Effect of culture age on drug metabolizing enzymes and their induction in primary cultures of rat hepatocytes

Lactate does not activate the carotid body of Wistar rat

The carotid body's glomus cells are the primary sensors of hypoxia in mammals. Previous studies suggested that the glomus cells' hypoxia sensitivity is mediated by lactate in mice. This molecule increases the intracellular [Ca²⁺] and induces exocytosis in glomus cells, activating the carotid sinus nerve (the axons of chemoreceptive petrosal neurons). On the other hand, how lactate affects the activity of carotid body of rats is still unknown. We hypothesized that lactate activates the carotid body of rats. In Wistar rats, we measured the changes in the electrical properties of isolated glomus cells and petrosal chemoreceptive neurons in in situ preparations in response to different concentrations of lactate. Superfusion of both physiological and supraphysiological concentrations of lactate did not affect the membrane conductance and potential of glomus cells. Moreover, lactate injected into the carotid body did not activate the anatomically and physiologically identified chemoreceptive petrosal neurons. We conclude that the carotid body of Wistar rats is not sensitive to lactate.

Multibiomarker responses upon exposure to tetrabromobisphenol A in the freshwater fish Carassius auratus

Tetrabromobisphenol A (TBBPA) is a widely used brominated flame retardant. It has been released into aquatic environments, where it is toxic to aquatic organisms. In the present study, five enzymes, glutathione-S-transferase (GST), aspartate aminotransferase (AST), alanine aminotransferase (ALT), UDP-glucuronosyltransferase (UDPGT), and the antioxidant enzyme glutathione reductase (GR) in serum and liver of crucian carp (Carassius auratus) were selected for screening. These enzymes may be suitable for use as early warning indicators of chronic TBBPA exposure. UDPGT, AST, ALT, and GR activities in serum were found to be as more sensitive to TBBPA as those of the liver. When the concentration of TBBPA exceeded 0.50-0.71 mg/L for an exposure period of 32 days, GST, AST, ALT, and UDPGT activities cannot be restored to normal levels, suggesting that fish exposed to TBBPA above this threshold may incur irreversible damage. The activities of AST, ALT, and GR increased more significantly than GST and UDPGT at the lowest concentration of 0.35 mg/L. AST showed the strongest activity with respect to toxic kinetics, followed by ALT and GR. This remained true from day 4 of exposure to TBBPA to day 32. However, GR showed the clearest and most significant dose-effect relationship. This shows that each of these three enzymes can be used as a biomarker for early warning applications focusing on TBBPA pollution. AST and ALT are suitable for use in conventional monitoring of water quality in areas at risk for TBBPA pollution, and GR is more suitable for use in burst TBBPA pollution accidents where GR activity in fish would change with the TBBPA concentration of the flowing water.

Longterm stability of phase I and phase II enzymes of porcine liver cells in flat membrane bioreactors

Recently, researchers have focused on the use of bioartificial liver devices to support patients with fulminant hepatic failure. Our team developed a cell-based flat membrane bioreactor (FMB). In this, porcine liver cells were maintained in 3D-coculture between two gel layers in a sandwich configuration for 3 weeks to study the influence of this bioreactor technique on the preservation of basic, not induced activities of phase I and phase II enzymes. First, the time and substrate dependencies of the following enzymes were measured: ethoxyresorufin-O-deethylase (EROD, CYP 1A1/1A2) and ethoxycoumarin-O-deethylase (ECOD, CYP 2B6) as phase I enzymes, and glutathione-S-transferase (GST), UDP-glucuronosyltransferase (UGT) and sulfotransferase (ST) as phase II enzymes. To find optimal test conditions Michaelis-Menten kinetics were calculated. Next, different potential inducers were tested to find out the most effective compounds. Based on these results, the basic, not induced levels of the different enzymes were determined in the flat membrane bioreactor. Furthermore, the response of these enzyme activities to the chosen inducers was investigated to examine whether the cells keep their ability for drug-drug interactions. Basic, not induced activities of both phase I enzymes and the phase II enzymes GST and UGT were maintained at nearly the initial levels during the complete period of study. In addition, it was possible to induce these enzymes twice or three times in a weekly interval. In contrast, the basic, not induced activity of ST increased during the first 10 days of culture. It stabilized then and was maintained steady. As in short-term investigations, no reaction of the ST-activity towards any inducer could be obtained. These results prove that porcine liver cells preserve their phase I and phase II activities and respond to inducing drugs over 3 weeks in culture. Therefore, the flat membrane bioreactor is not only suitable for investigating drug metabolism, drug-drug interactions, and enzyme induction but also for supporting liver functions.

