Decreased uptake of taurocholate and ouabain by hepatocytes isolated from cirrhotic rat liver

The microenvironment in hepatocyte regeneration and function in rats with advanced cirrhosis

In advanced cirrhosis, impaired function is caused by intrinsic damage to the native liver cells and from the abnormal microenvironment in which the cells reside. The extent to which each plays a role in liver failure and regeneration is unknown. To examine this issue, hepatocytes from cirrhotic and age-matched control rats were isolated, characterized, and transplanted into the livers of noncirrhotic hosts whose livers permit extensive repopulation with donor cells. Primary hepatocytes derived from livers with advanced cirrhosis and compensated function maintained metabolic activity and the ability to secrete liver-specific proteins, whereas hepatocytes derived from cirrhotic livers with decompensated function failed to maintain metabolic or secretory activity. Telomere studies and transcriptomic analysis of hepatocytes recovered from progressively worsening cirrhotic livers suggest that hepatocytes from irreversibly failing livers show signs of replicative senescence and express genes that simultaneously drive both proliferation and apoptosis, with a later effect on metabolism, all under the control of a central cluster of regulatory genes, including nuclear factor κB and hepatocyte nuclear factor 4α. Cells from cirrhotic and control livers engrafted equally well, but those from animals with cirrhosis and failing livers showed little initial evidence of proliferative capacity or function. Both, however, recovered more than 2 months after transplantation, indicating that either mature hepatocytes or a subpopulation of adult stem cells are capable of full recovery in severe cirrhosis.

CONCLUSION

Transplantation studies indicate that the state of the host microenvironment is critical to the regenerative potential of hepatocytes, and that a change in the extracellular matrix can lead to regeneration and restoration of function by cells derived from livers with end-stage organ failure.

Fractal and Fourier analysis of the hepatic sinusoidal network in normal and cirrhotic rat liver

The organization of the hepatic microvascular network has been widely studied in recent years, especially with regard to cirrhosis. This research has enabled us to recognize the distinctive vascular patterns in the cirrhotic liver, compared with the normal liver, which may explain the cause of liver dysfunction and failure. The aim of this study was to compare normal and cirrhotic rat livers by means of a quantitative mathematical approach based on fractal and Fourier analyses performed on photomicrographs and therefore on discriminant analysis. Vascular corrosion casts of livers belonging to the following three experimental groups were studied by scanning electron microscopy: normal rats, CCl(4)-induced cirrhotic rats and cirrhotic rats after ligation of the bile duct. Photomicrographs were taken at a standard magnification; these images were used for the mathematical analysis. Our experimental design found that use of these different analyses reaches an efficiency of over 94%. Our analyses demonstrated a higher complexity of the normal hepatic sinusoidal network in comparison with the cirrhotic network. In particular, the morphological changes were more marked in the animals with bile duct-ligation cirrhosis compared with animals with CCl(4)-induced cirrhosis. The present findings based on fractal and Fourier analysis could increase our understanding of the pathophysiological alterations of the liver, and may have a diagnostic value in future clinical research.

Enterohepatic circulation: physiological, pharmacokinetic and clinical implications

Enterohepatic recycling occurs by biliary excretion and intestinal reabsorption of a solute, sometimes with hepatic conjugation and intestinal deconjugation. Cycling is often associated with multiple peaks and a longer apparent half-life in a plasma concentration-time profile. Factors affecting biliary excretion include drug characteristics (chemical structure, polarity and molecular size), transport across sinusoidal plasma membrane and canniculae membranes, biotransformation and possible reabsorption from intrahepatic bile ductules. Intestinal reabsorption to complete the enterohepatic cycle may depend on hydrolysis of a drug conjugate by gut bacteria. Bioavailability is also affected by the extent of intestinal absorption, gut-wall P-glycoprotein efflux and gut-wall metabolism. Recently, there has been a considerable increase in our understanding of the role of transporters, of gene expression of intestinal and hepatic enzymes, and of hepatic zonation. Drugs, disease and genetics may result in induced or inhibited activity of transporters and metabolising enzymes. Reduced expression of one transporter, for example hepatic canalicular multidrug resistance-associated protein (MRP) 2, is often associated with enhanced expression of others, for example the usually quiescent basolateral efflux MRP3, to limit hepatic toxicity. In addition, physiologically relevant pharmacokinetic models, which describe enterohepatic recirculation in terms of its determinants (such as sporadic gall bladder emptying), have been developed. In general, enterohepatic recirculation may prolong the pharmacological effect of certain drugs and drug metabolites. Of particular importance is the potential amplifying effect of enterohepatic variability in defining differences in the bioavailability, apparent volume of distribution and clearance of a given compound. Genetic abnormalities, disease states, orally administered adsorbents and certain coadministered drugs all affect enterohepatic recycling.

