The effects of intracellular pH changes on the acetylcholine (ACh) release and cytoplasmic Ca2+ concentration at developing neuromuscular synapses were studied in Xenopus nerve-muscle co-cultures. 2. Spontaneous and evoked ACh release of motoneurons was monitored by using whole-cell voltage-clamped myocytes. Intracellular alkalinization with 15 mM NH4Cl slightly reduced the frequency of spontaneous synaptic currents (SSCs). However, cytosolic acidification following withdrawal of extracellular NH4Cl caused a marked and transient increase in spontaneous ACh release. 3. Another method of cytosolic acidification was used in which NaCl in Ringer solution was replaced with weak organic acids. The increase in spontaneous ACh release paralleled the level of intracellular acidification resulting from addition of these organic acids. Acetate and propionate but not isethionate, methylsulphate and glucuronate, caused an increase in intracellular pH and a marked increase in spontaneous ACh release. 4. Impulse-evoked ACh release was slightly augmented by intracellular alkalinization and inhibited by cytosolic acidification. 5. Cytosolic acidification was accompanied by an elevation in the cytoplasmic Ca2+ concentration ([Ca2+]i), resulting from both external Ca2+ influx and intracellular Ca2+ mobilization. In contrast, the increase in [Ca2+]i induced by high K+ was inhibited by cytosolic acidification. 6. We conclude that cytosolic acidification regulates spontaneous and evoked ACh release differentially in Xenopus motoneurons, increasing spontaneous ACh release but inhibiting evoked ACh release.
摘要
在非洲爪蟾神经-肌肉共培养物中研究了细胞内pH变化对发育中的神经肌肉突触处乙酰胆碱（ACh）释放和细胞质Ca2+浓度的影响。2. 通过使用全细胞电压钳制的肌细胞监测运动神经元的自发和诱发ACh释放。用15 mM NH4Cl进行细胞内碱化可轻微降低自发突触电流（SSCs）的频率。然而，去除细胞外NH4Cl后导致的细胞质酸化引起自发ACh释放显著且短暂增加。3. 使用了另一种细胞质酸化方法，即用弱有机酸替代林格溶液中的NaCl。自发ACh释放的增加与添加这些有机酸导致的细胞内酸化水平平行。乙酸盐和丙酸盐而非羟乙基磺酸盐、甲硫酸盐和葡萄糖醛酸盐，引起细胞内pH升高和自发ACh释放显著增加。4. 细胞内碱化使冲动诱发的ACh释放略有增加，而细胞质酸化则抑制该释放。5. 细胞质酸化伴随着细胞质Ca2+浓度（[Ca2+]i）升高，这是由细胞外Ca2+内流和细胞内Ca2+动员共同引起的。相比之下，高K+诱导的[Ca2+]i增加受到细胞质酸化的抑制。6. 我们得出结论，细胞质酸化在非洲爪蟾运动神经元中对自发和诱发的ACh释放有不同调节作用，增加自发ACh释放但抑制诱发ACh释放。
