Adenoviral transfer of human aquaporin-1 gene to rat liver improves bile flow in estrogen-induced cholestasis.

Estrogens can cause liver cholestatic disease. As downregulation of hepatocyte canalicular aquaporin-8 (AQP8) water channels has been involved in estrogen-induced bile secretory failure, we tested whether the archetypal water channel AQP1 improves 17α-ethinylestradiol (EE)-induced cholestasis. EE administration to rats reduced bile flow by 50%. A recombinant adenoviral (Ad) vector encoding human AQP1 (hAQP1), AdhAQP1, or a control vector was administered by retrograde bile ductal infusion. Hepatocyte canalicular hAQP1 expression was confirmed by liver immunostaining and immunoblotting in purified membrane fractions. Accordingly, canalicular osmotic water permeability was markedly increased. Bile flow, either basal or bile salt-stimulated was significantly augmented by over 50%. The choleretic efficiency of endogenous bile salts (that is, volume of bile per µmol of excreted bile salt) was significantly increased by 45% without changes in the biliary bile salt composition. Our data suggest that the adenoviral transfer of hAQP1 gene to the livers of EE-induced cholestatic rats improves bile flow by enhancing the AQP-mediated bile salt-induced canalicular water secretion. This novel finding might have potential therapeutic implications for cholestatic diseases.

Water movement across rat bile duct units is transcellular and channel-mediated.

In recent studies using freshly isolated rat cholangiocytes, we established that water crosses the cholangiocyte membrane by a channel-mediated mechanism involving aquaporins, a family of water-channel proteins. Our goal was to address the importance of channel-mediated water transport in ductal bile formation by employing a physiologic experimental model, the enclosed, polarized rat intrahepatic bile duct unit (IBDU). Expansion and reduction of luminal areas as a reflection of water movement into and out of IBDUs prepared from livers of normal rats were measured by quantitative computer-assisted image analysis. When enclosed IBDUs were exposed to inward or outward osmotic gradients, their luminal area rapidly increased (approximately 25%) or decreased (approximately 20%) reflecting net water secretion or absorption, respectively. These effects were specifically inhibited by 2 water channel blockers, DMSO and HgCl2. In both instances, beta-mercaptoethanol reversed the inhibitory effects. In the absence of an osmotic gradient, choleretic agents (secretin and forskolin) and a cholestatic hormone (somatostatin) induced a significant increase or decrease of IBDU luminal area by 21% and 22%, respectively. These effects were also inhibited by DMSO and reversed by beta-mercaptoethanol. Under our experimental conditions, DMSO did not interfere with either forskolin-induced cAMP synthesis or the generation of osmotic driving forces via the apical chloride-bicarbonate exchanger. Protamine, an inhibitor of the paracellular pathway, had no effect on hypotonic or forskolin-induced water secretion in IBDUs. These results in a physiologically relevant model of ductal bile formation provide additional support for the concept that osmotically driven and agonist-stimulated water movement into (secretion) and out of (absorption) the biliary ductal lumen is transcellular and water channel-mediated.

The water channel aquaporin-8 is mainly intracellular in rat hepatocytes, and its plasma membrane insertion is stimulated by cyclic AMP.

We previously found that water transport across hepatocyte plasma membranes occurs mainly via a non-channel mediated pathway. Recently, it has been reported that mRNA for the water channel, aquaporin-8 (AQP8), is present in hepatocytes. To further explore this issue, we studied protein expression, subcellular localization, and regulation of AQP8 in rat hepatocytes. By subcellular fractionation and immunoblot analysis, we detected an N-glycosylated band of approximately 34 kDa corresponding to AQP8 in hepatocyte plasma and intracellular microsomal membranes. Confocal immunofluorescence microscopy for AQP8 in cultured hepatocytes showed a predominant intracellular vesicular localization. Dibutyryl cAMP (Bt(2)cAMP) stimulated the redistribution of AQP8 to plasma membranes. Bt(2)cAMP also significantly increased hepatocyte membrane water permeability, an effect that was prevented by the water channel blocker dimethyl sulfoxide. The microtubule blocker colchicine but not its inactive analog lumicolchicine inhibited the Bt(2)cAMP effect on both AQP8 redistribution to cell surface and hepatocyte membrane water permeability. Our data suggest that in rat hepatocytes AQP8 is localized largely in intracellular vesicles and can be redistributed to plasma membranes via a microtubule-depending, cAMP-stimulated mechanism. These studies also suggest that aquaporins contribute to water transport in cAMP-stimulated hepatocytes, a process that could be relevant to regulated hepatocyte bile secretion.

