Gut microbes define liver cancer risk in mice exposed to chemical and viral transgenic hepatocarcinogens.

BACKGROUND AND AIMS: Hepatocellular carcinoma (HCC) frequently results from synergism between chemical and infectious liver carcinogens. Worldwide, the highest incidence of HCC is in regions endemic for the foodborne contaminant aflatoxin B1 (AFB1) and hepatitis B virus (HBV) infection. Recently, gut microbes have been implicated in multisystemic diseases including obesity and diabetes. Here, the hypothesis that specific intestinal bacteria promote liver cancer was tested in chemical and viral transgenic mouse models. METHODS: Helicobacter-free C3H/HeN mice were inoculated with AFB1 and/or Helicobacter hepaticus. The incidence, multiplicity and surface area of liver tumours were quantitated at 40 weeks. Molecular pathways involved in tumourigenesis were analysed by microarray, quantitative real-time PCR, liquid chromatography/mass spectrometry, ELISA, western blot and immunohistochemistry. In a separate experiment, C57BL/6 FL-N/35 mice harbouring a full-length hepatitis C virus (HCV) transgene were crossed with C3H/HeN mice and cancer rates compared between offspring with and without H hepaticus. RESULTS: Intestinal colonisation by H hepaticus was sufficient to promote aflatoxin- and HCV transgene-induced HCC. Neither bacterial translocation to the liver nor induction of hepatitis was necessary. From its preferred niche in the intestinal mucus layer, H hepaticus activated nuclear factor-kappaB (NF-kappaB)-regulated networks associated with innate and T helper 1 (Th1)-type adaptive immunity both in the lower bowel and liver. Biomarkers indicative of tumour progression included hepatocyte turnover, Wnt/beta-catenin activation and oxidative injury with decreased phagocytic clearance of damaged cells. CONCLUSIONS: Enteric microbiota define HCC risk in mice exposed to carcinogenic chemicals or hepatitis virus transgenes. These results have implications for human liver cancer risk assessment and prevention.

Role of Hepatitis C virus core protein in viral-induced mitochondrial dysfunction.

Hepatitis C virus (HCV) infection results in several changes in mitochondrial function including increased reactive oxygen species (ROS) production and greater sensitivity to oxidant, Ca(2+) and cytokine-induced cell death. Prior studies in protein over-expression systems have shown that this effect can be induced by the core protein, but other viral proteins and replication events may contribute as well. To evaluate the specific role of core protein in the context of viral replication and infection, we compared mitochondrial sensitivity in Huh7-derived HCV replicon bearing cells with or without core protein expression with that of cells infected with the JFH1 virus strain. JFH1 infection increased hydrogen peroxide production and sensitized cells to oxidant-induced loss of mitochondrial membrane potential and cell death. An identical phenomenon occurred in genome-length replicons-bearing cells but not in cells bearing the subgenomic replicons lacking core protein. Both cell death and mitochondrial depolarization were Ca(2+) dependent and could be prevented by Ca(2+) chelation. The difference in the mitochondrial response of the two replicon systems could be demonstrated even in isolated mitochondria derived from the two cell lines with the 'genome-length' mitochondria displaying greater sensitivity to Ca(2+) -induced cytochrome c release. In vitro incubation of 'subgenomic' mitochondria with core protein increased oxidant sensitivity to a level similar to that of mitochondria derived from cells bearing genome-length replicons. These results indicate that increased mitochondrial ROS production and a reduced threshold for Ca(2+) and ROS-induced permeability transition is a characteristic of HCV infection. This phenomenon is a direct consequence of core protein interactions with mitochondria and is present whenever core is expressed, either in infection, full-length replicon-bearing cells, or in over-expression systems.

Leukotriene D4 activates a chloride conductance in hepatocytes from lipopolysaccharide-treated rats.

