1 Center for Basic Research in Digestive Diseases, Departments of Internal Medicine and Biochemistry and Molecular Biology, Mayo Clinic and Foundation, Mayo Medical School, Rochester, Minnesota 55905; and 2 Departments of Biological Chemistry and Medicine, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
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ABSTRACT |
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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.,
-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
-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.
biliary epithelia; aquaporins; bile secretion
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INTRODUCTION |
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SECRETIN IS A GASTROINTESTINAL hormone that stimulates ductal bile secretion via specific receptors on bile duct epithelial cells, or cholangiocytes (9, 23, 28). Cholangiocytes, as polarized epithelial cells, have their plasma membrane divided by tight junctions into two domains: 1) the apical domain facing the ductal lumen, which functions as the secretory pole for ductal bile formation, and 2) the basolateral domain facing adjoining cells and underlying connective tissue. Although these two cholangiocyte domains contain different proteins (23), the water channel aquaporin-1 (AQP1) is expressed on both (26) and mediates the osmotically driven movement of water (22, 29). AQP1 is also expressed in erythrocytes, some endothelial cells, and in fluid transporting epithelia throughout the body at sites of constitutive (not regulated) water transport (1). Nevertheless, as we recently reported using an in vitro experimental model (i.e., isolated cholangiocytes), secretin increases the osmotic water transport and causes the redistribution of AQP1 from an intracellular location to the cell surface (24). However, the targeting (i.e., basolateral vs. apical domain) of the secretin-induced trafficking of AQP1 in cholangiocytes as well as the functional relevance of this process for the elaboration of ductal bile are unknown. To address these issues we used in vivo and in vitro experimental models in the rat and we found that secretin caused the insertion of AQP1 exclusively into the apical (secretory) domain of cholangiocytes, a mechanism that appears to be essential for the secretin-induced ductal bile secretion.
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MATERIALS AND METHODS |
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Animal model for study of ductal bile secretion.
Adult male Fisher rats after induction of cholangiocyte hyperplasia by
bile duct ligation (BDL) were used in these studies. The BDL rat is a
well-established experimental model for the study of
secretin-stimulated ductal bile secretion (4). Under pentobarbital anesthesia (5 mg/100 g body wt ip) the rat common bile duct was cannulated with polyethylene PE-50 tubing and heat sealed. After 1 wk
of bile duct obstruction, the cannula was externalized and clipped off
to allow bile collection, and the intravenous access was established
via the femoral vein using a PE-50 cannula. Forty minutes after the
biliary obstruction was released, 1.0 ml of 107 M secretin (Peninsula
Laboratories, Belmont, CA) or vehicle (PBS, pH 7.4) was administered
over 20 min, and bile was collected in two 10-min periods. Rectal
temperature was kept at 37°C, and bile volume was determined
gravimetrically by assuming a bile density of 1.0 g/ml. Bile osmolality
was measured by injecting 10 µl of sample into a Wescor 5100C vapor
pressure osmometer (Logan, Utah). The isolation of cholangiocytes was
begun immediately after secretin (or vehicle) administration as will be described.
Preparation of cholangiocytes.
Cholangiocytes were isolated from livers of BDL rats by enzymatic
digestion and mechanical disruption and purified by sequential counterflow elutriation with a J2-21 centrifuge equipped with a
JE-6B rotor (Beckman Instruments, Fullerton, CA). The final cell
suspension contained ~75% pure cholangiocytes [based on
staining with the cholangiocyte marker -glutamyltranspeptidase
(
-GT)], and the viability was >90% as assessed by trypan
blue exclusion. With the technique employed, three different
populations of cholangiocytes (i.e., small, medium, and large) can be
separated (5, 6). Small cholangiocytes do not express either secretin
receptor or key transporters involved in bile secretion (5, 6), and for
this reason they were excluded from our final cell preparation. Most of
the contaminating cells were sinusoidal endothelial cells (19), which
do not express secretin receptor (7) or AQP1 (29) and therefore did not
interfere in the studies.
