The accumulation and localisation of putrescine, spermidine, spermine and paraquat in the rat lung. In vitro and in vivo studies

Oral Supplementation with the Polyamine Spermidine Affects Hepatic but Not Pulmonary Lipid Metabolism in Lean but Not Obese Mice

The polyamine spermidine is discussed as a caloric restriction mimetic and therapeutic option for obesity and related comorbidities. This study tested oral spermidine supplementation with regard to the systemic, hepatic and pulmonary lipid metabolism under different diet conditions. Male C57BL/6 mice were fed a purified control (CD), high sucrose (HSD) or high fat (HFD) diet with (-S) or without spermidine for 30 weeks. In CD-fed mice, spermidine decreased body and adipose tissue weights and reduced hepatic lipid content. The HSD induced hepatic lipid synthesis and accumulation and hypercholesterolemia. This was not affected by spermidine supplementation, but body weight and blood glucose were lower in HSD-S compared to HSD. HFD-fed mice showed higher body and fat depot weights, prediabetes, hypercholesterolemia and severe liver steatosis, which were not altered by spermidine. Within the liver, spermidine diminished hepatic expression of lipogenic transcription factors SREBF1 and 2 under HSD and HFD and affected the expression of other lipid-related enzymes. In contrast, diet and spermidine exerted only minor effects on pulmonary parameters. Thus, oral spermidine supplementation affects lipid metabolism in a diet-dependent manner, with significant reductions in body fat and weight under physiological nutrition and positive effects on weight and blood glucose under high sucrose intake, but no impact on dietary fat-related parameters.

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.

Differences in epithelial-mesenchymal-transition in paraquat-induced pulmonary fibrosis in BALB/C and BALB/C (nu/nu) nude mice

Exposure to the toxic herbicide paraquat (PQ) can lead to the active absorption and enrichment of alveolar epithelial cells, resulting in pulmonary fibrosis and respiratory failure. At present, no effective clinical treatment is available. Notably, however, patients infected with human acquired immunodeficiency virus (HIV) (with T lymphocyte deficiency) do not show pulmonary fibrosis after PQ poisoning, suggesting that T lymphocytes may be involved in the occurrence and pathological development of lung fibers following PQ exposure, although relevant studies remain limited. Here, we found that the degree of pulmonary fibrosis induced by intragastric administration of PQ in congenital immunodeficiency BALB/C (nu/nu) nude (T lymphocyte loss) mice was lower than that in normal mice. However, pulmonary fibrosis was aggravated after transplantation of BALB/C (nu/nu) T lymphocytes into congenital immunodeficiency mice. This study is the first to report on the involvement of T lymphocytes in the occurrence and pathological development of lung fibers induced by PQ exposure. Thus, T cells may be an important cellular target for the clinical treatment of pulmonary fibrosis caused by PQ.

In VitroModels to Study Mechanisms of Lung Toxicity

Several functioning in vitro systems of varying complexity are currently in use for the study of mechanisms of lung toxicity. The isolated perfused lung is the model closest to the in vivo situation. It is a suitable model for combining metabolic and functional studies. It is, for instance, possible to relate changes in lung mechanics and lung perfusion flow to the release of various mediators during exposure of the lung to various agents. A simpler model may be constructed from lung slices which are less viable but suitable for uptake as well as metabolism studies.

Specific lung cells such as Clara cells and type II pneumocytes have been isolated and cultured and are valuable tools for studies of the molecular mechanisms of lung toxicity, particularly in cases of cell-specific toxicity. There is, however, a great need to develop techniques for the isolation and culture of other types of lung cells and also to improve the culturing techniques for those already isolated.

Spermidine supplementation and voluntary activity differentially affect obesity-related structural changes in the mouse lung

Obesity is associated with lung function impairment and respiratory diseases; however, the underlying pathophysiological mechanisms are still elusive, and therapeutic options are limited. This study examined the effects of prolonged excess fat intake on lung mechanics and microstructure and tested spermidine supplementation and physical activity as intervention strategies. C57BL/6N mice fed control diet (10% fat) or high-fat diet (HFD; 60% fat) were left untreated or were supplemented with 3 mM spermidine, had access to running wheels for voluntary activity, or a combination of both. After 30 wk, lung mechanics was assessed, and left lungs were analyzed by design-based stereology. HFD exerted minor effects on lung mechanics and resulted in higher body weight and elevated lung, air, and septal volumes. The number of alveoli was higher in HFD-fed animals. This was accompanied by an increase in epithelial, but not endothelial, surface area. Moreover, air-blood barrier and endothelium were significantly thicker. Neither treatment affected HFD-related body weights. Spermidine lowered lung volumes as well as endothelial and air-blood barrier thicknesses toward control levels and substantially increased the endothelial surface area under HFD. Activity resulted in decreased volumes of lung, septa, and septal compartments but did not affect vascular changes in HFD-fed mice. The combination treatment showed no additive effect. In conclusion, excess fat consumption induced alveolar capillary remodeling indicative of impaired perfusion and gas diffusion. Spermidine alleviated obesity-related endothelial alterations, indicating a beneficial effect, whereas physical activity reduced lung volumes apparently by other, possibly systemic effects.

