Conjugation of benzo(a)pyrene 7,8-dihydrodiol-9,10-epoxide in infant Swiss-Webster mice

Benzo(a)pyrene 7,8-dihydrodiol-9,10-epoxide (BPDE), accepted as the ultimate carcinogen of benzo(a)pyrene, has a very short half-life in aqueous solutions yet induces lung tumors when injected into infant mice. To evaluate the possibility that metabolites of BPDE, principally in the form of stable conjugates, contribute to binding to DNA in peripheral tissues, infant mice were injected i.p. with 39 nmol (+/- ) anti-BPDE. One h after injection, 5% of the dose was recovered in serum and appeared mostly as conjugated metabolites (54% as glucuronides and 16% as glutathione conjugates). Amounts of direct acting electrophiles in serum estimated by trapping with DNA comprised less than 0.02% of the injected dose. No more than 10% of the radioactivity in extracts of liver, lung, and kidney was recovered as BPDE. Glutathione conjugates predominated in the liver and lung, whereas glucuronides were the major metabolites in kidney. Radioactivity bound to DNA in liver, lung, and kidney was 21.5, 42.7, and 7.8 pmol/mg, respectively. Despite the rapid conversion of BPDE to stable conjugates, 32P-postlabeling profiles of DNA adducts in lung closely resembled that noted after addition of BPDE directly to lung homogenate. Thus, the reactive intermediate as well as stable conjugates of BPDE may be transported to target tissues where they initiate tumors.

Reversible and irreversible oxidant injury to PC12 cells by hydrogen peroxide

A simple and reproducible model to identify biochemical changes associated with the transition from reversible to irreversible oxidant injury and cell death was established using rat pheochromocytoma PC12 cells. Cells were subjected to a transient oxidative stress induced by exposure to hydrogen peroxide (H2O2). Reversible loss of high-energy phosphates, induced by exposing cells to 0.2 mM H2O2, was preceded by transient increases in cytosolic calcium with no loss of plasma membrane integrity, as indexed by release of cytosolic enzymes. In contrast, permanent loss of high-energy phosphates, induced by treating cells with 0.5 mH H2O2, was associated with sustained rises in cytosolic-free calcium and increased oxidation of pyruvate and palmitate, two mitochondrial substrates. Initial production of pyruvate and lactate was inhibited by exposure to 0.5 mM H2O2 but returned to values comparable to control values at one hour after treatment with H2O2. Compromise of the plasma membrane was a late event, occurring between 1 and 2 hours after exposure to 0.5 mM H2O2. Collectively, these data indicate that irreversible loss of high-energy phosphates and cell death caused by oxidative stress is more closely associated with altered mitochondrial function than with impaired glycolysis.

Suppression of benzo[a]pyrene metabolism by accumulation of triacylglycerols in rat hepatocytes: effect of high-fat and food-restricted diets

Diet has been implicated as a major determinant of chemical carcinogenesis. Accordingly, rates of benzo[a]pyrene (B[a]P) metabolism were compared in hepatocytes isolated from rats maintained on control, high-fat or food-restricted AIN-76A diets. Rats maintained on the food-restricted diet were given 65% of food consumed by the control group fed ad libitum. The high-fat diet group had free access to a modified AIN-76A diet in which the amount of corn oil was increased 4-fold at the expense of digestible carbohydrates. The triacylglycerol content in hepatocytes varied in direct proportion to dietary fat and calories and was 66 +/- 5, 105 +/- 7 and 192 +/- 16 nmol/mg dry wt in cells isolated from rats fed food-restricted, control and high-fat diets respectively. In contrast, the rate of B[a]P metabolism was highest in hepatocytes from rats maintained on the food-restricted diet and lowest in cells from animals given the high-fat diet (i.e. food-restricted greater than control greater than high-fat). Thus, an inverse correlation existed between the rate of B[a]P metabolism and the content of triacylglycerols in hepatocytes. At a cell density of approximately 2 mg dry wt/ml, rates of B[a]P (40 microM) metabolism were 1324 +/- 186, 1150 +/- 198 and 829 +/- 76 pmol/mg dry wt/h, respectively, in hepatocytes isolated from rats fed food-restricted, control and high-fat diets. When cells were incubated with a lower concentration of B[a]P (10 microM), the rate of B[a]P metabolism was greater than 2-fold higher in hepatocytes from rats fed the food-restricted diet compared to the rate measured in cells from the high-fat group. Glucuronidation of B[a]P metabolites in hepatocytes from rats fed high-fat diet was also approximately 30% lower than rates determined for control and food-restricted groups. These diet-induced alterations in rates of B[a]P metabolism occurred in the absence of changes in specific activity of arylhydrocarbon hydroxylase or UDP-glucuronosyltransferase in liver microsomes. Further, the rate of 7-ethoxycoumarin metabolism, a more hydrophilic substrate, was not affected by diet and B[a]P but not 7-ethoxycoumarin accumulated in hepatic lipid droplets. Thus, diet-induced changes in intracellular triacylglycerol, particularly in lipid droplets, may alter access of B[a]P to binding sites on arylhydrocarbon hydroxylase and thereby modulate B[a]P metabolism in intact hepatocytes.

