Relevance of NADPH depletion and mixed disulphide formation in rat lung to the mechanism of cell damage following paraquat administration

Evaluation of the efficiency of simvastatin loaded PLGA nanoparticles against acute paraquat-intoxicated rats

Here, we reported a novel nanotherapeutic platform for paraquat (PQ)-induced acute lung injury in animal models using simvastatin (SV) loaded into Poly (lactic-co-glycolic acid) (PLGA) nanoparticles (NPs). In this way, Male Wistar rats orally received PQ (120 mg / kg) plus saline, SV (20 mg / kg) or PLGA-SV NPs containing 5, 10 and 20 mg SV/ kg. The levels of TNFα, IL-1β, IL-6 and glutathione content were evaluated. In addition, the pathological changes in the lung were monitored. Our results indicated that PQ (120 mg/kg) significantly reduced the body weight of rats compared to the control group. The most decrease in the level of inflammatory cytokines, bleeding, alveolar destruction as well as lymphocytic infiltration in the lung was observed at group treated with PLGA-SV NPs (10 mg). Free SV (20 mg) as well as PLGA-SV NPs (5 mg) modulated the inflammatory factors and glutathione content, however, they could not prevent tissue damage of PQ. Interestingly, PLGA-SV NPs (20 mg) could not improve the PQ- induced pulmonary damage. In conclusion, PLGA-SV NPs (10 mg) attenuated PQ-induced lung injury. The underlying mechanism may be relevant to increasing glutathione levels and inhibition of the production of inflammatory factors.

Systemic inflammation and oxidative stress induced by inhaled paraquat in rat improved by carvacrol, possible role of PPARγ receptors

Control rats were exposed to saline aerosol, two groups were exposed to paraquat (PQ), 27 (PQ-L) and 54 (PQ-H) mg/m3 aerosols and six groups were treated with carvacrol, 20 (C-L) and 80 (C-H) mg/kg/day, pioglitazone, 5 (Pio-L) and 10 (Pio-H) mg/kg/day, C-L+Pio-L and dexamethasone, 0.03 mg/kg/day, for 16 days after the end of exposure to PQ-H. Different variables were measured after the end of treatment period. Total and differential white blood cells counts, nitrite, malondialdehyde, interleukin (IL)-10, and interferon-gamma levels were significant increased, but thiol, superoxide dismutase, catalase, IL-17, and tumor necrosis factor alpha were decreased in the blood due to both doses of PQ (p < 0.05-p < 0.001). Most measured parameters were significantly improved in treated groups with both doses of carvacrol, pioglitazone, the combination of C-L+Pio-L and dexamethasone compared to PQ-H group (p < 0.05-p < 0.001). Treatment with C-L+Pio-L showed significantly higher effects compared to each one alone (p < 0.05-p < 0.001). Systemic oxidative stress and inflammation due to inhaled PQ were improved by carvacrol and pioglitazone. Higher effects of C-L+Pio-L than each one alone suggests carvacrol modulating PPAR-γ receptors.

Zataria multiflora and Pioglitazone Affect Systemic Inflammation and Oxidative Stress Induced by Inhaled Paraquat in Rats

The effects of Zataria multiflora (Z. multiflora) and pioglitazone (a PPAR-γ agonist) alone and in combination, on systemic inflammation and oxidative stress induced by inhaled paraquat (PQ) as a herbicide, which induced inflammation in rats, were examined. Rats were exposed to (1) saline (control) and (2) 54 mg/m³ PQ aerosols (8 times, every other day, each time for 30 min) without treatment or treated with (3 and 4) two doses of Z. multiflora (200 and 800 mg/kg/day), (5 and 6) two doses of pioglitazone (5 and 10 mg/kg/day), (7) low doses of Z.multiflora + pioglitazone, (Pio-5+Z-200 mg/kg/day) or (8) dexamethasone (0.03 mg/kg/day) for 16 days, after the last PQ exposure. Different variables were measured at the end of the treatment period. Exposure to PQ significantly increased total and differential white blood cells (WBC) counts, serum levels of nitrite (NO₂), malondialdehyde (MDA), interleukin- (IL) 17, and tumor necrosis factor alpha (TNF-α), but reduced thiol, superoxide dismutase (SOD), catalase (CAT), IL-10, and interferon-gamma (INF-γ) (p < 0.05 to p < 0.001). Most measured parameters were significantly improved in groups treated with either doses of the extract, pioglitazone, Pio-5+Z-200 mg/kg/day, or dexamethasone compared to the PQ group (p < 0.05 to p < 0.001). The combination of low doses of Pio-5+Z-200 mg/kg/day showed significantly higher effects compared to each one alone (p < 0.05 to p < 0.001). Systemic oxidative stress and inflammation due to inhaled PQ were improved by Z. multiflora and pioglitazone. Higher effects of Pio-5+Z-200 mg/kg/day compared to each one alone suggest modulation of PPAR-γ receptors by the plant extract, but further studies using PPAR-γ antagonists need to be done in this regard.

Association between liberal oxygen therapy and mortality in patients with paraquat poisoning: A multi-center retrospective cohort study

Paraquat (N, N'-dimethyl-4, 4'-bipyridinium dichloride, PQ) intoxication is a common cause of lethal poisoning. This study aimed to identify the risk of using liberal oxygen therapy in patients with PQ poisoning. This was a multi-center retrospective cohort study involving four medical institutions in Taiwan. Data were extracted from the Chang Gung Research Database (CGRD) from January 2004 to December 2016. Patients confirmed to have PQ intoxication with a urine PQ concentration ≥ 5 ppm were analyzed. Patients who received oxygen therapy before marked hypoxia (SpO2 ≥ 90%) were defined as receiving liberal oxygen therapy. The association between mortality and patient demographics, blood paraquat concentration (ppm), and liberal oxygen therapy were analyzed. A total of 416 patients were enrolled. The mortality rate was higher in the liberal oxygen therapy group (87.8% vs. 73.7%, P = 0.007), especially in 28-day mortality (adjusted odds ratio [aOR]: 4.71, 95% confidence interval [CI]: 1.533–14.471) and overall mortality (aOR: 5.97, 95% CI: 1.692–21.049) groups. Mortality in patients with PQ poisoning was also associated with age (aOR: 1.04, 95% CI: 1.015–1.073), blood creatinine level (aOR: 1.49, 95% CI: 1.124–1.978), and blood paraquat concentration (ppm) (aOR, 1.51; 95% CI: 1.298–1.766). Unless the evidence of hypoxia (SpO2 < 90%) is clear, oxygen therapy should be avoided because it is associated with increased mortality.

Carvacrol and Zataria multiflora influenced the PPARγ agonist effects on systemic inflammation and oxidative stress induced by inhaled paraquat in rat

OBJECTIVES

The effects of PPAR-γ agonist alone and in combination with carvacrol and Zataria multiflora on inhaled paraquat (PQ) induced-systemic inflammation and oxidative stress were examined.

MATERIALS AND METHODS

Control group exposed to normal saline aerosol, one group exposed to 54 mg/m3 PQ aerosol and four groups exposed to PQ aerosol and treated with 5 mg/kg/day pioglitazone, pioglitazone + 200 mg/kg/day Z. multiflora extract, pioglitazone + 20 mg/kg/day carvacrol, and 0.03 mg /kg/day dexamethasone for 16 days after the end of exposure to PQ were studied. Exposure to normal saline or PQ was performed every other days for 30 min (8 times). Different variables were measured after the end of treatment period.

