title: H(+)-coupled alpha-methylaminoisobutyric acid transport in human intestinal Caco-2 cells.
abstract: Transepithelial apical-to-basal transport and cellular uptake of the non-metabolisable amino acid alpha-methylaminoisobutyric acid (MeAIB) across confluent monolayers of the human intestinal epithelial cell line Caco-2 are enhanced by a transepithelial pH gradient (apical pH 6.0, basolateral pH 7.4). In Na(+)-free conditions (apical pH 7.4, basolateral pH 7.4), net absorption (120 +/- 58 pmol/cm2 per h, n = 13) and uptake across the apical membrane (cell/medium ratio 0.56 +/- 0.06, n = 13) are low. However, in Na(+)-free conditions with apical pH 6.0, net absorption (685 +/- 95 pmol/cm2 per h, n = 15) and intracellular accumulation (cell/medium ratio 3.63 +/- 0.29, n = 14) were marked. Continuous monitoring of intracellular pH (pHi) in BCECF (2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein)-loaded Caco-2 cell monolayers indicated that apical addition of MeAIB (20 mM) was associated with H(+)-flow across the apical membrane in both Na+ and Na(+)-free conditions. This transport process is rheogenic in Na(+)-free media, stimulating an inward short-circuit current in voltage-clamped Caco-2 cell monolayers. On the basis of competition for MeAIB accumulation and pHi experiments, L-proline, glycine, L-alanine and beta-alanine are also substrates for H(+)-linked transport at the apical membrane of Caco-2 cells but L-valine, L-leucine and L-phenylalanine are not. These data are consistent with the expression, in the apical brush-border membrane of Caco-2 cells, of a H(+)-coupled, Na(+)-independent MeAIB carrier.
authors.affiliation: Department of Physiological Sciences, University of Newcastle upon Tyne, Medical School, UK.
authors.forename: D T
authors.initials: DT
authors.lastname: Thwaites
authors.forename: G T
authors.initials: GT
authors.lastname: McEwan
authors.forename: B H
authors.initials: BH
authors.lastname: Hirst
authors.forename: N L
authors.initials: NL
authors.lastname: Simmons
mesh_terms: D001692:Biological Transport / Q000187:drug effects; D002460:Cell Line; D016764:Cell Polarity; D003593:Cytoplasm / Q000502:physiology; D004848:Epithelium / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007408:Intestinal Absorption; D007413:Intestinal Mucosa / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D012964:Sodium / Q000494:pharmacology; D015091:beta-Alanine / Q000031:analogs & derivatives* / Q000378:metabolism
chemical_list: D015091:beta-Alanine; C012924:2,2-dimethyl-beta-alanine; D012964:Sodium