The immediate and long term effects of clofibrate on the metabolism of the perfused rat liver.

A study was made of the immediate effects of CPIB (chlorophenoxy-isobutyrate) and of the effects of clofibrate (ethyl-CPIB) pretreatment on the metabolism of the perfused liver. Both treatments caused an increased hepatic uptake of lactate and free fatty acids. Pretreatment with clofibrate resulted in a decrease in perfusate glucose, an increase in ketogenesis and a decreased output of very low density lipoprotein triacylglycerol. A more oxidized redox state of both the cytosol and the mitochondria was indicated by decreased ratios of perfusate [lactate]/[pyruvate] and [3-hydroxybutyrate]/[acetoacetate] respectively. Increased hepatic O₂ consumption was associated with the increased liver weight of rats treated with the drug for 1 week. The fate of free fatty acids was followed by infusing [1—¹⁴Cloleate. The increased oxidation of oleate to both CO₂ and ketone bodies in livers from animals pretreated with clofibrate was accompanied by a corresponding decreased incorporation of ¹⁴C into very low density lipoprotein triacylglycerol. Lipogenesis was depressed upon addition of CPIB to the perfusate, but was increased after pretreatment with clofibrate. No changes in cholesterol synthesis were detected. A hypothesis to account for the hypolipidaemic and other effects of clofibrate pretreatment is advanced. This is based on a primary enhancement of fatty acid oxidation accompanied by a reciprocal decrease in hepatic triacylglycerol secretion. It is suggested that increased peroxisomal oxidation of fatty acids may be a cause of the decreased redox potential. A consequent activation of pyruvate dehydrogenase would explain both the changes in carbohydrate metabolism and the increase in lipogenesis.     

Effects of S-8527 (1,1-bis(4'-(1'-carboxy-1'-methylpropoxy)phenyl) cyclohexane), a new hypolipidemic compound, on cholesterol and lipoprotein metabolism in rats.

The effects of S-8527 (1,1-bis[4'-(1Prime;-carboxy-1Prime;-methylpropoxy)phenyl]cyclohexane on cholesterol and lipoprotein metabolism were examined and compared with those of clofibrate in rats under various experimental conditions. When rats were given a daily oral dose of S-8527 for 8 days, the incorporation of [¹⁴C]acetate into liver cholesterol was not inhibited at the dose of 30 mg/kg of S-8527. which was reported to decrease the serum cholesterol significantly, but the higher dose (300 mg/kg) of S-8527 decreased the incorporation of [¹⁴C]acetate into liver cholesterol. Under these experimental conditions, clofibrate (300 mg/kg) caused a decrease in labeled cholesterol in the liver. Oral doses of S-8527 or clofibrate for 8 days did not affect the incorporation of [¹⁴C]mevalonate into liver cholesterol. Also, when the drugs were added to normal rat liver slices, the effects of S-8527 were not so marked as those seen with clofibrate. Oral doses of S-8527 (30 mg/kg) or clofibrate (300 mg/kg) for 8 days decreased the incorporation of [¹⁴C]lcucine into the protein of serum lipoproteins. S-8527 and clofibrate did not affect the body retention of radioactivity after the injection of labeled cholesterol into rats. In view of the above results, it is conceivable that S-8527 primarily inhibits either the secretion of lipoproteins containing cholesterol into plasma or the formation of lipoproteins containing cholesterol in the liver, or both, and secondarily interferes with the biosynthesis of liver cholesterol.     

Effects of chloroquine on lipid metabolism of mouse pancreatic islets

Pancreatic islets accumulate the amphiphilic drug, chloroquine, which leads to a marked impairment of the insulin production of the B-cells in vitro. In this study the effects of the drug on islet lipid metabolism in vitro were investigated. It was found that exposure of islets to chloroquine (10^?5 M) for one week induced an increase of d-[U-¹⁴C]glucose incorporation into phospholipids and triacylglycerols of about two-fold and into diacylglycerols of about six-fold. Furthermore, two-dimensional thin-layer chromatography showed an increased rate of d-[U-¹⁴C]glucose incorporation into all major phospholipid classes. A 17% increase of the total islet phospholipid content indicated that an accumulation of islet cell phospholipids occurred. Pulse-chase experiments with d[U-¹⁴C]glucose showed that the rate of degradation of islet phospholipids was decreased after prolonged exposure to chloroquine. No short-term effects of chloroquine upon either the de novo biosynthesis of islet lipids or upon phospholipid degradation could be demonstrated. The present data therefore suggest that chloroquine impairs the rate of islet lipid degradation in long-term experiments, which may be a primary lesion in the sequence of events leading to functional impairment of the B-cell.     