Altered transmembrane ionic flux in hepatocytes isolated from cirrhotic rats

BACKGROUND/AIMS

Although cirrhosis is known to be associated with many hepatocyte abnormalities, there is no well-established model to study the cellular drug uptake process independent of hemodynamic effects. The purpose of the present study was to test the following hypothesis: hepatocytes isolated from cirrhotic animals may be used as a model to study the cellular abnormalities associated with cirrhosis. Our hypothesis was tested by comparing the membrane potential (PD) of hepatocytes in anesthetized healthy and cirrhotic animals, and the PD and [3H]palmitic acid clearance rate of hepatocytes isolated from healthy and cirrhotic animals.

METHODS

Mild to moderate cirrhosis was induced in female Sprague-Dawley rats by CCl4 administration. PD was recorded in anesthetized animals using intracellular microelectrodes. Hepatocytes from those livers were subsequently isolated by collagenase perfusion for further determinations of PD and [3H]palmitic acid uptake.

RESULTS

The mean (+/-SEM) hepatocyte PD from intact rat livers was 38+/-1 mV (control) and -32+/-1 mV (cirrhosis; n=6/group, p<0.01). The PD (mean+/-SEM) in isolated hepatocytes was -21+/-1 mV (control) and -15+/-1 mV (cirrhosis, n=13/group, p<0.01). The clearance rate of [3H]palmitic acid was lower in hepatocytes isolated from cirrhotic animals (26%) than in those isolated from healthy control animals (p<0.01).

CONCLUSION

The results of this study indicate that hepatocytes isolated from cirrhotic animals may be used to study the cellular abnormalities associated with cirrhosis.

Acute carbon tetrachloride feeding selectively damages large, but not small, cholangiocytes from normal rat liver

The aim of this study was to develop a model of selective duct damage restricted to hormone-responsive segments corresponding to the ducts damaged in primary biliary cirrhosis (PBC). Carbon tetrachloride (CCl4) was fed by gavage to rats, and 2, 7, 14, and 28 days later, small and large cholangiocytes were isolated. Apoptosis was determined in situ by morphology and in purified cholangiocytes by assessment of nuclear fragmentation by 4, 6-diamidino-2-phenylindole (DAPI) staining. Cholangiocyte proliferation was evaluated in situ by morphometry of liver sections stained for cytokeratin-19 (CK-19) and by proliferating cellular nuclear antigen (PCNA) staining in liver sections and in purified cholangiocytes by PCNA gene expression. Ductal secretion was assessed by measurement of secretin receptor (SR) gene expression and secretin-induced cyclic adenosine 3',5'-monophosphate (cAMP) synthesis and secretin-induced choleresis. Two days after CCl4 administration, there was an increased number of small ducts, but a reduction of large ducts. Apoptosis, observed only in large ducts, was associated with decreased DNA synthesis and ductal secretion. Conversely, small cholangiocytes expressed de novo the SR gene and secretin-stimulated cAMP synthesis 2 days after CCl4 treatment. Proliferation of large cholangiocytes was delayed until 7 days, which was associated with a transient increase in ductal secretion in vivo. CCl4 effects on cholangiocytes were reversed by day 28. CCl4 treatment causes a decrease in large duct mass as a result of a higher rate of apoptosis and absence of initial proliferation in large cholangiocytes. These processes were concomitant with a decrease of ductal secretion in large cholangiocytes. Small cholangiocytes appear resistant to CCl4-induced apoptosis, and proliferate and transiently compensate for loss of proliferative and secretory activity of large cholangiocytes.

Assessment of the hepatic circulation in humans: new concepts based on evidence derived from a D-sorbitol clearance method

D-Sorbitol (SOR) is safe, is easy to measure, and has an exceptionally high extraction ratio in the normal liver of 0.93+/-0.05 (mean+/-SD). Together with the general interest in hepatic hemodynamics, these facts motivated us to review the usefulness of this compound for the assessment of liver plasma flow in humans. We concluded that in subjects without liver disease the nonrenal clearance of SOR-measured noninvasively-very closely approximates hepatic plasma flow. Because of its lower and more variable extraction ratio, indocyanine green should no longer be used without hepatic vein catheterization. Even in patients with cirrhosis, SOR exhibits higher hepatic extraction ratios than indocyanine green. To fully explore the potential of SOR in the evaluation of such patients attention needs to be paid to the complex changes in architecture and function occurring in this disease. In cirrhotics the noninvasively measured nonrenal clearance of SOR presumably approximates the flow through intact and capillarized sinusoids (functional flow) and reflects the amount of blood having functional contact with hepatocytes. The theoretic background of the method, its accuracy, further research needs, and potentials of various approaches are discussed in detail.

