Effects of organophosphates and nerve growth factor on muscarinic receptor binding number in rat pheochromocytoma PC12 cells

Muscarinic receptor binding in PC12 cells is influenced by both nerve growth factor (NGF) and organophosphates. Treatment of PC12 cells with a single dose of NGF (50 ng, 7S NGF/ml) increased [3H]N-methylscopolamine ([3H]-NMS) binding sites approximately two-fold at 48 hr but did not change the Kd for this ligand. Exposure of PC12 cells to soman, 50 microM, decreased [3H]-NMS binding in both undifferentiated and NGF-treated cells; however, decreases in muscarinic binding induced by the organophosphate were only minimal after the first hour after treatment and were maximal at about 24 hr. Other organophosphates including sarin, tabun, and VX as well as the carbamate, pyridostigmine, also reduced [3H]-NMS binding in PC12 cells measured 24-48 hr after treatment. The order of potency of organophosphates in lowering [3H]-NMS binding was soman greater than sarin greater than VX greater than tabun greater than DFP. High amounts of VX (200 microM) but not the other organophosphates inhibited [3H]-NMS binding when added to cells during the course of binding assays. Decreases in muscarinic receptor binding induced by the organophosphates differed markedly from that produced by carbamylcholine, which decreased [3H]-NMS binding maximally 30 min after addition to the cells. Decreases in [3H]-NMS binding produced by carbamylcholine were antagonized by atropine, but reductions in muscarinic binding produced by the organophosphates were not reversed by atropine. Thus, decreases in muscarinic receptor binding induced in PC12 cells by organophosphates occur via a novel mechanism that does not involve agonist-induced receptor desensitization.

Glucuronidation of 7-hydroxycoumarin in periportal and pericentral regions of the lobule in livers from untreated and 3-methylcholanthrene-treated rats

Rates of production of 7-hydroxycoumarin glucuronide were measured in specific zones of the liver lobule using micro-light guides placed on periportal and pericentral regions on the surface of livers from untreated and 3-methylcholanthrene-treated rats. Livers were perfused with sulfate-free buffer under normoxic conditions and fluorescence of free 7-hydroxycoumarin was monitored. The formation of nonfluorescent 7-hydroxycoumarin glucuronide was then inhibited completely by perfusion with N2-saturated perfusate containing 20 mM ethanol. The difference between fluorescence readings under normoxic and hypoxic conditions was used to calculate rates of glucuronidation. Maximal rates of glucuronidation (11.9-13.5 mumol/g/hr) did not differ significantly in periportal and pericentral regions in livers from either 3-methylcholanthrene-treated or untreated rats. In all regions of the liver lobule, glucuronidation was half-maximal with about 20 microM 7-hydroxycoumarin. Glucuronosyltransferase assayed in lyophilized tissue sections with saturating concentrations of UDPGA (9 mM) was 2.3-fold greater in pericentral than in periportal areas in livers from untreated rats. In livers from 3-methylcholanthrene-treated rats, activities were similar in periportal and pericentral regions but were 4- to 7-fold higher than values from untreated rats. In addition, glucuronosyltransferase activity assayed in native microsomes with physiological concentrations of UDP-glucuronic acid (UDPGA) (0.4 mM) with UDP-N-acetylglucosamine (0.3 mM) was 2-fold higher in preparations from 3-methylcholanthrene-treated than untreated rats. Thus, 3-methylcholanthrene treatment increased glucuronosyltransferase activity in vitro but did not alter rates of glucuronide formation in periportal and pericentral regions of the liver lobule of intact liver. Infusion of epinephrine (50 nM) into perfused livers from untreated and 3-methylcholanthrene-treated rats increased rates of glucuronidation by about 35%. Since epinephrine probably acts by increasing the supply of the cofactor UDPGA due to increased breakdown of glycogen, it follows that UDPGA supply limits rates of glucuronidation in perfused livers from both untreated and 3-methylcholanthrene-treated rats.

Effect of beta-naphthoflavone on mitochondrial supply of reducing equivalents for monooxygenation in periportal and pericentral regions of the liver lobule

