Spontaneous quantal and non-quantal release of acetylcholine at mouse endplate during onset of hypoxia

At 20 degrees C, both quantal and non-quantal spontaneous acetylcholine release (expressed as miniature endplate potential frequency [f-MEPPs] and the H-effect, respectively) increased during the first 30 min of hypoxia in solution with normal extracellular calcium ([Ca2+](o) = 2.0 mM). The hypoxia-induced tenfold increase of the f-MEPPs was virtually absent in low calcium solution ([Ca2+](o) = 0.4 mM) whereas there was still a significant increment of non-quantal release. This indicates that each of these two processes of acetylcholine release is influenced by mechanisms with different oxygen sensitivity. The rise of f-MEPPs during the onset of hypoxia apparently requires Ca2+ entry into the nerve terminal, whereas the non-quantal release can be increased by another factors such as a lower level of ATP.