Expression of aquaporin-4 water channels in rat cholangiocytes.

We recently reported that secretin induces the exocytic insertion of functional aquaporin-1 water channels (AQP1) into the apical membrane of cholangiocytes and proposed that this was a key process in ductal bile secretion. Because AQP1 is present on the basolateral cholangiocyte membrane in low amounts, we hypothesized that another AQP must be expressed at this domain to facilitate transbasolateral water movement. Thus, we investigated the expression, subcellular localization, possible regulation by secretin, and functional activity of AQP4, a mercury-insensitive water channel expressed in other fluid transporting epithelia. Using reverse transcription-polymerase chain reaction (RT-PCR) on RNA prepared from purified rat cholangiocytes, we amplified a product of 311 bp that was 100% homologous to the reported AQP4 sequence. RNase protection assay confirmed the presence of an appropriate size transcript for AQP4 in cholangiocytes. Immunoblotting detected a band of approximately 31 kd corresponding to AQP4 in basolateral but not apical membranes of cholangiocytes. Secretin did not alter the amount of plasma membrane AQP4 but, as expected, induced AQP1 redistribution from intracellular to apical plasma membranes. Functional studies showed that AQP4 accounts for about 15% of total cholangiocyte membrane water permeability. Our results indicate that: (1) cholangiocytes express AQP4 messenger RNA (mRNA) and protein and (2) in contrast to AQP1, which is targeted to the apical cholangiocyte membrane by secretin, AQP4 is constitutively expressed on the basolateral cholangiocyte membrane and is secretin unresponsive. The data suggest that AQP4 facilitates the basolateral transport of water in cholangiocytes, a process that could be relevant to ductal bile formation.

Taurocholate-induced inhibition of hepatic lysosomal degradation of horseradish peroxidase.

Endocytosed proteins in hepatocytes are transported to lysosomes for degradation. Metabolites accumulating in these organelles are released into bile by exocytosis, a process that seems to be regulated by the bile salt taurocholate (TC). In this study we examined if TC is also involved in the control of the lysosomal degradation of endocytosed proteins. We used [(14)C]sucrose-labeled horseradish peroxidase ([(14)C]S-HRP), a probe suitable to evaluate lysosomal proteolysis. TC-infused rats as well as isolated rat hepatocytes exposed to TC showed a significant inhibition in the lysosomal degradation of [(14)C]S-HRP (approximately 30%), with no change in either the uptake or the amount of protein reaching lysosomes. Under these conditions, the in vitro assay of lysosomal cathepsins B, L, H, and D revealed no change in their activities, suggesting that a reversible inhibition (lysosomal alkalinization?) was taking place in hepatocytes. Nevertheless, lysosomal pH measured using fluorescein isothiocyanate-dextran was shown not to be altered by TC. In addition, TC was unable to inhibit proteolysis in [(14)C]S-HRP loaded lysosomes or interfere in cathepsin assays. The results suggest that TC inhibits the lysosomal degradation of endocytosed proteins in hepatocytes and that the mechanism does not involve an effect of the bile salt per se or a rise in lysosomal pH.

Secretin induces the apical insertion of aquaporin-1 water channels in rat cholangiocytes.