RESULTS

PQ exposure significantly increased serum levels of NO2, MDA and IL-6 but dexreased CAT and IFN-γ levels and IFN-γ/IL-6 ratio compared to control group (P<0.01 to P<0.001). Treatment with pioglitazone only improved serum level of MDA (P<0.01). Treatment with combination of pioglitazone and carvacrol as well as treatment with dexamethasone improved all measured variables compared to PQ exposed group (P<0.05 to P<0.001). The effects of pioglitazone + Z. multiflura and pioglitazone + carvacrol on almost all measured variables were significantly higher than pioglitazone alone (P<0.05 to P<0.001).

CONCLUSION

The effects of combination therapy of pioglitazone with Z. multiflora or carvacrol on inhaled paraquat (PQ) induced-oxidative stress and systemic inflammation were higher than the effects of pioglitazone alone. These results suggested that the effects of the extract and carvacrol may mediated through PPAR-γ receptors.

Amelioration of Paraquat-Induced Pulmonary Injury by Mesenchymal Stem Cells

Acute paraquat (PQ) poisoning induces redox cycle and leads to fatal injury of lung. Clinical management is supportive in nature due to lack of effective antidote, and the mortality is very high. Mesenchymal stem cells (MSCs) process the properties of immunomodulation, anti-inflammatory, and antifibrotic effects and oxidative stress resistance. MSC transplantation may theoretically serve as an antidote in PQ intoxication. In this study, we examined the potential therapeutic effects of MSCs in PQ-induced lung injury. The degree of PQ toxicity in the rat type II pneumocyte cell line, L2, and MSCs was evaluated by examining cell viability, ultrastructural changes, and gene expression. L2 cells treated with 0.5 mM PQ were cocultured in the absence or presence of MSCs. For the in vivo study, adult male SD rats were administered an intraperitoneal injection of PQ (24 mg/kg body weight) and were divided into three groups: group I, control; group II, cyclophosphamide and methylprednisolone; group III, MSC transplantation 6 h after PQ exposure. MSCs were relatively resistant to PQ toxicity. Coculture with MSCs significantly inhibited PQ accumulation in L2 cells and upregulated the expression of antioxidative heme oxygenase 1 and metallothionein 1a genes, reversed epithelial-to-mesenchymal transition, and increased the viability of PQ-exposed L2 cells. Treatment with MSCs resulted in a significant reduction in severity of liver and renal function deterioration, alleviated lung injury, and prolonged the life span of rats. Altogether, our results suggest that MSCs possess antidote-like effect through multifactorial protection mechanism. The results of this preclinical study demonstrate that transplantation of MSCs may be a promising therapy and should be further validated clinically.

Mechanism of cytotoxicity of paraquat

Acute paraquat poisoning seems to be very complex because many possible mechanisms of paraquat cytotoxicity have been reported. Some may not be the cause of paraquat poisoning but the result or an accompanying phenomenon of paraquat action. The mechanism critical for cell damage is still unknown. Paraquat poisoning is probably a combination of several paraquat actions. Arguing which mechanism is more critical may not be important, and these clarified mechanisms should be connected and utilized in the development of treatment for paraquat poisoning. Many people still die of pulmonary fibrosis after paraquat exposure. The next target of study will be to verify the mechanism of pulmonary fibrosis by paraquat on the basis of the outcome of studies such as this review.

Medical management of paraquat ingestion

Poisoning by paraquat herbicide is a major medical problem in parts of Asia while sporadic cases occur elsewhere. The very high case fatality of paraquat is due to inherent toxicity and lack of effective treatments. We conducted a systematic search for human studies that report toxicokinetics, mechanisms, clinical features, prognosis and treatment. Paraquat is rapidly but incompletely absorbed and then largely eliminated unchanged in urine within 12-24 h. Clinical features are largely due to intracellular effects. Paraquat generates reactive oxygen species which cause cellular damage via lipid peroxidation, activation of NF-κB, mitochondrial damage and apoptosis in many organs. Kinetics of distribution into these target tissues can be described by a two-compartment model. Paraquat is actively taken up against a concentration gradient into lung tissue leading to pneumonitis and lung fibrosis. Paraquat also causes renal and liver injury. Plasma paraquat concentrations, urine and plasma dithionite tests and clinical features provide a good guide to prognosis. Activated charcoal and Fuller's earth are routinely given to minimize further absorption. Gastric lavage should not be performed. Elimination methods such as haemodialysis and haemoperfusion are unlikely to change the clinical course. Immunosuppression with dexamethasone, cyclophosphamide and methylprednisolone is widely practised, but evidence for efficacy is very weak. Antioxidants such as acetylcysteine and salicylate might be beneficial through free radical scavenging, anti-inflammatory and NF-κB inhibitory actions. However, there are no published human trials. The case fatality is very high in all centres despite large variations in treatment.

[Paraquat poisoning. Case report and overview]

Paraquat poisoning in Germany is rare. Because plasma levels do not necessarily match the ingested amount of paraquat, repeated measurement of plasma levels is imperative. There is a large potential in the prehospital phase to improve prognosis: further resorption must be terminated by rigorous charcoal administration and early tracheal intubation if necessary. Because paraquat can be resorbed by dermal contact, steps to ensure sufficient protection of emergency medical personnel must be taken.As soon as further resorption has been prevented sufficiently, forced diuresis, renal replacement therapy, and hemoperfusion can be of help, but still remain controversial. To reduce pulmonary fibrosis, inspiratory oxygen concentrations must be adjusted to the minimal amount needed to ensure satisfactory tissue oxygenation. Data supporting the advantageous use of cyclophosphamide combined with methylprednisolone for the treatment of pulmonary fibrosis were recently published. Since the toxic mechanism implies a mismatch of oxidants and anti-oxidants, co-administration of ascorbic acid and N-acetylcysteine are simple treatments with few side effects.

Nitric oxide in paraquat-mediated toxicity: A review

Paraquat, a cationic herbicide, produces degenerative lesions in the lung and in the nervous system after systemic administration to man and animals. Many cases of acute poisoning and death have been reported over the past few decades. Although a definitive mechanism of toxicity of paraquat has not been delineated, a cyclic single electron reduction/oxidation is a critical mechanistic event. The redox cycling of paraquat has two potentially important consequences relevant to the development of toxicity: the generation of the superoxide anion, which can lead to the formation of more toxic reactive oxygen species which are highly reactive to cellular macromolecules; and the oxidation of reducing equivalents (e.g., NADPH, reduced glutathione), which results in the disruption of important NADPH-requiring biochemical processes necessary for normal cell function. Nitric oxide is an important signaling molecule that reacts with superoxide derived from the paraquat redox cycle, to form the potent oxidant peroxynitrite, which causes serious cell damage. Although nitric oxide has been involved in the mechanism of paraquat-mediated toxicity, the role of nitric oxide has been controversial as both protective and harmful effects have been described. The present review summarizes recent findings in the field and describes new knowledge on the role of nitric oxide in the paraquat-mediated toxicity.

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.

P-glycoprotein induction: an antidotal pathway for paraquat-induced lung toxicity

The widespread use of the nonselective contact herbicide paraquat (PQ) has been the cause of thousands of deaths from both accidental and voluntary ingestion. The main target organ for PQ toxicity is the lung. No antidote or effective treatment to decrease PQ accumulation in the lung or to disrupt its toxicity has yet been developed. The present study describes a procedure that leads to a remarkable decrease in PQ accumulation in the lung, together with an increase in its fecal excretion and a subsequent decrease in several biochemical and histopathological biomarkers of toxicity. The administration of dexamethasone (100 mg/kg ip) to Wistar rats, 2 h after PQ intoxication (25 mg/kg ip), decreased the lung PQ accumulation to about 40% of the group exposed to only PQ and led to an improvement in tissue healing in just 24 h as a result of the induction of de novo synthesis of P-glycoprotein (P-gp). The involvement of P-gp in these effects was confirmed by Western blot analysis and by the use of a competitive inhibitor of this transporter, verapamil (10 mg/kg ip), which, given 1 h before dexamethasone, blocked its protective effects, causing instead an increase in lung PQ concentration and an aggravation of toxicity. In conclusion, the induction of P-gp, leading to a decrease in lung levels of PQ and the consequent prevention of toxicity, seems to be a new and promising treatment for PQ poisonings that should be further clinically tested.

