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.