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Various K ؉ and Cl ؊ channels are important in cell volume regulation and biliary secretion, but the specific role of cystic fibrosis transmembrane conductance regulator in cholangiocyte cell volume regulation is not known. The goal of this research was to study regulatory volume decrease (RVD) in bile duct cell clusters (BDCCs) from normal and cystic fibrosis (CF) mouse livers. Mouse BDCCs without an enclosed lumen were prepared as described (Cho, W. K. (2002) Am. J. Physiol. 283, G1320 -G1327). The isotonic solution consisted of HEPES buffer with 40% of the NaCl replaced with isomolar amounts of sucrose, whereas hypotonic solution was the same as isotonic solution without sucrose. The cell volume changes were indirectly assessed by measuring cross-sectional area (CSA) changes of the BDCCs using quantitative videomicroscopy. Exposure to hypotonic solutions increased relative CSAs of normal BDCCs to 1.20 ؎ 0.01 (mean ؎ S.E., n ‫؍‬ 50) in 10 min, followed by RVD to 1.07 ؎ 0.01 by 40 min. Hypotonic challenge in CF mouse BDCCs also increased relative CSA to 1.20 ؎ 0.01 (n ‫؍‬ 53) in 10 min but without significant recovery. Coadministration of the K ؉ -selective ionophore valinomycin restored RVD in CF mouse BDCCs, suggesting that the impaired RVD was likely from a defect in K ؉ conductance. Moreover, this valinomycin-induced RVD in CF mice was inhibited by 5-nitro-2 -(3-phenylpropylamino)-benzoate, indicating that it is not from nonspecific effects. Neither cAMP nor calcium agonists could reverse the impaired RVD seen in CF cholangiocytes. Our conclusion is that CF mouse cholangiocytes have defective RVD from an impaired K ؉ efflux pathway, which could not be reversed by cAMP nor calcium agonists.Under physiological conditions, osmoregulation plays a crucial role in cholangiocytes, which are exposed to various osmotic stresses from the uptake of solutes and electrolytes and bile secretion (1, 2). A recent study on a human cholangiocarcinoma cell line (3), as well as our results in a study on primary bile duct cell clusters (BDCCs) 1 from normal mouse livers (4), indicates that cholangiocytes can regulate their cell volumes back to base line from hypotonicity-induced swelling. As in other cell types, these adaptive mechanisms of regulatory volume decrease (RVD) in cholangiocytes are mediated by certain K ϩ and Cl Ϫ conductances (3, 4).Cystic fibrosis (CF) is the most common inherited multisystem disease in the Caucasian population and is caused by a defect in the cystic fibrosis transmembrane conductance regulator (CFTR). Many CF patients develop a spectrum of hepatobiliary diseases that are thought to be due to secretory dysfunctions from the CFTR Cl Ϫ channel defects. In recent years, CF liver diseases have become the second most common cause of mortality in CF patients as they live longer, but the underlying pathophysiological mechanisms are not well understood. Recent immunocytochemical studies of the liver have shown that CFTR is expressed only on the bile duct epithelium but not on hepatocytes (5). The CFTR is...

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