Relationship of hepatic cholate transport to regulation of intracellular pH and potassium

Sensitivity of bile acid transport by organic anion-transporting polypeptides to intracellular pH

We investigated the influence of intracellular pH (pHi) on [14C]-glycocholate (GC) uptake by human hepatoblastoma HepG2 cells that express sodium-independent (mainly OATP-A and OATP-8), but not sodium-dependent, GC transporters. Replacement of extracellular sodium by choline (Chol) stimulated GC uptake but did not affect GC efflux from loaded cells. Amiloride or NaCl replacement by tetraethylammonium chloride (TeACl) or sucrose also increased GC uptake. All stimulating circumstances decreased pHi. By contrast, adding to the medium ammonium or imidazole, which increased pHi, had no effect on GC uptake. In Chinese hamster ovary (CHO) cells expressing rat Oatp1, acidification of pHi had the opposite effect on GC uptake, that is, this was reduced. Changes in extracellular pH (pHo) between 7.40 and 7.00 had no effect on GC uptake at pHi 7.30 or 7.45 when pHo<pHi. However, GC uptake was inhibited at pHo 7.40 and 7.80 when pHo>pHi. Inhibition was not proportional to the pHo-pHi difference. Intracellular acidification decreased V(max), but had no effect on K(m). In sum, sodium-independent GC transport can be affected by intracellular acidification, possibly due both to modifications in the driving forces and to the particular response to protonation of carrier proteins involved in this process.

Uptake of bromosulfophthalein via SO2-4/OH- exchange increases the K+ conductance of rat hepatocytes

In confluent primary cultures of rat hepatocytes, micromolar concentrations of bromosulfophthalein (BSP) lead to a sizeable hyperpolarization of membrane voltage. The effect is a saturable function of BSP concentration yielding an apparent value of 226 micromol/l and a Vmax of -10.3 mV. The BSP-induced membrane hyperpolarization is inhibited by the K+ channel blocker Ba2+, and in cable-analysis and ion-substitution experiments it becomes evident that the effect is due to a significant increase in cell membrane K+ conductance. Voltage changes were attenuated by the simultaneous administration of SO2-4, succinate, and cholate (cis-inhibition) and increased after preincubation with SO2-4 and succinate (trans-stimulation), suggesting that the effect occurs via BSP uptake through the known SO2-4/OH- exchanger. Microfluorometric measurements reveal that BSP-induced activation of K+ conductance is not mediated by changes in cell pH, cell Ca2+, or cell volume. It is concluded that K+ channel activation by BSP (as well as by DIDS and indocyanine green) may reflect a physiological mechanism linking the sinusoidal uptake of certain anions to their electrogenic canalicular secretion.

Transport of fatty acids across membranes by the diffusion mechanism

In early research on fatty acid transport, passive diffusion seemed to provide an adequate explanation for movement of fatty acids through the membrane bilayer. This simple hypothesis was later challenged by the discovery of several proteins that appeared to be membrane-related fatty acid transporters. In addition, some biophysical studies suggested that fatty acids moved slowly through the simple model membranes (phospholipid bilayers), which would provide a rationale for protein-assisted transport. Furthermore, it was difficult to rationalize how fatty acids could diffuse passively across the bilayer as anions. Newer studies have shown that fatty acids are present in membranes in the un-ionized as well as the ionized form, and that the un-ionized form can cross a protein-free phospholipid bilayer quickly. This flip-flop mechanism has been validated in cells by intracellular pH measurements. The role of putative fatty acid transport proteins remains to be clarified.

