Adler E M, Augustine G J, Duffy S N, Charlton M P
Physiology Department, University of Toronto, Ontario, Canada.
J Neurosci. 1991 Jun;11(6):1496-507. doi: 10.1523/JNEUROSCI.11-06-01496.1991.
A number of calcium buffers were examined for their ability to reduce evoked transmitter release when injected into the presynaptic terminal of the squid giant synapse. Injection of EGTA was virtually ineffective at reducing transmitter release, even at estimated intracellular concentrations up to 80 mM. Conversely, the buffer 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), which has an equilibrium affinity for calcium similar to that of EGTA at pH 7.2, produced a substantial reduction in transmitter release when injected presynaptically. This effect of BAPTA was reversible, presumably because the buffer diffused out of the terminal and into uninjected regions of the presynaptic axon. BAPTA derivatives with estimated intracellular calcium dissociation constants (Kd) ranging from 0.18 to 4.9 microM were effective at reducing transmitter release at similar estimated concentrations. A BAPTA derivative with an estimated intracellular Kd of 31 mM was less effective. BAPTA did not affect presynaptic action potentials or calcium spikes in ways that could explain its ability to reduce transmitter release. The relative effects of presynaptic injections of BAPTA and derivatives are consistent with the calcium-buffering capability of these compounds if the presynaptic calcium transient that triggers release is hundreds of microM or larger. The superior potency of BAPTA compared to EGTA apparently results from the faster calcium-binding kinetics of BAPTA and suggests that the calcium-binding molecule that triggers release binds calcium in considerably less than 200 microsec and is located very close to calcium channels.
研究了多种钙缓冲剂在注入枪乌贼巨大突触的突触前终末时降低诱发递质释放的能力。即使在估计的细胞内浓度高达80 mM时，注入乙二醇双四乙酸（EGTA）对降低递质释放实际上也无效。相反，在pH 7.2时对钙的平衡亲和力与EGTA相似的缓冲剂1,2-双（2-氨基苯氧基）乙烷-N,N,N',N'-四乙酸（BAPTA），在突触前注入时能使递质释放大幅降低。BAPTA的这种作用是可逆的，大概是因为该缓冲剂从终末扩散到突触前轴突的未注入区域。估计细胞内钙解离常数（Kd）范围为0.18至4.9微摩尔的BAPTA衍生物在相似的估计浓度下能有效降低递质释放。估计细胞内Kd为31 mM的BAPTA衍生物效果较差。BAPTA对突触前动作电位或钙峰的影响并不能解释其降低递质释放的能力。如果触发释放的突触前钙瞬变是数百微摩尔或更大，那么突触前注入BAPTA及其衍生物的相对效果与这些化合物的钙缓冲能力是一致的。与EGTA相比，BAPTA的卓越效能显然源于其更快的钙结合动力学，这表明触发释放的钙结合分子结合钙的时间远少于200微秒，且位于非常靠近钙通道的位置。
