Ca2+ causes active contraction of bile canaliculi: direct evidence from microinjection studies

Bile canaliculi contract autonomously by releasing calcium into hepatocytes via mechanosensitive calcium channel

Drug-induced hepatocellular cholestasis leads to altered bile flow. Bile is propelled along the bile canaliculi (BC) by actomyosin contractility, triggered by increased intracellular calcium (Ca²⁺). However, the source of increased intracellular Ca²⁺ and its relationship to transporter activity remains elusive. We identify the source of the intracellular Ca²⁺ involved in triggering BC contractions, and we elucidate how biliary pressure regulates Ca²⁺ homeostasis and associated BC contractions. Primary rat hepatocytes were cultured in collagen sandwich. Intra-canalicular Ca²⁺ was measured with fluo-8; and intra-cellular Ca²⁺ was measured with GCaMP. Pharmacological modulators of canonical Ca²⁺-channels were used to study the Ca²⁺-mediated regulation of BC contraction. BC contraction correlates with cyclic transfer of Ca²⁺ from BC to adjacent hepatocytes, and not with endoplasmic reticulum Ca²⁺. A mechanosensitive Ca²⁺ channel (MCC), Piezo-1, is preferentially localized at BC membranes. The Piezo-1 inhibitor GsMTx-4 blocks the Ca²⁺ transfer, resulting in cholestatic generation of BC-derived vesicles whereas Piezo-1 hyper-activation by Yoda1 increases the frequency of Ca²⁺ transfer and BC contraction cycles. Yoda1 can recover normal BC contractility in drug-induced hepatocellular cholestasis, supporting that Piezo-1 regulates BC contraction cycles. Finally, we show that hyper-activating Piezo-1 can be exploited to normalize bile flow in drug-induced hepatocellular cholestasis.

Exposure to nitrate alters the histopathology and gene expression in the liver of Bufo gargarizans tadpoles

Nitrate is a ubiquitous component in aquatic environment and the concentrations of anthropogenic nitrate-nitrogen (NO₃N) can exceed 25 mg/L in surface waters and 100 mg/L in ground waters. The exceed nitrate has adverse effects on survival, development, and metamorphosis of amphibian. Liver is the hub of many biological processes, including lipid metabolism and bile salts secretion. However, there is little information about the effects of nitrate on the liver in amphibians during metamorphosis. In this study, B. gargarizans was exposed to different concentrations of nitrate from embryo to metamorphosis climax to investigate the effects of nitrate on the liver. The survival rate, metamorphosis percent, body mass, total length, and hind-limb length were measured. The histopathological changes and transcriptome responses in the liver of B. gargarizans to nitrate were examined. Results indicated exposure to 50 and 100 mg/L NO₃N delayed the metamorphosis and decreased the metamorphosis percent of B. gargarizans. The body size of B. gargarizans at 10 and 50 mg/L NO₃N groups were decreased while it was increased at 100 mg/L NO₃N group. In addition, exposure to 100 mg/L NO₃N caused severe histopathological changes, including cellular atrophy, increased intercellular areas, degraded lipid droplets, hepatic fibrosis, bile canaliculus contraction and degraded mitochondria in liver. The results of RNA-seq and qRT-PCR interpreted the molecular responses, which might be the factors to induce histopathological changes in the liver of B. gargarizans under the pressure of nitrate exposure.

A Predictive 3D Multi-Scale Model of Biliary Fluid Dynamics in the Liver Lobule

Bile, the central metabolic product of the liver, is transported by the bile canaliculi network. The impairment of bile flow in cholestatic liver diseases has urged a demand for insights into its regulation. Here, we developed a predictive 3D multi-scale model that simulates fluid dynamic properties successively from the subcellular to the tissue level. The model integrates the structure of the bile canalicular network in the mouse liver lobule, as determined by high-resolution confocal and serial block-face scanning electron microscopy, with measurements of bile transport by intravital microscopy. The combined experiment-theory approach revealed spatial heterogeneities of biliary geometry and hepatocyte transport activity. Based on this, our model predicts gradients of bile velocity and pressure in the liver lobule. Validation of the model predictions by pharmacological inhibition of Rho kinase demonstrated a requirement of canaliculi contractility for bile flow in vivo. Our model can be applied to functionally characterize liver diseases and quantitatively estimate biliary transport upon drug-induced liver injury.

Correlation between the liver temperature employed during machine perfusion and reperfusion damage: role of Ca2+

This study compares the effects of machine perfusion (MP) at different temperatures with simple cold storage. In addition, the role of Ca(2+) levels in the MP medium was evaluated. For MP, rat livers were perfused for 6 hours with Krebs-Henseleit (KH) solution (with 1.25 or 2.5 mM CaCl(2)) at 4 degrees C, 10 degrees C, 20 degrees C, 25 degrees C, 30 degrees C, or 37 degrees C. For cold storage, livers were perfused in situ and preserved with Celsior solution at 4 degrees C for 6 hours. The reperfusion period (2 hours at 37 degrees C) was performed under the same conditions used for MP-preserved and cold storage-preserved livers. Hepatic enzyme release, bile production, adenosine triphosphate (ATP) levels, and morphology were evaluated during MP and reperfusion. MP at 37 degrees C caused marked enzyme release; the same findings were obtained during reperfusion. By contrast, MP temperature lowering induced a significant decrease in liver damage. High levels of biliary gamma-glutamyltransferase and lactate dehydrogenase were found with MP at 4 degrees C and 10 degrees C but not with MP at 20 degrees C. When a KH-1.25 mM CaCl(2) solution was used during MP at 20 degrees C, very low enzyme release was observed and significantly lower hepatic damage was present at the end of the reperfusion period in comparison with cold storage. The same results were obtained when ruthenium red, a calcium uniporter blocker, was added to KH-2.5 mM CaCl(2). ATP levels were higher and morphology was better in liver preserved with KH-1.25 mM CaCl(2). MP at 20 degrees C with KH-1.25 mM CaCl(2) resulted in better quality liver preservation, improving hepatocyte and endothelial biliary cell survival, in comparison with cold storage. This raises the need to reconsider the temperature and calcium levels to be used during liver MP.

