J: Evidence for carrier-mediated chloride/bicarbonate exchange in canalicular rat liver plasma membrane vesicles

To determine whether anion exchangers might play a role in hepatic bile formation, we looked for the presence of Cl-:OHand Cl-:HCO3 exchange in highly purified canalicular (c) and basolateral (bl) rat liver plasma membrane (LPM) vesicles. In cLPM vesicles, a pH gradient (7.7 in/6.0 out) stimulated 36Cuptake twofold above values obtained during pH-equilibrated conditions (7.7 in = out). When 50 mM HCO3 was also present inside the vesicles, the same pH gradient (7.7 in/6.0 out) resulted in Cl- uptake to levels fourfold above pH- and HCO3-equilibrated controls and two- to threefold above Clequilibrium (overshoot). Initial rates of both pH and HCO3 gradient-stimulated Cl- uptake were completely inhibited by 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene (DIDS). A valinomycin-induced K+ diffusion potential (inside positive) also stimulated Cl- uptake in cLPM, but this conductive Clpathway was insensitive to DIDS. The DIDS-sensitive, pH and HCO3 gradient-stimulated Cl- uptake demonstrated: (a) saturation with Cl- (Km 6.3 mM; V.,. 51 nmol * mg- ' * min);, (b) partial inhibition by bumetanide (26%), furosemide (33%), probenecid (37%), and 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stilbene (49%); (c) cis-inhibition by chloride and nitrate but not by sulfate and various organic anions, and (d) independence from the membrane potential. These data demonstrate the presence of an electroneutral Cr-: OH- and Cl-:HCO3 exchanger in rat liver canalicular membranes that favors Cl-:HCO3 exchange. In contrast, no evidence was found for the presence of a Cl-:HCO- (OH-) exchange system in blLPM vesicles. Furthermore, neither blLPM nor cLPM vesicles exhibited Na+-stimulatable Cl- uptake, indicating the absence of a NaCI co-transport system in either LPM subfraction. These findings are consistent with a functional role for a Cl-:HCO3 (OH-) exchanger in canalicular bile formation