Rates of 7-ethoxycoumarin O-deethylation were determined in periportal and pericentral regions of the liver lobule in livers from corn oil- and beta-naphthoflavone-treated rats by monitoring the conversion of nonfluorescent 7-ethoxycoumarin to fluorescent 7-hydroxycoumarin with micro-light guides. Rates of monooxygenation in livers from fed, corn oil-treated rats of 1.4 mumol/g/hr were increased markedly to around 21 mumol/g/hr in both regions of the liver lobule after treatment of rats with beta-naphthoflavone. Fasting or treatment with 6-aminonicotinamide diminished the generation of NADPH by the pentose cycle, whereas KCN decreased NADPH generation via mitochondria. Fasting and 6-aminonicotinamide treatment decreased monooxygenation about 0.5 mumol/g/hr in both regions of the liver lobule in livers from corn oil-treated rats and around 5 mumol/g/hr in livers from beta-naphthoflavone-treated rats. KCN decreased rates about 0.5 mumol/g/hr in both regions of the lobule in livers from fed, corn oil-treated rats and nearly completely in livers from fasted rats. Rates declined from 14 to less than 2 mumol/g/hr in livers from fasted, beta-naphthoflavone-treated rats following 30-40 min of perfusion with cyanide. These data indicate that mitochondrial oxidations are the predominant source of reducing equivalents for monooxygenation in both regions of the liver lobule in livers from beta-naphthoflavone-treated rats. Activation of urea synthesis by infusion of ammonia, a process requiring mitochondrial NADPH, inhibited the metabolism of 7-ethoxycoumarin by 30%. Malate, which is a substrate for the malic enzyme shuttle mechanism involved in the transfer of reducing equivalents from the mitochondria to the cytosol, increased 10-fold during infusion of 7-ethoxycoumarin in livers from beta-naphthoflavone-treated rats but less than 3-fold in livers from control rats. Taken together, these data indicate that high rates of 7-hydroxycoumarin production in livers from beta-naphthoflavone-treated rats are sustained by increased rates of NADPH generation from mitochondrial sources.

Gluconeogenesis from fructose predominates in periportal regions of the liver lobule

Gluconeogenesis from fructose was studied in periportal and pericentral regions of the liver lobule in perfused livers from fasted, phenobarbital-treated rats. When fructose was infused in increasing concentrations from 0.25 to 4 mM, corresponding stepwise increases in glucose formation by the perfused liver were observed as expected. Rates of glucose and lactate production from 4 mM fructose were around 100 and 75 mumol/g/h, respectively. Rates of fructose uptake were around 190 mumol/g/h when 4 mM fructose was infused. 3-Mercaptopicolinate, an inhibitor of phosphoenolpyruvate carboxykinase, decreased glucose formation from fructose maximally by 20% suggesting that a fraction of the lactate formed from fructose is used for glucose synthesis. A good correlation (r = 0.92) between extra oxygen consumed and glucose produced from fructose was observed. At low fructose concentrations (less than 0.5 mM), the extra oxygen uptake was much greater than could be accounted for by glucose synthesis possibly reflecting fructose 1-phosphate accumulation. Furthermore, fructose diminished ATP/ADP ratios from about 4.0 to 2.0 in periportal and pericentral regions of the liver lobule indicating that the initial phosphorylation of fructose via fructokinase occurs in both regions of the liver lobule. Basal rates of oxygen uptake measured with miniature oxygen electrodes were 2- to 3-fold higher in periportal than in pericentral regions of the liver lobule during perfusions in the anterograde direction. Infusion of fructose increased oxygen uptake by 65 mumol/g/h in periportal areas but had no effect in pericentral regions of the liver lobule indicating higher local rates of gluconeogenesis in hepatocytes located around the portal vein. When perfusion was in the retrograde direction, however, glucose was synthesized nearly exclusively from fructose in upstream, pericentral regions. Thus, gluconeogenesis from fructose is confined to oxygen-rich upstream regions of the liver lobule in the perfused liver.

Metabolism via the pentose phosphate pathway in rat pheochromocytoma PC12 cells: effects of nerve growth factor and 6-aminonicotinamide

Exposure of rat pheochromocytoma PC12 cells to 0.1 mM 6-aminonicotinamide (6AN) for 24 hours resulted in a 500-fold increase in 6-phosphogluconate indicating active metabolism of glucose via the oxidative enzymes of the pentose phosphate pathway. Amounts of 6-phosphogluconate that accumulated in 6AN-treated cells at 24 hours were significantly increased by treatment of the cells with nerve growth factor (NGF) (100 ng 7S/ml) suggesting that metabolism of glucose via the pentose pathway at this time was enhanced by NGF. This stimulation of metabolism via the pentose pathway is probably a late response to NGF because initial rates of 6-phosphogluconate accumulation in 6AN-treated cells were the same in the presence and absence of NGF. Moreover, amounts of 14CO2 generated from 1-[14CO2]glucose during the initial six hour incubation period were the same in control and NGF-treated cells. Specific activities of hexose phosphates labeled from 1-[14CO2]glucose were also the same in control and NGF-treated cells. The observation that 6AN inhibited metabolism via the pentose phosphate pathway but failed to inhibit NGF-stimulated neurite outgrowth suggests that NADPH required for lipid biosynthesis accompanying NGF-stimulated neurite outgrowth from PC12 cells can be derived from sources other than, or in addition to, the oxidative enzymes of the pentose phosphate pathway.

