EFFECT OF URSODEOXYCHOLIC ACID ON INTRACELLULAR PH IN A BILE-DUCT EPITHELIUM-LIKE CELL-LINE

Recent studies in perfused livers and isolated hepatocytes indicate th at ursodeoxycholic acid-induced HCO3-rich hypercholeresis originates a t the ductule/duct level. The bile duct epithelium may be involved in bile alkalinization by passively reabsorbing the protonated unconjugat ed ursodeoxycholic acid, by directly secreting in response to an ursod eoxycholic acid-induced increase in acid/base transporter activity or by taking up UDCA- in exchange for a base equivalent. To investigate t hese processes in more detail, we studied the effects of ursodeoxychol ic acid on intracellular pH in SK-ChA-1, a well-differentiated human c holangiocarcinoma cell line similar to bile duct epithelium in terms o f intracellular pH regulatory mechanisms and morphological markers. In tracellular pH changes were monitored with a microfluorimetric setup u sing the fluorescent indicator 2'-7'-bis(2-carboxyethyl)-5,6,carboxy f luorescein. Administration of 50 to 1,000 mumol/L UDCA in the absence of HCO3 caused dose-dependent intracellular acidification (intracellul ar pH = - 0.13 +/- 0.03 pH/U after 500 mumol/L ursodeoxycholic acid). Acidification was not prevented by preincubation of cells with 0.5 mmo l/L 4,4-diisothiocyanatostilbene-2,2,-disulfonic acid (DIDS) for 30 mi n or by furosemide administration (1 mmol/L), thus ruling out the stim ulation of Cl/HCO3 exchange or the presence of an ursodeoxycholic acid /base exchange. Ursodeoxycholic acid also acidified human fibroblasts, a cell type with no transport capability for ursodeoxycholic acid. In addition, direct measurement of the activities of the three major aci d/base transporters in SK-ChA-1 cells (Na+/H+ exchange, sodium-depende nt and sodium-independent Cl/HCO3 exchange) failed to show significati ve differences between cells treated with 500 mumol/L UDCA and control s. In conclusion, ursodeoxycholic acid administration does not primari ly stimulate the activities of acid/base transporters responsible for HCO, secretion by bile duct epithelium. Rather, similar to what has pr eviously been shown in hepatocytes, ursodeoxycholic acid induces intra cellular acidification in SK-ChA-1 cells and in fibroblasts. This effe ct is likely mediated by nonionic diffusion of the weak acid ursodeoxy cholic acid. These data confirm that ursodeoxycholic acid can be passi vely reabsorbed by the biliary epithelium, consistent with the cholehe patic-shunt hypothesis.