title: Carrier-mediated γ-aminobutyric acid transport across the basolateral membrane of human intestinal Caco-2 cell monolayers.
abstract: The aim of the present study was to investigate the transport of γ-aminobutyric acid (GABA) across the basolateral membrane of intestinal cells. The proton-coupled amino acid transporter, hPAT1, mediates the influx of GABA and GABA mimetic drug substances such as vigabatrin and gaboxadol and the anticancer prodrug δ-aminolevulinic acid across the apical membrane of small intestinal enterocytes. Little is however known about the basolateral transport of these substances. We investigated basolateral transport of GABA in mature Caco-2 cell monolayers using isotope studies. Here we report that, at least two transporters seem to be involved in the basolateral transport of GABA. The basolateral uptake consisted of a high-affinity system with a K(m) of 290 μM and V(max) of 75 pmol cm(-2) min(-1) and a low affinity system with a K(m) of approximately 64 mM and V(max) of 1.6 nmol cm(-2) min(-1). The high-affinity transporter is Na(+) and Cl(-) dependent. The substrate specificity of the high-affinity transporter was further studied and Gly-Sar, Leucine, gaboxadol, sarcosine, lysine, betaine, 5-hydroxythryptophan, proline and glycine reduced the GABA uptake to approximately 44-70% of the GABA uptake in the absence of inhibitor. Other substances such as β-alanine, GABA, 5-aminovaleric acid, taurine and δ-aminolevulinic acid reduced the basolateral GABA uptake to 6-25% of the uptake in the absence of inhibitor. Our results indicate that the distance between the charged amino- and acid-groups is particular important for inhibition of basolateral GABA uptake. Thus, there seems to be a partial substrate overlap between the basolateral GABA transporter and hPAT1, which may prove important for understanding drug interactions at the level of intestinal transport.
authors.affiliation: Department of Pharmacy, Faculty of Health and Medical Sciences, University of Copenhagen, Universitetsparken, Copenhagen, Denmark. cun@farma.ku.dk
authors.forename: Carsten Uhd
authors.initials: CU
authors.lastname: Nielsen
authors.forename: Mette
authors.initials: M
authors.lastname: Carstensen
authors.forename: Birger
authors.initials: B
authors.lastname: Brodin
mesh_terms: D026905:Amino Acid Transport Systems / Q000378:metabolism*; D000622:Aminolevulinic Acid / Q000378:metabolism; D001692:Biological Transport; D018938:Caco-2 Cells; D002712:Chlorides / Q000378:metabolism; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D027981:Symporters / Q000378:metabolism*; D013654:Taurine / Q000378:metabolism; D015091:beta-Alanine / Q000378:metabolism; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D026905:Amino Acid Transport Systems; D002712:Chlorides; C475905:SLC36A1 protein, human; D027981:Symporters; D015091:beta-Alanine; D013654:Taurine; D005680:gamma-Aminobutyric Acid; D000622:Aminolevulinic Acid; D012964:Sodium