Experimental paraquat poisoning: a histological and electron-optical study of the changes in the lung

Paraquat toxicity in vitro. I. Pulmonary alveolar macrophages

When the herbicide paraquat (1,1'-dimethyl-4,4'-bipyridylium) was administered to adult rat pulmonary alveolar macrophages (PAM) in primary culture, both a time-dependent and a dose-dependent cytotoxic response (cell death) was observed. An LD50 value of 1 mM was calculated when these cells were exposed to paraquat in vitro for 12 h in Ham's F12 culture medium at 30 degrees C. Cell death was accompanied by the formation of TBA-reactive substances (lipid peroxidation) and was potentiated by hyperoxia (95% O2). In a 95% O2-5% CO2 atmosphere, an LD50 value of 0.1 mM was calculated. In addition, the presence of superoxide dismutase in the culture medium (1700 units/ml) inhibited the cytotoxic response. Since [14C]paraquat was not absorbed into these cells, extracellular superoxide anion radical formation was investigated as the cause of the observed cell death. Paraquat (0.5 mM) was found to stimulate extracellular O-2 generation, from PAM, but only in nonactivated cells. A sevenfold enhancement over the resting rate of radical generation was observed in the presence of paraquat. No increase in the O-2 generation rate of activated macrophages was observed upon the addition of paraquat to the culture medium. These data indicate that paraquat is cytotoxic to the pulmonary alveolar macrophage and further suggest that this cytotoxicity is mediated, at least in part, by an excess, extracellular production of active oxygen species. Implications of these findings with respect to the currently accepted hypothesis of paraquat poisoning in vivo are discussed.

Resolution after radiotherapy of severe pulmonary damage due to paraquat poisoning

A 29 year old man was admitted 36 hours after ingesting about 5 g paraquat. His arterial oxygen pressure fell progressively to 3.4 kPa (34 mm Hg), and pulmonary damage induced by paraquat was diagnosed. His condition did not improve after treatment with prednisolone and cyclophosphamide, but after irradiation both lungs cleared and arterial oxygen pressure started to improve. Irradiation of the lungs should be considered in patients who, after surviving the acute phase of poisoning with paraquat, show progressive deterioration of respiratory function.

Pathogenesis of paraquat-induced pulmonary hemorrhage in hamsters with special reference to arterial constriction

Three dimensional reconstructions of pulmonary arteries and the bronchial tree were made to evaluate the cause of hemorrhage in the lungs of hamsters treated with a single intraperitoneal injection of paraquat at a dose of 15 mg/kg b.w. The hemorrhage corresponding to the respiratory bronchiolar segment formed the minimum hemorrhagic unit; larger hemorrhages were composed of conglomerations of such units. The respiratory bronchiolar segment was fed by a single respiratory bronchiolar arteriole which had a distinct muscular medial coat. No evidence of breakage of the arterial wall passing through the hemorrhagic lesion could be detected even by careful observation with serial histological sections. Accordingly, the cause of the lung hemorrhage was sought in the combined effects of paraquat toxicity to the capillary endothelium and vasoconstriction of the respiratory bronchiolar arterioles.

Selective action of prostaglandin F2 alpha during paraquat-induced pulmonary edema in the perfused lung

