Competitive inhibition of NMDA receptor-mediated currents by extracellular calcium chelators. J Neurophysiol 84: 693-697, 2000. Calcium chelators have been widely used in electrophysiological recordings of N-methyl-D-aspartate (NMDA) receptor-mediated currents, as well as in studies of excitotoxicity. Intracellularly applied calcium chelators are known to inhibit, at least in part, such calciumdependent processes as calmodulin-dependent inactivation, calcineurin-dependent desensitization, and

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... ndown of NMDA receptors. On the other hand, the functional consequences and potential nonspecific effects of extracellularly applied chelators have not been extensively investigated. In whole-cell patch-clamp recordings from human embryonic kidney (HEK) 293 cells transiently transfected with recombinant NMDA receptors, we found that addition of calcium chelators such as EGTA shifted the glutamate dose-response curve to the right, from an EC 50 for NR1A/NR2A of 8 M in 1.8 mM Ca 2ϩ to ϳ24 M in a solution containing nominal 0 Ca 2ϩ /5 mM EGTA and further to ϳ80 M in 20 mM EGTA. A similar shift in glutamate dose-response was observed for NR1A/NR2B currents. This doseresponse shift was not due to a decrease in extracellular Ca 2ϩ concentration because there was no change in the glutamate EC 50 at Ca 2ϩ concentrations ranging from 10 mM to nominal 0/200 M EGTA. Moreover, addition of 5 mM EGTA fully chelated with 6.8 mM Ca 2ϩ did not produce any shift in the glutamate dose-response curve. We propose that calcium chelators, containing four free carboxyl moieties, competitively inhibit glutamate binding to NMDA receptors.