Iron and zinc differentially regulate Fe2+- and Zn2+-induced electrogenic transport pathways in human intestinal Caco-2 epithelia

We have previously reported on the stimulation of pH-dependent electrogenic transport pathways following exposure to apical Fe2+ and Zn2+ in Caco-2 epithelia (Scott et al. 2002). The present study set out to establish the effects of chronic Fe2+ and Zn2+ loading and Fe2+ depletion on these transport routes. Short circuit current (ISC) determinations were made on voltage-clamped Caco-2 epithelia, 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, apical pH was adjusted to 6.0. Iron ascorbate (1:10 molar ratio) or zinc histidine (1:5) was applied apically. For Fe2+ or Zn2+ loading, 100 µM of either Fe2+ or Zn2+ was added to culture medium for 24 h prior to experimentation. For Fe2+ depletion, 100 µM desferrioxamine was added to culture medium 40 h prior to experimentation. All conditions were accompanied by time-matched vehicle controls from the same cell batch. Under Fe2+-depleted conditions, the Fe2+-induced inward ISC was enhanced with Vmax rising significantly (P < 0.01, Student's unpaired t test, means ± S.E.M. (n)) from 0.97 ± 0.12 (5) to 2.04 ± 0.18 µA cm-2 (5). In contrast, Fe2+ depletion resulted in a reduction in the Zn2+-induced ISC, Vmax falling (P < 0.02) from 1.25 ± 0.07 (4) to 0.95 ± 0.05 µA cm-2 (4). Fe2+ loading reduced the Fe2+-evoked ISC at all Fe2+ concentrations tested (1-500 µM; P < 0.05-0.001) with transport failing to display saturation. However, Fe2+ loading had no effect on the Zn2+-induced ISC. Zn2+ loading enhanced the Fe2+-induced ISC by over 300 % with Vmax rising (P < 0.001) from 0.89 ± 0.13 (5) to 3.4 ± 0.19 µA cm-2. In striking contrast, Zn2+ loading reversed (P < 0.001) the inward ISC observed under control conditions (Vmax = 0.77 ± 0.12 µA cm-2 (5)) to a significant (P < 0.05) saturable outward ISC with a Vmax of 1.11 ± 0.27 µA cm-2. These data demonstrate that Fe2+ and Zn2+-evoked electrogenic transport pathways are differentially affected by altering the exposure levels to these metals. This provides evidence that some or all of the transporters involved in these pathways are regulated by both Fe2+ and Zn2+. The present data are consistent with previously published data demonstrating regulation of DMT-1 expression by Fe2+ and Zn2+ (Yamaji et al. 2001). However, it does not support the view that Fe2+ and Zn2+ are transported by a common pathway.