Lung prostaglandins (PGs) play a key role in normal pulmonary vascular regulation. We investigated PG metabolism during edema formation following paraquat-induced damage with an isolated perfused rat lung preparation. Lungs perfused with paraquat (PQ), 1 X 10(-7) M to 1 X 10(-2) M, showed significant increases in PGF2 alpha prior to detectable functional and pathological changes (increases in airway resistance, vascular resistance, and edema). No changes in PGE were observed. PGF2 alpha in perfused lungs showed a dose-related response following PQ exposure (up to 300% increase over control values). Lungs perfused with PQ and ventilated with high oxygen (95% O2-5% CO2) instead of air-5% CO2 showed a dramatic potentiation in the selective increase of PGF2 alpha, with levels reaching over 1 ng/ml (a 2600% increase over control values). The addition of exogenous PGF2 alpha to the perfusate without PQ initiated edema in a dose-related fashion, indicating the potential of PGF2 alpha as a causative agent in lung edema formation from PQ injury. The addition of ibuprofen (a nonsteroidal anti-inflammatory agent) to the perfusion medium blocked endogenous release of PGF2 alpha in lungs linked to oxidant-induced edema. These data show that in the perfused lung: (1) PQ caused a selective increase of PGF2 alpha; (2) this selective increase occurred prior to the onset of edema; (3) exogenous PGF2 alpha alone induced pulmonary edema; and (4) ibuprofen, in doses which blocked PGF2 alpha, also prevented edema formation.

Effect of paraquat on cytochrome P-450-dependent lipid peroxidation in bovine adrenal cortex mitochondria

We have investigated the effect of paraquat (methyl viologen) on lipid peroxidation in bovine adrenal cortex mitochondria. Incubation of a buffered aerobic mixture of mitochondria in the presence of Fe2+ or NADPH resulted in the formation of lipid peroxides whose accumulation could be followed at 532 nm as malondialdehyde. Fe2+ stimulates lipid peroxidation in normal mitochondria and those in which enzymes have been inactivated with heat. In contrast, NADPH has a stimulatory effect only in normal mitochondria, but not in heat-treated mitochondria. These results indicate that NADPH-dependent lipid peroxidation is an enzymatic process. Paraquat strongly inhibits this enzymatic lipid peroxidation, but has no effect on the non-enzymatic Fe2+-dependent process. The chemiluminescence that accompanies the NADPH-dependent lipid peroxidation is also markedly decreased in the presence of paraquat. Superoxide dismutase, which removes superoxide anion efficiently, does not inhibit malondialdehyde production. The mechanism of the inhibition of the lipid peroxidation by paraquat has been examined. Paraquat has no effect on NADPH-2,6-dichlorophenolindophenol reductase and on NADPH-cytochrome c reductase activities in bovine adrenal cortex mitochondria. However, paraquat strongly inhibits the NADPH-dependent reduction of cytochrome P-450. These results suggest that the inhibitory effect of paraquat on NADPH-dependent lipid peroxidation in adrenal cortex mitochondria is due to a decrease in the level of reduced cytochrome P-450 probably by diverting electrons from cytochrome P-450. Cytochrome c, which can compete with P-450 for available electrons from adrenodoxin, like paraquat had an inhibitory effect on NADPH-dependent lipid peroxidation. Lipid peroxidation was also strongly inhibited by steroid hydroxylase inhibitors, e.g., amphenone B, aminoglutethimide and metyrapone.

Repeated pulmonary function evaluation following bleomycin treatment

A computerized, nonsurgical, pulmonary function measurement method was tested for sensitivity and utility in detecting the development of fibrosis. Bleomycin, a fibrogenic agent, was intratracheally instilled into male Fisher 344 rats. Respiratory function was monitored in restrained, awake animals before treatment and for the subsequent 4 wk. In the first week, among responders, a significant (p less than 0.05) drop in body weight, tidal volume, and compliance was accompanied by a significant increase in respiratory frequency. Minute volume increased in the second week. Although body weight, tidal volume, and compliance returned to baseline values in the following weeks, respiratory frequency and minute volume remained significantly elevated. With the methods used here, respiratory rate change was the parameter most sensitive to the effects of bleomycin in vivo.

Studies on the effect of paraquat on glycogen mobilization in liver of common carp (Cyprinus carpio L.)

1. A herbicide, paraquat (1,1'dimethyl-4,4'-bipyridilium-dichloride) was administered to carp in 0.5-10.0 ppm concentrations, respectively, and blood sugar level, glucose-6-phosphatase and glycogen phosphorylase activities of liver were determined. 2. Paraquat treatment caused an increase of blood sugar level and enhanced phosphorylase and glucose-6-phosphatase activities. 3. Paraquat can induce alterations in endoplasmic reticulum that might contribute to the changes in glucose-6-phosphatase activity, resulting in an increase of blood glucose level and/or all the effects can be attributed to a high level of circulating epinephrine produced by paraquat toxicosis.