Effect of allyl alcohol on xanthine dehydrogenase activity in the perfused rat liver

Xanthine oxidase has been implicated in the production of reactive oxygen species and cell injury produced by various toxic compounds. Since allyl alcohol injuries the liver by an oxygen-dependent mechanism, we examined the actions of this hepatotoxicant on the conversion of xanthine dehydrogenase into xanthine oxidase in perfused livers. A microassay for NAD(+)-dependent xanthine dehydrogenase, based on measuring the production of NADH fluorometrically under anaerobic conditions, was developed and used to examine the actions of allyl alcohol on this activity in periportal and pericentral regions of the liver lobule. The oxygen-dependent activity, xanthine oxidase, was monitored in whole liver homogenates by uric acid formation at 302 nm under aerobic conditions. Perfusion of the liver with allyl alcohol (350 microM) increased xanthine oxidase and decreased xanthine dehydrogenase in whole liver consistent with the hypothesis that allyl alcohol enhanced calcium-dependent proteolytic conversion of the NAD(+)-dependent to the O2-dependent form. Xanthine dehydrogenase was higher in pericentral than in periportal regions of the liver lobule and tended to decrease selectively in periportal zones of livers exposed to allyl alcohol. O2 uptake was stimulated transiently by allyl alcohol followed by subsequent inhibition of respiration. These results are consistent with the idea that conversion of NAD(+)-dependent xanthine dehydrogenase to xanthine oxidase is involved in the zone-specific hepatotoxicity of allyl alcohol.

Inhibition of glucuronidation of benzo(a)pyrene phenols by long-chain fatty acids

Long-chain fatty acids inhibit glucuronidation of benzo(a)pyrene phenols in perfused liver; therefore, this study was designed to investigate interactions of fatty acids with beta-glucuronidase, glucuronosyl transferase, and energy supply. In beta-glucuronidase-deficient C3H/He mice, infusion of oleate (250 microM) increased the release of free benzo(a)pyrene phenols from 14 to 33 nmol/g/h and decreased release of glucuronides into the perfusate from 25 to 17 nmol/g/h. Rates of accumulation of glucuronides in the liver were also diminished from 11 to 4 nmol/g/h after infusion of oleate (250 microM). Fatty acids did not affect the release of benzo(a)pyrene metabolites into bile, and the ratio of free phenol to glucuronide production was increased from 0.57 to 1.30. A similar trend was observed in livers from DBA/2 mice that have beta-glucuronidase. Rates of hydrolysis of benzo(a)pyrene-O-glucuronide were not altered in isolated microsomes by addition of oleoyl coenzyme A (CoA) or octanoyl CoA (10- approximately 100 microM). Thus, we conclude that fatty acids do not alter glucuronidation by acting on beta-glucuronidase. The concentration of cofactors (UDP-glucuronic acid, UDP-glucose, and adenine nucleotides) involved in hepatic conjugation was not altered by infusion of concentrations of oleate (300 microM) that inhibited glucuronidation in perfused livers. When oleate concentrations were increased to 600 microM, UDP-glucuronic acid and UDP-glucose decreased 44 and 49%, respectively, and the ATP:ADP ratio declined concomitantly. Oleoyl CoA inhibited UDP-glucuronosyl transferase noncompetitively (half-maximal inhibition, 10 microM) in microsomes with 3-hydroxy-benzo(a)pyrene or p-nitrophenol as substrate. In contrast, octanoyl CoA was a very poor inhibitor of transferase activity. Inhibition of the transferase by oleoyl CoA was increased markedly by treatment with detergents (Triton X-100), i.e., half-inhibition of glucuronosyl transferase was obtained with about 2 microM oleoyl CoA. Inhibition of UDP-glucuronosyl transferase by oleoyl CoA was also increased in a dose-dependent manner by albumin, possibly due to increasing access of the CoA derivative to the enzyme. Collectively, these data indicate that fatty acids diminish glucuronidation via the formation of acyl CoA compounds that inhibit UDP-glucuronosyl transferase noncompetitively.

The liver plays a central role in the mechanism of chemical carcinogenesis due to polycyclic aromatic hydrocarbons

The important problem of whether metabolites and DNA adducts from benzo[a]pyrene (B[a]P) originate in the liver or target tissues was assessed using orthotopic liver transplantation. Following liver transplantation, the only source of metabolites for release into the blood and accumulation in target tissues is the liver. [3H]B[a]P (4 microM, 5 Ci/mmol) was infused into the portal vein of rats, and livers were perfused and either transplanted to a second rat or sham-operated and left in situ (non-transplant group). After 4 h, seven organs were collected and polar metabolites and DNA adducts were measured. In both groups, B[a]P in blood samples was below the limits of detection while levels of B[a]P in liver samples were approximately 5 pmol/g and polar metabolites were approximately 10 pmol/g. Concentrations of polar metabolites were also nearly identical in peripheral tissues from both groups. Phenols, glucuronides, sulfates and an unidentified metabolite of B[a]P were also similar, but GSH conjugate(s) had a tendency to be lower in blood of animals with transplanted livers. DNA adducts ranged from minimal values near levels of detection to approximately 0.2 pmol/mg DNA in lung, liver, and kidney. Importantly, there were no differences in DNA binding between the transplant and non-transplant groups. Taken together, these data provide compelling evidence that the liver is the predominant site of conversion of B[a]P into polar metabolites which are transported to target tissues and subsequently bind to DNA. Release of polar metabolites from the liver may represent a novel pathway for delivery of carcinogen conjugates to target tissues.