Glutathione S-transferase and UDP-glucuronyltransferase activity in primary cultures of rainbow trout gill epithelial cells

The objective of this study was to characterize rainbow trout (Oncorhynchus mykiss) gill epithelial cells in primary culture by evaluating their ability to maintain glutathione and glucuronide conjugating enzymes. The activity and inducibility of the phase II enzymes was investigated as a function of culture time. Glutathione S-transferase (GST) and UDP-glucuronyltransferase (UDPGT) enzyme activities were measured in freshly isolated cells and in cells cultured for 7 and 12 days. GST activity, determined with 1-chloro-2,4-dinitrobenzene, decreased gradually to 72% after 7 days and to 38% after 12 days in culture compared with freshly isolated cells. There was no significant difference between UDPGT activities in freshly isolated cells compared with cells cultured up to 12 days although a transient decrease in activity was observed at day 7. In vitro induction of the enzymes was studied using beta-naphtoflavone (BNF) and 3-methylcholanthrene (3-MC) as inducers. GST activity increased 2-fold after exposure to BNF and 1.5-fold after 3-MC exposure for 48 h in 7 days old cultures. No induction was observed in 12 days old cultures. UDPGT activity was not induced either at day 7 or 12.

Drug metabolism in hepatocyte sandwich cultures of rats and humans

Adult hepatocytes from rat and man were maintained for 2 weeks between two gel layers in a sandwich configuration to study the influence of this culture technique on the preservation of basal activities of xenobiotic-metabolizing phase I and phase II enzymes. The response of these enzyme activities to an enzyme inducer was investigated using rifampicin (RIF). Basal levels of cytochrome P-450 (CYP) isozymes were characterized by measuring ethoxyresorufin O-deethylation (EROD), ethoxycoumarin O-deethylation (ECOD), and the specific oxidation of testosterone (T). In hepatocytes from untreated rats, CYP isozyme levels, including the major form CYP 2C11, increased during the first 3 days in culture. After this period of recovery, the levels of CYP 2C11, CYP 2A1, and CYP 2B1 decreased, whereas CYP 3A1 increased. In contrast to these dynamic changes, CYP activities such as CYP 1A2 and the major isozyme CYP 3A4 were largely preserved until day 9 in cultures of human hepatocytes. In measuring phase II activities, a distinct increase in glucuronosyltransferase (UDP-GT) activity toward p-nitrophenol (PNP) was found for rat and human hepatocytes over 2 weeks in culture. Sulfotransferase (ST) activity toward PNP showed an initial increase, with a maximum at day 7 and day 9 in culture, respectively, and then decreased until day 14. Glutathione S-transferase (GST) activity decreased constantly during the time of culture. Effects of the enzyme-inducing drug rifampicin on phase I and phase II enzymes were investigated using cultured human hepatocytes. Rifampicin treatment (50 micromol/L) for 7 days resulted in a 3.7-fold induction of CYP 3A4 at day 9 in culture. ECOD activity was increased sixfold and phase II ST activity increased twofold compared to the initial value at day 3. No effect of rifampicin on CYP 3A was found in cultures of rat hepatocytes. These results demonstrate that rat and human hepatocytes preserve the major forms of CYP isozymes and phase II activities and respond to inducing drugs such as rifampicin. The novel hepatocyte sandwich culture is suitable for investigating drug metabolism, drug-drug interactions and enzyme induction.

