Influence of Sodium Ions on Transmitter Release

Abstract

WHEN the amplitudes of end-plate potentials (e.p.p.s) are depressed at frog neuromuscular junctions by low concentrations of calcium, lowering the sodium ion concentration in the medium causes a large increase in e.p.p. amplitude which has been shown to be due to an increased output of transmitter¹. In general, those changes in bathing media which increase the quantal content of e.p.p.s also increase the frequency of miniature end-plate potentials (m.e.p.p.s). It therefore seemed likely that m.e.p.p. frequency would be increased by reduction of the extracellular sodium ion concentration. This prediction was tested by measuring frequencies of spontaneous m.e.p.p.s at neuromuscular junctions in rat hemidiaphragms in vitro, before and after lowering the sodium ion concentration in the bathing solution.