Store-operated Ca(2+) channels and Stromal Interaction Molecule 1 (STIM1) are targets for the actions of bile acids on liver cells

Cholestasis is a significant contributor to liver pathology and can lead to primary sclerosis and liver failure. Cholestatic bile acids induce apoptosis and necrosis in hepatocytes but these effects can be partially alleviated by the pharmacological application of choleretic bile acids. These actions of bile acids on hepatocytes require changes in the release of Ca(2+) from intracellular stores and in Ca(2+) entry. However, the nature of the Ca(2+) entry pathway affected is not known. We show here using whole cell patch clamp experiments with H4-IIE liver cells that taurodeoxycholic acid (TDCA) and other choleretic bile acids reversibly activate an inwardly-rectifying current with characteristics similar to those of store-operated Ca(2+) channels (SOCs), while lithocholic acid (LCA) and other cholestatic bile acids inhibit SOCs. The activation of Ca(2+) entry was observed upon direct addition of the bile acid to the incubation medium, whereas the inhibition of SOCs required a 12 h pre-incubation. In cells loaded with fura-2, choleretic bile acids activated a Gd(3+)-inhibitable Ca(2+) entry, while cholestatic bile acids inhibited the release of Ca(2+) from intracellular stores and Ca(2+) entry induced by 2,5-di-(tert-butyl)-1,4-benzohydro-quinone (DBHQ). TDCA and LCA each caused a reversible redistribution of stromal interaction molecule 1 (STIM1, the endoplasmic reticulum Ca(2+) sensor required for the activation of Ca(2+) release-activated Ca(2+) channels and some other SOCs) to puncta, similar to that induced by thapsigargin. Knockdown of Stim1 using siRNA caused substantial inhibition of Ca(2+)-entry activated by choleretic bile acids. It is concluded that choleretic and cholestatic bile acids activate and inhibit, respectively, the previously well-characterised Ca(2+)-selective hepatocyte SOCs through mechanisms which involve the bile acid-induced redistribution of STIM1.

Clinical and pathological features of a prolonged type of acute intrahepatic cholestasis

AIM

We examined the clinical and pathological features of drug-induced acute intrahepatic cholestasis (AIC) to elucidate the pathogenesis of prolonged cases.

METHODS

Twenty-six cases of drug-induced AIC were divided into prolonged and non-prolonged groups. Serum bilirubin levels and other biochemical data were compared between the two groups. Biopsy liver specimens were examined by light and electron microscopy. The localization of multidrug resistance protein 2 (MRP2) was immunohistochemically assessed by the Envision technique.

RESULTS

The causative drugs of four prolonged cases were found to be tiopronin, chlorpromazine and diclofenac. Two of the patients either died or underwent liver transplantation. The maximal total bilirubin levels (35.2 +/-> 13.8 mg/dL) were significantly higher and a half-life of total bilirubin (78.8 +/-> 69.6 days) was markedly longer in the prolonged cases, in comparison to the non-prolonged cases (16.8 +/-> 8.1 mg/dL, 22.1 +/-> 12.7 days, respectively). The liverbiopsy specimens revealed canalicular cholestasis and a slight degree of lobular inflammation. In the prolonged cases, liver cell injury and cholestasis was marked, and the interlobular bile ducts disappeared in the portal triads. The reaction products of MRP2, recognized on the bile canaliculi in a control liver, were weakened and found in the pericanalicular vesicles in AIC.

CONCLUSION

These results indicated disturbances in the canalicular bilirubin transport through MRP2 in the prolonged cases, resulting from severe cholestasis, liver cell injury and vanishing bile ducts. The histological findings of the liver at the acute icteric phase may be important to understand the pathogenesis and to predict the prognosis in AIC.

Taurolithocholate impairs bile canalicular motility and canalicular bile secretion in isolated rat hepatocyte couplets

AIM

To investigate the effects of taurolithocholate (TLC) on the canalicular motility in isolated rat hepatocyte couplets (IRHC).

METHODS

TLC was added to IRHC at concentrations of 10 and 50 mumol/L, respectively. In each group, five time-lapse movies containing 3 representative bile canaliculi were taken under phase-contrast microscopy for 12 h. The number of bile canalicular contractions and the intervals between consecutive canalicular contractions were calculated. Furthermore, the effects of TLC on IRHC were examined by transmission electron microscopy.

RESULTS

The bile canalicular contractions were spontaneous and forceful in the controls. Active vesicular movement was observed in the pericanalicular region. Immediately after the addition of TLC, the bile canaliculi were deformed, and canalicular bile was incorporated into the vacuoles. The canaliculi were gradually dilated, and canalicular contractions were markedly inhibited by TLC. The vesicular movements became extremely slow in the pericanalicular region. The number of canalicular contractions significantly decreased in the TLC-treated groups, as compared with that in the controls. The time intervals were prolonged, as the TLC dosage increased, indicating that bile secretion into the canaliculi was impaired with TLC. Transmission electron microscopy revealed the lamellar transformation of the canalicular membranes in IRHC treated with TLC.

CONCLUSION

TLC impairs both the bile canalicular contractions and the canalicular bile secretion, possibly by acting directly on the canalicular membranes in TLC-induced cholestasis.

Coordinated Movement of Bile Canalicular Networks Reconstructed by Rat Small Hepatocytes

Hepatocytes in vivo have a potential for liver regeneration, but it has been very difficult to reconstruct hepatic organoids in vitro. Recent studies have shown that small hepatocytes (SHs) can reconstruct hepatic organoids including functional bile canaliculi (BC). In the present study we analyzed the movement of BC formed in the hepatic organoids, focusing on the coordination of contraction and dilation among cells and the mechanism producing the coordination. Hepatic cells, including SHs, were isolated from an adult rat liver and cultured. Time-lapse images of BC movements were taken and analyzed in cells treated with or without cytochalasin B (CB). Time-lapse images revealed that all BC, regardless of region contracted in a coordinated manner. Actin filaments were observed along the BC even after the BC networks treated with CB dilated markedly. Microinjection of dye was also carried out to investigate the flow thorough BC. Secreted fluorescein from the injected cell flowed along BC, and gap junctional protein connexin 32 was expressed along BC networks, suggesting cell-to-cell communication. Thus, groups of hepatocytes in the hepatic organoids act in a coordinated manner through intercellular communication.

Induction of cholestasis in the perfused rat liver by 2-aminoethyl diphenylborate, an inhibitor of the hepatocyte plasma membrane Ca2+ channels

BACKGROUND AND AIMS

An increase in the cytoplasmic free Ca2+ concentration in hepatocytes as a result of the release of Ca2+ from intracellular stores and Ca2+ inflow from the extracellular space is a necessary part of the mechanism by which bile acids are moved along the bile cannaliculus by contraction of the cannaliculus. 2-Aminoethyl diphenylborate (2-APB) is a recently discovered inhibitor of store-operated plasma membrane Ca2+ channels in hepatocytes. The aim of the present study was to test the ability of 2-APB to inhibit bile flow.

METHODS

Bile flow was measured in the isolated perfused rat liver using cannulation of the common bile duct. Measurements were carried out in the presence or absence of 2-APB in either the presence of taurocholic acid (to enhance basal bile flow) or in the absence of taurocholic acid and in the presence of the hormones vasopressin and glucagon, which are known to stimulate bile flow.

