ACTIVITY OF IONOTROPIC GLUTAMATE RECEPTORS IN RETINAL CELLS - EFFECT OF ASCORBATE FE2+-INDUCED OXIDATIVE STRESS/

The effect of oxidative stress induced by the oxidant pair ascorbate/F e2+ on the activity of ionotropic glutamate receptors was studied in c ultured chick retina cells. The release of [H-3]GABA and the increase of the intracellular free Na+ concentration ([Na+](i)), evoked by glut amate receptor agonists, were used as functional assays for the activi ty of the receptors. The results show that the maximal release of [H-3 ]GABA evoked by kainate (KA; similar to 20% of the total) or AMPA (sim ilar to 11% of the total) was not different in control and peroxidized cells, whereas the EC(50) values determined for peroxidized cells (33 .6 +/- 1.7 and 8.0 +/- 2.0 mu M for KA and AMPA, respectively) were si gnificantly lower than those determined under control conditions (54.1 +/- 6.6 and 13.0 +/- 2.2 mu M for KA and AMPA, respectively). The max imal release of [H-3]GABA evoked by NMDA under K+ depolarization was s ignificantly higher in peroxidized cells (7.5 +/- 0.5% of the total) a s compared with control cells (4.0 +/- 0.2% of the total), and the eff ect of oxidative stress was significantly reduced by a phospholipase A (2) inhibitor or by fatty acid-free bovine serum albumin. The change i n the intracellular [Na+](i) evoked by saturating concentrations of NM DA under depolarizing conditions was significantly higher in peroxidiz ed cells (8.9 +/- 0.6 mM) than in control cells (5.9 +/- 1.0 mM), KA, used at a subsaturating concentration (35 mu M), evoked significantly greater increases of the [Na+](i) in peroxidized cells (11.8 +/- 1.7 m M) than in control cells (7.1 +/- 0.8 mM). A saturating concentration (150 mu M) of this agonist triggered similar increases of the [Na+](i) in control and peroxidized cells. Accordingly, the maximal number of binding sites for 0,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine m aleate ([H-3]MK-801) was increased after peroxidation, whereas the max imal number of binding sites for [H-3]KA was not affected by oxidative stress. These data suggest that under oxidative stress the activity o f the ionotropic glutamate receptors is increased, with the NMDA recep tor being the most affected by peroxidation.