Cytochemical localization of Na+, K+-ATPase in the rat hepatocyte

Bile formation and secretion: An update

Bile formation is a fundamental physiological process that is vital to the survival of all vertebrates. However, little was known about the mechanisms of this secretion until after World War II. Initial studies involved classic physiologic studies in animal models and humans, which progressed to include studies in isolated cells and membrane vesicles. The advent of molecular biology then led to the identification of specific transport systems that are the determinants of this secretion. Progress in this field was reviewed in the American Physiologic Society's series on "Comprehensive Physiology" in 2013. Herein, we provide an in-depth update of progress since that time.

The Role of the Clathrin Adaptor AP-1: Polarized Sorting and Beyond

The selective transport of proteins or lipids by vesicular transport is a fundamental process supporting cellular physiology. The budding process involves cargo sorting and vesicle formation at the donor membrane and constitutes an important process in vesicular transport. This process is particularly important for the polarized sorting in epithelial cells, in which the cargo molecules need to be selectively sorted and transported to two distinct destinations, the apical or basolateral plasma membrane. Adaptor protein (AP)-1, a member of the AP complex family, which includes the ubiquitously expressed AP-1A and the epithelium-specific AP-1B, regulates polarized sorting at the trans-Golgi network and/or at the recycling endosomes. A growing body of evidence, especially from studies using model organisms and animals, demonstrates that the AP-1-mediated polarized sorting supports the development and physiology of multi-cellular units as functional organs and tissues (e.g., cell fate determination, inflammation and gut immune homeostasis). Furthermore, a possible involvement of AP-1B in the pathogenesis of human diseases, such as Crohn's disease and cancer, is now becoming evident. These data highlight the significant contribution of AP-1 complexes to the physiology of multicellular organisms, as master regulators of polarized sorting in epithelial cells.

Bile formation and secretion

Bile is a unique and vital aqueous secretion of the liver that is formed by the hepatocyte and modified down stream by absorptive and secretory properties of the bile duct epithelium. Approximately 5% of bile consists of organic and inorganic solutes of considerable complexity. The bile-secretory unit consists of a canalicular network which is formed by the apical membrane of adjacent hepatocytes and sealed by tight junctions. The bile canaliculi (∼1 μm in diameter) conduct the flow of bile countercurrent to the direction of portal blood flow and connect with the canal of Hering and bile ducts which progressively increase in diameter and complexity prior to the entry of bile into the gallbladder, common bile duct, and intestine. Canalicular bile secretion is determined by both bile salt-dependent and independent transport systems which are localized at the apical membrane of the hepatocyte and largely consist of a series of adenosine triphosphate-binding cassette transport proteins that function as export pumps for bile salts and other organic solutes. These transporters create osmotic gradients within the bile canalicular lumen that provide the driving force for movement of fluid into the lumen via aquaporins. Species vary with respect to the relative amounts of bile salt-dependent and independent canalicular flow and cholangiocyte secretion which is highly regulated by hormones, second messengers, and signal transduction pathways. Most determinants of bile secretion are now characterized at the molecular level in animal models and in man. Genetic mutations serve to illuminate many of their functions.

Physiological and molecular biochemical mechanisms of bile formation

This review considers the physiological and molecular biochemical mechanisms of bile formation. The composition of bile and structure of a bile canaliculus, biosynthesis and conjugation of bile acids, bile phospholipids, formation of bile micellar structures, and enterohepatic circulation of bile acids are described. In general, the review focuses on the molecular physiology of the transporting systems of the hepatocyte sinusoidal and apical membranes. Knowledge of physiological and biochemical basis of bile formation has implications for understanding the mechanisms of development of pathological processes, associated with diseases of the liver and biliary tract.

Deglycosylation of Na+/K+-ATPase causes the basolateral protein to undergo apical targeting in polarized hepatic cells

Polarized epithelia, such as hepatocytes, target their integral membrane proteins to specific apical or basolateral membrane domains during or after biogenesis. The roles played by protein glycosylation in this sorting process remain controversial. We report here that deglycosylation treatments in well-polarized hepatic cells by deglycosylation drugs, or by site-directed mutagenesis of the N-linked-glycosylation residues, all cause the Na+/K+-ATPase beta-subunit to traffic from the native basolateral to the apical/canalicular domain. Deglycosylated beta-subunits are still able to bind and therefore transport the catalytic alpha-subunits to the aberrant apical location. Such apical targeting is mediated via the indirect transcytosis pathway. Cells containing apical Na+/K+-ATPase appear to be defective in maintaining the ionic gradient across the plasma membrane and in executing hepatic activities that are dependent upon the ionic homeostasis such as canalicular excretion.

Do recommended textbooks contain adequate information about bile salt transporters for medical students?

