Ion permeation properties of the glutamate receptor channel in cultured embryonic Drosophila myotubes

J Physiol. 1994 Apr 1;476(1):1-16.

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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Cations / metabolism
  • Cell Membrane Permeability
  • Cells, Cultured
  • Culture Media
  • Drosophila / metabolism*
  • Embryo, Nonmammalian
  • Ion Channels / metabolism*
  • Membrane Potentials / physiology
  • Microtubules / metabolism*
  • Models, Biological
  • Receptors, Glutamate / metabolism*
  • Sodium / physiology
  • Sodium Chloride / metabolism

Substances

  • Cations
  • Culture Media
  • Ion Channels
  • Receptors, Glutamate
  • Sodium Chloride
  • Sodium