Chloride channels in the small intestinal cell line IEC-18

Small intestinal crypt cells play a critical role in modulating Cl ^- secretion during digestion. The types of Cl^- channels mediating Cl^- secretion in the small intestine was investigated using the intestinal epithelial cell line, IEC-18, which was derived from rat small intestine crypt cells. In initial radioisotope efflux studies, exposure to forskolin, ionomycin or a decrease in extracellular osmolarity significantly increased ³⁶Cl efflux as compared to control cells. Whole cell patch clamp techniques were subsequently used to examine in more detail the swelling-, Ca²⁺-, and cAMP-activated Cl^- conductance. Decreasing the extracellular osmolarity from 290 to 200 mOsm activated a large outwardly rectifying Cl^- current that was voltage-independent and had an anion selectivity of I^- > Cl^-. Increasing cytosolic Ca ²⁺ by ionomycin activated whole cell Cl^- currents, which were also outwardly rectifying but were voltage-dependent. The increase in intracellular Ca²⁺ levels with ionomycin was confirmed with fura-2 loaded IEC-18 cells. A third type of whole cell Cl^- current was observed after increases in intracellular cAMP induced by forskolin. These cAMP-activated Cl^- currents have properties consistent with cystic fibrosis transmembrane regulator (CFTR) Cl^- channels, as the currents were blocked by glibenclamide or NPPB but insensitive to DIDS. In addition, the current-voltage relationship was linear and had an anion selectivity of Cl ^- > I^-. Confocal immunofluorescence studies and Western blots with two different anti-CFTR antibodies confirmed the expression of CFTR. These results suggest that small intestinal crypt cells express multiple types of Cl^- channels, which may all contribute to net Cl^- secretion.