Time course of protection by N'N--diphenyl-p-phenylendiamine (DPPD) against carbon tetrachloride hepatotoxicy

Antioxidant-dependent inhibition of diquat-induced toxicity in vivo

The abilities of two experimental antioxidants (U-74006F and U-78517G), as well as the model antioxidant, diphenyl-p-phenylenediamine (DPPD), to protect against diquat-induced toxicity in male Fischer-344 rats were examined. Both experimental compounds afforded near complete protection against diquat-induced hepatotoxicity, as measured by clinical chemistry and histopathological indices. When observed, diquat-induced nephrotoxicity was also inhibited. Minimal protection was afforded by the model compound, DPPD. In follow-up studies with U-78517G, no effect on diquat-induced biliary excretion of oxidized glutathione was observed, suggesting that a shift in the thiol:disulfide ratio is not responsible for diquat-induced hepatotoxicity. These data are consistent with those from previous in vitro studies in our laboratory and are in agreement with studies by others which suggest that lipid peroxidation is an important event in diquat-induced hepatotoxicity in vivo. The antioxidant effects were largely route-independent as either oral pre-treatment alone (200 mg/kg, 24 h before diquat), intravenous pre-treatment alone (6 mg/kg, 5 min before diquat) or the combination of both treatments produced a similar degree of protection. While pre-treatment with antioxidants was quite effective, no significant U-78517G-dependent inhibition of toxicity was observed when administration was delayed by as little as 10 min post diquat. These latter data suggest that initiation of diquat-induced hepatotoxicity is rapid and that these compounds would therefore be unlikely to have clinical utility in the treatment of diquat intoxication.

Inhibition of carbon tetrachloride-induced lipid peroxidation by novel antioxidants in rat hepatic microsomes: dissociation from hepatoprotective effects in vivo

The ability of two novel antioxidants, U-74,006F and U-78,517G, as well as the known antioxidant N,N'-diphenyl-p-phenylenediamine to inhibit lipid peroxidation induced by carbon tetrachloride (CCl4) was investigated in Aroclor 1254-induced rat hepatic microsomes. All three compounds completely inhibited lipid peroxidation in microsomes as measured by the formation of thiobarbituric acid reactive substances (TBARS). Inhibition of lipid peroxidation was not a function of decreased bioactivation of CCl4, as the compounds did not substantially inhibit benzphetamine N-demethylase activity or covalent binding of [14-C]CCl4 to lipid or protein. Parallel studies examined the hepatoprotective effects of the compounds in vivo. Rats were pretreated with antioxidant or vehicle prior to administration of CCl4 (300 or 600 microL/kg i.p.). Sera were collected 24 h postadministration of CCl4 and analyzed for alanine aminotransferase (ALT) and alkaline phosphatase (ALP) activities and total bilirubin. Administration of CCl4 produced elevations in ALT, moderate changes in bilirubin, and no change in ALP activities. Histological examination of CCl4-treated livers revealed lipidosis and centrilobular necrosis. The antioxidants partially improved the clinical chemistry parameters, but had minimal effects on the histological lesion. In contrast to the complete inhibition of lipid peroxidation observed in the in vitro studies, none of the antioxidants markedly protected against CCl4-induced toxicity in vivo.

Antioxidant prevention of tumor promoter induced inhibition of mouse hepatocyte intercellular communication

The liver tumor promoters, phenobarbital (20-500 micrograms/ml), lindane (1,2,3,4,5,6-hexachlorocyclohexane, gamma-isomer; 0.1-5.0 micrograms/ml), and DDT (1,1-bis[4-chlorophenyl]-2,2,2-trichloroethane; 0.5-10.0 micrograms/ml), and the hydrogen peroxide-generating enzyme, glucose oxidase (0.01-0.10 units/ml) inhibited gap junctional intercellular communication between B6C3F1 mouse hepatocytes in primary culture. Addition of the antioxidants, superoxide dismutase (100 units/ml), DPPD (N,N'-diphenyl-1,4-phenylenediamine; 25 microM), and vitamin E (DL-alpha-tocopherol acetate; 100 microM), to tumor promoter-treated cultures prevented the inhibition of hepatocyte intercellular communication. DPPD and vitamin E, prevented the inhibition of hepatocyte intercellular communication by glucose oxidase. Superoxide dismutase had no effect on the inhibition of intercellular communication caused by glucose oxidase. These results suggest that activated oxygen species are produced during liver tumor promoter treatment of cultured mouse hepatocytes and are responsible for the inhibition of mouse hepatocyte intercellular communication by the promoters.

Mechanism of protection against carbon tetrachloride toxicity. I. Prevention of lethal effects by partial surgical hepatectomy

Both partial surgical hepatectomy and a challenge with a small dose of CCl4 depress the metabolism of xenobiotics in the liver. In fact, hepatocytes become provided with metabolic activity rates which are peculiar of either embryo or newborn rat liver. These experiments have shown that partial surgical hepatectomy prevents rats from death caused by otherwise lethal doses of CCl4. At the same time, sham-operated animals survive to a limited extent after a large dose of the halogen compound. Investigations carried out on the metabolic efficiency of liver microsomes, both in vito and in vivo, clearly demonstrate that the preventive effect against CCl4 depends mainly on the impaired metabolic activity of endoplasmic reticulum.

Ethanol-induced hepatotoxicity; experimental observations on the role of lipid peroxidation

Hepatic lipid peroxidation in vivo or in vitro as measured by UV absorption spectra of microsomal lipids or by production of TBA-reacting substances by whole liver homogenates, was studied after acute or during prolonged administration of ethanol. No evidence of peroxidative derangement of liver microsomal lipids in vivo was detected in either experimental situation, while the production of TBA-reacting substances by pooled liver homogenates incubated in vitro appeared slightly increased. Treatment with reduced glutathione (GSH and 2-mercaptopropionylglycine (2-MPG) was able to reduce fatty liver in acute and prolonged ethanol dosing, as well as the production of TBA-reacting compounds. Similar effects were obtained with 3-amino-1,2,4-triazole which was assayed only in acute experiments. By contrast, hepatic triglyceride accumulation induced by a single intoxicating dose of ethanol was not affected by preventive treatment with pyrazole which seemed to act as a pro-oxidant agent as far as the production of TBA-reacting substances is concerned. The role of lipid peroxidation as a pathogenic mechanism for acute and chronic ethanol-induced hepatotoxicity is discussed in relation to the action of anti-oxidant compounds which are active in preventing liver injury. It is concluded that lipid peroxidation is unlikely to be an important mechanism in alcohol hepatotoxicity.