Aquaporin-1 (AQP1) water channels are present in the apical and basolateral plasma membrane domains of bile duct epithelial cells, or cholangiocytes, and mediate the transport of water in these cells. We previously reported that secretin, a hormone known to stimulate ductal bile secretion, increases cholangiocyte osmotic water permeability and stimulates the redistribution of AQP1 from an intracellular vesicular pool to the cholangiocyte plasma membrane. Nevertheless, the target plasma membrane domain (i.e., basolateral or apical) for secretin-regulated trafficking of AQP1 in cholangiocytes is unknown, as is the functional significance of this process for the secretion of ductal bile. In this study, we used primarily an in vivo model (i.e., rats with cholangiocyte hyperplasia induced by bile duct ligation) to address these issues. AQP1 was quantitated by immunoblotting in apical and basolateral plasma membranes prepared from cholangiocytes isolated from rats 20 min after intravenous infusion of secretin. Secretin increased bile flow (78%, P < 0.01) as well as the amount of AQP1 in the apical cholangiocyte plasma membrane (127%, P < 0.05). In contrast, the amount of AQP1 in the basolateral cholangiocyte membrane and the specific activity of an apical cholangiocyte marker enzyme (i.e., gamma-glutamyltranspeptidase) were unaffected by secretin. Similar observations were made when freshly isolated cholangiocytes were directly exposed to secretin. Immunohistochemistry for AQP1 in liver sections from secretin-treated rats showed intensified staining at the apical region of cholangiocytes. Pretreatment of rats with colchicine (but not with its inactive analog beta-lumicolchicine) inhibited both the increases of AQP1 in the cholangiocyte plasma membrane (94%, P < 0.05) and the bile flow induced by secretin (54%, P < 0.05). Our results in vivo indicate that secretin induces the microtubule-dependent insertion of AQP1 exclusively into the secretory pole (i.e., apical membrane domain) of rat cholangiocytes, a process that likely accounts for the ability of secretin to stimulate ductal bile secretion.

Secretin promotes osmotic water transport in rat cholangiocytes by increasing aquaporin-1 water channels in plasma membrane. Evidence for a secretin-induced vesicular translocation of aquaporin-1.

Although secretin is known to stimulate ductal bile secretion by directly interacting with cholangiocytes, the precise cellular mechanisms accounting for this choleretic effect are unknown. We have previously shown that secretin stimulates exocytosis in cholangiocytes and that these cells transport water mainly via the water channel aquaporin-1 (AQP1). In this study, we tested the hypothesis that secretin promotes osmotic water movement in cholangiocytes by inducing the exocytic insertion of AQP1 into plasma membranes. Exposure of highly purified isolated rat cholangiocytes to secretin caused significant, dose-dependent increases in osmotic membrane water permeability (Pf) (e.g. increased by 60% with 10(-7) M secretin), which was reversibly inhibited by the water channel blocker HgCl2. Immunoblotting analysis of cholangiocyte membrane fractions showed that secretin caused up to a 3-fold increase in the amount of AQP1 in plasma membranes and a proportional decrease in the amount of the water channel in microsomes, suggesting a secretin-induced redistribution of AQP1 from intracellular to plasma membranes. Both the secretin-induced increase in cholangiocyte Pf and AQP1 redistribution were blocked by two perturbations that inhibit secretin-stimulated exocytosis in cholangiocytes, i.e. treatment with colchicine and exposure at low temperatures (20 and 4 degrees C). Our results demonstrate that secretin increases AQP1-mediated Pf in cholangiocytes. Moreover, our studies implicate the microtubule-dependent vesicular translocation of AQP1 water channels to the plasma membrane, a mechanism that appears to be essential for secretin-induced ductal bile secretion and suggests that AQP1 can be regulated by membrane trafficking.

Taurolithocholate can inhibit the biliary discharge of lysosomes in the rat.