Several studies have recently highlighted a number of limitations in medical textbooks. The aims of this study were to 1) to assess whether available medical textbooks provided students with adequate information about bile salt transporters, 2) compare the level of detail and the amount of information provided in current textbooks on hepatic transport mechanisms with those available in the literature, and 3) compare the amount of information provided in medical textbooks on hepatocyte transport mechanisms with those involving other transporters e.g., those found in the nephron. Seventy medical textbooks from disciplines including physiology, pathology, cell biology, medicine, pediatrics, pharmacology, pathophysiology, and histology published during the past six years were examined. The literature on bile salt transport has been searched mainly from the Internet (MEDLINE and PubMed). Most textbooks failed to provide any information on transporters found in the basolateral and canalicular membranes of hepatocytes. There are also deficiencies in information on bile salt transporters in the terminal ileum. However, up to the end of 2002, 3,610 articles and reviews had been published on hepatobiliary and enterocyte transport of bile salts. During the same period (from 1965), 10,757 articles had been published on renal transport. Thus the contents of textbooks may reflect the overall volume of research knowledge on renal transport. However, despite our current understanding of hepatic and intestinal transport of bile salts and extensive research, particularly over the past 12 years, there are major deficiencies in textbooks in this area. These findings indicate that there is an imbalance in the contents of current textbooks and a lack of information about hepatobiliary physiology, bile salt transporters, bile formation, and mechanisms underlying cholestasis and drug-induced injury. Authors, editors, and publishers of medical textbooks should consider the need to update the information provided on bile salt transporters.

The plasma membrane carbonic anhydrase in murine hepatocytes identified as isozyme XIV

BACKGROUND

Biochemical and histochemical studies have both previously indicated plasma membrane-associated carbonic anhydrase (CA) activity in hepatocytes which has been assumed to be CA IV. However, immunohistochemical data did not support this assignment. Recent northern blotting results indicated the presence of mRNA for the most recently discovered membrane-bound CA isozyme, CA XIV, in the liver. The present study was designed to examine whether CA XIV could contribute to the CA activity described in the hepatocytes.

METHODS

Tissue samples from mouse liver were subjected to immunohistochemical staining using the antibodies raised against recombinant mouse CA XIV and CA IV. RT-PCR and western blotting were also performed for CA XIV.

RESULTS

A strong immunofluorescent signal was observed in the plasma membrane of mouse hepatocytes. Although CA XIV was expressed on both the apical and basolateral surfaces, the staining was more prominent at the apical (canalicular) membrane domain. The expression of CA XIV in the liver was confirmed by RT-PCR and western blotting.

CONCLUSIONS

The presence of CA XIV in the hepatocyte plasma membrane places this novel enzyme at a strategic site to control pH regulation and ion transport between the hepatocytes, sinusoids and bile canaliculi.

A quantitative immunocytochemical study of Na+,K+-ATPase in rat hepatocytes after STZ-induced diabetes and dietary fish oil supplementation

Because diabetes causes alterations in hepatic membrane fatty acid content, these changes may affect the Na+,K+-ATPase. In this study we documented the effects of streptozotocin (STZ)-induced diabetes on hepatic Na+,K+-ATPase catalytic alpha1-subunit and evaluated whether these changes could be normalized by fish oil supplementation. Two groups of diabetic rats received fish oil or olive oil supplementation. Both groups had a respective control group. We studied the localization of catalytic alpha1-subunit on bile canalicular and basolateral membranes using immunocytochemical methods and confocal laser scanning microscopy, and the Na+, K+-ATPase activity, membrane fluidity, and fatty acid composition on isolated hepatic membranes. A decrease in the alpha1-subunit was observed with diabetes in the bile canalicular membranes, without changes in basolateral membranes. This decrease was partially prevented by dietary fish oil. Diabetes induces significant changes as documented by enzymatic Na+,K+-ATPase activity, membrane fluidity, and fatty acid content, whereas little change in these parameters was observed after a fish oil diet. In conclusion, STZ-induced diabetes appears to modify bile canalicular membrane integrity and dietary fish oil partly prevents the diabetes-induced alterations.

Species differences in hepatic bile acid uptake: comparative evaluation of taurocholate and tauroursodeoxycholate extraction in rat and rabbit

Dose-response curves for taurocholate and tauroursodeoxycholate were performed in rat and rabbit livers to get more insight into species differences in the hepatic bile acid uptake. The bile acids showed saturation kinetics in both animals, the Vmax in rat being higher than in rabbit and the Km being lower in the rat than in the rabbit for both the bile acids, with no significant difference in the hepatic cells morphometric parameters. Therefore, it is possible that differences in the kinetic parameters are related to the number and, to a lesser extent, to the affinity of the transporters on the sinusoidal plasma membranes.