Sulfotransferase gene expression in primary cultures of rat hepatocytes

Hepatocyte cultures have been used in pharmacotoxicological studies, and sulfotransferases (ST) are important drug-metabolizing enzymes in liver. The expression of sulfotransferases in hepatocyte cultures has not been examined systematically. In the present study, the mRNA levels of different sulfotransferases in male and female rat hepatocytes were examined by northern-blot analyses. Various culture conditions such as different matrices (collagen, matrigel, collagen sandwich, or co-culture with epithelial cells), medium (Way-mouth's MB 752/1 and Modified Chee's Medium) and glucocorticoid supplementation (dexamethasone, 0.1 microM) were compared. Phenol ST (ST1A1) mRNA levels decreased to about 50% of initial mRNA levels within 10 hr of culture. At 96 hr, ST1A1 mRNA levels were approximately 20% of initial values when cultured on collagen, matrigel or co-culture. The two media did not differ in ability to maintain ST1A1 mRNA levels in the absence of dexamethasone (DEX); however, DEX addition to either medium resulted in ST1A1 mRNA levels greater than 100% of the initial mRNA levels at 96 hr, with the greatest increase observed using the matrigel substratum and Chee's medium. In the absence of DEX, the mRNA levels of N-hydroxy-2-acetylaminoflurene sulfortransferase (ST1C1), estrogen sulfotransferase (ST1E2) and hydroxysteroid sulfotransferase (ST-20/21, ST-40/41, ST-60) fell to approximately 20% of their initial levels within 24 hr, and to less than 5% at 96 hr. The loss of expression of these sulfotransferases was observed with all culture conditions. Addition of DEX to the media resulted in ST-40/41 and ST-60 mRNA expression at 20 and 35% of their initial values, respectively, in cultures maintained on matrigel and Chee's medium at 96 hr. These data suggest that sulfotransferases lose their constitutive expression in hepatocyte culture, but retain their inducibility.

Collagen gel immobilization: a useful cell culture technique for long-term metabolic studies on human hepatocytes

1. Primary cultures of human hepatocytes have already been employed in various applications for the study of xenobiotic metabolism. Most of these approaches were performed either on freshly isolated cells or on short-term primary cultures. Standard culture techniques do not maintain functional stability of P450 enzymes for > 1 week in vitro. 2. The aim of this study was to demonstrate the beneficial effect of an easy to apply, extracellular matrix configuration on the long-term performance of cultured human liver cells. Light microscopical examination of the cultures indicated that the cells remained viable over 1 month. As revealed by electron microscopy, hepatocytes exhibited bile canaliculi and desmosomes and were rich in mitochondria and endoplasmatic reticulum, indicating metabolic activity. 3. An early culture phase (3 days after isolation) could be described with decreasing DNA content of the cultures, peak values of alanine-amino-transferase (ALAT), and increasing albumin synthesis. After this adaptive period stable levels for DNA content and albumin synthesis were noted; ALAT returned to low values. 4. Functional activity was monitored by measurements of P450 1A1-dependent O-demethylation of p-nitroanisole to p-nitrophenol, which appeared to be constant over 3 weeks and weakly inducible by 1 mM phenobarbital. Another set-up examined conjugation of acetaminophen at subtoxic concentrations: acetaminophen was metabolized to its glucuronide and sulphate; 3-(glutathione-S-yl)-acetaminophen was not detected. Almost identical metabolism was found, comparing day 3 with 16 of culture. 5. We concluded that collagen gel immobilization not only provides mechanical support to cultured hepatocytes, but also supports long-term differentiated function of the cells for metabolic studies.