Hydrolysis of 4-methylumbelliferyl sulfate in periportal and pericentral areas of the liver lobule

4-Methylumbelliferyl sulfate was used to characterize sulfatase activity in periportal and pericentral regions of the liver lobule in the perfused rat liver. Following infusion of 1.5 mM of this organic sulfatester, free 4-methylumbelliferone and 4-methylumbelliferyl glucuronide were formed at rates of 13 and 9 mumoles/g/h, respectively, in livers from fasted, phenobarbital-treated rats. 5-Pregnen-3 beta-ol, 20-one sulfate inhibited hydrolysis and metabolite production completely, whereas perfusion with nitrogen-saturated perfusate or FCCP decreased total metabolite formation by only 30%. 4-Methylumbelliferone formed from the hydrolysis of 4-methylumbelliferyl sulfate was monitored with micro-light guides placed on periportal and pericentral areas of the liver lobule. Detection of the desulfated product was always greater in the downstream region, i.e., infusion of 4-methylumbelliferyl sulfate produced a higher fluorescence signal in pericentral areas when perfusion was in the anterograde direction, while periportal areas demonstrated higher activity during perfusion in the retrograde direction. Perfusion with nitrogen-saturated perfusate abolished these differences. Taken together, these data suggest that uptake of organic sulfateesters is partially energy dependent, follows the hepatic oxygen gradient inversely, and is a major rate determinant for sulfatase activity in the liver.

Maintenance of nicotinamide dinucleotide phosphate content and oxidation-reduction state during mixed-function oxidation of p-nitroanisole in isolated perfused livers of various species

The influence of p-nitroanisole, a substrate for mixed-function oxidation, on total NADP+ and NADPH and NADP+/NADPH ratios was examined in perfused livers from three different species. Studies were performed using livers from Sprague-Dawley rats, Syrian golden hamsters and C57BL/6J mice. Although rates of p-nitroanisole O-demethylation varied more than 16-fold in perfused livers from these species, NADP+/NADPH ratios calculated from measured concentrations of NADP+ and NADPH and from ratios calculated from substrate pairs assumed to be in near equilibrium with NADP+-dependent dehydrogenases remained remarkably constant under most conditions. Thus, rates of NADPH utilization and generation must be tightly coupled in perfused livers during high rates of mixed-function oxidation. Exceptions to the general pattern noted above occurred in livers of fasted, phenobarbital-treated rats where carbohydrate reserves were depleted and in livers from 3-methyl-cholanthrene-treated mice where rates of p-nitroanisole O-demethylation were exceptionally high. Livers from fed phenobarbital-treated rats displayed a paradoxical decrease in NADP+/NADPH ratios reflecting reduction calculated from substrates assumed to be in near equilibrium with 6-phosphogluconate dehydrogenase during mixed-function oxidation, suggesting that NADPH generation exceeded NADPH utilization in the rat in the fed state. In contrast, the NADP+/NADPH ratio calculated from measured pyridine nucleotides increased in livers of 3-methylcholanthrene-treated mice perfused with p-nitroanisole, reflecting oxidation. Moreover, the NADP+/NADPH ratio calculated from substrates assumed to be near equilibrium with 6-phosphogluconate dehydrogenase increased in livers of fasted rats, suggesting that utilization of NADPH exceeded generation. Thus, adequate carbohydrate reserves appear essential for maintenance of NADPH during high rates of mixed-function oxidation.

Hepatic drug-metabolizing enzymes in primary and secondary tumors of human liver

Significant increases in activities of epoxide hydrolase, UDP-glucuronosyltransferase, and glutathione S-transferase, and marked reductions in cytochrome P-450 mixed-function oxidase systems occur in hyperplastic nodules induced in rat liver by chemical mutagens. In contrast, activities of both oxidative (Phase I) and conjugative (Phase II) enzymes are decreased in hepatocellular carcinomas induced by peroxisome proliferators. The present work compares alterations induced by chemical mutagens or peroxisome proliferators with changes in enzyme activities that occur in primary and secondary hepatic tumors in man. The above activities, along with beta-glucuronidase and arylsulfatase, were measured in liver samples from 6 normal livers obtained at immediate autopsy, and liver specimens obtained by surgical biopsy from the following patients: 8 with hepatomas, 5 with nonmetastatic colorectal carcinomas, and 14 with metastatic colorectal carcinomas. Cytochromes P-450MP and P-450NF in addition to epoxide hydrolase were measured by immunoquantitation. Enzymes involved in conjugation reactions were either assayed fluorometrically (UDP-glucuronosyltransferase, beta-glucuronidase, sulfotransferase, and sulfatase) or spectrophotometrically (glutathione S-transferase) using umbelliferyl substrates or 1-chloro-2,4-dinitrobenzene. Secondary hepatic tumors showed no significant change in drug-metabolizing enzymes, in contrast to primary hepatomas, which displayed decreases in all of the measured drug metabolizing enzymes. Arylsulfatase was markedly depressed in primary hepatomas (14% of normal values). Thus, activities of drug-metabolizing enzymes in human primary tumors resemble those associated with altered hepatic foci induced by peroxisome proliferators such as ciprofibrate. The marked decreases in sulfatase that occurred in primary but not in secondary human tumors suggest that sulfation of endogenous compounds and xenobiotics may differ in patients with primary and secondary hepatic tumors.

