Oxidative stress affects the selective ion permeability of voltage-sensitive Ca2+ channels in cultured retinal cells

The effect of ascorbate/Fe2+-induced oxidative stress on the intracellular Ca2+ concentration ([Ca2+](i)) and on the voltage-sensitive Ca2+ channels (VSCC) of chick retinal cells was evaluated in this study. We also analyzed the effect of oxidation on the intracellular Na+ concentration ([Na+](i)) and on the Ca2+-dependent release of [H-3]gamma-aminobutric acid (GABA) evoked by 50 mM KCl. The resting [Ca2+](i) was not affected by oxidation, but the [Ca2+](i) response (Delta[Ca2+](i)) to K+-depolarization was significantly inhibited under oxidative stress conditions. The Ca2+ influx stimulated by membrane depolarization was mediated by L- and N-type VSCC, and by N-metyl-D-aspartate (NMDA) receptor channel, activated by endogenous glutamate released by glutamatergic cells. In cultured retinal cells L-type channels are the major route of Ca2+ influx during depolarization and the most affected by oxidative stress. The N-type VSCC seem not to be affected by oxidant conditions; they were found to be involved in glutamatergic transmission and only indirectly in the release of [H-3]GABA evoked by K+-depolarization. Although the Ca2+-dependent release of [H-3]GABA evoked by 50 mM KCl is mediated by Ca2+ entry through L-type Ca2+ channels, it is not affected by pre-incubation with the oxidant pair. The oxidative stress conditions increased the [Na+](i) in Ca2+-free medium, by a process dependent of Na+ entry through L-type VSCC. The increased permeability of L-type VSCC to Na+ may increase the Ca2+-independent release of endogenous glutamate which, by activating the NMDA receptors, induces the release of [H-3]GABA by reversal of its transporter. The equilibrium between the release of GABA and glutamate may play an in important role in neuroprotection against excitotoxic insults. (C) 1997 Elsevier Science Ireland Ltd.