Role of ammonia in reversal of glutamate‐mediated Müller cell swelling in the rat retina

Abstract

Glutamate is thought to participate in a variety of retinal degenerative disorders. However, when exposed to glutamate at concentrations up to 1 mM, ex vivo rat retinas typically exhibit Müller cell swelling, but not excitotoxic neuronal damage. This Müller cell swelling is reversible following glutamate washout, indicating that the glial edema is not required for glutamate-induced neuronal injury. It is unclear whether glutamate directly induces the Müller cell swelling or whether a metabolite of glutamate such as glutamine acts as an osmolyte to generate the cellular edema. To examine this issue, ex vivo rat retinas were exposed to 1 mM glutamate or 1 mM glutamine and were evaluated histologically. Glutamate was also combined with 1 mM ammonia or with methionine sulfoximine (MSO), an inhibitor of glutamine synthetase, the enzyme that catalyzes the synthesis of glutamine from glutamate and ammonia. Glutamate-mediated Müller cell swelling was blocked by co-administration of ammonia and the reversibility of Müller cell swelling was inhibited by MSO administered following glutamate exposure. Glutamine alone failed to induce Müller cell swelling. These results indicate that glutamate-mediated Müller cell swelling is unlikely to result from glutamine accumulation. Rather, conversion of glutamate to glutamine in a reaction involving ammonia helps reverse Müller cell swelling following exposure to exogenous glutamate.

Effects of Acutely Elevated Hydrostatic Pressure in a Rat Ex Vivo Retinal Preparation

“…A previous report demonstrating a decline in GS immunoreactivity in Müller cells in a canine model of glaucoma is consistent with this notion. 11 As has been reported, 37 Müller cell swelling is reversible if glutamate is converted to glutamine via GS, as the inhibition of GS disrupts the reversibility. However, Müller cell swelling was irreversible in hyperbaric conditions.…”

“…We had observed in previous work that inhibition of glutamate uptake without adding exogenous glutamate leads to severe excitotoxic degeneration. 5,37 To examine these questions histologically, we incubated ex vivo rat retinal segments that were exposed to elevated hydrostatic pressure with 1.0 mM glutamate at normal pressure for short periods. In this protocol, Müller cell swelling was apparent, indicating that glial glutamate uptake is preserved after pressure loading.…”

Glutamate, Excitatory Amino Acid Transporters, Xc−Antiporter, Glutamine Synthetase, and γ-Glutamyltranspeptidase in Human Corneal Epithelium

“…The unique localization of glutathione-S-transferase in the wing/flat superficial epithelial cell layer, 38 also supports the detoxification potential of these interdigitating, tightly bound corneal epithelial cells. Further, conversion of glutamate by GS in wing and flat superficial epithelial cells may regulate cell volume and gap junctions 34,39 and protect against ammonia toxicity as reported for retinal astrocytes and Müller glial cells. 40 Superficial epithelial cells in some corneal sections contained high levels of glutamate and some expressed EAAT1-5, Xc − antiporter, and GGT.…”

Aloin Inhibits Müller Cells Swelling in a Rat Model of Thioacetamide-Induced Hepatic Retinopathy

“…However, Lucas and Newhouse reported that administration of ammonia did not induce acute toxicity in the retina [ 42 ]. Izumi et al also reported that when a retinal segment was incubated with 1 mM ammonia for 60 min, there was little effect on Müller cells [ 43 ]. Although a retina-specific inhibitory effect of aloin was not shown in this study, we demonstrated that aloin led to the reduction of gliotic alterations of Müller cells and prevented Müller cell swelling mediated by the downregulation of Kir4.1 channels.…”