Alterations of Na+,K(+)-ATPase activity after hypoxia and reoxygenation in the perfused rat liver: an electron microscopic cytochemical study

BACKGROUND/AIMS

Using the cerium technique for ultrastructural cytochemical studies, Na+,K(+)-ATPase activity was investigated in hypoxic and reoxygenated rat liver.

METHODS

In the control group, the livers were perfused with oxygenated hemoglobin-free Krebs-Henseleit buffer for 1 h. For hypoxia (60 min), the flow rate of the perfusate was decreased and oxygen was replaced by nitrogen. For reoxygenation, the liver was reperfused under oxygenated conditions for 5 min after 60 min of hypoxia.

RESULTS

In control livers, a strong Na+,K(+)-ATPase activity was detected at the basolateral membrane of hepatocytes while the apical membrane forming the bile canaliculi did not display any staining. In hypoxic livers, Na+,K(+)-ATPase activity had ceased in the plasma membrane of hepatocytes. In reoxygenated livers, Na+,K(+)-ATPase was rapidly reactivated in the basolateral hepatic membrane. The membrane of blebs generated during the hypoxic phase also showed enzyme activity. In addition, a striking accumulation of reaction product could be observed in about 10% of the apical membranes lining the bile canaliculi.

CONCLUSION

The results indicate a plasticity of the Na+,K(+)-ATPase in hypoxic and reoxygenated rat liver.

Distribution of the surface antigen HAM-4 and cytoskeleton during reformation of bile-canalicular structures in rat primary cultured hepatocytes

The distribution of rat bile-canalicular surface antigen (HAM-4 antigen) and cytoskeletal elements (microtubules, actin filaments, and cytokeratin filaments) was examined during the reformation of bile-canalicular structures (BC-structures) in primary cultures of dissociated hepatocytes obtained following collagenase perfusion. HAM-4 antigen, which initially dispersed after cell dissociation, became focused into regions of cell-to-cell contact even before formation of BC-structures. Typical bile-canalicular microvilli also appeared in these regions before the intercellular spaces were completely closed. Finally, after in vitro reformation of BC, HAM-4 antigen was localized specifically at the BC-surface. The process of BC-reformation and the intracellular organization of actin and cytokeratin filaments were not significantly affected by microtubule inhibitors (nocodazole, colcemid, and colchicine). However, the localization of HAM-4 antigen molecules at the surface of BC was disrupted by these inhibitors, suggesting that the distribution of HAM-4 antigen, which represents a marker for the reconstruction of surface polarity, is dependent on microtubule function.

Subcellular and molecular mechanisms of bile secretion

One of the liver's principal functions is the formation of bile, which is requisite for digestion of fat and elimination of detoxified drugs and metabolites. Bile is a complex fluid made up of water, electrolytes, bile acids, pigments, proteins, lipids, and a multitude of chemical breakdown products. In this review, we have summarized the source of various biliary components, the route by which they end up in bile, including the underlying subcellular and molecular mechanisms, and their contribution to bile formation. One of the reasons why bile formation is so complex is that there are many mechanisms with overlapping substrate specificities, i.e., many biochemically unrelated biliary constituents share common transport mechanisms. Additionally, biliary constituents may reach bile by more than one pathway. Some biliary components are critical for bile formation; others are of minor significance for bile formation but play a major physiological role. The major driving force for bile formation is the uptake and transcellular transport of bile salts by hepatocytes. The energy for bile formation comes from the sodium gradient created by the basolateral Na+/K(+)-ATPase, to which bile salt transport is coupled. The secretory pathway for bile salts involves uptake at the basolateral surface of the hepatocyte, vectorial transcellular movement, and transport across the canalicular membrane into the canalicular lumen. Hydrophilic bile salts are taken up via a sodium-dependent, saturable, carrier-mediated process coupled to the Na+/K(+)-ATPase. This uptake mechanism is also shared by other substrates, such as electroneutral lipids, cyclic oligopeptides, and a wide variety of drugs. Hydrophobic bile acids are taken up by a sodium-independent facilitated carrier-mediated mechanism in common with other organic ions, including sulfated bile acids, sulfobromophthalein, bilirubin, glutathione, and glucuronides, or by nonsaturable passive diffusion. Two major carrier proteins have been identified on the hepatocyte basolateral membrane: a 48-kDa protein that appears to be involved with Na(+)-dependent bile salt uptake, and a 54-kDa protein, thought to be associated with Na(+)-independent bile salt uptake. The intracellular transport of bile salts may involve cytosolic carrier proteins, of which several have been identified. Some evidence suggests a vesicular transport mechanism for bile salts. Since bile acids clearly do not enter the cell by endocytosis, formation of transport vesicles must be a more distal event in the transcellular translocation process. Some bile salts appear to be transported within the same unilamellar vesicles that are involved in the secretion of cholesterol and phospholipid.(ABSTRACT TRUNCATED AT 400 WORDS)