RESULTS

In livers perfused in the presence of taurocholic acid, 2-APB reversibly inhibited bile flow with a slow time of onset. The time of onset of inhibition was reduced by prior addition of the endoplasmic reticulum (Ca(2+) + Mg2+)adenosine triphosphatase inhibitor, 2,5-di-t-butylhydroquinone. In livers perfused in the absence of taurocholate, 2-APB had little effect on the basal rate of bile flow, but inhibited the ability of vasopressin and glucagon to stimulate bile flow.

CONCLUSIONS

It is concluded that an inhibitor of hepatocyte plasma membrane Ca2+ channels can induce cholestasis. The results provide evidence that suggests that, over a period of time, the normal function of hepatocyte store-operated Ca2+ channels is required to maintain bile flow. Future strategies directed at the regulation of bile flow might include pharmacological or other interventions that modulate Ca2+ inflow to hepatocytes.

Cellular mechanisms of direct-current electric field effects: galvanotaxis and metastatic disease

Endogenous direct-current electric fields (dcEFs) occur in vivo in the form of epithelial transcellular potentials or neuronal field potentials, and a variety of cells respond to dcEFs in vitro by directional movement. This is termed galvanotaxis. The passive influx of Ca2+ on the anodal side should increase the local intracellular Ca2+ concentration, whereas passive efflux and/or intracellular redistribution decrease the local intracellular Ca2+ concentration on the cathodal side. These changes could give rise to `push-pull' effects, causing net movement of cells towards the cathode. However, such effects would be complicated in cells that possess voltage-gated Ca2+ channels and/or intracellular Ca2+ stores. Moreover, voltage-gated Na+ channels, protein kinases, growth factors, surface charge and electrophoresis of proteins have been found to be involved in galvanotaxis. Galvanotactic mechanisms might operate in both the short term (seconds to minutes) and the long term (minutes to hours), and recent work has shown that they might be involved in metastatic disease. The galvanotactic responses of strongly metastatic prostate and breast cancer cells are much more prominent, and the cells move in the opposite direction compared with corresponding weakly metastatic cells. This could have important implications for the metastatic process and has clinical implications. Galvanotaxis could thus play a significant role in both cellular physiology and pathophysiology.

Bile canalicular formation in hepatic organoid reconstructed by rat small hepatocytes and nonparenchymal cells

The morphogenesis and movement of bile canaliculi (BC) are not well understood. This is because culture of hepatocytes that maintain polarity of cell membranes and possess highly differentiated functions has never been successful. We found that small hepatocytes (SHs), which are known to be hepatic progenitor cells, could proliferate and differentiate into mature hepatocytes and that BC-like structures developed between rising/piled-up cells. We investigated how BC-like structures developed with maturation of SHs and whether the structures were functionally active as BC. Hepatic cells, including SHs, were isolated from an adult rat liver and cultured. Immunocytochemistry and immunoblotting for BC proteins, such as ectoATPase, 5'-nucleotidase, dipeptidylpeptidase IV, and multidrug-resistance associated protein 2, were examined and time-lapse microscopy was used for the observation of BC contractions. Secretion of bilirubin into the reconstructed BC was also observed. The results of immunocytochemistry, immunoblots, and immunoelectron micrographs revealed that BC proteins were localized in the intercellular space that coincided with BC-like structures reconstructed between rising/piled-up cells. Tight junction-associated protein ZO-1 was also expressed along the BC-like structures. Bilirubin added to the medium were secreted into BC-like structure and accumulated without leakage. Time-lapse microscopy showed continuous contractions of reconstructed BC. In conclusion, BC-like structures reconstructed by SHs may be functional with membrane polarity, secretory ability, and motility. These results show that this culture system may suitable for investigating the mechanism of the formation of BC and their functions.

Intracellular calcium in the isolated rat liver: correlation to glucose release, K(+) balance and bile flow

This study correlates whole organ measurements of intracellular calcium concentration ([Ca(2+)](i)) with hormone-induced (epinephrine, vasopressin) changes of liver functions (glucose release, K(+) balance and bile flow). [Ca(2+)](i) was measured in the isolated perfused rat liver using the sensor Fura-2 and applying liver surface fluorescence spectroscopy. The technique was improved by (i) minimizing biliary elimination of the sensor by employing a rat strain deficient in canalicular organic anion transport (TR(-) mutation) and (ii) by correcting for changes of interfering intrinsic organ fluorescence that was shown to depend on the oxidation-reduction state (NAD(P)H content) of the organ. Epinephrine (50 nM) elicits an instantaneous peak rise of [Ca(2+)](i) to approx. 400 nM, followed by a sustained elevation that depends on the presence of extracellular Ca(2+). The rise of [Ca(2+)](i) coincides with initiation of glucose release, transient K(+) uptake, and transient stimulation of bile flow. Vasopressin (2 nM) exerts qualitatively similar effects. The transient rise of bile flow is attributed to Ca(2+)-mediated contraction of the pericanalicular actin-myosin web of hepatocytes.

A novel hepatic stellate (Ito) cell-derived protein, epimorphin, plays a key role in the late stages of liver regeneration

Limited data exist regarding morphogenesis and differentiation during liver regeneration. We examined the role of epimorphin on liver regeneration. After 70% partial hepatectomy, mouse liver was collected on days 1, 3, 7, and 14 for immunohistochemistry and the detection of epimorphin mRNA and connexin 32. Using primary cultured rat hepatocytes, morphogenesis and differentiation of cells were tested with or without epimorphin. Seven days after cell inoculation, the expression of connexin 32 and the cell-cell communication was tested as a marker of differentiation. Epimorphin was detected exclusively in hepatic stellate cells. Connexin 32 was detected only in hepatocytes. After partial hepatectomy, epimorphin mRNA was detected on day 3 and peaked at day 7, followed by protein expression. Connexin 32 expression showed a similar time course. Cultured hepatocytes formed multicellular spheroids in an active epimorphin-coated culture dish and showed positive dye coupling, whereas the cell-cell communication was lost without active epimorphin. Because epimorphin was expressed late in liver regeneration, it might play a role in morphogenesis and differentiation.

Carbon monoxide as a regulator of bile canalicular contractility in cultured rat hepatocytes

This study aimed to examine the mechanism(s) by which carbon monoxide (CO), a product of heme oxygenase reaction, controls the contractility of bile canaliculus (BC) in hepatocytes. When BCs associated with the couplet cells in cultured rat hepatocyte suspension were observed using time-lapse video microscopy, they exhibited periodical contractions with a most-probable interval of 6 minutes under our experimental conditions. The addition of 1 micromol/L zinc protoporphyrin IX (ZnPP), a potent inhibitor of heme oxygenase, to the culture medium elicited a 40% shortening of the interval time together with an increase in intracellular calcium concentrations, while the same concentration of iron protoporphyrin IX did not induce such changes. The production of CO, which was 0.5 nmol/h/10(8) cells in the absence of ZnPP, diminished to less than 0.1 nmol/h/10(8) cells upon application of ZnPP. The ZnPP-elicited increases in both contractile frequency and intracellular calcium concentrations were attenuated by the addition of 1 micromol/L CO or 50 micromol/L 1,2-bis(2-aminophenoxy) ethane-tetraacetate, a calcium chelator. Clotrimazole or metyrapone, inhibitors of cytochrome P450-dependent monooxygenase activities, also attenuated the ZnPP-induced elevation of the contractile frequency. On the other hand, intracellular cyclic guanosine monophosphate (cGMP) contents were not altered significantly by the application of ZnPP or by CO. These results indicate that CO generated by heme oxygenase controls the BC function by changing intracellular calcium concentrations presumably through a mechanism involving the cytochrome P450 reaction.

