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.