Changes in the serum concentration of cholesterol, triglycerides and phospholipids in the mouse and rat after administration of either chlorcyclizine or phenobarbital

Hepatic drug metabolizing enzyme induction and serum triacylglycerol elevation in rats treated with chlordiazepoxide, griseofulvin, rifampicin and phenytoin

Five days intraperitoneal administration of rats with chlordiazepoxide (0.4 mg/kg), griseofulvin (7 mg/kg), rifampicin (8. 6 mg/kg), phenytoin (4.3 mg/kg) and phenobarbitone (1.4 mg/kg; an established inducer of microsomal enzymes) caused a significant increase in serum triacylglycerol (P<0.001) and the activities of aniline hydroxylase, aminopyrine N-demethylase and p-nitroanisole O-demethylase (P<0.001). Aniline hydroxylase, aminopyrine N-demethylase and p-nitroanisole O-demethylase activities were increased 1.48-, 1.15- and 1.47-fold, respectively, in chlordiazepoxide-treated rats, 1.65-, 1.20- and 1.38-fold in griseofulvin-treated rats, 1.74-, 1.36- and 1.44-fold in rifampicin-treated rats, 1.56-, 1.29- and 1.62-fold in phenytoin-treated rats and 2.26-, 1.72- and 1.93-fold in phenobarbitone-treated rats. Chlordiazepoxide, griseofulvin, rifampicin, phenytoin and phenobarbitone increased the activity of cytosolic phosphatidate phosphohydrolase by 52, 58, 67, 73 and 82%, respectively, while the drugs elicited 50, 60, 60, 73 and 87% increases in the activity of the microsomal phosphatidate phosphohydrolase. Similarly, chlordiazepoxide, griseofulvin, rifampicin, phenytoin and phenobarbitone elicited 2.4-, 2.39-, 2.34-, 1.69- and 3.75-fold increases in serum triacylglycerol concentrations. The correlations between serum triacylglycerol concentrations and the activities of aniline hydroxylase, aminopyrine N-demethylase and p-nitroanisole O-demethylase were significant in all treatment groups (r=0.83, r=0.92 and r=0.87, respectively, n=30, P<0.001). Our results suggest that induction of hepatic enzymes by the administered drugs may lead to hypertriglyceridaemia as an adverse effect, possibly by inducing the activity of regulatory enzymes in the biosynthesis of triglyceride.

Diet-induced reduction in serum lipoproteins stimulates cell proliferation in weanling rats

Male weanling Wistar rats were fed a 4% cholestyramine diet and used as a model to demonstrate that a reduction in serum low density lipoproteins stimulates de novo cholesterogenesis leading to DNA synthesis and cell proliferation. Feeding this diet resulted in a decrease in serum very low density lipoproteins and low density lipoproteins, an increase in high density lipoproteins and an increase in de novo cholesterogenesis in liver, thymus, spleen, pancreas, kidney and lung. DNA synthesis increased only in the thymus and spleen. Histological examination of spleen, thymus and lymph nodes showed an increased number of immature cells and enhanced mitotic activity. These results suggest that a marked reduction in serum low density lipoproteins stimulates de novo cholesterogenesis, leading to enhanced DNA synthesis and cell proliferation.

Circadian rhythm of liver parameters (cellular structures, mitotic activity, glycogen and lipids in liver and serum) during three consecutive cycles in phenobarbital-treated rats

The circadian rhythm of gastric content, serum alkaline phosphatase (alk.P.), serum lipids, body weight (wt), relative (rel.) liver wt, cellular structures (by light- and electron-microscopy), mitotic activity of hepatocytes, glycogen content, protein and lipids in liver was studied in 180 male Sprague-Dawley rats orally treated at 0830-1030 with 50 mg/kg phenobarbital (PB) for 7 days. Thereafter, five PB-treated males and five controls each were studied at 4-hr intervals at 0600, 1000, 1400, 1800, 2200 and 0200 on 3 consecutive days. The lighting schedule in the colony was 12:12 = light/dark (light from 0600 to 1800). Following the rhythm of gastric emptying, the rel. liver wt showed a clear circadian rhythm with a peak at 0800. The rel. liver wt was raised in PB-treated rats at all times of the day. The circadian rhythm of cellular structures was closely related to the hepatic glycogen content which exhibited a clear rhythm with the peak also at 0800, but lowered values were found in PB-treated rats. The mitotic activity of hepatocytes was significantly increased in PB-treated rats but displayed the same circadian rhythm as controls with peaks at noon and troughs at midnight. The well-known hypertrophy of the smooth endoplasmic reticulum in PB-treated rats was not found at 0600, but was fully developed at 1400 and 2200. PB-treatment increased significantly the liver content of cholesterol, triglycerides and phospholipids. Liver cholesterol showed a clear circadian rhythm with peaks at 1800. No rhythm of liver protein, triglycerides and phospholipids was observed. In serum, levels of cholesterol were significantly elevated, those of triglycerides and alk.P. significantly lowered, while those of phospholipids were not affected by the treatment. The three serum lipids, alk.P. and beta-lipoprotein exhibited a clear circadian rhythm, while serum glucose and non-esterified fatty acids did not.

