N-METHYL-D-ASPARTATE RECEPTORS EXPRESSED IN A NONNEURONAL CELL-LINE MEDIATE SUBUNIT-SPECIFIC INCREASES IN FREE INTRACELLULAR CALCIUM

N-methyl-D-aspartate (NMDA) receptors can mediate cell death in neuron s and in non-neuronal cells that express recombinant NMDA receptors. I n neurons, increases in intracellular calcium correlate with NMDA rece ptor-mediated death, supporting a key role for loss of cellular calciu m homeostasis in excitotoxic cell death, In the present study, free in tracellular calcium concentrations were examined in response to activa tion of recombinant NMDA receptors expressed in human embryonic kidney 293 cells. Intracellular calcium was measured in transfected cell pop ulations by cotransfection with the calcium-sensitive, bioluminescent protein aequorin and by single cell imaging with the fluorescent calci um indicator fluo-3. Agonist application to NR1/2A or NR1/2B-transfect ed cells elicited robust rises in intracellular calcium. NR1/2A respon ses were inhibited by the noncompetitive antagonists MK-801 and dextro methorphan and were dependent on extracellular calcium but not on intr acellular calcium stores. In contrast, no detectable intracellular cal cium responses were observed in NR1/2C-transfected cells, These findin gs indicate that NMDA receptors in the absence of other neuron-specifi c factors can mediate increases in intracellular calcium with subunit specificity and extracellular calcium dependence.