N-methyl-D-aspartate (NMDA) receptors can mediate cell death in neurons and in non-neuronal cells that express recombinant NMDA receptors. In neurons, increases in intracellular calcium correlate with NMDA receptor- mediated death, supporting a key role for loss of cellular calcium homeostasis in excitotoxic cell death. In the present study, free intracellular calcium concentrations were examined in response to activation of recombinant NMDA receptors expressed in human embryonic kidney 293 cells. Intracellular calcium was measured in transfected cell populations by cotransfection with the calcium-sensitive, bioluminescent protein aequorin and by single cell imaging with the fluorescent calcium indicator fluo-3. Agonist application to NR1/2A or NR1/2B-transfected cells elicited robust rises in intracellular calcium. NR1/2A responses were inhibited by the noncompetitive antagonists MK-801 and dextromethorphan and were dependent on extracellular calcium but not on intracellular calcium stores. In contrast, no detectable intracellular calcium responses were observed in NR1/2C- transfected cells. These findings indicate that NMDA receptors in the absence of other neuron-specific factors can mediate increases in intracellular calcium with subunit specificity and extracellular calcium dependence.
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