Emetine increases giant miniature endplate potential population at the frog neuromuscular junction

Intracellular recordings from 90 endplates in untreated cutaneous pectoris muscles of the frog showed that giant miniature endplate potentials (gMEPPs) occurred at a low frequency of 0.017 +/- 0.002 s-1. These spontaneous potentials are several times larger in amplitude and have time to peak twice longer than the normal MEPPs (nMEPPs). The frequency of gMEPPs was not significantly influenced by the presence or absence of tetrodotoxin, calcium or high Mg2+. The frequency of gMEPPs was not dependent on the frequency of nMEPPs nor was it influenced by prior prolonged motor nerve stimulation. After a delay of 10-20 min, the antiamebic drug, emetine, (10 microM) increased the frequency of gMEPPs by about 10-fold without affecting nMEPP frequency. The emetine-induced gMEPPs were very similar in characteristics to gMEPPs occurring in untreated muscles. The ability of emetine to increase the gMEPP frequency was not affected by the absence of extracellular calcium. The results indicate that emetine enhances the frequency of the calcium-independent gMEPPs that are normally seen in untreated muscles.