When frog nerve-muscle preparations are stimulated tetanically in saline solutions containing Ca2+, Mn2+, Co2+, or Ni2+ there is a substantial rise in miniature end-plate potential (MEPP) frequency. After stimulation is ended, there is a decline back toward pre-stimulus MEPP frequencies. If we assume that MEPP frequencies are an index of the concentration of the divalent cations at some critical site in the nerve terminal, then the fall in MEPP frequency reflects the clearing of the divalent metal cation from the critical region. As a first step in investigating the clearing mechanism, we have studied the effects of temperature on the decline in MEPP frequencies following a tetanus. The Q10s for the fall in MEPP frequencies following the tetanus in Ca2+ or Mn2+ range between 1.2 and 1.6. The results can be interpreted in terms of a two-stage model for the fall-off in release probability following stimulation. In Co2+ or Ni2+ containing solutions at 7 degrees C or lower the MEPP frequency is sustained at an elevated level following the tetanus; at higher temperatures the decline does not reach the initial control level.
Animals Anura