Isolation of apical and basolateral cholangiocyte plasma membrane domains. Apical and basolateral plasma membranes were prepared from isolated cholangiocytes as previously described by us (31). Briefly, cholangiocytes were washed and sonicated in 0.3 M sucrose containing 0.01% soybean trypsin inhibitor, 0.1 mM phenylmethylsulfonyl fluoride, and 0.1 mM leupeptin (Sigma, St. Louis, MO). The plasma membrane fraction (designated total cholangiocyte plasma membrane) was obtained by centrifugation at 200,000 g for 60 min on a discontinuous 1.3 M sucrose gradient (24). An aliquot of these membranes was saved, and the remainder was further subfractionated by high-speed centrifugation through discontinuous sucrose gradients to obtain two subfractions enriched in either apical or basolateral cholangiocyte plasma membrane domains (31). Cholangiocytes exposed (in vitro or in vivo) to secretin and corresponding controls were processed in parallel, and the membrane enrichments did not differ between groups. The apical and basolateral plasma membrane preparations were essentially devoid of intracellular membranes (31). Nevertheless, according to the distribution of marker enzymes (31), a minor degree of contamination of the basolateral by the apical membrane (about 11%) was expected, which did not affect the interpretation of the results.
Protein concentration was determined by the fluorescamine method using BSA as standard (32).Immunohistochemistry for AQP1 in liver sections. Rats (normal or 1 wk BDL) were handled and treated with secretin as previously mentioned, and then the livers were perfused via the portal vein with PBS to eliminate the blood, removed, sliced, and fixed by immersion with 4% paraformaldehyde. Paraffin sections (4 µm) were placed in 10 mM citrate buffer (pH 6.0) and microwaved twice for 2 min to improve staining by antigen unmasking. After the sections underwent washing and quenching of endogenous peroxidase, they were blocked and incubated with rabbit affinity-purified antibodies against AQP1 (2 µg/ml; Alpha Diagnostics International, San Antonio, TX) for 2 h at room temperature. The following steps were carried out by an immunoperoxidase procedure (Vectastain Elite ABC kit; Vector Laboratories, Burlingame, CA). The peroxidase was visualized by reaction with diaminobenzidine and hydrogen peroxidase (Sigma) and counterstain with hematoxylin. Controls using nonimmune rabbit IgG (Vector Laboratories) or omission of primary or secondary antibody revealed no labeling.
Immunoblotting for AQP1. Solubilized plasma membrane fractions from cholangiocytes as well as from erythrocytes (positive controls) and hepatocytes (negative controls) were subjected to SDS-PAGE and transferred to nitrocellulose sheets. After being blocked, blots were incubated overnight at 4°C with AQP1 antiserum (17) diluted 1:500. The blots were then washed and incubated with horseradish peroxidase-conjugated goat antirabbit immunoglobulin (Tago, Burlingame, CA), and bands were detected by the enhanced chemiluminescence detection system (ECL, Amersham). Autoradiographs obtained by exposing nitrocellulose sheets to Kodak XAR film were scanned and quantitated using appropriate software program (Molecular Analyst, Bio-Rad).
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RESULTS |
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Apical vs. basolateral insertion of AQP1 in secretin-treated
cholangiocytes.
To study the membrane domain involved in the secretin-induced
insertion of AQP1 in cholangiocytes, we isolated fractions enriched in
apical and basolateral plasma membranes from secretin-exposed isolated
cholangiocytes. As we previously reported (24), the exposure of cells
to secretin caused a significant increase of AQP1 in total
cholangiocyte plasma membranes (189%,
P < 0.05; Fig.
1), whereas the specific activity of the
apical cholangiocyte markers -GT and alkaline phosphatase was
unaffected (not shown). Further subfractionation of these membranes
showed that the increase of AQP1 was almost exclusively associated with
the apical membrane domain (120%, P < 0.05; Fig. 1). Figure 1 shows that the exposure of cells to
secretin caused a significant increase of AQP1 in total cholangiocyte
plasma membranes (189%, P < 0.05)
due almost exclusively to the insertion of this protein into the apical
membrane domain (120%, P < 0.05).
In contrast, the amount of AQP1 in basolateral membranes was not
significantly altered by secretin.
