N-methyl-D-aspartate excitotoxicity:: Relationships among plasma membrane potential, Na+/Ca2+ exchange, mitochondrial Ca2+ overload, and cytoplasmic concentrations of Ca2+, H+, and K+

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.