title: Transport characteristics of L-citrulline in renal apical membrane of proximal tubular cells.
abstract: L-Citrulline has diagnostic potential for renal function, because its plasma concentration increases with the progression of renal failure. Although L-citrulline extracted by glomerular filtration in kidney is mostly reabsorbed, the mechanism involved is not clearly understood. The present study was designed to characterize L-citrulline transport across the apical membranes of renal epithelial tubular cells, using primary-cultured rat renal proximal tubular cells, as well as the human kidney proximal tubular cell line HK-2. L-Citrulline was transported in a Na(+)-dependent manner from the apical side of both cell types cultured on permeable supports with a microporous membrane. Kinetic analysis indicated that the transport involves two distinct Na(+)-dependent saturable systems and one Na(+)-independent saturable system in HK-2 cells. The uptake was competitively inhibited by neutral and cationic, but not anionic amino acids. Relatively large cationic and anionic compounds inhibited the uptake, but smaller ones did not. In HK-2 cells, mRNA expression of SLC6A19 and SLC7A9, which encode B(0)AT1 and b(0,+)AT, respectively, was detected by RT-PCR. In addition, L-citrulline transport was significantly decreased in HK-2 cells in which either SLC6A19 or SLC7A9 was silenced. Hence, these results suggest that amino acid transporters B(0)AT1 and b(0,+)AT are involved in the reabsorption of L-citrulline in the kidney, at least in part, by mediating the apical membrane transport of L-citrulline in renal tubule cells.
authors.affiliation: Department of Membrane Transport and Pharmacokinetics, Faculty of Pharmacy, Institute of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kakuma-machi, Kanazawa, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Mitsuoka
authors.forename: Yoshiyuki
authors.initials: Y
authors.lastname: Shirasaka
authors.forename: Akimasa
authors.initials: A
authors.lastname: Fukushi
authors.forename: Masanobu
authors.initials: M
authors.lastname: Sato
authors.forename: Toshimichi
authors.initials: T
authors.lastname: Nakamura
authors.forename: Takeo
authors.initials: T
authors.lastname: Nakanishi
authors.forename: Ikumi
authors.initials: I
authors.lastname: Tamai