Characterization of cytosolic Ca2+ signaling in rat bile duct epithelia

Bile duct epithelia play an important role in the formation and conditioning of bile. However, hormonal responses in this epithelial tissue are incompletely understood. Secretin increases ductular secretion through the intracellular messenger adenosine 3',5'-cyclic monophosphate (cAMP), but whether hormones increase cytosolic Ca2+ (Ca2+(i)) in these cells and whether Ca2+(i) regulates duct secretion is unknown. To address these questions, we examined Ca2+(i) signaling in isolated rat bile duct units using ratio microspectrofluorometry and confocal microscopy. We also used videomicroscopy to examine secretion and cell volume in isolated bile duct cells and duct units. Acetylcholine (ACh) and ATP both increased Ca2+(i) in bile duct units and elicited patterns of Ca2+(i) increases and oscillations that were distinct and dose dependent. In contrast, Ca2+(i) was not increased by the hepatocyte Ca2+(i) agonists vasopressin, angiotensin, and phenylephrine or by the exocrine pancreas agonists cholecystokinin (CCK) and bombesin. In addition, secretin did not increase Ca2+(i) in the isolated bile duct units, whereas ACh did not increase Ca2+(i) in isolated hepatocytes. Mobilization of internal, thapsigargin-sensitive Ca2+ stores contributed more than influx of extracellular Ca2+ to the Ca2+(i) increases induced in the duct units, and ATP-induced increases in Ca2+(i) could be blocked by microinjection of heparin but not de-N-sulfated heparin. ACh transiently decreased bile flow in the isolated perfused rat liver, although neither ACh nor ATP altered secretion in isolated ducts or changed the volume of single isolated bile duct cells. These findings demonstrate that bile duct epithelial cells possess both muscarinic and purinergic receptors that activate Ca2+(i) signaling pathways similar to those seen in other types of epithelia, but that the two types of receptors elicit distinct patterns of Ca2+(i) signals. Increases in Ca2+(i) have minimal direct effects on bile duct secretion, although it remains to be determined whether such signals selectively modulate other aspects of bile duct epithelial cell function.