Hermans M P, Schmeer W, Henquin J C
Endocrinology. 1987 May;120(5):1765-73. doi: 10.1210/endo-120-5-1765.
Mouse islets were used to test the hypothesis that the B cell membrane must be depolarized for acetylcholine to increase insulin release. The resting membrane potential of B cells (at 3 mM glucose) was slightly decreased (5 mV) by acetylcholine, but no electrical activity appeared. This depolarization was accompanied by a Ca-independent acceleration of 86Rb and 45Ca efflux but no insulin release. When the B cell membrane was depolarized by a stimulatory concentration of glucose (10 mM), acetylcholine potentiated electrical activity, accelerated 86Rb and 45Ca efflux, and increased insulin release. This latter effect, but not the acceleration of 45Ca efflux, was totally dependent on extracellular Ca. If glucose-induced depolarization of the B cell membrane was prevented by diazoxide, acetylcholine lost all effects but those produced at low glucose. In contrast, when the B cell membrane was depolarized by leucine or tolbutamide (at 3 mM glucose), acetylcholine triggered a further depolarization with appearance of electrical activity, accelerated 86Rb and 45Ca efflux, and stimulated insulin release. Acetylcholine produced similar effects (except for electrical activity) in the presence of high K or arginine which, unlike the above test agents, depolarize the B cell membrane by a mechanism other than a decrease in K+ permeability. Omission of extracellular Ca abolished the releasing effect of acetylcholine under all conditions but only partially decreased the stimulation of 45Ca efflux. The results show thus that acetylcholine stimulation of insulin release does not result from mobilization of cellular Ca but requires that the B cell membrane be sufficiently depolarized to reach the threshold potential where Ca channels are activated. This may explain why acetylcholine alone does not initiate release but becomes active in the presence of a variety of agents.
即B细胞膜必须去极化，乙酰胆碱才能增加胰岛素释放。乙酰胆碱使B细胞的静息膜电位（在3 mM葡萄糖浓度下）略有降低（5 mV），但未出现电活动。这种去极化伴随着86Rb和45Ca外流的钙非依赖性加速，但没有胰岛素释放。当B细胞膜被刺激浓度的葡萄糖（10 mM）去极化时，乙酰胆碱增强电活动，加速86Rb和45Ca外流，并增加胰岛素释放。后一种效应，但不是45Ca外流的加速，完全依赖于细胞外钙。如果用二氮嗪阻止葡萄糖诱导的B细胞膜去极化，乙酰胆碱就会失去所有作用，只剩下在低葡萄糖浓度下产生的作用。相反，当B细胞膜被亮氨酸或甲苯磺丁脲（在3 mM葡萄糖浓度下）去极化时，乙酰胆碱会引发进一步去极化并出现电活动，加速86Rb和45Ca外流，并刺激胰岛素释放。在高钾或精氨酸存在的情况下，乙酰胆碱产生了类似的效应（除电活动外），与上述测试剂不同的是，高钾或精氨酸通过降低钾离子通透性以外的机制使B细胞膜去极化。在所有条件下，去除细胞外钙都会消除乙酰胆碱的释放作用，但只会部分降低对45Ca外流的刺激。因此，结果表明，乙酰胆碱对胰岛素释放的刺激不是由细胞内钙的动员引起的，而是需要B细胞膜充分去极化以达到激活钙通道的阈电位。这可能解释了为什么乙酰胆碱单独不能引发释放，但在多种试剂存在时会变得活跃。