Differential Ca2+ signaling in neonatal and adult rat hepatocyte doublets

BACKGROUND/AIMS

Intracellular Ca2+ ([Ca2+]i) is important in various cellular functions, including cellular proliferation and differentiation. To elucidate the relationship between [Ca2+]i oscillations and physiological hepatocyte proliferation, phenylephrine-evoked [Ca2+]i responses were sequentially investigated using short-term cultured hepatocyte doublets obtained from 1-, 3-, 6- and 8-week-old rats.

METHODS/RESULTS

DNA synthesis in hepatocytes, determined by BrdU incorporation, was approximately 20% in 1-week-old rats, and decreased to <1% as the rats aged. Correspondingly, [Ca2+]i responses evoked by 10 micromol/l phenylephrine in hepatocyte doublets shifted from transient to sinusoidal-type [Ca2+]i oscillations and then to a sustained increase in [Ca2+]i, followed by a gradual return to baseline. The incidence of [Ca2+]i oscillations was 100+/-0.0%, 83.3+/-16.7%, 38.7+/-0.6% and 5.5+/-5.0% in 1-, 3-, 6- and 8-week-old rats, respectively. Removal of extracellular Ca2+ did not abolish [Ca2+]i oscillations, indicating that [Ca2+]i oscillations were caused primarily by Ca2+ mobilization from internal sites of the cells. The [Ca2+]i level in each of the adjacent cells was synchronous in sustained increase in [Ca2+]i, but asynchronous in [Ca2+]i oscillations. In proliferating doublets obtained from 1-week-old rats, the frequency of oscillations increased in a dose-dependent manner for phenylephrine concentrations of 1 to 100 micromol/l.

CONCLUSIONS

Phenylephrine-evoked [Ca2+]i oscillations were directly related to hepatocyte proliferation and were mediated by frequency modulation. These results suggest that phenylephrine-evoked [Ca2+]i oscillations may contribute to cell-cycle progression of hepatocytes in physiological liver growth.

Introduction to the biliary tract, the gallbladder, and gallstones

This paper serves to introduce a topical section of fifteen invited original research contributions dealing with normal and pathological development of the human biliary tract. This section also includes comparative anatomy of the gallbladder and the cystic duct as well as, the formation of gallstone. This series of reports have used advanced microscopic and ancillary techniques to study adaptative changes in gallbladder epithelial cell changes regarding permeability, renewal, mucous secretion as well as cholesterol uptake and nucleation. Several contributions deal with the blood and lymphatic drainage of the gallbladder. The gallbladder contractility is clarified by recent findings about its innervation, elegantly demonstrated and supported by complementary immunohistochemical and neurophysiological techniques. In vivo models for production of cholelithiasis in the ground squirrel and the Syrian hamster are introduced. Recent in vitro cellular and molecular models have substantially increased the understanding of biliary tract calculi formation. Finally, a survey and new data about progesterone gene regulation of both cholesterol metabolism and gallstone formation obtained in the Syrian hamster model are compared with cholelithogenesis in human.

Dimethylsulfoxide maintains intercellular communication by preserving the gap junctional protein connexin32 in primary cultured hepatocyte doublets from rats

Intercellular communication via gap junctions is one of the differentiated functions of cells. Dimethylsulfoxide (DMSO) is known to induce cell differentiation and maintain differentiated cellular functions in primary hepatocyte culture, but the mechanism of action of DMSO is unknown. Therefore, we investigated the effect of DMSO on cell-cell communication via gap junctions of hepatocyte doublets, which are differentiated cells that lose differentiated functions with time in culture. In isolated rat hepatocyte doublets, we assessed the effects of 1, 2 and 3% DMSO in culture medium on morphological changes and dye-coupling activity between pairs of cells by microinjection with fluorescent dye (Lucifer Yellow CH). The distribution of gap junction protein connexin32 (Cx32) was assessed by indirect immunofluorescence analysis and the Cx32 mRNA was detected by the reverse transcription-polymerase chain reaction method. Dimethylsulfoxide delayed the morphological change of hepatocyte doublets from a spherical to a flattened shape. Dye-coupling efficiency significantly decreased with time in culture in the control group, whereas in groups treated with 2 and 3% DMSO, dye-coupling efficiency was retained after 6 and 9 h of inoculation (P < 0.05 and P < 0.01, respectively). Analysis by indirect immunofluorescence showed few fluorescent spots for Cx32 in the control group at 9 h of incubation, whereas many punctate fluorescent spots were seen in the 3% DMSO group at 9 h of incubation. The detection of Cx32 mRNA in the 3% DMSO group was also stronger than in controls. Dimethylsulfoxide significantly maintained intercellular communication via gap junctions in primary cultured rat hepatocytes through the preservation of functional Cx32 protein, thus maintaining cell differentiation.

Disturbed structural interactions between microfilaments and tight junctions in rat hepatocytes during extrahepatic cholestasis induced by common bile duct ligation

Microfilaments in epithelial cells are important for the structural and functional integrity of tight junctions. In the present study, we examined the relationship between microfilaments and tight junctions in hepatocytes of rat liver following common bile duct ligation (CBDL) for up to 2 weeks. Actin filaments and tight junctions were studied by fluorescence microscopy using 7-nitrobenzene-2-oxa-1,3-diazole phallacidin (NBD-ph) and an anti-ZO-1 antibody, respectively. Double-stained sections were examined with confocal laser scanning microscopy (CLSM). Electron microscopy was applied for the assessment of structural alterations in microfilaments and in tight junctions with detergent-extraction and freeze-fracture preparations. Our results showed that F-actin was present at the entire plasma membrane of hepatocytes in control liver, whereas CBDL increased the amount of F-actin mainly at the bile canalicular and lateral plasma membranes. Simultaneously, the immunofluorescence of ZO-1 underwent striking changes, i.e., from a uniform to an irregular staining pattern with various fluorescence intensities. CLSM demonstrated a colocalization of ZO-1 and F-actin in control liver and its deterioration in CBDL liver. Electron microscopy showed marked alterations of microfilaments and tight junctions due to CBDL. It is concluded that actin filaments are intimately associated with tight junctions in normal hepatocytes. CBDL impairs this association by progressively diminishing the structural interaction between F-actin and ZO-1, which may in turn lead to functional disturbances of tight junctions.