Endotoxin (LPS) can cause hepatocellular injury under several circumstances, and leukotrienes have been implicated as a contributing factor. Since ion channel activation has been associated with cytotoxicity, the aim of this study was to determine the circumstances under which LPS and/or leukotrienes activate ionic conductances in hepatocytes. LPS treatment of rats increased Cl- conductance in hepatocytes from 232+/-42 to 1236+/-134 pS/pF. Voltage dependence and inhibitor specificity of this conductance were similar to that of a swelling-activated Cl- conductance, and internal dialysis with nucleoside analogues suggested control by an inhibitory G protein. The lipoxygenase inhibitor nordihydroguaiaretic acid, the specific leukotriene D4 (LTD4) receptor antagonist MK-571, and the 5-lipoxygenase activating protein inhibitor MK-886 all significantly inhibited the conductance. Intracellular dialysis with LTD4 (1.5 microM) elevated intracellular Ca2+ from 143+/-6.5 to 388+/-114 nM within 6 min and stimulated an outwardly rectifying conductance from 642+/-159 to 1669+/-224 pS/pF (n = 9, P < 0.001). In hepatocytes prepared from untreated rats, this concentration of intracellular LTD4 neither raised intracellular Ca2+ nor activated the conductance. The LTD4 response could be induced in normal hepatocytes by culture with either conditioned medium from LPS-treated macrophages or purified TNF-alpha. In conclusion, intracellular LTD4 activates a chloride conductance in hepatocytes isolated from rats treated with LPS or primed in vitro with TNF-alpha. Changes in the hepatocellular accumulation of leukotrienes therefore mediate channel activation and may contribute to liver injury during sepsis and other inflammatory conditions.

The interaction between acetylation and serine-574 phosphorylation regulates the apoptotic function of FOXO3.

The multispecific transcription factor and tumor suppressor FOXO3 is an important mediator of apoptosis, but the mechanisms that control its proapoptotic function are poorly understood. There has long been evidence that acetylation promotes FOXO3-driven apoptosis and recently a specific JNK (c-Jun N-terminal kinase)-dependent S574 phosphorylated form (p-FOXO3) has been shown to be specifically apoptotic. This study examined whether acetylation and S574 phosphorylation act independently or in concert to regulate the apoptotic function of FOXO3. We observed that both sirtuins 1 and 7 (SIRT1 and SIRT7) are able to deacetylate FOXO3 in vitro and in vivo, and that lipopolysaccharide (LPS) treatment of THP-1 monocytes induced a rapid increase of FOXO3 acetylation, partly by suppression of SIRT1 and SIRT7. Acetylation was required for S574 phosphorylation and cellular apoptosis. Deacetylation of FOXO3 by SIRT activation or SIRT1 or SIRT7 overexpression prevented its S574 phosphorylation and blocked apoptosis in response to LPS. We also found that acetylated FOXO3 preferentially bound JNK1, and a mutant FOXO3 lacking four known acetylation sites (K242, 259, 290 and 569R) abolished JNK1 binding and failed to induce apoptosis. This interplay of acetylation and phosphorylation also regulated cell death in primary human peripheral blood monocytes (PBMs). PBMs isolated from alcoholic hepatitis patients had high expression of SIRT1 and SIRT7 and failed to induce p-FOXO3 and apoptosis in response to LPS. PBMs from healthy controls had lower SIRT1 and SIRT7 and readily formed p-FOXO3 and underwent apoptosis when similarly treated. These results reveal that acetylation is permissive for generation of the apoptotic form of FOXO3 and the activity of SIRT1 and particularly SIRT7 regulate this process in vivo, allowing control of monocyte apoptosis in response to LPS.

Serine 574 phosphorylation alters transcriptional programming of FOXO3 by selectively enhancing apoptotic gene expression.

