REDUCTION BY INTRACELLULAR CALCIUM CHELATION OF ACETYLCHOLINE SECRETION WITHOUT OCCLUDING THE EFFECTS OF ADENOSINE AT FROG MOTOR-NERVE ENDINGS

1 The calcium chelators bis-(aminophenoxy)ethane-tetraacetic acid (BAP TA) or dimethyl-BAPTA. (MBAPTA) were introduced into the cytoplasm of frog motor nerve endings by use of the AM loading technique. The effec ts of intracellular Ca2+ chelation were studied on quantal acetylcholi ne (ACh) release and on the action of adenosine. 2 Intracellular BAPTA or DMBAPTA prevented the increases in quantal ACh secretion normally evoked by caffeine. 3 Intracellular DMBAPTA decreased the number of AC h quanta released by individual nerve impulses and virtually eliminate d the fast phase of facilitation in response to paired nerve impulses. 4 Adenosine reduced both spontaneous and evoked secretion of ACh quan ta with its usual potency and efficacy in the presence of intracellula r DMBAPTA. Adenosine had no significant effect on facilitation. 5 The results, which suggest that adenosine and intracellular DMBAPTA reduce ACh secretion by different mechanisms, are consistent with the hypoth esis that adenosine inhibits ACh release by reducing the ability of Ca 2+ to promote ACh secretion from frog motor nerve endings.