Paraquat-induced oxidative stress and dysfunction of cellular redox systems including antioxidative defense enzymes glutathione peroxidase and thioredoxin reductase

We examined if paraquat-induced oxidative stress and cytotoxicity in pulmonary microvascular endothelial cells are associated with cellular redox systems such as the glutathione system and the thioredoxin system. Loss of viability, accompanied by marked decreases in glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and thioredoxin reductase activities, occurred 48 h after exposure to 1mM paraquat. These changes were preceded by an increased production of hydrogen peroxide after the decrease in glutathione peroxidase activity. Glutaredoxin activity was not decreased even after exposure to paraquat for 48 h, whereas thioredoxin activity was slightly decreased at 48 h. Unexpectedly, the activity of peroxiredoxin, a non-selenoenzyme, was almost completely lost at 24h. Loss of GAPDH activity and viability was notably aggravated by mercaptosuccinate. Selenium supplementation suppressed the loss of activities of glutathione peroxidase and thioredoxin reductase and alleviated paraquat-induced cytotoxicity. An in vitro experiment demonstrated that GAPDH was highly susceptible to reactive oxygen species generated in the xanthine-xanthine oxidase system, whereas thioredoxin reductase was considerably resistant. Taken together, the results suggest that the reduced regenerative ability of oxidatively damaged proteins including GAPDH due to the inactivation of thioredoxin reductase and glutathione peroxidase by paraquat may contribute to increasing oxidative stress, leading to cell death.

Identification of early molecular pathways affected by paraquat in rat lung

We have used global gene expression profiling, combined with pathway analysis tools, to identify in rats the molecular events associated with paraquat toxicity in the lung. Early (2, 8 and 18h) gene expression changes induced following intraperitoneal (i.p.) exposure to paraquat were measured in the caudal lobe of lungs using Affymetrix rat genome GeneChips (31,042 probe sets). A single high dose of paraquat dichloride (20mg/kg) was used that has been shown previously to cause in rats extensive lung fibrosis after 10 days. Hierarchical clustering of 543 paraquat-responsive genes (false discovery rate<0.05) revealed that under these conditions of exposure paraquat induces a staged transcriptional response in the rat lung that precedes the appearance of lung damage. We report here that many of the transcriptional responses to paraquat were rapid (being maximal at 2h post-dose), and that the predominant molecular functions and biological processes associated with these genes include membrane transport, oxidative stress, lung development, epithelial cell differentiation and transforming growth factor beta (TGF-beta) signalling. These data provide novel insights into the molecular pathways that lead to toxicity after exposure of the rat lung to paraquat.

Understanding the mechanisms of drug-associated interstitial lung disease

Drugs have been implicated in lung injury as a result of direct pharmacological action, persistence or metabolism in the tissue, or via the production of a reactive metabolite or metabolites. The result of this apparent drug-associated injury ranges from cellular dysfunction through to cell death (apoptosis) and alteration of repair mechanisms that are essential in replacing critical tissue elements and function. There is limited knowledge on how timing of drug administration or drug interactions may interfere with the repair mechanisms or modulate the expression of pulmonary toxicity. Chemotherapeutic drugs and novel agents, such as those targeting the epidermal growth factor receptor (EGFR), appear to affect both normal and neoplastic cells. However, unlike chemotherapy, where the actions are systemic and directly as a result of biotransformation or cell injury, it has been postulated that effects of EGFR-targeting agents are more likely to be focused on epithelia via a pharmacological effect. Furthermore, risk factors for the development of adverse pulmonary reactions, as well as biological markers indicating incipient toxicity, need to be prospectively identified. Proteomics, through the identification of ⩾1000 proteins or peptides in blood samples, will hopefully identify candidates for this role.

Increased synaptosomal dopamine content and brain concentration of paraquat produced by selective dithiocarbamates

Exposure to pesticides may be a risk factor for Parkinson's disease based on epidemiologic data in humans, animal models and in vitro studies. Different dithiocarbamate pesticides potentiate the toxicity of both 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and paraquat in mouse models of Parkinsonism by an unknown mechanism. This study examined the effects of commercially used dithiocarbamates on [3H]dopamine transport in striatal synaptosomal vesicles and on the concentration of [14C]paraquat in vivo in mice. Different ethylenebis-dithiocarbamates and diethyl-dithiocarbamate increased dopamine accumulation in synaptosomes, whereas dimethyl-dithiocarbamate and methyl-dithiocarbamate did not. Increased dopamine accumulation in synaptosomes was dose dependent and was related to the carbon backbone of these molecules. The dithiocarbamates that increased accumulation of dopamine did not alter the influx of dopamine, but rather delayed the efflux out of synaptosomes. These same dithiocarbamates also increased the tissue content of [14C]paraquat in vivo by a mechanism that appeared to be distinct from the dopamine transporter. There was a consistent relationship between the dithiocarbamates that increased synaptosomal accumulation of dopamine and tissue content of paraquat, with those previously demonstrated to enhance paraquat toxicity in vivo. These results suggest that selective dithiocarbamates may alter the kinetics of different endogenous and exogenous compounds to enhance their neurotoxicity.

Modulation of bile acids induced by paraquat in rabbits

Rabbit bile was examined for changes in composition induced by paraquat. Paraquat was administered intraperitoneally and changes in bile components were monitored by high performance liquid chromatography. Alterations in the ratios of total glycine/taurine conjugated bile acids (TGC/TTC), cholic acid/deoxycholic acid (CA/DC), cholic acid/chenodeoxycholic acid (CA/CDC) and cholic acid/cholesterol (CA/CH) were measured as an index of paraquat toxicity. A statistically significant increase in the ratio of TGC/TTC was observed, while CA/DC, CA/CDC and CA/CH showed a decrease. Phospholipids, protein, sugar, bilirubin, beta-carotene, vitamin A and vitamin E in the bile and serum of the experimental animals were also monitored. In bile, the levels of cholesterol, phospholipids, protein, sugar, and total bile acids increased while the levels of the antioxidants beta-carotene, vitamin A and vitamin E decreased. A decrease in the bilirubin content of the bile was also observed. These modifications may be useful clinically for assessment of paraquat toxicity.

Paraquat-induced oxidative stress and dysfunction of the glutathione redox cycle in pulmonary microvascular endothelial cells

Oxidative stress and changes in the antioxidant defense system that include the glutathione redox cycle in cultured pulmonary microvascular endothelial cells after exposure to paraquat at 0.1 and 0.5 mM were examined as a function of time. Cell viability was substantially lost 72 h after exposure to 0.5 mM paraquat, but not 0.1 mM paraquat. Viability loss was accompanied by increased glutathione-protein mixed disulfide formation, as well as a loss in glyceraldehyde-3-phosphate dehydrogenase activity, indicating a low defense potential. At 4 h after exposure to paraquat at both doses, however, a marked loss in NADPH was found, together with a decrease in aconitase activity. With 0.5 mM paraquat, increased NADP(+) accompanied by NADPH loss diminished constantly after 48 h without recovery of lost NADPH, suggesting destruction of pyridine nucleotides under oxidative stress. NAD(+) decreased 72 h after exposure to 0.5 mM paraquat, but NADH was not influenced. 3-Aminobenzamide did not protect the loss in NADP(+) or NAD(+) and cell viability. Although oxidized glutathione did not increase by exposure to paraquat at both doses through a 96-h exposure period, reduced glutathione increased at 48 to 72 h, with an increase in glutathione disulfide reductase activities. In contrast, a marked loss in glutathione peroxidase activity was produced 48 h after exposure to 0.5 mM paraquat, preceding cell injury. Mercaptosuccinate, an inhibitor of glutathione peroxidase, distinctly hastened viability loss by paraquat. These results indicate that the reduced ability of the glutathione redox cycle, leading to high oxidative stress, is closely associated with paraquat-induced cytotoxicity.

