Quelling of spontaneous transmitter release by nerve impulses in low extracellular calcium solutions

1. The effect of nerve stimulation on spontaneous transmitter release was studied at the frog neuromuscular synapse which was bathed in a solution containing very low extracellular calcium concentration. Conventional methods for intracellular and extracellular recording were used and the pattern of quantal liberation following the nerve stimulus was determined. 2. Stimulation of the motor nerve (at rates between 0.09 and 2Hz) caused a reduction in the frequency of the miniature e.p.p.s in comparison to the prestimulation values. 3. The mean distribution of the time of occurrence of the miniature e.p.p.s during the interstimulus period showed periodic oscillations. 4. The quelling effect of nerve stimulation on transmitter release is explained by the hypothesis that a low [Ca]o a reversed electrochemical gradient for calcium occurs and nerve stimulation causes an increased calcium conductance leading to calcium efflux which in turn temporarily reduces [Ca]i and transmitter release.