Liver X Receptor Agonist TO901317 Attenuates Paraquat-Induced Acute Lung Injury through Inhibition of NF-κB and JNK/p38 MAPK Signal Pathways

Paraquat (PQ) is a widely used herbicide with extremely high poisoning mortality mostly from acute lung injury (ALI) or progressive pulmonary fibrosis. Toxicity mechanisms remain unclear, but a redox cycling process that generates reactive oxygen species (ROS) is involved, as are inflammation and cell apoptosis. We established an ALI mouse model by intraperitoneal injection of PQ (28 mg/kg) and then investigated the effects of a potent liver X receptor (LXR) agonist, TO901317 (5 or 20 mg/kg), injected intraperitoneally 30 min after PQ administration. Poisoned mice exhibited severe lung tissue lesions and edema, significant neutrophilic (PMNs) infiltration, and release of the proinflammatory cytokines tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). PQ administration also decreased activity of antioxidases, including superoxide dismutase (SOD), catalase (CAT), and glutathione S-transferases (GSTs), and increased lipid peroxidation as evaluated by malondialdehyde (MDA) levels. PQ exposure induced upregulation of the proapoptotic gene Bax and downregulation of the antiapoptotic gene Bcl-2, leading to marked cell apoptosis in the lung tissues. TO901317 treatment reversed all these effects through inhibition of PQ-induced nuclear factor kappa B (NF-κB) and JNK/p38 mitogen-activated protein kinase (MAPK) activation. The LXR agonist TO901317 had potent antioxidant, anti-inflammatory, and antiapoptotic effects against PQ-induced ALI.

Efficacy of vitamin C against liver and kidney damage induced by paraquat toxicity

Paraquat has been demonstrated to be a highly toxic compound for humans and animals and many cases of acute poisoning and death have been reported over the past few decades. The current experiment aimed to examine if vitamin C (ascorbic acid) alleviates the morphological changes induced by paraquat (PQ) administration in the liver and kidney of male albino rats. Male adult rats received paraquat (PQ) (1.5 mg/kg body weight) daily for three weeks. Vitamin C (VC) at a dose of 20 mg/kg body weight was given concomitantly with PQ to rats. Animals were divided into three groups in this experiment (control, PQ and PQ+VC). The morphopathological manifestations were investigated in tissues from liver and kidney. As expected, PQ administration induced marked changes in the morphological structure of the liver and kidney in PQ demonstrated animals. Importantly, vitamin C administration restored PQ-induced changes in the studied organs. Vitamin C administration attenuated the morphological damages induced by PQ in the liver and kidney of experimental animals. Our results suggest an antitoxic effect of vitamin C against paraquat.

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.

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.

A perspective of polyamine metabolism

Polyamines are essential for the growth and function of normal cells. They interact with various macromolecules, both electrostatically and covalently and, as a consequence, have a variety of cellular effects. The complexity of polyamine metabolism and the multitude of compensatory mechanisms that are invoked to maintain polyamine homoeostasis argue that these amines are critical to cell survival. The regulation of polyamine content within cells occurs at several levels, including transcription and translation. In addition, novel features such as the +1 frameshift required for antizyme production and the rapid turnover of several of the enzymes involved in the pathway make the regulation of polyamine metabolism a fascinating subject. The link between polyamine content and human disease is unequivocal, and significant success has been obtained in the treatment of a number of parasitic infections. Targeting the polyamine pathway as a means of treating cancer has met with limited success, although the development of drugs such as DFMO (alpha-difluoromethylornithine), a rationally designed anticancer agent, has revolutionized our understanding of polyamine function in cell growth and provided 'proof of concept' that influencing polyamine metabolism and content within tumour cells will prevent tumour growth. The more recent development of the polyamine analogues has been pivotal in advancing our understanding of the necessity to deplete all three polyamines to induce apoptosis in tumour cells. The current thinking is that the polyamine inhibitors/analogues may also be useful agents in the chemoprevention of cancer and, in this area, we may yet see a revival of DFMO. The future will be in adopting a functional genomics approach to identifying polyamine-regulated genes linked to either carcinogenesis or apoptosis.

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.

Effects of oxygen pressure and medium volume on the toxicity of paraquat in rat and human type II pneumocytes

The herbicide, paraquat is highly toxic for mammals, with the lungs being the main target organ, because of the active accumulation of the compound in this organ. The cellular toxicity of paraquat has been shown to be an O2-driven process and hyperoxia is known to increase the lethality of paraquat. In this study we have examined the effect of various O2 concentrations on the toxicity of paraquat in rat and human type II pneumocytes in culture, and we have tested whether the thickness of the liquid layer above the cells would influence the toxicity of paraquat. Type II pneumocytes were isolated from rat or human lung tissue using trypsin digestion, percoll density gradient centrifugation and differential attachment. Adherent cells (day 2) were incubated for 20 h in different volumes of culture medium (thickness of liquid layer), whether or not in the presence of paraquat, in the presence of different O2 tensions. The viability of the cells was assessed by the release of LDH in the culture medium. In both rat and human type II pneumocytes the toxicity of paraquat was independent of the thickness of the liquid layer (2.5 to 10 mm height). The toxicity of paraquat in rat type II pneumocytes decreased from a TC50 value of 28 microM paraquat at 21% O2 to 107 microM at 10% O2 and increased to 12 microM and 8 microM at 60% and 85% O2, respectively. For human type II pneumocytes the TC50 values were 7 microM; 25 microM and > 1000 microM paraquat at 60%, 21% and 10% O2, respectively. In this study we have shown that the diffusion of O2 through a liquid layer does not limit the toxicity of paraquat and that, as in vivo, increasing O2 partial pressure enhances the toxicity of paraquat.

