The Journal of Neurophysiology Vol. 84 No. 2 August 2000, pp. 693-697
Copyright ©2000 by the American Physiological Society

Competitive Inhibition of NMDA Receptor-Mediated Currents by Extracellular Calcium Chelators

 ¹Kinsmen Laboratory of Neurological Research, Department of Psychiatry,  ²Department of Physiology, and  ³Department of Medicine, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada

Chen, Nansheng, Timothy H. Murphy, and Lynn A. Raymond. 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 calcium-dependent processes as calmodulin-dependent inactivation, calcineurin-dependent desensitization, and rundown 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 EC₅₀ for NR1A/NR2A of 8 µM in 1.8 mM Ca²⁺ to ~24 µM in a solution containing nominal 0 Ca²⁺/5 mM EGTA and further to ~80 µM in 20 mM EGTA. A similar shift in glutamate dose-response was observed for NR1A/NR2B currents. This dose-response shift was not due to a decrease in extracellular Ca²⁺ concentration because there was no change in the glutamate EC₅₀ at Ca²⁺ concentrations ranging from 10 mM to nominal 0/200 µM EGTA. Moreover, addition of 5 mM EGTA fully chelated with 6.8 mM Ca²⁺ 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.