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P.N. Martin, P. Roon, S.B. Smith, V. Ganapathy; Expression Pattern of SMCT, a Sodium–Coupled Monocarboxylate Transporter, in Retina . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5150.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Neuronal cells in retina have high demands for metabolic substrates. In addition to glucose, these cells use lactate as energy substrate. Müller cells in retina produce lactate from glucose and release it into extracellular space. Neuronal cells take up lactate and use it for energy production. Monocarboxylate transporters (MCTs) have been implicated both in the release and uptake of lactate in retinal cells. Recently, we identified a new transporter (SMCT) that is capable of Na+–coupled electrogenic transport of short–chain fatty acids and lactate. SMCT is expressed abundantly in the kidney, intestine and colon; however, its expression in retina has not been studied. In the present study, we asked whether SMCT is expressed in retina, and if so, in which particular retinal cell types. Methods: SMCT was localized in intact retina by in situ hybridization and immunohistochemistry. Expression of SMCT mRNA in neural retina and RPE/eyecup was determined by RT–PCR. Similar techniques were used to analyze the expression of SMCT mRNA and protein in three retinal cell types: ARPE–19 (RPE), RGC–5 (ganglion), and RMC–1 (Müller) cells. Results:RT–PCR analysis and subsequent sequencing of the resultant products established that SMCT mRNA is expressed in both neural retina and RPE/eyecup in mouse eyes. Immunohistochemical analysis of intact mouse retina localized SMCT protein in the RPE, photoreceptor cell inner segments, outer nuclear layer, inner nuclear layer as well as in the retinal ganglion cell layer. In situ hybridization performed on intact mouse retina revealed a similar pattern of expression. Expression of SMCT protein was readily evident in RGC–5 and ARPE–19 cells; in contrast, only a weak expression was detectable in RMC–1 cells. Conclusions: SMCT is expressed mostly in neuronal cells and RPE cells in retina. Since SMCT functions in the Na+–coupled active uptake of lactate into cells, we postulate that this transporter is primarily responsible for the entry of lactate into retinal neuronal cells. These studies establish for the first time that MCTs are not the only transporters responsible for lactate uptake in retina.
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