title: Na+-dependent and Na+-independent betaine transport across the apical membrane of rat renal epithelium.
abstract: The low renal excretion of betaine indicates that the kidney efficiently reabsorbs the betaine filtered by the glomeruli but the mechanisms involved in such a process have been scarcely investigated. We have detected concentrative and non-concentrative betaine transport activity in brush-border membrane vesicles (BBMV) from rat renal cortex and medulla. The concentrative system is the Sodium/Imino-acid Transporter 1 (SIT1) because it is Na+- and Cl--dependent, electrogenic and is inhibited by an anti-SIT1 antibody. Its apparent affinity constant for betaine, Kt, is 1.1±0.5 mM and its maximal transport velocity, Vmax, 0.5±0.1 nmol betaine/mg protein/s. Inhibitors of the Na+/Cl-/betaine uptake are L-proline (75%) and cold betaine, L-carnitine and choline (40-60%). Neither creatine, TEA, taurine, β-alanine, GABA nor glycine significantly inhibited Na+/Cl-/betaine uptake. The non-concentrative betaine transport system is Na+- and H+-independent, electroneutral, with a Kt for betaine of 47±7 μM and a Vmax of 7.8±1 pmol betaine/mg protein/s. Its transport activity is nearly abolished by betaine, followed by L-carnitine (70-80%) and proline (40-50%), but a difference from the Na+/Cl-/betaine transport is that it is inhibited by TEA (approx. 50%) and unaffected by choline. The underlying carrier functions as an antiporter linking betaine entry into the BBMV with the efflux of either L-carnitine or betaine, an exchange unaffected by the anti-SIT1 antibody. As far as we know this is the first work reporting that betaine crosses the apical membrane of rat renal epithelium by SIT1 and by a Na+- and H+-independent transport system.
authors.affiliation: Department of Physiology, University of Seville, Spain.
authors.forename: Mercedes
authors.initials: M
authors.lastname: Cano
authors.affiliation: Department of Physiology, University of Seville, Spain. Electronic address: calonge@us.es.
authors.forename: María L
authors.initials: ML
authors.lastname: Calonge
authors.affiliation: Department of Physiology, University of Seville, Spain.
authors.forename: Anunciación A
authors.initials: AA
authors.lastname: Ilundáin
mesh_terms: D000818:Animals; D001622:Betaine / Q000378:metabolism*; D001693:Biological Transport, Active; D002462:Cell Membrane / Q000378:metabolism*; D002478:Cells, Cultured; D004847:Epithelial Cells / Q000378:metabolism*; D007668:Kidney / Q000378:metabolism*; D008297:Male; D008657:Metabolic Clearance Rate; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism*
chemical_list: D001622:Betaine; D012964:Sodium
keywords: Betaine; Brush-border membrane vesicles; Kidney; Transport