Disruption of phospholipid metabolism by paraquat in cultured pneumocytes

Paraquat (PQ) has specific pneumotoxicity in humans. Its effects on phospholipid metabolism in type 2 pneumocytes, A-549, and in lung fibroblasts, WI-38, were examined. PQ inhibited the incorporation of palmitic acid into phosphatidyl-ethanolamine by 42%, but did not inhibit the incorporation into phosphatidylcholine. PQ also inhibited the incorporation of acetate into phosphatidylcholine and ethanolamine by 82-88% in A-549 cells and by 92% in WI-38 cells. PQ inhibited the incorporation of choline into phosphatidylcholine and ethanolamine by 55 and 73%, respectively. Phosphatidylcholine was detected in the medium of A-549 cells. PQ completely inhibited this phosphatidylcholine secretion. These results suggest that PQ has inhibitory effects on phospholipid synthesis and excretion in human type 2 pneumocytes.

Young Scientists Award lecture 1981: The identification of an accumulation system for diamines and polyamines into the lung and its relevance to paraquat toxicity

The energy dependent accumulation of the herbicide paraquat into the lung is known to be a major factor responsible for the selective toxicity of paraquat to this organ. The studies reported in this paper were designed to examine the hypothesis that the transport process responsible for the accumulation of paraquat is present to accumulate endogenous substrates from the plasma. Paraquat is accumulated into the lung by a process which is different from that responsible for the uptake of the monoamine, 5-hydroxytryptamine (5HT). Furthermore, 5HT is known to be accumulated into the endothelial cells of the lung whereas paraquat was accumulated, at least in part, by the alveolar type I and type II epithelial cells. In the search for compounds which would reduce the uptake of paraquat into the lung a series of diamines were found to be the most effective inhibitors. In particular the diamine, putrescine, effectively inhibited the uptake of paraquat into the lung and was itself accumulated by a process which obeyed saturation kinetics. The apparent Km for the process was 7 microM with a Vmax of 330 mumoles/g wet weight lung/h. This apparent Km is an order of magnitude lower than that for the uptake of paraquat. The uptake of putrescine was inhibited when paraquat was present in the incubation medium or when the metabolic inhibitors rotenone, or iodoacetate together with KCN were added. Putrescine was not accumulated by slices of liver, kidney, heart or spleen. It was taken up into brain slices by a KCN sensitive process although the accumulation was much less than that which occurred in lung slices. Thus, in these respects the uptake of putrescine is similar to that which has been described for paraquat. The uptake of putrescine into lung slices with damaged type I and type II alveolar epithelial cells was reduced as was the uptake of paraquat. The reduction was similar for both compounds suggesting they were both taken up into the same cellular compartment. The studies described in this paper suggest that (i) the process in the lung which accumulates paraquat is that which is normally responsible for the uptake of putrescine in particular and endogenous diamines and polyamines in general and (ii) this uptake process is located in the alveolar type I and type II epithelial cells.

Ultrastructural evidence of pulmonary capillary endothelial damage from paraquat

Because of lack of agreement concerning the toxicity of paraquat to the pulmonary microvasculature, we have undertaken an electron microscopic study of lungs of paraquat-treated rats. Rats were injected with paraquat or sterile water (controls) intraperitoneally; the animals were then killed at 24-h intervals for 10 days post-injection. In the control animals, lung ultrastructure remained normal throughout the study. In treated animals, the initial evidence of alveolar epithelial injury occurred 24 h post-paraquat. By 48 h, severe fragmentation and desquamation of membranous pneumocytes occurred, and both alveolar and interstitial edema were present. Epithelial damage was maximal 72-96 h post-paraquat. Pulmonary capillary endothelial abnormalities were less extensive than the alveolar epithelial lesions. Endothelial damage was first observed 48 h post-paraquat. In endothelial cells on the septal (thick) side of the capillaries, the number of pinocytotic vesicles was significantly increased (P less than 0.05) from 48 to 96 h post-paraquat. In endothelium adjacent to damaged epithelium, abnormalities included hydration, fragmentation, discontinuity, and widened intercellular junctions; these were maximal 72-96 h post-paraquat. Although other mechanisms are probably important, damaged pulmonary capillary endothelium seems to be a factor favoring paraquat-induced pulmonary edema.

