Role of inorganic electrolytes in bile acid-independent canalicular bile formation

Theoretical considerations on the role of membrane potential in the regulation of endosomal pH

Na+,K(+)-ATPase has been observed to partially inhibit acidification of early endosomes by increasing membrane potential, whereas chloride channels have been observed to enhance acidification in endosomes and lysosomes. However, little theoretical analysis of the ways in which different pumps and channels may interact has been carried out. We therefore developed quantitative models of endosomal pH regulation based on thermodynamic considerations. We conclude that 1) both size and shape of endosomes will influence steady-state endosomal pH whenever membrane potential due to the pH gradient limits proton pumping, 2) steady-state pH values similar to those observed in early endosomes of living cells can occur in endosomes containing just H(+)-ATPases and Na+,K(+)-ATPases when low endosomal buffering capacities are present, and 3) inclusion of active chloride channels results in predicted pH values well below those observed in vivo. The results support the separation of endocytic compartments into two classes, those (such as early endosomes) whose acidification is limited by attainment of a certain membrane potential, and those (such as lysosomes) whose acidification is limited by the attainment of a certain pH. The theoretical framework and conclusions described are potentially applicable to other membrane-enclosed compartments that are acidified, such as elements of the Golgi apparatus.

Solvent isotope effect on bile formation in the rat

2H2O affects many membrane transport processes by solvent and kinetic isotope effects. Since bile formation is a process of osmotic filtration where such effects could be important, we investigated the effects of 2H2O on bile formation in the in situ perfused rat liver. Dose finding experiments showed that at high concentrations, 2H2O increased vascular resistance and induced cholestasis; at 60% 2H2O however, a clear dissociation between the vascular and biliary effects was observed. Therefore, further experiments were carried out at this concentration. The main finding was a reduction in bile salt-independent bile flow from 0.99 +/- 0.04 to 0.66 +/- 0.04 microliters.min-1.g-1 (P < 0.001). This was associated with a 40% reduction in biliary bicarbonate concentration (P < 0.001). Choleretic response to neither taurocholate nor ursodeoxycholate was altered by 2H2O; in particular, there was a similar stimulation of bicarbonate secretion by ursodeoxycholate in the presence of 60% 2H2O. To further elucidate this phenomenon, the effect of 2H2O on three proteins potentially involved in biliary bicarbonate secretion was studied in vitro. 2H2O slightly inhibited cytosolic carboanhydrase and leukocyte Na+/H(+)-exchange, these effects reached statistical significance at 100% 2H2O only, however. In contrast, Cl-/HCO(3-)-exchange in canalicular membrane vesicles was already inhibited by 50% (P < 0.001) at 60% 2H2O. Finally, there was a slight reduction in biliary glutathione secretion while that of the disulphide was not affected. Our results are compatible with an inhibition of canalicular Cl-/HCO(3-)-exchange by 2H2O. Whether this is due to altered hydration of the exchanger and/or of the transported bicarbonate remains to be determined.

Rat hepatocytes exhibit basolateral Na+/HCO3- cotransport

Primary cultures and plasma membrane vesicles were used to characterize Na+ and HCO3- transport by rat hepatocytes. Na+ uptake into hepatocytes was stimulated approximately 10-fold by 25 mM extracellular HCO3-.HCO3--stimulated Na+ uptake was saturable, abolished by 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stilbene (SITS), and unaffected by amiloride or Cl- removal. Neither propionate nor acetate reproduced this effect of HCO3-. 22Na efflux from preloaded hepatocytes was similarly increased approximately 10-fold by an in greater than out HCO3- concentration gradient. 22Na efflux was also increased by valinomycin and an in greater than out K+ concentration gradient in the presence but not absence of HCO3-. Intracellular pH (pHi) measured with the pH-sensitive fluorochrome 2',7'-bis-(2-carboxyethyl)-5-(and 6-)carboxyfluorescein (BCECF) decreased at a rate of 0.227 (+/- 0.074 SEM) pH units/min when extracellular HCO3- concentration was lowered from 25 to 5 mM at constant PCO2. This intracellular acidification rate was decreased 50-60% in the absence of Na+ or presence of SITS, and was unaffected by amiloride or Cl- removal. Membrane hyperpolarization produced by valinomycin and an in greater than out K+ concentration gradient caused pHi to fall; the rate of fall was decreased 50-70% by Na+ removal or SITS, but not amiloride. An inside positive K+ diffusion potential and a simultaneous out greater than in HCO3- gradient produced a transient 4,4'-diisothiocyano-2,2' disulfonic acid stilbene (DIDS) sensitive, amiloride-insensitive 22Na accumulation in basolateral but not canalicular membrane vesicles. Rat hepatocytes thus exhibit electrogenic basolateral Na+/HCO3- cotransport.

