A possible interpretation for difference in neostigmine-induced changes of spontaneous activities and evoked muscle potentials between rat medial gastrocnemius and soleus muscles

Recycling of acetylcholine following impulse transmission in rat muscle revealed in the presence of neostigmine

1. The recycling process of acetylcholine (ACh) following impulse transmission was studied in terms of muscle potentials evoked by repetitive stimulation in the presence of neostigmine. 2. Wistar rats were anaesthetized with urethane and muscle potentials were recorded with concentric electrodes from their exposed medial gastrocnemius muscles before and after the injection of neostigmine. 3. All potentials before neostigmine treatment were similar in amplitude. A set of 10 stimuli was given at 0.5 Hz 6-8 min after drug administration. The first potential was as large as that before it. The second potential was greatly depressed. Thereafter, potentials gradually recovered. 4. Two sets of 10 stimuli were delivered at a 1 min interval (i.e. with a 40 s rest period between them). The second potential was not depressed so severely in the second set as in the first set. The same procedure was repeated in some rats and the aforementioned phenomenon was noted. When two sets of 10 stimuli were given at an interval of 2 min or more, the second potential was equally depressed in the both sets of stimuli. 5. The recycling of ACh following impulse transmission in the junctional region was revealed in terms of muscle potentials in the presence of neostigmine. This process was activated due to repetitive stimulation. Moreover, the activated state seemed to be maintained for a while after the cessation of stimuli. These results suggest the possible existence of a neural mechanism that can exert an influence on the recycling of ACh following impulse transmission beyond a short period of time.