Comparative metabolism of SC-42867 and SC-51089, two PGE2 antagonists, in rat and human hepatocyte cultures

1. The metabolism of SC-42867 and SC-51089, two PGE2 antagonists, was studied in cultured rat and human hepatocytes. Both compounds possess an 8-chlorodibenzoxazepine moiety, but differ from each other by the nature of the side chain connected to the nitrogen atom. SC-42867 and SC-51089 and their in vitro metabolites were separated by reversed-phase hplc. The major metabolites of both compounds were identified by mass spectrometry (ms) analysis. 2. SC-42867 was metabolized on the tricyclic moiety only. Oxidative N-dealkylation with opening of the oxazepine ring was the major metabolic pathway obtained in rat hepatocytes. The metabolic profile obtained in cultured human hepatocytes was comparable with that of cultured rat hepatocytes. However, the compound was metabolized to a much lower extent by the human cells. 3. SC-51089 was extensively metabolized by both cultured rat and human hepatocytes. Human cells metabolized this compound quite differently than cultured rat hepatocytes. Aromatic hydroxylation with consequent glucuronidation and sulphation were the main metabolic pathways observed in cultured human hepatocytes. Oxidative N-dealkylation with opening of the oxazepine ring and consequent glucuronidation was the major metabolic pathway observed in rat hepatocytes. Further metabolism occurred, in contrast with the human hepatocytes, mainly on the side chain. 4. The present in vitro results are compared with data of previous in vivo studies performed in rat.

Rat hepatocyte cultures and co-cultures in biotransformation studies of xenobiotics

Long-term cultures of hepatocytes could represent a suitable in vitro model for biotransformation studies of xenobiotics. At present however, no ideal culture system can be proposed since, in all existing models, phenotypic changes occur, affecting selectively some components of phase I and/or phase II xenobiotic metabolism. From the authors' own results and recent studies of several other investigators, carried out on rat hepatocytes, it becomes clear that four groups of factors may affect biotransformation capacity: soluble medium factors, extracellular matrix components, cell-cell interactions and factors affecting replication. For the maintenance of liver-specific functions, it seems of utmost importance that the tridimensional shape of the hepatocytes is kept. Usually, phase II enzymatic activity is better kept than that of phase I. The cytochrome P450 dependent monoxygenases, in particular, are easily lost. Interesting is the observation that co-cultures of rat hepatocytes with rat liver epithelial cells exhibit higher and much better preserved phase I and phase II biotransformation than monocultures. Clearly, further research is needed to improve this promising in vitro model.

A mechanism for the loss of cytochrome P-450 in primary mouse hepatocytes

This study examined various biochemical parameters such as mitochondria and mitochondrial DNA (mtDNA), total heme and cyto P450 content in fresh hepatocytes and dedifferentiated hepatocytes. These parameters were chosen in order to understand the dramatic decrease in drug metabolism in cultured hepatocytes. The data in this study shows a temporal decrease in cytochrome P450, a total heme and also a decrease in mitochondria. Also, the ratio of mtDNA content to mitochondrial density was found to increase as hepatocytes underwent dedifferentiation. Stereological analysis of cell preparations provided a measure of mitochondrial density per cell area and mtDNA content was assessed by the use of a specific radiolabelled probe. This study demonstrates that a loss of the organelle which is partially responsible for synthesis of heme correlates with a decrease in cytochrome P450.

Sulfation and glucuronidation of acetaminophen by cultured hepatocytes reproducing in vivo sex-differences in conjugation on Matrigel and type 1 collagen