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Effect of secretin on bile flow and cholangiocyte plasma
membrane AQP1 in BDL rats.
The above results were consistent with the concept that the
insertion of AQP1 into the apical (secretory) domain of cholangiocytes is involved in the secretin-regulated mechanism of ductal bile secretion. To begin to investigate this issue, we studied the in vivo
effect of secretin. The hormone was infused into BDL rats (a model for
the study of ductal bile secretion), and bile flow and content of AQP1
in cholangiocyte plasma membranes were assessed. As shown in Fig.
2, secretin increased bile flow (78%,
P < 0.01) and the amount of AQP1 in
the cholangiocyte plasma membrane (121%, P < 0.05) without affecting the
specific activity of -GT.
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Apical vs. basolateral insertion of AQP1 in secretin-treated BDL
rats.
In agreement with the results in hormone-treated isolated
cholangiocytes (see above), after infusion of secretin in vivo, AQP1
was predominantly targeted to the apical domain (Fig.
3). Treatment of rats with secretin
resulted in a significant increase in the amount of AQP1 in apical
(127%, P < 0.05) but not in
basolateral membranes as assessed by immunoblotting (Fig. 3).
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Effect of colchicine on secretin-induced bile flow and AQP1 increase
in cholangiocyte plasma membranes.
To provide support for the functional significance of the regulated
increase of cholangiocyte surface AQP1 in the elaboration of ductal
bile, we made use of the microtubule blocker colchicine, which we had
previously shown inhibits secretin-induced insertion of AQP1 into the
plasma membrane of isolated cholangiocytes (24). As shown in Fig.
5, pretreatment of rats with colchicine
abolished the increase of AQP1 in cholangiocyte plasma membrane and
decreased the secretin-induced bile flow by 25 ml · min1 · kg
body wt
1 (~54%). In
contrast, under these conditions, bile osmolality increased by 6 mosmol/kg. The inactive analog
-lumicolchicine did not affect the
action of secretin compared with rats receiving no treatment (data not
shown).
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DISCUSSION |
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The major findings reported here relate to the molecular mechanisms of water transport in the liver. Our results using an established in vivo rodent model for studying ductal bile secretion indicate that 1) the ductal choleresis induced by secretin is associated with an increase in the amount of AQP1 protein in the apical plasma membrane of cholangiocytes and 2) the disruption of cholangiocyte microtubules by colchicine inhibits both the secretin-induced choleresis and the secretin-induced increase in AQP1 in the apical cholangiocyte membrane. These observations, confirmed in vitro using an isolated cholangiocyte experimental model, provide a plausible molecular explanation for secretin-induced ductal bile secretion.
Bile is initially secreted at the canalicular membrane of hepatocytes and then modified by cholangiocytes during passage through bile ducts. Although this ductal bile secretion results from the osmotically driven transport of water across biliary epithelia, the regulatory and mechanistic details of this process are unknown (9, 22, 23, 28). Cholangiocytes (unlike hepatocytes) express the water channel AQP1, which, in the unstimulated state, is present in both apical and basolateral plasma membrane domains and, based on accumulating data, seems to be the major pathway for the movement of water across these cells (29, 35). In a recent study using isolated cholangiocytes, we showed that secretin (which stimulates ductal bile secretion) induces the insertion of functional AQP1 water channels in cholangiocyte plasma membranes. This finding was the first reported evidence for hormone-regulated membrane insertion of AQP1 in any epithelial cell and led us to propose that this process played an important role in secretin-induced ductal bile secretion. In the present study, we have extended those observations using primarily an in vivo rat model. The biochemical and morphological results reported here indicate that secretin causes the insertion of AQP1 into the apical (but not the basolateral) domain of cholangiocytes and that this process likely accounts for the ability of secretin to stimulate ductal bile secretion.