Biotransformation Enzymes as Determinants of Xenobiotic Toxicity in Domestic Animals

After coming in contact with living organisms, the majority of foreign compounds undergo a number of chemical reactions known as biotransformations. These are performed by hepatic and extra-hepatic enzyme systems and usually yield more polar derivatives, referred to as 'metabolites', which may leave the body via the urinary and biliary routes or be excreted in animal products such as milk and eggs. Biotransformation does not always imply detoxification because in certain instances metabolites will be produced that are capable of reacting with tissue macromolecules or acquiring toxic properties different to or greater than those of the parent molecule. In this review, which is focused on domestic animals, the role played by oxidative, reductive, hydrolytic and conjugative biotransformation enzymes in the activation/detoxification of xenobiotics is examined. The relationship between extra-hepatic metabolism and target organ toxicity as well as the action of rumen microflora on feed additives, phytotoxins, and pesticides are then discussed. Some of the most important metabolic-based species-related susceptibilities to different poisons, and the influence of enzyme inducers or inhibitors on xenobiotic toxicity and drug safety are also reviewed.

Polyamines in the lung: polyamine uptake and polyamine-linked pathological or toxicological conditions

The natural polyamines putrescine, cadaverine, spermidine, and spermine are found in all cells. These (poly)cations exert interactions with anions, e.g., DNA and RNA. This feature represents their best-known direct physiological role in cellular functions: cell growth, division, and differentiation. The lung and, more specifically, alveolar epithelial cells appear to be endowed with a much higher polyamine uptake system than any other major organ. In the lung, the active accumulation of natural polyamines in the epithelium has been studied in various mammalian species including rat, hamster, rabbit, and human. The kinetic parameters (Michaelis-Menten constant and maximal uptake) of the uptake system are the same order of magnitude regardless of the polyamine or species studied and the in vitro system used. Also, other pulmonary cells accumulate polyamines but never to the same extent as the epithelium. Although different uptake systems exist for putrescine, spermidine, and spermine in the lung, neither the nature of the carrier protein nor the reason for its existence is known. Some pulmonary toxicological and/or pathological conditions have been related to polyamine metabolism and/or polyamine content in the lung. Polyamines possess an important intrinsic toxicity. From in vitro studies with nonpulmonary cells, it has been shown that spermidine and spermine can be metabolized to hydrogen peroxide, ammonium, and acrolein, which can all cause cellular toxicity. In hyperoxia or after ozone exposure, the increased polyamine synthesis and polyamine content of the rat lung is correlated with survival of the animals. Pulmonary hypertension induced by monocrotaline or hypoxia has also been linked to the increased polyamine metabolism and polyamine content of the lung. In a small number of studies, it has been shown that polyamines can contribute to the suppression of immunologic reactions in the lung.

Selenium-dependent cellular glutathione peroxidase protects mice against a pro-oxidant-induced oxidation of NADPH, NADH, lipids, and protein

Since our prior work indicated that Se-dependent cellular glutathione peroxidase (GPX1) was necessary for protection against paraquat lethality, the present studies were to elucidate the biochemical mechanisms related to that protection. Four groups of mice [Se-deficient or -adequate GPX1 knockout and wild-type (WT)] were injected (i.p.) with 50 mg paraquat/kg body weight and tissues were collected 0, 0.5, 1, 2, 3, or 4 h after the injection. Whereas the ratios of NADPH/NADP and NADH/NAD in lung were reduced by 50-70% only 0.5 h after the injection in all groups, these two ratios in liver of the Se-adequate WT were significantly higher than those of the three GPX1 knockout or deficient groups 2-4 h after the injection. The paraquat-induced pulmonary lipid peroxidation and hepatic protein oxidation, measured as F(2)-isoprostanes and carbonyl contents, respectively, peaked at 1 h in these three groups. No such oxidative events were shown in any tissue of the Se-adequate WT throughout the time course. Whereas the F(2)-isoprostane formation was accelerated by both GPX1 knockout and Se deficiency in liver, it was not significantly elevated by the paraquat treatment in brain of any group. The paraquat injection also resulted in temporal changes in lung GPX activity and GPX1 protein in the Se-adequate WT, and significant reductions in lung total SOD activity in the GPX1 knockout or deficient groups. In conclusion, GPX1 plays a critical role in maintaining the redox status of mice under acute oxidative stress, and protects against paraquat-induced oxidative destruction of lipids and protein in vivo. These protections of GPX1 seem to be inducible and coordinated with those of other antioxidant enzymes.

Biochemical studies on the toxicity of 1, 1'-dimethyl-4, 4'-bipyridylium dichloride in the rat

The effect of intraperitoneal administration of lethal dose (50 mg/kg) of paraquat on the microsomal cysteine levels in the plasma, liver and lung of adult male Wistar rats has been investigated using Rank Chromaspek amino acid analyzer. The microsomal alanine levels were also determined to help in assessing the extent of paraquat interference with cellular protein. DL-Buthionine-[S,R]-Sulfoximine (BSO) and Diethyl maleate (DEM) were used to potentiate the toxic effect of the bipyridyl. The microsomal cysteine levels were significantly (P < or = 0.05) depressed in the plasma, liver and lung of the paraquat-treated rats compared with the saline-injected group but the alanine levels were not similarly affected. Probably, paraquat poisoning interferes specifically with the cellular cysteine content in the rat. These findings could provide a valuable information on the biochemical mechanism of paraquat intoxication.

Paraquat toxicity and oxidative damage. Reduction by melatonin

The ability of melatonin to protect against paraquat-induced oxidative damage in rat lung, liver, and serum was examined. Changes in the levels of malondialdehyde (MDA) plus 4-hydroxyalkenals (4-HDA) and reduced and oxidized glutathione concentrations were measured. Paraquat (50 mg/kg) was injected i.p. into either Sprague-Dawley or Wistar rats with or without the co-administration of 5 mg/kg melatonin. Paraquat alone increased MDA + 4-HDA levels in serum and lungs of both rat strains, with these increases being abolished by melatonin co-treatment. Paraquat also decreased reduced glutathione levels and increased oxidized glutathione concentrations in lung and liver; these changes were negated by melatonin. The effect of melatonin on paraquat-induced mortality was also studied. Paraquat at a dose of 79 mg/kg was lethal for 50% of animals within 24 hr; when administered together with melatonin, the LD50 for paraquat increased to 251 mg/kg.