Utilization of ATP-depleted cells in the analysis of taurocholate uptake by isolated rat hepatocytes

The usefulness of ATP-depleted rat hepatocytes in transport studies was examined. ATP-depleted hepatocytes were prepared by incubating cell suspensions with 30 microM rotenone. In ATP-depleted hepatocytes, plasma membrane permeability was increased and mitochondrial membrane potential decreased, while both intracellular volume and pH remained normal. Furthermore, in the presence of valinomycin, the initial uptake rates of 3H-tetraphenyl phosphonium (TPP+) with varied medium concentrations of potassium were predicted according to the Goldman-Hodgkin-Katz equation, which demonstrated that a potassium diffusion potential could be produced in this system. Using the thus-characterized ATP-depleted cells, the uptake mechanism of taurocholate was investigated. In the presence of an inwardly directed Na gradient, the taurocholate uptake was markedly stimulated and bile acid was transiently accumulated at a concentration 3-times higher than at equilibrium ('overshoot') in ATP-depleted cells. No overshoot was observed in viable cells, however, which suggests that in ATP-depleted cells the Na gradient, a driving force for taurocholate uptake, decreased with time. In both viable and ATP-depleted cells, the relationship between medium concentrations of Na and the Na-dependent initial uptake rate were sigmoidal, and the Hill coefficients were close to 2. The Na-dependent initial uptake rate of taurocholate was stimulated by a valinomycin-induced inside negative potassium-diffusion potential in ATP-depleted cells, and the movement of a 'one plus' (as a net) charge was revealed by fitting the data to the Goldman-Hodgkin-Katz equation. These results support the hypothesis that sodium-coupled hepatic uptake of taurocholate occuthrough an electrogenic process with the stoichiometry of 2 Na: 1 taurocholate, although this issue is controversial. In the presence of an outwardly directed sodium gradient, efflux of taurocholate from ATP-depleted cells was not stimulated. Consequently, the physiological transport vector of taurocholate from blood to cell is not only due to the direction of the sodium gradient (blood to cell) but also to membraneous orientation of transport carriers. In conclusion, kinetic analysis using ATP-depleted hepatocytes allowed the formulation of a new approach to clarify the as yet unresolved issues concerning transport stoichiometry and the mechanism for vectorial transport of taurocholate.

Isolation of Raja erinacea basolateral liver plasma membranes: characterization of lipid composition and fluidity

Developing a method for isolating skate (Raja erinacea) basolateral liver plasma membranes, as well as characterizing the lipid composition and fluidity of these membranes, was the primary purpose of this study. Membranes were isolated using self-generating Percoll gradients. Marker enzyme studies indicate that this preparation is highly enriched in the basolateral domain of the liver plasma membrane and largely free of contamination by intracellular organelles or canalicular membranes. Further, these membranes contain the agency responsible for Na(+)-dependent alanine transport. This finding indicates that this membrane preparation will be useful for the study of skate liver plasma membrane transport processes. The lipid composition and fluidity (as assessed by the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene) of the skate basolateral liver plasma membrane shows little variation among preparations. Further, DPH anisotropy plotted as a function of temperature yields a straight line (r = 0.99) which indicates that there is no lipid phase change in these membranes from 4 degrees to 37 degrees C. The membrane preparation does contain substantial phospholipase A2 activity. The function of this enzyme is, in part, to modify membrane lipid composition and fluidity in response to temperature variations; therefore, this finding suggests that in situ lipid metabolizing enzymes may play a central role in the adaptation of skate basolateral liver plasma membranes to changes in the ambient temperature.

Na/K-ATPase in rabbit paranasal sinus mucosa during induced sinusitis

Sinusitis was produced in rabbits, after which animals were separated into three groups: allergic sinusitis, induced purulent sinusitis, and spontaneous purulent sinusitis. Mucosal specimens were taken from these animals and normal controls. Na/K-ATPase was localized cytochemically and its activity studied in order to define the energy metabolism of secretion. The Na/K-ATPase reaction was unable to be clearly distinguished in either the allergic sinusitis specimens or the normal mucosa. In both purulent sinusitis groups, an intensive reaction was observed in the subepithelial glands and a weak reaction was found in the goblet cells. The Na/K-ATPase activity in the purulent sinusitis groups was significantly higher than that in the normal control group. The increased Na/K-ATPase activity may be an affect of hyperactivity of the secretory cells.

