Abstract
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 ^36^Cl efflux as compared to control cells. Whole cell patch clamp techniques were subsequently used to examine in more detail the swelling‐, Ca^2+^‐, 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 Ca^2+^ 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. © 2005 Wiley‐Liss, Inc.