Ursodeoxycholate-induced hypercholeresis in cirrhotic rats: further evidence for cholehepatic shunting

The aim of the investigation was to explore whether ursodeoxycholate, a tertiary bile acid with potential for treatment of chronic cholestasis in cirrhotic liver disease, has the same physiological effects in cirrhotic as in normal rats. Furthermore, we wanted to investigate whether ductular proliferation, as it occurred in this situation, increases the bicarbonate stimulatory effect of ursodeoxycholate. Rats (n = 16) were rendered cirrhotic by continuous exposure to phenobarbital-carbon tetrachloride; untreated animals (n = 13) served as controls. In cirrhotic rats in vivo, ursodeoxycholate (20 mumoles/min/kg) stimulated bile salt secretion and bile flow less than in controls. Nevertheless, the increment in ursodeoxycholate-induced biliary bicarbonate--the bicarbonate stimulatory potency--was increased by 29% in cirrhotic animals (0.55 +/- 0.08 mmol vs. 0.71 +/- 0.11 mmol; p < 0.05). This finding could be related to ductular proliferation because the volume fraction of bile ductules, determined stereologically, increased from 0.3% +/- 0.1% to 2.7% +/- 0.6% in cirrhotic rats (p < 0.005). To explore further the behavior of ductules during ursodeoxycholate stimulation, we carried out experiments in the in situ perfused rat liver. In the portally perfused organ, replacement of bicarbonate by tricine-acetate abolished ursodeoxycholate-induced hypercholeresis. In the dually perfused organ (perfusion of both portal vein and hepatic artery) perfusion of the hepatic artery with bicarbonate-containing buffer, ursodeoxycholate had a similar stimulatory effect as in vivo in both control and cirrhotic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Liver function improvement following increased portal blood flow in cirrhotic rats

BACKGROUND/AIMS

Liver microcirculation in cirrhosis is characterized by development of intrahepatic shunts and capillarization of sinusoids secondary to cell necrosis and deposition of new collagen, resulting in both decreased drug elimination and increased vascular resistance with portal hypertension. The aim of this study was to examine the effects of increased portal blood flow on hepatic microcirculation and drug elimination in 13 perfused livers from cirrhotic rats.

METHODS

Intrahepatic resistance was assessed under basal conditions (21.2 +/- 0.3 mL/min) and 1 hour after doubling the flow (41.6 +/- 1.0 mL/min). A multiple indicator dilution technique was used at both flow rates to measure sinusoidal volume, albumin and sucrose extravascular volumes, and cellular water volume. Hepatic elimination of labeled taurocholate and propranolol was also measured, and the recovery of 15-microns microspheres was used to evaluate large intrahepatic shunts.

RESULTS

After doubling the flow, intrahepatic resistance decreased by 31%. Sinusoidal and extravascular volume increased significantly without a change in microsphere recovery. However, there was a marked increase in taurocholate and propranolol elimination by cirrhotic livers. Moreover, during high flow, significant correlations were found between changes in albumin extravascular volume and taurocholate and propranolol elimination.

CONCLUSIONS

Increased portal blood flow in cirrhotic rats induces a decrease in intrahepatic resistance without changes in intrahepatic shunting and improves drug elimination by the liver without deleterious effects on hepatocyte viability.

Hepatocellular Na+/H+ exchange is activated at transcriptional and posttranscriptional levels in rat biliary cirrhosis

BACKGROUND/AIMS

Rat hepatocyte Na+/H+ exchange is activated in vitro by growth factors and in vivo following partial hepatectomy. This study explored by which mechanism(s) it is activated in a cirrhosis model characterized by chronic stimulation of hepatocyte proliferation.

METHODS

Rat hepatocytes were isolated 4 weeks after bile duct ligation or sham operation. Intracellular pH (pHi) was fluorimetrically determined, and plasma membranes and messenger RNA (mRNA) were prepared from isolated hepatocytes by standard methods.

