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Masatoshi Tomi, Masahiko Mori, Masanori Tachikawa, Kazunori Katayama, Tetsuya Terasaki, Ken-ichi Hosoya; L-Type Amino Acid Transporter 1–Mediated l-Leucine Transport at the Inner Blood–Retinal Barrier. Invest. Ophthalmol. Vis. Sci. 2005;46(7):2522-2530. doi: 10.1167/iovs.04-1175.
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purpose. L-type amino acid transporters (LATs) prefer branched-chain and aromatic amino acids, including neurotransmitter precursors. The objective of this study was to clarify the expression and function of LAT at the inner blood–retinal barrier (BRB).
methods. [3H]l-Leucine transport at the inner BRB was characterized by using in vivo integration plot analysis and a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2). The expression of the LAT1 was demonstrated by quantitative real-time RT-PCR, immunoblot, and immunohistochemical analyses.
results. The apparent influx permeability clearance of [3H]l-leucine in the rat retina was found to be 203 μL/(min · g retina), supporting a carrier-mediated influx transport of l-leucine at the BRB. [3H]l-Leucine uptake by TR-iBRB2 cells was an Na+-independent and concentration-dependent process with a K m of 14.1 μM. This process was more potently cis inhibited by substrates of LAT1, d-leucine, d-phenylalanine, and d-methionine, than those of LAT2, l-alanine, and l-glutamine. [3H]l-Leucine efflux from TR-iBRB2 cells was trans-stimulated by substrates of LAT1. The expression of LAT1 mRNA was 100- and 15-fold greater than that of LAT2 in TR-iBRB2 and magnetically isolated rat retinal vascular endothelial cells, respectively. The expression of LAT1 protein was observed in TR-iBRB2 and primary cultured human retinal endothelial cells and immunostaining of LAT1 was observed along the rat retinal capillaries.
conclusions. LAT1 is expressed at the inner BRB and mediates blood-to-retina l-leucine transport. This transport system plays a key role in maintaining large neutral amino acids as well as neurotransmitters in the neural retina.
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