Circadian rhythm of the liver of male rats pre-treated with phenobarbital--II. Hexobarbital sleeping time and lipids content in liver and serum

The circadian rhythm of hexobarbital sleeping time and lipids content in liver and serum were studied in 226 male Sprague-Dawley rats pretreated daily at 0800-0900 with 70 mg/kg (study 1 or 3) or 50 mg/kg (study 2) phenobarbital (PB) orally for 7 days. Thereafter, eight (study 1) or five (study 2 and 3) rats each were studied at 4-hr intervals at 1000, 1400, 1800, 2200, 0200, 0600 and 1000 through the following day. The lighting schedule in the colony was 12:12 +/- light:dark (light from 0600 to 1800). The hexobarbital sleeping times of PB-pretreated rats were generally shortened compared to the controls and no circadian rhythm was observed. PB-treatment increased slightly the liver content of cholesterol, and significantly that of triglycerides and phospholipids. Liver cholesterol and phospholipids showed circadian rhythms with peaks during the dark phase. No circadian rhythm of liver triglycerides existed. In serum, levels of triglycerides and phospholipids were slightly lowered by PB-treatment, while levels of cholesterol and beta-lipoprotein were not influenced. Serum values did not exhibit circadian rhythms.

Dose-related effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in C57BL/6J and DBA/2J mice

The dose-related effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) were studied in B6D2F1/J (B6D), C57BL/6J (C57), and DBA/2J (DBA) mice. A 14-fold difference in lethality was observed in C57 and DBA mice, based upon 30-day LD50 values of 182 and 2570 micrograms TCDD/kg body wt, respectively. The 30-day LD50 for B6D mice was 296 micrograms TCDD/kg body wt. A progressive loss of body weight in all strains of mice was observed during the 30-day LD50 studies, with maximal weight losses of 24.7, 34.0, and 33.4% prior to death of C57, B6D, and DBA mice, respectively. In separate experiments, it was found that decreased feed consumption did not contribute to weight loss in C57 mice exposed to lethal or sublethal doses of TCDD until the animals were moribund. Time-course studies in C57 mice treated with 200 micrograms TCDD/kg body wt indicated that decreases in serum glucose and triglyceride concentrations and increases in hepatic triglyceride content occurred within 4 to 8 days of exposure, and were maximally altered within 17 to 21 days postexposure, concomitant with a 25% body weight loss. C57 mice fasted for 24 to 96 hr lost 18% of body weight and also exhibited alterations in glucose and lipid parameters; however, these changes were substantially different than the effects of TCDD exposure. In concert, these observations demonstrate that decreased feed consumption (hypophagia) does not account for weight loss and changes in carbohydrate and lipid metabolism in TCDD-treated C57 mice. Dose-response experiments resulted in comparable changes in glucose and lipid parameters when DBA mice were exposed to 10-fold higher doses of TCDD than C57 mice. Parallel LD50 responses and parallel changes in carbohydrate and lipid metabolism, at 10- to 15-fold differences in dose range, are indicative of a common mechanism of toxicity in TCDD-treated C57 and DBA mice.

Sex and strain differences in response to cocaine

After pretreatment with phenobarbital, female B6AF1 mice showed considerably higher serum glutamic oxaloacetic transaminase (SGOT) elevations and more periportal necrosis from a single injection of cocaine than males. This sex difference was androgen dependent. Castration or treatment with flutamide made males respond like females, while testosterone made females behave like males. There was no significant sex difference in enzymes of cocaine metabolism. When the mice were induced by exposure to pine bedding, males showed higher SGOT elevations and more centrilobular necrosis after cocaine than females. In this case, the sex difference could be attributed to increased levels of cytochrome P-450 and cocaine N-demethylase in liver microsomes. BALB/cBy mice on pine bedding showed much less liver damage from cocaine than B6AF1 mice, but they were more sensitive to norcocaine and N-hydroxynorcocaine. This difference was correlated with low levels of cocaine N-demethylase in the BALB/cBy mice. Liver microsomes from phenobarbital-induced BALB/cBy mice had less norcocaine N-hydroxylase activity than those from B6AF1 mice. These studies demonstrate that the pattern of sex and strain differences in liver damage from cocaine depends on the inducing agent and can be related to a large extent to the microsomal enzymes induced by that agent.