Cell membrane and transepithelial voltages and resistances in isolated rat hepatocyte couplets

The basic electrical properties of an isolated rat hepatocyte couplet (IRHC) system have been analyzed using classical techniques of epithelial electrophysiology, including measurement of electric potentials, resistances and intracellular ion activities. Applications of these techniques are discussed with respect to their limitations in small isolated cells. Mean intracellular and intracanalicular membrane potentials ranged from -23.7 to -46.7 and -4.3 to -5.9 mV, respectively. Membrane resistances were determined using an equivalent circuit analysis modified according to the geometry of the IRHC system. Resistances of the sinusoidal (basolateral) and canalicular (luminal) cell membranes and tight junctions averaged 0.15 and 0.78 G omega and 25 m omega, respectively. The cells are electrically coupled via low resistance intercellular communications (approximately 58 M omega). Intracellular ion activities for Na+, K+ and Cl- averaged 12.2, 88.1 and 17.7 mmol/liter, respectively. The basolateral membrane potential reveals a permeability sequence of PK greater than PCl greater than PNa. The luminal potential showed minimal dependence on changes in transjunctional ion gradients, indicating a poor ion selectivity of the paracellular pathway. The electrogenic (Na+-K+)-ATPase contributes little to the luminal and cellular negative electric potential. Therefore, the luminal potential probably results from the secretion of impermeant ions and a Donnan distribution of permeant ions, a mechanism which provides the osmotic driving force for bile formation. By providing the unique opportunity to measure luminal potentials, this isolated hepatocyte system permits study of secretory mechanisms for the first time in a mammalian gland using electrophysiologic techniques.

Ursodeoxycholate stimulates Na+-H+ exchange in rat liver basolateral plasma membrane vesicles

Na+:H+ and Cl-:HCO3- exchange are localized, respectively, to basolateral (blLPM) and canalicular (cLPM) rat liver plasma membranes. To determine whether these exchangers play a role in bile formation, we examined the effect of a choleretic agent, ursodeoxycholate (UDCA), on these exchange mechanisms. 22Na (1 mM) and 36Cl (5 mM) uptake was determined using outwardly directed H+ and HCO3- gradients, respectively. Preincubation of blLPM vesicles with UDCA (0-500 microM) resulted in a concentration-dependent increase in initial rates of amiloride-sensitive pH-driven Na+ uptake, with a maximal effect at 200 microM. UDCA (200 microM) increased Vmax from 23 +/- 2 (control) to 37 +/- 7 nmol/min per mg protein; apparent Km for Na+ was unchanged. Preincubation with tauroursodeoxycholate (200 microM), taurocholate (10-200 microM) or cholate, chenodeoxycholate, or deoxycholate (200 microM) had no effect on pH-driven Na+ uptake. UDCA (200 microM) had no effect on either membrane lipid fluidity, assessed by steady-state fluorescence polarization using the probes 1,6-diphenyl-1,3,5-hexatriene, 12-(9-anthroyloxy) stearic acid, and 2-(9-anthroyloxy) stearic acid (2-AS), or Na+,K+-ATPase activity in blLPM vesicles. In cLPM vesicles, UDCA (0-500 microM) had no stimulatory effect on initial rates of HCO3(-)-driven Cl- uptake. Enhanced basolateral Na+:H+ exchange activity, leading to intracellular HCO3- concentrations above equilibrium, may account for the bicarbonate-rich choleresis after UDCA infusion.

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-:OH- and 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 36Cl- uptake 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 Cl- equilibrium (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 Cl- pathway was insensitive to DIDS. The DIDS-sensitive, pH and HCO3- gradient-stimulated Cl- uptake demonstrated: saturation with Cl- (Km approximately 6.3 mM; Vmax approximately 51 nmol X mg-1 X min-1); partial inhibition by bumetanide (26%), furosemide (33%), probenecid (37%), and 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stilbene (49%); cis-inhibition by chloride and nitrate but not by sulfate and various organic anions, and independence from the membrane potential. These data demonstrate the presence of an electroneutral Cl-: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-:HCO3- (OH-) exchange system in blLPM vesicles. Furthermore, neither blLPM nor cLPM vesicles exhibited Na+-stimulatable Cl- uptake, indicating the absence of a NaCl 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.