The sulfate and glucuronide conjugation of acetaminophen (APAP) by hepatocytes cultured on Matrigel or type 1 collagen was compared to APAP metabolism in vivo. The metabolic fate of low (15 mg/kg), medium (125 mg/kg), and high (300 mg/kg) doses of APAP injected intraperitoneally were determined in male and female rats. Males excreted more APAP as the sulfate conjugate than females, which correlated with the twofold greater APAP sulfotransferase activity in the male vs. females (301 +/- 24 vs. 156 +/- 18 pmol.mg-1 protein.min-1). Also, as sulfate conjugation became saturated, there was a dose-related shift in APAP metabolism from sulfate to glucuronide conjugation in both sexes. After death, the livers of the same animals were perfused with collagenase and the hepatocytes cultured in modified Waymouth's medium on either Matrigel or rat-tail collagen, with various doses of APAP (0, 0.125, 0.25, 0.5, and 1.0 mM). Sex differences in APAP sulfation and glucuronidation persisted in culture for up to 4 days, with sulfation predominating in the male similar to in vivo. With increasing APAP concentration (dose), there was a saturation of sulfate conjugation and a shift to glucuronidation as observed in vivo. Sex differences in APAP sulfation and glucuronidation were no longer significant by Day 4 in culture. Sulfation, and to a lesser extent, glucuronidation, were more stable on Matrigel than collagen. We concluded that APAP metabolism of freshly isolated hepatocytes could replicate in vivo sex differences in conjugation, and that Matrigel was superior to collagen as substrate.

Maintenance of mixed-function oxidase and conjugation enzyme activities in hepatocyte cultures prepared from normal and diseased human liver

Isolated hepatocytes were prepared from normal and diseased human livers and maintained in primary monolayer culture for up to 96 h. The viability and yields of cell preparations obtained from diseased livers did not differ significantly from those obtained from normal livers. During the culture period a significant increase in cell protein/DNA ratio was observed in both normal and diseased hepatocytes. The maintenance of a number of drug metabolising enzyme activities was determined in these hepatocytes during 96 h of culture. In normal hepatocytes the maintenance pattern of mixed-function oxidase activities (ethoxycoumarin-O-deethylase and ethoxyresorufin-O-deethylase) was clearly different from that of the conjugating enzymes (sulfotransferase and glutathione transferase). Whereas ethoxycoumarin-O-deethylase and ethoxyresorufin-O-deethylase activities declined sharply over the first 24 h in culture and then either totally or partially recovered, sulfotransferase and glutathione transferase activities were found to be relatively more stable initially but thereafter decline progressively. In diseased hepatocytes mixed-function oxidase activities were maintained less well than the corresponding activities in normal hepatocytes whereas conjugation enzyme activities were maintained to a similar extent.

Role of heme in phenobarbital induction of cytochromes P450 and 5-aminolevulinate synthase in cultured rat hepatocytes maintained on an extracellular matrix

When hepatocytes are cultured on matrigel, a reconstituted basement membrane matrix, mRNAs for cytochrome P450 class IIB1/2 and class III genes can be induced by treatment with phenobarbital. We took advantage of this new system to critically evaluate the role of heme as a regulator of these cytochromes P450 and of 5-aminolevulinate synthase (ALA-S), the rate-limiting enzyme in heme biosynthesis. Phenobarbital treatment of rat cultures increased the total amount of cytochrome P450, activities catalyzed by IIB1/2 (benzyloxy- and pentoxyresorufin O-dealkylases) and ALA-S activity, and ALA-S mRNA. Treatments with phenobarbital combined with succinyl acetone, an inhibitor of heme biosynthesis at the step of 5-aminolevulinate dehydrase, blocked the induction of the proteins for cytochrome P450IIB1/2 and cytochrome P450IIIAI, as indicated by spectral, immunological, and enzymatic assays. However, at the same time, succinyl acetone cotreatment failed to inhibit the induction of the mRNAs for cytochrome P450IIB1/2 and cytochrome P450IIIA. Lack of effect on the cytochrome P450 mRNAs was selective inasmuch as treatment with phenobarbital combined with succinyl acetone synergistically increased both ALA-S activity and ALA-S mRNA, presumably by blocking formation of heme, the feedback repressor of ALA-S. Indeed, the increase in ALA-S mRNA caused by the combined treatment was abolished by adding heme itself to the cultures. In contrast to earlier concepts, we conclude that in the intact hepatocyte, phenobarbital-induced cytochrome P450 induction is independent of changes in heme synthesis.