Forkhead box O3 (FOXO3) is a multispecific transcription factor that is responsible for multiple and conflicting transcriptional programs such as cell survival and apoptosis. The protein is heavily post-translationally modified and there is considerable evidence that post-transcriptional modifications (PTMs) regulate protein stability and nuclear-cytosolic translocation. Much less is known about how FOXO3 PTMs determine the specificity of its transcriptional program. In this study we demonstrate that exposure of hepatocytes to ethanol or exposure of macrophages to lipopolysaccharide (LPS) induces the c-Jun N-terminal kinase (JNK)-dependent phosphorylation of FOXO3 at serine-574. Chromatin immunoprecipitation (ChIP), mRNA and protein measurements demonstrate that p-574-FOXO3 selectively binds to promoters of pro-apoptotic genes but not to other well-described FOXO3 targets. Both unphosphorylated and p-574-FOXO3 bound to the B-cell lymphoma 2 (Bcl-2) promoter, but the unphosphorylated form was a transcriptional activator, whereas p-574-FOXO3 was a transcriptional repressor. The combination of increased TRAIL (TNF-related apoptosis-inducing ligand) and decreased Bcl-2 was both necessary and sufficient to induce apoptosis. LPS treatment of a human monocyte cell line (THP-1) induced FOXO3 S-574 phosphorylation and apoptosis. LPS-induced apoptosis was prevented by knockdown of FOXO3. It was restored by overexpressing wild-type FOXO3 but not by overexpressing a nonphosphorylatable S-574A FOXO3. Expression of an S-574D phosphomimetic form of FOXO3 induced apoptosis even in the absence of LPS. A similar result was obtained with mouse peritoneal macrophages where LPS treatment increased TRAIL, decreased Bcl-2 and induced apoptosis in wild-type but not FOXO3(-/-) cells. This work thus demonstrates that S-574 phosphorylation generates a specifically apoptotic form of FOXO3 with decreased transcriptional activity for other well-described FOXO3 functions.

Na+-H+ exchange at the apical membrane of Necturus gallbladder. Extracellular and intracellular pH studies.

The mechanism of luminal solution acidification was studied in Necturus gallbladder by measurement of mucosal solution and intracellular pH with glass electrodes. When the gallbladder was bathed by a Na-Ringer's solution it acidified the luminal side by a Na+-dependent, amiloride-inhibitable process. In the presence of ouabain, acidification was reduced but could be stimulated to a rate greater than that under control conditions by the imposition of an inwardly directed Na+ gradient. These results suggest that luminal acidification results from Na+-H+ exchange at the apical membrane and not by diffusion of metabolic CO2. Li+ can substitute for Na+ but K+, Rb+, Cs+, and tetramethylammonium (TMA+) cannot. The maximal rate of exchange was about five times greater for Na+ than for Li+. Intracellular pH (pHi) was measured with recessed-tip glass microelectrodes; with the tissue bathed in Na-Ringer's solution (pH 7.75), pHi was 7.51 +/- 0.04. After inhibition of Na+-H+ exchange by mucosal perfusion with amiloride (1 mM) or by complete Na+ replacement with TMA+, phi fell reversibly by 0.15 and 0.22 pH units, respectively. These results support the conclusion that Na+-H+ exchange at the apical membrane is the mechanism of luminal acidification and is involved in the maintenance of steady state pHi.

Na+-H+ exchange and Na+ entry across the apical membrane of Necturus gallbladder.