Cationic amphiphilic drugs as a potential tool for modifying phospholipids of tumor cells. An in vitro study of chlorpromazine effects on Krebs II ascites cells

[³²P]orthophosphate and [U-¹⁴C]glycerol incorporation into Krebs ascites cell lipids was studied in vitro in the presence of chlorpromazine (CPZ)¹. At concentrations not exceeding 0.1 mM, the drug produced no cell damage within 3 hr incubation. Under these conditions, CPZ inhibited the incorporation of [³²P]orthophosphate into phosphatidylcholine and phosphatidylethanolamine and of [U-¹⁴C]glycerol into phosphatidylcholine and triglycerides, in a dose-dependent manner. On the other hand, synthesis of phosphatidic acid and phosphatidylglycerol was greatly enhanced, whereas phosphatidylinositol showed no major change. These results are compatible with an inhibition of phosphatidate phosphohydrolase, redirecting phospholipid biosynthesis towards the anionic classes. In vitro treatment of cells for 3 hr induced profound changes of phospholipid composition, which displayed a relative increase of phosphatidylglycerol and phosphatidic acid at the expense of phosphatidylcholine and phosphatidylethanolamine. The use of amphipathic cationic drugs can thus offer an interesting model for studying the relationship between cell proliferation and membrane phospholipid biosynthesis.     

Low density lipoprotein-lowering and high density lipoprotein-elevating effects of nicardipine in rats

Oral administration of a calcium antagonist, nicardipine hydrochloride (simply designated as nicardipine), in doses of 10-100 mg/kg tended to decrease total serum cholesterol and to increase high density lipoprotein (HDL) cholesterol in the normal rat. These effects of nicardipine were much greater than those of clofibrate, a standard cholesterol-lowering drug. Neither nicardipine nor clofibrate caused significant alteration in serum triglyceride and phospholipid. In hypercholesterolemic rats, nicardipine increased significantly HDL cholesterol with a reduction of total serum cholesterol, whereas clofibrate did not change HDL cholesterol. Separation of serum lipoproteins either by ultracentrifugation in various densities of KBr-NaCl solution or by polyacrylamide gel electrophoresis demonstrated that nicardipine increased preferentially HDL2 (density: 1.063-1.125), with a reduction of the low density lipoprotein (LDL) (density: 1.006-1.063) level in hypercholesterolemic rats. Serum triglyceride and liver phospholipid were increased slightly by nicardipine with no clear dose-dependency. Clofibrate also increased serum triglyceride. In normal rats, neither nicardipine nor nicotinic acid inhibited sterol biosynthesis from [1-14C]acetate in the liver, whereas clofibrate inhibited sterol production. Oral administration of [4-14C]cholesterol to hypercholesterolemic rats indicated that nicardipine had no inhibitory effect on the intestinal absorption of cholesterol.     