Futile cycling of a sulfate conjugate by isolated hepatocytes

The sulfate conjugate of the model compound 4-methylumbelliferone was taken up and hydrolyzed considerably more rapidly by isolated hepatocytes than was the glucuronide conjugate. Using intact hepatocytes or homogenates of hepatocytes, compounds were identified that either inhibited 4-methylumbelliferyl sulfate hydrolysis via arylsulfatase or impaired its uptake into cells. For example, sodium sulfate inhibited hydrolysis of 4-methylumbelliferyl sulfate by intact hepatocytes (half-maximal inhibition, 0.1 mM) but not by homogenates, suggesting a selective action on organic sulfate uptake at the plasma membrane. In contrast, cholesterol sulfate inhibited hydrolysis of 4-methylumbelliferyl sulfate by homogenates but not by hepatocytes, consistent with the hypothesis that cholesterol sulfate does not readily enter intact cells. Compounds that inhibited hydrolysis of 4-methylumbelliferyl sulfate by both isolated hepatocytes and microsomes include sodium sulfite (half-maximal inhibition, 0.1 mM), pregnenolone sulfate (half-maximal inhibition, 1 microM), and estrone sulfate (half-maximal inhibition, 10 microM). To test whether production of sulfate conjugates could be modified by agents affecting arylsulfatase in intact hepatocytes, we examined the effects of pregnenolone sulfate on the production of 4-methylumbelliferyl sulfate from 4-methylumbelliferone. Addition of pregnenolone sulfate (100 microM) to intact cells increased rates of 4-methylumbelliferone sulfate production and decreased the fraction of 4-methylumbelliferone converted into the glucuronide. Hydrolysis of 4-methylumbelliferyl sulfate by isolated microsomes was inhibited in a dose-dependent manner by adenosine 3'-phosphate 5'-phosphosulfate (PAPS) when cytosol, a source of sulfotransferase was present. Furthermore, addition of low concentrations of PAPS (0.5 microM) to a reconstituted system of microsomes and cytosol impaired the formation of fluorescent product from 4-methylumbelliferyl sulfate until PAPS was consumed, indicating that futile cycling via arylsulfatase and sulfotransferase occurred. Subsequent futile cycling of free 4-methylumbelliferone and 4-methylumbelliferyl sulfate occurred upon repeated additions of PAPS and was prevented by sodium sulfite, an inhibitor of arylsulfatase. These results argue strongly that sulfate conjugate production within hepatocytes is regulated by futile cycling via sulfotransferase and arylsulfatase. Thus, drugs and endogenous substances that affect arylsulfatase may have marked effects on sulfate conjugate production by the liver.

Enzyme activities associated with carcinogen metabolism in liver and nonhepatic tissues of rats maintained on high fat and food-restricted diets

The influence of high fat or food-restricted diets on key enzymes associated with polycyclic aromatic hydrocarbon metabolism was assessed in liver, lung, kidney and stomach of rats. Animals had access ad libitum to the AIN-76A purified diet (control) or were given 65% of the intake of controls for 3 wk. The high fat diet was isoenergetic to the control diet by substituting corn oil for equal energy from carbohydrate and addition of cellulose to obtain equal energy density. Activities of arylhydrocarbon hydroxylase and 7-ethoxycoumarin O-deethylase were significantly increased in lungs of food-restricted rats and decreased by the high fat diet but were not altered in liver. Both diets increased arylhydrocarbon hydroxylase approximately twofold in kidney. Glucose 6-phosphate and 6-phosphogluconate dehydrogenase were lowered in lung, kidney and liver by the high fat diet. Hepatic glutathione S-transferase was increased by high fat feeding. Food restriction decreased activities of arylsulfatase and beta-glucuronidase about 40% in lung. Hepatic arylsulfatase was also decreased about 40% by this treatment. Changes in hydrolase activities in livers and lungs of animals maintained on restricted diets raises in the interesting possibility that net production of glucuronide and sulfate conjugates of carcinogens by the liver and their hydrolysis in lung is altered by food restriction.

Regulation of urea synthesis in sublobular regions of the liver lobule by oxygen

Periportal and pericentral regions of the liver lobule were isolated from perfused rat liver using a micropunch and incubated in Krebs-Henseleit buffer (pH 7.6) containing 2% poly(ethylene glycol) in Eagle's basal medium, PMSF (50 micrograms/ml) and leupeptin (20 micrograms/ml) for 2 h at 25 degrees C under and O2/CO2 (95:5%) gas phase. Maximal rates of urea production from ammonium chloride were 96.4 +/- 8.7 and 32.8 +/- 5.4 mumol/g per h at 800 and 200 microM O2. Thus, urea synthesis was 2-3-times greater at high than low O2 tension in plugs from periportal and pericentral regions of the liver lobule.

