The molecular biology of chloride secretion in epithelia.

The ion transport mechanisms involved in electrogenic Cl secretion include a Cl exit step through the Cl channel at the apical membrane. In cystic fibrosis (CF), Cl secretion is impaired due to a missing Cl permeability of the apical membrane. Patch-clamp experiments have demonstrated that Cl channels are present in CF cells; however, the regulation of channel activity via the cAMP-pathway is impaired. Recent studies have shown that the CF defect is very closely associated with the Cl channel itself or with an associated, membrane-bound regulatory subunit of the Cl channel complex. Because not much is known about the molecular details of the individual transport mechanisms and the signaling pathways involved in Cl secretion in epithelial cells, recent findings on the molecular biology of these mechanisms obtained in other cellular systems are discussed.

Apical membrane chloride channels in a colonic cell line activated by secretory agonists.

We characterized the anion channel responsible for the increase in apical membrane Cl secretion using a model salt-secreting epithelium, the T84 colonic cell line. The adenosine 3',5'-cyclic monophosphate (cAMP)-mediated secretagogues, prostaglandin E2, forskolin, and 8-bromo-cAMP, evoked activity of an outwardly rectifying Cl channel in previously quiet cell-attached membrane patches. The channel remained active in excised, inside-out membranes, where its single-channel conductance was 40-45 pS at 0 mV with 160 mM NaCl in pipette and bath. Selectivities were PCl/PNa = 50 and for halides I(1.8)/Br(1.4)/Cl(1.0)/F(0.4). This halide sequence illustrates that the ability of various anions to undergo transepithelial secretion is determined by the selectivity of the basolateral membrane Cl entry step rather than by the apical Cl channel. Open-channel probability increased with depolarization, an effect that would adjust the rate of Cl exit across secretory cell apical membranes with agonist-induced changes in apical membrane potential. Comparison with the properties of Cl channels detected in other cell types suggests that this cAMP-stimulated Cl channel is uniquely present in the apical membranes of salt-secreting epithelial cells.