Protection from paraquat-induced lung damage and lethality in adult rats pretreated with clofibrate

ADME/T-based strategies for paraquat detoxification: Transporters and enzymes

Paraquat (PQ) is a toxic, organic herbicide for which there is no specific antidote. Although banned in some countries, it is still used as an irreplaceable weed killer in others. The lack of understanding of the precise mechanism of its toxicity has hindered the development of treatments for PQ exposure. While toxicity is thought to be related to PQ-induced oxidative stress, antioxidants are limited in their ability to ameliorate the untoward biological responses to this agent. Summarized in this review are data on the absorption, distribution, metabolism, excretion, and toxicity (ADME/T) of PQ, focusing on the essential roles of individual transporters and enzymes in these processes. Based on these findings, strategies are proposed to design and test specific and effective antidotes for the clinical management of PQ poisoning.

Paraquat poisonings: mechanisms of lung toxicity, clinical features, and treatment

Paraquat dichloride (methyl viologen; PQ) is an effective and widely used herbicide that has a proven safety record when appropriately applied to eliminate weeds. However, over the last decades, there have been numerous fatalities, mainly caused by accidental or voluntary ingestion. PQ poisoning is an extremely frustrating condition to manage clinically, due to the elevated morbidity and mortality observed so far and due to the lack of effective treatments to be used in humans. PQ mainly accumulates in the lung (pulmonary concentrations can be 6 to 10 times higher than those in the plasma), where it is retained even when blood levels start to decrease. The pulmonary effects can be explained by the participation of the polyamine transport system abundantly expressed in the membrane of alveolar cells type I, II, and Clara cells. Further downstream at the toxicodynamic level, the main molecular mechanism of PQ toxicity is based on redox cycling and intracellular oxidative stress generation. With this review we aimed to collect and describe the most pertinent and significant findings published in established scientific publications since the discovery of PQ, focusing on the most recent developments related to PQ lung toxicity and their relevance to the treatment of human poisonings. Considerable space is also dedicated to techniques for prognosis prediction, since these could allow development of rigorous clinical protocols that may produce comparable data for the evaluation of proposed therapies.

Inhibitory activity of epigallocatechin gallate (EGCg) in paraquat-induced microsomal lipid peroxidation--a mechanism of protective effects of EGCg against paraquat toxicity

Recently we have reported that epigallocatechin gallate (EGCg), a major component of Japanese green tea, significantly increased the survival rate of paraquat (Pq) poisoned mice. This paper describes two biochemical activities of EGCg, which relate to its protective effects against Pq toxicity. EGCg inhibited Pq-induced microsomal malondialdehyde (MDA) productions in rat liver microsome system containing 40 microM FeSO(4). Forty micromolar EGCg inhibited MDA production significantly. EGCg may inhibit the Pq-induced MDA production by at least two mechanisms. One may be iron-chelating activity as the inhibition disappeared when excess amounts of FeSO(4) were added to the reaction mixture, which indicated that EGCg reduced iron driven lipid peroxidation by pulling out available irons in the reaction mixture. The other is radical scavenging activity. EGCg scavenged DMPO-OOH spin adducts generated by the microsome-Pq system. The dose response curve of EGCg was similar to that obtained by ascorbic acid which is a typical water-soluble radical scavenger. Although ascorbic acid had a potential activity of scavenging superoxide radicals, it can not be recommended to use for the treatment of Pq poisoning, because ascorbic acid acts as a pro-oxidant in the presence of free transition metal ions by accelerating the Fenton reaction (Fe(2+)+H(2)O(2)-->Fe(3+)+OH(-)+OH*), which is responsible for lipid peroxidation. On the contrary, EGCg inhibited iron-driven lipid peroxidation presumably not only by chelating to Fe ions but also by scavenging superoxide radicals, which are responsible for the reduction of ferric (Fe(3+)) to ferrous (Fe(2+)) that catalyzes the Fenton reaction. Chelating and radical scavenging activity of EGCg can be expected simultaneously in the occurrence of Pq toxicity, which may explain the protective effects of EGCg against Pq toxicity.