Isolated rat hepatocytes can signal to other hepatocytes and bile duct cells by release of nucleotides

Intercellular communication among certain cell types can occur via ATP secretion, which leads to stimulation of nucleotide receptors on target cells. In epithelial cells, however, intercellular communication is thought to occur instead via gap junctions. Here we examined whether one epithelial cell type, hepatocytes, can also communicate via nucleotide secretion. The effects on cytosolic Ca2+ ([Ca2+]i) of mechanical stimulation, including microinjection, were examined in isolated rat hepatocytes and in isolated bile duct units using confocal fluorescence video microscopy. Mechanical stimulation of a single hepatocyte evoked an increase in [Ca2+]i in the stimulated cell plus an unexpected [Ca2+]i rise in neighboring noncontacting hepatocytes. Perifusion with ATP before mechanical stimulation suppressed the [Ca2+]i increase, but pretreatment with phenylephrine did not. The P2 receptor antagonist suramin inhibited these intercellular [Ca2+]i signals. The ATP/ADPase apyrase reversibly inhibited the [Ca2+]i rise induced by mechanical stimulation, and did not block vasopressin-induced [Ca2+]i signals. Mechanical stimulation of hepatocytes also induced a [Ca2+]i increase in cocultured isolated bile duct units, and this [Ca2+]i increase was inhibited by apyrase as well. Finally, this form of [Ca2+]i signaling could be elicited in the presence of propidium iodide without nuclear labeling by that dye, indicating that this phenomenon does not depend on disruption of the stimulated cell. Thus, mechanical stimulation of isolated hepatocytes, including by microinjection, can evoke [Ca2+]i signals in the stimulated cell as well as in neighboring noncontacting hepatocytes and bile duct epithelia. This signaling is mediated by release of ATP or other nucleotides into the extracellular space. This is an important technical consideration given the widespread use of microinjection techniques for examining mechanisms of signal transduction. Moreover, the evidence provided suggests a novel paracrine signaling pathway for epithelia, which previously were thought to communicate exclusively via gap junctions.

Effects of wortmannin, a novel myosin light-chain kinase inhibitor, on bile canalicular contraction in vitro and in vivo

BACKGROUND

The cytoskeletal system is believed to play an important role in normal bile formation. The effects of wortmannin, a new myosin light-chain kinase inhibitor, on bile canalicular contraction and bile flow have been observed.

METHODS

The bile canalicular contraction of cultured hepatocyte doublets was investigated, using an image analyzer with a phase contrast microscope, and the intracellular Ca2+ concentration was measured, using microscopic fluorometry. We also investigated bile flow by in vivo intraportal infusion of the drug in rats.

RESULTS

Treatment with wortmannin inhibited norepinephrine-induced canalicular contraction and caused a decrease in bile flow without changing systematic and portal blood pressure. Morphologic examination of the electron microscopic study showed that most bile canaliculi were dilated, with loss of microvilli, but no other apparent damage was seen in parenchymal hepatocytes.

CONCLUSIONS

These data suggest that the integrity of the phosphorylation system of myosin is essential for normal bile flow.

Monoclonal antibodies to rabbit hepatocyte myosin that cross-react with human liver myosin

We prepared polyclonal and monoclonal antibodies against myosin purified from rabbit hepatocytes. Immunoblotting analyses revealed that the polyclonal antibody and four (HM1, HM2, HM3 and HM4) of five monoclonal antibodies reacted with myosin heavy chain. Chymotryptic cleavage of myosin yielded a 130 kDa fragment comprising the tail portion of the myosin heavy chain and a 67 kDa fragment comprising the ATP-binding active site of the myosin head. All active antibodies reacted with epitopes localized in the 130 kDa fragment. Monoclonal antibodies HM3 and HM4 and the polyclonal antibody reacted strongly with myosin heavy chains from a human liver homogenate prepared from a surgically resected liver specimen. Immunocytochemical analyses showed that myosin is localized along the plasma membrane as well as around the bile canaliculi in both rabbit and human hepatocytes. Immunocytochemical analyses on liver blocks obtained from those patients who suffered various types of diseases accompanying cholestasis clearly indicated a marked increase in pericanalicular myosin. This altered myosin localization is analogous to that observed in phalloidin-treated liver. Thus, myosin localization, determined using these antibodies, can provide a valid morphological basis for diagnosing the pathological state of the patient liver.

Structural and functional features of bile canaliculi in adult rat hepatocyte spheroids

Spheroids of adult rat hepatocytes are spherical cell aggregates which retain three-dimensional architecture and hepatocyte specific functions. In this study, we investigated the detailed structure and function of bile canaliculi in spheroids. Hepatocytes were prepared from adult rat liver and cultured with epidermal growth factor (50 ng/ml). Hepatocytes formed floating spheroids 4 days after inoculation. The morphology of hepatocyte spheroids was investigated after fluorescent staining for actin using confocal laser scanning microscopy and electron microscopy. To study the function of bile canaliculi, the transcellular transport of fluorescein diacetate was observed. These experiments were performed in a control group and in a group treated with the actin inhibitor cytochalasin B. In a control group, spheroids contained bile canalicular structures which were surrounded by actin filaments. Added fluorescent dye was secreted and pooled in bile canaliculi. Cytochalasin B caused marked distention of bile canaliculi and prominent accumulation of secreted fluorescent dye in dilated bile canaliculi. This phenomenon was based on the impairment of contractile movement of bile canaliculi. These results demonstrate that hepatocyte spheroids maintain functional and morphological peculiarity, and therefore this model may be useful in investigation of the mechanism of bile formation and intrahepatic cholestasis.

Isolation of the bile canalicular actin-myosin II motor

Cytoskeleton-rich canalicular membranes (CCMs) with preserved cytoskeleton and demembranated CCMs, consisting only of cytoskeletal elements, were used to examine the relationship of pericanalicular microfilaments, myosin II phosphorylation, and canalicular contraction. The components of CCMs were visualized by fluorescence microscopy using the filamentous actin probe rhodamine-phalloidin and by electron microscopy, before and after incubation in 1 microM Ca2+/1 mM ATP (contraction solution). Canalicular contraction (luminal closure) was evaluated by morphometric analysis. Myosin II was extracted from CCMs, purified by immunoprecipitation, and analyzed on Western blots. In sequential experiments, autoradiographs of gels from [gamma-32P]-ATP-treated CCMs in the presence or absence of Ca2+ were examined after 0.25, 0.50, 1, 2, 3, 5, and 10 min, and the effects of W7 (a calmodulin antagonist) and ML9 (a myosin light chain kinase inhibitor) were evaluated. The results showed that phosphorylation of the 20-kDa protein was low in controls but enhanced beginning 0.25-0.50 min after addition of contraction solution. Both W7 and ML9 significantly inhibited this reaction and inhibited canalicular contraction. The results indicate that phosphorylation of the regulatory 20-kDa myosin light chain of canaliculus-associated myosin II coincides with or precedes contraction of the canaliculus. We conclude that the canalicular contractile apparatus is composed of actin filaments and a myosin II motor.