The role of Na+-H+ exchange in Na+ transport across the apical membrane was evaluated in Necturus gallbladder epithelium by means of intracellular Na+ activity (aNai) and 22Na+ uptake measurements. Under control conditions, complete replacement of Na+ in the mucosal solution with tetramethylammonium reduced aNai from 14.0 to 6.9 mM in 2 min (P less than 0.001). Mucosal addition of the Na+-H+ exchange inhibitor amiloride (10(-3) M) reduced aNai from 15.0 to 13.3 mM (P less than 0.001), whereas bumetanide (10(-5) and 10(-4) M) had no effect. Na+ influx across the apical membrane was studied by treating the tissues with ouabain, bathing them in Na-free solutions, and suddenly replacing the mucosal solution with an Na-containing solution. When the mucosal solution was replaced with Na-Ringer's, aNai increased at approximately 11 mM/min. This increase was inhibited by 54% by amiloride (10(-3) M, P less than 0.001) and was unaffected by bumetanide (10(-5) M). Amiloride-inhibitable Na+ fluxes across the apical membrane were also induced by the imposition of pH gradients. Na+ influx was also examined in tissues that had not been treated with ouabain. Under control conditions, 22Na+ influx from the mucosal solution into the epithelium was linear over the first 60 s and was inhibited by 40% by amiloride (10(-3) M, P less than 0.001) and by 19% by bumetanide (10(-5) M, P less than 0.025). We conclude that Na+-H+ exchange is a major pathway for Na+ entry in Necturus gallbladder, which accounts for at least half of apical Na+ influx both under transporting conditions and during exposure to ouabain. Bumetanide-inhibitable Na+ entry mechanisms may account for only a smaller fraction of Na+ influx under transporting conditions, and cannot explain influx in ouabain-treated tissues. These results support the hypothesis that NaCl entry results primarily from the operation of parallel Na+-H+ and Cl--HCO-3 exchangers, and not from a bumetanide-inhibitable NaCl cotransporter.

Intracellular K+ activity and its relation to basolateral membrane ion transport in Necturus gallbladder epithelium.

A study of the mechanisms of the effects of amphotericin B and ouabain on cell membrane and transepithelial potentials and intracellular K activity (alpha Ki) of Necturus gallbladder epithelium was undertaken with conventional and K-selective intracellular microelectrode techniques. Amphotericin B produced a mucosa-negative change of transepithelial potential (Vms) and depolarization of both apical and basolateral membranes. Rapid fall of alpha Ki was also observed, with the consequent reduction of the K equilibrium potential (EK) across both the apical and the basolateral membrane. It was also shown that, unless the mucosal bathing medium is rapidly exchanged, K accumulates in the unstirred fluid layers near the luminal membrane generating a paracellular K diffusion potential, which contributes to the Vms change. Exposure to ouabain resulted in a slow decrease of alpha Ki and slow depolarization of both cell membranes. Cell membrane potentials and alpha Ki could be partially restored by a brief (3-4 min) mucosal substitution of K for Na. Under all experimental conditions (control, amphotericin B, and ouabain), EK at the basolateral membrane was larger than the basolateral membrane equivalent emf (Eb). Therefore, the K chemical potential difference appears to account for Eb and the magnitude of the cell membrane potentials, without the need to postulate an electrogenic Na pump. Comparison of the rate of Na transport across the tissue with the electrodiffusional K flux across the basolateral membrane indicates that maintenance of a steady-state alpha Ki cannot be explained by a simple Na,K pump-K leak model. It is suggested that either a NaCl pump operates in parallel with the Na,K pump, or that a KCl downhill neutral extrusion mechanism exists in addition to the electrodiffusional K pathway.

Emergency management of drug overdose.

Successful management of patients presenting after intentional, accidental, acute or chronic overdose depends on prompt identification of drugs ingested, an organized approach of care that centers around stabilization, patient assessment, gastric evacuation, GI/systemic elimination, patient education and psychiatric evaluation.

Automobile air bag-mediated injury: a case presentation.

Because of the growing number of automobiles equipped with air bags, it is virtually certain that patients will seek treatment in the emergency department for air bag-mediated injury. The emergency nurse must be aware of air bag-mediated injury patterns and prepared for triage and management of any of the potential concurrent injuries caused by air-bag deployment.

Electrogenicity of Na(+)-coupled bile acid transporters.

The Na(+)-bile acid cotransporters NTCP and ASBT are largely responsible for the Na(+)-dependent bile acid uptake in hepatocytes and intestinal epithelial cells, respectively. This review discusses the experimental methods available for demonstrating electrogenicity and examines the accumulating evidence that coupled transport by each of these bile acid transporters is electrogenic. The evidence includes measurements of transport-associated currents by patch clamp electrophysiological techniques, as well as direct measurement of fluorescent bile acid transport rates in whole cell patch clamped, voltage clamped cells. The results support a Na+:bile acid coupling stoichiometry of 2:1.