Effects of Fuller's Earth and activated charcoal on oral absorption of paraquat in rabbits

1. Thirty male rabbits of local strain (weighing 1.5-2 kg) were divided into five groups. Four groups were treated with an oral dose of paraquat, which was followed by either Fuller's Earth or activated charcoal 0.5 or 2.0 h later. The remaining group acted as the control group and was treated only with an oral dose of paraquat. The dose of paraquat was 20.0 mg/kg given in a concentration of 20.0 mg/mL. 2. Both adsorbents were administered in 15 mL normal saline as a 30% slurry. Blood was sampled from the ear vein 0.5, 1, 2, 4, 8 and 24h after the administration of paraquat. 3. Paraquat concentration was determined spectophotometrically at 600 nm by comparing against a standard curve of paraquat obtained by the addition of standard paraquat into normal rabbit serum and extracting interfering substances with ether. 4. The results of the present study show that either adsorbent can bring down the serum paraquat level. There was no significant difference found in the effectiveness of either adsorbent. 5. It is concluded that the administration of an adsorbent as early as possible will help in the reduction of paraquat absorption from the gastrointestinal tract. 6. Activated charcoal is still effective in lowering serum paraquat concentration when given more than 1 h after ingestion of paraquat.

A novel pharmacological approach for paraquat poisoning in rat and A549 cell line using ambroxol, a lung surfactant synthesis inducer

Paraquat (PQ) is a widely used herbicide that causes acute adult respiratory distress syndrome (ARDS) and chronic lung damage (diffuse fibrosis). One of the earliest biochemical effects induced by PQ is damage to type II pneumocytes with consequent depletion of surfactant. With the aim of counteracting the toxic effects of PQ, a series of investigations were performed into the possible protective effect of the drug ambroxol, which induces the synthesis of surfactant in lung alveolar type II cells. The number of survivors and survival time of rats treated ip with 35 mg PQ/kg was significantly increased by 3 days of ambroxol pretreatment and by ambroxol treatment 30 min or 2 hr after PQ. Total phospholipid content in lung and bronchoalveolar lavage fluid (BALF) was significantly reduced 30 hr after treatment with PQ alone. The association of ambroxol with PQ significantly antagonized this reduction. In BALF the ratio between palmitic acid and stearic acid concentrations was significantly lower in animals treated with PQ alone but was returned to normal by the association with ambroxol. The cell line A549, exposed in vitro to PQ concentrations from 0.5 x 10(-4) to 2 x 10(-3) M, showed a significant dose-dependent loss of viability. Cells pretreated with ambroxol (10 mg/ml) were more resistant to PQ and their viability started to decrease significantly only from a PQ concentration of 0.8 x 10(-3) M. Membrane microviscosity was measured on the same cells. Cells treated with PQ alone showed a reduction of membrane microviscosity, which was significantly counteracted by ambroxol pretreatment. The curves of modification of membrane microviscosity of cells treated with PQ and with ambroxol plus PQ paralleled those of cell viability, indicating that the stimulation of surfactant synthesis in vitro may be a prerequisite for counteracting some of the early effects of PQ.

Paraquat poisoning. An overview of the current status

Paraquat is a bipyridyl compound with no known chronic toxicity or teratogenicity. It is poorly absorbed when inhaled, but causes severe illness when ingested orally, death usually occurring within 2 days of ingestion of 50 mg/kg. At lower doses death may be delayed for several weeks. The toxic compound accumulates in lung tissue where free radicals are formed, lipid peroxidation is induced and nicotinamide adenine dinucleotide phosphate (NADPH) is depleted. This produces diffuse alveolitis followed by extensive pulmonary fibrosis. The most important prognostic indicator is the quantity of paraquat absorbed, as shown by the plasma paraquat concentration. While renal failure will develop in the majority of those patients who eventually die, it may not, if present alone, indicate a fatal outcome. The absence of caustic burns in the upper digestive tract indicates a good prognosis. Treatment of paraquat poisoning remains ineffective, but Fuller's earth, activated charcoal and resins may prevent some absorption of the toxin. When tubular necrosis occurs, renal excretion of the compound decreases rapidly. A 3-compartment pharmacokinetic model has been described following ingestion of tracer doses including a 'deep' compartment for active pulmonary accumulation. Haemodialysis, haemoperfusion and forced dialysis have been attempted, with no clear improvement in survival rates. Superoxide dismutase, glutathione peroxidase, N-acetylcysteine and other 'free radical scavengers' have failed to alter the outcome in poisoned patients. Other theoretical treatments, such as deferoxamine, immunotherapy, NADPH repletion and lung transplantation still require clinical validation.