Evidence for redox cycling of diquat in rat small intestine

It has previously been established that acute diquat (1,1'-ethylene, 2,2'-bipyridilium) toxicity in the rat is associated with stimulation of net fluid secretion into the gastrointestinal tract. We have examined the possibility that the mechanism of diquat toxicity in the small intestine involves redox cycling of the bipyridyl leading to a disturbance of biochemical function and oxidative stress. Experiments performed in vitro showed that diquat (10 microM to 1 mM) produced an increase in activity of the pentose phosphate pathway in rat small intestinal tissue slices, suggesting that there was oxidation of NADPH even at concentrations of diquat which do not cause intestinal fluid secretion in anaesthetized rats. When the effect of diquat on pentose phosphate activity was measured in rats in situ at a dose which causes maximal fluid secretion [50 mM diquat dibromide (DQBr2)], production of 14CO2 from [1-14C]-glucose increased by 278 +/- 28% (N = 4) within 1 hr of exposure to diquat. Under these same conditions, the tissue content of NADPH in the proximal small intestine was significantly depleted, though there was no corresponding increase in NADP+ concentration. Diquat had no effect on tissue concentrations of either the reduced or oxidized forms of NAD. It is likely that NADPH oxidation at low diquat concentrations can be adequately compensated for by mechanisms within the tissue which protect against oxidative stress. However, the data also suggest that diquat-induced fluid secretion in the rat small intestine is associated with redox cycling of bipyridyl leading to depletion of NADPH.

Mechanism of cytotoxicity of paraquat. I. NADH oxidation and paraquat radical formation via complex I

The mechanism of cytotoxicity by paraquat was studied focusing attention on its effect on the mitochondrial electron transport system. Paraquat inhibited both mitochondrial and cytoplasmic malate dehydrogenase activities. NADH oxidation was verified in NADH: ubiquinone oxidoreductase (complex I) reaction mixture in which paraquat was an only electron acceptor, and paraquat radical formation was observed as turning blue of the reaction mixture. A kinetic characteristic of this enzyme reaction was that Km was so high as 4.1 mM. The maximum reaction velocity was defined in the range over pH 9. NADH autoxidation with complex I, but without paraquat, was not observed in any pH range. The maximum reaction velocity of the NADH autoxidation by paraquat without complex I was observed in pH 8.5, but the figure was so small as to be negligible. With these results, we propose the hypothesis that paraquat does not promote the autoxidation with complex I, but accepts electrons via complex I to induce paraquat radical formation.

Mechanism of paraquat-stimulated lipid peroxidation in mouse brain and pulmonary microsomes

Paraquat-stimulated NADPH-dependent lipid peroxidation in mouse brain and pulmonary microsomes was inhibited by superoxide dismutase and singlet oxygen quenchers, but not by catalase or hydroxyl radical scavengers. MnCl2, which might form a salt with unsaturated lipid, inhibited the lipid peroxidation in brain microsomes, but not that in pulmonary microsomes. These findings suggest that activated oxygen species, especially superoxide and singlet oxygen, may play a major role in the stimulation of microsomal lipid peroxidation by paraquat in both brain and lung, and that the nature of the lipids exposed to peroxidative attack may be different in microsomes of the two organs.

Comparison of one-electron reduction activity against the bipyridylium herbicides, paraquat and diquat, in microsomal and mitochondrial fractions of liver, lung and kidney (in vitro)

The first one-electron reduction steps of paraquat and diquat were compared using microsomal and mitochondrial fractions of rat liver, lung and kidney. Both fractions reduced each herbicide effectively, with the order of the Vmax values in microsomes and mitochondria being liver greater than lung greater than kidney and kidney greater than liver greater than lung, respectively. Although similar Vmax values were obtained from the liver and lung with the two subcellular fractions, the affinity of mitochondrial enzymes was lower, suggesting that the reduction of both herbicides in a microsomal site would be dominant in these two organs. The Vmax values for radical formation of paraquat were higher than those of diquat in all the endogenous one-electron reducing systems. The apparent Km values for diquat, however, were lower than those for paraquat in both subcellular fractions from the three tissues, indicating the superiority of the reduction for diquat to that for paraquat at low concentrations. This difference in the Km values supported the finding that the reduction velocity for diquat was significantly higher than that for paraquat at 1 mM concentration. Thus, at low concentrations, diquat would be reduced more easily than paraquat. In addition, tissue enzymatic specificity for paraquat was not obtained. From these data, it seems reasonable to conclude that the tissue-selective accumulation of paraquat previously proposed determines its toxicity.

Effects of pneumotoxic trialkylphosphorothioates on the pentose phosphate pathway in rat lung slices

The extent of glucose oxidation via the pentose phosphate pathway (PPP), a possible indicator of oxidative stress, was assessed in rat lung slices by measuring the production of 14CO2 on incubation with either [6-14C]glucose or [1-14C]glucose. Incubation in the presence of the pneumotoxins O,S,S-trimethyl phosphorodithioate (OSSMe, 10(-5) -10(-3) M), O,O,S-triethyl phosphorothioate (10(-4) -10(-2) M) or S,S,S-trimethyl phosphorotrithioate (10(-4) -10(-2) M) resulted in a significant, but small stimulation of PPP (max. 276%), compared to that found in the presence of the model oxidant, paraquat (1089%, 10(-5) M). Following in vivo treatment with an LD50 dose of OSSMe, PPP was unchanged (after 6 h) or decreased (after 24 h) compared to saline-treated rats. PPP was also decreased in slices which had been preincubated with OSSMe (10(-3) M) and then transferred to fresh medium. Following treatment with phorone (250 mg/kg i.p.) pulmonary levels of non-protein sulfhydryls were first reduced (20% of control at 3 h) and then increased (288% of control at 24 h), but at neither time was lung slice PPP activity affected, thus suggesting that in the rat lung PPP activity does not directly depend on pulmonary glutathione content.

Cellular response to oxidative damage in lung induced by the administration of dimethylarsinic acid, a major metabolite of inorganic arsenics, in mice

Oral administration of dimethylarsinic acid (DMAA), a major metabolite of inorganic arsenics, induces DNA damage in the mouse and rat lung due to both active oxygens and dimethylarsenic peroxyl radical produced in the metabolism of DMAA. Our paper describes the cellular response to DMAA in the mouse lung. In male ICR mice given a single po dose (1500 mg/kg) of DMAA-Na, the activities of mitochondrial superoxide dismutase, glutathione peroxidase, and glucose-6-phosphate dehydrogenase significantly increased at 6 hr or longer after dosing, while cytosolic superoxide dismutase and catalase did not. With regard to cellular sulfhydryls after DMAA dosing, levels of reduced glutathione and nonprotein sulfhydryl decreased, while mixed disulfides significantly increased. Further, NADPH markedly decreased at 6-9 hr after DMAA dosing. These cellular variations suggest that the mouse pulmonary cell produced active oxygens, i.e., superoxide anion radical, hydrogen peroxide, and subsequent radicals in the metabolism of DMAA and that these and also the dimethylarsenic peroxyl radical were responsible for pulmonary DNA damage.

Different effects of paraquat on microsomal lipid peroxidation in mouse brain, lung and liver

Paraquat stimulates NADPH-Fe(2+)-dependent microsomal lipid peroxidation in mouse brain and strongly inhibits it in the liver. In lung microsomes, the lipid peroxidation was stimulated by paraquat at 10(-4) M, but not at higher doses. An antioxidant action of paraquat seemed to account, at least in part, for the lack of stimulation in lung microsomes, but it was inappropriate to explain the result in hepatic microsomes. There was no apparent correlation between the effects of paraquat on the lipid peroxidation and on the activity of NADPH-cytochrome P-450 reductase, the enzyme which initiates redox cycling of paraquat, resulting in generation of active oxygen species. In fact, the effect of paraquat on the lipid peroxidation was independent of paraquat radical production, an intermediate in the cycle. However, the inhibitory potency of N-ethylmaleimide on NADPH-cytochrome P-450 reductase activity paralleled that on the lipid peroxidation stimulated by paraquat in brain and lung. These findings indicate that the effect of paraquat on microsomal lipid peroxidation differs among the organs and that other factors, besides NADPH-cytochrome P-450 reductase, might be involved in the stimulation of lipid peroxidation by paraquat.

