Facilitation and delayed release at single frog neuromuscular junctions

Facilitation and delayed spontaneous release were recorded from neuromuscular junctions at 2 temperatures. In some experiments, Sr2+ substituted for Ca2+. We measured the ratio of delayed release to resting spontaneous release (D), and the ratio of the second end-plate potential amplitude to the first (F). An extension of the model of Barton et al. (1983) relating Ca2+ to evoked and spontaneous release is presented. If residual Ca2+ accounts for facilitation and delayed release, then D must be greater than F. In all of our experiments, D was greater than F. We conclude that residual Ca2+ is sufficient to account for facilitation and delayed release at the frog neuromuscular junction. However, D falls abruptly at a time when F is almost unaltered, which raises serious difficulties in accepting the residual calcium hypothesis.