Stimulation of mixed-function oxidation by NADPH in perfused mouse livers. Studies with saponin-permeabilized tissue

In perfused livers from fed and fasted beta-naphthoflavone-treated C57BL/6J mice, maximal rates of p-nitroanisole O-demethylation were 30-40 mu moles/g/hr and 15-20 mu moles/g/hr respectively. The detergent saponin, at concentrations ranging from 0.001 to 0.005%, was infused between 2 and 30 min to establish optimal conditions to permeabilize plasma membranes. Permeabilization was assessed by release of lactate dehydrogenase and stimulation of p-nitroanisole O-demethylation by citrate. Saponin (0.005% for 5 min) alone had little effect on the rates of p-nitroanisole O-demethylation or conjugation of p-nitrophenol by perfused livers. Further, dicarboxylates or NADPH had no effect on rates of monooxygenation by perfused mouse liver in the absence of saponin. In saponin-treated livers from fasted mice, however, rates of monooxygenation were increased rapidly by infusion of dicarboxylates (10 mM malate, citrate, or isocitrate) or an NADPH-generating system (60 and 110% respectively), over a 6-8 min period. During this time period, cellular energetics were not comprised as reflected by normal rates of glucuronidation of p-nitrophenol. Thus, non-permeable metabolites can enter saponin-permeabilized cells in the perfused liver. Rates of monooxygenation were increased 40-60% in livers from fed mice by citrate, NADPH (200 microM) or an NADPH-generating system. In contrast, saponin decreased mixed-function oxidation assayed in isolated microsomes incubated with an NADPH-generating system. Taken together, these data support the hypothesis that maximal rates of monooxygenation in intact hepatocytes from fed as well as fasted mice is limited by the availability of NADPH.

Mechanism of hepatotoxicity in periportal regions of the liver lobule due to allyl alcohol: studies on thiols and energy status

The possible involvement of thiols and adenine nucleotides in the selective toxicity to periportal regions by allyl alcohol was evaluated in isolated perfused rat livers. Infusion of allyl alcohol (350 microM) for 20 min depleted hepatic glutathione content by 95% in both regions of the liver lobule yet damage was undetectable as indexed by release of lactate dehydrogenase or uptake of trypan blue. Perfusion for an additional 40 min in the absence of allyl alcohol resulted in lactate dehydrogenase release (2400 U/l) and uptake of trypan blue by 75% of hepatocytes in periportal regions of the liver lobule; however, dye was not taken up by cells in pericentral areas. Because thiol content was depleted in the undamaged pericentral area, it was concluded that thiol depletion alone cannot explain local toxicity to periportal regions by allyl alcohol. Perfusion with dithioerythritol (1.5 mM) prevented damage due to allyl alcohol totally. In contrast, addition of dithioerythritol 20 min after allyl alcohol did not prevent allyl alcohol-induced damage to periportal regions indicating that irreversible changes occur during the first 20 min which ultimately lead to damage. Fasting or pretreatment of rats with diethylmaleate (0.7 g/kg; 1 hr) to deplete glutathione decreased the T1/2 required for release of lactate dehydrogenase from 45 to 35 and 22 min, respectively. When methionine was infused into livers from diethylmaleate-treated rats, the T1/2 for release of lactate dehydrogenase by allyl alcohol was increased to 45 min. Infusion of allyl alcohol for 60 min also produced a significant decrease in ATP content and in the ATP/ADP ratio in periportal but not pericentral regions of the liver lobule.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanism of hepatotoxicity to periportal regions of the liver lobule due to allyl alcohol: role of oxygen and lipid peroxidation