An evaluation of the redox cycling potencies of paraquat and nitrofurantoin in microsomal and lung slice systems

The redox cycling abilities of the pulmonary toxins paraquat and nitrofurantoin have been compared with those of the potent redox cyclers, diquat and menadione in lung and liver microsomes by using the oxidation of NADPH and consumption of oxygen. The relative potencies of these compounds to undergo redox cycling were in the order: diquat approximately menadione much greater than paraquat congruent to nitrofurantoin. This was partly attributed to the much lower affinity (Km) of lung and liver microsomes for paraquat and nitrofurantoin than for diquat and menadione. The potential to redox cycle was assessed in an intact cellular system by determining the oxygen consumption of rat lung slices in the presence (10(-6), 10(-5) and 10(-4) M) or absence of each of the four substrates. At concentrations of paraquat (10(-5) M) known to be accumulated by lung slices, a small but significant stimulation of lung slice oxygen uptake was observed. Nitrofurantoin (10(-4)-10(-6) M) did not affect lung slice oxygen uptake in lung slices, an observation consistent with its being a poor redox cycling compound, which is not actively accumulated into lung cells. This data has important implications in assessing the risk of exposure to paraquat. Low levels of paraquat would not be expected to cause lung damage because insufficient compound is present in the lung to exert its toxicity by redox cycling (due to the high Km observed).

Free malondialdehyde levels in the urine of rats intoxicated with paraquat

We examined the excretion of free malondialdehyde (MDA) in the urine of rats to which a herbicide, Gramoxone, had been orally administered. The herbicide was administered for 2 days at a dose of 60 mg paraquat/kg body weight/day. As a result, the concentration of free MDA decreased following the intake of Gramoxone. The total amount of free MDA increased temporarily, but then it decreased significantly to below normal values. Rats that died during this experimental period did not excrete any free MDA. In the surviving animals, the MDA concentration in serum and lung microsomes decreased, while that in liver microsomes increased slightly after intake of the poison. Although the cause of the decrease in the urinary free MDA level remains unclear, the marked changes may provide valuable information regarding a toxic mechanism of paraquat intake.

Reduction of hexavalent chromium by ascorbic acid and glutathione with special reference to the rat lung

The reduction of 20 microM hexavalent chromium [chromium(VI)] by L-ascorbic acid (AsA) (0.06-2 mM) and/or glutathione (GSH) (2-15 mM) in buffer solutions, cell-free bronchoalveolar lavage fluids or soluble fractions of rat lungs was investigated at physiological pH (37 degrees C). The reduction in AsA solution was pseudo-first-order in a single phase with respect to chromium(VI), but that in GSH solution showed a two-phase process. The half-life of chromium(IV) ranged from seconds to hours. The reducing ability of AsA was markedly higher than that of GSH. Coexistence of equimolar GSH with AsA accelerated the reduction rate slightly, in comparison with that in the corresponding AsA solution. Lavage fluids containing 0.06 mM AsA showed pH-dependent reactions similar to those of the corresponding AsA solutions. The lung-soluble fractions reduced chromium(VI) in a process composed of phase I and phase II, characterized by the reducing ability of AsA-GSH cooperation and of AsA alone, respectively. Reduction in the former was 30-40% more rapid than in the latter. The biological half-life of chromium(VI) in the lung was estimated to be 0.6 min, on the basis of the reducing activity in the first phase. However, the apparent biological half-life of chromium(VI) was about 2 min in rat lungs after intratracheal injection of chromate, involving depletion of AsA, but no significant changes in GSH. The difference is discussed in terms of AsA-induced initiative reduction in the alveolar lining fluid and subsequent obstructive effects of the resulting trivalent species on trans-membrane permeability of chromate anions.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation of the cell specific toxicity of paraquat by 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). Implications for the heterogeneous distribution of glutathione (GSH) in rat lung

In order to study oxidative stress in the lung, we have developed a rat lung slice model with compromised oxidative defences. Lung slices with markedly inhibited glutathione reductase activity (approximately 80% inhibition) were prepared by incubating slices, with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) (100 microM) in an amino acid-rich medium for 45 min at 37 degrees. These lung slices had similar levels of GSH and ATP and polyamine uptake (a marker of alveolar epithelial type I and II cell function) to control rat lung slices. We have utilized these BCNU pretreated slices to study the effects of the herbicide, paraquat, in comparison to those of 2,3-dimethoxy-1,4-naphthoquinone, a potent redox cycler. Paraquat (10-100 microM) caused only minimal changes in the levels of GSH or ATP in control or compromised slices. In contrast, 2,3-dimethoxy-1,4-naphthoquinone caused a decrease in GSH in control slices but a markedly enhanced decrease in both GSH and ATP in compromised slices. Both compounds had only limited effects on putrescine and spermidine uptake in control slices. However, they caused a marked inhibition in compromised slices. Paraquat had little effect on 5-hydroxytryptamine uptake (a marker of endothelial cell function) in either control or compromised slices whereas the quinone inhibited uptake in the compromised slices. Thus, the lack of effect of paraquat on GSH and ATP does not support the involvement of oxidative stress in its toxicity. In contrast, using polyamine uptake, as a functional marker of alveolar epithelial cell damage, suggests a role for redox cycling. As paraquat is known to be accumulated primarily in alveolar type I and II cells (a small fraction of the lung cell population), our data suggest that only a small proportion of pulmonary GSH and ATP is present in alveolar epithelial type I and II cells but that much larger amounts may be present in endothelial cells. These studies highlight the problem of gross tissue measurements in heterogeneous tissues such as the lung.

Correlation between lipid peroxidation and morphological manifestation of paraquat-induced lung injury in rats

Biochemical and morphological studies of rat lung were performed to determine the role of lipid peroxidation in the in vivo lung toxicity of paraquat. Two injections of 20 mg/kg paraquate were administered intraperitoneally every other day. While notable epithelial damage in the lungs was observed on the day after the second paraquat injection and progressed through the 5th day, the concentration of lipid peroxides in the rat lungs did not increase by the 3rd day after the injection. The lipid peroxide concentrations increased after the 5th day post-injection, and reached the maximum concentrations on the 7th day, when the damaged alveolar surface had been mostly repaired by regenerative pneumocytes. On the other hand, the delayed increase of lung lipid peroxides in paraquat-treated rats paralleled the increased number of macrophages in the lung, which reached maximum numbers on the 7th day. Glutathione peroxidase activity in the lungs also increased with a similar time course. Macrophages from the lungs contained a large amount of engulfed degradation products and cellular debris, and immunohistochemical study showed high glutathione peroxidase content on the 5th and 7th days. These results suggest that lipid peroxidation is a relatively late event in the in vivo paraquat-treated lung and that the delayed increase of lipid peroxides in the lungs occurs from the phagocytic activities of macrophages rather than from toxic cell injury.

