OXIDATIVE STRESS, HYPOXIA, AND ISCHEMIA-LIKE CONDITIONS INCREASE THE RELEASE OF ENDOGENOUS AMINO-ACIDS BY DISTINCT MECHANISMS IN CULTURED RETINAL CELLS

The aim of this study was to elucidate the mechanisms by which retinal cells release endogenous amino acids in response to ascorbate/Fe2+-in duced oxidative stress, as compared with chemical hypoxia or ischemia. In the absence of stimulation, oxidative stress increased the release of aspartate, glutamate, taurine, and GABA only when Ca2+ was present . Under hypoxia or ischemia, the release of aspartate, glutamate, glyc ine, alanine, taurine, and GABA increased mainly by a Ca2+-independent mechanism. The increased release observed in N-methyl-D-glucamine(+) medium suggested the reversal of the Na+-dependent amino acid transpor ters. Upon oxidative stress, the release of aspartate, glutamate, and GABA, occurring through the reversal of the Na+-dependent transporters , was reduced by about 30%, although the release of taurine was enhanc ed. An increased release of [H-3]arachidonic acid and free radicals se ems to affect the Na+-dependent transporters for glutamate and GABA in oxidized cells. All cell treatments increased [Ca2+](i) (1.5 to twofo ld), although no differences were observed in membrane depolarization. The energy charge of cells submitted to hypoxia or oxidative stress w as not changed. However, ischemia highly potentiated the reduction of the energy charge, as compared with hypoglycemia or hypoxia alone. The present work is important for understanding the mechanisms of amino a cid release that occur in vivo upon oxidative stress, hypoxia, or isch emia, frequently associated with the impairment of energy metabolism.