Ethanol potentiates oxygen uptake and toxicity due to menadione bisulfite in perfused rat liver

Menadione bisulfite is a hepatotoxicant that damages periportal regions of the lobule in perfused liver in an oxygen-dependent manner. The effect of ethanol on menadione bisulfite toxicity was examined in perfused rat liver. Addition of menadione bisulfite (3 mM) alone to the perfusate increased oxygen uptake by 20-30 mumols/g/hr. Lactate dehydrogenase was released into the effluent after 60 min of perfusion and reached values around 100 units/g/hr. Under these conditions, trypan blue was taken up exclusively in periportal regions of the liver lobule; 44% of periportal cells were stained. In the presence of ethanol, maximal increases in oxygen uptake due to menadione bisulfite were much larger (about 90 mumols/g/hr), and lactate dehydrogenase release occurred earlier and reached higher maximal values (330 units/g/hr). Trypan blue staining was also more extensive; 90% of periportal cells were stained. The effect of ethanol on menadione bisulfite-induced oxygen uptake required metabolism via alcohol dehydrogenase (ADH), because ethanol increased oxygen uptake due to menadione bisulfite from 44 to 81 mumols/g/hr in deermice with ADH but had no effect in deermice lacking ADH. Other agents that increase NADH (xylitol and 2-ethyl-1-hexanol) also potentiated the stimulation of oxygen uptake due to menadione bisulfite, suggesting that ethanol was working by increasing the NADH redox state. Cyanide abolished the increase in oxygen uptake due to menadione bisulfite, both in the absence and in the presence of ethanol, supporting the hypothesis that the effect of ethanol on menadione bisulfite-mediated oxygen uptake involves the mitochondrial respiratory chain. Further, the stimulation of oxygen uptake by menadione bisulfite in isolated mitochondria was enhanced when matrix NADH was increased by addition of beta-hydroxybutyrate. These data indicate that ethanol potentiates oxygen uptake and toxicity due to menadione bisulfite most likely by generation of NADH for redox cycling of this model quinone.

Unique role of oxygen in regulation of hepatic monooxygenation and glucuronidation

The purpose of this study was to evaluate the hypothesis that NADPH supply in intact cells is regulated by oxygen tension. This was accomplished by studying monooxygenation in perfused livers from Ah locus-responsive C57BL/6J mice, where rates of monooxygenation are high. Elevation of flow rate decreases the hepatic O2 gradient and increases O2 delivery to the organ. Under these conditions, rates of p-nitroanisole O-demethylation were 2-3 times higher in perfused livers from fed or fasted mice at high (10 ml/min) compared with normal (5 ml/min) flow rates. Rates of monooxygenation were directly proportional to oxygen tension (half-maximal rates occurred with approximately 400 microM O2). On the other hand, rates were independent of oxygen concentration in isolated microsomes where NADPH was supplied in excess. The decrease in rate due to diminished O2 concentration in the intact organ could not be attributed to hypoxia, because O2 tension in the effluent perfusate exceeded 50 microM even when influent perfusate was saturated with 25% O2 and ATP/ADP ratios were in the normal range. Thus, monooxygenation of p-nitroanisole in perfused mouse liver is dependent on oxygen tension. Similarly, glucuronidation of p-nitrophenol was oxygen dependent in the intact organ but not in isolated microsomes supplemented with UDP-glucuronic acid. Taken together, these data support the hypothesis that, at high oxygen tensions (e.g., in periportal regions of the liver lobule), mitochondrial activity is increased, which in turn enhances NADPH and UDP-glucuronic acid turnover, leading to accelerated rates of monooxygenation and glucuronidation in intact cells. In support of this idea, NH4Cl, which utilizes NADPH for urea synthesis, inhibited monooxygenation in the perfused mouse liver at high but not low flow rates. Thus, important phase I and II detoxification reactions are regulated indirectly by the hepatic oxygen gradient, via mechanisms involving cofactor supply, when cytochrome P-450 is not limiting.

Increase in survival time of liver transplants by protease inhibitors and a calcium channel blocker, nisoldipine

Kupffer cells are activated by calcium and release a variety of toxic mediators, including proteases. The purpose of these studies, therefore, was to determine if protease inhibitors and a calcium channel blocker could increase survival time in the rat model of orthotopic liver transplantation. Survival for 30 days was greater than 90% in this model when livers were stored for 1 hr in Ringer's solution (survival conditions)--however, grafts stored for 4 hr in Euro-Collins solution or 8 hr in University of Wisconsin (UW) solution survived postoperatively only 1.2 and 0.7 days, respectively (nonsurvival conditions). When livers were stored for 4 hr in Euro-Collins containing a cocktail of protease inhibitors (leupeptin, pepstatin A, phenylmethylsulfonyl fluoride, 20 ng/ml each; diisopropyl fluorophosphate, 100 microM) and subsequently transplanted, however, survival time was increased significantly to 11.5 days. Inclusion of a calcium channel blocker, nisoldipine (1.4 microM), in the protease inhibitor cocktail increased survival time to 23 days. Actually, nisoldipine alone increased survival time to 25 days. Nisoldipine alone also increased survival time in livers stored for 8 or 16 hr in UW solution to between 15 and 20 days. Serum transaminase levels reached peak values greater than 2400 U/L one day postoperatively in the nonsurvival groups, and liver injury assessed histologically was apparent. Under these conditions, pulmonary infiltration of inflammatory cells was observed in about 60% of the lungs examined and was associated with massive bleeding. Inclusion of the protease cocktail, nisoldipine, or both in the storage solutions decreased maximal SGOT levels and injury to both liver and lung significantly by about 50% postoperatively. Nisoldipine also decreased phagocytosis of carbon particles by the perfused liver 2- to 3-fold following storage under nonsurvival conditions (half-maximal effect = 0.3-0.4 microM nisoldipine). Moreover, nisoldipine improved hepatic microcirculation. It accelerated blood flow into the liver, as indexed by hemoglobin reflectance from the liver surface. These data support the hypothesis that Kupffer cells are activated early in the sequence of events that causes graft failure leading to endothelial cell-mediated alterations in the microcirculation. This work demonstrates clearly that dihydropyridine-type calcium channel blockers such as nisoldipine may be clinically useful in storage solutions for liver prior to transplantation.

