Permeation of organic cations and ammonium through the glutamate receptor channel in Drosophila larval muscle.

The inside-out configuration of the patch-clamp technique was used to study the effects of large organic cations on the single-channel current through the glutamate receptor channel in muscles of Drosophila larvae. Control experiments with symmetrical Na+ showed slightly supra-linear I-V curves. When external Na+ was equiosmolarly replaced with either arginine+ or N-methyl-D-glucamine+ (NMDG+), the reversal potential changed almost according to VNa, suggesting that both these ions are only slightly permeant through the channel. However, both ions strongly reduced the current amplitude in a voltage-dependent manner, the effect being most pronounced for the inward current. Tris+ had similar effects on the Na+ current, although the reversal potential indicated that this ion is somewhat permeant. All of these results could be fitted with a "one-ion" Eyring model for the channel with internal binding sites. The ion-channel dissociation constants for arginine+, NMDG+, and Tris+ were found to be 17, 27, and 23 mM, respectively. In order to acquire evidence for the "one-ion" hypothesis used in the model, I-V data were taken with different mixtures of Na+ and a comparably permeant ion, NH4+. All of the data could be accurately fitted with the results of the Eyring theory for singly occupied channels.