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In the present study, we investigated the effect of naturally occurring and synthetic peroxides on K+-depolarization-evoked release of [3H]D-aspartate from bovine isolated retinae. Furthermore, effect of peroxides on endogenous glutamate concentrations were measured by HPLC in bovine neural retinae and vitreous humor of eyes treated with hydrogen peroxide (H2O2) ex vivo. Both naturally occurring H2O2 (1-100 microM) and synthetic (cumene hydroperoxide, cuOOH; 1-100 microM) peroxides caused a concentration-dependent inhibition of K+-evoked [3H]D-aspartate release without affecting basal tritium efflux. The antioxidant, trolox (2 mM) prevented the inhibition of evoked [3H]D-aspartate overflow elicited by both H2O2 (30 microM) and cuOOH (10 microM). Inhibition of catalase by 3-amino-triazole (3- AT 100 mM) enhanced an inhibitory effect of a low concentration of H2O2 (1 microM) but antagonized the effect of H2O2 (30 microM) on K+-induced [3H]D-aspartate release. In ex vivo experiments, exogenously applied H2O2 (1-100 microM) also caused a concentration-related decrease in glutamate levels in the bovine retina. We conclude that peroxides can inhibit K+-evoked release of [3H]D-aspartate and also decrease endogenous glutamate concentrations in the bovine retina.

The glucose deprivation-induced release of [3H]D-aspartate was studied in bovine and human retinas in a superfusion apparatus. [3H]D-aspartate release was significantly increased upon omitting glucose in the superfusion buffer. This effect was dependent on external Ca2+ because L- and N-type Ca2+-channel blockers, such as diltiazem (1 microM), nitrendipine (1 microM), and omega-conotoxin (100 nM), significantly reduced the effect of glucose-deprivation induced release of [3H]D-aspartate. Furthermore, while glutamate receptor agonists (L-glutamate, N-methyl-D-aspartate, but not kainate) potentiated the effects of glucose deprivation, antagonists (MK-801, MCPG, ifenprodil, and L-AP3) at these receptors blocked the glucose deprivation-induced release process. Taken together, these studies have demonstrated that under conditions of glucose deprivation, as may happen during ischemic events in vivo, the retinal glutamatergic nerve endings and/or glial cells promote the efflux of [3H]D-aspartate into the extracellular environment. This process appears to be receptor-mediated and dependent on extracellular Ca2+ and is similar to previous reports pertaining to brain tissues.

The glucose deprivation-induced release of [[sup 3]H]D-aspartate was studied in bovine and human retinas in a superfusion apparatus. [[sup 3]H]D-aspartate release was significantly increased upon omitting glucose in the superfusion buffer. This effect was dependent on external Ca[sup 2+] because L- and N-type Ca[sup 2+]-channel blockers, such as diltiazem (1 μM), nitrendipine (1 μM), and ω-conotoxin (100 nM), significantly reduced the effect of glucose-deprivation induced release of [[sup 3]H]D-aspartate. Furthermore, while glutamate receptor agonists (L-glutamate, N-methyl-D-aspartate, but not kainate) potentiated the effects of glucose deprivation, antagonists (MK-801, MCPG, ifenprodil, and L-AP3) at these receptors blocked the glucose deprivation-induced release process. Taken together, these studies have demonstrated that under conditions of glucose deprivation, as may happen during ischemic events in vivo, the retinal glutamatergic nerve endings and/or glial cells promote the efflux of [[sup 3]H]D-aspartate into the extracellular environment. This process appears to be receptor-mediated and dependent on extracellular Ca[sup 2+] and is similar to previous reports pertaining to brain tissues. [ABSTRACT FROM AUTHOR]

Purpose. The aim of the present study was two-fold: (a) to examine the effect of hypoxia on [³H]D-aspartate release from isolated bovine and human retinae, and (b) to investigate the regulation of hypoxia-induced neurotransmitter release by glutamate receptor agonists and antagonists. Methods. Isolated neural retinae were incubated in oxygenated Krebs buffer solution containing [³H]D-aspartate and then prepared for studies of neurotransmitter release using the superfusion method. Release of [³H]D-aspartate was evoked by K[sup +] (50mM) applied at 90 minutes (S[sub 1]) and hypoxia (induced by exposure of tissues to solutions pregassed with 95% N[sub 2]: 5% CO[sub 2] for 60 minutes) at 108 minutes (S[sub 2]) after onset of superfusion. Results. Under hypoxic conditions, pO[sub 2] in normal Krebs buffer solution was reduced from 14.53 ± 0.26 ppm (n = 6) to 0.54 ± 0.04 ppm (n = 9) after one hour of gassing with 95% N[sub 2]: 5% CO[sub 2]. Exposure to hypoxia elicited an overflow of [³H]D-aspartate yielding S[sub 2]/S[sub 1] ratios of 0.62 ± 0.06 (n = 12) and 0.54 ± 0.03 (n = 8) in bovine and human tissues respectively. In isolated bovine retinae, L- and N-calciumchannel antagonists diltiazem, nitrendipine, verapamil and ω-conotoxin significantly (p < 0.01 or higher) attenuated hypoxia-induced [³H]D-aspartate release. L-glutamate (30 µM) significantly (p < 0.001) potentiated hypoxiainduced [³H]D-aspartate release whereas kainate (30 µM) inhibited this response. NMDA (in concentrations up to 1 mM) had no effect on hypoxia-induced [³H]D-aspartate release. Antagonists of glutamate receptors and the polyamine site on the NMDA receptor inhibited hypoxia-induced release of [³H]D-aspartate in bovine retina with the following rank order of activity: ifenprodil ≅ MCPG > L-AP3 > MK-801. At an equimolar concentration (10 µM), L-AP3 but not ifenprodil, MCPG, MK 801 or arcaine,... [ABSTRACT FROM AUTHOR]