1.   Hydrochloric acid secretion by gastric parietal cells entails complex ion transport events at the apical membrane.

2.   Movement of Cl - across the apical membrane of stimulated parietal cells seems to be mediated by Cl - channels.

3.   They are present on the apical membranes of gastric parietal cells and, in some species, colon epithelial cells, where they mediate acid secretion.

4.   Furthermore, selective isolation of vesicles from basolateral and apical membranes has often revealed a polarised distribution of these transport systems between the two membranes.

5.   This is partly due to the appearance of Cl - conductive pathways in the apical membrane.

6.   This is in contrast to the apical membrane of the absorptive ileal villus cells, which contains both exchangers.

7.   These include activation of apical membrane Cl - recycling by opening of Cl - channels.

8.   The luminal pH near the intestinal apical membrane is maintained at a more acid level than bulk luminal pH.

9.   It favours conversion of these weak acids to their protonated uncharged form which can then diffuse across the apical membrane into the cell.