Characterization of the basolateral charge carrier during Fe2+ and Zn2+-induced electrogenic transport in human intestinal Caco-2 epithelia

Apical exposure to Fe2+ and Zn2+ is associated with electrogenic transport processes in Caco-2 epithelia (Scott et al. 2002). These processes are dependent upon the pH of the apical bathing medium, consistent with proton-coupled uptake mechanisms for each metal. The present study set out to further characterise these Fe2+ and Zn2+-induced electrogenic processes by attempting to identify the nature of the basolateral charge carrier(s) in Caco-2 epithelia. Short-circuit current (ISC) determinations were performed on voltage-clamped Caco-2 epithelia (passages 38-45), grown on permeable supports (Anotec, Nunc). Cells were bathed with isotonic mannitol/Hepes buffer (37 °C; pH 7.4). At the onset of the experiment, the apical medium was adjusted to pH 6.0. Zinc-histidine (100 µM; 1:5 molar ratio) or iron-ascorbate (100 µM; 1:10 molar ratio) was added to the apical medium. Either Fe2+ or Zn2+ (100 µM) was added to the basolateral medium when the effects of basolateral metal were studied. With apical pH 6.0 and basolateral pH 7.4, application of Fe2+ to the apical bath resulted in an increased (P < 0.001; Student's unpaired t test) inward ISC of 0.12 ± 0.02 µA cm-2 (21) (mean ± S.E.M. (n)). Reducing basolateral pH to 6.0 resulted in a decreased (P < 0.05) ISC response to Fe2+, falling to 0.06 ± 0.02 µA cm-2 (21). Similarly, lowering basolateral pH from 7.4 to 6.0 resulted in a reduced (P < 0.05) inward ISC in response to apical Zn2+ exposure (pH 7.4, 0.33 ± 0.07 µA cm-2 (5); pH 6.0, 0.07 ± 0.02 µA cm-2 (5)). The presence of 100 µM Fe2+ in the basolateral medium (apical pH 6.0, basolateral pH 7.4) caused no significant difference to the ISC response following addition of apical Fe2+ (control: 0.31 ± 0.03 µA cm-2 (6); basolateral Fe2+: 0.26 ± 0.03 µA cm-2 (6)). This was also the case when apical Zn2+ was added in the presence of 100 µM basolateral Zn2+ (control: 0.45 ± 0.06 µA cm-2 (7); basolateral Zn2+: 0.40 ± 0.05 µA cm-2 (7)). These data support the view that H+ is the basolateral charge carrier during both Fe2+ and Zn2+-induced electrogenic transport in Caco-2 cells. The data do not support a role for either Fe2+ or Zn2+ involvement at the basolateral membrane since dissipating/ reversing the basolateral gradient for either metal had no effect on the ISC responses. This suggests the existence of a novel H+-conductive pathway in the basolateral membrane of Caco-2 cells, which is electrogenic in nature.