The natural bile salt taurolithocholate (TLC) impairs the biliary excretion of lipids and proteins, which are known to reach the canaliculus via vesicles. In this study we examined whether these observations could be extended to the exocytic discharge of lysosomal contents into bile. The single intravenous injection of a cholestatic dose of TLC, 3 micromol/100 g body wt., markedly inhibited the biliary excretion of the lysosomal enzymes acid phosphatase and beta-glucuronidase, despite the excretion of bile salts being normalized after a transient diminution. Under such a condition, TLC did not affect the normal transport to and the processing in lysosomes of the exogenously administered [14C]sucrose-labeled horseradish peroxidase. However, the biliary excretion of the radioactive lysosomal metabolites of the protein was significantly reduced. The results indicate that TLC can inhibit the biliary discharge of lysosomes in the rat without altering the functional integrity of these organelles. Possible explanations for these findings are discussed.

Rat cholangiocytes absorb bile acids at their apical domain via the ileal sodium-dependent bile acid transporter.

Although bile acid transport by bile duct epithelial cells, or cholangiocytes, has been postulated, the details of this process remain unclear. Thus, we performed transport studies with [3H]taurocholate in confluent polarized monolayers of normal rat cholangiocytes (NRC). We observed unidirectional (i.e., apical to basolateral) Na+-dependent transcellular transport of [3H]taurocholate. Kinetic studies in purified vesicles derived from the apical domain of NRC disclosed saturable Na+-dependent uptake of [3H]taurocholate, with apparent Km and Vmax values of 209+/-45 microM and 1.23+/-0.14 nmol/mg/10 s, respectively. Reverse transcriptase PCR (RT-PCR) using degenerate primers for both the rat liver Na+-dependent taurocholate-cotransporting polypeptide and rat ileal apical Na+-dependent bile acid transporter, designated Ntcp and ASBT, respectively, revealed a 206-bp product in NRC whose sequence was identical to the ASBT. Northern blot analysis demonstrated that the size of the ASBT transcript was identical in NRC, freshly isolated cholangiocytes, and terminal ileum. In situ RT-PCR on normal rat liver showed that the message for ASBT was present only in cholangiocytes. Immunoblots using a well-characterized antibody for the ASBT demonstrated a 48-kD protein present only in apical membranes. Indirect immunohistochemistry revealed apical localization of ASBT in cholangiocytes in normal rat liver. The data provide direct evidence that conjugated bile acids are taken up at the apical domain of cholangiocytes via the ASBT, and are consistent with the notion that cholangiocyte physiology may be directly influenced by bile acids.

Solute and water transport pathways in cholangiocytes.

Cholangiocytes possess specific membrane transport systems for a large number of substrates. As a result of the secretion or absorption of solutes, water moves passively across plasma membranes, very likely through water channels; as a result, the volume and composition of primary bile are modified. Cholangiocytes express receptors for secretin and somatostatin, hormones that alter intracellular levels of cAMP in opposite directions; these alterations very likely alter the activation (i.e., phosphorylation state) or number (i.e., by coupled exocytic-endocytic membrane modification) of transporters in the apical or basolateral cholangiocyte membranes. Although our understanding of these mechanisms is sill embryonic, it is anticipated that accelerated research efforts by laboratories utilizing recently available experimental models will provide important advances in the knowledge of the molecular mechanisms involved in the transport of water and solutes by cholangiocytes. The challenge will be to integrate the emerging, often isolated, findings in a physiologically relevant manner to understand how cholangiocytes function normally and in pathological conditions.

Rat hepatocytes transport water mainly via a non-channel-mediated pathway.