Effect of dimethylthiourea on plasma paraquat concentration

In a previous study, administration of the hydroxyl radical scavenger, dimethylthiourea (DMTU), and paraquat was associated with higher mortality in rats than was paraquat alone. In the present study, the possibility was evaluated that administration of DMTU increased plasma paraquat levels. Plasma paraquat concentrations were measured in Sprague-Dawley rats 1, 2, 4, 8 and 24 h after intraperitoneal (i.p.) injection of 29 mg paraquat cation/kg body wt. Another group of rats was treated identically except that they received i.p. injections of DMTU before injections of paraquat. Administration of DMTU was associated with increased plasma paraquat concentration (P less than 0.01). Pharmacokinetic analyses indicated that, compared to rats receiving paraquat alone, rats given paraquat and DMTU showed: (1) greater area under the paraquat concentration time-curve; (2) lower total body paraquat clearance; and (3) smaller apparent volume of distribution. Plasma biochemical studies indicated that paraquat caused hyperglycemia as well as an early reduction (compared to controls) in hepatic enzymes. We conclude that: (1) DMTU administration is associated with increased plasma paraquat concentrations; and (2) impaired synthesis or inhibition of release of hepatic proteins may be an early effect of paraquat.

Methylene blue competes with paraquat for reduction by flavo-enzymes resulting in decreased superoxide production in the presence of heme proteins

Methylene blue competes 100 to 600 times more effectively than paraquat for reduction by three different flavo-containing enzymes; xanthine oxidase, NADH cytochrome c reductase, and NADPH cytochrome c reductase. Paraquat and methylene blue both interact with deflavo xanthine oxidase, indicating that neither electron acceptor reacted at the FAD site of the enzyme where molecular oxygen is reduced to superoxide. As the paraquat radical also directly reduced acetylated cytochrome c the hemeprotein could not be utilized for measuring superoxide production in the presence of the herbicide. In the presence of cytochrome c the methylene blue caused a sharp decrease in both paraquat-induced superoxide and hydroxyl radical production.

Two hypolipidemic peroxisome proliferators increase the number of lamellar bodies in alveolar cells type II of the rat lung

Male rats treated with either clofibrate or nafenopin, two peroxisome proliferating compounds with potent hypolipidemic properties, show identical structural changes in their lungs. In both cases, two types of lung cells were affected by these agents: (i) the alveolar epithelial cells type II and (ii) the intraalveolar macrophages. These lung cells are known to be involved in the metabolism of the pulmonary surfactant which serves to reduce the surface tension within alveoli. The size of the alveolar cells type II was conspicuously increased in the treated lungs. Compared to controls, intraalveolar macrophages apparently were slightly more numerous. Their enlarged cytoplasm was strongly vacuolated. The osmiophilic lamellar bodies within alveolar cells type II represent the intracellular presecretory pulmonary surfactant. Their number per individual alveolar cell type II was estimated by means of light microscopic morphometry on Epon-embedded semithin sections. Compared to control lungs, the number was increased by about 30% in rats treated with clofibrate (11.4 +/- 0.5 lamellar inclusions in the control cells; P less than 0.001). The increase in the number of lamellar bodies per type II cell was close to 60% in animals fed the nafenopin diet. In contrast the frequency of alveolar cells type II, estimated per area of lung tissue, remained unchanged. These results demonstrate that clofibrate and nafenopin, two drugs with hypolipidemic properties, cause identical structural changes in the rodent lung. It is concluded from these data that (i) the morphological changes observed in the surfactant metabolizing cells represent a specific action of hypolipidemic agents at the lungs and (ii) hypolipidemic peroxisome proliferators influence the metabolism of the pulmonary surfactant.

Selective inhibition of bipyridyl-stimulated NADPH oxidation by ascorbic acid

The effect of the bipyridyl herbicides, paraquat and diquat (0.01-1.0 mM), on NADPH oxidation was determined in vitro using rat lung microsomal preparations. Experiments were performed in the absence of mixed function oxidation (MFO) substrates, in the presence of substrates (ethylmorphine or benzphetamine), and also in the presence of ascorbic acid (0.1-10.0 mM). NADPH oxidation was stimulated by both herbicides in the absence or presence of either substrate in a concentration-dependent manner. When ascorbic acid was included in incubations along with either bipyridyl, the stimulated rate of NADPH oxidation decreased in the presence of benzphetamine but the stimulation was unaltered in the presence of ethylmorphine or in the absence of substrate. These studies indicate that ascorbic acid may offer some protection from bipyridyl-mediated NADPH oxidation in rat lung microsomal fractions, but that protection appears to be dependent upon the simultaneous presence of specific MFO substrates.

