Permeation properties of Na+ and Ca2+ ions through the mouse epsilon2/zeta1 NMDA receptor channel expressed in Xenopus oocytes.
Ion permeation properties of the mouse e2/zeta1 NMDA receptor channel expressed in Xenopus oocytes were studied using the outside-out patch-clamp technique. In symmetrical Na+ solutions, the single-channel I-V relations were almost linear at low electrolyte concentrations, but rectified inwardly for Na+ concentrations above 50 mm. In symmetrical Na+ solutions, the "zero-current conductance" increased with Na+ concentration and saturated according to a hyperbolic curve, the half-maximal saturating activity, KM(Na), being 14.2 mm and the maximal conductance, Gmax(Na), 53.9 pS. When Ca2+ was present with Na+ in the external solution, the single-channel current was lower than in pure Na+, although the reversal potential indicated a higher permeability for Ca2+ than for Na+. Using ion activities, PCa/PNa was found to be about 17. The I-V data were fitted with a model based on the Eyring's rate theory, assuming a one-ion pore with three energy barriers and two sites. The KM(Ca) and Gmax (Ca) were 76.5 microm and 21.2 pS, respectively. According to the estimated rate constants, KM for Ca2+ is mainly determined by the binding strength of a site located 80% away from the channel opening at the external membrane-solution interface, a position similar to that postulated previously for the Mg2+ blocking site.
Iino
M
M
Department of Physiology, Gunma University School of Medicine, 3-39-22 Showa-machi, Maebashi, 371, Japan.
Ciani
S
S
Tsuzuki
K
K
Ozawa
S
S
Kidokoro
Y
Y
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004553:Electric Conductivity; D005260:Female; D007473:Ion Channels / Q000502:physiology*; D008564:Membrane Potentials; D051379:Mice; D008954:Models, Biological; D009865:Oocytes / Q000502:physiology*; D018408:Patch-Clamp Techniques; D010539:Permeability; D016194:Receptors, N-Methyl-D-Aspartate / Q000096:biosynthesis*; D012964:Sodium / Q000378:metabolism
D007473:Ion Channels; D016194:Receptors, N-Methyl-D-Aspartate; D012964:Sodium; D002118:Calcium