Differential effects of hyperoxia and hydrogen peroxide on DNA damage, polyadenosine diphosphate-ribose polymerase activity, and nicotinamide adenine dinucleotide and adenosine triphosphate contents in cultured endothelial cells and fibroblasts

The effects of oxidative stress on DNA damage and associated reactions, increased polyadenosine diphosphate-ribose polymerase (PARP) activity and decreased nicotinamide adenine dinucleotide (NAD) and adenosine triphosphate (ATP) contents, have been tested in primary cultures of porcine aortic endothelial cells. The cells were treated with 50-500 microM H2O2 for 20 min or 100 microM paraquat for 3 days or were exposed to 95% O2 for 2 and 5 days. The administration of 250-500 microM H2O2 resulted in a marked increase in PARP activity and a profound depletion of ATP and NAD. Although hyperoxia had no effect on PARP activity and reduced only slightly the ATP and NAD stores, it markedly reduced the ability of endothelial cells to increase PARP activity upon exposure to DNase. Paraquat had a similar effect. Human dermal fibroblasts were also exposed to 50-500 microM H2O2 for 20 min or 95% O2 for 5 days. Their response to H2O2 differed from that of endothelial cells by their ability to maintain the ATP content at a normal level. Fibroblasts were also insensitive to the effect of hyperoxia. These results suggest that the oxidant-related DNA damage is a function of the type of oxidative stress used and may be cell-specific.

Studies on the metabolism of the pneumotoxin O,S,S-trimethyl phosphorodithioate--I. Lung and liver microsomes

The metabolism of O,S,S-trimethyl phosphorodithioate (OSSMe), a pneumotoxic impurity in some organophosphorus insecticides, was investigated in rat lung and liver microsomal preparations, using OSSMe labelled with 3H or 14C on one of its thiolo-methyl (CH3S-) groups. Production of O,S-dimethyl phosphorothioate (OSMeO-) and binding of radioactivity to protein were NADPH-dependent and were shown to be, at least partly, cytochrome P-450-dependent processes in both lung and liver microsomes. Incubation with reduced glutathione prevented the binding of radioactivity without affecting OSMeO- production. The Km for the conversion of OSSMe to OSMeO- was 15-fold lower in lung (0.30 +/- 0.07 mM) than in liver (4.63 +/- 2.42 mM) microsomes. These results show that cytochrome P-450-dependent mixed-function oxidase is implicated in at least part of the metabolic activation of OSSMe, and suggest that the pulmonary isozyme(s) are more active at metabolizing OSSMe than hepatic isozymes. It is speculated, on the basis of literature data on other sulphur-containing chemicals, that the metabolic activation of OSSMe involves oxidation of a thiolo-sulphur, with subsequent formation of CH3-S-S-protein disulphides.

Gastric mucosal lesions induced by hemorrhagic shock in baboons. Role of oxygen-derived free radicals

In this study we sought to define the role of oxygen-derived free radicals during ischemia and reperfusion in the production of acute damage to the gastric mucosa of baboons. The protective effect of the xanthine oxidase inhibitor, allopurinol, the superoxide scavenger, superoxide dismutase (SOD), and a long-acting SOD-albumin was determined. Mucosal damage was evaluated using light and scanning electron microscopy. Evidence for oxidative insult to the gastric mucosa was sought by measuring tissue concentrations of reduced (GSH) and oxidized (GSSG) glutathione. Gastric mucosal blood flow was estimated using the microsphere technique. A similar pattern of tissue damage was found at the end of ischemia in all three groups. Thirty minutes after reperfusion, severe mucosal damage (grade 3) increased only in the untreated control. In the two treated groups, grade 3 damage remained unchanged during reperfusion and a decrease in the percentage of moderate damage (grade 2) was seen. Both GSH and GSSG tissue concentrations were lower in the untreated controls as compared to the scavenger-treated groups, making it questionable whether GSH/GSSG tissue levels adequately reflect oxidative stress. We conclude that in our ischemia-reperfusion model the generation of oxygen-derived free radicals produces mucosal damage and prevents the restitution of moderate mucosal damage during reperfusion. In ischemia, factors other than free radicals seem to be responsible for mucosal damage. The protective effect of allopurinol and SOD was not mediated by changes in gastric mucosal blood flow.

Prolonged, intravenous paraquat infusion in the rat. I. Failure of coinfused putrescine to attenuate pulmonary paraquat uptake, paraquat-induced biochemical changes, or lung injury

Paraquat (PQ) was administered to rats for 7 days by iv infusion from osmotic minipump at dosage rates of 250 and 500 nmol PQ/hr. The efficacy of putrescine in attenuating pulmonary PQ accumulation in vivo and the resulting PQ-induced biochemical changes and lung injury were assessed in these animals by coinfusion of putrescine at rates of 2500 or 5000 nmol/hr. Dose-dependent, steady-state blood levels of both PQ and putrescine were achieved by 18 hr and maintained throughout the infusion period. Lung PQ content at 7 days was dose-dependent and up to 18-fold greater than corresponding blood levels. No evidence of toxicity was observed in low-dose PQ animals while weight loss and overt toxicity was observed in high-dose PQ rats between Days 4 and 5. Histopathological examination of high-dose PQ rat lungs revealed qualitative changes typical of PQ toxicity. Significant (p less than 0.05) increases in lung glutathione and activities of glucose-6-phosphate dehydrogenase and GSSG reductase resulted from both PQ doses, reflecting PQ-induced oxidant stress and increased demand on lung NADPH. A net decrease in lung NADPH (p less than 0.05) was directly measured in high-dose PQ rats and may have contributed to the PQ-induced lung injury. Although putrescine is an effective inhibitor of pulmonary PQ uptake in vitro, the blood putrescine levels achieved in this study did not appear to inhibit this process in vivo. This was evidenced by putrescine's failure to decrease 7-day lung PQ content, PQ-induced biochemical changes, or lung injury.

Influence of bipyridylium compounds on microsomal mixed-function oxidation activities

The two principal bipyridyl herbicides, paraquat and diquat, were investigated for their influence on microsomal mixed-function oxidation (MFO) activities and on NADPH oxidation rates in lung, liver, and kidney preparations. In lung microsomal preparations, benzphetamine N-demethylation was found to be inhibited by paraquat and diquat in a concentration-dependent manner, but ethylmorphine N-demethylation was unaffected by these bipyridyls. In liver microsomal fractions, both benzphetamine and ethylmorphine N-demethylases were inhibited by paraquat and diquat. Neither bipyridyl affected MFO activity in kidney preparations. A kinetic investigation of the enzyme inhibition showed that only Vmax was affected by paraquat and diquat, providing the first evidence for noncompetitive inhibition by the bipyridyls. In all microsomal preparations, NADPH oxidation was stimulated significantly by paraquat and to an even greater extent by diquat in the absence or presence of benzphetamine or ethylmorphine. The influence of MFO substrates on the stimulation varied widely among the three organ systems. In lung, paraquat- or diquat-mediated stimulation of NADPH oxidation was equal in the absence of MFO substrates and in the presence of ethylmorphine, but the stimulation was increased in the presence of benzphetamine. Stimulation of NADPH oxidation by the bipyridyls, in liver as well as in kidney preparations, was equal in all situations in the absence of MFO substrates and in the presence of benzphetamine or ethylmorphine, although the quantity of this stimulation was greater in liver than in kidney fractions. It is apparent that the bipyridyls are potent stimulators of in vitro NADPH oxidation in microsomal preparations from several organs. The quantity of the NADPH oxidation stimulation seems to be a decisive factor in the inhibition of xenobiotic metabolism. Whether the stimulation of NADPH oxidation and the non-competitive inhibition of xenobiotic metabolism play a significant role in bipyridyl toxicity are under further investigation.