Modification of surfactant metabolizing cells in rat lung by clofibrate, a hypolipidemic peroxisome proliferating agent. Evidence to suggest that clofibrate influences pulmonary surfactant metabolism

The influence of clofibrate (ethyl-alpha-p-chlorophenoxy-isobutyrate), a hypolipidemic peroxisome proliferating agent, has been tested on the lungs of adult male rats. Drug administration for 7 days caused structural changes in two types of lung cells, both of which are involved in the metabolism of the pulmonary surfactant. By light microscopy the prominent features were the presence of enlarged type II alveolar epithelial cells and foamy intraalveolar macrophages. Compared with controls, type II cells in treated rats apparently contained more numerous surfactant-containing lamellar bodies, as visualized in semi-thin sections of Epon-embedded tissue. This difference was quantified morphometrically by light microscopy: the number of lamellar bodies was estimated as the profile number per individual type II alveolar cell, transsected at its nucleus. Clofibrate administration for 7 days resulted in a significant increase in the number of the lamellar inclusions. In contrast the number of type II alveolar cells per area of lung remained unchanged. There was no evidence of atelectasis or inflammatory infiltration in the drug-treated lungs, a finding confirmed in sections of perfusion-fixed, paraffin-embedded whole lung-lobes. By electron microscopy the lamellar inclusion bodies in the type II alveolar cells in treated rats, apart from being more numerous and sometimes smaller, were morphologically identical to those in controls. The vacuolated alveolar macrophages seen in treated rats also contained various lamellar phospholipid inclusions.(ABSTRACT TRUNCATED AT 250 WORDS)

Glutathione depleting agents and lipid peroxidation

The mechanisms by which glutathione (GSH) depleting agents produce cellular injury, particularly liver cell injury have been reviewed. Among the model molecules most thoroughly investigated are bromobenzene and acetaminophen. The metabolism of these compounds leads to the formation of electrophilic reactants that easily conjugate with GSH. After substantial depletion of GSH, covalent binding of reactive metabolites to cellular macromolecules occurs. When the hepatic GSH depletion reaches a threshold level, lipid peroxidation develops and severe cellular damage is produced. According to experimental evidence, the cell death seems to be more strictly related to lipid peroxidation rather than to covalent binding. Loss of protein sulfhydryl groups may be an important factor in the disturbance of calcium homeostasis which, according to several authors, leads to irreversible cell injury. In the bromobenzene-induced liver injury loss of protein thiols as well as impairment of mitochondrial and microsomal Ca2+ sequestration activities are related to lipid peroxidation. However, some redox active compounds such as menadione and t-butylhydroperoxide produce direct oxidation of protein thiols.

Measurement of lipid peroxidation in vivo: a comparison of different procedures

A study was undertaken to investigate whether some of the methods commonly used to detect lipid peroxidation of cellular membranes in vivo correlate with each other. The study was performed with the livers of bromobenzene-intoxicated mice, in which lipid peroxidation develops when the depletion of glutathione (GSH) reaches a threshold value. The methods tested and compared were the following: i) measurement of the malondialdehyde (MDA) content of the liver; ii) detection of diene conjugation absorption in liver phospholipids; iii) measurement of the loss of polyunsaturated fatty acids in liver phospholipids; and iv) determination of carbonyl functions formed in acyl residues of membrane phospholipids as a result of the peroxidative breakdown of phospholipid fatty acids. Correlations among the values obtained with these methods showed high statistical significances, indicating that the procedures measure lipid peroxidation in vivo with comparable reliability. Analogously, the four methods appeared also to correlate when applied to in vitro microsomal lipid peroxidation.

