Is extracellular calcium buffering involved in regulation of transmitter release at the neuromuscular junction?

During synaptic activity at the neuromuscular junction, sodium, potassium and calcium ions flow through both the postsynaptic and presynaptic membrane. These ionic fluxes can cause changes in the local extracellular concentration in the synaptic gap: a decrease in the concentration of the inwardly flowing ions (sodium and calcium) and an increase in the outwardly flowing potassium ions. To check whether depletion of calcium ions in the synaptic gap is involved in transmitter release, we have used calcium buffers to keep the extracellular calcium concentration almost constant. The expectation was that if depletion does occur, transmitter release will increase; if no depletion occurs, there will be no change in quantal release when the calcium concentration is the same in buffered and unbuffered bathing solutions. We report here that, surprisingly, perfusing the frog neuromuscular preparation with a calcium-buffered solution caused a decrease in transmitter release compared with that in an unbuffered solution with the same calcium concentration. This presumably indicates that the calcium level in the synaptic cleft is higher than that in the bulk extracellular medium. If such a mechanism operates physiologically, it may provide an energetically economical way to determine the level of evoked transmitter release and thus synaptic efficiency.