Methanol poisoning in the emergency department.

The mainstay of effective treatment in methanol poisoning involves prompt identification of the ingestion, decreasing the metabolism of methanol, rapid removal of methanol from the system, treating the acidosis, and arranging psychiatric follow-up.

Free concentrations of intracellular fluorescent anions determined by cytoplasmic dialysis of isolated hepatocytes.

Intracellular organic ions exist in free solution bound to cytoplasmic proteins, partitioned within intracellular membranes, and enclosed in intracellular vesicles and organelles. The aim of this study was to develop a method for measurement of the free cytosolic concentration of organic ions. This was accomplished by measuring initial rates of diffusion between patch-clamp pipettes and cell cytoplasm and determining the null-point concentration of this process. Carboxydimethylfluorescein (CF) was used as a model compound. It readily diffused between cytoplasm and pipette, and there was a linear relationship between concentration in the pipette and equilibrium cell fluorescence. When cells previously loaded with CF were patched, intracellular fluorescence rapidly changed in a positive or a negative direction, depending on the concentration of CF in the pipette. The null point, defined as the concentration at which cells neither gained nor lost fluorescence, described the same relationship between free concentration and total cell fluorescence as that determined by direct loading of the cells to equilibrium. In hepatocytes preloaded with a fluorescent bile acid derivative, cholylglycylamidofluorescein (CGamF), by exposure (0.05 microM) for 30 min, the null point occurred at a CGamF concentration in the pipette of 0.6 microM. This value is 12 times greater than that in the bath. In conclusion, a new method is described that can measure free cytosolic concentrations of fluorescent molecules. It should prove useful in determining the intracellular location and state of transported organic ions.

Electrogenicity of Na-coupled bile salt transport in isolated rat hepatocytes.

The importance of membrane voltage in uptake of bile salts into hepatocytes is not known. Electrogenicity of the primary bile salt transport process, Na-bile salt cotransport, has been difficult to determine because the large K and Cl conductances of the sinusoidal membrane (GK and GCl, respectively) obscure any transport associated currents. In the present study hepatocytes were treated to reduce these membrane conductances and electrogenic entry of taurocholate and glycocholate was demonstrated. Intracellular voltage and resistance changes resulting from bile salt transport were measured in hepatocytes in which GK and GCl were blocked by impalement with Na acetate microelectrodes and external exposure to quinine (400 microM). This increased the cell input resistance from 153 +/- 17 to 230 +/- 17 M omega (n = 14, P < 0.001). Under these conditions, exposure to 100 microM of taurocholate or glycocholate produced Na-dependent depolarizations of 3.0 +/- 0.5 and 4.2 +/- 0.8 mV, respectively. These correspond to transport currents of 13.9 and 7.6 pA/cell, which are comparable to those predicted from known [3H]taurocholate uptake rates if one positive charge enters the cell with each bile salt molecule. Although uptake of these two bile salts was electrogenic, this was not the case for all bile salts. Na-dependent transport of taurodehydrocholate, which occurs at similar rates to that for taurocholate, produced no voltage change. The unconjugated bile salts cholate and ursodeoxycholate also produced no measurable voltage or resistance changes. In conclusion, Na-dependent uptake of taurocholate and glycocholate is electrogenic, whereas uptake of taurodehydrocholate, ursodeoxycholate, and cholate is predominantly electroneutral.(ABSTRACT TRUNCATED AT 250 WORDS)

cAMP- and swelling-activated chloride conductance in rat hepatocytes.

