INTRACELLULAR FREE NA+ CONCENTRATION INCREASES IN CULTURED RETINAL CELLS UNDER OXIDATIVE STRESS CONDITIONS

The effect of oxidative stress, induced by ascorbate/Fe2+, on the intr acellular free Na+ concentration ([Na+](i)) of cultured chick retina c ells was determined using the fluorescent indicator Na+-binding benzof uran isophthalate (SBFI). The resting [Na+](i) of retina cells submitt ed 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 in duced a sustained [Na+](i) increase (Delta[Na+](i)), which was signifi cantly higher in peroxidized cells (8.1 +/- 0.7 mM) than in control ce lls (4.9 +/- 0.9 mM). The glutamate receptor antagonists, MK-801 and C NQX, 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 p lateau. The Na+ channel blocker, tetrodotoxin (TTX), also reduced the sustained increase of the [Na+](i) evoked by 50 mM KCl in both experim ental conditions. However, TTX and glutamate receptor antagonists test ed together failed to abolish the Delta[Na+](i) upon K+-depolarization , indicating that TTX-resistant Na+ channels were involved in the Nainflux. The entry of Na+ through these channels contributed mainly to the early phase of the [Na+](i) rise upon K+-depolarization, whereas t he glutamate receptors seem to contribute more significantly to the [N a+](i) response for stimulations longer than 30-50 s. The results sugg est that an excessive activation of glutamate receptors increases the influx of Na+ and the resting [Na+](i) under oxidative stress conditio ns.