Vol. 56, Issue 3, 619-632, September 1999

N-methyl-D-aspartate Excitotoxicity: Relationships among Plasma Membrane Potential, Na⁺/Ca²⁺ Exchange, Mitochondrial Ca²⁺ Overload, and Cytoplasmic Concentrations of Ca²⁺, H⁺, and K⁺

Lech Kiedrowski

The Psychiatric Institute, Departments of Psychiatry and Pharmacology, College of Medicine, The University of Illinois at Chicago, Chicago, Illinois

A high cytoplasmic Na⁺ concentration may contribute to N-methyl-D-aspartate (NMDA)-induced excitotoxicity by promoting Ca²⁺ influx via reverse operation of the Na⁺/Ca²⁺ exchanger (NaCaX), but may simultaneously decrease the electrochemical Ca²⁺ driving force by depolarizing the plasma membrane (PM). Digital fluorescence microscopy was used to compare the effects of Na⁺ versus ions that do not support the NaCaX operation, i.e., N-methyl-D-glucamine⁺ or Li⁺, on: PM potential; cytoplasmic concentrations of Ca²⁺, H⁺, and K⁺; mitochondrial Ca²⁺ storage; and viability of primary cultures of cerebellar granule cells exposed to NMDA receptor agonists. In the presence of Na⁺ or Li⁺, NMDA depolarized the PM and decreased cytoplasmic pH (pH_C); in the presence of Li⁺, Ca²⁺ influx was reduced, mitochondrial Ca²⁺ overload did not occur, and the cytoplasm became more acidified than in the presence of Na⁺. In the presence of N-methyl-D-glucamine⁺, NMDA instantly hyperpolarized the PM, but further changes in PM potential and pH_C were Ca-dependent. In the absence of Ca²⁺, hyperpolarization persisted, pH_C was decreasing very slowly, K⁺ was retained in the cytoplasm, and cerebellar granule cells survived the challenge; in the presence of Ca²⁺, pH_C dropped rapidly, the K⁺ concentration gradient across the PM began to collapse as the PM began to depolarize, and Ca²⁺ influx and excitotoxicity greatly increased. These results indicate that the dominant, very likely excitotoxic, component of NMDA-induced Ca²⁺ influx is mediated by reverse NaCaX and that direct Ca²⁺ influx via NMDA channels is curtailed by Na-dependent PM depolarization.