Influence of oxidative stress on membrane potential and on K+ channels in neuronal cells

Effect of glucose deprivation and acute glutamate exposure in cultured retinal cells

The relationship between bioenergetics and the glutamate system was analyzed in a neuronal model of retinal cells in culture, submitted to glucose deprivation and exposed to glutamate for 2 h, and compared with exposure to glutamate in the presence of glucose. Under glucose deprivation, a reduction (about 1.1-fold) in the energy charge of the cells occurred, probably as a result of a decrement (by about 75%) in the cellular redox efficacy, as determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) test. In the absence of glucose, exposure of retinal cells to 10 microM glutamate potentiated the reduction in the energy charge (by about 1.2-fold) and induced a significant increase in the uptake of 45Ca2+ by the cells (1.3-fold), although no significant changes were observed in the presence of glucose. Under glucose deprivation, 100 microM glutamate caused an irreversible cell membrane damage, as shown by the significant increase in lactate dehydrogenase (LDH) leakage (about 1.8-fold). A significant increase in membrane depolarization, measured by the reduction of [3H]tetraphenylphosphonium+ ([3H]TPP+) uptake, was also observed after glutamate exposure in the absence of glucose. In the presence of glucose, high glutamate concentrations (10 mM) induced a major increase in Ca2+ entry into the cells and membrane depolarization, without affecting the energy charge or cell survival. In contrast, in the absence of glucose, 10 mM glutamate did not alter Ca2+ accumulation by the cells and a smaller decrease in membrane potential occurred, as compared to 100 microM glutamate exposure. Data shown in this study suggest that during a prolonged (2 h) and acute exposure to high glutamate (10 mM), under glucose deprivation conditions, the neuronal systems have "adaptive" mechanisms that allow the survival of cells. These findings may have implications in neuronal degeneration occurring during brain ischemia.

Intracellular free Na+ concentration increases in cultured retinal cells under oxidative stress conditions

The effect of oxidative stress, induced by ascorbate/Fe2+, on the intracellular free Na+ concentration ([Na+]i) of cultured chick retina cells was determined using the fluorescent indicator Na(+)-binding benzofuran isophthalate (SBFI). The resting[Na+]i of retina cells submitted to oxidative stress (15.5 +/- 1.9 mM) was significantly higher than that of control cells (8.9 +/- 0.8 mM). KCl (50 mM) depolarization induced a sustained [Na+]i increase (delta[Na+]i), which was significantly higher in peroxidized cells (8.1 +/- 0.7 mM) than in control cells (4.9 +/- 0.9 mM). The glutamate receptor antagonists, MK-801 and CNQX, reduced more significantly the initial delta[Na+]i induced by K(+)-depolarization under oxidative stress conditions (65% of inhibition), than in control cells (20% of inhibition). Moreover, in the presence of MK-801 and CNQX the increase in the [Na+]i, which was similar in control and peroxidized cells, was followed by a decrease towards a plateau. The Na+ channel blocker, tetrodotoxin (TTX), also reduced the sustained increase of the [Na+]i evoked by 50 mM KCl in both experimental conditions. However, TTX and glutamate receptor antagonists tested together failed to abolish the delta[Na+]i upon K(+)-depolarization, indicating that TTX-resistant Na+ channels were involved in the Na+ influx. The entry of Na+ through these channels contributed mainly to the early phase of the [Na+]i rise upon K(+)-depolarization, whereas the glutamate receptors seem to contribute more significantly to the [Na+]i response for stimulations longer than 30-50 s. The results suggest that an excessive activation of glutamate receptors increases the influx of Na+ and the resting [Na+]i under oxidative stress conditions.