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

Authors


Corresponding author W.-M. Fu: Department of Pharmacology, College of Medicine, National Taiwan University, Taipei, Taiwan. Email: wenmei@ccms.ntu.edu.tw

Abstract

  • 1The 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.
  • 2Spontaneous 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.
  • 3Another 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.
  • 4Impulse-evoked ACh release was slightly augmented by intracellular alkalinization and inhibited by cytosolic acidification.
  • 5Cytosolic 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.
  • 6We 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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