The objective of this study was to evaluate the role of local oxygen tension in the zone-specific hepatotoxicity due to allyl alcohol. Infusion of allyl alcohol (350 microM) for 60 min into livers from normal fed rats perfused in the anterograde direction damaged virtually all cells in periportal areas of the liver lobule as indexed by trypan blue uptake (half-maximal uptake at 38 min). Under these conditions, oxygen uptake was inhibited only in periportal hepatocytes. Increasing the time of infusion of allyl alcohol to 90 min, however, caused dye uptake in virtually all cells across the liver lobule, with half-maximal staining in pericentral regions occurring at 70 min, indicating that hepatocytes in pericentral areas are not immune to damage by allyl alcohol. In livers from diethylmaleate-treated rats, the half-time for staining of periportal and pericentral regions was 27 and 39 min, respectively. Perfusion in the retrograde direction reverses the oxygen gradient in the liver. When allyl alcohol was infused in the retrograde direction for 60 min, cells in pericentral regions took up trypan blue whereas those in periportal areas were not damaged. Concomitantly, oxygen uptake was decreased only in pericentral regions. Infusion of allyl alcohol in oxygen-saturated perfusate in either direction caused release of lactate dehydrogenase and malondialdehyde. If oxygen tension was decreased by lowering the flow rate or perfusing with air-saturated buffer in the anterograde direction, however, malondialdehyde release and dye uptake due to allyl alcohol was reduced markedly.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-dependent activation of glycogen phosphorylase in rat pheochromocytoma PC12 cells by nerve growth factor

Glycogen phosphorylase in PC12 cells exists in two forms analogous to those found in brain and muscle. The active phosphorylated form of the enzyme, phosphorylase-a, represents about 20-30% of total glycogen phosphorylase in these cells. Incubation of PC12 cells with 100 ng 7S nerve growth factor/ml increased phosphorylase-a within minutes. In contrast to nerve growth factor, insulin (6 ng/ml) and epidermal growth factor (6 ng/ml) decreased phosphorylase-a. Activation of phosphorylase-a by nerve growth factor was not accompanied by increases in cyclic AMP; however, removal of extracellular Ca2+ or incubation of cells with calcium channel blockers inhibited activation of glycogen phosphorylase by nerve growth factor.

Phenolic antioxidants: potent inhibitors of the (Ca2+ + Mg2+)-ATPase of sarcoplasmic reticulum

Bis(2-hydroxy-3-tert-butyl-5-methylphenyl)methane (bis-phenol) is the most potent inhibitor of the (Ca2+ + Mg2+)-ATPase of skeletal muscle sarcoplasmic reticulum yet identified. The compound behaves as a reversible, tight-binding inhibitor with apparent Ki = 0.3 microM. Butylated hydroxytoluene, butylated hydroxyanisole, and 4-nonylphenol are also effective inhibitors. These observations are of particular interest in light of the widespread use of such phenolic antioxidants and stabilizers in the food industry and in the manufacture of rubbers and plastics and the ease with which the compounds are extracted into organic solvents.

Hydrolysis of organic sulfates in periportal and pericentral regions of the liver lobule: studies with 4-methylumbelliferyl sulfate in the perfused rat liver

The hydrolysis of 4-methylumbelliferyl sulfate by liver sulfatases to free fluorescent 4-methylumbelliferone and the subsequent formation of the glucuronide conjugate were studied in the isolated perfused rat liver. In livers from fed, phenobarbital-treated rats, 4-methylumbelliferyl sulfate (0.25-1.5 mM) was hydrolyzed rapidly to free 4-methylumbelliferone at maximal rates of about 5 mumol/g/hr. A major fraction of the free 4-methylumbelliferone formed was converted to the glucuronide at maximal rates around 20 mumol/g/hr. Similar rates of hydrolysis were observed in livers from fasted, phenobarbital-treated or normal rats, although the ratio of glucuronide to free product was decreased markedly by fasting. In liver homogenates, however, rates of organic sulfate hydrolysis exceeded those observed in the perfused liver by at least 2-fold, suggesting that 4-methylumbelliferyl sulfate content is an important determinant of rates of hydrolysis in the perfused liver. There was a good correlation (r = 0.91) between rates of product formation and fluorescence of 4-methylumbelliferone detected from the liver surface with fiber optic light guides. Fluorescence of 4-methylumbelliferone produced from hydrolysis of 4-methylumbelliferyl sulfate was also monitored with micro-light guides placed on periportal and pericentral areas of the liver lobule for the estimation of local rates of product formation. When perfusions were in the anterograde direction, desulfation of 4-methylumbelliferyl sulfate was about 50% higher in pericentral (28.8 +/- 9.3 mumol/g/hr) than in periportal (18.2 +/- 2.7 mumol/g/hr) areas. Furthermore, 4-methylumbelliferyl sulfate content determined in microdissected samples was 1.5- to 2-fold higher in pericentral than in periportal regions of the liver lobule but the activity of 4-methylumbelliferyl sulfate sulfatase was identical in both zones of the liver lobule. We conclude, therefore, that the local substrate content is an important determinant of rates of 4-methylumbelliferyl sulfate hydrolysis in sublobular zones of the liver.