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-induced formation of leukotriene B4 in rat lungs: modulation by N-acetylcysteine

The present work is focused on the formation of the inflammatory mediator leukotriene B4 (LTB4) in the lungs of paraquat (PQ)-intoxicated rats. The levels of LTB4 and the number of neutrophils in lung lavages of PQ-intoxicated rats, measured 12 h after 30 mg/kg PQ, increased significantly compared with those of control animals; administration of 50 mg/kg IP N-acetylcysteine (NAC), 8 h after PQ, inhibited this effect. The release of LTB4 from alveolar macrophages (AM) or alveolar epithelial type II cells from healthy animals incubated with PQ and/or NAC did not offer' an explanation for the effect of these chemicals on LTB4 in the bronchoalveolar lavage fluid (BALF). The PQ-enhanced, NAC-inhibited release of arachidonic acid (AA) by alveolar epithelial type II cells did, however, explain our in vivo results, when one assumes that the AM synthesize their 5-lipoxygenase products from alveolar epithelial cell-derived AA, an hypothesis demonstrated already by other researchers.

Putrescine accumulation in human pulmonary tumours

Type II pneumocytes and Clara cells, both epithelial cells that possess an active uptake system for polyamines, have been identified as possible precursor cells of at least some types of lung tumours. In this study we have investigated whether human pulmonary tumours exhibit putrescine uptake. Lung slices from both tumoral tissue and non-tumoral tissue, obtained from patients undergoing surgery for lung cancer, were incubated with radiolabelled putrescine at both 37 degrees C and 4 degrees C. The accumulation of putrescine was evaluated by its apparent kinetic parameters, in the presence or absence of cystamine, and by autoradiography. The investigated tumoral tissue (six squamous carcinomas and five adenocarcinomas) did not show accumulation of putrescine above that attributable to simple diffusion, except for one adenocarcinoma. In this specimen autoradiography showed that the accumulation was not specifically associated with any particular cell type, but that practically every cell accumulated putrescine. We conclude that human pulmonary tumours do not accumulate polyamines in a manner similar to normal pulmonary epithelial cells.

N-acetylcysteine increases the glutathione content and protects rat alveolar type II cells against paraquat-induced cytotoxicity

A protective effect of N-acetylcysteine in oxidative lung damage was reported by a number of workers; however, the mechanism underlying this effect was not thoroughly elucidated. The present research investigates the protection by N-acetylcysteine against paraquat-induced cytotoxicity to alveolar type II cells, which are known to be specific targets of paraquat toxicity in vivo. We found that addition of 1 mM N-acetylcysteine to alveolar type II cells incubated with 1 mM paraquat reduced the cytotoxic index from 17.4 +/- 1.3% to 9.3 +/- 1.5%. This effect could not be explained by the interference of N-acetylcysteine with the active uptake of paraquat by type II cells. Incubation of these cells with N-acetylcysteine enhances their glutathione content, thus reducing the paraquat- induced depletion of glutathione in these cells. These results suggest that N-acetylcysteine exerts its protective effect by acting as a precursor for glutathione in alveolar type II cells.

Further evidence that the blood/brain barrier impedes paraquat entry into the brain

The distribution of the non-selective herbicide paraquat was examined in the brain following subcutaneous administration of 20 mg kg-1 paraquat ion containing [14C]paraquat to male adult rats in order to determine whether paraquat crosses the blood/brain barrier. Following administration, [14C]paraquat reached a maximal concentration in the brain (0.05% of administered dose) within the first hour and then rapidly disappeared from the brain. However, 24 h after administration of the herbicide, about 13% of the maximal recorded concentration of paraquat remained in the brain (1.6 nmol g-1 wet weight) and could not be removed by intracardiac perfusion. Using measurements of [14C]paraquat in dissected brain regions and using quantitative autoradiography we demonstrated an asymmetrical distribution in and around the brain at 30 min (maximal concentration) and 24 h after administration. Most of the paraquat was associated with five structures, two of which, the pineal gland and linings of the cerebral ventricles lie outside the blood/brain barrier whilst the remaining three brain areas, the anterior portion of the olfactory bulb, hypothalamus and area postrema do not have a blood/brain barrier. Overall, the distribution of [14C]paraquat in the brain 24 h after systemic administration was highly correlated to the blood volume. These data indicate that any remaining paraquat in the brain 24 h after systemic administration is associated with elements of the cerebro-circulatory system, such as the endothelial cells that make up the capillary network and that there is a limited entry of paraquat into brain regions without a blood/brain barrier.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the lung in accumulation and metabolism of xenobiotic compounds--implications for chemically induced toxicity

The mammalian lung is exposed to and affected by many airborne and bloodborne foreign compounds. This review summarizes the role of lung in accumulation and metabolism of xenobiotics, some of which are spontaneously reactive or are metabolically activated to toxic intermediates. The specific architectural arrangement of mammalian lung favors that so-called pneumophilic drugs are filtered out of the blood and are retained within the tissue as shown in particular for amphetamine, chlorphentermine, amiodarone, imipramine, chlorpromazine, propranolol, local anaesthetics, and some miscellaneous therapeutics. There is strong evidence that intrapulmonary distribution activity and regulation of drug-metabolizing enzymes in lung is distinct from liver. This review focuses on the metabolic rate of selected compounds in lung such as 5-fluoro-2'-deoxyuridine, local anesthetics, nicotine, benzo(alpha)pyrene, ipomeanol, 4-methylnitrosamino-1-(3-pyridyl)-1-butanone. It is widely accepted that the formation of radical species is a key event in the pneumotoxic mechanisms induced by bleomycin, paraquat, 3-methylindole, butylhydroxytoluene, or nitrofurantoin. Finally, methodological approaches to assess the capacity of lung to eliminate foreign compounds as well as biochemical features of the pulmonary tissue are evaluated briefly.