Disposition of clofibrate in the rat: Acute and chronic administration

The disposition of 1-[¹⁴C]clofibrate (0.4 $\text{mmole}\text{kg}$) was studied in rats after acute (single dose) and chronic (b.i.d., for 14 days) administration. With a single dose (orally or by intraperitoneal injection) of clofibrate, most (~90 per cent) of the ¹⁴C-dose appeared in the urine within 24 hr and the recovery of ¹⁴C from the urine and feces was nearly quantitative within 72 hr. Little fecal excretion of ¹⁴C (< 5 per cent) occurred after a single or chronic clofibrate administration. Clofibrate was readily absorbed and eliminated, as evidenced by a rapid increase in plasma ¹⁴C level within 90 min and a calculated biological half-life of 4.1 hr. The pharmacokinetic profile of ¹⁴C-elimination in rats was unaffected by pretreatment with cholestyramine. Clofibric acid [2-(4-chlorophenoxy)-2-methylpropionic acid] was identified as the major metabolite in plasma (~97 per cent) whereas the glucuronide of clofibric acid was the main urinary and biliary metabolite (~96 per cent). Clofibric acid, as the free acid and glucuronide form, accounted for 99 per cent of the total ¹⁴C-dose in rats, and unchanged clofibrate was not detected in any of the biological samples. Two unidentified, minor urinary metabolites were also detected. In cannulated bile duct studies, it was found that [¹⁴C]clofibrate, as clofibric acid, was rapidly and efficiently excreted in the bile. The biliary excretion rates of ¹⁴C and of the glucuronide of clofibric acid were also not altered by phenobarbital pretreatment. Chronic treatment with [¹⁴C]clofibrate did not alter the qualitative or quantitative nature of biotransformation in vivo. An increased rate of urinary ¹⁴C-elimination was observed following chronic 1-[¹⁴C]clofibrate treatment, with concomitant reductions in blood and heart ¹⁴C-content and an elevation in ¹⁴C-content of epididymal fat tissue. Subcellular fractionation of liver, from rats given [¹⁴C]clofibrate chronically, indicated an increased distribution of ¹⁴C into mitochondria and peroxisomes. Tissue ¹⁴C-levels, achieved in these in vivo studies, were an order of magnitude lower than those required for the pharmacological activities of clofibrate and clofibric acid in vitro.     

Effects of various antihypertensive agents on lipid metabolism: alterations in the pattern of lipids synthesized from [14C]oleate in rat liver in vitro

The effects of five antihypertensive agents on lipid biosynthesis from [1-14C]oleate were studied in rat liver minces. At a level of 1 mM, propranolol and prazosin increased the incorporation of [14C]oleate into diglycerides and cholesteryl esters by two- to fourfold and increased total phospholipid labeling by 20-30%. Chlorthalidone and metoprolol at 1 mM also stimulated the incorporation of [14C]oleate into phospholipids and diglycerides (20-50%) but did not affect its incorporation into triglycerides or cholesteryl esters. All four of the compounds statistically significantly inhibited the incorporation of [14C]oleate into phosphatidylcholine by 12-37% but stimulated incorporation into phosphatidylinositol by 17-95%. Nadolol differed from the other compounds in that it did not show selective effects but rather inhibited the incorporation of [14C]oleate into all lipid classes by approximately 50%. The data are discussed in terms of possible mechanisms involved in the lipid synthesis patterns and suggest the possibility that plasma lipid/lipoprotein changes observed in patients undergoing antihypertensive therapy may reflect, in part, altered hepatic lipid synthesis.     

Effect of polychlorinated biphenyls (PCBs) administration on phospholipid biosynthesis in rat liver

The effect of PCBs or phenobarbital on the biosynthesis of phospholipids in hepatic endoplasmic reticulum of rats was studied by the intraperitoneal injection of [³²P]orthophosphate, [Me?¹⁴ C]choline or [2?³H]glycerol. Significant increases in liver microsomal phospholipid content after the administration of either PCBs or phenobarbital indicated the actual proliferation of endoplasmic reticulum membranes. The rate of both [³²P] and [¹⁴C] incorporations into microsomal choline-containing phospholipids, such as phosphatidylcholine, sphingomyelin and lysophosphatidylcholine, was reduced to one fifth by PCBs administration compared with control animals. The incorporation of [³²P]orthophosphate into phosphatidylethanolamine or other phospholipid classes was less or not affected, respectively, by PCBs administration. The specific inhibitory effect of PCBs on the incorporation into cholinecontaining phospholipids was not observed when [2?³-H]glycerol was used as a precursor. Phenobarbital administration, however, increased significantly the rate of [³²P] incorporation into liver phospholipids, especially phosphatidylcholine. It is suggested that the increase in microsomal phospholipid content by PCBs administration is not due to the stimulation of synthesis but to the inhibition of the catabolism of membrane phospholipids and that the increase in content caused by phenobarbital is due at least in part, to the stimulation of synthesis. The possible site(s) of PCBs-induced inhibition of phospholipid biosynthesis in rat liver is discussed.     