Although secretin is well known to stimulate ductal bile secretion by
interacting with specific cAMP-coupled receptors on the basolateral
domain of cholangiocytes (9, 23, 28), little as well as controversial
information is available on the stimulus-secretion coupling. For
example, it has been proposed that secretin induces exocytosis of
vesicles containing bafilomycin
A1-inhibitable
H+-ATPase into the basolateral
domain of pig cholangiocytes. The H+ secretion would increase
intracellular pH and, in turn, the secretion of
HCO3 and water into bile.
Nevertheless, rat and human cholangiocytes lack bafilomycin
A1-sensitive
H+-ATPase activity (13, 30),
indicating that the mechanisms involved in ductal bile secretion are
not the same in all species. Based on recently published observations
(9, 22, 23, 28) and those reported here, it can be proposed that the
secretin-induced increase in cAMP triggers the exocytic insertion of
AQP1 and the activation of apical cystic fibrosis transmembrane
regulator Cl
channels. The
resulting efflux of Cl
would cause the exit of HCO
3 (via
activation of apical
Cl
/HCO
3
exchanger) and the paracellular transport of sodium ions. The vectorial
transport of these electrolytes would in turn drive the osmotic
transcellular movement of water via AQP1 into the biliary space (Fig.
6). As we discussed elsewhere (22), no
substantial fraction of the transepithelial water flow seems to be
paracellular. Importantly, the identification of a growing number of
new apical transport proteins (3, 20, 21) raises the possibility that
the movement of other osmotically active solutes are involved in
hormone-regulated ductal bile formation; for example, it has been
reported that the Na+-coupled
transport of bile salts may be regulated by secretin (2). Thus we would
propose that on secretin stimulation AQP1 is targeted toward and
inserted into the apical (secretory) domain of cholangiocytes to
optimize efficient coupling between biliary transport of solutes and
water.
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Cholangiocytes are cells with an unusually high cholesterol content in
their plasma membrane, i.e., cholesterol-to-phospholipid ratio 1 (31,
35). Although a physiological role for the high amount of cholesterol
in the cholangiocyte plasma membrane is unknown, it has been well
documented that cholesterol reduces membrane water permeability (16).
These facts suggest that for cholangiocytes to maintain osmotic
equilibrium under basal conditions they are required to express AQPs in
their plasma membranes.
Our data indicate that under basal (nonsecretin-stimulated) conditions, cholangiocytes possess a greater density of AQP1 in the apical than the basolateral domain. If AQP1 were the only water channel protein present in cholangiocytes, there would be an imbalance in the membrane water permeability of these two domains. This apparent disparity in the polarized distribution of AQP1 strongly suggests that other members of the AQP family of proteins are also expressed in the basolateral domain of cholangiocytes. In this regard, we recently found that cholangiocytes contain mRNA for aquaporin-3 and aquaporin-4 (Marinelli, Pham, and LaRusso, unpublished data), two water channels that have been described to be present exclusively in the basolateral domain of the epithelial cells in which they are expressed (22). Nevertheless, the fact that AQP1 increases in the apical but not in the basolateral domain of cholangiocytes on secretin stimulation suggests that the apical domain is the limiting barrier for net transepithelial water transport in cholangiocytes.
Our finding that secretin only modulates the apical insertion of AQP1 suggests that AQP1 is constitutively inserted into the basolateral domain and/or that signaling pathways other than the cAMP cascade are involved. Evidence for the cAMP-mediated stimulation of apical targeting of vesicles containing other transporters has been shown in a number of epithelia, including colonocytes, hepatocytes, and kidney tubule cells (8, 10, 11, 25, 34). In kidney collecting tubule cells, for example, it has been well documented that vasopressin (via cAMP) induces the exocytic insertion of aquaporin-2 (an AQP1- related water channel) exclusively into the apical domain of these cells (25, 34).
We found that the in vivo administration of the microtubule
depolymerizing drug colchicine inhibited the increase of AQP1 in
cholangiocyte plasma membrane induced by secretin. These observations are in agreement with those previously reported by us using an in vitro
model (i.e., isolated cholangiocytes), as well as with studies in
kidney proximal tubule cells indicating that microtubules are required
for insertion of AQP2 into the plasma membrane (18, 24). The finding
that colchicine also inhibited the secretin-induced bile flow is in
conflict with one previous report (15) describing the lack of
inhibitory action of colchicine in a different model for the study of
ductal bile secretion (i.e., -naphthyl-isothiocyanate-fed rats). The
reason for this discrepancy is unclear, although differences in the
experimental models as well as the doses of colchicine may be involved.
