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

Abstract

A high cytoplasmic Na(+) concentration may contribute to N-methyl-D-aspartate (NMDA)-induced excitotoxicity by promoting Ca(2+) influx via reverse operation of the Na(+)/Ca(2+) exchanger (NaCaX), but may simultaneously decrease the electrochemical Ca(2+) 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 Ca(2+), H(+), and K(+); mitochondrial Ca(2+) 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(+), Ca(2+) influx was reduced, mitochondrial Ca(2+) 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 Ca(2+), 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 Ca(2+), pH(C) dropped rapidly, the K(+) concentration gradient across the PM began to collapse as the PM began to depolarize, and Ca(2+) influx and excitotoxicity greatly increased. These results indicate that the dominant, very likely excitotoxic, component of NMDA-induced Ca(2+) influx is mediated by reverse NaCaX and that direct Ca(2+) influx via NMDA channels is curtailed by Na-dependent PM depolarization.

Glutamate activates cation currents in the plasma membrane of Arabidopsis root cells

“…Common features include voltage-independence, rapid activation upon changes in voltage and low selectivity among cations. The Ca 2+ permeability of the plant glutamate-activated currents is similar to that of NMDA-and unedited kainate/AMPA-selective receptors (Leinderszufall et al 1994;Kiedrowski 1999), rather than to edited AMPA receptors, which are poorly permeable for Ca 2+ . In our experiments, the sensitivity to glutamate was comparatively low, with 0.2-0.5 mM glutamate causing half-maximal activation.…”

Concurrent measurements of the free cytosolic concentrations of H+ and Na+ ions with fluorescent indicators

“…Changes in pH i and [Na + ] i may be linked directly through mechanisms as diverse as Na + /H + exchange [15], Na + -dependent Cl -/HCO 3 -exchange [29], electrogenic Na + /HCO 3 -cotransport [9] and the activities of transmitter reuptake mechanisms [35], or indirectly via the coordinated activities of two or more transport mechanisms. In a number of cell types, for example, a rise in [Na + ] i promotes reverse-mode Na + /Ca 2+ exchange and the subsequent rise in intracellular free Ca 2+ concentration ([Ca 2+ ] i ) can cause a fall in pH i by activating the acid-loading Ca 2+ ,H + -ATPase [16]. Furthermore, changes in pH i and [Na + ] i can influence the activities of not only pH i regulating transporters [13,15] but also mechanisms that contribute to Na + flux across biological membranes, including Na + /Ca 2+ exchange and Na + /K + /Cl -cotransport [5,30].…”

Normal brain ageing: models and mechanisms

“…Evidence for age-dependent alterations in the function of the plasma membrane Ca 2C ATPase (PMCA) has been presented, and might involve changes in the phosphorylation properties or calmodulin-binding properties (Zaidi et al 1998). Another Ca 2C removal system of significance in neuronal physiology is the PM Na C / Ca 2C exchanger (Hoyt et al 1998;Kiedrowski 1999). Recent exciting data revealed a new mechanism through which the activity of the Na C /Ca 2C exchanger can be modulated, involving a Ca 2C -dependent activation of proteolysis and resulting in a loss of function (Bano et al 2005).…”