RESULTS

Resting pHi was higher in bile duct-ligated than in control rats (7.42 +/- 0.03 vs. 7.06 +/- 0.04; P < 0.001). Although plasma membrane lipid composition and intracellular buffering capacity were similar, initial Na+/H+ exchange-mediated rates of pHi recovery following acid loading were higher in bile duct-ligated than in control rats (0.098 +/- 0.011 vs. 0.055 +/- 0.005 pH units/min; P < 0.05). The antiporter's set point was shifted approximately 0.3 pH units towards more alkaline values and its steady-state mRNA levels were doubled after bile duct ligation.

CONCLUSIONS

Hepatocellular Na+/H+ exchange is transcriptionally and posttranscriptionally activated in rat biliary cirrhosis further supporting a relationship between hepatocyte proliferation and Na+/H+ exchange activation.

Alterations in mitochondrial function and morphology in chronic liver disease: pathogenesis and potential for therapeutic intervention

Studies assessing mitochondrial function and structure in livers from humans or experimental animals with chronic liver disease, including liver cirrhosis, revealed a variety of alterations in comparison with normal subjects or control animals. Depending on the etiology of chronic liver disease, the function of the electron transport chain and/or ATP synthesis was found to be impaired, leading to decreased oxidative metabolism of various substrates and to impaired recovery of the hepatic energy state after a metabolic insult. Changes in mitochondrial structure include megamitochondria with reduced cristae, dilatation of mitochondrial cristae and crystalloid inclusions in the mitochondrial matrix. The most important strategies to maintain an adequate mitochondrial function per liver are mitochondrial proliferation and increases in the activity of critical enzymes or in the content of cofactors per mitochondrion. Possibilities to assess hepatic mitochondrial function and to treat mitochondrial dysfunction in patients with chronic liver disease are discussed.

Structure-function relationship in secondary biliary cirrhosis in the rat. Stereologic and hemodynamic characterization of a model

Secondary biliary cirrhosis in the rat is an attractive model since unlike other models it does not rely on exogenous toxic compounds to induce cirrhosis. However, because little is known about the microcirculatory abnormalities of this model, this study investigated hemodynamics in rats with predefined functional impairment and related them to different aspects of stereologically quantified structure. All animals with at least 50% reduction in microsomal function, assessed by the aminopyrine breath test, had portal hypertension. The sinusoidal space, as assessed by multiple indicator dilution in the perfused liver, was reduced whereas large vessel space was increased. This reduction in sinusoidal space could contribute to increased portal resistance. The degree of intrahepatic shunting varied as assessed by a microsphere technique (13.9 vs. 0.5% in controls). These alterations were confirmed by stereological analysis. Numerically, there was excellent agreement between functional indicator dilution data and anatomic quantitation. Microvascular exchange was impaired as in other models of cirrhosis as shown by a reduced extravascular albumin space (4.5 vs. 2.2%, p < 0.01). In contrast to alterations in vascular space, this functional impairment was not reflected in the stereologically assessed space of Disse which averaged 5% of liver volume in both groups. Finally, in spite of reduced microsomal function in vivo (aminopyrine breath test) and in vitro (aminopyrine N-demethylase activity), the smooth endoplasmic reticulum was maintained (4.3 vs. 3.5 m2/ml cytosol, n.s.), which demonstrates that microsomal function in this model is reduced per unit hepatocyte. This suggests that the sick-cell hypothesis applies to secondary biliary cirrhosis in the rat.

Alterations induced by fascioliasis and cirrhosis on the biliary excretion of cefmetazole in wistar rats

1. Alterations induced by fascioliasis and cirrhosis on the biliary excretion of cefmetazole have been studied in Wistar rats. 2. Both infestation with Fasciola hepatica and experimental cirrhosis originated a significant decrease in the biliary excretion and in bile flow increase induced by the drug. 3. Administration of the beta-lactam antibiotic induced a lower degree of uncoupling of biliary lipid secretion in the cirrhotic and fasciolotic animals, but the effect was evident in all experimental groups.

Enhanced vasoconstrictor response of the isolated perfused cirrhotic rat liver to humoral vasoconstrictor substances found in portal venous blood