Ethanol consumption decreases alanine uptake by rat basolateral liver plasma membrane vesicles

Alterations of amino acid metabolism may play an important role in the pathogenesis of ethanol-induced liver disease. Previous studies indicate that ethanol added in vitro inhibits amino acid uptake by cultured hepatocytes and liver plasma membrane vesicles; however, the effect of chronic ethanol consumption on amino acid uptake by the liver remains unknown. Therefore, the present studies were performed to determine if chronic ethanol consumption impairs alanine uptake by rat basolateral liver plasma membrane vesicles. Male Sprague-Dawley rats were pair-fed for 6 weeks a diet containing 36% of calories as ethanol or a control diet in which ethanol was isocalorically replaced with carbohydrate. Chronic ethanol consumption reduced basolateral liver plasma membrane sodium-dependent alanine transport activity by 36.3% +/- 15.9% (p less than 0.01). This reduction was caused primarily by impaired activity of amino acid transport system A. The response of system A to glucagon was reduced in the ethanol-fed rats, suggesting that impaired hormonal regulation is at least partially responsible for the lower system-A activity. Kinetic analysis shows that ethanol consumption reduces the Vmax of sodium-dependent alanine transport without affecting the Km. These studies indicate that chronic ethanol consumption reduces alanine uptake by the rat liver. They further show that the reduced uptake is at least partially caused by an intrinsic defect in membrane-transport processes rather than another regulatory mechanism.

Cryptic Na+,K(+)-ATPase activity in rat liver canalicular plasma membranes: evidence for its basolateral origin

Controversy exists concerning the localization of the enzyme Na+,K(+)-ATPase to canalicular membranes in hepatocytes. Most studies find enzyme activity only at the basolateral plasma membrane domain of the hepatocyte. However, Na+,K(+)-ATPase activity has been detected recently in a canalicular membrane fraction prepared by Mg++ precipitation, suggesting that differences in membrane domain fluidity account for these discrepancies. To reinvestigate this question, we used free-flow electrophoresis to further purify canalicular liver plasma membranes originally separated by sucrose density centrifugation. With this technique, canalicular membranes devoid of Na+,K(+)-ATPase activity by routine assay were separated into six subfractions. More than 80% of the activities of canalicular marker enzymes was recovered in two subfractions closest to the anode, which were totally devoid of Na+,K(+)-ATPase activity. However, Na+,K(+)-ATPase activity could now be detected in the four other fractions that contained only small amounts of canalicular marker enzymes. The basolateral marker enzyme, glucagon-stimulated adenyl cyclase, comigrated with this cryptic Na+,K(+)-ATPase activity. Furthermore, addition of 6 mumol/L [12-(2-methoxyethoxy)-ethyl-8-(cis-2-n-octylcyclopropyl)-octanoate ], a membrane-fluidizing agent, to the original canalicular membrane preparation and to all subfractions did not stimulate or unmask latent Na+,K(+)-ATPase activity. Finally, when canalicular membranes isolated by Mg++ precipitation were subjected to free-flow electrophoresis, they could not be separated from the more positively charged Na+,K(+)-ATPase-containing fractions, probably because of alterations in surface charge. Together these findings suggest that Na+,K(+)-ATPase is a basolateral enzyme, that represents a small contaminant when present in canalicular liver plasma membranes and that methodological differences may account for previous discrepancies.

Extrahepatic obstructive cholestasis reverses the bile salt secretory polarity of rat hepatocytes

To elucidate the consequences of extrahepatic cholestasis on the structure and function of hepatocytes, we studied the effects of bile duct ligation on the turnover, surface distribution, and functional activity of the canalicular 100-kD bile salt transport protein (cBSTP). Basolateral (blLPM) and canalicular (cLPM) liver plasma membrane vesicles were purified to the same degree from normal and cholestatic rat livers and the membrane bound cBSTP identified and quantitated using polyclonal anti-cBSTP antibodies. Cholestasis of 50 h resulted in an increased release of cBSTP into bile, thereby decreasing its in vivo half-life from 65 to 25 h. Furthermore, a significant portion of cBSTP accumulated at the basolateral surface and in intracellular vesicles of cholestatic hepatocytes. This redistribution of cBSTP was functionally paralleled by decreased and increased electrogenic taurocholate anion transport in cLPM and blLPM vesicles, respectively. These results demonstrate that biliary obstruction causes a reversal of the bile salt secretory polarity of rat hepatocytes. The resulting increase in basolateral (sinusoidal) bile salt efflux might protect hepatocytes from too high an accumulation of toxic bile salts within the cell interior.

