Permeation Properties of Na⁺ and Ca²⁺ Ions through the Mouse ε₂/ζ₁ NMDA Receptor Channel Expressed in Xenopus Oocytes

Abstract.

Ion permeation properties of the mouse e₂/ζ₁ 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, K _M (Na), being 14.2 mm and the maximal conductance, G _max(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, P_Ca/P_Na 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 K _M (Ca) and G _max (Ca) were 76.5 μm and 21.2 pS, respectively. According to the estimated rate constants, K _M 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.