On the other hand, our data do agree with those of Cho and Boyer (12)
who provided direct evidence for the colchicine-induced inhibition of
secretin-stimulated ductal bile flow using isolated rat bile duct units.
We believed that the inability of secretin to increase AQP1 in
cholangiocyte plasma membranes in microtubule-disrupted rats prevented
the ductular choleresis. A possible colchicine-induced inhibition in
the biliary transport of osmotically active solutes, as described for
HCO3 in pig (33), may significantly contribute to the failure of secretin to stimulate bile flow. Nevertheless, the associated increase in bile osmolality is making this
possibility uncertain.
In conclusion, the data presented in this study, together with our previous observations (24), support the concept that on secretin stimulation vesicles containing AQP1 are directed (via microtubules) to the secretory pole (i.e., apical membrane domain) of cholangiocytes to facilitate the osmotic movement of water and in turn, the elaboration of ductal bile. These results provide the first cohesive and comprehensive molecular explanation for hormone-induced ductal bile secretion (Fig. 6).
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ACKNOWLEDGEMENTS |
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We thank D. Lubinski for secretarial help.
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FOOTNOTES |
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This work was supported in part by National Institutes of Health Grants DK-24031, HL-33991, and HL-48268.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests: N. F. LaRusso, Center for Basic Research in Digestive Diseases, Mayo Clinic, 200 First St. SW, Rochester, MN 55905.
Received 25 February 1998; accepted in final form 1 October 1998.
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REFERENCES |
---|
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---|
1.
Agre, P.,
G. M. Preston,
B. L. Smith,
J. S. Jung,
S. Raina,
C. Moon,
W. B. Guggino,
and
S. Nielsen.
Aquaporin CHIP: the archetypal molecular water channel.
Am. J. Physiol.
265 (Renal Fluid Electrolyte Physiol. 34):
F463-F476,
1993
2.
Alpini, G.,
S. Glaser,
J. Phinizy,
A. Caligiuri,
W. Robertson,
J. Lasater,
R. Rodgers,
Z. Tretjak,
and
G. LeSage.
Secretin-dependent insertion of the Na+-dependent bile acid transporter on the cholangiocyte apical membrane regulates ductal reabsorption of bile acids and ductal bile secretion in bile duct ligated (BDL) rats (Abstract).
Hepatology
26:
261A,
1997.
3.
Alpini, G.,
S. S. Glaser,
R. Rodgers,
J. L. Phinizy,
W. E. Robertson,
J. Lasater,
A. Caligiuri,
Z. Tretjak,
and
G. D. LeSage.
Functional expression of the apical Na+-dependent bile acid transporter in large but not small rat cholangiocytes.
Gastroenterology
113:
1734-1740,
1997[Medline].
4.
Alpini, G.,
J. O. Phillips,
and
N. F. LaRusso.
The biology of biliary epithelia.
In: The Liver: The Biology and Pathobiology, edited by I. M. Arias,
J. L. Boyer,
N. Fausto,
W. B. Jakoby,
D. A. Schachter,
and D. A. Shafritz. New York: Raven, 1994, p. 623-653.
5.
Alpini, G.,
S. Roberts,
S. M. Kuntz,
Y. Ueno,
S. Gubba,
P. V. Podila,
G. LeSage,
and
N. F. LaRusso.
Morphological, molecular, and functional heterogeneity of cholangiocytes from normal rat liver.
Gastroenterology
110:
1636-1643,
1996[Medline].
6.
Alpini, G.,
C. Ulrich,
S. Roberts,
J. O. Phillips,
Y. Ueno,
P. V. Podila,
O. Colegio,
G. D. LeSage,
L. J. Miller,
and
N. F. LaRusso.
Molecular and functional heterogeneity of cholangiocytes from rat liver after bile duct ligation.
Am. J. Physiol.
