Kinetic differences in the effect of calcium on quantal and non-quantal acetylcholine release at the murine diaphragm.

The effects of Ca2+ withdrawal on non-quantal, evoked quantal and spontaneous quantal release of acetylcholine (ACh) from the motor nerve terminals were studied with standard intracellular recording techniques. Anticholinesterase-treated mouse diaphragms were used. In a Ca2(+)-free solution all forms of ACh release decreased, but with different kinetics. As expected, evoked quantal release declined to zero within a few minutes. Spontaneous quantal release, i.e. the frequency of the miniature end-plate potentials (MEPPs), decreased to 15% of the control within 20 min after calcium withdrawal. The slowest decay was that of non-quantal release which declined very slowly and reached zero after 45-50 min. Following Ca2+ re-admission, both quantal types of ACh release were rapidly restored (evoked in 10 min, spontaneous in 20 min). However, recovery of non-quantal release did not occur until after 50 to 60 min.