Calcium-dependent regulation of secretion in biliary epithelial cells: The role of apamin-sensitive SK channels

Background & Aims: Increases in intracellular Ca2+ are thought to complement CAMP in stimulating Cl- secretion in cholangiocytes, although the site(s) of action and channels involved are unknown. We have identified a Ca2+-activated K+ channel (SK2) in biliary epithelium that is inhibited by apamin. The purpose of the present studies was to define the role of SK channels in Ca2+-dependent cholangiocyte secretion. Methods: Studies were performed in human Mz-Cha-1 cells and normal rat cholangiocytes (NRC). Currents were measured by whole-cell patch clamp technique and transepithelial secretion by Ussing chamber. Results: Ca2+-dependent stimuli, including purinergic receptor stimulation, ionomycin, and increases in cell volume, each activated K+-selective currents with a linear IV relation and time-dependent inactivation. Currents were Ca2+ dependent and were inhibited by apamin and by Ba2+. In intact liver, immunoflourescence with an antibody to SK2 showed a prominent signal in cholangiocyte plasma membrane. To evaluate the functional significance, NRC monolayers were mounted in a Ussing chamber, and the short-circuit current (I-sc) was measured. Exposure to ionomycin caused an increase in I-sc 2-fold greater than that induced by CAMP. Both the basal and ionomycin-induced I-sc were inhibited by basolateral Ba2+, and similar to58% of the basolateral K+ current was apamin sensitive. Conclusions: These studies demonstrate that cholangiocytes exhibit robust Ca2+-stimulated secretion significantly greater in magnitude than that stimulated by CAMP. SK2 plays an important role in mediating the increase in transepithelial secretion due to increases in intracellular Ca2+. SK2 channels, therefore, may represent a target for pharmacologic modulation of bile flow.