Regulation of glutamate metabolism by hydrocortisone and branched chain keto acids in cultured rat retinal Muller cells (TR-MUL)

Glutamate released from retinal neurons during neurotransmission is taken up by retinal Muller cells, where much of the amino acid is subsequently amidated to glutamine or transaminated to alpha-ketoglutarate for oxidation. Muller cell glutamate levels may have to be carefully maintained at fairly low concentrations to avoid excesses of glutamate in extracellular spaces of the retina that would otherwise cause excitotoxicity. We employed a cultured rat retinal Muller cell line in order to study the metabolism and the role of Muller cell specific enzymes on the glutamate disposal pathways. We found that the TR-MUL cells express the glial specific enzymes, glutamine synthetase, the mitochondrial isoform of branched chain aminotransferase (BCATm) and pyruvate carboxylase, all of which are involved in glutamate metabolism and homeostasis in the retina. Hydrocortisone treatment of TR-MUL cells increased glutamine synthetase expression and the rate of glutamate amidation to glutamine. Addition of branched chain keto acids (BCKAs) increased lactate and aspartate formation from glutamate and also oxidation of glutamate to CO2 and H2O. The two glutamate disposal pathways (amidation and oxidation) did not influence each other. When glutamate levels were independently depleted within TR-MUL cells, the uptake of glutamate from the extracellular fluid increased compared to uptake from control (undepleted) cells suggesting that the level of intracellular glutamate may influence clearing of extracellular glutamate. (C) 2011 Elsevier B.V. All rights reserved.