Transplacental effects of carcinogens and non-carcinogens on activities of pyruvate kinase and lactate dehydrogenase as well as isozymic pattern of LDH in mouse lung

The carcinogens, urethane (URTH), 3-methylcholanthrene (MCA) and dimethylnitrosamine (DMN) given to pregnant mice enhanced permanently the activities of pyruvate kinase (PK) and lactate dehydrogenase (LDH) in the lungs of offspring even well before the appearance of lung tumours. The noncarcinogenic analogues phenylurethane (PHUR) and pyrene (PYR), had no effect on PK or LDH activity. The non-carcinogenic pulmonary toxicant Paraquat (PAR) elicited only a temporary elevation in the activities of the enzymes tested. The H:M ratio of LDH sub-units in the lung tissue was permanently decreased by URTH administered transplacentally. PAR caused only a temporary decrease in the H:M ratio, while PHUR had no effect on the isozymic pattern of LDH.

Phospholipids of the lung in normal, toxic, and diseased states

The highly pulmonary concentration of 1,2-dipalmitoyl-sn-glycerol-3-phosphorylcholine (dipalmitoyllecithin) and its implication as an important component of lung surfactant have promoted investigation of phospholipid metabolism in the lung. This review will set the contents including recent informations for better understanding of phospholipid metabolism of the lung in normal state (physiological significances of lung phospholipids, characteristics of phospholipids in lung tissue and alveolar washing, biosynthetic pathways of dipalmitoyllecithin, etc.) as well as in toxic states (pulmonary oxygen toxicity, etc.) and in diseased states (idiopathic respiratory distress syndrome, pulmonary alveolar proteinosis, etc.) Since our main concern has been to clarify the most important route for supplying dipalmitoyllecithin, this review will be focused upon the various biosynthetic pathways leading to the formation of different molecular species of lecithin and their potential significance in the normal, toxic, and diseased lungs.

Activity of pyruvate kinase and lactic acid dehydrogenase in mouse lung after transplacental exposure to carcinogenic and non-carcinogenic chemicals

The carcinogenic urethane (URTH), dimethylnitrosamine (DMN), 3-methylcholanthrene (MCA), benzo[a]pyrene (BP), 7,12-dimethylbenz[a]anthracene (DMBA) and aflatoxin B1 (B1) administered to pregnant CFLP mice increased the activity of pyruvate kinase (PK) and lactate dehydrogenase (LDH) and decreased the ratio of LDH H and M subunits in the lungs of offsprings. However, under the same conditions, the non-carcinogenic phenylurethane (PHUR), ethylformate (EF), chrysene (CHRY), perylene (PER) and pyrene (PYR), as well as the toxic Paraquat (PAR), butylated hydroxytoluene (BHT) and cadmium chloride (CdCl2), did not influence the activities of the enzymes tested.

