1. Intracellular recordings from the cutaneous pectoris muscle fibres of the frog showed that giant miniature end-plate potentials (gMEPPs) occurred in untreated preparations. Emetine (10 microM), after a 15-20 min delay, increased the frequency of gMEPPs. In both cases gMEPPs disappeared in the presence of (+)-tubocurarine. 2. There was no correlation between the frequency of gMEPPs and the frequency of normal MEPPs (nMEPPs) in untreated or emetine-treated fibres. 3. Tetraphenylborate (TPB, 50 microM) applied to muscles pre-treated with emetine caused the frequency and amplitude of both nMEPPs and gMEPPs to decrease gradually. All MEPPs disappeared in about 10 min. 4. Chloride permeability was modified by changing the pH of the Ringer solution. Changing the pH from 7.2 to 8.2 or 6.2, which in both cases caused marked increases in nMEPP frequency, had no significant effect on gMEPP frequency in untreated muscles. 5. Decreasing the pH from 7.2 to 6.2 blocked the ability of emetine to increase gMEPP frequency. Increasing the pH from 7.2 to 8.2 had no significant effect on emetine ability to increase gMEPP frequency. 6. Treatment with the Cl- channel blocker SITS (0.5 mM) had no effect on gMEPPs in untreated muscle or on the ability of emetine to increase the frequency of these potentials. 7. The Ca2+ sensitivity of gMEPPs in untreated or emetine-treated muscles was tested by treatments which are known to alter intracellular Ca2+. Raising extracellular Ca2+ (10 mM), treatment with Mn2+ (10 mM), Mg2+ (10 mM), K+ (7.5 mM), hypotonic solution, ouabain (0.2 mM) or ethanol (0.5 M), although causing profound changes in nMEPP frequency had no significant effect on gMEPP frequency in untreated fibres or on the ability of emetine to increase gMEPP frequency. 8. It is concluded that at the frog neuromuscular junction generation of the normally occurring or emetine-induced gMEPPs is independent of Ca2+ and does not seem to be influenced by changing membrane C1- permeability.
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