During bile formation by the liver, large volumes of water are transported across two epithelial barriers consisting of hepatocytes and cholangiocytes (i.e. intrahepatic bile duct epithelial cells). We recently reported that a water channel, aquaporin-channel-forming integral protein of 28 kDa, is present in cholangiocytes and suggested that it plays a major role in water transport by these cells. Since the mechanisms of water transport across hepatocytes remain obscure, we performed physiological, molecular, and biochemical studies on hepatocytes to determine if they also contain water channels. Water permeability was studied by exposing isolated rat hepatocytes to buffers of different osmolarity and measuring cell volume by quantitative phase contrast, fluorescence and laser scanning confocal microscopy. Using this method, hepatocytes exposed to hypotonic buffers at 23 degrees C increased their cell volume in a time and osmolarity-dependent manner with an osmotic water permeability coefficient of 66.4 x 10(-4) cm/s. In studies done at 10 degrees C, the osmotic water permeability coefficient decreased by 55% (p < 0.001, at 23 degrees C; t test). The derived activation energy from these studies was 12.8 kcal/mol. After incubation of hepatocytes with amphotericin B at 10 degrees C, the osmotic water permeability coefficient increased by 198% (p < 0.001) and the activation energy value decreased to 3.6 kcal/mol, consistent with the insertion of artificial water channels into the hepatocyte plasma membrane. Reverse transcriptase polymerase chain reaction with hepatocyte RNA as template did not produce cDNAs for three of the known water channels. Both the cholesterol content and the cholesterol/phospholipid ratio of hepatocyte plasma membranes were significantly (p < 0.005) less than those of cholangiocytes; membrane fluidity of hepatocytes estimated by measuring steady-state anisotropy was higher than that of cholangiocytes. Our data suggests that the osmotic flow of water across hepatocyte membranes occurs mainly by diffusion via the lipid bilayer (not by permeation through water channels as in cholangiocytes).

Assessment of the in vivo hepatic lysosomal processing of horseradish peroxidase.

The lysosomal processing of horseradish peroxidase (HRP) was assessed in this study, i.e., its lysosomal proteolysis and the biliary output of its possible lysosomal metabolites by rat liver in vivo. HRP was covalently linked to [14C]sucrose to provide a label that remains trapped within lysosomes after proteolysis. The [14C]sucrose-labelled HRP was injected into the portal vein of rat, and after 30 min about 34% of the injected radiolabel was present in the liver. Subcellular fractionation by differential centrifugation and further purification of lysosomes in a Percoll gradient showed that radiolabel was concentrated in lysosomes and indicated that about 91% of the total proteolysis of HRP in liver could be attributed to these organelles. The in vivo lysosomal degradation rate of HRP at 30 min was about 40%/h, decreasing over time. The lysosomal inhibitors chloroquine and leupeptin suppressed proteolysis of HRP by about 30 and 60%, respectively. Analysis of the 14C excreted in bile by trichloroacetic acid precipitation and by SDS-polyacrylamide gel electrophoresis showed a minor fraction, which was intact HRP (40 kDa), and a major fraction, which was associated with material smaller than 3 kDa. The biliary output of these low molecular mass products, in contrast to that of intact HRP, did not gradually decline with time and represented about 3% of the corresponding amounts in liver. Chloroquine and leupeptin specifically decreased their biliary excretion (about 60%), giving additional support to their lysosomal origin. In addition, the overall hepatic processing of [14C]sucrose-labelled HRP did not differ from that of the native HRP measured by enzyme assay, indicating no significant alteration caused by the labelling procedure.

Effect of cholephilic dyes on hepatic tight junctional permeability in the rat.

Changes in biliary permeability during cholephilic dye-induced choleresis, as assessed by measuring the movement into bile of two permeability probes, [14C]sucrose and horseradish peroxidase, were analyzed following an i.v. infusion (60 nmol/min per 100 g body wt) of the model cholephilic organic anion sulfobromophthalein in rats. Dye infusion led to a progressive increase of the [14C]sucrose bile-to-plasma ratio, which reached a maximum value after 100 min of dye infusion (+97%). Paracellular entry of horseradish peroxidase, as evaluated by the early peak of its biliary appearance curve, was also selectively increased (+69%), without changes in the later (transcytotic) access of the protein. Additional dose-response studies of biliary permeability to [14C]sucrose, using sulfobromophthalein and rose bengal, showed that this effect was dose-dependent and rapidly reversed by interruption of dye administration. The influence of hydrophobic/hydrophilic balance on this effect was also studied by infusing four dyes covering a broad range of hydrophobicity (phenol red, bromocresol green, sulfobromophthalein, and rose bengal), so as to attain a similar value of dye hepatic content at the end of the experiment (approximately 150 nmol/g liver wt). Under these conditions, a strong positive correlation was found between the increase in biliary permeability to [14C]sucrose and dye hydrophobicity. These results suggest that cholephilic dyes increase tight junctional permeability in a reversible and dose-dependent manner, and that this effect depends on the hydrophobic/hydrophilic balance of the dye.