Stability and optimization of canalicular function in hepatocyte couplets

An enriched preparation of rat hepatocyte couplets was obtained by collagenase perfusion and subsequent elutriation (> 85 per cent couplets and triplets; viability of over 95 per cent). Canalicular secretory activity (the ability to accumulate cholyl-lysyl-fluorescein, CLF) was first apparent after 2 h of culture at 37 degrees C and was present in over 80 per cent of the total population after 5-6 h. This remained almost constant for at least 4 h in both elutriated and directly plated cells. Initial storage of freshly prepared couplets at 4 degrees C for up to 6 h prior to incubation had no adverse effect upon secretory function. Reduction of canalicular secretory activity occurred at a concentration of the hepatotoxic agent menadione (IC50 17 microM) that was lower than that required to induce mild plasma-membrane blebbing (IC50 43 microM). This study has optimized and characterized the canalicular secretory effectiveness and stability of an enriched preparation of hepatocyte couplets, and established the feasibility of studies of toxic agents on hepatobiliary function in a heterogeneous population of hepatocytes. In this preparation other biochemical parameters can be assessed, thus complementing previous techniques using individual couplets.

Periportal- and perivenous-enriched hepatocyte couplets: differences in canalicular activity and in response to oxidative stress

Unlike isolated single hepatocytes, hepatocyte couplets retain their apical polarity, and, during short-term culture form an enclosed canalicular space or vacuole between the two adjacent cells into which biliary secretion is initiated. Hepatocyte couplets were prepared after partial collagenase perfusion of rat liver. Centrifugal elutriation was used to fractionate the preparation into six couplet-containing suspensions. Image analysis was used to determine the size of cultured couplets. The size of the couplets ranged from 34.1 +/- 0.76 microns and 684 +/- 24.1 microns 2 (mean length and area respectively +/- S.E.M.) in Fraction 2, to 43.7 +/- 0.57 microns and 1033 +/- 33.8 microns 2 length and area respectively in Fraction 7. Glutamine synthetase activity was assessed in each freshly eluted fraction and was shown to be predominant in Fractions 6 and 7. Pretreatment of rats with CCl4, which selectively destroys perivenous hepatocytes, decreased the proportion of couplets in these fractions by over 67%, and their glutamine synthetase activity by over 97%. It was concluded that Fractions 2 and 3 contained predominantly couplets of Zone 1 (periportal) origin, Fractions 4 and 5 those from Zone 2, and Fractions 6 and 7 predominantly couplets of Zone 3 (perivenous) origin. The development of canalicular secretory activity was assessed in the couplets after a 15 min incubation with a fluorescent bile acid, cholyl-lysyl-fluorescein (CLF). This was sigmoidal in all fractions, but slower in the periportal couplets, taking 5.1 h for 50% to show secretory activity in Fraction 2, compared with 2.7 h for Fraction 7. Incubation of hepatocyte couplets with 1 or 10 microM taurodehydrocholate, a non-toxic bile acid analogue, did not influence the rate of development of accumulation of CLF by the couplets or the area of the canalicular vacuole in any fraction. However, it did decrease the CLF content of couplets incubated with CLF for 15 min to a greater extent in those of perivenous origin. After subjecting the couplets to oxidative stress by incubation with 20 microM menadione (2-methyl-1,4-naphthoquinone), it was evident that periportal couplets were less able to maintain canalicular secretory activity than perivenous couplets.

Motility of bile canaliculi in the living animal: implications for bile flow

Modern fluorescence microscopic techniques were used to image the bile canalicular system in the intact rat liver, in vivo. By combining the use of sodium fluorescein secretion into bile, with digitally enhanced fluorescence microscopy and time-lapse video, it was possible to capture and record the canalicular motility events that accompany the secretion of bile in life. Active bile canalicular contractions were found predominantly in zone 1 (periportal) hepatocytes of the liver. The contractile movements were repetitive, forceful, and appeared unidirectional moving bile in a direction towards the portal bile ducts. Contractions were not seen in the network of canaliculi on the surface of the liver. Cytochalasin B administration resulted in reduced canalicular motility, progressive dilation of zone 1 canaliculi, and impairment of bile flow. Canalicular dilations invariably involved the branch points of the canalicular network. The findings add substantively to previous in vitro studies using couplets, and suggest that canalicular contractions contribute physiologically to bile flow in the liver.

Myosin in hepatocytes is essential for bile canalicular contraction

Active dynamic contraction of bile canaliculi has been observed in cultured doublet hepatocytes using time-lapse cinephotomicrography. This contractile movement plays an important role in normal bile formation. The mechanism of bile canalicular contraction has been proved to involve the Ca(2+)-calmodulin system and pericanalicular actin filaments. However, the role of myosin in this system is still unknown. In this study, using the newly synthesized myosin light-chain kinase inhibitor ML-9, we found that the treatment of cultured doublet hepatocytes with ML-9 inhibited canalicular contraction. This inhibitory effect suggests that myosin is involved in this complex cellular function and that the integrity of the actin-myosin system, as well as the Ca(2+)-calmodulin system is essential for normal bile canalicular contraction.

Phalloidin-induced accumulation of myosin in rat hepatocytes is caused by suppression of autolysosome formation

Administration of phalloidin in vivo to rats causes marked changes in the distribution of actin and myosin in hepatocytes, which accompanies reduced bile flow. We have found that in hepatocytes treated with phalloidin for 3 and 7 days, cellular myosin content increased about 1.5-fold and 4.7-fold, respectively. In addition, total cell protein content and several marker enzyme activities were also elevated by 30-120% depending on the duration of phalloidin treatment. These observations allow us to speculate that phalloidin somehow elicits inhibition of cellular protein degradation, which results in the increase of these protein levels. To examine this possibility further, we analyzed leupeptin-induced density shift of phagolysosomes. In normal liver, the injection of leupeptin/E64c caused an increase in the density of both heterolysosomes and autolysosomes, due to retarded digestion of sequestered proteins as a result of the inhibition of lysosomal cathepsins. Accumulation, in these denser autolysosomes, of lactic dehydrogenase, pyruvate kinase, aldolase, and myosin was demonstrated by enzyme assays and immunoblot analysis. In the phalloidin-treated liver, the increase in the density of autolysosomes and the accumulation of above cytoplasmic enzymes were markedly inhibited. However, phalloidin did not affect the shift in the density of heterolysosomes. From these data, we concluded that autolysosome formation was specifically hindered in phalloidin-treated rat hepatocytes, which results in the reduction of autophagic protein degradation and eventual increase in intracellular protein levels.