O2-dependent hepatotoxicity due to ethylhexanol in the perfused rat liver: mitochondria as a site of action

Toxicity of 2-ethylhexanol, a metabolite of diethylhexyl phthalate, was assessed in the perfused rat liver. Livers from starved rats were perfused with ethylhexanol (3 mM) dissolved in Krebs-Henseleit buffer (pH 7.4, 37 degrees C) saturated with 95% O2-5% CO2 in both the anterograde and retrograde direction. Following infusion of ethylhexanol, O2 uptake and ketone body formation were diminished by 50 and 80%, respectively, and cell damage, as assessed by the appearance of lactate dehydrogenase in the effluent perfusate, was apparent. Both inhibition of O2 uptake by ethylhexanol and the appearance of lactate dehydrogenase in the perfusate were dose-dependent. Only O2-rich upstream regions of the liver lobule were damaged as reflected by trypan blue uptake. Inhibition of O2 uptake by ethylhexanol was also reflected by a 60% decrease in the ATP/ADP ratio. Local rates of O2 uptake, measured using miniature electrodes placed on the liver surface, indicated that ethylhexanol only diminished O2 uptake in O2-rich upstream regions of the liver lobule regardless of the direction of flow. This phenomenon apparently can be explained by a direct effect of ethylhexanol on mitochondria in upstream regions since active state 3 rates of respiration were inhibited by ethylhexanol in isolated mitochondria. Ethylhexanol also caused a dose-dependent decrease in the mitochondrial membrane potential and an increase in the beta-hydroxybutyrate/acetoacetate (B/A) ratio. However, infusion of radical scavengers such as allopurinol, cianidanol and uric acid did not alter lactate dehydrogenase release due to ethylhexanol. Thus, the toxicity of ethylhexanol in the liver is dependent on local O2 tension and mitochondrial are primary targets.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between plasticizers and fatty acid metabolism in the perfused rat liver and in vivo. Inhibition of ketogenesis by 2-ethylhexanol

Rates of ketone body (beta-hydroxybutyrate plus acetoacetate) production by perfused livers from starved rats were decreased about 60% from 39 +/- 2 to 17 +/- 3 mumol/g/hr by 2-ethylhexanol (200 microM), a primary metabolite of the plasticizer diethylhexyl phthalate. Inhibition of ketogenesis by ethylhexanol was dose dependent (half-maximal inhibition occurred with 25 microM) in the presence or absence of 4-methylpyrazole, an inhibitor of alcohol dehydrogenase. Concentrations of beta-hydroxybutyrate relative to acetoacetate (B/A) increased in a step-wise manner from 0.32 to 0.75 in the effluent perfusate when ethylhexanol was infused. In contrast, the B/A ratio decreased in parallel with inhibition of ketone body production when alcohol dehydrogenase was inhibited. Pretreatment of rats with phenobarbital, an inducer of omega and omega-1 hydroxylases, diminished inhibition of ketone body production by low (less than 50 microM) of ethylhexanol. Thus, ethylhexanol is oxidized via phenobarbital-inducible pathways to metabolites which do not inhibit ketogenesis. Studies were conducted to determine the site of inhibition of fatty acid oxidation by ethylhexanol. Rates of ketone body production in the presence of oleate (250 microM), which requires transport of the corresponding CoA compound into mitochondria, were reduced from 80 +/- 6 to 58 +/- 8 mumol/g/hr by ethylhexanol. In contrast, ketone body production from hexanoate, which is activated in the mitochondria, was not affected by ethylhexanol. Basal and oleate-stimulated rates of H2O2 production were not affected by ethylhexanol, indicating that peroxisomal beta-oxidation was not altered by the compound. Based on these data it is concluded that 2-ethylhexanol inhibits beta-oxidation of fatty acids in mitochondria but not in peroxisomes. Treatment of rats with ethylhexanol (0.32 g/kg, i.p.) decreased plasma ketone bodies from 1.6 to 0.8 mM, increased hepatic triglycerides and increased lipid predominantly in periportal regions of the liver lobule. These data indicate that alterations in hepatic fatty acid metabolism in periportal regions of the liver lobule may be early events in peroxisome proliferation.

Activation of glycogen phosphorylase in rat pheochromocytoma PC12 cells and isolated hepatocytes by organophosphates

Several organophosphates including diisopropylfluorophosphonate (DPF) and a variety of compounds used as chemical warfare agents produced dose- and time-dependent increases in phosphorylase-a, the phosphorylated form of glycogen phosphorylase in rat pheochromocytoma cells, PC12, and isolated hepatocytes. Increases in phosphorylase-a did not occur in cells exposed to the carbamates, physostigmine or pyridostigmine, or to O-ethyl S-2-diisopropylaminoethylmethyl-phosphonathiolate (VX), an organophosphate which is protonated at physiological pH. When extracellular pH was increased to pH 8, VX acted like the other organophosphates and increased phosphorylase-a activity. The possibility that organophosphates increase phosphorylase-a in intact cells by releasing Ca2+ from intracellular binding sites is supported by the following findings: organophosphate-induced increases in phosphorylase-a did not correlate with changes in cyclic AMP in the two cell types studied; in PC12 cells, increases in this activity occurred in the absence of extracellular calcium and were not inhibited by the calcium channel blocker, verapamil; fluorescence of the calcium sensitive dye, Quin-2, in PC12 cells preloaded with the acetoxymethyl ester of the dye was increased by soman; finally, addition of the calcium ionophore, A23187, to PC12 cells maintained in calcium-free medium caused sarin-stimulated phosphorylase-a activity to return rapidly to basal levels. Collectively, these data argue strongly that organophosphates increase phosphorylase-a activity in intact cells via a novel mechanism involving release of calcium from intracellular binding sites.

