Permeation Properties of Na + and Ca 2+ Ions through the Mouse ε 2 /ζ 1 NMDA Receptor Channel Expressed in Xenopus Oocytes

Abstract

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.

Glutamate Receptor Ion Channels: Structure, Regulation, and Function

“…Potassium channels can accommodate multiple permeating ions, and this is a key component of their high selectivity and transport rates (Doyle et al, 1998;Morais-Cabral et al, 2001;. Studies of NMDA receptors suggest that the pore contains only a single permeating ion (Zarei and Dani, 1994;Schneggenburger, 1996;Iino et al, 1997), which may reflect the nonselective nature of the pore. In K ϩ channels as well as in other highly selective channels, the interaction between the narrow constriction and permeating ions is tight and this reduces their free energy in this region.…”

Permeant ion regulation of N -methyl- d -aspartate receptor channel block by Mg 2+

“…If they were the only, or the highest-affinity, monovalent permeant ion-binding sites in the channel, then half-saturation of NMDA-receptor singlechannel current should occur when half these sites are occupied. Previous estimates of the permeant monovalent cation concentrations at which half-maximal saturation of NMDA-receptor single-channel currents occurs range from 0.5 to 129 mM (8,10,23,40). The variation in these measurements, which were made under differing conditions and with different permeant ions, prevents their use in assessing the potential role of the permeant ion-binding sites in channel permeation.…”

Evaluation and Comparison of Ion Permeation and Agonist Selectivities for N-Methyl- -Aspartate Receptor Channels with Different Subunit Compositions in Bilayer Lipid Membranes Based on Integrated Single-Channel Currents

“…The criterion described under Materials and Methods ascertained that for all the records the L-glutamate-induced channel current arose from the same ion channel in each BLM. It will be interesting to compare the single-channel response of the ⑀2/1 channel in BLMs with the ⑀2/1 channel expressed in Xenopus oocytes (14,28,29). With the ⑀2/1 channel in Xenopus oocytes, a single-channel conductance of 47.4 pS was mainly observed in 1 mM Ca 2ϩ , while in the present BLM system, several lower conductance levels out of ϳ16, ϳ22, ϳ28, and ϳ33 pS, accompanied by the maximum level of ϳ44 pS, were observed.…”