Putrescine and paraquat uptake in human lung slices and isolated type II pneumocytes

Paraquat is accumulated into the lungs of various species by an active uptake system which also appears to mediate the uptake of endogenous polyamines, such as putrescine. The accumulation of putrescine in the human lung has been previously shown to be mainly located in the type II cells. In the present study, we have studied the mutually competitive inhibition of putrescine and paraquat in human lung slices and the inhibition of putrescine by paraquat or cystamine in isolated human type II pneumocytes. Peripheral lung tissue taken from patients undergoing pneumectomy or lobectomy was used. The initial steps of the cell isolation procedure differed from the literature in that the tissue was first sliced in 0.7 mm thick slices, which were washed in phosphate buffered saline without calcium and magnesium (PBS-), followed by incubations with trypsin. The type II cells were purified and isolated by differential adherence on plastic followed by Percoll gradient centrifugation. Uptake was determined 48 hr after cell isolation. The accumulation of radiolabelled putrescine showed saturation kinetics, with the following apparent kinetic parameters: Km 6.7 and 6.2-7.6 microM and Vmax 2.7 and 3.0-3.4 mumol/g prot/hr for slices and isolated cells, respectively. In the presence of paraquat, putrescine uptake was reduced, in both systems, in a manner compatible with competitive inhibition, with calculated inhibition constants (Ki) of 549-614 and 659-895 microM paraquat for slices and isolated cells, respectively. The accumulation of putrescine in isolated human pneumocytes was strongly reduced in the presence of cystamine, with calculated Ki of 3.7 microM cystamine. These data indicate that putrescine, paraquat and cystamine accumulate in the human lung by the same uptake system, but that the affinities for the three substrates differ. The presence of an uptake system for putrescine in cultured human pulmonary type II is probably useful as a functional viability test.

Potentiation of oxidant-induced toxicity in hamster lung slices by dimethylthiourea

Dimethylthiourea (DMTU) is an effective scavenger of reactive oxygen metabolites. This property has been successfully exploited, experimentally, in the protection of cells and tissues against oxidative damage. In this study, however, we have observed that levels of nonprotein sulfhydryls (NPSH) in hamster lung slices were markedly decreased by incubation with 10 or 40 mM DMTU. These changes were associated with morphological signs of injury, increased levels of oxidised glutathione (GSSG), and an increased activity of the pentose phosphate pathway (PPP), suggesting that the loss of NPSH was due to their oxidation. Incubation with 40 mM, but not 10 mM DMTU, also resulted in a decreased ability to oxidise [6-14C]glucose or to synthesise proteins, suggesting that at the high concentration, DMTU may cause functional impairment of the tissue. Furthermore, the ability of the slices to accumulate putrescine decreased after incubation with the oxidative toxins paraquat (PQ), tert-butyl hydroperoxide (t-BOOH) or hydrogen peroxide (H2O2) and was further decreased by co-incubation with DMTU. Putrescine uptake, a function specific to the alveolar type I and II epithelial cells, was not affected by incubation with DMTU alone. DMTU did not exacerbate the effect of the nonoxidative toxin iodoacetamide (IAA) on putrescine uptake but it did affect markers of general cell damage or dysfunction. We suggest, therefore, that the toxicity of oxidants toward lung tissue is potentiated in alveolar epithelial cells by DMTU.

Kinetics and cellular localisation of putrescine uptake in human lung tissue

BACKGROUND

The polyamines (putrescine, spermidine, and spermine) are involved in cellular growth, proliferation, and differentiation. In the lungs of various species, polyamines are accumulated by an active uptake system which also mediates the uptake of cystamine and paraquat. In the rat lung putrescine uptake has been shown to be cell-specific, occurring predominantly in the alveolar epithelium. The aim of this study was to characterise the uptake of putrescine in human lung.

METHODS

Lung tissue was obtained from 31 patients undergoing surgery for lung cancer. Slices (0.7 mm thick) from non-tumour containing lung parenchyma were incubated for 15-60 minutes in Krebs-Ringer phosphate buffer with various concentrations of putrescine (2.5 to 80 mumol/l) containing 0.1 microCi [1,4-14C]-putrescine. Uptake was assessed from tissue radioactivity. For autoradiographic imaging, slices were incubated for 30 minutes with 2.5 mumol/l putrescine containing 2.5 mCi [1,4n-3H]-putrescine.

RESULTS

The accumulation of [14C]-putrescine into slices was time-dependent and energy-dependent, and obeyed saturation kinetics, with mean calculated values for Vmax (maximal rate of uptake) of 414 nmol/g/hour and for Km (medium concentration at which the rate of uptake is half Vmax) of 7.2 mumol/l, with a large interindividual variation. Competitive inhibition was observed on incubation with cystamine, which appears to have a high affinity for the uptake system since its calculated Ki (concentration of inhibitor at which the Km is doubled) was 3.2 mumol/l. Ultrastructural autoradiography showed labelling over both type I and type II cells of the alveolar epithelium, but not over the endothelium or any cells of the interstitium. Alveolar macrophages were also devoid of label.

CONCLUSIONS

These results show that the human lung possesses an active uptake system for putrescine, and probably also cystamine, which is located in both cell types of the alveolar epithelium. These findings may be used to develop tests for the assessment of the alveolar epithelium.