Humoral vasoconstrictor factors in portal venous blood have an important influence on hepatic vascular tone. The aim of this study was to determine whether there is altered reactivity of the intrahepatic portal vascular bed of cirrhotic livers to such factors. Isolated perfused rat liver preparations (IPRLP) obtained from rats with carbon tetrachloride-induced cirrhosis and from normal controls were treated with small aliquots of fresh, heparinized venous blood (4% vol/vol) added to a synthetic perfusate composed of 2.5% bovine serum albumin in Krebs-Henseleit buffer. Compared with blood from the inferior vena cava, portal venous blood produced a greater increase in perfusion resistance of normal IPRLP (2.8 +/- 0.7 vs 15 +/- 3%, P less than 0.05). There was no significant difference in the response of normal IPRLP to portal venous blood obtained from cirrhotic animals compared with portal blood from normal controls (10 +/- 4 vs 15 +/- 3%). However, cirrhotic IPRLP were significantly (P less than 0.05) more responsive to portal venous blood than were control livers, regardless of whether the blood was obtained from control (28 +/- 6%) or cirrhotic (24 +/- 6%) rats. The response of both control and cirrhotic IPRLP to portal blood could be partially inhibited by the alpha-adrenoceptor antagonist phentolamine (5 x 10(-6) mol/L) and cirrhotic IPRLP were more responsive than controls to exogenous noradrenaline (518 +/- 27 vs 363 +/- 21%, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation of mitochondrial metabolism in liver cirrhosis. Different strategies to maintain a vital function

Mitochondrial function and structure in cirrhotic livers from humans or rats show a variety of changes as compared to control livers. Mitochondrial ATP production is reduced in rats with CCl4- or thioacetamide-induced liver cirrhosis and in rats with secondary biliary cirrhosis. Activity of the electron transport chain is decreased in rats with secondary biliary cirrhosis. In rats with CCl4-induced cirrhosis, the mitochondrial content of certain constituents of the respiratory chain (cytochrome a + a3, cytochrome b and ubiquinone) is increased and activities of cytochrome c oxidase and ATPase are elevated. Similarly, in humans with liver cirrhosis, mitochondrial cytochrome a + a3 content is elevated and has been used to assess the risk for hepatectomy. In rats with secondary biliary cirrhosis, compensatory strategies include increased mitochondrial volume per hepatocyte and possibly increased extramitochondrial ATP production (increased glycolysis). Thus, a variety of adaptive mechanisms are used to maintain mitochondrial function in cirrhotic livers.

The role of capillarization in hepatic failure: studies in carbon tetrachloride-induced cirrhosis

During the cirrhotic process, the hepatic microvascular phenotype is transformed from sinusoids (discontinuous capillaries) into continuous capillaries. This transformation has been termed capillarization. Many hepatic functions depend on the rapid, bidirectional exchange of macromolecules between plasma and hepatocytes. To determine whether capillarization contributes to hepatic failure in cirrhosis, we decided to study the plasma clearance (125I) and hepatocyte uptake (electron microscopy) of three tracers in normal and cirrhotic rats. The tracers chosen were a hemeundecapeptide with peroxidatic activity (fluid-phase pinocytosis), asialofetuin (receptor-mediated endocytosis of a medium size protein) and ferritin (receptor-mediated endocytosis of a large size protein). The results demonstrate a decreased hepatocyte uptake of hemeundecapeptide; a significant delay in plasma clearance of asialofetuin; and a minor delay in plasma clearance of ferritin, but a striking trapping of ferritin in the cirrhotic capillary basement membrane. The delayed plasma clearance in cirrhosis cannot be ascribed to a decreased number of surface receptors because, in isolated hepatocytes, the number of molecules bound per cell was equivalent in normal and cirrhotic livers. These findings support the concept of capillarization, with the formation of continuous diffusion and filtration barriers between plasma and hepatocytes, representing a significant hindrance to the bidirectional macromolecular exchange normally taking place between these two compartments. Furthermore, at least in the case of ferritin, the capillary basement membrane of cirrhotic livers seems to be the major filtration barrier. This hindrance to hepatocyte uptake, and presumably also to secretion, may be the cause (or at least a major determinant) of the hepatic failure characteristic of cirrhosis.