Metabolic effects of clofibrate and of cholestyramine administration to dogs

Dogs were given clofibrate, 50 mg/kg of body weight/day for 6 days or 50 mg/kg of body weight/day for 2 days, followed by 40 mg/kg of body weight every 30–36 hr over an additional 9-day period. Cholestyramine, 0.7 g/kg of body weight/day, was administered to an additional group of dogs over an 11-day period. Plasma cholesterol concentrations in dogs treated with clofibrate for either 6 or 11 days decreased 24 per cent and decreased 16 per cent in dogs given cholestyramine. ¹⁴C incorporation into bicarbonate from 3-¹⁴C-pyruvate, from [U-¹⁴C]-L-alanine and from [1-¹⁴C]glucose was significantly increased by liver slices from clofibrate-treated dogs when compared with tissue from control animals, but cloflbrate did not result in any changes in [¹⁴C]cholesterol or [¹⁴C]fatty acid formation from either [3-¹⁴C]pyruvate or from [U-¹⁴C]alanine. [¹⁴C]Cholesterol formation from 6-[¹⁴C]glucose was virtually abolished in liver slices taken from dogs treated with clofibrate for 11 days. Liver slices from dogs treated with cholestyramine showed a 3.5- to 4.0-fold increase in ¹⁴C incorporation into cholesterol from both [3-¹⁴C]pyruvate and [6-¹⁴C] glucose, and a significant increase in ¹⁴C incorporation from [1-¹⁴C]glucose into bicarbonate. Clofibrate administration resulted in decreased liver glycogen concentrations and decreased ¹⁴C incorporation into glycogen from [¹⁴C]glucose, but ¹⁴C incorporation into glycogen from [U-¹⁴C]L-alanine was unaffected. If the hypocholesterolemic effect of clofibrate in dogs results from decreased cholesterol synthesis, changes in glucose metabolism may form the metabolic basis rather than alterations in pyruvate (or acetyl CoA) metabolism.     

In vivo meal model for the evaluation of agents which affect the absorption of triglyceride and cholesterol

A meal model in which rats were trained to consume within 2 hr a high fat meal containing glycerol tri[1-¹⁴C]oleate and [³H]cholesterol was compared to the corn oil bolus model. In the meal model, dietary triglyceride was absorbed from the small intestine faster during the first 6–8 hr and more completely than intubated corn oil, as determined by analysis of intestinal contents, serum radioactivity, serum triglycerides, adipose tissue and liver lipids. The effects of Cholestyramine, Colestipol and Pluronic L-101 (1% dietary admixes) on these variables were evaluated for 5 days in the meal model. Lipid absorption during a 72-hr period was reduced by all compounds. The per cent excretion of glycerol tri[1-¹⁴C]oleate was increased significantly by Pluronic L-101 (10-fold), Cholestyramine (5.7-fold) and Colestipol (2.7-fold). The excretion of [³ H]cholesterol was enhanced significantly by Cholestyramine (1.6-fold) and Colestipol (1.3-fold). The following observations were made 4hr after the initiation of the meal. Pluronic L-101 increased significantly the retention of glycerol tri[1-¹⁴C]oleate and [³H]cholesterol in stomach (380 and 375 per cent, respectively), and of glycerol tri[1-¹⁴C]oleate in the small intestine (1100 per cent). The percent of intestinal lipid remaining as triglyce-ride from the intestinal lumen was increased significantly by Pluronic L-101 (160 per cent). Pluronic L-101 reduced significantly [¹⁴C]lipid and [³H]cholesterol in liver; Cholestyramine and Colestipol suppressed only [³H]cholesterol. In adipose tissue, Pluronic L-101 treatment reduced significantly [¹⁴C]lipid content only; Cholestyramine and Colestipol suppressed selectively [³H]cholesterol. After 5 days of treatment, only Pluronic L-101 treatment resulted in significantly reduced serum triglycerides (32 per cent), cholesterol (21 per cent) and glucose (15 per cent). These data suggest that this in vivo meal-feeding model provides a physiological technique for evaluating agents affecting lipid absorption.     

Effect of phenobarbital on methyl transfer between methylated drugs and hepatic microsomal phospholipids.

