Lasiocarpine: factors influencing its toxicity and effects on liver cell division

Acute Toxicity of Acrylonitrile: Effect of Diet on Tissue Nonprotein Sulfhydryl Content and Distribution of 1–14C-Acrylonitrile or Its Metabolites

The influence of diet on the acute toxicity (lethality) and distribution of glutathione as measured by tissue nonprotein sulfhydryl (NPSH) concentration following acrylonitrile (ACN) treatment was studied. The tissue distribution of radiolabeled (14C) acrylonitrile or its radioactive metabolites was also determined. The diets tested were a standard laboratory rat chow; a casein-based, complete diet; and 2 diets high in saturated fat content that were either lipotrope deficient or lipotrope supplemented. The latter 2 diets were associated with decreased weight gain in the absence of ACN and increased lethality after orally administratered ACN. The increase in lethality, while most pronounced in the group fed the lipotrope-deficient, high-fat diet, was also seen in rats fed the purified control diet. The high-fat diets were associated with increases in brain and liver NPSH concentrations (mg NPSH/100 g body weight). Following oral administration of 1–14C-ACN, blood contained the highest specific activity. In terms of fraction of administered dose, liver was found to contain the highest total fraction. In the tissues measured, a majority of radioactivity appeared bound to trichloroacetic acid (TCA) insoluble components. The sequence of tissue activity (nCi 14C per g or ml) was as follows: blood > stomach > liver, duodenum, lung, kidney > brain > adrenal. While diet affected toxicity, it did not modify this general ranking.

Identity of a biliary metabolite formed from monocrotaline in isolated, perfused rat liver

A pneumotoxic pyrrolic metabolite, previously isolated from the bile when rat liver was perfused with the pyrrolizidine alkaloid, monocrotaline, has been identified as 7-glutathionyl-dehydroretronecine. The metabolite showed a TLC spot and HPLC peak corresponding with the latter compound, and a procedure for replacing the thioether group with an ethoxy group converted the metabolite to dehydroretronecine 7-ethyl ether, confirming that the glutathionyl moiety was attached to the 7-position of dehydroretronecine. The same metabolite was detected in bile from rat liver perfused with retrorsine, which is a diester alkaloid similar to monocrotaline, whereas it was not formed from heliotrine, an alkaloid lacking the 7-ester function.

Recovery of the pyrrolic nucleus of pyrrolizidine alkaloid metabolites from sulphur conjugates in tissues and body fluids

Reactive pyrrolic metabolites formed when pyrrolizidine alkaloids are given to rats can alkylate soluble and tissue-bound thiol groups. Pyrrolic thioethers thus formed are relatively stable, and may persist in tissues for long periods. A simple procedure has been developed for recovering the nucleus of the pyrrolic metabolite from such S-binding, whether in solution or attached to solid tissues, in an easily identifiable form. The thioether bond was broken by silver nitrate and the pyrrolic moiety allowed to react with ethanol or methanol to form an ethoxy or methoxy derivative. The chemical basis of the procedure was established by model experiments on a preparative scale, but for small scale recovery from tissues, pyrrolic ethers were extracted and identified by TLC, HPLC, capillary GC and mass spectrometry. Because the pyrrolic derivatives thus formed were easily recognised and unrelated to any physiological compound, the recovery method described, especially when applied to blood samples, provided a way to monitor animals for previous exposure to toxic pyrrolizidine alkaloids.

Influences of various xenobiotic inducers on cytocidal toxicity of lasiocarpine and senecionine in primary cultures of rat hepatocytes

The influences of in vivo pretreatment with phenobarbitone (PB), 3-methylcholanthrene (3-MC), 2,2',4,4',5,5'-hexachlorobiphenyl (HCBP), and 3,3',4,4'-tetrachlorobiphenyl (TCBP) on cytocidal hepatotoxicity of two pyrrolizidine alkaloids, lasiocarpine (LC) and senecionine (SC), were compared in short-term primary cultures of rat hepatocytes. Toxicity was measured by release of lactate dehydrogenase (LDH) into culture medium at 24 h. LC was slightly more toxic to control hepatocytes than SC in the graded response range of 10-160 microM. PB and HCBP (a PB-type polychlorobiphenyl inducer) similarly potentiated toxicity of SC, and each diminished the degree to which cell killing by LC and SC was inhibited by SKF-525-A. By comparison, 3-MC and TCBP (a 3-MC-type PCB inducer) each diminished toxicity of SC but had little effect on toxicity of LC. Alpha-naphthoflavone (ANF) potentiated toxicity of both LC and SC in hepatocytes induced by 3-MC or TCBP but had little effect on responses of hepatocytes induced by either PB or HDBP. These results indicate that xenobiotics that induce similar patterns of cytochrome P-450 isozymes have qualitatively similar modulating influences on cytocidal hepatotoxicity of pyrrolizidine alkaloids in primary cultures. However, the observed modulating effects could not be explained solely on the basis of altered activation rates by the cytochrome P-450 species known to be induced by the various xenobiotics.

