Different Mechanisms of Ca(2+) Transport in NMDA and Ca(2+)-permeable AMPA Glutamate Receptor Channels

The channel of the glutamate N-methyl-d-aspartate receptor (NMDAR) transports Ca(2+) approximately four times more efficiently than that of Ca(2+)-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPAR). To investigate the basis of this difference in these glutamate receptors (GluRs), we measured the ratio of Cs(+) efflux and Ca(2+) influx in recombinant NMDAR and Ca(2+)-permeable AMPAR channels expressed in human embryonic kidney 293 (HEK 293) cells over a wide voltage range. At any one potential, this biionic flux ratio was measured by quantifying the total charge and the charge carried by Ca(2+) using whole-cell currents and fluorometric techniques (dye overload) with Cs(+) internally and Ca(2+) externally (1.8 or 10 mM) as the only permeant ions. In AMPAR channels, composed of either GluR-A(Q) or GluR-B(Q) subunits, the biionic flux ratio had a biionic flux-ratio exponent of 1, consistent with the prediction of the Goldman-Hodgkin-Katz current equation. In contrast, for NMDAR channels composed of NR1 and NR2A subunits, the biionic flux-ratio exponent was ∼2, indicating a deviation from Goldman-Hodgkin-Katz. Consistent with these results, in NMDAR channels under biionic conditions with high external Ca(2+) and Cs(+) as the reference ions, Ca(2+) permeability (P(Ca)/P(Cs)) was concentration dependent, being highest around physiological concentrations (1–1.8 mM; P(Ca)/P(Cs )≈ 6.1) and reduced at both higher (110 mM; P(Ca)/P(Cs )≈ 2.6) and lower (0.18 mM; P(Ca)/P(Cs )≈ 2.2) concentrations. P(Ca)/P(Cs) in AMPAR channels was not concentration dependent, being around 1.65 in 0.3–110 mM Ca(2+). In AMPAR and NMDAR channels, the Q/R/N site is a critical determinant of Ca(2+) permeability. However, mutant AMPAR channels, which had an asparagine substituted at the Q/R site, also showed a biionic flux-ratio exponent of 1 and concentration-independent permeability ratios, indicating that the difference in Ca(2+) transport is not due to the amino acid residue located at the Q/R/N site. We suggest that the difference in Ca(2+) transport properties between the glutamate receptor subtypes reflects that the pore of NMDAR channels has multiple sites for Ca(2+), whereas that of AMPAR channels only a single site.