Phase I and phase II xenobiotic biotransformation in cultures and co-cultures of adult rat hepatocytes

The aim of this study was to measure the activity of phase I and II key enzymes in the biotransformation of xenobiotics and their inducibility by phenobarbital (2 mM) in two currently used in vitro models, namely adult rat hepatocytes, conventionally cultured or co-cultured with rat epithelial cells derived from primitive biliary duct cells. For phase I, the cytochrome P450 content and the enzymic activities of 7-ethoxycoumarin O-deethylase and aldrin epoxidase have been determined, for phase II glutathione S-transferase activity was measured. In conventional cultures, all phase I parameters investigated declined continuously as a function of culture time. Two mM phenobarbital had inducing effects on 7-ethoxycoumarin O-deethylase and glutathione S-transferases but not on aldrin epoxidase. In co-cultures, after an initial decrease, a steady state situation developed for all the parameters measured, lasting for at least 10 days. The cytochrome P450 content, the 7-ethoxycoumarin O-deethylase, aldrin epoxidase and glutathione S-transferase activities were maintained from 3 to 4 days on at 25, 100, 15 and 50%, respectively, of their corresponding value obtained for freshly isolated hepatocytes. After phenobarbital treatment, the parameters mentioned were significantly increased with the exception of the aldrin epoxidase activity of which the inducibility was nearly completely lost.

Effect of xenobiotics on monooxygenase activities in cultured human hepatocytes

The activity of human cytochrome P450 monooxygenases, aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase can be increased by 3-methylcholanthrene, phenobarbital and ethanol in human hepatocytes maintained in primary culture. Total cytochrome P450 content increased two-fold after 48 hr of incubation with methylcholanthrene or phenobarbital and 1.5-fold after incubation with ethanol. The three chemicals elicited different effects on cytochrome P450 dependent activities. Addition of 3-methylcholanthrene caused a time- and concentration-dependent increase in both monooxygenase activities, aryl hydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase, while phenobarbital and ethanol increased 7-ethoxycoumarin O-deethylase activity but had no effect on aryl hydrocarbon hydroxylase. Dexamethasone per se had little or no effect on either monooxygenase activities, but potentiated the effect of the three chemicals on 7-ethoxycoumarin O-deethylase.

Metabolism of the liver tumor promoter ethinyl estradiol by primary cultures of rat hepatocytes

Previously, we reported that relatively high micromolar concentrations of the liver tumor promoter 17 alpha-ethinyl estradiol (EE2) stimulated DNA synthesis and enhanced the DNA synthetic response to epidermal growth factor (EGF) in primary cultures of female rat hepatocytes [J.D. Yager, B.D Roebuck, T.L. Paluszcyk, and V.A. Memoli, Carcinogenesis 7, 2007-2014 (1986); Y.E. Shi and J.D. Yager, Cancer Res. 49, 3574-3580 (1989)]. In this study, our goal was to examine the metabolism of EE2 in cultured hepatocytes. After 4, 24, and 48 hr of culture, hepatocytes maintained their ability to convert up to 95% of a 4 nM concentration of [3H]EE2 to polar conjugates within 4 hr. EE2 at 2 microM was also 95% metabolized within 4 hr. HPLC analysis of the metabolites confirmed the rapid disappearance of [3H]EE2 and the formation of polar conjugates as detected by organic extraction. HPLC separation of hydrolyzed conjugates indicated that the major aglycone was the parent compound, EE2. In general, the metabolites differed both qualitatively and quantitatively from those reported in vivo in the rat. The rapid metabolism of EE2 by hepatocytes in culture may, at least in part, explain the high concentrations of EE2 required to stimulate DNA synthesis in cultured hepatocytes and to potentiate the response to EGF.

