Effects of intra-axonal injection of Ca²⁺ buffers on evoked release and on facilitation in the crayfish neuromuscular junction

Ca²⁺ buffers were injected into the excitatory axon of the crayfish opener muscle. The magnitude and time course of evoked release and of facilitation were measured. EGTA (on-rate about 10⁶ M^-1s^-1 had no effect on evoked release but reduced facilitation. BAPTA and nitr-5, buffers with similar K_d's but faster on-rates, reduced both evoked release and facilitation. However, these buffers had no effect on the time course of evoked release. These results show that fast Ca²⁺ buffers reduce the Ca²⁺ transient associated with evoked release and also the level of residual Ca²⁺ involved in facilitation. However, Ca²⁺ buffering is not the mechanism which controls the time course of release.