Involvement of opioid receptors in shaking behaviour induced by paraquat in rats

Paraquat (30-70 mg/kg intraperitoneally) caused typical shaking behaviour in rats in a dose-dependent manner. Myoclonus also appeared after the shaking behaviour in several rats treated with the highest dose of paraquat. Morphine (5 mg/kg intraperitoneally, 30 min. before paraquat) significantly reduced the frequency of shaking behaviour. The alleviation by morphine disappeared when naloxone (1.5 mg/kg intraperitoneally 15 min. after morphine) was coadministered. Although there was no histological change in brain slices of paraquat-treated rats (70 mg/kg intraperitoneally), the fluorescein uptake into brain was increased by the treatment. Morphine prevented the increase of fluorescein uptake, but naloxone failed to antagonize this effect. On the other hand, intracerebroventricularly administered paraquat (25.7 micrograms/rat) caused tremor in all rats, but not shaking behaviour nor myoclonus. These findings suggest that paraquat administered systemically as well as centrally may be toxic to the brain. Although the actions of paraquat on the brain seem to be complicated, opioid receptors may play a role in the actions.

Polyamine transport and ornithine decarboxylase activity in hypoxic pulmonary artery smooth muscle cells

Hypoxia causes remodeling of the pulmonary circulation that is dependent on increases in lungs polyamine contents. Mechanisms by which polyamines are regulated in hypoxic lung cells are unknown, but ornithine decarboxylase (ODC) activity, the initial enzyme in de novo biosynthesis, is depressed and polyamine transport is augmented in lungs from hypoxic rats (R.-T. Shiao et al. 1990. Am. J. Physiol. 259:L351-L358). To determine if hypoxia directly influences polyamine regulatory mechanisms in pulmonary vascular cells, we examined [14C]spermidine (SPD) transport and ODC activity in bovine main pulmonary artery smooth muscle cells (PASMCs) cultured under standard (culture medium Po2: greater than 100 mm Hg), "normoxic" (culture medium Po2: 50 to 70 mm Hg), or "hypoxic" (culture medium Po2: 18 to 30 mm Hg) conditions. Uptake of [14C]SPD in cells cultured under standard conditions was temperature- and concentration-dependent, exhibited saturation kinetics, and was abolished by metabolic inhibition. Modeling of transport according to Michaelis-Menten kinetics revealed that [14C]SPD uptake in cells cultured under standard conditions was characterized by Km and Vmax values of 0.78 microM and 4.5 pmol/min/10(6) cells, respectively. In comparison to cells cultured under standard conditions, Km was unaffected by culture under normoxic or hypoxic conditions while Vmax was increased to 18 pmol/min/10(6) cells in normoxic cells and to 33 pmol/min/10(6) cells in preparations cultured under hypoxic conditions. Inhibition of ODC with alpha-difluoromethylornithine (DFMO) also induced SPD transport, as evidenced by an increase in the Vmax to 65 pmol/min/10(6) cells. Both hypoxia- and DFMO-induced increases in [14C]SPD transport were suppressed by cycloheximide and actinomycin D, thus highlighting the importance of protein and RNA synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Cutaneous toxicity and absorption of paraquat in porcine skin

Paraquat, a commonly used herbicide, has been shown to be toxic in exposed field workers. The objectives of this study were to (a) assess the cutaneous toxicity of paraquat in vivo in pig skin and in vitro in the isolated perfused porcine skin flap (IPPSF) and (b) quantitate its absorption in the IPPSF. The amounts of 3, 24, and 200 mg of paraquat were topically applied (5 cm2 surface area) on the ventral abdomen of pigs and biopsied after 6-8 hr for light microscopy (LM) and transmission electron microscopy (TEM). IPPSFs were topically dosed with the same concentrations and perfused for 8 hr (n = 4/treatment). The dosed area of the skin was sampled for LM, TEM, and enzyme histochemistry. IPPSFs were also treated topically with [14C]paraquat dichloride at the aforementioned concentrations (n = 4/dose) and hourly perfusate samples were collected for radiolabel determination and assessment of biochemical and physiological parameters. The epidermal changes were similar both in vivo and in vitro. The changes included epidermal intercellular edema which increased with dose and epidermal-dermal separation at the 200-mg dose. Acid phosphatase and nonspecific esterase activities were increased in the upper layers of the epidermis, while alkaline phosphatase showed a greater activity in the stratum basale layer. Glucose utilization of all treated IPPSFs was lower than that of the controls and a variation in the vascular resistance profiles was seen in all the treated flaps. Radiotracer studies indicated that a majority of the compound remained on top of the application site and minimal absorption or penetration into skin was observed. Thus, at high concentrations and prolonged exposure, paraquat may have deleterious effects on epidermal morphology in the absence of significant percutaneous absorption.

Characteristics of putrescine uptake and subsequent GABA formation in primary cultured astrocytes from normal C57BL/6J and epileptic DBA/2J mouse brain cortices