Release of Ca2+ from the endoplasmic reticulum is not the mechanism for bile acid-induced cholestasis and hepatotoxicity in the intact rat liver

The hypothesis that monohydroxy bile acids exert their cholestatic and hepatotoxic effects via a sustained elevation of cytosolic [Ca2+] was tested in the isolated perfused rat liver. Infusion of the specific inhibitor of microsomal Ca2+ sequestration, 2,5-di(tert-butyl)-1,4-benzohydroquinone (tBuBHQ) (25 microM for 10 min) produced efflux of Ca2+ from the liver and a sustained (20 min) increase in cytosolic [Ca2+] as indicated by the threefold increase in hepatic glucose output. Release of the endoplasmic reticular Ca2+ pool was demonstrated by the complete abolition of vasopressin- and phenylephrine-induced Ca2+ exchange between the liver and perfusate. Despite the profound perturbation of intracellular Ca2+ homeostasis produced by tBuBHQ, there was no decrease in bile flow and no evidence of hepatocellular injury (for 60 min), as indicated by lactate dehydrogenase release. In contrast, lithocholic acid (25 microM for 10 or 30 min) or taurolithocholic acid (5 microM for 10 or 30 min) produced an 80-90% inhibition of bile flow and a progressive increase in perfusate lactate dehydrogenase activity. During and after bile acid infusion, there was no change in Ca2+ fluxes between liver and perfusate, no stimulation of glucose output from the liver, and hormone-stimulated Ca2+ responses were preserved. It is concluded that the mechanisms for bile acid-induced cholestasis and hepatotoxicity in the intact liver are not attributable to changes in intracellular Ca2+ homeostasis, and especially not to prolonged release or depletion of Ca2+ sequestered in the endoplasmic reticulum.

Cellular mechanisms of intrahepatic cholestasis

Most forms of intrahepatic cholestasis are caused by a failure of hepatocytes to secrete osmotically active bile constituents into the minute channels of bile canaliculi. This overall vectorial bile secretory process is dependent upon a variety of polarised active transport functions at the basolateral (sinusoidal and lateral) and canalicular plasma membrane domains, as well as upon the coordinated vectorial movement of intracellular vesicles. Although considerable progress has been made in recent years in the identification, characterisation and exact localisation of a number of polarised hepatocellular transport systems, the primary mechanisms and targets leading to defective bile secretion and cholestasis are still not completely understood. For example, not all reported experimental data are compatible with the concept that estrogen-induced cholestasis represents a predominant sinusoidal disease process. In addition, the pathophysiological significance of disturbed transcytotic pathways and/or disrupted intracellular calcium homeostasis are not yet clear. For many forms of cholestasis, it remains uncertain as to whether leaky tight junctions represent a primary cause rather than a secondary phenomenon of the cholestatic state. However, the ongoing progress in the understanding of the normal mechanisms involved in the establishment, maintenance and regulation of ion homeostasis and polar transport functions in hepatocytes will, undoubtedly, improve our knowledge of the pathogenesis of intrahepatic cholestasis and, it is hoped, lead to better therapeutic strategies in the near future.

Current concepts of biliary secretion

Biliary secretion is reviewed. Bile acids pass along the biliary tract and small intestine without undergoing passive absorption because of their hydrophilicity and size. Active ileal absorption leads to the development of a large circulating pool of molecules and thus dissociates biliary secretion from bile acid biosynthesis (which is synonymous with cholesterol degradation). Man differs from most vertebrates in having little bile acid-independent flow; bile acid-dependent flow is also less in man than many other vertebrates. The hypercholeretic effects of certain bile acids are reviewed; the most likely explanation is cholehepatic shunting of the unconjugated, lipophilic species. Biliary lipid secretion involves bile acid-stimulated microtubule-dependent movement of phospholipid-cholesterol-rich vesicles from the Golgi to the canaliculus. Bile acid biotransformation during hepatic transport involves reconjugation (with glycine or taurine) of C24 bile acids (deconjugated during enterohepatic cycling), conjugation with glucuronate of lipophilic C23-nor bile acids, reduction of oxo groups, and epimerization of iso-(3 beta-hydroxy) bile acids. Glucose and amino acids enter bile from plasma as secondary solutes and are absorbed efficiently in the biliary ductular system. The biliary system is almost freely permeable to plasma Ca2+; in bile, Ca2+ is bound to bile acid monomers and micelles. Alteration of biliary lipid secretion by orally administered bile acids is a major first step in the medical treatment of calculous biliary disease.

Hepatocyte gap junctions are permeable to the second messenger, inositol 1,4,5-trisphosphate, and to calcium ions

Hepatocytes are well coupled by gap junctions, which allow the diffusion of small molecules between cells. Although gap junctions in many tissues are permeable to molecules larger than cAMP and in several preparations gap junctions pass cAMP itself, little direct evidence supports permeation by other second-messenger species. Ca2+, perhaps the smallest second messenger, would be expected to cross gap junctions, but the issue is complicated because gap-junction channels are closed when intracellular free Ca2+ concentration, [Ca2+]i, is elevated to micromolar levels or above. Inositol 1,4,5-trisphosphate (InsP3), a second messenger that can evoke Ca2+ release, might also reduce junctional permeability by this mechanism. We report here evidence for transjunctional flux of Ca2+ and InsP3 in freshly isolated pairs or small clusters of rat hepatocytes. The Ca2+ indicator fura-2 was used to monitor transjunctional diffusion of Ca2+ directly or to detect passage of InsP3 by localized Ca2+ release. Fura-2 injected as the free acid passed between cells. Injection of InsP3 or CaCl2 immediately increased [Ca2+]i in the injected cell (peak values less than 1 microM), and [Ca2+]i increased rapidly in contacting cells (within seconds). The initial rise in [Ca2+]i induced by InsP3 was greater at discrete regions in the cytoplasm of both injected and uninjected cells and was inconsistent with simple diffusion of Ca2+. In the coupled cells the regions of greatest increase were not necessarily near the contact zone. In contrast, the rise induced in [Ca2+]i by CaCl2 injection when cells were bathed in normal Ca2+ was always more diffuse than with InsP3 injection, and in cells coupled to a cell injected with CaCl2 the earliest and maximal increases occurred at the region of cell contact. This difference in distribution indicates that injected InsP3 (or an active metabolite, but not Ca2+) diffused between cells to cause localized release of Ca2+ from intracellular stores. Ca2+ injection induced a rise in [Ca2+]i in coupled cells even when cells were maintained in Ca2+-free saline, suggesting that changes in [Ca2+]i seen in adjacent cells were due to transjunctional diffusion from the injected cell and not to uptake from the extracellular solution. However, in Ca2+-free saline, [Ca2+]i distribution was nonuniform, indicating that Ca2+-releasing mechanisms contribute to the observed changes. No increase in [Ca2+]i was seen in adjacent cells when Ca2+ was injected after treatment with the uncoupling agent octanol (500 microM), which itself did not change [Ca2+]i. These data provide evidence that the second messengers Ca2+ and InsP3 can be transmitted from cell to cell through gap junctions, a process that may have an important role in tissue function.