Protective role of sulfhydryl reagents in oxidant lung injury

Recently there has been a great deal of interest in exploring possible ways to protect the lung from oxidant damage. Since sulfhydryl compounds are among the most important endogenous antioxidants, their therapeutic use has been proposed. Glutathione (GSH), the main intracellular nonprotein sulfhydryl, plays an important role in the maintenance of cellular proteins and lipids in their functional state. With oxidant stress, GSH acts to protect cell constituents as evidenced by increased turnover to GSSG, formation of mixed disulfides with proteins, utilization of NADPH, and utilization of glucose in the pentose pathway. When GSH is experimentally lowered (e.g., by protein deficiency or with diethylmaleate) the toxic effects of oxidant stress are exacerbated as evidenced by increased membrane and cell damage, pulmonary edema, and mortality. Several recent investigations have shown that sulfhydryl reagents (particularly N-acetyl cysteine, a cell-permeable GSH precursor) can provide significant protection against certain pulmonary toxins. N-acetyl cysteine reduced the lethal effects of 100% O2 in rats by 65%. Therefore, the therapeutic potential of sulfhydryl reagents in the treatment and prevention of oxidant injury and the mechanisms involved are an important direction for lung research.

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.

Glutathione redox status of control and cadmium oxide-exposed rat lungs during oxidant stress

Activities of enzymes responsible for the maintenance of reduced glutathione (GSH) levels have been shown in a previous study to be increased in rat lungs following a 3-h exposure to cadmium oxide aerosols at 5.0 mg/m3. In this study, the ability of the lung to maintain levels of GSH during challenge with tert-butyl hydroperoxide (tBuOOH) was evaluated in isolated perfused lungs from control and cadmium oxide-exposed rats. Changes in glutathione redox status were indicated by measurements of nonprotein sulfhydryls (NPSH), total glutathione (1/2 GSH + GSSG), and glutathione disulfide (GSSG) in liquid nitrogen freeze-clamped lungs after 3-min infusions with 0-0.6 mM tBuOOH. In control and cadmium oxide-exposed lungs, levels of 1/2 GSH + GSSG remained constant over the range of 0-0.6 mM tBuOOH, indicating that no loss of glutathione from the system had occurred. In experiments with control lungs, levels of NPSH fell from 8.04 +/- 0.22 to 3.09 +/- 0.40 mumol/g dry weight when tBuOOH concentrations were increased from 0 to 0.6 mM (n = 20-23). In cadmium oxide-exposed lungs, NPSH levels also decreased proportionally to increases in GSSG. However, at concentrations of 0.075 and 0.15 mM tBuOOH, significantly smaller decreases in NPSH levels were observed in cadmium oxide-exposed lungs compared with controls. This protection against the GSH-depleting effects of tBuOOH might be explained by increased tissue levels of GSH-related enzymes.

Comparative study on the selenium- and N-acetylcysteine-related effects on the toxic action of hyperoxia, paraquat and the enzyme reaction hypoxanthine-xanthine oxidase in cultured endothelial cells

The potential protective effect of N-acetylcysteine against various types of oxidative stress (exposure to hyperoxia, treatment with paraquat, incubation in the presence of the hypoxanthine-xanthine oxidase system) was tested in primary cultures of porcine aortic endothelial cells. It was compared to that of selenomethionine (Se-Met), known to increase glutathione peroxidase activity, when given either alone or in combination with N-acetylcysteine. LDH release, 3H-thymidine (TdR) incorporation into DNA and DNA content were measured to assess the cytotoxic effect of the conditions tested. Total and oxidized glutathione content was also determined. Whereas Se-Met had a partial protective effect on all the conditions but paraquat treatment, N-acetylcysteine administration had no effect on the hyperoxia induced changes and significantly worsened the cytotoxic action of paraquat. On the other hand, LDH release following an incubation in the presence of the hypoxanthine-xanthine oxidase was significantly reduced after N-acetylcysteine treatment. No major change in total nor in oxidized glutathione followed N-acetylcysteine treatment in control and experimental conditions. A dose-dependent protective effect of N-acetylcysteine was obtained when this agent was given concomitantly with the xanthine oxidase system. These data suggest that in cultured endothelial cells a N-acetylcysteine-related protective effect, if present, is most likely to result from the direct scavenging action of N-acetylcysteine.

Formation and reduction of glutathione-protein mixed disulfides during oxidative stress. A study with isolated hepatocytes and menadione (2-methyl-1,4-naphthoquinone)

Incubation of isolated rat hepatocytes with menadione (2-methyl-1,4-naphthoquinone) resulted in a dose-dependent depletion of intracellular reduced glutathione (GSH), most of which was oxidized to glutathione disulfide (GSSG). Menadione metabolism was also associated with a dose- and time-dependent inhibition of glutathione reductase, impairing the regeneration of GSH from GSSG produced during menadione-induced oxidative stress. Inhibition of glutathione reductase by pretreatment of hepatocytes with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) greatly potentiated both GSH depletion and GSSG formation during the metabolism of low concentrations of menadione. Concomitant with GSH oxidation, mixed disulfides between glutathione and protein thiols were formed. The amount of mixed disulfides produced and the kinetics of their formation were dependent on both the intracellular GSH/GSSG ratio and the activity of glutathione reductase. The mixed disulfides were mainly recovered in the cytosolic fraction and, to a lesser extent, in the microsomal and mitochondrial fractions. The removal of glutathione from protein mixed disulfides formed in hepatocytes exposed to oxidative stress was dependent on GSH and/or cysteine and appeared to occur predominantly via a thiol-disulfide exchange mechanism. However, incubation of the microsomal fraction from menadione-treated hepatocytes with purified glutathione reductase in the presence of NADPH also resulted in the reduction of a significant portion of the glutathione-protein mixed disulfides present in this fraction. Our results suggest that the formation of glutathione-protein mixed disulfides occurs as a result of increased GSSG formation and inhibition of glutathione reductase activity during menadione metabolism in hepatocytes.

Mechanism of paraquat toxicity in lung and its relevance to treatment

The symptoms of paraquat poisoning depend largely on the amount of compound consumed, although in many cases the most characteristic feature of poisoning is lung damage, causing severe anoxia which leads to death. Studies in experimental animals have demonstrated that paraquat produces an acute damaging phase in the lung, followed by a reparative phase dominated by an extensive fibrosis. The latter is a major contributor to the lung lesion that causes anoxia. The specific toxicity in the lung can be explained in part by the selective accumulation of paraquat into this organ in comparison with other tissues. The accumulation is energy-dependent and probably specific to certain lung cells. It is now known that paraquat is accumulated into the lung by a recently described diamine transport process located in the alveolar epithelial cells and the Clara cells of the airways. When accumulated, paraquat undergoes a NADPH-dependent one-electron reduction to for its free radical which almost instantly reacts with molecular oxygen to reform the cation and concomitantly produce superoxide anion. This species of oxygen radical can contribute to the formation of more toxic species of radical which may directly damage vital cellular constituents. Paraquat has been shown to stimulate rapidly the pentose phosphate pathway and inhibit the synthesis of fatty acids in the lung in a dose-dependent manner. In addition there is a rapid increase in the pulmonary levels of mixed disulphides and the eventual reduction of NADPH levels in the lung. These results are consistent with the suggestion that paraquat causes a rapid and pronounced oxidation of NADPH which initiates compensatory biochemical responses in the lung.(ABSTRACT TRUNCATED AT 250 WORDS)

Alteration of the redox state of NADPH and glutathione in perfused rabbit lung by paraquat

The glutathione and NADP+ status of perfused rabbit lung was determined both before and after perfusion with 1 mM paraquat. The pulmonary glutathione redox state was similar to that of liver, having a glutathione/glutathione disulfide ratio of about 240. This ratio was lower in lungs perfused with glucose-free medium, a condition in which NADPH probably becomes limiting. Perfusion with paraquat significantly increased the pulmonary glutathione disulfide content, particularly in the absence of glucose, resulting in a glutathione/glutathione disulfide ratio of less than 100. NADP+ levels in rabbit lung were increased approximately two-fold by perfusion with paraquat in medium containing glucose and three-fold in the absence of glucose.