Grohovaz F, Fesce R, Haimann C
C.N.R. Center of Cytopharmacology, University of Milano, Italy.
Cell Biol Int Rep. 1989 Dec;13(12):1085-95. doi: 10.1016/0309-1651(89)90022-2.
The time course of exocytosis of quanta of acetylcholine induced by 20 mM K+ was studied at the frog neuromuscular junction. Images of vesicle fusion on freeze-fracture replicas were mostly localized at the active zones in resting preparations fixed in 20 mM K+. Fusions appeared also outside the active zones in preparations fixed after 1 min exposure to 20 mM K+ and were evenly distributed over the presynaptic membrane after 5 min in 20 mM K+ (even though secretion was prevented by withdrawing Ca2+ until 30 s before fixation). The mean densities of vesicle fusions were comparable in all conditions, as were the total number of quanta released during the fixation period. This indicates that fusions outside active zones represent ectopic exocytosis, slowly activated by potassium. Partial inactivation of K(+)-induced quantal release (time and concentration-dependent) was observed electrophysiologically; this may be related to the observed decrease in density of vesicle fusions along the active zones, with time. Consistently, after 5 min in 15 mM K+ fusion density at the active zones remained high. It is concluded that active zone-associated and ectopic fusions are two exocytotic processes activated with differential time courses and concentration-dependence by K+.
在青蛙神经肌肉接头处研究了由20 mM K⁺诱导的乙酰胆碱量子胞吐作用的时间进程。在固定于20 mM K⁺的静息标本中，冷冻蚀刻复制品上的囊泡融合图像大多位于活性区。在暴露于20 mM K⁺ 1分钟后固定的标本中，融合也出现在活性区之外，并且在20 mM K⁺中孵育5分钟后均匀分布于突触前膜（尽管在固定前30秒通过去除Ca²⁺来阻止分泌）。在所有条件下，囊泡融合的平均密度相当，固定期间释放的量子总数也相当。这表明活性区外的融合代表异位胞吐作用，由钾缓慢激活。通过电生理学观察到K⁺诱导的量子释放部分失活（时间和浓度依赖性）；这可能与观察到的沿活性区囊泡融合密度随时间降低有关。一致地，在15 mM K⁺中孵育5分钟后，活性区的融合密度仍然很高。得出的结论是，与活性区相关的融合和异位融合是由K⁺以不同的时间进程和浓度依赖性激活的两个胞吐过程。
