Selenite-induced protection of bromobenzene hepatotoxicity in male rats

Acute treatment with sodium selenite effectively reduces bromobenzene hepatotoxicity in male, Sprague-Dawley rats. Hepatocellular damage was ameliorated as shown by marked decreases in plasma alanine and aspartate aminotransferase (ALT and AST) activities. A single dose of selenite (12.5 or 30 mumol Se/kg, ip) was administered to rats at 4, 24, 48, or 72 hr before injection of bromobenzene (7.5 mmol/kg, ip). Plasma ALT and AST activities and hepatic glutathione (GSH) content were measured 24 hr after bromobenzene treatment. As the length of time of selenite pretreatment increased, the extent of reduction of bromobenzene-induced elevation in plasma enzyme activities by selenite was enhanced, and generally, in a dose-related manner with optimal protection occurring in rats pretreated 72 hr prior with selenite. However, depletion of liver GSH by bromobenzene was not affected by selenite treatment. Hepatic GSH levels and GSH detoxication enzyme activities were measured at various intervals in rats treated with selenite alone. Selenite increased hepatic GSH content 20 to 25% at both 24 and 48 hr after injection, with a return to GSH control levels at 72 hr. Selenite treatment produced slight decreases in GSH peroxidase activity but did not alter GSH S-transferase activity. These studies suggest that the reduction of bromobenzene hepatotoxicity by selenite does not involve alterations in the activity of hepatic GSH detoxication enzymes; however, the data suggest that factors in addition to selenite-induced changes in hepatic glutathione levels are also involved.

Circadian rhythm in acetaminophen toxicity: role of nonprotein sulfhydryls

A circadian rhythm in the toxicity of acetaminophen administered at different times of the day was exhibited in male mice. Peak lethality (70% dead) occurred at 1800 hr and nadir lethality (10% dead) occurred at 1000 hr. Hepatic nonprotein sulfhydryl (NPS) concentrations also varied in a circadian manner and were found to exhibit an inverse relationship to the acetaminophen lethality rhythm, with the highest hepatic concentrations occurring at 1000 hr and the lowest concentrations occurring at 1800 hr. At 1800 hr when acetaminophen lethality was greatest and NPS levels the lowest, more reactive acetaminophen metabolites were found covalently bound to hepatic macromolecules as compared to those bound at 1000 hr when NPS levels were highest and acetaminophen lethality was lowest. Hepatic cytochrome P-450 levels measured at these two daily time periods did not differ. The plasma half-life of acetaminophen was significantly decreased at the 1000 hr time period as compared to the 1800 hr time period. These findings suggest that the normal daily changes in NPS levels may influence the lethality of acetaminophen.

Circadian variations in glutathione-S-transferase and glutathione peroxidase activities in the mouse

Circadian variations in hepatic glutathione-S-transferase and glutathione peroxidase activities were found in male Swiss-Webster mice maintained under a 12/12 lighting schedule (L: 06.00-18.00 h) for 3 weeks prior to use. Hepatic glutathione-S-transferase activity was significantly elevated during the dark phase as compared to the light phase. The glutathione peroxidase activity, using two different substrates, was biphasic in nature. Using cumene hydroperoxide, the enzyme activity was significantly elevated at 18.00 and 06.00 h; nadirs occurred at 10.00, 14.00 and 22.00 h. Using hydrogen peroxide as the substrate, enzyme activity was greatest at 14.00 and 06.00 h; nadirs occurred at 10.00 and 22.00 h. Glutathione concentration also varied in a circadian manner with peak levels occurring between 06.00 and 10.00 h and the nadir occurring at 18.00 h. These studies have shown that glutathione levels and two enzymes utilizing glutathione as a cosubstrate vary in a similar circadian manner, with the highest values primarily occurring during the dark phase. Thus, these results may have considerable toxicological importance.

Effect of acute and repeated selenium treatment on hepatic monooxygenase enzyme activity in male rats

The effect of sodium selenite administered acutely or repeatedly on the biochemical components of the hepatic microsomal monooxygenase enzyme system was examined in male rats. 72 h following acute administration of selenium (2.4 mg Se/kg, i.p.), there was a significant decrease in ethylmorphine-N-demethylase activity and cytochrome P-450 levels but no change in aniline hydroxylase or NADPH cytochrome c reductase activity. Following repeated administration of selenite in the drinking water (1, 2, or 4 ppm Se) for 30 days, there was no alteration in any of the parameters measured. Following the in vitro additions of selenite to microsomes obtained from untreated rats, ethylmorphine-N-demethylase and aniline hydroxylase activities were inhibited at selenium concentrations of 10(-4) M or greater, but the inhibition achieved was less than 50%. No alterations in cytochrome P-450 levels were observed. These results indicate that selenium is a rather weak, indirect, and substrate-specific inhibitor of the hepatic monooxygenase enzyme system.

Species and sex differences in selenium inhibition of hepatic drug metabolism in rodents

Hepatic microsomal drug metabolism was assessed in male and female rats, mice and hamsters 72 hr after administration of sodium selenite. The greatest inhibitory effect of selenium on hepatic drug metabolism occurred in male rats where the magnitude of reduction in microsomal metabolism by selenium treatment of ethylmorphine and aminopyrine was disproportionately greater than the decrease in either cytochrome P-450 levels or metabolism of aniline. In contrast, the inhibition of metabolism of the three substrates in female mice and the decrease in ethylmorphine-N-demethylase in male mice were similar in magnitude to the reduction observed in cytochrome P-450 content. Hepatic microsomal metabolism in male and female hamsters was relatively resistant to the inhibitory effects of selenium. The inhibition of drug metabolism by selenium may reflect a specific effect of selenium on selected isozymes of cytochrome P-450.

Direct vasodilator activity of atropine in the rat perfused hindlimb preparation

1. The autoperfused hindlimb technique in the rat was used to determine the effects of atropine upon the peripheral vasculature. 2. Injection of atropine (0.1-0.8 mumoles) into the hindlimb perfusion circuit resulted in a dose-dependent decrease in perfusion pressure which was immediate in onset and always preceded systemic pressure changes. The duration of vasodilation lengthened with increasing doses of atropine and ranged from 1.5 to 9 min. 3. During atropine-induced vasodilation, challenging doses of acetylcholine did not alter the perfusion pressure. 4. These experiments suggest that atropine is capable of direct vasodilator activity upon the peripheral vasculature in which a muscarinic mechanism appears unlikely.