Influence of stimulation on Ca2+ recruitment triggering [3H]acetylcholine release from the rat motor-nerve endings

The influence of rat phrenic nerve stimulation frequency (5-50 Hz) and of pulse duration (0.04-1 ms) on Ca2+ mobilization triggering [H-3]acetylcholine release was investigated. The P-type voltage-dependent Ca2+ channel(VDCC) blocker, omega -agatoxin IVA (100 nM), decreased [H-3]acetylcholine release evoked by pulses of 0.04-ms duration delivered at 5 Hz frequency. When the stimulus pulse duration was increased to 1 ms (5 Hz frequency) or the stimulation frequency to 50 Hz (0.04-ms duration), inhibition of [H-3]acetylcholine release became evident after blockade of L-type VDCC, with nifedipine (1 muM), and/or depletion of thapsigargin-sensitive internal stores. The inhibitory effect of thapsigargin (2 muM) was still observed in Ca2+-free medium. Neither omega -conotoxin GVIA (1 muM) nor omega -conotoxin MWC (150 nM) modified neurotransmitter release. The results suggest that, depending on the stimulus paradigm, both internal (thapsigargin-sensitive) and external (either P- or L-type channels) Ca2+ pools can be mobilized to promote acetylcholine release from motor nerve terminals. (C) 2000 Elsevier Science B.V. All rights reserved.