Pharmacological treatments of paraquat poisoning

A large number of pharmacological techniques aimed at modifying paraquat toxicity have been investigated. There is no convincing controlled evidence that any are unequivocally useful. Studies with an ascorbic acid and riboflavin combination appear effective in rats, and there is a suggestion that cyclophosphamide and dexamethasone may in some way alter paraquat toxicity in man and by pretreatment, but not concurrent treatment, also in the rat. Further controlled studies are required of these treatments in patients who are potentially salvageable. There is a need for a rapid paraquat assay for clinical use in order that patients in this category can be identified quickly and included in appropriate controlled studies.

Detection of in vivo lipid peroxidation using the thiobarbituric acid assay for lipid hydroperoxides

Thiobarbituric acid (TBA) assays which have been modified for detection of lipid hydroperoxides appear to be useful for demonstration of in vivo lipid peroxidation. Since these methods require heating tissue membranes with the buffered TBA, there is a possibility of interference from the detection of autoxidation that occurs during heating. These studies were undertaken to investigate conditions which favor TBA color production from hydroperoxide while limiting autoxidation during the assay. An acetic acid-sodium acetate buffered (pH 3.6) TBA assay was used. Heating linoleic acid hydroperoxide with 50 microM ferric iron or under nitrogen nearly doubled color production compared to heating it with no added iron or under air. The lipid antioxidant butylated hydroxytoluene inhibited color production from fatty acid hydroperoxides. When tissue fractions, including liver and lung microsomes and lung whole membranes, were heated in the assay, color production was greater under air than under nitrogen and was much greater under oxygen. When liver microsomes from carbon tetrachloride-exposed rats were used, color was increased only when oxygen was present in the heating atmosphere. The results with tissue fractions appear to demonstrate autoxidation during color development rather than the presence of preformed hydroperoxides. Finally, it was found that color production from membrane fractions was dependent on the vitamin E content of the membranes. It appears that autoxidation during heating should be limited by heating under nitrogen and not by adding antioxidants, which inhibit color production from hydroperoxides. As the vitamin E effect demonstrates, antioxidant status must be considered, since a change in color production could result from a change in antioxidant content without the accumulation of lipid hydroperoxides.

The toxic effect of diquat on the rat lung after intratracheal administration

Diquat toxicity to the lung was evaluated in rats. Diquat was administered intratracheally (i.t.) and its toxic potential was compared with that of paraquat. Diquat showed toxic effects in the lung when administered i.t. In previous research, diquat administered by the intravenous (i.v.) or oral (p.o.) route had little effect on the lung. a sufficient dose of diquat administered i.t. would be potentially toxic to the lung, but diquat is much less toxic to the lung than paraquat, which is actively taken up by lung tissue. This suggests that diquat is not actively taken up by lung tissue when administered i.t., but may be passively taken up by the lung tissue and induce lung damage.

Failure of desferrioxamine to modify the toxicity of paraquat in rats

The feasibility of using desferrioxamine (DF), an iron chelator, as a therapeutic agent against paraquat (PQ++) toxicity in male Sprague-Dawley rats was explored, based on the rationale of limiting toxic hydroxyl radical production from hydrogen peroxide by removing redox-active iron. Body weights, mortality, and lung histopathology were followed for periods up to 14 days after intraperitoneal injection of PQ++ (20 or 25 mg/kg body weight) with or without concurrent daily subcutaneous injections of DF (300 mg/day). Animals receiving PQ++ showed the expected typical patterns of mortality and of lung histopathology, namely: marked edema, subpleural hemorrhage, acute inflammation, perivascular mononuclear cell infiltrates, sloughing of alveolar and bronchiolar lining cells, and diffuse interstitial fibrosis. Desferrioxamine alone was non-toxic. Surprisingly, results when both PQ++ and DF were administered indicated a failure of DF to ameliorate toxic effects of PQ++ in the lung, and even suggested an accentuation of PQ++-induced damage by DF. Mortality data showed that PQ++/DF animals died in greater numbers (20 mg PQ++/kg) or died earlier (25 mg PQ++/kg) than animals receiving DF alone. Qualitative histopathology in PQ++/DF animals was comparable to PQ++ animals in early stages, but damage was more severe in both incidence and severity of lesions in PQ++/DF animals, particularly at the 25 mg PQ++/kg dose level. After 14 days, surviving animals receiving PQ++ alone showed almost complete resolution of previous inflammation and other acute effects, whereas in the only surviving PQ++/DF animal initial fibrosis had persisted and become more generalized. Thus, chelation therapy with DF may not be straightforward in its effects on PQ++ toxicity.