Regulation of Ca²⁺ signaling in rat bile duct epithelia by inositol 1,4,5-trisphosphate receptor isoforms

Cytosolic Ca²⁺ (Ca_i²⁺) regulates secretion of bicarbonate and other ions in the cholangiocyte. In other cell types, this second messenger acts through Ca²⁺ waves, Ca²⁺ oscillations, and other subcellular Ca²⁺ signaling patterns, but little is known about the subcellular organization of Ca²⁺ signaling in cholangiocytes. Therefore, we examined Ca²⁺ signaling and the subcellular distribution of Ca²⁺ release channels in cholangiocytes and in a model cholangiocyte cell line. The expression and subcellular distribution of inositol 1,4,5-trisphosphate (InsP₃) receptor (InsP₃R) 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 Ca²⁺ signaling in cholangiocytes was examined by confocal microscopy. All 3 InsP₃R isoforms were expressed in cholangiocytes, whereas RyR was not expressed. The type III InsP₃R 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 InsP₃R was expressed even more heavily in NRC cells but was concentrated apically in these cells as well. Adenosine triphosphate (ATP), which increases Ca²⁺ via InsP₃ in cholangiocytes, induced Ca²⁺ oscillations in both cholangiocytes and NRC cells. Acetylcholine (ACh) induced apical-to-basal Ca²⁺ waves. In conclusion, Ca²⁺ signaling in cholangiocytes occurs as polarized Ca²⁺ waves that begin in the region of the type III InSP₃R. Differential subcellular localization of InsP₃R isoforms may be an important molecular mechanism for the formation of Ca²⁺ waves and oscillations in cholangiocytes. Because Ca_i²⁺ is in part responsible for regulating ductular secretion, these findings also may have implications for the molecular basis of cholestatic disorders.