Induction of UDP-glucuronosyltransferase activities in Gunn, heterozygous, and Wistar rat livers by pregnenolone-16 alpha-carbonitrile

The effect of pregnenolone-16 alpha-carbonitrile (PCN) on UDP-glucuronosyltransferase (UDP-GT) activity was comprehensively examined in Wistar (JJ), heterozygous (Jj) and Gunn (jj) rats with eleven different acceptors for glucuronic acid. UDP-GT activity after 3-methylcholanthrene (3-MC) and phenobarbital (PB) treatment was studied in additional rats for comparative purposes. Conjugation of group-1 aglycones (1-naphthol and p-nitrophenol) was much lower in Gunn than in Wistar rats. PCN did not alter UDP-GT conjugation of these acceptors. UDP-GT activity toward group-1 aglycones was increased by 3-MC in Wistar and heterozygous rats but was not enhanced in Gunn rats by any inducer. Activity toward group-2 aglycones (morphine, chloramphenicol, valproic acid) was similar in control rats of all genotypes. PCN increased chloramphenicol conjugation, whereas PB enhanced the glucuronidation of all group-2 aglycones in Wistar, heterozygous, and Gunn rats. Conjugation of group-3 acceptors (bilirubin and digitoxigenin monodigitoxoside, DIG) was deficient in Gunn rats and was not inducible. PCN increased glucuronidation of bilirubin and DIG in Wistar and heterozygous rats. The concentration of UDP-glucuronic acid (UDPGA) in liver was similar in control animals of all genotypes and was increased in rats treated with 3-MC. The other inducers did not affect hepatic UDPGA levels. Thus, 3-MC, PB, and PCN induce UDP-GT activities toward different groups of acceptors of glucuronic acid. The results support the hypothesis that PCN induces a form of UDP-GT that preferentially conjugates the group-3 acceptors, bilirubin and DIG.

On the disulfiram-like activity of moxalactam

A three-way crossover study was undertaken in 10 healthy subjects to characterize the reported disulfiram-like activity of moxalactam and to assess its influence on ethanol and acetaldehyde metabolism. On different occasions separated by at least 2 wk subjects were given in random order: 0.5 gm/kg ethanol orally, 0.5 gm/kg ethanol followed in 1 hr by 1.0 gm IV moxalactam, and 1.0 gm IV moxalactam every 8 hr for four doses followed by 0.5 gm/kg ethanol. Mean ethanol elimination rates of 13.1 +/- 0.76, 10.1 +/- 1.11, and 10.9 +/- 1.06 mg/dl/hr (mean +/- SEM) were observed in the three protocols, respectively. Corresponding mean estimated acetaldehyde clearance rates were 103.7 +/- 15.55, 92.8 +/- 13.79, and 97.3 +/- 10.41 l/min (mean +/- SEM). While no consistent moxalactam effect on ethanol or acetaldehyde elimination was observed, two subjects experienced mild disulfiram-like reactions to ethanol after moxalactam pretreatment. In one subject this reaction was associated with markedly elevated blood acetaldehyde concentrations. We conclude that moxalactam pretreatment may induce a disulfiram-like reaction after ethanol ingestion in some, probably due to inhibition of aldehyde dehydrogenase, and that alcoholic beverages are contraindicated in patients receiving moxalactam. We suggest, however, that such reactions will not occur when moxalactam is given after ethanol ingestion.

Cadmium toxicity in the newborn rat

Newborn rats have about a 20-fold higher concentration of metallothionein in the liver than does the adult rat. This marked increase in hepatic metallothionein does not result in a lower acute toxicity to cadmium in newborn rats; it does not markedly increase the distribution of cadmium to the liver, decrease its concentration in other tissues, or produce a slower removal of the cadmium from younger animals. The toxicity of cadmium in the newborn rat appears to be more dependent on other factors such as the development of the blood-brain barrier and the stage of development of the testes than on the amount of metallothionein in the liver. The results of this study suggest that we should probably reevaluate the role of metallothionein in producing tolerance to cadmium.

Role of ligandin as a binding protein and as an enzyme in the biliary excretion of sulfobromophthalein

The effect of butylated hydroxyanisole (BHA; 600 mg/kg i.p. daily, for 10 days) and trans-stilbene oxide (TSO; 400 mg/kg i.p. daily, for 4 days) on the in vitro hepatic activity of glutathione transferases, the hepatic content of organic anion binding proteins and the plasma disappearance and biliary excretion of sulfobromophthalein (BSP), phenol-3,6-dibromsulphthalein disulfonate and [3H]ouabain was investigated in mice (BHA) and rats (TSO). Both BHA and TSO increased glutathione transferase activity toward BSP (360 and 200%), hepatic ligandin content (160 and 120%) and the biliary excretion of BSP (370 and 85%). BSP-glutathione excretion was enhanced, indicating that BSP conjugation was also stimulated in vivo. In contrast to BSP, biliary excretion of phenol-3,6-dibromsulphthalein disulfonate and organic anion which is not biotransformed but binds to ligandin, was unaltered or slightly increased (29%) after BHA or TSO treatment, respectively. TSO administration also did not affect the excretion of ouabain, a compound that neither binds to ligandin nor is biotransformed before excretion. Induction of ligandin failed to influence the initial disappearance of BSP, phenol-3,6-dibromsulphthalein disulfonate or ouabain from plasma, suggesting that induction had no marked effect on the hepatic uptake of these compounds. These studies suggest that ligandin plays a more important role in the biliary excretion of BSP due to its enzymatic rather than its binding properties.