Intrabronchial instillation of paraquat in rats: lung morphology and retention study

Various amounts of paraquat (10(-5) to 10(-12) g) in 0.1 ml saline were instilled directly into the left bronchus of male adult rats. Gravimetric, macroscopic, and microscopic studies on the left lobe of the lung showed that 10(-5) g of paraquat produced lung oedema and macroscopic lesions two and 14 days after doing. The pathology of the lung was similar to that seen after systemic poisoning. When 10(-6) g of paraquat was instilled, some animals developed lung oedema and macroscopic lesions. Microscopic examination showed subtle changes in the parenchyma of the lung. With amounts of paraquat equal to or less than 10(-7) g (doses as little as 10(-12) g were used), no changes in the lung were seen. This is contrary to published accounts in which amounts as low as 10(-12) g (1 Pg) were reported to cause acute damage to the rabbit lung. When 3H paraquat was instilled into the left lobe (doses of 10(-5) to 10(-10) g were used), the loss of paraquat from the lung was biphasic. The initial half-life was less than one hour. The secondary phase obeyed first-order kinetics, and the half-life was dependent on the dose of paraquat instilled. This half-life was as short as 11 hours when 10(-5) g paraquat was instilled and was 76 hours after the instillation of 10(-10) g paraquat. The decrease in the half-life of the secondary phase with increasing doses of paraquat is possibly associated with the production of oedema or lung cell damage, or both. After the instillation of 10(-8) g 3H paraquat, the initial half-life was less than 15 minutes, and paraquat was detected in the urine and plasma at that time. This suggests that 50% of the instilled paraquat was rapidly absorbed from the lung into the plasma.

The effects of paraquat on neonatal rat lung: a histological and biochemical study

The effects of paraquat on morphological, histological, and biochemical parameters in neonatal rat lung were studied. One-day-old rat pups were injected (IP) with 25 mg paraquat per kg body weight and sacrificed after 24 hr. At the end of the experimental period, the body weight in control and herbicide-treated animals slightly increased and decreased, respectively. The lung weight in the paraquat group was not significantly lower than those of the control. Histologically, the lungs from the paraquat group showed an increase in the thickness of the alveolar wall with much intra-alveolar infiltration of cells and cell debris. In the paraquat-treated group, while the total lung protein increased by about 18%, the enzyme phosphatidic acid phosphatase activity was reduced nearly 30%. These results indicate that paraquat induces both histological and biochemical changes in the neonatal rat lung.

Effect of carcinogenic and non-carcinogenic chemicals on the activities of four glycolytic enzymes in mouse lung

The activities of four glycolytic enzymes were measured in the lung homogenate of CFLP mice treated with a variety of carcinogens and non-carcinogens for mouse lung. The carcinogenic urethane, dimethylnitrosamine (DMNA), 3-methylcholanthrene (MCA), benzo[a]pyrene (BP), 7,12-dimethylbenz[a]anthracene (DMBA) and aflatoxin B1 enhanced the activity of hexokinase (HK), phosphofructokinase (PFK), pyruvate kinase (PK) and lactate dehydrogenase (LDH) 28 days after a single intraperitoneal administration. These carcinogens also altered the ratio of LDH H and M subunits. In contrast, under the same conditions the non-carcinogenic phenylurethane, ethylformate, chrysene, perylene and pyrene, as well as the pulmonarily toxic Paraquat, butylated hydroxytoluene (BHT) and cadmium chloride (CdCl2), did not influence either the activities of the enzymes tested or the isozyme pattern of LDH.

The influence of paraquat on the in vivo incorporation of lecithin precursors in lung tissue and 'alveolar' lecithin

Rats were injected with paraquat (1,1'-dimethyl-4,4'-bipyridyliumdichloride, 35 mg/kg) and killed 14 or 48 h later. Twelve hours prior to sacrifice labelled choline or palmitic acid was injected. Immediately after sacrifice alveolar lavage was performed in situ and the lungs were removed and homogenized. Lipid extracts from lavage fluid and lung homogenates were prepared and their lecithin content analysed by thin layer chromatography. Lecithin was quantitated by phosphorous determination, the incorporation rate of the lecithin precursors in 'alveolar' and lung tissue lecithin was measured by liquid scintillation, and the fatty acid composition of lecithin was determined by gas liquid chromatography. In comparison with controls, the content of lecithin was reduced in lavage fluid from paraquat-injected rats. These animals also had reduced incorporation of choline in alveolar lecithin. The rate of incorporation of palmitic acid was the same in experimental animals and controls, so was the incorporation of choline in lung tissue lecithin. The findings suggest that paraquat interferes with the synthesis of alveolar lecithin by the phosphocholine transferase pathway, but not with the formation of lung tissue lecithin.

