THE ANTAGONISM OF Ca²⁺ BY Na⁺ AND OTHER MONOVALENT IONS AT THE FROG NEUROMUSCULAR JUNCTION

The quantal content of the e.p.p. and the frequency of the m.e.p.p.s in the presence of enhanced extracellular K + have been investigated by intracellular recording from frog sartorius muscle fibres. In the presence of 0·23 mM Ca 2+ (one eighth of the normal), the replacement of NaCl by sucrose or glycine led to a large increase in quantal content; the size of this increase appeared to be related to the amount of NaCl replaced. The quantal content in 0·9 mM Ca 2+ , however, was not appreciably altered by the replacement of NaCl by sucrose. When LiCl was used to replace NaCl, the immediate change in the quantal content was small. With methyl‐ and ethylammonium or tris chlorides as substitutes for NaCl, the immediate effect was a reduction in quantal content. A delayed effect of LiCl was a reduction, and that of methylammonium an increase, in quantal content. The replacement of NaCl by sucrose accelerated the m.e.p.p. frequency in the presence of enhanced K + (12 mM); in contrast the replacement of NaCl by methylammonium chloride caused a reduction in frequency. With LiCl as a substitute the frequency of the m.e.p.p.s increased in the normal K + concentration, the increase being slowly reversible in the normal Ringer solution. It is suggested that antagonism of Ca 2+ by Na + is non‐specific and that other monovalent cations will also compete with Ca 2+ .