Protective action of zinc against pyrrolizidine alkaloid-induced hepatotoxicity in rats

The influence of Zn on the acute hepatotoxicity of pyrrolizidine alkaloids (PAs) was determined in male rats. Zinc, 72 mumol/kg as ZnCl2, was administered ip for 3 consecutive days, followed 16 h after the last dose by a single ip injection of purified mixed PAs (80, 120, or 160 mg/kg) obtained from tansy ragwort (Senecio jacobaea). Hepatotoxicity of the PAs was assessed by measuring the activities of plasma glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) and by histological examination of the liver. There was a dose-dependent increase in plasma GOT and GTP 24 h after PA administration, whereas no significant increase of these enzymes was seen after administering Zn alone. The 7-fold increase in plasma GOT and 12-fold increase in GPT after PA (120 mg/kg) were reduced to 2.4- and 2.1-fold, respectively, by Zn pretreatment. The PA-induced liver necrosis was either reduced in severity or abolished by Zn when the PA dose was 80 or 120 mg/kg. These results suggest a protective effect of Zn against PA hepatotoxicity. The protective effect was associated with a marked increase in liver metallothionein and a significant decrease in hepatic cytochrome P-450 content, aminopyrine N-demethylase activity, and in vitro microsomal conversion of the PAs to pyrroles. Liver nonprotein sulfhydryls were unchanged. The possible role of metallothionein in the sequestration of pyrrole metabolites merits further investigation.

Rapid hepatoma induction in rats by dipentylnitrosamine and its modification by other carcinogens

Dipentylnitrosamine (DPN), administered in the diet at a concentration of 2,000 ppm to Fischer-344 rats, produced hepatomas in 27% of both males and females within 8 weeks. Bile-duct carcinomas were also produced. All animals also showed liver nodular hyperplasia and bile-duct cell proliferation. Feeding DPN at 1,500 ppm produced hepatomas or bile-duct carcinomas in 7% of the males and varying degrees of nodular hyperplasia and duct cell proliferation after 8 weeks of feeding. The total cumulative dose after feeding 2,000 ppm DPN was of the order of 7,700 mg/kg body weight in males and 8,200 mg/kg in females. There was a 28% weight decrement in animals feed 2,000 ppm DPN when compared to controls but no mortality. DPN carcinogenicity was enhanced when DPN was combined with a liver carcinogen, cycasin (fed as cycad not flour, equivalent to 160 ppm cycasin). No effect on DPN carcinogenicity was found in rats fed DPN and lasiocarpine, another known liver carcinogen with antimitotic activity. Neither was there any effect of DPN carcinogenicity in rats fed DPN and N-butyl-N-(4-hydroxybutyl) nitrosamine or N-methyl-N'-nitro-N-nitrosoguanidine. In contrast, the trisodium salt of nitrilotriacetic acid reduced the hepatocarcinogenic action of DPN. The enhancement of DPN carcinogenicity with cycasin is compatible with the hypothesis that DPN-induced liver cell populations (hyperplastic nodules) can resist the cytotoxic effect of cycasin and differentiate rapidly towards hepatomas, while other areas of the liver are suppressed.

The role of liver glutathione in the acute toxicity of retrorsine to rats

(1) Two procedures have been used to change the glutathione concentration in the livers of male rats. The glutathione level is increased to about double that of the controls, 0.5 h after the administration of cysteine (200 mg/kg, i.p.) and to about 25% that of controls, 1 h after the administration of 2-chloroethanol (30 mg/kg, i.p.). (2) The acute LD50 of retrorsine to rats (42 mg/kg) is increased by pretreatment with cysteine to 83 mg/kg and decreased by pre-treatment with chloroethanol to 23 mg/kg. In all three groups, deaths are accompanied by haemorrhagic centrilobular necrosis of the liver. (3) 2 h after the administration of retrorsine to rats (60 mg/kg), the levels of pyrrolic metabolites in the livers of animals pre-dosed with cysteine or chloroethanol are respectively about 60% and 200% those of rats given no pre-treatment. (4) Neither in the normal nor in the pre-treated rats dose retrorsine (60 mg/kg) cause a detectable fall in liver glutathione concentration 0.5-4 h after dosing. By 24 h, the glutathione concentration in the livers of the retrorsine-dosed rats is higher than those of the corresponding controls. There was no significant change in the liver weights of the treated rats relative to the controls. (5) Treatment of rats with retrorsine (60 mg/kg) causes a fall in the liver concentrations of cytochrome P-450, 24 h after dosing. This loss of cytochrome P-450 is increased in rats pre-treated with chloroethanol. The concentrations of cytochrome b5 in the same animals are not significantly reduced.