Permeability changes induced by L-glutamate at the crayfish neuromuscular junction

Two different methods are described which allow the reversal potential (Er) for the channels opened by L-glutamate at the voltage-clamped, crayfish neuromuscular junction to be measured accurately. In both cases the value of Er was found to be about +6 mV. Reversal potentials were also measured in solutions where Na+ was replaced by K+, Ca2+, or Mg2+; or in which Cl- was replaced by isethionate. In solutions where Na+ was partially replaced by K+, the measured reversal potentials were compared to theoretical values predicted by both the constant-field and equivalent-circuit equations. The experimental values were more accurately described by the constant-field equation. Permeability ratios (PX/PNa) for K+, Ca2+, Mg2+, and Cl- were calculated using the constant-field equation. K+ and Na+ were equally permeant while Ca2+ and Mg2+ were about half as permeant as the monovalent cations. Cl- was impermeant. The results of these experiments indicate that the L-glutamate activated channel is non-selective for cations. Furthermore, the value of the permeability ratios for the physiological cations tested are very similar to those obtained for the acetylcholine activated channel in vertebrate skeletal muscle.