Submitted on June 12, 2009
Revised on August 31, 2009
Accepted on September 10, 2009
Identification and functional characterization of the intermediate conductance Ca2+-activated K+ channel (IK-1) in biliary epithelium.
Amal K. Dutta1, Al-Karim Khimji1, Meghana Sathe1, Charles Kresge1, Vinay Parameswara2, Victoria Esser1, Don C Rockey1, and Andrew P. Feranchak3*
1 University of Texas Southwestern Medical Center
2 UNiversity of Texas Southwestern Medical Center
3 The University of Texas Southwestern Medical Center
* To whom correspondence should be addressed. E-mail: Drew.Feranchak{at}UTSouthwestern.edu.
In the liver, adenosine triphosphate (ATP) is an extracellular signaling molecule that is released into bile and stimulates a biliary epithelial cell secretory response via engagement of apical P2 receptors. The molecular identities of the ion channels involved in ATP-mediated secretory responses have not been fully identified. Intermediate conductance Ca2+-activated K+ channels (IK) have been identified in biliary epithelium, but functional data is lacking. The aim of these studies therefore was to determine the location, function, and regulation of IK channels in biliary epithelial cells and to determine their potential contribution to ATP-stimulated secretion. Expression of IK-1 mRNA was found in both human Mz-Cha-1 biliary cells and polarized normal rat cholangiocyte (NRC) monolayers and immunostaining revealed membrane localization with a predominant basolateral signal. In single Mz-Cha-1 cells, exposure to ATP activated K+ currents, increasing current density from 1.6 ± 0.1 pA/pF to 7.6 ± 0.8 pA/pF. Currents were dependent on intracellular Ca2+ and sensitive to clotrimazole and TRAM-34 (specific IK channel inhibitors). Single channel recording demonstrated that clotrimazole-sensitive K+ currents had a unitary conductance of 46.2 ± 1.5 pS consistent with IK channels. In separate studies, 1-EBIO (an IK activator) stimulated K+ currents in single cells that were inhibited by clotrimazole. In polarized NRC monolayers, ATP significantly increased transepithelial secretion which was inhibited by clotrimazole. Lastly, ATP-stimulated K+ currents were inhibited by the P2Y receptor antagonist suramin and by the IP3 receptor inhibitor 2-APB. Together these studies demonstrate that IK channels are present in biliary epithelial cells and contribute to ATP-stimulated secretion through a P2Y-IP3 receptor pathway.