O2 uptake in periportal and pericentral regions of liver lobule in perfused liver

O2 uptake by the perfused liver decreased at O2 concentrations considerably higher than levels that caused NADH reduction when the input O2 concentration was varied. The maximal rate of O2 uptake was two- to threefold higher in periportal (137 +/- 8 mumol . g-1 . h-1; O2 concentration = 478 +/- 37 microM) than pericentral regions (59 +/- 5 mumol . g-1 . h-1; O2 concentration = 263 +/- 21 microM); however, the O2 concentration required for half-maximal O2 uptake was similar (approximately 20 microM) in the two areas. The infusion of atractyloside, antimycin A, or KCN inhibited O2 uptake in both zones by 50-85%, indicating that O2 uptake in both regions was largely dependent on mitochondrial electron transport. The content of ATP and ADP and ATP:ADP were similar in microdissected samples from periportal and pericentral areas. In contrast, when livers were perfused in the retrograde direction, O2 uptake was two- to threefold greater in pericentral than in periportal regions. Maximal rates of O2 uptake correlated with the local O2 concentration irrespective of the direction of flow when the electrode was moved across the liver lobule with a micromanipulator. Lower rates of O2 uptake in pericentral areas were not altered appreciably by infusion of agents known to uncouple oxidative phosphorylation (DNP), increase ADP supply (fructose), or increase the NADH redox state (ethanol or octanoate). These data are consistent with the hypothesis that maximal rates of O2 uptake are regulated, in part, in the perfused liver by O2 concentrations far above the Km of cytochrome oxidase for O2.

Sublobular distribution of transferases and hydrolases associated with glucuronide, sulfate and glutathione conjugation in human liver

Activities of glucuronosyltransferase, sulfotransferase, glutathione S-transferase, beta-glucuronidase and sulfatase were determined in microdissected samples of periportal and pericentral sublobular regions from four human livers obtained at immediate autopsy. New methods are presented for the microdetermination of sulfotransferase and sulfatase activities in microdissected samples weighing 0.1 to 4 micrograms dry weight using umbelliferone and 4-methylumbelliferone sulfate as substrates. The three transferases were distributed heterogeneously across the liver lobule. Glucuronosyltransferase and glutathione S-transferase were localized predominantly in pericentral regions. In contrast, sulfotransferase activity was greater in periportal than pericentral regions. Average activities for glucuronosyltransferase and sulfotransferase were 23, and 50 mumoles X gm dry wt-1 X hr-1, respectively, in periportal regions, and 34 and 38 mumoles X gm dry st-1 X hr-1, respectively, in pericentral regions. Activities of glutathione S-transferase were considerably higher than those of the other transferases and were 8.3 mmoles X gm dry wt-1 X hr-1 in periportal areas and 12.2 mmoles X gm dry wt-1 hr-1 in pericentral areas. The two hydrolases studied, beta-glucuronidase and sulfatase, were evenly distributed across the liver lobule. The presence of significant hydrolase and transferase activities in both zones of the liver lobule supports the idea that net production of both sulfate and glucuronide conjugates may be influenced by futile cycling of conjugation-deconjugation reactions in both zones of the liver. Based on enhanced formation of sulfate but not glucuronide conjugates in homogenates of human liver treated with inhibitors of the hydrolases, it is suggested that futile cycling is more pertinent to the regulation of sulfation than glucuronidation.

Effectiveness of physostigmine as a pretreatment drug for protection of rats from organophosphate poisoning

The effectiveness of physostigmine and atropine pretreatment against the lethal effects of sarin was studied in rats given lethal subcutaneous injections (130 micrograms/kg) of the organophosphate. Pretreatment of these animals with physostigmine 30 min prior to injection of sarin reduced mortality to 28% and when the drug coadministered with atropine only 4% of the animals died. The latter treatment also reduced significantly the extent and duration of symptoms due to sarin; however, atropine, pyridostigmine, and neostigmine injected alone did not protect animals against the lethal effects of sarin. Physostigmine caused only slight inhibition of cholinesterase in blood and skeletal muscle. Cholinesterase activity in blood and muscle of rats pretreated with physostigmine before sarin administration was significantly higher than in tissues from rats injected with sarin alone. In rats receiving sarin following pretreatment with physostigmine, twitch potentiation of extensor muscles and maintenance of tension during tetanic stimulation of the nerve recovered to near control levels. Muscle function recovered despite significant inhibition of cholinesterase. Effective protection against lethality by physostigmine could be related to protection of cerebral cholinesterase since inhibition of this enzyme by sarin was lowered significantly after pretreatment with physostigmine. Alternatively, physostigmine may also interact with the nicotinic acetylcholine receptor ion-channel complex directly.

