EGTA Can Inhibit Vesicular Release in the Nanodomain of Single Ca2+ Channels

The exogenous Ca 2+ chelator EGTA (ethylene glycol tetraacetic acid) has been widely used to probe the coupling distance between Ca 2+ channels and vesicular Ca 2+ sensors for neurotransmitter release. Because of its slow forward rate for binding, EGTA is thought to not capture calcium ions in very proximity to a channel, whereas it does capture calcium ions at the remote distance. However, in this study, our reaction diffusion simulations (RDSs) of Ca 2+ combined with a release calculation using vesicular sensor models indicate that a high concentration of EGTA decreases Ca 2+ and vesicular release in the nanodomain of single channels. We found that a key determinant of the effect of EGTA on neurotransmitter release is the saturation of the vesicular sensor. When the sensor is saturated, the reduction in the Ca 2+ concentration by EGTA is masked. By contrast, when the sensor is in a linear range, even a small reduction in Ca 2+ by EGTA can decrease vesicular release. In proximity to a channel, the vesicular sensor is often saturated for a long voltage step, but not for a brief Ca 2+ influx typically evoked by an action potential. Therefore, when EGTA is used as a diagnostic tool to probe the coupling distance, care must be taken regarding the presynaptic Ca 2+ entry duration as well as the property of the vesicular Ca 2+ sensor.