Helmchen F, Borst J G, Sakmann B
Abteilung Zellphysiologie, Max-Planck-Institut für medizinische Forschung, Heidelberg, Germany.
Biophys J. 1997 Mar;72(3):1458-71. doi: 10.1016/S0006-3495(97)78792-7.
Calcium dynamics associated with a single action potential were studied quantitatively in the calyx of Held, a large presynaptic terminal in the rat brainstem. Terminals were loaded with different concentrations of high- or low-affinity Ca2+ indicators via patch pipettes. Spatially averaged Ca2+ signals were measured fluorometrically and analyzed on the basis of a single compartment model. A single action potential led to a total Ca2+ influx of 0.8-1 pC. The accessible volume of the terminal was about 0.4 pl; thus the total calcium concentration increased by 10-13 microM. The Ca(2+)-binding ratio of the endogenous buffer was about 40, as estimated from the competition with Fura-2, indicating that 2.5% of the total calcium remained free. This is consistent with the peak increase in free calcium concentration of about 400 nM, which was measured directly with MagFura-2. The decay of the [Ca2+]i transients was fast, with time constants of 100 ms at 23 degrees C and 45 ms at 35 degrees C, indicating Ca2+ extrusion rates of 400 and 900 s-1, respectively. The combination of the relatively low endogenous Ca(2+)-binding ratio and the high rate of Ca2+ extrusion provides an efficient mechanism for rapidly removing the large Ca2+ load of the terminal evoked by an action potential.
在大鼠脑干中的一个大型突触前终末—— Held 壶腹，对与单个动作电位相关的钙动力学进行了定量研究。通过膜片吸管将不同浓度的高亲和力或低亲和力 Ca2+ 指示剂加载到终末中。通过荧光法测量空间平均 Ca2+ 信号，并基于单室模型进行分析。单个动作电位导致总的 Ca2+ 内流为 0.8 - 1 pC。终末的可及体积约为 0.4 pl；因此总钙浓度增加了 10 - 13 microM。根据与 Fura - 2 的竞争估计，内源性缓冲液的 Ca(2+) 结合比率约为 40，这表明总钙的 2.5% 保持游离状态。这与用 MagFura - 2 直接测量的游离钙浓度峰值增加约 400 nM 一致。[Ca2+]i 瞬变的衰减很快，在 23 摄氏度时时间常数为 100 ms，在 35 摄氏度时为 45 ms，分别表明 Ca2+ 外排速率为 400 和 900 s-1。相对较低的内源性 Ca(2+) 结合比率与高 Ca2+ 外排速率相结合，为快速清除动作电位诱发的终末中大量 Ca2+ 负荷提供了一种有效机制。
