Voltage and concentration dependence of Ca²⁺ permeability in recombinant glutamate receptor subtypes

The channels associated with glutamate receptor (GluR) subtypes, namely N-methyl-D-aspartate receptors (NMDARs), and Ca²⁺-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) and kainate receptors (KARs), are to varying degrees permeable to Ca²⁺. To compare the mechanism of Ca²⁺ influx, we measured Ca²⁺ permeability relative to that of Na⁺ (P_Ca/P_Na) using fractional Ca²⁺ currents (P_f) and reversal potential measurements over a wide voltage and Ca²⁺ concentration range in recombinant NMDAR NR1-NR2A, AMPAR GluR-A(Q) and KAR GluR-6(Q) channels. For NR1-NR2A channels, P_Ca/P_Na derived from P_f measurements was voltage independent but showed a weak concentration dependence. A stronger concentration dependence was found when P_Ca/P_Na was derived from changes in reversal potentials on going from a Na⁺ reference solution to a solution with Ca²⁺ as the only permeant ion ('biionic' condition). In contrast, P_Ca/P_Na was concentration independent when derived from changes in reversal potentials on going from a Na⁺ reference solution to the same solution with added Ca²⁺ ('high monovalent' condition). For GluR-A(Q) channels, P_Ca/P_Na derived from all three approaches was concentration independent, and for the reversal potential-based approaches were of comparable magnitude. Their most distinctive property was that P_Ca/P_Na derived from P_f measurements was strongly voltage dependent. For GluR-6(Q) channels, P_Ca/P_Na derived from P_f measurements was weakly voltage dependent. On the other hand, P_Ca/P_Na derived from all three approaches was the most strongly concentration dependent of any GluR subtype and, except for low Ca²⁺ concentrations, the values were of comparable magnitude. Thus, the three Ca²⁺-permeable GluR subtypes showed unique patterns of Ca²⁺ permeability, indicating that distinct biophysical and molecular events underlie Ca²⁺ influx in each subtype.