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.
Ciani
S
S
Department of Physiology, UCLA School of Medicine 90024, USA.
Nishikawa
K
K
Kidokoro
Y
Y
D000818:Animals; D001120:Arginine / Q000378:metabolism; D001693:Biological Transport, Active / Q000502:physiology; D004331:Drosophila melanogaster; D007473:Ion Channels / Q000378:metabolism* / Q000502:physiology; D007814:Larva; D008274:Magnesium / Q000378:metabolism; D008536:Meglumine / Q000378:metabolism; D008564:Membrane Potentials / Q000502:physiology; D008962:Models, Theoretical; D009469:Neuromuscular Junction / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010539:Permeability; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D017470:Receptors, Glutamate / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D014325:Tromethamine / Q000378:metabolism
D007473:Ion Channels; D000644:Quaternary Ammonium Compounds; D017470:Receptors, Glutamate; D014325:Tromethamine; D008536:Meglumine; D001120:Arginine; D012964:Sodium; D008274:Magnesium