EFFECT OF UDCA ON INTRACELLULAR AND BILIARY PH IN ISOLATED RAT HEPATOCYTE COUPLETS AND PERFUSED LIVERS

To study how ursodexoycholic acid (UDCA) increases biliary HCOBAR3 concentration and alkalinizes bile, intracellular pH (pH(i)) and canalicular pH (pH(c)) were measured microfluorimetrically in isolated rat hepatocyte couplets (IRHC). Isolated perfused rat livers (IPRL) were also used to assess the roles of ClBAR, HCOBAR3, and zone III hepatocytes. In IRHC, UDCA diminished pH(i) only when HCOBAR3 was omitted. pH(i) recovery was inhibited by amiloride. UDCA did not affect pH(i) recovery from an acid load (NH4Cl) nor modify pH(c) (+HCOBAR3). In IPRL, biliary HCOBAR3 concentration increased following UDCA despite removal of ClBAR (to inhibit ClBAR -HCOBAR3 exchanger) or destruction of zone III hepatocytes with digitonin. Moreover, when HCOBAR3 was omitted from the perfusate, biliary pH rose following UDCA even though the hypercholeresis was abolished. Thus 1) hepatic UDCA uptake represents an acid load that is counteracted by Na+ - H+ exchange when HCOBAR3 is absent; 2) UDCA does not alkalinize pH(c); and 3) alkalinization of biliary pH in IPRL is not HCOBAR3 dependent, does not involve C1BAR - HCOBAR3 exchange, or zonal differences in UDCA metabolism or excretion. UDCA appears to alkalinize bile by protonation within the bile duct lumen. UDCAH may then cross the biliary epithelium.