Functional activity of a large neutral amino acid transporter (LAT) in rabbit retina: A study involving the in vivo retinal uptake and vitreal pharmacokinetics of l-phenyl alanine

Abstract

Purpose The purpose of this study is to elucidate the functional activity of large neutral amino acid transporter (LAT) in rabbit retina and to delineate its role in the retinal uptake and intravitreal pharmacokinetics of l-phenylalanine ( l-Phe). Methods In vivo retinal uptake of l-Phe and l-alanine ( l-Ala) was determined in the presence and absence of specific transport inhibitors following intravitreal administration. l and d isomers of amino acids were employed as inhibitors to determine the stereo-selectivity of LAT. Reverse transcription-polymerase chain reaction (RT-PCR) was carried out for LAT isoforms (LAT1 and LAT2). Vitreal disposition of l-Phe following administration in rabbit vitreous was studied in the presence of an other competing LAT substrate d-methionine using microdialysis. Results Retinal uptake of l-Phe was significantly inhibited in presence of a specific LAT inhibitor, 2-aminobicyclo-[2,2,1]-heptane-2-carboxylic acid (BCH), l isomers of large neutral amino acids and LAT1 specific but not by LAT2 specific and charged amino acids. No significant inhibition of l-Ala retinal uptake was observed with LAT substrates. LAT isoforms (LAT1 and LAT2) were identified by RT-PCR in rabbit retina. The mean residence time (MRT) and area under curve (AUC) values of l-Phe following intravitreal adminstration were significantly increased in the presence of d-methionine, a LAT substrate. Conclusions This study demonstrates the functional activity and molecular expression of large neutral amino acid transporter in the rabbit retina. Furthermore, based on these studies it can be concluded that LAT is involved in the retinal uptake and intravitreal elimination of l-Phe.