Effects of digitoxin and lithium, used as a marker of passive Na transport, on secretin-dependent bile flow in the pig

The present study was performed in anaesthetized pigs, and the first aim was to assess the role of Na,K-ATPase in secretin-dependent biliary HCO3 secretion (JbHCO3). Intra-arterial administration of the cardiac glycoside digitoxin (0.2 mg/kg-1) reduced hepatic Na K-ATPase activity, JbHCO3 and secretin-dependent bile flow by 24, 55 and 34% respectively. In the second part of this study lithium (Li) was used as a marker of passive Na transport to assess the electrochemical gradient for Na flux into bile duct lumen during secretin-stimulated bile flow and impeded biliary osmotic water flow by i.v. infusion of glucose. At plasma glucose 85 (73-96) mmol l-1, bile [Na] and [Li] exceeded their concentrations in plasma by 57 and 47% respectively. By using the Nernst equation, transepithelial potential difference (PD) during hyperglycaemia was estimated to be -6.2 (0 to -10.8) mV (ductal lumen negative), which corresponds to a [Li]bile/[Li]plasma ratio of 1.3 (1.0-1.5). The ratio was not significantly different from the observed [Li]bile/[Li]plasma ratio of 1.4 (1.3-1.5). It is concluded (1) that Na, K-ATPase is necessary for JbHCO3, probably by sustaining the cell membrane PD (cell interior negative) which is a driving force for apical electrogenic HCO3 secretion, and (2) transepithelial Li (and hence Na) flux is driven solely by the negative transcellular PD during secretin-stimulated bile flow in the pig.

Biochemical localization of hepatic surface-membrane Na+,K+-ATPase activity depends on membrane lipid fluidity

Membrane proteins of transporting epithelia are often distributed between apical and basolateral surfaces to produce a functionally polarized cell. The distribution of Na+,K+-ATPase [ATP phosphohydrolase (Na+/K+-transporting), EC 3.6.1.37] between apical and basolateral membranes of hepatocytes has been controversial. Because Na+,K+-ATPase activity is fluidity dependent and the physiochemical properties of the apical membrane reduces its fluidity, we investigated whether altering membrane fluidity might uncover cryptic Na+,K+-ATPase in bile canalicular (apical) surface fractions free of detectable Na+,K+-ATPase and glucagon-stimulated adenylate cyclase activities. Apical fractions exhibited higher diphenylhexatriene-fluorescence polarization values when compared with sinusoidal (basolateral) membrane fractions. When 2-(2-methoxyethoxy)ethyl 8-(cis-2-n-octylcyclopropyl)octanoate (A2C) was added to each fraction, Na+,K+-ATPase, but not glucagon-stimulated adenylate cyclase activity, was activated in the apical fraction. In contrast, further activation of both enzymes was not seen in sinusoidal fractions. The A2C-induced increase in apical Na+,K+-ATPase approached 75% of the sinusoidal level. Parallel increases in apical Na+,K+-ATPase were produced by benzyl alcohol and Triton WR-1339. All three fluidizing agents decreased the order component of membrane fluidity. Na+,K+-ATPase activity in each subfraction was identically inhibited by the monoclonal antibody 9-A5, a specific inhibitor of this enzyme. These findings suggest that hepatic Na+,K+-ATPase is distributed in both surface membranes but functions more efficiently and, perhaps, specifically in the sinusoidal membranes because of their higher bulk lipid fluidity.

Effects of chlorpromazine on taurocholate transport in isolated rat hepatocytes

Chlorpromazine has been shown to have no effect on the uptake of the endogenous bile salt substrate, taurocholate, by isolated rat hepatocytes. It has been shown, however, to inhibit directly release of taurocholate from pre-loaded cells over extended incubation. However, there was no inhibition of the efflux process per se as shown by similar initial rates of taurocholate efflux in the presence or absence of chlorpromazine. Pretreatment of rats with chlorpromazine (100 mumoles/kg) resulted in no change in the ability to transport (that is, accumulate or secrete) taurocholate by hepatocytes isolated 2, 24, 36, 48, or 60 hr later. The data indicate that, if a direct effect on bile acid transport is important in chlorpromazine induced biliary dysfunction, then it involves release rather than uptake at the cell membrane. However, as efflux itself is not inhibited chlorpromazine may interfere with release of taurocholate from intracellular sites.

Properties of phallotoxin uptake by basolateral plasma membrane vesicles from rat liver: evidence for a carrier-mediated transport

The mechanism and driving forces for hepatocellular phallotoxin uptake were studied by a rapid-filtration technique using basolateral liver plasma membrane vesicles (blLPM). An inwardly directed Na+ gradient but not K+-gradient transiently stimulated taurocholate uptake into blLPM 1.4-1.7-fold above the equilibrium value (overshoot), demonstrating functionally intact vesicles. In contrast, overshooting phallotoxin uptake (1.15-1.2-fold intravesicular accumulation above equilibrium value) was observed in the presence of a K+ as well as of a Na+ gradient. Na+ could be replaced by K+ or Li+. In the presence of choline a distinct uptake reduction of 57% was seen. Counter-transport phenomena suggest phallotoxin transport rather than binding. Phallotoxin uptake was inhibited significantly by taurocholate, iodipamide and antamanide, but only slightly by alpha-amanitin. Creation of a negative intravesicular potential by altered accompanying anions or by valinomycin-induced K+ diffusion potential enhanced the initial uptake rate for phallotoxin, demonstrating rheogenic solute uptake. These findings provide evidence that hepatocellular uptake of phallotoxin is due to carrier-mediated transport. Hepatic uptake of phallotoxin is assumed to be driven by both a monovalent cation gradient (Na+ or K+) and a transmembranal potential difference.