Brain maturation and GABA metabolism are known to play a key role in epileptogenesis. The metabolism of the polyamines (putrescine, spermidine and spermine) is closely linked to the process of brain maturation. Putrescine has been shown to be catabolized to GABA in brain tissue and astrocytes. In order to better understand the importance of glial putrescine transport and metabolism, a model of age-dependent epilepsy was used to study the kinetic properties of [14C]putrescine uptake into cultured astrocytes from normal C57/BL and audiogenic DBA/2 newborn mice, and the subsequent GABA formation. (1) Putrescine uptake exhibited non-Michaelian allosteric kinetics with positive co-operativity (Hill factor = 2), suggesting a physiological importance of putrescine uptake by astrocytes. (2) The Vmax of putrescine uptake was significantly higher in C57/BL astrocytes than in DBA/2J, but the uptake affinity for putrescine was higher in DBA/2J than in C57/BL. (3) Higher K+ concentrations (18 mM) had little effect on putrescine uptake in either strain. (4) Ten-micromolar N-acetylputrescine, the first putrescine metabolite, stimulated putrescine uptake into astrocytes of both strains, but to a different degree: +46% in C57/BL and + 102% in DBA/2J. (5) The specific radioactivity of the GABA formed from labelled putrescine was four times higher in astrocytes from DBA/2J than from C57/BL mice. (6) The molar ratio of glutamate/GABA in the cerebral cortex of the DBA/2J mice was significantly higher during the period of audiogenic seizure susceptibility than in age-matched C57/BL mice. Our results show characteristics of putrescine uptake into astrocytes; we demonstrated distinct kinetic properties between normal and epileptic strains of mice.(ABSTRACT TRUNCATED AT 250 WORDS)

The accumulation of pentamidine into rat lung slices and its interaction with putrescine

The aromatic diamidine, pentamidine, accumulated into rat lung slices by an uptake system that obeyed saturation kinetics, with an average Km value of 554 microM and a Vmax value of 4077 nmol/g lung wet wt/30 min, respectively. This system was not inhibited by metabolic inhibitors but was greatly diminished by lowering the temperature from 37 degrees to 4 degrees. Both compounds, pentamidine and putrescine, inhibited the uptake of the other and the inhibition of pentamidine accumulation by putrescine was demonstrated to be non-competitive. Uptake of putrescine was inhibited by increasing concentrations of pentamidine. As putrescine accumulates in epithelial type 1 and type 2 cells and in Clara cells, it is likely that pentamidine is also accumulated in these cell types but does not utilize the pulmonary uptake system for polyamine transport. Within the time period studied, toxic effects of the drug were not observed.

Indices of oxidative stress in hamster lung following exposure to cobalt(II) ions: in vivo and in vitro studies

Cobalt, a metal with numerous industrial applications, has been associated with lung disease, an extreme form of which is an interstitial fibrosis. The biochemical mechanisms underlying this toxicity are not understood. In vitro studies have suggested that cobalt(II) ions are able to generate reactive oxidant species (possibly hydroxyl radical) in a reaction with hydrogen peroxide, and we have hypothesized that the occurrence of such an event in lung tissue, and the subsequent development of oxidative damage, may contribute to this pulmonary toxicity. The intratracheal instillation of CoCl2 into hamster lungs resulted after 3 h in decreased levels of reduced glutathione and increases in levels of oxidized glutathione and in the activity of the pentose phosphate pathway. These changes, which are compatible with the generation of oxidative stress, were reversed by 48 h at low Co2+ doses (1.0 to 1,000 micrograms/kg). Irreversible changes at higher doses coincided with the onset of pulmonary edema. Incubation of lung slices with CoCl2 (0.1 to 10 mM) resulted in time- and Co2+ concentration-dependent increases in levels of oxidized glutathione and protein-mixed disulfides and a decrease in reduced glutathione. A concentration-dependent stimulation of the pentose phosphate pathway was also observed. These changes preceded the detection of overt cell toxicity, as assessed by various biochemical parameters. These data indicate that thiol oxidation constitutes an early event in the pulmonary toxicity of cobalt(II) ions and are compatible with the hypothesis that the generation of oxidative stress may be of significance to the toxic process.

Molecular features necessary for the uptake of diamines and related compounds by the polyamine receptor of rat lung slices

The influence of 17 putrescine analogues on the uptake of putrescine and/or paraquat by rat lung slices has been determined. Most of these compounds are competitive inhibitors of putrescine and/or paraquat uptake, but three show no inhibiting activity. Apparent Ki values of the putrescine derivatives increase, and thus the inhibitory effects decrease, with increasing N-methylation. Comparison of N-methyl-1,4-diaminobutane (Ki = 8 microM) with N,N'-bis-methyl-1,4-diaminobutane (Ki = 25.5 microM) shows that a single primary amino group is desirable for high inhibiting activity. Dimethylation at one amino function does not greatly decrease inhibitory potential (thus N,N-dimethyl-1,4-diaminobutane has Ki = 11.5 microM). Increasing the size of N-alkyl substituents in putrescine derivatives, decreased their inhibitory action on the uptake of putrescine. Investigation of the effect of conformationally-restricted analogues of putrescine shows that both (E) and (Z) isomers of 1,4-diaminobut-2-ene are poor inhibitors of putrescine uptake. Analogues of putrescine with bulky substituents on the butyl chain, i.e. the meso- and rac-isomers of 1,1-dichloro-2,3-diaminomethylcyclopropane, do not inhibit putrescine uptake. Inhibiting putrescine derivatives which contain aziridine groups are competitive inhibitors of putrescine and paraquat uptake. Surprisingly, N-(4-aminobutyl)aziridine is the most effective inhibitor of putrescine uptake studied, and is a better inhibitor of paraquat uptake than the endogenous polyamine, putrescine. N-(4-Aminobutyl)aziridine binds reversibly to the polyamine transporter and its inhibitory effects do not appear to be due to any cytotoxic activity of the aziridine. The parameter A (mM)-1 defined as 1000/Ki (where Ki units are microM) was taken as a measure of the affinity of a compound for the polyamine receptor in this paper.