An outwardly rectifying Cl- conductance was identified in primary isolated rat hepatocytes, and the whole cell patch-clamp technique was used to characterize its properties and mechanisms of activation. With symmetrical Cl(-)-containing solutions on both sides and adenosine 3',5'-cyclic monophosphate (cAMP; 100 microM) in the pipette solution, a large outwardly rectifying conductance (1,014 +/- 153 pS/pF, n = 20) developed in all cells within 3 min. This cAMP-activated conductance was highly anion selective and slowly inactivated at voltages > 80 mV. It was completely inhibited by the anion channel blocker 5-nitro-2-(3-phenylpropylamino) benzoic acid (200 microM, n = 6) and partially inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (150 microM, n = 7). It displayed a halide selectivity of I- > Br- > Cl-. In the absence of cAMP, a functionally similar conductance was activated by cell swelling. Reduction of bath osmolality from 300 to 250 mosmol/kg increased membrane conductance from 64 +/- 16.4 to 487 +/- 23 pS/pF (n = 4). This swelling-activated conductance was also highly anion selective and had identical halide selectivity and blocker sensitivity as the cAMP-activated conductance. Although cell swelling was not necessary for cAMP activation, cell shrinkage with hyperosmotic bath (350 mosmol/kg), either before or after exposure to cAMP, inhibited the cAMP-activated conductance. By the determination of conductance as a function of bath osmolality in the presence and absence of cAMP, it was observed that cAMP shifted the osmotic set point for conductance activation without changing either the maximum or minimum conductance. In conclusion, both cAMP and cell swelling activate a large outwardly rectifying Cl- conductance in rat hepatocytes. Its ionic selectivity and sensitivity to channel blockers are identical to those seen for swelling-activated Cl- conductances in many cell types. The conductive properties are not those of cystic fibrosis transmembrane conductance regulator-mediated Cl- conductance. cAMP appears to activate this conductance by altering the volume set point of a swelling-activated channel.

Intracellular ionic activities and transmembrane electrochemical potential differences in gallbladder epithelium.

Intracellular ion activities in Necturus gallbladder epithelium were measured with liquid ion-exchanger microelectrodes. Mean values for K, Cl and Na activities were 87, 35 and 22 mM, respectively. The intracellular activities of both K and Cl are above their respective equilibrium values, whereas the Na activity is far below. This indicates that K and Cl are transported uphill toward the cell interior, whereas Na is extruded against its electrochemical gradient. The epithelium transports NaCl from mucosa to serosa. From the data presented and the known Na and Cl conductances of the cell membranes, we conclude that neutral transport driven by the Na electrochemical potential difference can account for NaCl entry at the apical membrane. At the basolateral membrane, Na is actively transported. Because of the low Cl conductance of the membrane, only a small fraction of Cl transport can be explained by diffusion. These data suggest that Cl transport across the basolateral membrane is a coupled process which involves a neutral NaCl pump, downhill KCl transport, or a Cl-anion exchange system.

Maintenance of cellular acidification in cyanide-treated hepatocytes results from inhibition of Na+/H+ exchange.

Inhibition of respiration by metabolic inhibitors or hypoxia is accompanied by intracellular acidification. Although this acidification is known to promote cell survival during hypoxia, little is known about its mechanism. Given that the Na+/H+ exchanger is known to be a major component of pH regulation in normal hepatocytes, the aim of this study was to determine the effects of inhibition of mitochondrial respiration on intracellular pH (pHi) regulation and Na+/H+ exchange. Cyanide (CN-; 5 mM) plus fructose (20 mM) were used as a model of hypoxic acidosis. pHi was measured with quantitative fluorescence microscopy of cells loaded with the pH indicator, 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein. In control cells, pHi was 7.09 +/- 0.01 SE (n = 106). After 60 min in CN(-)-fructose, pHi fell to 6.74 +/- 0.01 (n = 129, P < 0.001). The pHi recovery rate (expressed as mmol H+.l-1.min-1) was determined under both conditions after acid loading by transient exposure and removal of 20 mM NH4Cl. Control and CN(-)-treated cells recovered at 3.59 +/- 0.25 (n = 42) and 0.69 +/- 0.09 (n = 38, P < 0.001), respectively. Amiloride treatment (1 mM) in the absence of CN- reduced pHi recovery similarly to that caused by CN- (0.34 +/- 0.07, n = 14). CN(-)-treated cells exposed to amiloride demonstrated no additional inhibition (efflux rate 0.65 +/- 0.11, n = 27), suggesting that the inhibition is directed at Na+/H+ exchange. Twenty minutes after CN- removal, CN(-)-treated cells regained their ability to recover from an acid load, thus demonstrating the reversibility of this effect.(ABSTRACT TRUNCATED AT 250 WORDS)

