Electrophysiology and morphology have been combined to investigate the time course of the exocytosis of quanta of neurotransmitter induced by elevated concentrations of K+ at the frog neuromuscular junction. 2. Replicas of freeze-fractured resting nerve terminals fixed in the presence of 20 mM-K+ showed images of fusion of synaptic vesicles with the presynaptic axolemma which were closely associated with the active zones. After 1 min in 20 nM-K+ fusions appeared also outside the active zones, and by 5 min they became uniformly distributed over the presynaptic membrane. 3. The average total density of fusions was not significantly different at the various times examined since it decreased at the active zones while it increased over the rest of the membrane. 4. Resting terminals fixed in 20 mM-K+ released 33,000-45,000 quanta after the addition of fixative; terminals stimulated by 20 mM-K+ for 1-5 min released 50,000-100,000 quanta during fixation. The fixative potentiated K+-induced transmitter release. 5. Fusions were uniformly distributed in terminals pre-incubated for 5 min in 20 mM-K+ without added Ca2+, stimulated by adding Ca2+ for 30 s, and then fixed. Conversely, after 5 min stimulation in hypertonic Ringer solution fusions remained predominantly located near the active zones. A similar distribution was observed after 15 min stimulation by a lower concentration of K+ (15 mM). 6. At all concentrations of K+ tested (10, 15, 20, 25 mM) miniature end-plate potential (MEPP) rate attained a steady-state value within 10-15 min. Values from a single junction were generally lower at higher concentrations of K+, which indicates partial inactivation of the secretion-recycling process. 7. The data indicate that K+ initially activates exocytosis at the active zones. Subsequently, ectopic exocytosis is activated while sites at the active zones appear to undergo partial inactivation. These phenomena are not related to the intensity or to the amount of previous secretion.
摘要
电生理学和形态学已结合起来，研究在青蛙神经肌肉接头处，高浓度钾离子诱导神经递质量子释放的胞吐作用的时间进程。2. 在20 mM - K⁺存在下固定的冷冻断裂静息神经末梢的复制品显示，突触小泡与突触前轴膜融合的图像与活性区密切相关。在20 nM - K⁺中放置1分钟后，融合也出现在活性区之外，到5分钟时，它们均匀分布在突触前膜上。3. 在检查的不同时间，融合的平均总密度没有显著差异，因为它在活性区减少，而在膜的其余部分增加。4. 在添加固定剂后，在20 mM - K⁺中固定的静息末梢释放33,000 - 45,000个量子；在20 mM - K⁺中刺激1 - 5分钟的末梢在固定过程中释放50,000 - 100,000个量子。固定剂增强了钾离子诱导的递质释放。5. 在不添加钙离子的20 mM - K⁺中预孵育5分钟的末梢中，融合均匀分布，添加钙离子刺激30秒后固定。相反，在高渗林格溶液中刺激5分钟后，融合主要仍位于活性区附近。在较低浓度的钾离子（15 mM）刺激15分钟后也观察到类似的分布。6. 在所有测试的钾离子浓度（10、15、20、25 mM）下，微小终板电位（MEPP）频率在10 - 15分钟内达到稳态值。在较高钾离子浓度下，单个接头的值通常较低，这表明分泌 - 再循环过程部分失活。7. 数据表明，钾离子最初在活性区激活胞吐作用。随后，异位胞吐被激活，而活性区的位点似乎经历部分失活。这些现象与先前分泌的强度或量无关。
