title: Ion permeation properties of the glutamate receptor channel in cultured embryonic Drosophila myotubes.
abstract: Ion permeation properties of the glutamate receptor channel in cultured myotubes of Drosophila embryos were studied using the inside-out configuration of the patch-clamp technique. Lowering the NaCl concentration in the bath (intracellular solution), while maintaining that of the external solution constant, caused a shift of the reversal potential in the positive direction, thus indicating a higher permeability of the channel to Na+ than to Cl- (PCl/PNa < 0.04), and suggesting that the channel is cation selective. With 145 mM Na+ on both sides of the membrane, the single-channel current-voltage relation was almost linear in the voltage range between -80 and +80 mV, the conductance showing some variability in the range between 140 and 170 pS. All monovalent alkali cations tested, as well as NH4+, permeated the channel effectively. Using the Goldman-Hodgkin-Katz equation for the reversal potential, the permeability ratios with respect to Na+ were estimated to be: 1.32 for K+, 1.18 for NH4+, 1.15 for Rb+, 1.09 for Cs+, and 0.57 for Li+. Divalent cations, i.e. Mg2+ and Ca2+, in the external solution depressed not only the inward but also the outward Na+ currents, although reversal potential measurements indicated that both ions have considerably higher permeabilities than Na+ (PMg/PNa = 2.31; PCa/PNa = 9.55). The conductance-activity relation for Na+ was described by a hyperbolic curve. The maximal conductance was about 195 pS and the half-saturating activity 45 mM. This result suggests that Na+ ions bind to sites in the channel. All data were fitted by a model based on the Eyring's reaction rate theory, in which the receptor channel is a one-ion pore with three energy barriers and two internal sites.
authors.affiliation: Department of Physiology, Jerry Lewis Neuromuscular Research Center, University of California at Los Angeles, CA 90024.
authors.forename: H
authors.initials: H
authors.lastname: Chang
authors.forename: S
authors.initials: S
authors.lastname: Ciani
authors.forename: Y
authors.initials: Y
authors.lastname: Kidokoro
mesh_terms: D000818:Animals; D002412:Cations / Q000378:metabolism; D002463:Cell Membrane Permeability; D002478:Cells, Cultured; D003470:Culture Media; D004330:Drosophila / Q000378:metabolism*; D004625:Embryo, Nonmammalian; D007473:Ion Channels / Q000378:metabolism*; D008564:Membrane Potentials / Q000502:physiology; D008870:Microtubules / Q000378:metabolism*; D008954:Models, Biological; D017470:Receptors, Glutamate / Q000378:metabolism*; D012964:Sodium / Q000502:physiology; D012965:Sodium Chloride / Q000378:metabolism
chemical_list: D002412:Cations; D003470:Culture Media; D007473:Ion Channels; D017470:Receptors, Glutamate; D012965:Sodium Chloride; D012964:Sodium