Regulation of Ca(2+) Signaling in Rat Bile Duct Epithelia by Inositol 1,4,5-trisphosphate Receptor Isoforms

Cytosolic Ca(2+) (Ca(i)(2+)) regulates secretion of bicarbonate and other ions in the cholangiocyte. In other cell types, this second messenger acts through Ca(2+) waves, Ca(2+) oscillations, and other subcellular Ca(2+) signaling patterns, but little is known about the subcellular organization of Ca(2+) signaling in cholangiocytes. Therefore, we examined Ca(2+) signaling and the subcellular distribution of Ca(2+) release channels in cholangiocytes and in a model cholangiocyte cell line. The expression and subcellular distribution of inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) isoforms and the ryanodine receptor (RyR) were determined in cholangiocytes from normal rat liver and in the normal rat cholangiocyte (NRC) polarized bile duct cell line. Subcellular Ca(2+) signaling in cholangiocytes was examined by confocal microscopy. All 3 InsP(3)R isoforms were expressed in cholangiocytes, whereas RyR was not expressed. The type III InsP(3)R was the most heavily expressed isoform at the protein level and was concentrated apically, whereas the type I and type II isoforms were expressed more uniformly. The type III InsP(3)R was expressed even more heavily in NRC cells but was concentrated apically in these cells as well. Adenosine triphosphate (ATP), which increases Ca(2+) via InsP(3) in cholangiocytes, induced Ca(2+) oscillations in both cholangiocytes and NRC cells. Acetylcholine (ACh) induced apical-to-basal Ca(2+) waves. In conclusion, Ca(2+) signaling in cholangiocytes occurs as polarized Ca(2+) waves that begin in the region of the type III InsP(3)R. Differential subcellular localization of InsP(3)R isoforms may be an important molecular mechanism for the formation of Ca(2+) waves and oscillations in cholangiocytes. Because Ca(i)(2+) is in part responsible for regulating ductular secretion, these findings also may have implications for the molecular basis of cholestatic disorders.