The changing pattern of paraquat poisoning in man

The clinical and pathological findings are described in 14 patients who died between 6 hours and 26 days after drinking paraquat. Respiratory failure and delayed pulmonary fibrosis have become the hallmark of this poison, but were not the common mode of death in this series. Toxic myocarditis, renal tubular necrosis and centrilobular liver cell damage were significant factors in the eight deaths which occurred within 5 days of paraquat ingestion. Similar abnormalities plus respiratory failure caused the two deaths which occurred 5 and 6 days after consumption of the poison. Respiratory failure was the sole cause of death in only four patients who died 8 to 26 days after drinking paraquat, although the lungs showed pathological changes in all cases. The patients who died in multisystem failure, with one exception, had drunk larger quantities of paraquat than those who survived for a longer period and died in respiratory failure.

Cerebral damage in paraquat poisoning

This is the first report on cerebral changes in eight patients who died of paraquat poisoning. These included generalized oedema, haemorrhages (both subependymal and subarachnoid), glial reactions (microglial activity and astrocytic response) and meningeal inflammation. Oedema and haemorrhage were the most consistent and significant findings; they suggest that paraquat may damage the cerebral blood vessels. The distribution of haemorrhages was unusual and resembled that seen in thiamine deficiency.

A comparison of the effects of paraquat and diquat on lung compliance, lung volumes and single breath diffusing capacity in the rat

Paraquat intoxication in its initial stage is characterized histologically in the lungs by atelectasis, hyaline membrane formation, alveolar edema and vascular hemorrhage often into the interstitium or air spaces. Information on the functional modification of paraquat-damaged lungs has been lacking. We evaluated lung volumes, single breath diffusing capacity of the lungs for carbon monoxide (DLCO) and static lung compliance (Cst(L)) in rats treated with paraquat or diquat. Measurements were made 12, 24, 48, and 72 h after treatment. Paraquat by intratracheal (i.t.) instillation 0.5 mg/kg or by intraperitoneal injection (i.p.) 27 mg/kg significantly decreased (P less than 0.01) the body weight, total lung capacity (TLC), functional residual capacity (FRC), vital capacity (VC), residual volume (RV), DLCO, apparent alveolar volume (VA) and Cst(L). At a lower dose level (13.5 mg/kg), the effects of paraquat peaked at about 24 h following treatment, causing a significantly decreased (P less than 0.01) VC and TLC. Diquat i.t. or i.p. had little effect on the lungs. However, diquat i.p. decreased body weight (P less than 0.01) and caused a slight increase (P less than 0.05) in VC. The data obtained are consistent with the known pathological changes seen in paraquat-damaged lungs in that, by both routes, paraquat caused severe lung damage associated with decreased elasticity of the lungs and thorax, destruction of gas exchanging alveolar surfaces, and edema. These changes were detected reliably by lung function measurements.

Biochemical and morphological correlation of oxidant-induced pulmonary injury: low dose exposure to paraquat, oxygen, and ozone

The authors compared the temporal pattern of low-dose oxidant-induced lung injury in rats after exposure to either 1 ppm ozone or 100% oxygen for 24 hr or from treatment with paraquat (20 mg/kg, intraperitoneally). Histological abnormalities in airways, parenchyma, and blood vessels were evaluated from coded and randomized sections and compared with appropriate controls. Drug metabolism by lung endoplasmic reticulum was studied in similarly treated rats as another index of lung injury. Exposure to oxygen caused no discernible morphological or biochemical abnormalities. Exposure to ozone caused histological lesions which appeared early and resolved by 7 to 14 days, whereas paraquat-induced lesions were first evident at about 7 to 14 days. Abnormalities in drug metabolism followed a similar pattern. Low-dose oxidant exposure from ozone and paraquat produce similar histological and biochemical lesions in rat lungs but with distinct temporal patterns.