Effect of epinephrine on glycogen phosphorylase-alpha in various preparations of rat liver

1. Glycogen phosphorylase-alpha, a commonly used index of cytosolic free calcium, was compared in various preparations of rat liver in the absence and presence of 0.1 microM epinephrine. 2. Total phosphorylase in isolated perfused livers and freshly-isolated hepatocytes were the same as that observed in liver in situ; however, phosphorylase-alpha was 50% higher in perfused liver and 80% higher in hepatocytes than activities measured in situ. Total phosphorylase was reduced approximately 50% in hepatocytes maintained in primary culture for 24 hr. 3. Epinephrine increased phosphorylase-alpha approximately 2-fold in livers perfused for 30 min but only about 20% in hepatocytes incubated for 30 min. After 90 min of perfusion or incubation, epinephrine increased phosphorylase-alpha nearly 4-fold in perfused livers and only 30% in isolated hepatocytes. The results suggest that amounts of free calcium and calcium-dependent coupling of adrenergic receptors to phosphorylase-alpha differ markedly between the intact liver and isolated hepatocytes.

Adenine nucleotides and carbohydrates in subpopulations of hepatic nodules with normal and compromised microcirculation

Fluorescein-isothiocyanate dextran (FITC-dextran), a dye confined to the vascular space, was infused via the hepatic artery and portal vein into perfused livers from fed rats treated with diethylnitrosamine for 4 to 5 months. Fluorescence due to FITC-dextran was detected with fiberoptic microlight guides placed on surface nodules of about 5 mm in diameter. Nodules were categorized into groups with normal and compromised microcirculation based on their fluorescence following infusion of FITC-dextran. Similar results were obtained when nodules were classified based on reflectance of trypan blue. Despite compromised microcirculation, ATP and ADP levels as well as ATP/ADP ratios were comparable in both groups of nodules; however, AMP was elevated in FITC-dextran-negative nodules (i.e., those with compromised microcirculation). Nodules with compromised microcirculation also contained higher glucose and lactate levels than nodules that were well perfused; however, glycogen was five times lower than in FITC-dextran-positive nodules. Fasting reduced ATP/ADP ratios in poorly perfused nodules in comparison to well-perfused nodules. In perfused livers from fed rats where glycogen was high, however, ATP/ADP ratios and rates of ATP depletion during ischemia were the same in well-perfused and poorly perfused nodules. Products of glycogen breakdown (e.g., glucose and lactate) were elevated in nodules from livers of fed but not fasted rats. The results indicate that alteration of perfusion of hepatic nodules does not change ATP levels nor the capacity of nodules to utilize high energy phosphate during anoxia. Thus, near normal energy status is maintained from glycogen metabolism in poorly perfused nodules via glycolysis. Since basal ATP content and utilization is comparable in well and poorly perfused nodules, compromised energy status is unlikely to explain selection of nodules that regress to near normal hepatocytes.

Effect of fatty acids on formation, distribution, storage, and release of benzo(a)pyrene phenols and glucuronides in the isolated perfused rat liver

The hydroxylation of benzo(a)pyrene and conjugation, storage, and release of benzo(a)pyrene phenols and glucuronides by the perfused rat liver were studied in the presence and absence of acute addition of physiological concentrations of common dietary fatty acids. The actions of fatty acids on the oxidation and conjugation of benzo(a)pyrene in the intact liver were compared with their actions on microsomes isolated from rat liver. Rats were treated with beta-naphthoflavone to stimulate polycyclic aromatic hydrocarbon metabolism. Long-chain fatty acyl CoA compounds (palmitoyl CoA, oleoyl CoA, linolenoyl CoA; 50 microM) inhibited hydroxylation of benzo(a)pyrene by isolated microsomes by about 45%; however, long-chain fatty acids did not affect overall rates of hydroxylation of benzo(a)pyrene by the perfused liver at concentrations ranging up to 300 microM. The medium-chain acyl CoA compound, octanoyl CoA, also did not affect benzo(a)pyrene hydroxylation in microsomes or liver. Although fatty acids did not alter rates of hydroxylation, the ratio of free benzo(a)pyrene phenols to glucuronides (F/G ratio) increased about 60% (P less than 0.05) in livers perfused with long-chain fatty acids (palmitate, oleate, linolenate). Inhibition of glucuronidation was not observed with the medium-chain fatty acid, octanoate. Benzo(a)pyrene phenols and glucuronides accumulated linearly in the liver at rates of approximately 40 nmol/g/h. A second action of both long- and medium-chain length fatty acids was to increase rates of release of benzopyrene phenols into the perfusate by 50 to 80%. Fatty acids did not effect release of benzo(a)pyrene phenols and glucuronides into bile. Taken together, these data support the hypothesis that fatty acids displace carcinogenic metabolites of benzo(a)pyrene from binding sites in the liver which enter the circulation and travel to target tissues.

