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 [3H])gamma-aminobutric acid (GABA) evoked by 50 mM KCI. 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 [3H]GABA evoked by K+-depolarization. Although the Ca2+-dependent release of [3H]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 [3H]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.
Agostinho
P
P
Center for Neurosciences of Coimbra, University of Coimbra, Portugal.
Duarte
C B
CB
Carvalho
A P
AP
Oliveira
C R
CR
D000818:Animals; D000975:Antioxidants / Q000494:pharmacology; D001205:Ascorbic Acid / Q000494:pharmacology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D002470:Cell Survival / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D002642:Chick Embryo; D004594:Electrophysiology; D018683:Excitatory Amino Acid Agents / Q000494:pharmacology; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D015227:Lipid Peroxidation / Q000187:drug effects / Q000502:physiology; D018377:Neurotransmitter Agents / Q000378:metabolism; D018384:Oxidative Stress / Q000187:drug effects*; D010539:Permeability; D011189:Potassium Chloride / Q000494:pharmacology; D012160:Retina / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D000975:Antioxidants; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D018683:Excitatory Amino Acid Agents; D018377:Neurotransmitter Agents; D005680:gamma-Aminobutyric Acid; D011189:Potassium Chloride; D001205:Ascorbic Acid