Heidelberger R, Heinemann C, Neher E, Matthews G
Abteilung Membranbiophysik, Max-Planck-Institut für biophysikalische Chemie, Göttingen, Germany.
Nature. 1994 Oct 6;371(6497):513-5. doi: 10.1038/371513a0.
Rapid calcium-dependent exocytosis underlies neurotransmitter release from nerve terminals. Despite the fundamental importance of this process, neither the relationship between presynaptic intracellular calcium ion concentration ([Ca2+]i) and rate of exocytosis, nor the maximal rate of secretion is known quantitatively. To provide this information, we have used flash photolysis of caged Ca2+ to elevate [Ca2+]i rapidly and uniformly in synaptic terminals, while measuring membrane capacitance as an index of exocytosis and monitoring [Ca2+]i with a Ca(2+)-indicator dye. When [Ca2+]i was abruptly increased to > 10 microM, capacitance rose at a rate that increased steeply with [Ca2+]i. The steepness suggested that at least four calcium ions must bind to activate synaptic vesicle fusion. Half-saturation was at 194 microM, and the maximal rate constant was 2,000-3,000 s-1. A given synaptic vesicle can exocytose with high probability within a few hundred microseconds, if [Ca2+]i rises above 100 microM. These properties provide for the extremely rapid signalling required for neuronal communication.
快速的钙依赖性胞吐作用是神经递质从神经末梢释放的基础。尽管这一过程至关重要，但突触前细胞内钙离子浓度（[Ca2+]i）与胞吐作用速率之间的关系，以及分泌的最大速率，目前都尚无定量认识。为了获取这些信息，我们利用笼锁Ca2+的闪光光解来迅速且均匀地提高突触末梢内的[Ca2+]i，同时测量膜电容作为胞吐作用的指标，并使用Ca(2+)指示剂染料监测[Ca2+]i。当[Ca2+]i突然增加到>10微摩尔时，电容以随[Ca2+]i急剧增加的速率上升。这种陡峭程度表明，至少有四个钙离子必须结合才能激活突触小泡融合。半饱和浓度为194微摩尔，最大速率常数为2000 - 3000 s-1。如果[Ca2+]i上升到100微摩尔以上，一个给定的突触小泡能够在几百微秒内以高概率发生胞吐作用。这些特性为神经元通讯所需的极其快速的信号传递提供了条件。