Bile canalicular contraction in the isolated hepatocyte doublet is related to an increase in cytosolic free calcium ion concentration

Dynamic contractions of bile canaliculi have been observed in cultured doublet hepatocytes by means of time-lapse cinephotomicrography, and this contractile movement plays an important role in bile secretion. Although details of the mechanism are still unknown, the Ca2+-calmodulin system is believed to play a main role in this mechanism. In this study we measured the intracellular Ca2+ concentration of individual doublet hepatocytes using the Ca2+ indicator "fura 2" and microscopic fluorometry. We also observed the effects of A23187, norepinephrine and epinephrine on bile canalicular contraction and intracellular Ca2+ concentration. After loading 1 mumol/l fura 2 in doublet cells, we added A23187, epinephrine or norepinephrine and then measured the Ca2+ concentration in a given small area in the cytoplasm of individual doublet cell. A23187, norepinephrine and epinephrine caused a prompt increase of the intracellular Ca2+ concentration and also caused bile canalicular contraction. The present study indicates that the sudden increase of intracellular Ca2+ concentration causes bile canalicular contraction through the Ca2+-calmodulin system.

Electrical properties of the gap junctional membrane studied in rat liver cell pairs

Cell pairs isolated from adult rat liver were used to study the electrical properties of gap junctions. Each cell of a cell pair was connected to a suction pipette so as to enable whole cell tight-seal recording. A double voltage-clamp approach was adopted to control the voltage gradient across the gap junction and measure the transjunctional current. The current-voltage relationship of the gap junctional membrane was linear over the voltage range tested (+/- 50 m V). Under control conditions, the resistance of the gap junction, rj, was 15 M omega (n = 27; range, 4.6 to 45.8 M omega), corresponding to a conductance gj of 67 nS. rj was insensitive to the nonjunctional membrane potential, Vm (voltage range,-90 m V to + 40 m V). There was no indication of a time-dependent gating of rj (time range, 20 ms to 10 s). Dialysis with 1 mM CaCl2 produced irreversible electrical uncoupling without affecting the linearity of the relationship Vj/Ij.

Pacemaker region in a rhythmically contracting embryonic epithelium, the enveloping layer of Oryzias latipes, a teleost

The primary objectives of this study were to determine the embryonic stage at which the Oryzias latipes enveloping layer (EVL) begins to contract rhythmically, and to determine where these contractions arise within the EVL. Using time-lapse video recording, we showed that the contractions begin at stage 14 (the stage of the embryonic shield) and arise in the ventral region of the EVL, which is centered at 180 degrees longitude from the embryonic shield. We have called this the pacemaker region for the contractions. Using fluorescein diacetate as a vital stain, we showed that the ventral region of the EVL continues to act as a pacemaker even after the EVL is detached from the rest of the egg. Rhythmic contractile activity ceased when we removed a group of about 130 cells--10% of the total EVL--from the pacemaker region; comparably large wounds elsewhere had no effect on the contractions. When we cut detached EVLs into ten pieces, only 2.4 +/- 1.8 (mean +/- SD, N = 11) of them contracted rhythmically, even though a considerably larger proportion of the EVL cells participate in the contractions in undisturbed blastoderms. We conclude that the pacemaker cells are necessary for rhythmic contractile activity and that cells outside this region do not contract spontaneously. The contractile waves are propagated at a velocity of 14-54 microns sec-1. This value, which is two to three orders of magnitude slower than the propagation of epithelial action potentials, is similar to the rate of propagation of waves of increased cytosolic Ca2+ in other systems. We propose that the medaka EVL is a good system in which to study certain aspects of epithelial morphogenesis.

Effects of calmodulin antagonists on secretion of bile and bile acid

In the isolated perfused rat liver, the effects of calmodulin (CaM) antagonists, chlorpromazine (CPZ), trifluoperazine (TFP), W-7 and W-5, on secretion of bile and bile acid were compared. Without addition of taurocholic acid to the perfusate, TFP (200 microM or higher), CPZ (200 microM) and W-7 (400 microM) decreased the bile flow transiently. In contrast, W-5 did not decrease the bile flow. Taking into consideration the binding of TFP and CPZ to bovine serum albumin in the perfusate, they diminished the bile flow by inhibiting CaM function. This was also supported by the difference between the effects of W-7 and W-5. These findings suggested that CaM was involved in the secretion of bile. Under the constant infusion of taurocholic acid into the perfusate, CaM antagonists decreased the secretion of bile acid. However this might be due to the inhibition of bile acid uptake, because these agents inhibited the uptake into isolated rat hepatocytes. The concentrations required for inhibition of the uptake were near to those which decreased the viability, suggesting that the inhibition was due to their non-specific cytotoxic effect. Future studies must be carried out to determine whether CaM is involved in the secretion of bile acid.

Electrophysiological properties of gap junctions between dissociated pairs of rat hepatocytes

Physiological properties of isolated pairs of rat hepatocytes were examined within 5 h after dissociation. These cells become round when separated, but cell pairs still display membrane specializations. Most notably, canaliculi are often present at appositional membranes which are flanked by abundant gap and tight junctions. These cell pairs are strongly dye-coupled; Lucifer Yellow CH injected into one cell rapidly diffuses to the other. Pairs of hepatocytes are closely coupled electrically. Conductance of the junctional membrane is not voltage sensitive: voltage clamp studies demonstrate that gj is constant in response to long (5 s) transjunctional voltage steps of either polarity (to greater than +/- 40 mV from rest). Junctional conductance (gj) between hepatocyte pairs is reduced by exposure to octanol (0.1 mM) and by intracellular acidification. Normal intracellular pH (pHi), measured with a liquid ion exchange microelectrode, was generally 7.1-7.4, and superfusion with saline equilibrated with 100% CO2 reduced pHi to 6.0-6.5. In the pHi range 7.5-6.6, gj was constant. Below pH 6.6, gj steeply decreased and at 6.1 coupling was undetectable. pHi recovered when cells were rinsed with normal saline; in most cases gj recovered in parallel so that gj values were similar for pHs obtained during acidification or recovery. The low apparent pK and very steep pHi-gj relation of the liver gap junction contrast with higher pKs and more gradually rising curves in other tissues. If H+ ions act directly on the junctional molecules, the channels that are presumably homologous in different tissues must differ with respect to reactive sites or their environment.