Rates of pentose cycle flux in perfused rat liver. Evaluation of the role of reducing equivalents from the pentose cycle for mixed-function oxidation

Rates of NADPH production via the pentose phosphate cycle were determined in perfused livers from phenobarbital-treated rats by measuring 14CO2 production from [1-14C]glucose infused in the presence and absence of p-nitroanisole (0.2 mM), a substrate for mixed-function oxidation. In the fed state, basal rates of NADPH generation were 34-44 mumol/g/hr. p-Nitroanisole, which was metabolized at rates of 8.9 mumol/g/hr, stimulated pentose cycle-dependent NADPH production by 21-24 mumol/g/hr. Fasting for 24 hr prior to perfusion diminished pentose cycle flux by 80% and largely abolished the stimulation of the pentose cycle by p-nitroanisole. In contrast, rates of p-nitroanisole O-demethylation were only diminished slightly, to 5.7 mumol/g/hr. Fasting decreased hepatic glucose, glucose 6-phosphate, and 6-phosphogluconate contents drastically as expected. Pretreatment of rats with 6-aminonicotinamide, which is metabolized to a potent inhibitor of 6-phosphogluconate dehydrogenase, decreased rates of NADPH generation via the pentose cycle to 6.9 mumol/g/hr but did not alter rates of p-nitroanisole metabolism (8.8 mumol/g/hr). Basal rates of NADPH generation decreased from 38 to 26 mumol/g/hr during infusion of potassium cyanide (2 mM), an inhibitor of mitochondrial energy metabolism. Cyanide also decreased rates of p-nitroanisole O-demethylation by over 60%; however, stimulation of NADPH generation via the pentose cycle by p-nitroanisole was as great in the presence (17-21 mumol/g/hr) as in the absence of cyanide. Since rates of mixed-function oxidation were unaffected after virtually complete inhibition of the pentose cycle with 6-amino-nicotinamide, it is concluded that reducing equivalents for the mixed-function oxidation of p-nitroanisole are not provided by the pentose cycle under these conditions.

Effect of bile salts on rates of formation, accumulation, and export of mutagenic metabolites from benzo(a)pyrene produced by the perfused rat liver

The effect of sodium taurocholate on the biliary export of stable mutagenic phenolic glucuronide metabolites of benzo(a)pyrene from livers of corn oil- or 3-methylcholanthrene-treated rats was studied using a nonrecirculating perfusion system. Sterile bile samples were collected every 4 min and assayed for mutagens using the Ames Salmonella (Ta 98) test without addition of microsomes but containing beta-glucuronidase. Rates of export of mutagens produced from benzo(a)pyrene (20 microM) into the bile were stimulated 5-fold by the bile salt sodium taurocholate, concomitant with a 2- to 3-fold increase in bile flow. Steady-state rates of 60 and 90 revertants/g/h were observed in bile when 20 microM benzo(a)pyrene was infused into livers from corn oil or 3-methylcholanthrene-treated rats, respectively. These rates of efflux were increased to 250 and 550 revertants/g/h by the addition of taurocholate. Rates of production of mutagenic phenolic metabolites which account for the mutagenic activity were determined by adding rates of efflux into bile and effluent perfusate with rates of accumulation of metabolites in the cell. In livers from 3-methylcholanthrene-treated rats, rates (8 min) of benzo(a)pyrene phenol formation averaged 300 nmol/g/h during the initial 20 min of perfusion but increased to 450 nmol/g/h after 1 h. The addition of taurocholate increased maximal rates of phenol efflux in the bile from 6 to 148 nmol/g/h and decreased rates of phenol accumulation in intracellular stores from 342 to 220. Rates of efflux into the vena cava effluent averaged 120 nmol/g/h and were not affected by taurocholate. Infusion of dehydrotaurocholate increased the appearance of metabolites of benzo(a)pyrene in the effluent perfusate but did not change rates of efflux into bile. Taurocholate doubled rates of output of phenolic metabolites into the effluent perfusate when bile flow was arrested by perfusion with calcium-free buffer. Thus, mutagenic glucuronides from benzo(a)pyrene phenols accumulated in hepatocytes much faster than rates at which they were exported. Total rates of production of phenolic glucuronides by the liver were not affected by bile salts; however, taurocholate stimulated their export into bile, while dehydrotaurocholate increased their concentration in the effluent perfusate. Both salts probably act by displacing metabolites from intracellular binding sites.