Effects of injury on [3H]putrescine uptake by types I and II cells in rat lung slices

The capacity of different lung parenchymal cells to accumulate putrescine was investigated by incubating slices of rat lung in a medium containing the tritiated compound. Quantitative examination of autoradiographs, by electron microscopy, indicated that accumulation of putrescine occurred in both the Type I and Type II cells of the alveolar epithelium. Putrescine uptake was abolished by the addition of spermidine to the medium or by incubating at 0 degrees C. Lung samples from rats dosed with the pneumotoxin O,S,S-trimethyl phosphorodithioate (OSSMeO), which selectively damages Type I pneumocytes, showed a large reduction in the uptake of label by both Type I and Type II cells. This treatment also resulted in an increase in the labeling of alveolar macrophages. Control samples, from undosed rats, were incubated in medium containing tritiated 5-hydroxytryptamine; this compound did not accumulate in epithelial cells but it was concentrated in the endothelium of the alveolar capillaries and in the blood cells within these vessels. The demonstration of putrescine uptake by both Type I and Type II pneumocytes, together with its reduction by dosing with OSSMeO, has vindicated the use of this activity, in lung slices, as an index of damage to the alveolar epithelium.

Diamine uptake by rat lung type II cells in vitro

Lung epithelial type II cells are responsible for synthesising and secreting pulmonary surfactant which reduces surface tension and prevents lung collapse. Type II cells replace type I cells and can proliferate in response to alveolar injury. An important aspect of this proliferation may be the ability of type II cells to accumulate amines actively, particularly the endogenous diamine putrescine. Putrescine is accumulated into isolated alveolar type II cells by an energy-dependent process. The uptake obeys saturation kinetics for which an apparent Km of 14.7 microM and Vmax of 130 pmol/micrograms DNA/hr was derived. The inhibitory effects of structurally similar amines on putrescine accumulation are described. As the herbicide paraquat has been suggested to share the same uptake system as putrescine from lung slice studies, this phenomenon was investigated in type II cell cultures. The results demonstrated that paraquat is a partially competitive inhibitor of putrescine accumulation in the cells. The Ki for the inhibition of putrescine uptake by paraquat in type II cells was calculated to be 69 microM, a value which closely matches the Km for paraquat (70 microM) predicted from lung slice studies. In molecular terms, the partial nature of the competition indicates that paraquat and putrescine do not occupy identical sites. Saturation of its site by paraquat reduced the affinity of putrescine 3.6-fold, but did not abolish it.

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).

Interactions between paraquat, endogenous lung amines' antioxidants and isolated mouse Clara cells

The ability of paraquat to damage mouse lung Clara cells in the presence and absence of herbicide inhibitors is investigated using a cell culture system. Clara cell damage is assessed on the loss of nitroblue tetrazolium reductase activity and the inability to attach and spread on an extracellular matrix. Endogenous amines such as putrescine and spermidine reduce paraquat-induced damage at low concentrations indicating that they compete for the same cell surface receptor as paraquat and thus potentially block the accumulation of the herbicide. The efficacy of 10 microM exogenous putrescine as a protectant is reduced the longer the time before it is added to the cultures. Clara cells contain high levels of NADPH-dependent P-450 reductase which is required to redox cycle the paraquat and generate reactive oxygen radicals. Compounds with antioxidant properties are examined for their ability to reduce the Clara cell damage. Cystamine, the disulphide form of the naturally occurring thiol, cysteamine, and taurine, a metabolite of cystamine, both of which are accumulated in the lung, do not reduce paraquat-induced Clara cell damage. Another antioxidant, alpha-tocopherol is also ineffective but reduced glutathione (GSH), present in high quantities (3.2 mM) in clara cells, could reduce damage to the cultured cells. Cysteine, a precursor of GSH, can also prevent Clara cell damage when the concentration of paraquat is low.

Mechanism of protection of alveolar type II cells against paraquat-induced cytotoxicity by deferoxamine

Paraquat toxicity has been associated with the generation of free radicals in alveolar epithelial cells in which paraquat specifically accumulates via a polyamine uptake system. In the present study we investigated whether deferoxamine (DF), an iron chelator that has antioxidant capacity and that also has a polyamine-like structure, could protect alveolar type II cells (ATTC) against injury by paraquat. Radiolabeled [3H]adenine ATTC were incubated in a medium containing 75 microM paraquat in the absence or presence of DF (500 microM). After 3 hr of incubation paraquat-mediated cytotoxicity of ATTC, as measured by [3H]adenine release, was significantly (P less than 0.005) decreased by addition of DF (26.6 +/- 2.6% vs 7.4 +/- 1.7%). Accumulation of radiolabeled [14C]paraquat at a concentration of 75 microM was also decreased (70%) by 500 microM DF from 94.8 +/- 2.1 to 28.9 +/- 6.7 nmoles paraquat/2.5 x 10(5) ATTC. This effect of DF was dose dependent and comparable with the protective effect of equimolar concentrations of putrescine. However, per cent uptake of paraquat at a concentration of 500 microM was not significantly inhibited by DF (1 mM), whereas paraquat-induced injury was still markedly reduced (36.2 +/- 2.5% vs 2.6 +/- 4.2%). This indicated that the protective effect of DF could not be explained by its competition with paraquat on uptake alone. In the same series of experiments using another iron chelator, pyridoxal benzoyl hydrazone (PBH), which has antioxidant properties similar to DF but does not show its polyamine-like structure, ATTC lysis was also prevented although paraquat uptake was not reduced. These in vitro data indicate that the mechanism of protection by DF against paraquat toxicity in lung epithelial type II cells is two-fold: inhibition of paraquat uptake through its compliance with the structural requirements necessary for transport, and inhibition of paraquat-induced iron-catalysed free radical generation.

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.