Clinical pharmacokinetics in patients with liver disease

From considerations of hepatic physiology and pathology coupled with pharmacokinetic principles, it appears that altered drug elimination in liver disease may result from the following mechanisms: reduction in absolute cell mass, in cellular enzyme content and/or activity, in portal vein perfusion due to extrahepatic/intrahepatic shunting, or of portal perfusion of hepatocyte mass due to decreased portal flow or sinusoidal perfusion; increase in arterial perfusion relative to portal perfusion; preferential perfusion of the sinusoidal midzone and terminal zones by arterioles; potential for direct mixing of arterial blood within the space of Disse; reduced exchange across the endothelial lining; and impaired diffusion within the space of Disse. In general, oxidative drug metabolism is impaired in liver disease and the degree of impairment of oxidisation differs between drugs but correlates best with the degree of sinusoidal capillarisation, i.e. the degree of access of the drug from the sinusoid to the hepatocyte. Drug conjugation appears to be relatively unaffected by liver disease, whereas elimination by biliary excretion correlates best with the degree of intrahepatic shunting and not with sinusoidal capillarisation. As the latter should impair hepatocyte access of all compounds similarly, a potentially important mechanism could be impaired access of oxygen to hepatocytes as oxidative metabolism is much more sensitive to oxygen supply than are conjugation or biliary excretion. This suggests a potentially important therapeutic role for agents which increase the hepatic oxygen supply. Useful adjunctive strategies may also derive from the oxygen limitation hypothesis. Anaemia should be targeted as a critically important variable, as should oxygen-carrying capacity, i.e. modification of the smoking habit. Additionally, enzyme inducers such as barbiturates may be used if overriding hypoxic constraints are removed by oxygen supplementation. Agents likely to seriously compromise arterial perfusion of the hepatic vascular bed should be avoided, e.g. those causing postural hypotension or vasospasm. Vasodilators can be used to actively promote arterial perfusion. While the effect of liver disease on drug handling is highly variable and difficult to predict, there are well recognised principles for modifying dosage. These include halving the dose of drugs given systemically (or of low clearance drugs given orally) and a 50 to 90% reduction in the dose of drugs with a high hepatic clearance given orally. Changes in the pharmacodynamic effects of drugs (either alone or in addition to pharmacokinetic changes) can also be profound, and awareness of this possibility should be increased.

Galactose removal kinetics during hypoxia in perfused pig liver: reduction of Vmax, but not of intrinsic clearance Vmax/Km

The galactose elimination kinetics was examined in five perfused pig livers of 1.2 kg during hypoxia induced by administration of 2, 4 or 7% oxygen in the oxygenator instead of 20% as used in nine control experiments, previously published. Galactose was given as four to five successive constant infusion rates so that successive steady-state period with galactose concentrations from 0.04 to 5 mmol l-1 were obtained in each experiment. From the relationship between the calculated elimination rate and the perfusate galactose concentration, values of the maximal elimination rate Vmax and the half saturation concentration Km were calculated. Both Vmax and Km were reduced by hypoxia: the lower the oxygen supply, the greater the reduction. Vmax was about 0.08 mmol min-1 kg-1 liver at 2% oxygen and about 0.18 mmol min-1 kg-1 liver at 4-7% oxygen; both being significantly lower than the value of 0.43 mmol min-1 kg-1 liver at 20% oxygen. Km was about 0.07 mmol l-1 at 2% oxygen and 0.13 mmol l-1 at 7% oxygen; both significantly lower than the value of 0.23 mmol l-1 at 20% oxygen. A nearly parallel reduction of liver ATP concentration and galactose Vmax indicates that the galactose Vmax may reflect the phosphorylation capacity of the liver cells. The Vmax/Km ratio (intrinsic hepatic clearance) was unchanged during hypoxia.

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.

Decrease in the number of receptors for epidermal growth factor in the liver of D-galactosamine-intoxicated rats

Hepatic transport of epidermal growth factor (EGF) was studied in D-galactosamine-intoxicated rats by the multiple-indicator dilution (MID) method. The extraction ratio of 125I-labeled EGF in the intoxicated rats, obtained from a model-independent analysis of the dilution curves, decreased to 45% of the control values. A distributed two-compartment model was fitted to the dilution data by nonlinear least-squares regression, and the kinetic parameters, kon.PT (product of on-rate constant and receptor density), koff (off-rate constant) and ks (sequestration rate constant) were determined. The values of kon.PT and ks in the intoxicated rats decreased to approximately one-half and one-third of those in the control rats respectively. Similar decreases in the kon.PT and ks values in the intoxicated rats were also observed for the transport of 125I-labeled insulin, a positive control, into the liver. The 125I-labeled EGF binding experiment at equilibrium using liver homogenates revealed that the intoxication reduced the receptor density (PT) to one-third of the control values, whereas the equilibrium dissociation constant (kd) did not change significantly. The activities of Na+,K+-ATPase, cytochrome P-450 and glutathione S-transferase decreased in the intoxicated rats to 70-80% of the control values. The number of nuclei per unit area of tissue slices was also reduced to 70% of the control. Thus, the extent to which the enzyme activities and the number of nuclei decreased in the intoxicated liver was smaller than that of the number of EGF receptors. It is concluded that the reduction of EGF receptors cannot be explained by the "intact hepatocyte hypothesis" but rather by the functional change of hepatocytes induced by the administration of D-galactosamine.