Effect of glucose on 7-hydroxycoumarin glucuronide production in periportal and pericentral regions of the liver lobule

The effect of starvation and glucose addition on glucuronidation was assessed in sublobular regions of the lobule in perfused livers from phenobarbital-treated rats. Fibre-optic micro-light guides were placed on periportal and pericentral areas on the surface of livers to monitor the fluorescence (excitation 366 nm, emission 450 nm) of free 7-hydroxycoumarin from the tissue surface. After infusion of 7-hydroxycoumarin (80 microM) under normoxic conditions, steady-state increases in fluorescence were reached in 6-8 min in both regions. Subsequently, the formation of non-fluorescent 7-hydroxycoumarin glucuronide was inhibited completely by perfusion with N2-saturated perfusate containing 20 mM-ethanol. The difference in fluorescence between anoxic and normoxic perfusions was due to glucuronidation under these conditions. In livers from fed rats, rates of glucuronidation in periportal and pericentral regions of the liver lobule were 8 and 19 mumol/h per g, respectively. In contrast, rates of glucuronidation were 3 and 9 mumol/h per g, respectively, in periportal and pericentral regions of livers from starved rats. Infusion of glucose (20 mM) had no effect on rates of glucuronidation in livers from fed rats; however, glucose increased rates of glucuronidation rapidly (half-time, t0.5 = 1.5 min) in periportal and pericentral regions to 7 and 17 mumol/h per g, respectively in livers from starved rats. These results indicate that the rapid synthesis of the cofactor UDP-glucuronic acid derived from glucose is an important rate-determinant for glucuronidation of 7-hydroxycoumarin in both periportal and pericentral regions of livers from starved rats.

Inhibition of p-nitroanisole O-demethylation in perfused rat liver by oleate

p-Nitroanisole O-demethylation in perfused livers from fasted, phenobarbital-treated rats was rapidly and reversibly inhibited by sodium oleate (0.3 to 0.6 mM). Xylitol partially reversed this inhibitory effect. The inhibition was not mediated by a direct effect of oleate on microsomal components since concentrations of oleate ranging up to 1.0 mM did not affect p-nitroanisole O-demethylation by isolated microsomes. Infusion of 0.6 mM oleate did not alter the measured intracellular NAD+/NADH ratio but did cause a significant increase in the intracellular NADP+/NADPH ratio. A significant decrease in the ATP/ADP ratio was also observed. Oleoyl CoA inhibited p-nitroanisole O-demethylation in microsomes (Ki about 30 microM), and both oleoyl CoA and palmitoyl CoA inhibited the energy-linked nicotinamide nucleotide transhydrogenase in submitochondrial particles (Ki about 1 microM). Thus, inhibition of mixed-function oxidation in the intact liver by oleate is most likely mediated by oleoyl CoA. Oleoyl CoA inhibits mixed-function oxidation in the intact liver by acting directly on cytochrome P-450 and by decreasing generation of NADPH via inhibition of key enzymes of the citric acid cycle and the energy-linked transhydrogenase.

Stimulation of 3-benzo[a]pyrenyl glucuronide hydrolysis by calcium activation of microsomal beta-glucuronidase

Rates of hydrolysis of 3-hydroxybenzo[a]pyrenyl glucuronide by microsomal beta-glucuronidase from rat liver were 397 +/- 17 nmol/min per g protein and were half-maximal with about 100 microM substrate. Treatment of rats with phenobarbital or 3-methylcholanthrene, which elevates activities of glucuronosyltransferase(s), lowered rates of hydrolysis of benzo[a]pyrene glucuronide by 25%. Hydrolysis of the glucuronide by microsomal beta-glucuronidase was stimulated by micromolar concentrations of calcium in the range existing in cytosol of hepatocytes (apparent Km approximately 0.2 microM). Thus, humoral factors that change intracellular concentrations of free calcium may alter the production and export of glucuronides of benzo[a]pyrene metabolites from the liver.

Sublobular compartmentation of pharmacologic events (SCOPE): metabolic fluxes in periportal and pericentral regions of the liver lobule

New techniques have been developed employing microlight guides and miniature O2 electrodes which permit metabolic events to be studied noninvasively in periportal and pericentral zones of the liver lobule. These events include O2 uptake, fat and carbohydrate metabolism, monooxygenation and glucuronidation. The lobular distribution of maximal enzyme activities measured by immunohistochemical or microchemical techniques does not always correlate with metabolic flux rates as measured in periportal and pericentral regions with the new noninvasive methods. The region of the liver lobule exhibiting highest metabolic flux rates can be shifted to another region of the lobule by changing the direction of flow from anterograde to retrograde. These data are consistent with the hypothesis that many metabolic pathways (e.g., oxygen, carbohydrate and fat metabolism which are under dynamic short-term regulation) operate below maximal velocity in intact hepatocytes.