To quantify the ion-permeation ability of the recombinant ε1-4/ζ1 channel activated by agonists, the magnitude of agonist-induced integrated single-channel currents for the ε2-4/ζ1 N-methyl-D-aspartate (NMDA) channels in bilayer lipid membranes (BLMs) was evaluated electrochemically based on the single-channel recordings. The recombinant ε2/ζ1 channels were purified from Chinese hamster ovary cells expressing each channel and incorporated in BLMs formed by the tip-dip method. Three typical agonists, L-glutamate, NMDA, and (2S, 3R, 4S) isomer of 2-(carboxycyclopropyl)glycine (L-CCG-IV, were investigated at a concentration of 50 μM. The magnitude of L-glutamate-induced integrated current was found to depend on the ε-subunit composition and to increase in the order of ε2/ζ1 > ε1/ζ1 ≃ ζ4/ζ1 > ζ3/ζ1, which differs from that of the reported binding affinities (EC50) between L-glutamate and each channel type. On the other hand, the magnitude of the integrated currents induced by NMDA and L-CCG-IV did not vary among the four channel types. The order of agonist selectivity toward the ε2-4/ζ1 channels in terms of the magnitude of the integrated current was L-glutamate > L-CCG-IV ≃ (NMDA for the ε2/ζ1 channel, L-CCG-IV > NMDA > L-glutamate for the ε3/ζ1 channel, and L-CCG-IV ≃ L-glutamate > NMDA for the ε4/ζ1 channel, suggesting that the agonist selectivity also depends on the ε-subunit composition. The present study shows that each ε1-4/ζ1 channel has its own ability of ion permeation, i.e., its own signal transduction ability, which is not parallel to its binding ability. (C) 2000 Academic Press.
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