The effect of phenobarbital on the incorporation of the label from N-[¹⁴C-Me]nicotine and [¹⁴C]formaldehyde into hepatic phospholipids of the rat has been studied. ¹⁴C was utilized for the formation of methylated phospholipids from both precursors. Phenobarbital elicited no significant action either on the synthesis of total hepatic phospholipids or on the incorporation of radioactivity into the total or individual liver phospholipid fractions. However, this treatment increased phospholipid content and the uptake of the label from nicotine into microsomal phospholipids. Phenobarbital raised microsomal phosphatidylethanolamine, -choline (PC), -serine (PS), and lysophosphatidylcholine contents and the incorporation of ¹⁴C-labeled methyl groups from nicotine into PC and PS fractions. Radioactivity from [¹⁴C]formaldehyde was also incorporated into hepatic phospholipids. Phenobarbital however, had no significant effect on the incorporation either into total or microsomal phospholipids. Comparing the utilization of ¹⁴C for synthesis of liver microsomal phospholipids from N-[¹⁴C-Me]nicotine or [¹⁴C]formaldehyde with the natural methyl donor, l-[¹⁴C]-Me]methionine, greater amounts were taken up from methionine than nicotine or formaldehyde. The methyl group of nicotine was probably incorporated into phospholipids via the metabolic pool; the enhancing effect of phenobarbital on this process was associated with increased metabolism and with increased methyl transfer into methyl group containing microsomal phospholipids.     

Studies on the effect of xylitol on oxalate formation.

The excretion of [¹⁴C]oxalate after parenteral administration of [U-¹⁴C]xylitol, [U-¹⁴]fructose, [U-¹⁴C]glucose [U-¹⁴C]sorbitol and [U-¹⁴C]glycine has been studied in normally fed rats. All of these compounds were about equally effective as precursors of the urinary oxalate. The effect of xylitol on the NAD⁺-dependent catalytic oxidation of glyoxylate to oxalate has also been investigated in rat liver cytosol, this reaction being the last step on the oxalate biosynthetic pathway. Xylitol slightly decreased oxalate production from glyoxylate in this system. These results are discussed in relation to the observation that the clinical use of xylitol for parenteral nutrition is sometimes complicated by renal failure with deposits of calcium oxalate in the renal tubules.     

AMIODARONE-INDUCED PHOSPHOLIPIDOSIS: AN IN VIVO [14C]-ACETATE UPTAKE STUDY IN RAT

ABSTRACT

Amiodarone (AD), a potent antiarrhythmic drug, is often associated with several adverse effects. It is shown to accumulate phospholipids in various tissues, and the impaired catabolism of phospholipids has been implicated in AD-induced phospholipidosis. The synthesis of phospholipids in tissues has not been dealt with. Hence, the incorporation of [¹⁴C]-acetate into phospholipids has been studied to understand the AD-induced phospholipidosis in lung and liver. A significant increase in lung and liver phospholipids was observed after 21 and 28 days of AD (175 mg/kg body weight/day) treatment. In the lung and liver, the incorporation of [¹⁴C]-acetate into all phospholipid fractions was elevated, while in the lung mitochondria phosphatidylcholine, phosphatidyl ethanolamine and the cardiolipin levels were significantly increased. The results indicate that, in addition to the impaired catabolism of phospholipid, AD treatment resulted in increased phospholipid synthesis.     

The effect of partial hepatectomy on the toxicity of ethylene glycol, glycolic acid, glyoxylic acid and glycine

Glycine, glycolate, ethylene glycol and glyoxylate were force-fed to male rats on which partial hepatectomy had been performed. Urine was collected for 48 hr and assayed for oxalate and glyoxylate. Partial hepatectomy increased the toxicity of glyoxylate, decreased the toxicity of ethylene glycol and glycolate, and increased the urinary oxalate of rats fed glyoxylate, but not of rats fed ethylene glycol, glycine and glycolate. Liver regeneration was not altered by the oxalate precursors employed. [U-¹⁴C]Glycine, [U-¹⁴C]ethylene glycol, [U-¹⁴C]-glycolate, and [U-¹⁴C]glyoxylate were oxidized to [U-¹⁴C]oxalate by the isolated perfused liver. The order for increased oxalate synthesis was glycine, ethylene glycol, glycolate and glyoxylate. The results suggest that the toxicity of ethylene glycol and glycolate is due to the formation of metabolic products such as glyoxylate or oxalate. The liver was identified as the major source of endogenous oxalate synthesis in the rat.     