Taurocholate transport by liver plasma membrane vesicles is not altered in cirrhotic rats

Recently, we have shown that taurocholate transport is impaired in hepatocytes isolated from CCl4 cirrhotic rats. Na+,K+-ATPase activity depends on the lipid composition of the surrounding membrane. Therefore, we performed this study in order to detect differences in plasma membrane composition and membrane functions between livers of CCl4 cirrhotic (n = 17) and of control rats (n = 15). After biochemical characterization of the animals we isolated basolateral and canalicular membrane vesicles and determined membrane enzyme activities, transport functions and lipid composition. We found no differences in the isolation characteristics of the plasma membranes between the two groups. The lipid composition of the membrane fractions was not altered, except for a lower cholesterol content in the canalicular membranes of the cirrhotic group (200 +/- 15 vs. 246 +/- 18 micrograms/mg protein, P less than 0.05). Taurocholate transport into basolateral membrane vesicles and marker enzyme activities of the membrane fractions were also equal in control and cirrhotic animals. We conclude that the plasma membrane composition and membrane enzyme/transport activities have returned to normal in CCl4 cirrhotic rats 14 days after cessation of exposure to CCl4. Thus, a disturbed transport system is not the cause for the observed decreased taurocholate transport into hepatocytes from cirrhotic rats. Even a cirrhotic liver has a high potential for recovery after acute CCl4 intoxication.

Evidence of the intact hepatocyte theory in alcoholic cirrhosis

To improve our understanding of the predominant operational model involved in the decreased clearance in cirrhosis, hepatic clearance, the extraction ratio of indocyanine green, and liver plasma flow were estimated in cirrhosis, either with a method based on the Fick principle or with a pharmacokinetic method assuming a bicompartmental plasma elimination of the dye. The two methods gave similar values for clearance. In contrast, the pharmacokinetic analysis gave significantly (p less than 0.01) lower hepatic plasma flow values and significantly (p less than 0.01) higher extraction values than those obtained with the reference (Fick principle) method. The main finding of this work is that in these cirrhotic patients, as in normal subjects, 'cellular' extraction estimated by the pharmacokinetic method is in the range of 0.60-0.80, whereas the extraction by the entire liver, assessed by the reference method, is low. In chronic liver diseases such as cirrhosis these data support the predominance of the intact hepatocyte theory, which assumes the existence of intrahepatic shunts associated with normally perfused and normally extracting hepatocytes. In acute liver disease, a cellular damage could be superadded.

In vitro screens from CNS, liver and kidney for systemic toxicity

The development and evaluation of in vitro systems from target organs for preliminary assessments of the potential for systemic toxic effects has been receiving increased attention. This review presents a synopsis of progress made in developing toxicity screens for three common target organs and identifies further work needed for more complete validation.

Alterations in the functional expression of receptors on cirrhotic rat hepatocytes

Reduced hepatic uptake and clearance of macromolecules in liver cirrhosis is due to two major factors: increased diffusional barriers, resulting primarily from the deposition of excessive connective tissue in the space of Disse, and hepatocellular dysfunction, manifested by receptor and/or postreceptor defects. To probe the mechanisms underlying hepatocellular dysfunction in liver cirrhosis, we have investigated receptor-ligand interactions for asialoorosomucoid, insulin and epidermal growth factor in hepatocytes isolated from the livers of rats chronically exposed to phenobarbital and carbon tetrachloride for up to 12 weeks. Viable cells were allowed to attach at 37 degrees C and the high-affinity cell surface binding sites for each ligand were assessed at 4 degrees C in the presence of [125I]-ligand. In parallel incubations, digitonin (0.055%) was added to the binding medium to assess total cellular binding sites. Results demonstrated that chronic treatment of rats with phenobarbital increased hepatocyte asialoorosomucoid surface receptor affinity (p less than 0.05) but had no affect on the number of asialoglycoprotein binding sites. Treatment with CCl4 and phenobarbital significantly reduced the number of surface binding sites for asialoorosomucoid (p less than 0.05) and epidermal growth factor (p less than 0.02), although this treatment had no effect on either the binding affinity or the number of binding sites for insulin. The decrease in cell surface binding sites for asialoorosomucoid and epidermal growth factor was not due to a redistribution of the surface sites to intracellular locations, since the total number of cellular binding sites also was reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of hepatic function in liver cirrhosis in the rat. Multivariate analysis