Early bronchiolar damage following paraquat poisoning in mice

Paraquat causes focal intracellular oedema of the terminal bronchiolar epithelial cells and focal subpleural atelectasis with thickening of the interalveolar septa 1 hr after the administration of an LD50. These changes are progressive, and lead to panacinar atelectasis with necrosis plus sloughing of epithelial cells in many terminal bronchioles. Radioactive phosphatidyl choline (PC) is recoverable by lavage within 90 s of the administration of tritiated palmitate, which supports previous suggestions that one source of pulmonary surfactant is rapid secretion by the terminal bronchiole. Paraquat causes a reduction in the relative amounts of radioactive PC that are recoverable from the airways within 90 s of giving tritiated palmitate. A deficiency of pulmonary surfactant of bronchiolar origin is implicated, at least in part, in the pathogenesis of the acute phase of the paraquat lesion in mice.

Toxicity of 1,1'-alkyl-4,4'-bipyridylium salts in the rat

Methyl (paraquat), propyl, isopropyl, butyl, methyl-pentyl, hexyl, octyl and benzyl viologens (1,1'-Alkyl-4,4'-bipyridylium salts) were administered subcutaneously to female Sprague-Dawley rats to determine relative toxicities. These compounds all produce the spectrum of effects previously reported for paraquat and additionally produce a focal necrosis at the injection site, nonemptying of the stomach and adrenal enlargement. A lethal dose of propyl, hexyl or benzyl viologen often produced a yellow to red serous fluid in the pleural cavity. Many of the signs observed with viologen poisoning are similar to adrenal hormone effects and the suggestion is made that the adrenals may be contributing to toxicity.

The pathology and biochemistry of paraquat

After the administration of paraquat to rats the lung is the organ most severely damaged. The pathology in the lung can be divided into two distinct phases: (1) a destruction phase lasting a few days with damage to the type I and type II alveolar epithelial cells, oedema and haemorrhage (most of the rats which die after dosing with paraquat do so during this phase); (2) a reparative phase with regeneration of the epithelium and, in areas of severe damage, a characteristic proliferation of fibroblasts. In both phases of the lesion the death of the rats results from anoxia. Paraquat is selectively accumulated by the rat lung in comparison with other tissues and this accounts, at least in part, for the specific toxic effect in this organ. The accumulation into the lung was shown by in vitro studies to depend on energy and is inhibited by various endogenous and exogenous compounds. This uptake process is not that which has been described for 5-hydroxytryptamine and evidence is presented to suggest that the type I and type II alveolar epithelial cells are sites of accumulation. When paraquat is present in lung cells, it undergoes a cyclical reduction and oxidation with the production of superoxide anion. This radical may lead directly or indirectly to the formation of lipid peroxides and hence to cell death. However, paraquat stimulates the pentose-phosphate pathway and both reduces the level of NADPH and inhibits fatty acid synthesis in the lung. These effects occur when there is only minimal ultrastructural damage to the lung cells. It is suggested, therefore, that the primary mechanism of toxicity of paraquat is the extreme oxidation of NADPH which inhibits vital physiological processes and renders the cell more susceptible to attack from lipid hydroperoxides.

Injury and repair of the lung: response to intravenous Freund's adjuvant

Tissue from the lungs of rabbits was examined at intervals up to 24 weeks after the animals had received a single intravenous injection of Freund's complete adjuvant. Though this is not a conventional method for damaging the peripheral lung, it had the advantage of producing multiple lesions in which most tissue components were altered for a prolonged period. White blood cells were present within the tissue and air spaces of these damaged areas. They persisted for 6 weeks in large numbers and gradually decreased over the next 12 weeks. There was replacement of type A by type B alveolar lining cells. Basement membranes were displaced and lost. Elastic and collagen fibres were distorted and destroyed. Blood vessels were occluded. Epithelioid cell and foreign body granulomas developed. Interalveolar septa disappeared, and air spaces were compressed. Despite all these changes the lungs regained near normal structure by 24 weeks after the initial injury. These results do not support the importance that has been placed on damage to various structural components of the lung as an explanation for chronic pulmonary disease. They do give some insight into the capacity of peripheral lung tissue for regenerationa following a single injury that induces a prolonged inflammatory response.