Organic anion transporting polypeptide mediates organic anion/HCO3- exchange

Organic anion transporting polypeptide (oatp) is an integral membrane protein cloned from rat liver that mediates Na+-independent transport of organic anions such as sulfobromophthalein and taurocholic acid. Previous studies in rat hepatocytes suggested that organic anion uptake is associated with base exchange. To better characterize the mechanism of oatp-mediated organic anion uptake, we examined transport of taurocholate in a HeLa cell line stably transfected with oatp under the regulation of a zinc-inducible promoter (Shi, X., Bai, S., Ford, A. C., Burk, R. D., Jacquemin, E., Hagenbuch, B., Meier, P. J., and Wolkoff, A. W. (1995) J. Biol. Chem. 270, 25591-25595). Whereas noninduced transfected cells showed virtually no uptake of [3H]taurocholate, taurocholate uptake by induced cells was Na+-independent and saturable (Km = 19.4 +/- 3.3 microM; Vmax = 62.2 +/- 1.4 pmol/min/mg protein; n = 3). To test whether organic anion transport is coupled to HCO3- extrusion, we compared the rates of taurocholate-dependent HCO3- efflux from alkali-loaded noninduced and induced cells. Monolayers grown on glass coverslips were loaded with the pH-sensitive dye 2', 7'-bis(carboxyethyl)-5(6)-carboxyfluorescein; intracellular pH (pHi) was measured by excitation ratio fluorometry. Noninduced and induced cells were alkalinized to an equivalent pHi ( approximately 7.7) by transient exposure to a 50 mM HCO3-, Cl--free solution. In the absence of extracellular Cl- and taurocholate, isohydric reduction of superfusate HCO3- concentration from 50 to 25 mM resulted in an insignificant change in pHi over time (dpHi/dt) in both groups. Addition of 25 microM taurocholate to the superfusate led to a rapid fall in pHi in induced (-0.037 +/- 0.011 pH units/min to pHi of 7.41 +/- 0.14) but not in noninduced (0.003 +/- 0.006 pH units/min to pHi of 7.61 +/- 0.08) cells (p < 0.03). These data indicate that oatp-mediated taurocholate transport is Na+-independent, saturable, and accompanied by HCO3- exchange. We conclude that organic anion/base exchange is an important, potentially regulatable component of oatp function.

Bile canalicular cationic dye secretion as a model for P-glycoprotein mediated transport

This study explores properties of P-glycoprotein dependent membrane transport in rat liver with the use of acridine orange as the substrate. We studied the biliary secretion of the dye, its binding to canalicular membrane P-glycoprotein, and effects of the inhibitor cyclosporin A: acridine orange is excreted into bile together with less hydrophobic and glucuronidated metabolites. Cyclosporin A inhibited both the secretion of acridine orange and of its metabolites. In TR- animals, a rat strain that is deficient of the canalicular multi-specific organic anion transport system, non-metabolized acridine orange is the predominant species in bile and its secretion is also inhibited by cyclosporin A. Binding of acridine orange to liver P-glycoprotein was analyzed by photoaffinity labeling with azidopine, a substrate of P-glycoprotein dependent transport in multi-drug resistant tumor cells. Labeling of the immunoprecipitated P-glycoprotein was inhibited by acridine orange, verapamil, and by cyclosporin A. The results show that biliary secretion of acridine orange is highly analogous to P-glycoprotein mediated membrane drug transport in tumor cells that exhibit multi-drug resistance.

Movement of fatty acids, fatty acid analogues, and bile acids across phospholipid bilayers

How lipophilic acids move across membranes, either model or biological, is the subject of controversy. We describe experiments which better define the mechanism and rates in protein-free phospholipid bilayers. The transbilayer movement of lipophilic acids [fatty acids (FA), covalently-labeled FA, bile acids, and retinoic acid] was monitored by entrapping pyranin, a water-soluble, pH-sensitive fluorescent molecule to measure pH inside unilamellar vesicles [Kamp, F., & Hamilton, J.A. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 11367-11370]. Equations for the pseudo-unimolecular rate constants for transbilayer movement of un-ionized (kappa FAH) and ionized (kappa FA-) acids are derived. All FA studied (octanoic, lauric, myristic, palmitic, stearic, oleic, elaidic, linoleic, linolelaidic, and arachidonic) and retinoic acid exhibited rapid transbilayer movement (t 1/2 < 1 s) via the un-ionized form across small unilamellar egg phosphatidylcholine (PC) vesicles. FA produced by phospholipase A2 in the outer leaflet of PC vesicles equilibrated rapidly to the inner leaflet. Ionized FA showed enhanced transbilayer movement (kappa FA- = 0.029 s-1) in the presence of equimolar valinomycin. The three FA analogues [12-(9-anthroyloxy)stearic acid, 5-doxylstearic acid, and 1-pyrenenonanoic acid] moved across PC bilayers via the un-ionized form; except for the anthroyloxy FA (kappa FAH = 4.8 x 10(-3) s-1), the rates were too fast to measure (t 1/2 < 1 s). The rate for cholic acid (CA) transbilayer movement was slow (kappa CAH = 0.056 s-1) compared to that of the more hydrophobic bile acids, deoxy- and chenodeoxycholic acid (t 1/2 < 1 s). The taurine conjugates of the three bile acids did not cross the bilayer (t 1/2 > 1 h). A further application of the pyranin method was to measure the partitioning of FA and bile acids among water, albumin, and PC vesicles. Our results show that the ability of lipophilic acids to permeate a PC bilayer rapidly is dependent on the presence of the un-ionized acid in the membrane interface. Considering the fast unfacilitated movement of FA across protein-free phospholipid bilayers, it is unlikely that there is a universal need for a transport protein to enhance movement of FA across membrane bilayers. Physiological implications of proton movement accompanying fast movement of un-ionized lipophilic acids (and the consequent generation of a pH gradient) are discussed.