Biochemical changes in alpha-naphthyl isothiocyanate-induced chronic cholangitis in the rat

Male Wistar rats were treated for 3 weeks with alpha-naphthyl isothiocyanate (ANIT, 5.4 mmol per kg food). Chronic necrotic cholangitis without pronounced transaminasemia and hyperbilirubinemia developed. The activity of 5'-nucleotidase in liver plasma membrane preparations was strongly depressed (3.5 times) while the activities of K+-, Na+- and Mg++-ATPases were not affected. The liver microsomal cytochrome P-450 and cytochrome b5 contents decreased. Elevation of reduced glutathione liver content after challenge with ANIT was recorded. The observed biochemical changes may be important for the pathogenesis of ANIT-induced chronic cholangitis.

Anion transport in basolateral (sinusoidal) liver plasma-membrane vesicles of the little skate (Raja erinacea)

The mechanism(s) of [35S]sulphate transport was investigated in basolateral liver plasma-membrane vesicles of the little skate elasmobranch, Raja erinacea. Imposition of an intravesicular alkaline pH gradient (pH 8.0 in/pH 6.0 out) stimulated sulphate uptake 5-10-fold compared with pH-equilibrated (pH 8.0 in = out) conditions and 2-3-fold over equilibrium sulphate uptake (overshoot). This pH-gradient-stimulated sulphate uptake was temperature-dependent, saturable with increasing concentrations of sulphate and could be inhibited by the protonophore carbonyl cyanide m-chlorophenylhydrazone and the anion-transport inhibitors 4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid (DIDS) and probenecid, cis-Inhibition of pH-gradient-driven sulphate uptake was observed with sulphate, oxalate, cholate and bromosulphophthalein, but not with chloride and taurocholate. In addition, sulphate and oxalate trans-stimulated [35S]sulphate uptake under pH-equilibrated conditions. Although also stimulated by an inside-alkaline pH gradient, transmembrane transport of [3H]cholate was not inhibited by DIDS, suggesting that its pH-gradient-driven uptake is not mediated by an anion-transport 'carrier'. In conclusion, these studies indicate that a basolateral plasma-membrane sulphate-transport system has evolved in skate hepatocytes and is similar to that in mammalian liver cells. This archaic anion-exchange system co-transports certain organic anions such as oxalate and has developed early in vertebrate evolution.

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.

Three differentially expressed Na,K-ATPase alpha subunit isoforms: structural and functional implications

We have characterized cDNAs coding for three Na,K-ATPase alpha subunit isoforms from the rat, a species resistant to ouabain. Northern blot and S1-nuclease mapping analyses revealed that these alpha subunit mRNAs are expressed in a tissue-specific and developmentally regulated fashion. The mRNA for the alpha 1 isoform, approximately equal to 4.5 kb long, is expressed in all fetal and adult rat tissues examined. The alpha 2 mRNA, also approximately equal to 4.5 kb long, is expressed predominantly in brain and fetal heart. The alpha 3 cDNA detected two mRNA species: a approximately equal to 4.5 kb mRNA present in most tissues and a approximately equal to 6 kb mRNA, found only in fetal brain, adult brain, heart, and skeletal muscle. The deduced amino acid sequences of these isoforms are highly conserved. However, significant differences in codon usage and patterns of genomic DNA hybridization indicate that the alpha subunits are encoded by a multigene family. Structural analysis of the alpha subunits from rat and other species predicts a polytopic protein with seven membrane-spanning regions. Isoform diversity of the alpha subunit may provide a biochemical basis for Na,K-ATPase functional diversity.