Effects of phospholipase A2 inhibitors on diethyl maleate-induced lipid peroxidation and cellular injury in isolated rat hepatocytes

Isolated hepatocytes provide a suitable system for investigation of various aspects of the mechanism of a toxic response. The mechanism by which most chemicals induce hepatotoxicity is still not known. Evidence that phospholipases may play a role in cellular injury has been reported. In the present study the effects of reported inhibitors of phospholipase A2 (quinacrine, chlorpromazine, dexamethasone, and dibutyryl cyclic AMP) on diethyl maleate (DEM)-induced lipid peroxidation, reduced glutathione (GSH) depletion, and cellular injury were examined in isolated hepatocyte suspensions. Hepatocytes were incubated for 7 h under control conditions or with (1) DEM (4 mM), (2) one of the inhibitors (quinacrine, 10, 50, or 150 microM; chlorpromazine, 50 microM; dexamethasone, 0.1, 0.5, 1, or 2.5 mM; dibutyryl cyclic AMP, 0.1, 0.5, 1, or 2.5 mM) or aspirin (500 microM), or (3) a combination of DEM and one of the inhibitors or aspirin to determine their effect on DEM toxicity. Samples were withdrawn at hourly intervals for estimation of cellular injury (loss of intracellular K+ and lactate dehydrogenase and trypan blue exclusion index), lipid peroxidation (thiobarbituric acid reactants assay), and GSH concentration. Quinacrine and chlorpromazine inhibited DEM-induced lipid peroxidation but not cellular injury or GSH loss. This suggests that phospholipase A2 may be involved in DEM-induced lipid peroxidation but not cell damage. However, dexamethasone and dibutyryl cyclic AMP enhanced both lipid peroxidation and loss of cell viability due to DEM, suggesting novel aspects of the biochemical mechanisms of chemically induced cytotoxicity.

Determination of hepatic uridine 5'-diphosphoglucuronic acid concentration by conjugation with diethylstilbestrol

A sensitive and reliable assay for uridine 5'-diphosphoglucuronic acid (UDPGA) was developed that involved conjugation of diethylstilbestrol (DES) in vitro. This conjugation reaction is solely dependent upon UDPGA concentration. The assay uses 0.13 M Tris-HCl, pH 7.4, 6.7 mM MgCl2, 0.05% Brig 58, 0.25 mg guinea pig liver microsomal protein, 0.13 mM 3H-DES (0.2 microCi/ml), and 200 microliters of boiled 10% liver homogenate in a total volume of 0.5 ml. After a 60-min incubation at 37 degrees C, unconjugated DES is extracted into 5 ml of chloroform and the residual metabolized 3H-DES in the aqueous phase is determined by liquid scintillation spectrometry. After addition of beta-glucuronidase to the aqueous phase, about 90% of the radioactivity could be extracted into chloroform, demonstrating the DES-glucuronic acid is the primary metabolite. Thus, this method easily permits quantitation of UDPGA in rat liver in the 1-10 nmol range.

Effect of inducers and inhibitors of glucuronidation on the biliary excretion and choleretic action of valproic acid in the rat

Valproic acid (VPA) induces an immediate choleresis in the rat which may be attributable to the osmotic properties of VPA-glucuronic acid conjugates in bile. The influence of inducers and inhibitors of glucuronidation of VPA on the biliary excretion and choleretic effect of VPA was studied. Hepatic UDP-glucuronyltransferase activity toward VPA was determined in vitro. Pretreatment with phenobarbital (75 mg/kg/day for 4 days) enhanced VPA glucuronidation; borneol (750 mg/kg) decreased VPA conjugation; 3-methylcholanthrene (20 mg/kg/day for 4 days) and galactosamine (600 mg/kg) had no effect on glucuronidation of VPA in vitro. Hepatic UDP-glucuronic acid content was decreased by borneol and galactosamine administration and was enhanced by phenobarbital and 3-methylcholanthrene pretreatment. The enzyme inducers increased the plasma disappearance of VPA in vivo but did not augment its biliary excretion or choleretic effect. Borneol and galactosamine, which inhibited the conjugation and plasma disappearance of VPA, decreased its biliary excretion and inhibited the VPA-induced increase in bile flow. Thus, the bile flow rate after VPA administration is closely related to the excretion of VPA-glucuronic acid. These data support the conclusion that the choleretic effect of VPA is due to the osmotic activity of VPA conjugates in bile.

