title: Transport of L-leucine hydroxy analogue and L-lactate in rabbit small-intestinal brush-border membrane vesicles.
abstract: Substitution of the alpha-amino group of amino acids by hydroxyl groups yields hydroxy analogues (HA), which have been ascribed beneficial effects in nitrogen-sparing diets for uremic patients. In this study, intestinal uptake of L-leucine HA (L-LeuHA) and L-lactate into rabbit jejunal brush-border membrane vesicles was investigated. An inward-directed H+ or Na+ gradient stimulated uptake of both labelled substrates in a voltage-clamped assay. The H+ gradient was the major driving force of uptake as compared with the Na+ gradient, and it led to a transient accumulation of both L-LeuHA and L-lactate. The proton ionophore carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP) reduced the initial H(+)-gradient-driven uptake rates of both substrates, but was without effect on Na(+)-gradient-driven uptakes. The H(+)-gradient-driven L-LeuHA uptake was saturable (apparent Kt = 15.4 mM). Alpha-HA of L-leucine, L-isoleucine, L-valine, D-leucine, D-valine or L-lactate inhibited the H(+)-gradient-driven L-LeuHA or L-lactate uptakes whereas free branched-chain amino acids had no effect. Preloading the vesicles with one of the L- or D-HA of branched-chain amino acids or with L-lactate stimulated tracer L-LeuHA and also tracer L-lactate uptakes in the presence of a H+ gradient. It is concluded that H(+)-gradient-driven transport of L- and D-stereoisomeric HA of branched-chain amino acids as well as of L-lactate across rabbit intestinal brush-border membranes is mediated by the same carrier. Furthermore, there exists a Na+ gradient-driven L-lactate transport system in the rabbit intestinal brush-border membrane.
authors.affiliation: Institute of Physiology, University of Zürich, Switzerland.
authors.forename: M
authors.initials: M
authors.lastname: Friedrich
authors.forename: H
authors.initials: H
authors.lastname: Murer
authors.forename: E G
authors.initials: EG
authors.lastname: Berger
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D007421:Intestine, Small / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D007425:Intracellular Membranes / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D007773:Lactates / Q000493:pharmacokinetics*; D007930:Leucine / Q000493:pharmacokinetics*; D008871:Microvilli / Q000378:metabolism / Q000502:physiology / Q000648:ultrastructure; D011817:Rabbits; D013997:Time Factors
chemical_list: D007773:Lactates; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D007930:Leucine