Uptake and metabolic rate of liposome-entrapped [U-14C]glutamate in rats

The metabolic fate of [U-¹⁴C]glutamate administered intravenously to rats in aqueous solutions or entrapped in vescicles of phospholipids extracted from rat brain has been studied. When [U-¹⁴C]glutamate was administered entrapped in liposomes, higher serum radioactivity levels and higher radioactivity uptake by liver and brain were observed. Rats treated with [U-¹⁴C]glutamate entrapped in liposomes exhibited an increased glutamate oxidation shown by increased expired [¹⁴C]CO₂ and a modified elimination of the radioactivity in urine.     

Influence of clofibrate on liver microsomal hydroxylation of cholesterol and androstenedione

The liver microsomal metabolism of 4-[4-¹⁴C]androstene-3,17-dione and [4-¹⁴C]cholesterol was studied in control and clofibrate-treated rats.In the control rat 25 per cent of androstenedione metabolites were hydroxylated at the 6β-position. Another 25 per cent were recovered as 16-oxygenated derivatives and minor amounts (5 per cent) were hydroxylated at the 6α- or a 7α-position. Clofibrate stimulated all the hydroxylation reactions of this compound. The 6β-hydroxylation was elevated by 100 per cent, the 7α-hydroxylation by 70 per cent, and the 6α- and 16α-hydroxylations by 50 per cent. Furthermore, following treatment with clofibrate, the ratio between 17β-hydroxy-4-androstene-3,16-dione and 16α-hydroxy-5-androstene-3, 17-dione increased from 0.15 to 0.68. The activity of the 17β-hydroxysteroid oxido-reductase increased by 100 per cent, whereas the 3β-hydroxysteroid oxidoreductase and 5α-reductase activities were only slightly affected.The 7α-hydroxylation of labelled cholesterol was uninfluenced by treatment with clofibrate.It is suggested that clofibrate stimulates the activity of the enzyme system involved in the hydroxylation of drugs in the liver.     

Studies on the metabolic pathway of the acetyl group for acetylcholine synthesis

Glucose and pyruvate are the most effective precursors of the acetyl moiety of acetylcholine in mammalian brain; the metabolic intermediates between pyruvate and acetylcholine, however, are unknown. The following data suggest that citrate is not the sole intermediate of the acetyl group for acetylcholine synthesis in rat brain slices or synaptosomes: (1) 2.5 mM (?)-hydroxycitrate decreased acetylcholine synthesis from [U-¹⁴C]glucose by only 25 per cent; (2) inhibition of citrate transport out of mitochondria by n-butylmalonate or 1,2,3-benzenetricarboxylate variably affected acetylcholine synthesis; and (3) high concentrations of nonradioactive citrate decreased the synthesis of acetylcholine but did not decrease the specific activity of the acetylcholine synthesized from [U-¹⁴C]glucose. even though the uptake of citrate into the synaptosomes under these experimental conditions was approximately five times greater than the uptake of glucose. Other possible acetyl donors altered acetylcholine synthesis. Acetylcarnitine stimulated synthesis in brain slices, and carnitine stimulated synthesis by synaptosomes.The specificactivity of the acetylcholine synthesized from [U-^14Cglucose by synaptosomes was decreased by N-acetyl-l-aspartate (10mM), acetyl CoA (1 mM), and acetyl phosphate (10mM) which is consistent with these compounds acting as direct acetyl donors. Acetate (10 mM) did not affect either the amount or specific activity of the acetylcholine synthesized. Further evidence of compartmentation of cytoplasmic acetyl CoA is presented. The cytoplasmic acetyl CoA for acetylcholine synthesis is distinguishable from the cytoplasmic acetyl CoA for lipid synthesis. (?)-Hydroxycitrate inhibited acetylcholine synthesis without inhibiting lipid synthesis from [U-¹⁴C]glucose. However, when 3-hydroxy[3-¹⁴C]butyrate was used as substrate, (?)-hydroxycitrate inhibited incorporation into lipids twice as much as incorporation into acetylcholine. [U-¹⁴C]Glucose metabolism by infant brain slices was more sensitive than adult brain slices to (?)-hydroxycitrate. However, the response to the other compounds which interfere with citrate metabolism was similar in slices from adult and infant brains.     