Triacylglycerol metabolism in the phenobarbital-treated rat

1. Various aspects of triacylglycerol metabolism were compared in rats given phenobarbital at a dose of 100mg/kg body wt. per day by intraperitoneal injection; controls were injected with an equal volume of 0.15m-NaCl by the same route. Animals were killed after 5 days of treatment. 2. Rats injected with phenobarbital demonstrated increased liver weight, and increased microsomal protein per g of liver. Other evidence of microsomal enzyme induction was provided by increased activity of aminopyrine N-demethylase and cytochrome P-450 content. Increased hepatic activity of gamma-glutamyltransferase (EC 2.3.2.2) occurred in male rats, but not in females, and was not accompanied by any detectable change in the activity of this enzyme in serum. 3. Phenobarbital treatment increased the hepatic content of triacylglycerol after 5 days in starved male and female rats, as well as in non-starved male rats; non-starved females were not tested in this regard. At 5 days after withdrawal of the drug, there was no difference in hepatic triacylglycerol content or in hepatic functions of microsomal enzyme induction between the treated and control rats. 4. After 5 days, phenobarbital increased the synthesis in vitro of glycerolipids in cell-free liver fractions fortified with optimal concentrations of substrates and co-substrates when results were expressed per whole liver. The drug caused a significant increment in the activity of hepatic diacylglycerol acyltransferase (EC 2.3.1.20), but did not affect the activity per liver of phosphatidate phosphohydrolase (EC 3.1.3.4) in cytosolic or washed microsomal fractions. A remarkable sex-dependent difference was observed for this latter enzyme. In female rats, the activity of the microsomal enzyme per liver was 10-fold greater than that of the cytosolic enzyme, whereas in males, the activities of phosphohydrolases per liver from both subcellular fractions were similar. 5. The phenobarbital-mediated increase in hepatic triacylglycerol content could not be explained by a decrease in the hepatic triacylglycerol secretion rate as measured by the Triton WR1339 technique. Since the hepatic triacylglycerol showed significant correlation with microsomal enzyme induction functions, with hepatic glycerolipid synthesis in vitro and with diacylglycerol acyltransferase activity, it is likely to be due to enhanced triacylglycerol synthesis consequent on hepatic microsomal enzyme induction. 6. In contrast with rabbits and guinea pigs, rats injected with phenobarbital showed a decrease in serum triacylglycerol concentration in the starved state; this decrease persisted for up to 5 days after drug administration stopped, and did not occur in non-starved animals. It seems to be independent of the microsomal enzyme-inducing properties of the drug, and may be due to the action of phenobarbital at an extrahepatic site.

Effects of phenobarbital upon triacylglycerol metabolism in the rabbit

1. The association between hepatic microsomal enzyme induction and triacylglycerol metabolism was examined in fasting male rabbits (2kg body wt.) injected intra-peritoneally with 50 mg of phenobarbital per kg for 10 days. 2. Occurrence of enzyme induction was established by a significant increase in hepatic aminopyrine N-demethylase activity and cytochrome P-450 content, as well as a doubling of microsomal protein per g of liver and a 54% increase in liver weight. Parallel increments in hepatic gamma-glutamyltransferase (EC 2.3.2.2) activity occurred; these were more pronounced in the whole homogenate than in the microsomes, which only accounted for 12.5% of the total enzyme activity in the controls and 17.0% in the animals given phenobarbital. Increased activity of gamma-glutamyltransferase activity was also observed in the blood serum of the test animals. 3. The rabbits given phenobarbital manifested increased hepatic triacylglycerol content and the triacylglycerol concentration of blood serum was also elevated. These changes were accompanied by a significantly enhanced ability of cell-free fractions of liver from the test animals (postmitochondrial supernatant and microsomal fractions) to synthesize glycerolipids in vitro from sn-[14C] glycerol 3-phosphate and fatty acids, when expressed per whole liver. Relative to the protein content of the fraction, glycerolipid synthesis in vitro was significantly decreased in the microsomes, presumably consequent upon the dramatic increase in their total protein content, whereas no change occurred in the postmitochondrial supernatant, possibly due to the protective effect of cytosolic factors present in this fraction and known to enhance glycerolipid synthesis. 4. Microsomal phosphatidate phosphohydrolase accounted for 85% of the total liver activity of this enzyme and its specific activity was 20-fold higher than that of the cytosolic phosphatidate phosphohydrolase (EC 3.1.3.4), when each was measured under optimal conditions. A significant increase in the activity of both enzymes per whole liver occurred in the rabbits given phenobarbital. A closer correlation between hepatic triacylglycerol content and and microsomal phosphatidate phosphohydrolase, as well as the above observation, suggest that this, rather than the cytosolic enzyme, may be rate-limiting for triacylglycerol synthesis in rabbit liver. 5. Significant correlations were observed between the various factors of hepatic microsomal-enzyme induction (aminopyrine N-demethylase and gamma-glutamyltransferase activity as well as cytochrome P-450 content) and hepatic triacylglycerol content, suggesting that that microsomal enzyme induction may promote hepatic triacylglycerol synthesis and consequently hypertriglyceridaemia in the rabbit.

Phenobarbital as a hypocholesterolemic agent in the rat and rabbit

The administration of phenobarbital was observed to inhibit the rise in serum cholesterol that occurred in rats after thyroidectomy or hypophysectomy and in rabbits after inclusion of cholesterol in their diet. This hypocholesterolemic effect of phenobarbital was found to be due to its capacity to inhibit the intestinal absorption of cholesterol in both these species. We found no evidence to suggest that phenobarbital altered the endogenous process of cholesterol metabolism in either of these two species of animals.