalian motor nerve endings: the suppression of neostigmine-induced fasciculations by vecuronium and isoflurane.

The occurrence of fasciculations following administration of agents is a well-known pharmacologic phenomenon. Using the cat soleus nerve-muscle preparation, intravenous neostigmine doses between 20-200 micrograms/kg evoked fasciculations in a dose-related manner. The data demonstrate that the fasciculations were the result of the direct effect of neostigmine acting at the motor nerve endings. Vecuronium in a dose-related manner (3 and 5 micrograms/kg iv) suppressed this prejunctional activity of neostigmine. The prejunctional effect of vecuronium explains its effectiveness in preventing succinylcholine-induced fasciculations. In the presence of isoflurane (end-tidal concentration 0.20-0.25%), the suppressant effect of vecuronium on motor nerve endings was enhanced. The prejunctional action of isoflurane may be a major contribution to the additive effects of non-depolarizing muscle relaxants and potent inhalation agents.