Organic solvents increase membrane fluidity and affect bile flow and K+ transport in rat liver

Following the earlier observation that inhalation of volatile lipid solvents and of narcotic gases causes cholestasis, we studied the effects of various organic solvents on bile flow, plasma membrane fluidity and potassium movement in rat liver. Both in vivo and in the isolated perfused liver, applications of CCl4, CHCl3, dichloromethane, trichloroethylene, halothane, benzene and cyclohexane elicited rapid and sustained but reversible cholestasis. A transient phase of choleresis was observed prior to and after cholestasis, during the increase and fall in liver tissue solvent concentrations, respectively. Tissue concentrations required to produce cholestasis were lower the higher the lipophilicity of the solvent. Membrane fluidity was measured in isolated basolateral liver cell membranes by fluorescence polarization. Fluidity increased with increasing solvent concentration, the increase being associated with either biphasic stimulation and inhibition of membrane enzymes (Na+,K(+)-ATPase, 5'nucleotidase) or with inhibition alone (Mg(2+)-ATPase). In the isolated perfused liver, application of organic solvents caused hepatic uptake of K+ that was followed by K+ release upon withdrawal of the solvent. The magnitude of K+ uptake elicited by the solvent was comparable with the effect of blocking K+ channels with 2 mM Ba2+, but Ba2+ was ineffective in the presence of the solvent. In contrast, application of ouabain caused K+ release in equal amounts in the absence and presence of the solvent, indicating that K+ uptake elicited by the solvent results from inhibition of K+ efflux through K+ channels rather than stimulation of the Na+,K+ pump. The data show that cholestasis elicited by lipid solvents is associated with an increase in membrane fluidity and with disturbance of liver K+ homeostasis. The significance of these observations is discussed with respect to other models of experimental cholestasis.

ATP-dependent bile-salt transport in canalicular rat liver plasma-membrane vesicles

The present study identifies and characterizes a novel ATP-dependent bile-salt transport system in isolated canalicular rat liver plasma-membrane (cLPM) vesicles. ATP (1-5 mM) stimulated taurocholate uptake into cLPM vesicles between 6- and 8-fold above equilibrium uptake values (overshoot) and above values for incubations in the absence of ATP. The ATP-dependent portion of taurocholate uptake was 2-fold higher in the presence of equilibrated KNO3 as compared with potassium gluconate, indicating that the stimulatory effect of ATP was not due to the generation of an intravesicular positive membrane potential. Saturation kinetics revealed a very high affinity (Km approximately 2.1 microM) of the system for taurocholate. The system could only minimally be stimulated by nucleotides other than ATP. Furthermore, it was preferentially inhibited by conjugated univalent bile salts. Further strong inhibitory effects were observed with valinomycin, oligomycin, 4,4'-di-isothiocyano-2,2'-stilbene disulphonate, sulphobromophthalein, leukotriene C4 and N-ethylmaleimide, whereas nigericin, vanadate, GSH, GSSG and daunomycin exerted only weak inhibitory effects or none at all. These results indicate the presence of a high-affinity primary ATP-dependent bile-salt transport system in cLPM vesicles. This transport system might be regulated in vivo by the number of carriers present at the perspective transport site(s), which, in addition to the canalicular membrane, might also include pericanalicular membrane vesicles.