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Regulation of acetylcholine release by intracellular acidification of developing motoneurons in Xenopus cell cultures

Yu-Hwa Chen, Mei-Lin Wu * and Wen-Mei Fu

1. 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.

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