Effect of lysosomotropic agents on the taurocholate-stimulated biliary excretion of horseradish peroxidase.

The effects of the lysosomotropic agents chloroquine and leupeptin on the taurocholate-stimulated biliary excretion of horseradish peroxidase (HRP) was studied in bile fistula rats. HRP (0.5 mg/100 g body wt) was injected into the portal vein during taurocholate (0.4 mumol/min/100 g body wt) or saline infusion. HRP appeared in bile showing both an early (approx. 5 min) and a late (approx. 25 min) excretion peak. The late peak, which represented about 95% of the total HRP excreted, is due to transcellular vesicular transport. The early peak is mainly due to paracellular leakage although a rapid vesicular transport also contributes. Taurocholate infusion significantly increased the biliary output of HRP (both peaks) and of the endogenous lysosomal enzyme acid phosphatase. Pretreatment with chloroquine or leupeptin inhibited the taurocholate-stimulated late excretion of HRP into bile, without affecting its early excretion. The lysosomotropic agents did not affect the biliary excretion of bile salts but significantly inhibited the taurocholate-stimulated biliary excretion of acid phosphatase. The results are consistent with a role of lysosomes in the taurocholate-stimulated major transcellular vesicular transport of HRP into bile.

Taurolithocholate-induced inhibition of biliary lipid and protein excretion in the rat.

Taurolithocholate (TLC), a natural bile salt, induces selective impairment on canalicular membrane of the hepatocyte, which seems to be a major determinant of its cholestatic effect in experimental animals. In order to extend existing studies about the effects of TLC on bile secretion, we examined in TLC-treated rats the biliary excretion of compounds that are transported to canalicular membrane via vesicles, such as lipids and proteins. The single intravenous injection of TLC (3 mumol/100 g body wt.) inhibited transiently the biliary bile salt excretion, while the biliary excretion of lipids (i.e., cholesterol and phospholipids) and proteins remained inhibited even though the biliary excretion and composition of bile salts were normalized. Under such a condition, TLC also inhibited the transcellular vesicular pathway to the exogenous protein horseradish peroxidase entry into bile, without altering the paracellular biliary access of the protein. The hepatic uptake of horseradish peroxidase was unaffected by TLC-treatment. The results indicate that TLC can inhibit the biliary excretion of compounds that reach the canaliculus via a vesicular pathway, such as lipids and proteins, by a mechanism not related to a defective bile salt excretion. Possible explanations for these findings are discussed.

Biliary excretion of polyethylene glycol molecular weight 900. Evidence for a bile salt-stimulated vesicular transport mechanism.