cAMP stimulates fluorescent bile acid uptake into hepatocytes by membrane hyperpolarization.

Elevation of intracellular adenosine 3',5'-cyclic monophosphate (cAMP) hyperpolarizes hepatocytes and increases the uptake rate of bile acids. The purpose of this study was to determine to what extent these two phenomena are linked. Fluorescent bile acid analogues (FBA) were used to probe bile acid transport into whole cell patch-clamped hepatocytes. Na(+)-dependent uptake of cholyl-nitrobenz-2-oxa-1,3-diazol-4-yl-lysine (C-NBD-L), an FBA with a net charge of -1, was shown to be electrogenic, whereas uptake of cholylglycylamidofluorescein (CGamF), an FBA with a net charge of -2, was neutral. Incubation of hepatocytes with 8-bromo-cAMP (8-BrcAMP; 100 microM) increased the uptake rate of the electrogenically transported FBA by 25% (P = 0.002), but had no effect on the uptake rate of the electroneutrally transported FBA. Microelectrode impalements revealed that 8-BrcAMP or forskolin hyperpolarized hepatocytes by 6-8 mV. To determine if hyperpolarization is responsible for the cAMP-induced increase in uptake rate, cAMP was directly introduced into hepatocytes during whole cell patch clamp under voltage-clamp conditions. As long as voltage clamp was maintained at -30 mV there was no stimulation of C-NBD-L uptake. However, when voltage clamp was terminated by either pipette removal or current clamp, cAMP increased the uptake rate by 25-34% (P < 0.002). In both of these protocols, cAMP had no effect on uptake of the electroneutrally transported FBA, CGamF. Finally, in voltage-clamped hepatocytes in the absence of cAMP, a 10-mV hyperpolarization increased the uptake rate of C-NBD-L by 23%. We therefore conclude that short-term cAMP-induced stimulation of fluorescent bile acid uptake in hepatocytes is a direct consequence of membrane hyperpolarization.

Bile acid uptake via the human apical sodium-bile acid cotransporter is electrogenic.

Intestinal absorption of bile acids depends on a sodium-bile acid cotransport protein in the apical membrane of the ileal epithelial cell. Transport is Na+-dependent, but the Na+-bile acid stoichiometry and electrogenicity of transport are not known. Studies in whole intestine, isolated cells, and ileal membrane vesicles have been unable to resolve this issue because transport currents are small and can be obscured by other ionic conductances and transport proteins present in these membranes. In this study, the human apical sodium-bile acid transporter was expressed in stably transfected Chinese hamster ovary cells that lack other bile acid transporters. The Na+-dependent transport of a fluorescent bile acid analog, chenodeoxycholyl-Nepsilon-nitrobenzoxadiazol-lysine, was monitored by fluorescence microscopy in single, voltage-clamped cells. Bile acid movement was bidirectional and voltage-dependent with negative intracellular voltage-stimulating influx. A 3-fold reduction in extracellular Na+ produced a negative 52 mV shift of the flux-voltage relationship, consistent with a 2:1 Na+:bile acid coupling stoichiometry. No Na+- or voltage-dependent uptake was observed in nontransfected Chinese hamster ovary cells. These results indicate that the cotransport of bile acids and Na+ by human apical sodium-bile acid transporter is electrogenic and bidirectional and is best explained by a 2:1 Na+:bile acid coupling stoichiometry. These results suggest that membrane potential may regulate bile acid transport rates under physiological and pathophysiological conditions.