272 (Gastrointest. Liver Physiol. 35):
G289-G297,
1997
7.
Alpini, G.,
C. D. Ulrich,
L. D. Pham,
J. O. Phillips,
L. J. Miller,
and
N. F. LaRusso.
Upregulation of secretin receptor gene expression in rat cholangiocytes after bile duct ligation.
Am. J. Physiol.
266 (Gastrointest. Liver Physiol. 29):
G922-G928,
1994
8.
Benedetti, A.,
M. Strazzabosco,
O. C. Ng,
and
J. L. Boyer.
Regulation of activity and apical targeting of the Cl/HCO
3 exchanger in rat hepatocytes.
Proc. Natl. Acad. Sci. USA
91:
792-796,
1994[Abstract].
9.
Boyer, J. L.
Bile duct epithelium: frontiers in transport physiology.
Am. J. Physiol.
270 (Gastrointest. Liver Physiol. 33):
G1-G5,
1996
10.
Boyer, J. L.,
and
C. J. Soroka.
Vesicle targeting to the apical domain regulates bile excretory function in isolated rat hepatocyte couplets.
Gastroenterology
109:
1600-1611,
1995[Medline].
11.
Bradbury, N. A.,
and
R. J. Bridges.
Role of membrane trafficking in plasma membrane solute transport.
Am. J. Physiol.
267 (Cell Physiol. 36):
C1-C24,
1994
12.
Cho, W. K.,
and
J. L. Boyer.
Microtubule dependence of VIP-stimulated fluid secretion from cholangiocytes: implications for membrane recycling (Abstract).
Hepatology
24:
147A,
1996.
13.
Cho, W. K.,
A. Mennone,
and
J. L. Boyer.
Forskolin stimulates secretion in isolated polarized bile ductular units by mechanisms primarily lacking a H+-ATPase responseevidence for heterogeneity in the biliary epithelium (Abstract).
Hepatology
8:
296A,
1993.
14.
Coffey, R. J. J.,
L. J. Kost,
R. M. Lyons,
H. L. Moses,
and
N. F. LaRusso.
Hepatic processing of transforming growth factor- in the rat. Uptake, metabolism, and biliary excretion.
J. Clin. Invest.
80:
750-757,
1987[Medline].
15.
Dallenbach, A.,
and
E. L. Renner.
Colchicine does not inhibit secretin-induced choleresis in rats exhibiting hyperplasia of bile ductules: evidence against a pivotal role of exocytic vesicle insertion.
J. Hepatol.
22:
338-348,
1995[Medline].
16.
De Gier, J.
Osmotic properties of liposomes.
In: Water Transport in Biological Membranes. From Model Membranes to Isolated Cells, edited by G. Benga. Boca Raton, FL: CRC, 1989, p. 77-98.
17.
Denker, B. M.,
B. L. Smith,
F. P. Kuhajda,
and
P. Agre.
Identification, purification, and partial characterization of a novel Mr 28,000 integral membrane protein from erythrocytes and renal tubules.
J. Biol. Chem.
263:
15634-15642,
1988
18.
Elkjaer, M.,
H. Birn,
P. Agre,
E. I. Christensen,
and
S. Nielsen.
Effects of microtubule disruption on endocytosis, membrane recycling, and polarized distribution of aquaporin-1 and gp330 in proximal tubule cells.
Eur. J. Cell Biol.
67:
57-72,
1995[Medline].
19.
Ishii, M.,
B. T. Vroman,
and
N. F. LaRusso.
Isolation and morphologic characterization of bile duct epithelial cells from normal rat liver.
Gastroenterology
97:
1236-1247,
1989[Medline].
20.
Lazaridis, K.,
L. Pham,
B. Vroman,
P. C. de Groen,
and
N. F. LaRusso.
Kinetic and molecular identification of sodium-dependent glucose transporter in normal rat cholangiocytes.
Am. J. Physiol.
272 (Gastrointest. Liver Physiol. 35):
G1168-G1174,
1997
21.