Polyethylene glycol molecular weight 900 (PEG-900) has been used as a marker of vectorial water transport into bile canaliculus. However, the mechanisms by which this compound is excreted have not been clarified. To gain more information on this process, we studied the biliary excretion of [3H]PEG-900 in rats during choleresis induced by canalicular choleretics. In addition, the effects of the microtubule inhibitors colchicine and vinblastine, and of the acidotropic agent chloroquine, on PEG-900 excretion were studied to determine whether a vesicular pathway is involved. Continuous i.v. infusion of either dehydrocholate (DHC, a non-micelle forming bile salt choleretic) or 4-methylumbelliferone (4-MU, a non-bile salt canalicular choleretic) at stepwise-increasing rates [0.7, 1.0 and 1.2 mumol.min-1.(100 g body wt)-1] induced a gradual increment in bile flow, whereas a transient increment of [3H]PEG-900 excretion was observed only during DHC-induced choleresis. Furthermore, studies in which two consecutive i.v. injections of DHC (10 mumol/100 g body wt) were administered showed that [3H]PEG-900 excretion induced by a second administration of DHC was 54% lower than that induced by the first one, despite a similar excretion in bile flow. Finally, colchicine (0.5 mumol/100 g body wt), vinblastine (0.5 mumol/100 g body wt) and chloroquine (50 mg/kg body wt) pretreatments inhibited the DHC-induced increment in biliary [3H]PEG-900 output, while DHC-induced choleresis was almost unaffected. Conversely, excretion of [14C]sucrose, when coadministered with [3H]PEG-900, was not impaired by the treatments. These results suggest that, unlike sucrose, PEG-900 excretion is not associated with canalicular water movements. Instead, it may be related to a vesicular transport process followed by a bile acid-stimulated discharge of secretory vesicles into bile through the lysosomal compartment.

Biliary excretion of proteins in the rat during dehydrocholate choleresis.

Choleresis induced by dehydrocholate (DHC) stimulates the discharge into bile of lysosomes, which are implicated in the biliary excretion of proteins. Contrary to taurocholate-induced choleresis, DHC choleresis is not affected by microtubule (mt) inhibition. Therefore, the role of mt's in the biliary protein excretion during bile salt choleresis was analyzed in this study. Normal rats and rats treated with the mt poisons colchicine or vinblastine or with the acidotropic agent chloroquine (Cq) were used. The analysis of the protein component in bile was made on SDS-polyacrylamide gel, and the individual polypeptides were quantitated by densitometry. The excretion of bile polypeptides were compared with that of lysosomal acid phosphatase. Bile flow and bile salt output did not show changes on account of treatments. The biliary excretion of acid phosphatase was stimulated by DHC, and it was not affected by mt inhibitors but was markedly diminished by Cq. DHC choleresis produced different effects on the bile polypeptides. The biliary excretion of polypeptide of high molecular mass (84-140 kDa) was stimulated by DHC. Cq treatment increased their basal biliary excretions, whereas DHC-induced secretion was qualitatively and quantitatively similar to that of controls. The 69-kDa polypeptide (albumin) also increased during DHC-induced choleresis, but it showed a different excretory pattern. Cq treatment inhibited such an increase but no correlation with the excretory pattern of the lysosomal marker was found. The biliary excretion of polypeptides of low molecular mass (down to 14 kDa) suffered a transitory decrease and then a subsequent increase over basal values during the DHC choleresis.(ABSTRACT TRUNCATED AT 250 WORDS)

A simple method for the correction of biliary excretion curves distorted by the biliary dead space.

Biliary solute concentrations measured at the tip of the cannula suffer a delay with respect to bile flow due to the transit time through the biliary tree volume. This study proposes a simple method, which is valid under variable bile flow conditions, to correct the distortion introduced by the biliary tree volume on the kinetic curves of the biliary excretion rate. The biliary transit time (tt) was calculated as the time needed to excrete a bile volume equal to the biliary tree volume by means of the interpolation of biliary cumulative volume versus time curves. Such tt permits one to estimate the canalicular concentration at time t, interpolating the biliary concentration curves at time t-tt. The product between the estimated canalicular concentration and the bile flow allows the calculation of the corrected biliary excretion rate. This method was evaluated by a comparison between biliary excretion rate curves of [14C]taurocholate [( 14C]TC) injected as a bolus under basal and sodium dehydrocholate (DHC)-induced choleresis conditions. Since the canalicular excretion rate of [14C]TC is considered independent of bile flow, the significant differences observed in its excretion kinetics under both conditions were attributed to distortion due to the biliary tree volume. After the correction, both curves showed a significant overlapping. This result indicates that the method improves the time-course representation of canalicular events in biliary excretion kinetic studies.