Localization of Na+,K+-ATPase alpha-subunit to the sinusoidal and lateral but not canalicular membranes of rat hepatocytes

Controversy has recently developed over the surface distribution of Na+,K+-ATPase in hepatic parenchymal cells. We have reexamined this issue using several independent techniques. A monoclonal antibody specific for the endodomain of alpha-subunit was used to examine Na+,K+-ATPase distribution at the light and electron microscope levels. When cryostat sections of rat liver were incubated with the monoclonal antibody, followed by either rhodamine or horseradish peroxidase-conjugated goat anti-mouse secondary, fluorescent staining or horseradish peroxidase reaction product was observed at the basolateral surfaces of hepatocytes from the space of Disse to the tight junctions bordering bile canaliculi. No labeling of the canalicular plasma membrane was detected. In contrast, when hepatocytes were dissociated by collagenase digestion, Na+,K+-ATPase alpha-subunit was localized to the entire plasma membrane. Na+,K+-ATPase was quantitated in isolated rat liver plasma membrane fractions by Western blots using a polyclonal antibody against Na+,K+-ATPase alpha-subunit. Plasma membranes from the basolateral domain of hepatocytes possessed essentially all of the cell's estimated Na+,K+-ATPase catalytic activity and contained a 96-kD alpha-subunit band. Canalicular plasma membrane fractions, defined by their enrichment in alkaline phosphatase, 5' nucleotidase, gamma-glutamyl transferase, and leucine aminopeptidase had no detectable Na+,K+-ATPase activity and no alpha-subunit band could be detected in Western blots of these fractions. We conclude that Na+,K+-ATPase is limited to the sinusoidal and lateral domains of hepatocyte plasma membrane in intact liver. This basolateral distribution is consistent with its topology in other ion-transporting epithelia.

Effect of complete sulfation of bile acids on bile formation in rats

The effect of sulfation of common bile acids on the formation of bile was investigated in male rats by infusing them with the sulfate esters of cholic, chenodeoxycholic, deoxycholic, lithocholic or dehydrocholic acid in four step-wise, increasing doses. Each dose was infused for 30 min and bile collected every 10 min. Control studies were performed by using either albumin solution (the bile acid carrier) or corresponding nonsulfated bile acids at concentrations similar to those of the sulfated products. The secretion of sulfated bile acids was slower and less than that of nonsulfated bile acids, demonstrating transport maximum kinetics rather than the secretory rate maximum characteristic of nonsulfated bile acids. Sulfation significantly increased bile salt-independent bile flow and the choleretic potency of the bile acids tested. With the exception of deoxycholic acid, which had a slight stimulatory effect, bile acid sulfation generally prevented a rise in bile acid-dependent phospholipid and cholesterol secretion. In fact, it reduced biliary phospholipid and cholesterol secretion associated with the secretion of endogenous bile acids. These data are in agreement with the physicochemical properties of sulfated bile acids. They indicate that sulfation prevents the cholestatic action of nonsulfated bile acids, perhaps by increasing bile flow via a high choleretic potential and/or by stimulating bile acid-independent bile flow. The effect of sulfated bile acids on the secretion of biliary phospholipids may protect the canalicular membrane from the detergent properties of bile acids and may thus block the cholestasis which results from high, nonsulfated bile acid concentrations.

Drug-induced cholestasis

Intrahepatic cholestasis, defined as arrested bile flow, mimics extrahepatic obstruction in its biochemical, clinical and morphological features. It may be due to hepatocyte lesions of which there are three types, termed canalicular, hepatocanalicular and hepatocellular, respectively; or it may be due to ductal lesions at the level of the cholangiole or portal or septal ducts. Defective bile flow due to hepatic lesions reflects abnormal modification of the ductular bile. Defective formation of canalicular bile may involve bile acid-dependent or independent flow. It appears to result most importantly from defective secretion of bile acid-dependent flow secondary to defective uptake from sinusoidal blood, defective transcellular transport and defective secretion; or from regurgitation of secreted bile via leaky tight junctions. An independent defect in bile acid-independent flow is less clear. Defective flow of bile along the canaliculus may reflect increased viscosity and impaired canalicular contractility secondary to injury of the pericanalicular microfibrillar network. Impaired flow beyond the canaliculus may result from ductal injury. Sites of lesions that contribute to cholestasis include the sinusoidal and canalicular plasma membrane, the pericanalicular network and the tight junction and, less certainly, microtubules and microfilaments and Golgi apparatus. A number of drugs that lead to cholestasis have been found to lead to injury at one or more of these sites. Other agents (alpha-naphthylisothiocyanate, methylenedianiline, contaminated rapeseed oil, paraquat) lead to ductal injury resulting in cholestasis. Reports of inspissated casts in ductules (benoxaprofen jaundice) and injury to the major excretory tree (5-fluorouridine after hepatic artery infusion) have led to other forms of ductal cholestasis. Most instances of drug-induced cholestasis present as acute, transient illness, although important chronic forms also occur. The clinical features include the reflection of the cholestasis (pruritus, jaundice), systemic manifestations and extrahepatic organ involvement. While nearly all classes of medicinal agents include some that can lead to cholestasis, there are differences among the various categories. Phenothiazines and related antipsychotic and 'tranquillizer' drugs characteristically lead to cholestatic hepatic injury. The tricyclic antidepressants may lead to cholestatic or hepatocellular injury.(ABSTRACT TRUNCATED AT 400 WORDS)