Drug metabolism by periportal and perivenous rat hepatocytes. Comparison of phase I and phase II reactions and their inducibility during culture

Hepatocytes were aseptically isolated from either the periportal (pp; zone 1) or the perivenous (pv; zone 3) region by digitonin-collagenase perfusion and cultured on type I collagen for 4 to 9 days. In freshly isolated cells the pp:pv activity ratios of the acinar marker enzymes gamma-glutamyltranspeptidase (gamma-GT), alanine aminotransferase (ALAT) and glutamate dehydrogenase (GLDH) were 2.8, 1.6 and 0.76, respectively. During culture ALAT and GLDH activities gradually declined, but the pp-pv difference was retained for at least 4 days. In contrast, the difference in the gamma-GT activity was rapidly lost, due to its fast initial activation in pv cells. The initial 7-ethoxycoumarin O-deethylase (ECDE) activity was higher in pv cells; this difference was retained for several days of culture and was increased by induction in vitro with either phenobarbital (PB) or beta-naphthoflavone (beta NF). Although the basal UDP-glucuronyltransferase (UDPGT) activity with either p-nitrophenol (pNP) or hydroxybiphenyl (HBP) as substrate did not differ significantly, the in-vitro PB- or beta NF-induced activity was higher in pv cells. Both glucuronidation and sulfation of methylumbelliferone tended to be higher in pv cells. Glutathione S-transferase was initially significantly higher in pv cells and this difference was augmented after in vitro induction by PB or beta NF. After six days in culture all the observed pp-pv differences had disappeared. These results suggest that hepatocytes isolated from the perivenous region seem to maintain their initially higher capacity for phase I and phase II drug reactions during culture and also respond more strongly than periportal cells to in vitro induction.

Gamma-glutamyltransferase induction by glucocorticoids in rat liver: age-dependence, time-dependence, dose-dependence, and intralobular distribution

Postnatal responsiveness of rat-liver gamma-glutamyltransferase (GGT) to glucocorticoids (GC) has been defined by investigating: age-dependence, time-dependence, hormonal dose-dependence, and lag-time of the enzyme re-expression; half-life of the induced enzyme activity; dynamics of the enzyme reappearance in the liver tissue. Hydrocortisone-acetate (HC) or dexamethasone (DEX) were administered to the animals starting 1, 2, 3, 4, or 5 d before killing, at the doses of 25 micrograms or 1 microgram/(g b. w. x d), respectively. In 14 d old rats, after a lag-time of about 20 h (DEX) or 30 h (HC), GGT activity progressively increased up to 38 and 31 times the control value, respectively, at 5th d; the enzyme re-expression was linearly hormone dose-dependent; half-life of the induced enzyme activity was about 36 h. In 21 d old rats, GGT re-induction behaved as in 14 d old animals, except that the induced activity was about half that of each correspondent treatment. In 28 d old rats, a very low but significant GGT activity was re-expressed only after hormonal treatments longer than 48 h. In 35 and 77 d old rats, significant GGT activity was never re-induced. GGT was re-expressed in liver parenchyma, with a defined space-course. In 14 d old rats, GGT reappeared first in periportal areas, then in acinar zone 1, finally in acinar zone 2. While the animals were ageing, GGT re-expression occurred to lesser and lesser extents in liver tissues, because of a progressive space-restriction from acinar zones 1 and 2 to zone 1 and finally, in 35 d old rats, to periportal areas. In adults, GGT was re-expressed only by rare hepatocytes in periportal spaces. Acinar zone-3 hepatocytes did never re-express GGT, irrespectively of the animal age. Thus, 2 rat hepatocyte populations could be distinguished (1 responsive, the other unresponsive to GC for GGT re-expression), the relative proportion of which changes in favour of the unresponsive one while the animal ages. Hepatic GGT re-induction by GC, occurring after a long lag-time, does not follow the typical model of hormonal induction. Previous permissive cell changes seem to be required. Hepatocyte-GGT re-expression by GC appears to be inversely correlated with the differentiation level and the cytochrome P-450 amount (activity) of the cell as limiting factors for the triggering of the enzyme induction.

