Transintestinal absorption of gamma-aminobutyric acid (GABA) via a pH-dependent mechanism is demonstrated in the human intestinal epithelial cell line Caco-2. Experiments with BCECF [2′,7′,-bis(2-carboxyethyl)-5(6)-carboxyfluorescein]-loaded Caco-2 cells show that GABA transport across the apical membrane is coupled to proton flux into the cell.
A number of GABA analogues were tested for their ability to: (a) inhibit pH-dependent [3H]GABA uptake across the apical membrane; (b) stimulate H+ flux across the apical surface of BCECF-loaded Caco-2 cell monolayers; (c) increase inward ISC across voltage-clamped Caco-2 cell monolayers.
Nipecotic acid, isonipecotic acid, d,l-β-aminobutyric acid, and 3-amino-1-propanesulfonic acid each caused a marked acidification of intracellular pH and an increase in ISC when superfused to the apical surface of Caco-2 cell monolayers. In contrast, l-α-amino-n-butyric acid failed to induce proton flux or ISC. The ability of these compounds to induce proton or current flow across the apical surface of this intestinal epithelium was closely related to the relative inhibitory effects on [3H]GABA uptake.
These observations demonstrate H+/GABA sympatry and suggest that this transport mechanism may be accessible as a pathway for oral absorption of therapeutically useful GABA analogues.