Ca2+‐dependent K+ (BK) channels at varicosities in Xenopus nerve–muscle cell cultures were used to quantify experimentally the instantaneous active zone [Ca2+]AZ resulting from different rates and durations of Ca2+ entry in the absence of extrinsic buffers and correlate this with neurotransmitter release. Ca2+ tail currents produce mean peak [Ca2+]AZ ~ 30 μM; with continued influx, [Ca2+]AZ reaches ~45–60 μM at different rates depending on Ca2+ driving force and duration of influx. Both IBK and release are dependent on Ca2+ microdomains composed of both N‐ and L‐type Ca channels. Domains collapse with a time constant of ~0.6 ms. We have constructed an active zone (AZ) model that approximately fits this data, and depends on incorporation of the high‐capacity, low‐affinity fixed buffer represented by phospholipid charges in the plasma membrane. Our observations suggest that in this preparation, (1) some BK channels, but few if any of the Ca2+ sensors that trigger release, are located within Ca2+ nanodomains while a large fraction of both are located far enough from Ca channels to be blockable by EGTA, (2) the IBK is more sensitive than the excitatory postsynaptic current (EPSC) to [Ca2+]AZ (K1/2–26 μM vs. ~36 μM [Ca2+]AZ); (3) with increasing [Ca2+]AZ, the IBK grows with a Hill coefficient of 2.5, the EPSC with a coefficient of 3.9; (4) release is dependent on the highest [Ca2+] achieved, independent of the time to reach it; (5) the varicosity synapses differ from mature frog nmjs in significant ways; and (6) BK channels are useful reporters of local [Ca2+]AZ.

中文翻译：

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通过实验监测神经元-肌肉细胞培养物中神经递质释放过程中突触活性区的钙动态

钙2+‐依赖性 K+(BK) 静脉曲张处的通道爪蟾属神经肌肉细胞培养物用于通过实验量化瞬时活动区 [Ca2+]AZ由不同的 Ca 速率和持续时间造成2+在没有外源缓冲的情况下进入并将其与神经递质释放相关联。钙2+尾电流产生平均峰值 [Ca2+]AZ〜30μM；随着持续流入，[Ca2+]AZ根据 Ca 以不同速率达到 ~45–60 μM2+流入的驱动力和持续时间。两个都我巴克和释放取决于 Ca2+由 N 型和 L 型 Ca 通道组成的微域。域崩溃的时间常数约为 0.6 ms。我们构建了一个大致适合该数据的活性区（AZ）模型，并且取决于质膜中磷脂电荷代表的高容量、低亲和力固定缓冲液的掺入。我们的观察表明，在这种制备中，(1)一些 BK 通道，但几乎没有 Ca2+触发释放的传感器位于 Ca 内2+纳米域，而两者的大部分都距离 Ca 通道足够远，可以被 EGTA 阻断，(2)我巴克比兴奋性突触后电流 (EPSC) 对 [Ca2+]AZ（K1/2–26 μM 与 ~36 μM [Ca2+]AZ）；(3) 随着[Ca2+]AZ， 这我巴克Hill 系数增长为 2.5，EPSC 系数增长为 3.9；(4) 释放取决于最高[Ca2+] 已实现，与达到目标的时间无关；(5) 静脉曲张突触与成熟青蛙 nmjs 有显着差异；(6) BK 频道是当地 [Ca2+]AZ。