Calcium-dependent regulation of secretion in biliary epithelial cells: the role of apamin-sensitive SK channels.
OBJECTIVE
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.
Feranchak
Andrew P
AP
University of Colorado Health Sciences Center, 4200 East 9th Avenue, Denver, Colorado 80262, USA. drew.feranchak@UCHSC.edu
Doctor
R Brian
RB
Troetsch
Marlyn
M
Brookman
Kathryn
K
Johnson
Sylene M
SM
Fitz
J Gregory
JG
D000818:Animals; D001030:Apamin / Q000494:pharmacology; D001514:Bee Venoms / Q000494:pharmacology; D001653:Bile Ducts, Intrahepatic / Q000187:drug effects / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D015759:Ionomycin / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D024681:Potassium Channels, Calcium-Activated; D051381:Rats; D051640:Small-Conductance Calcium-Activated Potassium Channels
D001514:Bee Venoms; D007476:Ionophores; C496897:KCNN2 protein, human; C496899:Kcnn2 protein, rat; D015221:Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D051640:Small-Conductance Calcium-Activated Potassium Channels; D001030:Apamin; D015759:Ionomycin; D011188:Potassium; D002118:Calcium