Glutamate is the excitatory neurotransmitter in the retina, but it is neurotoxic in excessive amounts. A decrease in the ability of Müller cells to remove glutamate from the extracellular space may play a crucial role in the disruption of glutamate homeostasis that occurs in the diabetic retina. Previously we have shown that taurine has protective effects against diabetes-induced glutamate dysmetabolism in retinal Müller cells. The aim of this study is to examine the effects and underlying mechanism of taurine on high glucose-induced alterations of Müller cells glutamate uptake and degradation. Müller cells cultures were prepared from 5- to 7-day-old Sprague-Dawley rats. Glutamate uptake was measured as (3)H-glutamate content of the lysates. Glutamine synthetase (GS) activity was assessed by a spectrophotometric assay. The expressions of glutamate transporters (GLAST) and GS were examined by RT-PCR and western-blot. In 25 mmol/l high glucose-treated cultures, Müller cells glutamate uptake, GS activity and GLAST, GS expressions were decreased significantly compared with 5 mmol/l normal glucose cultures (p<0.05). Taurine (1 and 10 mmol/l) significantly inhibited the high glucose-induced decreases in glutamate uptake, GS activity and GLAST, GS expressions (p<0.05). The generation of TBARS, ROS and NO in Müller cells increased significantly after treatment with high glucose compared with normal glucose. However, treatment of 1 and 10 mmol/l taurine resulted in a significant decrease in TBARS, ROS and NO levels (p<0.05). The high glucose treatment decreased antioxidant enzyme (catalase, SOD and GSH-px) activities compared with normal glucose. Taurine treatment increased the catalase, SOD and GSH-px activity in a dose-dependent manner. These findings suggest that taurine may regulate Müller cells' glutamate uptake and degradation under diabetic conditions via its antioxidant mechanism.
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