We investigated the determinants of hepatic clearance functions in a rat model of liver cirrhosis induced by phenobarbital/CCl4. Aminopyrine N-demethylation (ABT), galactose elimination (GBT), and serum bile acids (SBA) were determined in vivo. The livers were then characterized hemodynamically: intrahepatic shunting (IHS) was determined by microspheres and sinusoidal capillarization by measuring the extravascular albumin space (EVA) by a multiple indicator dilution technique. The intrinsic clearance was determined by assaying the activity of the rate-limiting enzymes in vitro. Hepatocellular volume (HCV) was measured by morphometry. ABT and SBA, but not GBT, differentiated cirrhotic from normal liver. IHS ranged from normal to 10%; all cirrhotic livers showed evidence of sinusoidal capillarization (reduced EVA). The cirrhotic livers showed a bimodal distribution of HCV, HCV being decreased in 50% of the cirrhotic livers. Multivariate analysis showed EVA and portal flow to be the main determinants of microsomal (ABT) and cytosolic (GBT) clearance function; SBA, by contrast, were determined solely by IHS. We conclude that sinusoidal capillarization is the main determinant of hepatic clearance, while serum bile acids reflect intrahepatic shunting. These findings emphasize the importance of alterations of hepatic nutritional flow to explain reduced clearance function in cirrhosis of the liver.

Canalicular bile flow and bile salt secretion are maintained in rats with liver cirrhosis. Further evidence for the intact cell hypothesis

Different aspects of biliary physiology were studied in rat model of liver cirrhosis induced by CCl4/phenobarbitone. We measured bile flow, bile salt secretion, biliary secretion pressure and bile-to-plasma ratios of inert solutes under basal conditions and during infusion of taurocholate (0.4 and 0.8 mumol.min-1.100 g body wt.-1) in 11 cirrhotic and 10 control male Sprague-Dawley rats. Bile flow and biliary bile salt secretion did not differ between the two groups. Analyzing the relationship between bile salt secretion and bile flow, however, we found an increased slope (P less than 0.02) in the cirrhotic animals, suggesting a higher apparent osmotic activity of the bile salts secreted. Maximal biliary secretion pressure was maintained in cirrhotic animals (22.5 +/- 2.5 vs. 25.0 +/- 2.9 cmH2O) in the absence of exogenous bile salt. During taurocholate infusion it decreased to a lesser extent (P less than 0.001) in cirrhotic animals (13.5 +/- 3.4 vs. 19.3 +/- 3.8 cmH2O). Bile-to-plasma ratios of [3H]sucrose and [14C]ferrocyanide were markedly increased in cirrhotic rats. Biliary [14C]erythritol clearance was equal to bile flow in both groups. In the cirrhotic group, the [3H]sucrose bile/plasma ratio was positively correlated with spleen weight (r = 0.744, P less than 0.01), serum concentration of alkaline phosphatase (r = 0.583, P less than 0.05) and basal maximum biliary secretion pressure (r = 0.801, P less than 0.001). We conclude that chronic portal hypertension is associated with increased permeability of the blood/bile barrier. Nevertheless, bile flow and bile salt secretion are maintained in cirrhotic rats, giving support to the intact cell hypothesis for this important hepatocellular function.

Aminopyrine N-demethylation by rats with liver cirrhosis. Evidence for the intact cell hypothesis. A morphometric-functional study

The intact cell hypothesis states that a reduced number of intrinsically normal hepatocytes, together with hemodynamic alterations, explains decreased drug metabolism in cirrhosis. We explored this hypothesis by comparing results of the aminopyrine breath test with in vitro measurements of aminopyrine N-demethylation and morphometrically determined liver cell volume in a rat model of cirrhosis. Aminopyrine N-demethylation in vivo (ABT-k) was 0.98 +/- 0.10/h (mean +/- SD) in controls. The cirrhotic rats were separated into those with normal (NCR) and those with abnormal ABT-k (PCR). Microsomal aminopyrine N-demethylase averaged 2.08 +/- 0.77 and 2.09 +/- 0.54 mumol/min in controls and NCRs, respectively; it was reduced to 1.00 +/- 0.81 mumol/min (p less than 0.02) in PCRs. Morphometrically determined hepatocellular volume was 18.8 +/- 2.8, 17.1 +/- 1.9, and 11.6 +/- 6.1 ml in controls, NCRs, and PCRs, respectively, PCRs being lower than controls (p less than 0.01) and NCRs (p less than 0.05). When N-demethylase and cytochrome P450 were related to hepatocellular volume (in milliliters), no significant difference between the three groups was apparent. We conclude that reduced aminopyrine N-demethylation in progressed cirrhosis is mainly due to a loss of liver cell volume. The function per liver cell volume remains constant, however, thus favoring the intact cell hypothesis for the handling of slowly metabolized compounds such as aminopyrine.