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N.N. Osborne, C.J. Layton, J.P. Wood, M. Graham, G. Chidlow; Localisation of Monocarboxylate Transporter Subtypes 1-4 in the Rat Eye . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4584.
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Purpose: To determine the presence and localisation of the monocarboxylate transporter (MCT) subtypes 1-4 in the rat eye. Methods: Retina, iris, lens and cornea were procured from Wistar rat eyes. In order to analyse the retinal pigmented epithelium (RPE), monolayer cultures were produced from Lister Hooded rats and these were grown from initial harvest through to the fifth passage. Tissues were analysed to determine the presence of mRNA and proteins for MCTs 1-4 using electrophoresis/Western blotting and both real-time (Taqman) and reverse transcription-polymerase chain reaction (RT-PCR) procedures. Sections from whole Wistar rat eyes that had been fixed in Davidson’s were immunohistochemically processed for the localisation of MCTs 1-4. Ocular tissues were also analysed from Wistar rats that had been exposed to intense light for 48 hours and in aged dystrophic RCS rats to determine the effect of photoreceptor loss on retinal MCTs. Results: MCT1 mRNA and protein are present in the iris and in cultured RPE cells and appear to be particularly located to the photoreceptors of the retina. MCT1-immunoreactivity is restricted to the apical side of the RPE in situ, to photoreceptor outer segments and to iris epithelial cells. MCT2 mRNA and proteins are barely detectable in the iris epithelium but clearly present in cultured RPE cells, the whole of the retina and the cornea. MCT3 was expressed at high levels in primary RPE cultures, but unlike MCT1 and MCT2, mRNA levels declined through increasing passage and were no longer detectable by passage 5. Surprisingly, MCT3 mRNA was also faintly detectable in the iris and in the retina. mRNA for MCT3 also appeared to exist in the inner retina, since the signal was elevated in both intense light-treated Wistar and dystrophic RCS rats, both of which had extensive photoreceptor loss. PCR and immunoblotting experiments indicated that MCT4 expression was widespread (retina, iris, lens epithelium, cornea). MCT4 expression in cultured RPE cells also appeared to increase with passage time, reaching the highest level by passage 5. Conclusions: MCTs 1-4 are widely expressed in a differential manner in ocular tissues. The distribution of the individual MCT subtypes likely reflects their specific transport properties for defined monocarboxylates such as lactate or pyruvate.
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