Pulmonary histological appearances in fatal paraquat poisoning

The paper describes the histological appearances in the lungs from 11 fatal cases of paraquat poisoning. The study originated as an attempt to define the sequential changes in the condition and, from them, to assess the mode of action of the herbicide in the human. The main features of the "paraquat lung" include haemorrhage, extrusion of macrophages, oedema, "honeycombing", fibrosis and, rarely, epithelial hyperplasia. The indications are that the changes are progressive once a threshold tissue concentration has been reached. The appearances are compared to those in poisoning by hyperbaric oxygen and it is hypothesized that the toxic action of paraquat is to sensitize the lungs to oxygen at atmospheric pressure.

[A contribution to the pathogenesy of letal paraquat poisoning (author's transl)]

Case reports of severe lung damage in patients treated with oxygen following paraquat poisoning lead the autors to design an experiment, in which the contributory role of oxygen was tested. It was shown on rats that administration of oxygen in paraquat - intoxicated rats causes, already in the LD50/20 dosage of paraquat, death in all experimental animals during the first 24 hours. Light and electron microscopic examination revealed edema, desquamation of alveolar epithelium with cell destruction of typ I and Typ II pneumocytes and total denudation of the alveolar basement membrane. Alveolar macrophages participated in the removal of the cellular debris. The toxic substance itself was not detectable in the blood of the rats 24 hours or later after the oral application.

Pulmonary ultrastructure after oral and intravenous dosage of paraquat to rats

Paraquat was administered to rats by gavage or intravenously at doses which were approximately equitoxic (680 mu. moles/kh and 65 mu. moles/kg respectively) and the lungs examined by light and electron microscopy at intervals up to 48 hours. No significant changes were observed in alveolar endothelial cells at any of the time intervals studied. After intravenous administration the first ultrastructural changes were observed at 4 hr in the type I cells which were less electron dense and contained few organelles. At 8 hr these lesions were more marked and in some places the basement memebrane was denuded. Type II cells were also showing damage to mitochondria and loss of microvilli. After oral dosing, the type and sequence of changes was similar but the first changes were not seen until 22 hr. Intravenous injection of 0-03 micron carbon particles 1 hr before killing showed no significant leakage from the alveolar endothelium. This study provides no morphological evidence that the oedema of the lung caused by paraquat in rats is due to damage to endothelial cells. It appears that, following dosing by the two routes, the difference in interval between dosing and the development of lesions is due to the accumulation of paraquat. Lesions in type I cells therefore occurred when a certain concentration of paraquat is known to be present in the lung. It is suggested that a prime compartment into which paraquat is accumulated is the alveolar epithelial cell.

Mitosis of type B alveolar cells in the early hyperplastic response to Freund's adjuvant

Numerous areas of granulomatous inflammation develop in the lungs of rabbits following the intravenous injection of Freund's complete adjuvant (FCA). Within a few days after FCA injection, hyperplasia of type B (type I) alveolar cells is present on the surface of the septa in which an inflammatory reaction is developing. Mitosis of type B cells is detected 12 h after FCA injection and is common over the next 120 h. In addition, there are morphologic changes that are consistent with migration of these cells. The type B cells in mitosis extend across alveolar septa as well as along the alveolar surface. The extension of type B cells through alveolar septa is not limited to cells in mitosis, but is also observed in non-mitotic type B cells. Stimulation of mitosis and hyperplasia of type B cells is discussed in relation to the focal tissue injury and inflammatory response.