Lazaridis, K. N.,
L. Pham,
P. Tietz,
R. A. Marinelli,
P. C. de Groen,
S. Levine,
P. A. Dawson,
and
N. F. LaRusso.
Rat cholangiocytes absorb bile acids at their apical domain via the ileal sodium-dependent bile acid transporter.
J. Clin. Invest.
100:
2714-2721,
1997
22.
Marinelli, R. A.,
and
N. F. LaRusso.
Aquaporin water channels in liver: their significance in bile formation.
Hepatology
26:
1081-1084,
1997[Medline].
23.
Marinelli, R. A.,
and
N. F. LaRusso.
Solute and water transport pathways in cholangiocytes.
In: Seminars in Liver Disease. New York: Thieme, 1996, p. 221-229.
24.
Marinelli, R. A.,
L. Pham,
P. Agre,
and
N. F. LaRusso.
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.
J. Biol. Chem.
272:
12984-12988,
1997
25.
Nielsen, S.,
C. L. Chou,
D. Marples,
E. I. Christensen,
B. K. Kishore,
and
M. A. Knepper.
Vasopressin increases water permeability of kidney collecting duct by inducing translocation of aquaporin-CD water channels to plasma membrane.
Proc. Natl. Acad. Sci. USA
92:
1013-1017,
1995[Abstract].
26.
Nielsen, S.,
B. L. Smith,
E. I. Christensen,
and
P. Agre.
Distribution of aquaporin CHIP in secretory and resorptive epithelia and capillary endothelia.
Proc. Natl. Acad. Sci. USA
90:
7275-7279,
1993[Abstract].
27.
Orlowski, M.,
and
A. Meister.
-Glutamyl-p-nitroanilide: a new convenient substrate for determination and study of L- and D-
-glutamyltranspeptidase activities.
Biochim. Biophys. Acta
73:
679-681,
1963.
28.
Raeder, M. G.
Mechanisms of fluid and electrolyte transport by the biliary epithelium and their contribution to bile formation.
Curr. Opin. Gastroenterol.
11:
439-444,
1995.
29.
Roberts, S. K.,
M. Yano,
Y. Ueno,
L. Pham,
G. Alpini,
P. Agre,
and
N. F. LaRusso.
Cholangiocytes express the aquaporin CHIP and transport water via a channel-mediated mechanism.
Proc. Natl. Acad. Sci. USA
91:
13009-13013,
1994
30.
Strazzabosco, M.,
R. Joplin,
A. Szembery,
A. Granato,
C. Spirli,
and
G. Crepaldi.
Characterization of acid/base transport systems in human intrahepatic bile duct cells (Abstract).
Gastroenterology
106:
A991,
1994.
31.
Tietz, P.,
L. Pham,
and
N. F. LaRusso.
Isolation of cholangiocyte plasma membrane vesicles enriched in apical or basolateral plasma membrane domains.
Biochemistry
34:
15436-15443,
1995[Medline].
32.
Udenfriend, S.,
S. Stein,
P. Bohlen,
W. Dairman,
W. Leimgruber,
and
M. Weigele.
Fluorescamine: a reagent for assay of amino acids, peptides, proteins, and primary amines in the picomole range.
Science
178:
871-872,
1972[Medline].
33.
Veel, T.,
T. Buanes,
T. Grotmol,
E. Engeland,
and
M. G. Raeder.
Colchicine blocks the effects of secretin on bile duct cell tubulovesicles and plasma membrane geometry and impairs ductular HCO3 secretion in the pig.
Acta Physiol. Scand.
139:
603-607,
1990[Medline].
34.
Yamamoto, T.,
S. Sasaki,
and
K. Fushimi.
Vasopressin increases AQP-CD water channel in apical membrane of collecting duct cells in Brattleboro rats.
Am. J. Physiol.
268 (Cell Physiol. 37):
C1546-C1551,
1995
35.
Yano, M.,
R. A. Marinelli,
S. K. Roberts,
V. Balan,
L. Pham,
J. E. Tarara,
P. C. de Groen,
and
N. F. LaRusso.
Rat hepatocytes transport water mainly via a non-channel mediated pathway.
J. Biol. Chem.
271:
6702-6707,
1996