Characterization of L-arginine transporters in rat renal inner medullary collecting duct

Previous work from our laboratory has demonstrated that the inner medullary collecting duct (IMCD) expresses a large amount of nitric oxide synthase ( NOS) activity. The present study was designed to characterize the transport of NOS substrate, L-arginine, in a suspension of bulk-isolated IMCD cells from the Sprague-Dawley rat kidney. Biochemical transport studies demonstra ted an L-arginine transport system in IMCD cells that was saturable and Na independent (n = 6). L-Arginine uptake by IMCD cells was inhibited by the cationic amino acids L-lysine, L-homoarginine, and L-ornithine (10 mmol/l e ach) and unaffected by the neutral amino acids L-leucine, L-serine, and L-g lutamine. Both L-ornithine (n = 6) and L-lysine (n = 6) inhibited NOS enzym atic activity in a dose-dependent manner in IMCD cells, supporting the impo rtant role of L-arginine transport for NO production by this tubular segmen t. Furthermore, RT-PCR of microdissected IMCD confirmed the presence of cat ionic amino acid transporter CAT1 mRNA, whereas CAT2A, CAT2B, and CAT3 were not detected. These results indicate that L-arginine uptake by IMCD cells occurs via system y(+), is encoded by CAT1, and may participate in the regu lation of NO production in this renal segment.