Interactions between glycolysis and mixed function oxidation: studies with 7-ethoxycoumarin in perfused livers from beta-naphthoflavone-treated rats

Interaction between glycolysis and mitochondrial oxidations to supply reducing equivalents at high rates for mixed function oxidation was evaluated in the perfused liver after treatment of rats with beta-naphthoflavone. Livers from fasted beta-naphthoflavone-treated rats were employed because rates of 7-ethoxycoumarin O-deethylation were constant (16 mumol/g/hr) for at least 1 hr of perfusion. Preinfusion with KCN, an inhibitor of oxidative phosphorylation, caused the rate of 7-ethoxycoumarin O-deethylation to decline by 60% over 30 min of perfusion. The decline in rates of mixed function oxidation in the intact liver was not due to a direct effect of KCN on cytochrome P-450, inasmuch as cyanide did not diminish rates of 7-ethoxycoumarin O-deethylation by isolated microsomes. Cyanide rapidly decreased hepatic oxygen uptake by 70% and increased rates of glycolysis (lactate plus pyruvate production) from less than 10 to over 60 mumol/g/hr. Rates of glycolysis and mixed function oxidation subsequently declined in parallel during infusion of KCN. Infusion of ethanol (20 mM), a known inhibitor of glycolysis, decreased the stimulation of glycolysis caused by KCN to 20 mumol/g/hr and lowered maximal rates of 7-hydroxycoumarin production to about 6 mumol/g/hr. Both mixed function oxidation and glycolysis also declined in parallel over 30 min of perfusion in the presence of ethanol and KCN. When cyanide infusion was terminated, rates of oxygen uptake returned rapidly to basal values; however, rates of mixed function oxidation remained low. In contrast, infusion of ethanol in the absence of cyanide had no effect on rates of mixed function oxidation. Infusion of glucose (30 mM) or pyruvate (1 mM) after KCN restored maximal rates of mixed function oxidation in parallel with increases in rates of glycolysis. In contrast to results obtained in livers from fasted rats, cyanide and ethanol had little effect on 7-ethoxycoumarin O-deethylation in livers from fed rats. Taken together, these results argue strongly that rates of mixed function oxidation in the intact livers of fasted rats are sustained by reducing equivalents derived from mitochondrial oxidations. Glycolysis can supply substrates needed for the transport of reducing equivalents from the mitochondria into the cytosol for mixed function oxidation. Because glycogen reserves are minimal in the fasted state, rates of glycolysis and mixed function oxidation declined in parallel during the infusion of cyanide, because reducing equivalents derived from mitochondria are not available.

Effect of nerve growth factor on the synthesis of amino acids in PC12 cells

Radioactive short-chain fatty acids preferentially label glutamine relative to glutamate in brain due to compartmentation of glutamine and glutamate. To determine whether this phenomenon occurs in a single cell culture model, we examined the effect of fatty acid chain length on the synthesis as well as pool size of selected amino acids in rat pheochromocytoma PC12 cells, a cell culture model of the large glutamate compartment in neurons. Intracellular 14C-amino acids were quantitated by HPLC, and the incorporation of [U-14C]-glucose, [1-14C]-butyrate, [1-14C]-octanoate, and [1-14C]-palmitate into five amino acids was measured in native and NGF-treated PC12 cells. NGF pretreatment decreased the intracellular concentration of amino acids as did addition of fatty acids but these effects were not additive. Specific activities of amino acids in native cells labelled by 14C-octanoate were 1,300 DPM/nmol, 490 DPM/nmol, 200 DPM/nmol, and 110 DPM/nmol for glutamate, aspartate, glutamine, and serine, respectively. No radioactivity was detected in alanine. Similar specific activities were noted when 14C-butyrate was the precursor; however, there was at least 5-fold less if 14C-palmitate was the precursor. Pretreatment of cells with NGF decreased the specific activity of amino acids by 25-65%. Specific activities of amino acids synthesized from 14C-glucose decreased in the following order: glutamate, 1,640 DPM/nmol; aspartate, 1,210 DPM/nmol; alanine, 580 DPM/nmol; glutamine, 275 DPM/nmol; and serine, 80 DPM/nmol for native cells. NGF pretreatment decreased the specific activities of glutamate and glutamine, but not of the other 3 amino acids. The preferred precursor for glutamate synthesis in native PC12 cells was glucose followed by octanoate, butyrate and palmitate (16:6:3:1).

Hepatotoxicity of menadione predominates in oxygen-rich zones of the liver lobule

This study was designed to investigate the mechanism of zone-specific hepatotoxicity due to menadione. Infusion of menadione (64-1000 microM) into perfused livers from fasted rats caused a concentration-dependent increase in O2 uptake. During perfusion in the anterograde direction, menadione (1 mM) increased O2 uptake from 115 +/- 11 to 142 +/- 10 mumol/g/hr within 30 min, followed by a decrease to 92 +/- 11 mumol/g/hr over the next 30 min. Trypan blue was taken up by 90% of cells in periportal regions reflecting irreversible cell death, whereas cells in pericentral areas were not damaged. When the hepatic O2 gradient was reversed by perfusing in the retrograde direction, menadione increased O2 uptake initially from 114 +/- 11 to 132 +/- 14 mumol/g/hr, followed by a decline to 51 +/- 12 mumol/g/hr, qualitatively similar to data obtained from perfusions in the natural, anterograde direction. During perfusions in the retrograde direction, however, 95% of cells in pericentral regions were stained with trypan blue whereas those in periportal areas were spared. O2 uptake in specific zones of the liver lobule was then measured with miniature O2 electrodes. When menadione was infused during anterograde perfusions, O2 uptake increased in O2-rich periportal areas from 128 +/- 6 to 156 +/- 12 mumol/g/hr, but was not altered in pericentral regions. Conversely, during perfusions in the retrograde direction, menadione did not affect O2 uptake in periportal areas, but stimulated uptake in O2-rich pericentral regions from 120 +/- 4 to 150 +/- 14 mumol/g/hr.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen-dependent hepatotoxicity due to doxorubicin: role of reducing equivalent supply in perfused rat liver

