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

BACKGROUND & AIMS: Increases in intracellular Ca 2+ 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 Ca 2+ -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 Ca 2+ -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: Ca 2+ -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 Ca 2+ dependent and were inhibited by apamin and by Ba 2+. 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 Ba 2+, and approximately 58% of the basolateral K + current was apamin sensitive.
CONCLUSIONS: These studies demonstrate that cholangiocytes exhibit robust Ca 2+ -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 Ca 2+. SK2 channels, therefore, may represent a target for pharmacologic modulation of bile flow.