Figure 4. Probing the effect of presynaptically added Ca²⁺ buffers on Ca²⁺ current facilitation, transmitter release facilitation, and residual [Ca²⁺]_i.

A, average time courses of paired-pulse facilitation of transmitter release recorded in the presynaptic presence of 100 μm fura-6F (n = 4; black), 100 μm fura-6F and 75 μm EGTA (n = 4; blue), and 50 μm fura-2 (n = 4; red). B, average time course of Ca²⁺ current facilitation (same colour code as in A). C, average residual [Ca²⁺]_i signals in response to single depolarizations measured under the three different presynaptic Ca²⁺-buffering conditions. The [Ca²⁺]_i traces were offset on the y-axis such that the baseline [Ca²⁺]_i values before stimulus onset overlapped. Note that the time course of transmitter release facilitation (A) and the decay of [Ca²⁺]_i (C) are more sensitive to adding EGTA than the time course of Ca²⁺ current facilitation (B). D, average control EPSC amplitudes (top), and control Ca²⁺ charges (bottom) under the different Ca²⁺-buffering conditions. In the presence of fura-2, the Ca²⁺ charge was significantly larger as compared with fura-6F alone (P = 0.04), but the EPSC amplitudes were not significantly different between the various Ca²⁺-buffering conditions, indicating a slight suppression of transmitter release by 50 μm fura-2. The average data in A–D are from the same data set (n = 4 cells for each Ca²⁺-buffering condition).