Doxorubicin is an important anticancer drug that undergoes redox cycling leading to the production of oxygen radicals; however, its clinical use is limited by toxicity. Redox cycling due to doxorubicin was assessed in the perfused rat liver from increases in O2 uptake by the organ, and toxicity was determined from lactate dehydrogenase release and trypan blue uptake. Doxorubicin increased O2 uptake in a concentration-related manner with half-maximal increases at about 100 microM drug. Within 5 min after addition of 300 microM doxorubicin, lactate dehydrogenase was detected in the effluent perfusate. Enzyme release increased steadily and reached values of 600 units/liter after 60 min. Rates of O2 uptake due to redox cycling of doxorubicin (300 microM) increased by 57 mumol/g/hr in oxygen-rich (mean [O2] = 473 microM) periportal regions of the liver lobule, but did not change in pericentral regions where O2 tension was lower [( O2] = 247 microM). Concomitantly, fluorescence of NAD(P)H measured from the liver surface decreased in periportal but not pericentral regions. The zone-specific decrease in NADPH was attributed to redox cycling of doxorubicin. Trypan blue was taken up exclusively by cells in periportal regions of the liver lobule after perfusion with doxorubicin. When the average O2 tension was lowered from 550 to 200 microM, O2 uptake due to redox cycling of doxorubicin in periportal regions was reduced 3-fold and toxicity was abolished, indicating that toxicity due to doxorubicin is oxygen-dependent. Redox cycling of doxorubicin was minimal in regions of the perfused liver where the O2 concentration was below 400 microM. In contrast, isolated microsomes displayed maximal changes in O2 uptake due to redox cycling of doxorubicin at O2 tensions of about 10 microM. Thus, oxygen per se is not rate-limiting for redox cycling of doxorubicin in the intact organ. Since NADPH is also required for redox cycling of doxorubicin, the effect of oxygen on the ability of mitochondria and the pentose cycle to supply reducing equivalents for redox cycling of doxorubicin was examined. NADPH supply from the pentose cycle was reduced by fasting while that from mitochondria was inhibited by cyanide. The increase in O2 uptake due to redox cycling of doxorubicin was around 60 mumol/g/hr in livers from fed or fasted rats. In the presence of potassium cyanide, stimulation of O2 uptake by doxorubicin was reduced by about one-half in livers from fed rats (29 mumol/g/hr) yet was abolished nearly completely in livers from fasted rats (7 mumol/g/hr).(ABSTRACT TRUNCATED AT 400 WORDS)

Selective depletion and measurement of glutathione in periportal and pericentral regions in the perfused rat liver

A method was developed to selectively deplete and measure glutathione (GSH) in periportal and pericentral regions of the liver lobule based on the formation of chlorodinitrobenzene-glutathione (CDNB-GSH) adducts. Using microlight guides, we demonstrated that small pulses of CDNB caused reflected light at 366 nm to decline only in upstream regions of the liver lobule. This indicates that GSH was only depleted in upstream periportal or pericentral regions following perfusions in the anterograde or retrograde direction, respectively. Infusion of repeated pulses of CDNB in alternating directions of perfusion (up to eight) allowed selective and complete depletion of GSH in specific sublobular regions. Summation of local rates of adduct formation indicated that GSH content averaged 3.6 +/- 0.8 mumol/g in periportal regions and was 3.3 +/- 0.8 mumol/g in pericentral areas. Total values for GSH calculated from GSH-CDNB adduct formation were nearly identical to levels of GSH measured chemically. This new method may be useful in evaluating the mechanism of toxic chemicals which interact with GSH in discrete regions of the liver lobule.

A new method for the isolation of fresh hepatocytes from periportal and pericentral regions of the liver lobule

A simple method which avoids the use of perfusion with calcium free buffer, hydrolytic enzymes and detergents has been developed to obtain fresh hepatocytes from periportal and pericentral regions of the liver lobule. Cylindrical plugs (200 x 500 microns) of periportal and pericentral areas of the rat liver lobule weighing about 1 mg were collected with a micropunch from fresh or perfused liver. Ninety percent of cells were intact as assessed from trypan blue staining. Glutamine synthetase activity was detected predominantly (ca. 85%) in plugs isolated from pericentral regions indicating that this method allows selective harvesting of pure sublobular zones of the liver lobule. Rates of oxygen uptake measured at 25 degrees C by plugs from livers perfused in the anterograde direction were 56 +/- 5 and 33 +/- 7 mumol/g/h by periportal and pericentral plugs, respectively, values similar to data obtained from the intact organ. This method provides new opportunities to study the regulation of basic metabolic processes in cells from sublobular areas under nearly physiological conditions.