In vivo studies on the effect of garlic oil in mice

The in vivo effect of garlic oil on protein, glycogen and lipid metabolism as well as on oxidative phosphorylation has been studied in liver and kidneys of mice. The incorporation of [¹⁴C] leucine into proteins of the liver is virtually unchanged by treatment with garlic oil whereas the labelling of proteins of the kidneys is reduced. Garlic oil enhances the incorporation of [¹⁴C] glucose into hepatic glycogen, produces a temporary rise in blood glucose levels and depletes liver glycogen. Renal glycogen levels and incorporation of [¹⁴C] glucose into renal glycogen remain unchanged following garlic oil treatment. The compound has little or no effect on the incorporation of [¹⁴C] acetate into hepatic and renal lipids. Garlic oil impairs partially the oxidative phosphorylation associated with the oxidation of ascorbate in hepatic mitochondria without affecting noticeably the P/O ratio during the oxidation of glutamate and succinate.     

Effect of typical inducers of microsomal drug-metabolizing enzymes on phospholipid metabolism in rat liver.

The effect of administration of phenobarbital (PB), polychlorinated biphenyls(PCBs) or 3-methyl-cholanthrene (3-MC) on the metabolism of phospholipids in rat liver was studied by the i.p. injection of [³²P]orthophosphate or [Me-¹⁴C]choline chloride. The inducers were given to animals for 2 successive days PB had no significant effect on the incorporation rate of ³²Pi into liver microsomal phospholipid classes 48 hr after the first administration. The rate of incorporation of [¹⁴C]choline into phosphatidylcholine (PC) in both subcellular components of the liver and blood plasma decreased slightly at this experimental period. In addition, the ratio of [¹⁴C] sp. act. of phosphorylcholinc to that of microsomal phosphulipid was considerably higher on PB-trcated rats as compared with the ratio in control rats. These and previous findings strongly suggest that proliferation of liver endoplasmic reticulum (ER) membranes induced by PB would be accompanied by the stimulation of phospholipid synthesis at the early process of induction and subsequently followed by the decrease in turnover rate of microsomal phospholipids. The administration of PCBs, on the other hand, caused strong inhibition of both ³²Pi and ¹⁴C incorporation into PC in liver subcellular fractions. The secretion of PC from liver cells to blood plasma was also strongly depressed. In addition, phospholipid catabolizing activity was found to be depressed in the liver. These results indicate that hypertrophy of ER membranes in the liver after PCBs administration could be due to a depression of both secretion of lipoprotein from liver cells to plasma and catabolic activity toward membranous phospholipids in liver cells. The decrease in ³²Pi incorporation into liver microsomal PC was also observed in rats treated with 3-MC. There occurred a considerable accumulation of ¹⁴C activity in phosphorylcholine in the liver of rats treated with either PCBs or 3-MC, suggesting strongly that these drugs caused an inhibition of PC synthesis at the site of CDP-choline formation, namely the inhibition of the reaction catalyzed by cholinephosphate cytidylyltransferase.     

Sequential first-pass elimination of a metabolite derived from a precursor

We examined data from our previous studies in which we not only delivered perfusate containing tracer concentrations of [¹⁴C]phenacetin and its metabolite [³H]acetaminophen under constant perfusate flow (10 ml/ min/ liver) into the rat liver preparation just once, but also recirculated fresh reservoir perfusate containing a tracer dose of [¹⁴C]phenacetin through the same rat liver preparation. From the single-pass studies, estimates of f_m, the fractional rate of conversion for [¹⁴C]phenacetin to form [¹⁴C]acetaminophen, and F_(M.P), the apparent availability of [¹⁴C]acetaminophen, were obtained by determining the concentrations of [¹⁴C]acetaminophen in the perfusate before and after incubation with Glusulase. These estimates were f_m=0.871±0.16 and F_(M.P)=0.43±0.10. These and the steady-state clearance values of phenacetin (9.1±0.8ml/min) and acetaminophen (6.7±0.7ml/min) from the single-pass studies were used to predict the concentrations of [¹⁴C]acetaminophen in the reservoir perfusate on recirculation of [¹⁴C]phenacetin. We found that the sequential first-pass elimination of the metabolite must be considered when the metabolite is highly extracted by the liver. If we had neglected to take this into account, the fractional rate of conversion of a precursor to form a metabolite and the rate of formation of the metabolite would have been underestimated by the factor F_(M.P).