Lack of protection against chemically induced injury to isolated hepatocytes by omission of calcium from the incubation medium

Recent evidence has renewed interest in the hypothesis that Ca plays a central role in cell death. It was previously found that Cd and CuCl2 cause loss of viability of isolated hepatocytes. It was therefore of interest to determine whether Ca was intimately involved with the toxic effect of these metals. Some of the chemicals that were previously shown to be toxic through a mechanism involving Ca (amphotericin B, lysolecithin, and Ca ionophore A23187) were also included in the study. Hepatocytes were incubated with one of these chemicals and samples taken at various time points up to 120 min for estimation of cell viability (intracellular K+ and leakage of aspartate aminotransferase) and lipid peroxidation. The toxic effects due to Cd or Cu were not ameliorated on omission of Ca from the incubation medium. Furthermore, of the other three chemicals investigated, only the toxic properties of the Ca ionophore were effectively blocked by incubation in a Ca-free medium. The results of this study do not support the hypothesis that Ca plays a ubiquitous role in the death of liver cells.

Hepatic extraction of bile acids in rats

The hepatic extraction ratio (Eh) of five common bile acids was determined by comparison of the area under the plasma concentration time curve (AUC) after administration into the femoral and portal veins of rats. Taurocholic acid (TCA), glycocholic acid (GCA), cholic acid (CA), deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA) were studied. Using equimolar doses of the bile acids (9.3mumoles), the effects of both conjugation and the number and position of hydroxyl groups on Eh were compared. TCA had the highest Eh (about 80 per cent) followed in order by GCA (65 per cent), CA (55 per cent) and CDCA (55 per cent, with DCA exhibiting the lowest Eh (40 per cent). This suggests that conjugation has a more important effect on hepatic extraction of bile acids than the number of hydroxyl groups.

Uptake of bile acids by isolated rat hepatocytes

Hepatic uptake of five common bile acids was examined in isolated rat hepatocytes. Taurocholic acid (TCA), glycocholic acid (GCA), cholic acid (CA), deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA) were studied. Uptake was extremely rapid and was linear for at least 45 sec for all bile acids tested at substrate concentrations from 1 to 400 micrometers. Both nonsaturable binding to the cell surface membrane and the initial rate of uptake (V0) of the dihydroxy bile acids (DCA and CDCA) were about ten times greater than those of the trihydroxy bile acids (TCA, GCA and CA) which correlates with the higher lipophilicity of the dihydroxy bile acids. Kinetic analysis demonstrated that uptake of these bile acids was due both to a saturable process and a linear process. The apparent diffusion constant (Dapp) of the unsaturable process for the dihydroxy bile acids was also ten times greater than that for the trihydroxy bile acids. After correction for the nonsaturable binding to the cell membrane and linear entry, the Km and Vmax for the uptake was determined. Conjugation with taurine decreased the Km of CA but not the Vmax, while glycine conjugation did not alter either parameter, suggesting that conjugation with taurine may increase its affinity for the transport system. The trihydroxy bile acids have a higher affinity but a lower transporting capacity for the saturable process than the dihydroxy bile acids. In vivo hepatic extraction appears to be more dependent on the affinity of the bile acid for the transport system than the capacity at which it can be transported.

Evaluation of hepatic storage of sulfobromophthalein in rats and dogs

Sulfobromophthalein (BSP) is stored in the liver during its passage from plasma to bile. The soluble hepatic anion-binding protein, ligandin, may be responsible for BSP storage since BSP binds to this protein. Studies were undertaken in rats and dogs to determine whether the amount of ligandin in the liver is a major determinant of storage. Ligandin content in dog liver was found to be 0.252 mg BSP bound/kg body wt which was about one-half that in rat liver (0.504 mg BSP bound/kg body wt). Relative to plasma, BSP accumulation in liver tissue (or storage of BSP) following single bolus injection of BSP was about 5 times greater in rats than in dogs. Lack of correlation between BSP accumulation and ligandin content suggests that intracellular storage of this anion is not solely determined by its binding to ligandin. Differences in the carrier-mediated transport processes on either/or both sinusoidal and canalicular sides of the hepatocyte may be responsible for this affect.

Effect of repeated oral administration on taurocholate on hepatic excretory function in the rat

The effect of repeated administration of taurocholate on bile acid pool size, biliary composition and biliary excretory capacity for bile acids and two xenobiotics was determined. The bile acid pool was increased 50 to 60% by oral administration of sodium taurocholate (300--900 mg/kg, 10 ml/kg) every 12 hr for 2 days to male Sprague-Dawley rats. Bile flow, biliary excretion of bile acids, cholesterol and phospholipid and the concentrations of phospholipid and bile acids in bile were increased in rats treated with 750 mg of taurocholate per kg. No effect was observed on Na+,K+ or Cl- levels. The biliary transport maximum for taurocholate was increased by 30% in rats treated with 750 mg/kg. In contrast, the plasma disappearance and biliary excretion of phenol-3,6-dibromphthalein and ouabain were not affected by taurocholate administration.