Localization of spermidine uptake in rabbit lung slices

The lungs have a high polyamine transport capability, and the type II pneumocyte has recently been identified as a major site of putrescine uptake and localization (N. A. Saunders, P. J. Rigby, K. F. Ilett, and R. F. Minchin. Lab. Invest. 59: 380-386, 1988). However, recent evidence suggests that multiple polyamine transport systems exist. In the present study, localization of spermidine uptake in rabbit lung was investigated. Although [14C]spermidine was rapidly accumulated by lung slices, it was not significantly metabolized, and no efflux of the accumulated polyamine was apparent. Autoradiographs prepared after [3H]spermidine transport revealed a localization of uptake activity to cells identified by electron microscopy as type II pneumocytes. Spermidine uptake occurred in all type II cells examined and thus appeared to be a characteristic function of this cell type. In contrast, spermidine uptake was virtually absent in the major airways and blood vessels, whereas moderate uptake was associated with pulmonary alveolar macrophages and alveolar tissue. Subsequent purification and culture of type II pneumocytes showed these cells to have significant polyamine uptake activity. In addition, spermidine uptake activity was positively correlated with the proportion of type II cells present at the various stages of their purification. In other studies, cultured pulmonary alveolar macrophages possessed similar uptake activity to that of cultured type II cells. Combined, these data suggest that both type II cells and pulmonary alveolar macrophages may represent major sites of spermidine uptake in vivo. We also suggest that the transport of polyamines by type II cells may reflect a critical role for polyamines in a characteristic function of this cell type.

Pulmonary alveolar macrophages express a polyamine transport system

Polyamine transport is an important mechanism by which cells regulate their intracellular polyamine content. It is well established that the lung has a high capacity for polyamine transport, and recently the polyamine putrescine has been shown to be selectively accumulated into the type II pneumocyte of rabbit lung slices (Saunders et al.: Lab. Invest., 95:380-386, 1988). In addition, it has been suggested that there may be more than one polyamine transport system in lung tissue (Byers et al.: Am. J. Physiol., 252:C663-C669, 1987). In the present study, we have examined whether there are differences in the distribution of putrescine and spermidine uptake activities in isolated rabbit lung cells. We report that pulmonary alveolar macrophages have a greater rate of uptake of both putrescine and spermidine than the total lung cell population. Kinetic analysis of the polyamine uptake system present in macrophages showed putrescine uptake consisted of a saturable (Km = 2.1 microM) and nonsaturable component whilst spermidine uptake consisted of both a high- and a low-capacity saturable component (Km = 0.16 microM and 1.97 microM, respectively). The rate of polyamine transport was similar to those reported for many proliferative or tumor cell-lines and appears to be greater than any other major lung cell type. Inhibition studies of the transport of polyamines into pulmonary alveolar macrophages suggested that the uptake of both putrescine and spermidine was mediated by the same system, which could not be described by simple Michaelis-Menten kinetics. The transport appears to be reversible due to significant efflux. This is the first study to describe the presence of multiple polyamine transport systems in pulmonary alveolar macrophages.

Competition for polyamine uptake into rat lung slices by WR2721 and analogues

The objective of these studies was to determine whether a series of structurally related radioprotective agents could act as substrates for the recently identified polyamine system in the lung. We have shown that WR2721 (S-2(3-aminopropylamino)ethyl phosphorothioate), S-2(4-aminobutylamino)ethyl phosphorothioate (S-ABEP or WR2822) and S-2(7-aminoheptylamino)ethyl phosphorothioate (S-AHEP) competitively inhibit the uptake of putrescine into rat lung slices. The ability of the radioprotectors to act as substrates for the polyamine uptake system was expressed as the Ki for each compound. The Ki values for WR2721, S-ABEP and S-AHEP in the absence of dithiothreitol were 48, 57 and 7 mumol dm-3 compared to 155, 88 and 15 mumol dm-3 in the presence of dithiothreitol, indicating that the disulphide form may have a higher affinity for the transport system. By analogy with other substrates for the polyamine uptake system we have concluded that it should be possible to target radioprotectors to the alveolar epithelial type I and II cells and the Clara cells in the lung, as they prossess this uptake system, and thus protect these cells from oxidative stress.

Mechanistic studies: their role in the toxicological evaluation of pesticides

To date, studies on the mechanism of toxicity of pesticides are not yet an integral part of the toxicological evaluation process. However, in recent years mechanistic studies have played an increasing role in the assessment of toxicological hazards to man, and in this paper we have described two examples where an understanding of mechanism has contributed positively to risk assessment or has provided a surer scientific basis for the judgement of whether a potential hazard will be expressed in man. In the first example, an evaluation of the scientific literature leads to the conclusion that hepatic peroxisome proliferation in rats and mice is directly, or indirectly, related to the development of hepatocellular tumours. A wide range of non-mutagenic chemicals elicit peroxisome proliferation in mouse and rat liver, but not the guinea pig or marmoset liver. Using one of the diphenyl ether herbicides, fomesafen, we have shown that isolated hepatocytes from mice and rats, but not those from guinea pigs, marmosets and significantly man, undergo peroxisome proliferation. Therefore, it seems reasonable to conclude that although fomesafen causes peroxisome-related tumours in the mouse, man is neither susceptible nor sensitive to this mechanism. Consequently, we can conclude that fomesafen will not cause liver tumours in humans exposed to this herbicide. The herbicide paraquat, although safe in normal agricultural use, has been responsible for numerous human fatalities, almost exclusively as a result of the intentional ingestion of the concentrated commercial product.(ABSTRACT TRUNCATED AT 250 WORDS)