Abstract
The effects of Cs ions on the frequency of miniature end-plate potentials (MEPPs) were. investigated at the frog neuromuscular junction. The frequency of MEPPs increased gradually with time, after a delay, when part of the NaCl in normal Ringer's solution was replaced isotonically with CsCl. The delay was greatly prolonged and the rate of increase in MEPP frequency slowed when a low concentration-between 6 and 40mM-of CsCl was applied. As the external concentration of CaCl2 (0.4-6.3mM) was increased, the delay was prolonged and the rate of increase became slower. The effect of Mg ions was qualitatively the same as that of Ca ions. In the case of reapplication, Cs ions produced a prompt increase in MEPP frequency and the rate was much faster than before. When the solution contained no added Ca and 1mM EGTA, no appreciable change in MEPP frequency was observed when CsCl was applied. It is suggested that the first step in the gradual increase in MEPP frequency is the slow entry of Cs ions into the nerve terminal, following which transmitter release is mainly induced by the influx of external Ca ions.