Aminopyrine metabolism in primary monolayer cultures of diabetic rat hepatocytes

1. A new support system has been used which provides long-term maintenance of rat liver parenchymal cells in monolayer cultures. The cells, maintained on collagen gel/polychlorinated vinylidene film, expressed aminopyrine metabolizing activity for up to 5 days. This culture system was utilized to study the metabolism of aminopyrine in the liver cells isolated from normal, alloxan- and streptozotocin-diabetic rats. 2. Aminopyrine was metabolized at a slower rate in both types of cultured diabetic rat hepatocytes than in cultured normal rat hepatocytes, as judged from higher levels of the unchanged drug in the culture medium. 3. The formation of the metabolites 4-monoaminoantipyrine, 4-acetylaminoantipyrine and 4-formylaminoantipyrine decreased in the cultured diabetic rat hepatocytes, while that of 4-aminoantipyrine was at the same levels as controls. In contrast, 3-hydroxymethyl-2-methyl-4-dimethylamino-1-phenyl-3-pyrazolin-5-on e (AM-CH2OH) formation in the cultured diabetic rat hepatocytes increased over control value. These findings agree with in vivo results which have been reported by the authors. 4. The increase in AM-CH2OH was prevented by insulin in a dose-dependent manner. However, insulin did not affect the formation of other metabolites. These findings indicate that the amount of cytochrome P-450 isozyme involved in the oxidation of 3-methyl group may be regulated by insulin. 5. The present results, indicate that this primary culture system is a useful tool for the study of drug metabolism in diabetes.

The content and activity of cytochrome P-450 in long-term culture of hepatocytes from male and female rats

The content of cytochrome P-450 and the capacity for O-demethylation have been measured in cultures of hepatocytes from male and female rats for a period of 21 days. The effect of dexamethasone, insulin, glucagon, phenobarbital and hemin was investigated. In hepatocytes from female rats the content of cytochrome P-450 was unchanged after one day of culture. From day 1 to day 3 the content of cytochrome P-450 decreased by 65% and only the combined addition of dexamethasone, phenobarbital and hemin diminished the fall. After the initial fall, addition of 0.1 microM dexamethasone resulted in a stable value. Addition of 1 microM dexamethasone or 1 mM phenobarbital gave rise to an induction of cytochrome P-450 (285%). The high level of cytochrome P-450 was maintained for 3 weeks. In hepatocytes from male rats the content of cytochrome P-450 decreased by 40% after one day of culture. From day 1 to day 3 the content decreased by 45% and the decrease continued irrespective of the presence of hormones and/or phenobarbital. The O-demethylase activity in cultures of hepatocytes from female rats correlated to the cytochrome P-450 content independent of medium composition and age of the cultures, whereas no correlation was found in cultures from male rats. The present study demonstrates that hepatocytes from female rats in cultures retain O-demethylase activity for at least 3 weeks and that, with the experimental conditions used, the response to the hormones and inducers is different for hepatocytes from male and female rats.

Induction of 2-carboxybenzaldehyde reductase by phenobarbital in primary culture of rat hepatocytes

When rats were treated with phenobarbital (PB), the activity of CBA reductase, which catalyzes the conversion of 2-carboxybenzaldehyde (CBA) to 2-hydroxymethylbenzoic acid (HMB), in the liver was markedly enhanced. Likewise, addition of PB to the primary culture of rat hepatocytes increased the activity of CBA reductase. The enzyme recovered from cell lysate of cultured cells showed the same characteristics in molecular and catalytic properties as the enzyme purified from the livers of the rats treated with PB. Experiments with cycloheximide suggest that de novo synthesis of the enzyme protein is enhanced by PB in primary culture.