Ca2+ currents through single channels in acutely dissociated nerve terminals from rat neurohypophyses were recorded using cell-attached patch recordings with 110 mM Ba2+ as the charge carrier. 2. One type (Nt, where the t denotes terminal) of single Ca2+ channel current was evoked only by depolarizing steps from holding potentials less negative than -50 mV. Because this channel opened primarily at the beginning of a 180-ms-long voltage pulse, the averaged ensemble current decayed rapidly (approximately 30 ms). Infrequently, the channel opened throughout such a long pulse, resulting in a long-lasting averaged ensemble current. The averaged channel open time constant (tau) was 0.34 ms and the two averaged closed time constants were 1.78 (tau 1) and 86.57 (tau 2) ms. The mean unitary slope conductance for this channel was 11 pS and its threshold for activation was approximately -10 mV. 3. The other type (L) of single Ca2+ channel current could be evoked in isolation by depolarizations from holding potentials more positive than or equal to -50 mV. This channel opened throughout an entire 180-ms-long voltage pulse. The averaged ensemble current, therefore, showed little inactivation. The averaged channel open-time constant was 0.49 ms and the two average closed time constants were 2.02 (tau 1) and 79.91 (tau 2) ms. The mean unitary slope conductance for this channel was 25 pS. 4. Bay K 8644 (5 microM), a dihydropyridine (DHP) Ca2+ channel agonist, increased the open probability of the larger-conductance L-type Ca2+ channel by prolonging the average duration (to 2.79 ms) of channel openings, but did not alter the single channel slope conductance. In contrast, the same concentration of Bay K 8644 did not affect the smaller-conductance Nt-type Ca2+ channel. The DHP Ca2+ channel antagonist nicardipine (5 microM), but not nifedipine (5 microM), reduced the open probability of the large-conductance L-type Ca2+ channel by shortening the duration (to 0.36 ms) of channel openings. 5. The voltage- and time-dependent properties of these two types of single Ca2+ channel currents are in close agreement with those of the two components of macroscopic Ca2+ currents previously reported using the "whole-terminal" recording method. Therefore these two types of single channels appear to underlie the macroscopic current. 6. Our studies suggest that the terminal Nt-type Ca2+ channel differs from the conventional somatic N- and T-type Ca2+ channels in some respects, and that the terminal L-type Ca2+ channel is similar to the conventional somatic L-type Ca2+ channel.(ABSTRACT TRUNCATED AT 400 WORDS)
摘要
使用细胞贴附式膜片钳记录技术，以110 mM Ba2+作为电荷载体，记录大鼠神经垂体急性分离神经末梢中单个通道的Ca2+电流。2. 一种类型（Nt，其中t表示末梢）的单个Ca2+通道电流仅在从比 -50 mV更正的保持电位进行去极化步骤时被诱发。由于该通道主要在180毫秒长的电压脉冲开始时打开，平均整体电流迅速衰减（约30毫秒）。偶尔，该通道会在整个这样长的脉冲期间打开，导致持久的平均整体电流。平均通道开放时间常数（tau）为0.34毫秒，两个平均关闭时间常数分别为1.78（tau 1）和86.57（tau 2）毫秒。该通道的平均单位斜率电导为11 pS，其激活阈值约为 -10 mV。3. 另一种类型（L）的单个Ca2+通道电流可通过从比 -50 mV更正或等于 -50 mV的保持电位进行去极化单独诱发。该通道在整个180毫秒长的电压脉冲期间打开。因此，平均整体电流几乎没有失活。平均通道开放时间常数为0.49毫秒，两个平均关闭时间常数分别为2.02（tau 1）和79.91（tau 2）毫秒。该通道的平均单位斜率电导为25 pS。4. 二氢吡啶（DHP）Ca2+通道激动剂Bay K 8644（5 microM）通过延长通道开放的平均持续时间（至2.79毫秒）增加了较大电导L型Ca2+通道的开放概率，但未改变单通道斜率电导。相比之下，相同浓度的Bay K 8644对较小电导的Nt型Ca2+通道没有影响。DHP Ca2+通道拮抗剂尼卡地平（5 microM）而非硝苯地平（5 microM）通过缩短通道开放的持续时间（至0.36毫秒）降低了大电导L型Ca2+通道的开放概率。5. 这两种类型的单个Ca2+通道电流的电压和时间依赖性特性与先前使用“全末梢”记录方法报道的宏观Ca2+电流的两个成分的特性密切一致。因此，这两种类型的单个通道似乎是宏观电流的基础。6. 我们的研究表明，末梢Nt型Ca2+通道在某些方面与传统的体细胞N型和T型Ca2+通道不同，并且末梢L型Ca2+通道与传统的体细胞L型Ca2+通道相似。（摘要截短为400字）
