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Tissue distribution of paraquat and diquat after oral administration in rats
165
Forensic Science International, 14 (1979) 165 - 170 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
TISSUE DISTRIBUTION OF PARAQUAT ADMINISTRATION IN RATS
AND DIQUAT AFTER ORAL
EMIKO KURISAKI and HIROKO SAT0 Department
of Legal Medicine, Fukushima
(Received November 3,1978;
Medical College, Fukushima
in revised form February 27,1979;
(Japan)
accepted May 1,1979)
Summary The differential distribution of paraquat and diquat in liver, kidney, lung, brain and heart has been studied after oral administration of the LD~,J or LD50/2 to rats. Paraquat concentrations were highest in all organs at 24 hours; at this time concentrations in kidney and lung were 2 to 3 times higher than at 2 hours. In contrast, with the exception of kidney, tissue diquat concentrations were highest at 2 hours. There was severe lung damage at 24 hours after paraquat; diquat did not produce severe lung lesions, but caused intestinal distention and diarrhea. These findings suggest that the difference in the toxicity of paraquat and diquat is related to their difference in tissue distribution and excretion.
Introduction Paraquat (l,l’-dimethyl-4,4’-bipyridylium) and diquat (l,l’-ethylene4,4’-bipyridylium) are nonselective contact herbicides available under the commercial names of Gramoxone and Reglox. These compounds are widely used throughout the world, and accidental and suicidal poisoning has increased proportional to usage [ 11. These two bipyridylium compounds show differences in toxicity [2 71. For example, paraquat causes severe pulmonary lesions [ 8 - lo] , whereas diquat does not have a marked pulmonary toxicity. To study further the qualitative difference in the toxicity of the two compounds, we have determined their tissue distributions after oral administration in rats. Materials and methods Animals
and materials
Male Wistar rats (180 - 225 g) were used; they were allowed free access
to food and water. Rats were given either the LD50 (150 mg paraquat cation per kg or 231 mg diquat cation per kg) or LDs0/2, as determined by Clark et al. [ 11 - 121 ; a dose-volume of 0.2 ml/kg was used.
166
For biochemical analysis, thirteen rats were each given the LD5c of paraquat and another thirteen the LD5e of diquat. For each group, five animals were sacrificed at 2 hours, five at 24 hours, three at 48 hours. The LD5s/2 was given to groups containing five animals, of which two were sacrificed at 2 days, two at 5 days and one at 9 days. For histological study, nine rats were given the LD5c of paraquat and another nine rats the LD5a of diquat. For each group, two animals were sacrificed at 2 hours and two at 24 hours; five died between 3 and 5 days after paraquat and another five died on the 3rd or 4th day after diquat. Reagents
Gramoxone (containing 24% paraquat, Nippon Noyaku Co. Ltd.) and Reglox (containing 30% diquat, Nippon Noyaku Co. Ltd.) were used. The formulated solutions were appropriately diluted with distilled water. Determination
of paraquat
and diquat
Animals were lightly anesthetized with ether, and exsanguinated by cutting the inferior vena cava. Brain, heart, lungs, liver and kidneys were removed and stored in a freezer. Paraquat
[13]
Frozen organ samples were rapidly thawed, homogenized in 1 N HCl, centrifuged at 3000 rpm for 15 min and the supernatant retained. The precipitate was resuspended in 1 N HCl and recentrifuged. The combined supernatants were subjected to ion-exchange resin column chromatography. A column (1 cm i.d.) was prepared from Dowex 5OW-X8 (H+) cation-exchange resin and packed 5 cm high. The column was washed with water and the 1 N HCl supernatant passed through the column. The column was then washed successively with 25 ml portions of water, 1 N HCl and water. Subsequently, the column was eluted with 5 N HCl, the eluate evaporated to dryness on the water-bath, the residue dissolved in 4 ml of water and used for paraquat determination. 0.375 ml of 1 N NaOH and 0.75 ml of freshly prepared 10% sodium dithionite were added to 3 ml of the eluate, and the absorbance measured spectrophotometrically at 630 nm. The standard curve was constructed on the basis of an accurately prepared paraquat solution. Recovery of paraquat added to blood was 90%. Diquat
[14]
Samples for diquat determinations were prepared as for paraquat except that 5% trichloroacetic acid was used in place of 1 N HCl. The column was the same as that used for paraquat. The column was washed with water, then with 6 M sodium chloride to convert the resin to the sodium form, and then with water. After passing the trichloroacetic acid supematant through the column, the column was washed with 25 ml portions of water, 0.6 M sodium chloride, and water and then eluted with 20 ml of 5 M ammonium chloride. 0.8 ml of freshly prepared 1% sodium dithionite in 5% aqueous sodium
167
tetraborate was added to 4 ml of the eluate, and the absorbance measured at 380 nm. The standard curve was constructed on the basis of an accurately prepared diquat solution. Recovery of diquat was 81%. Histological
study
Two and 24 hours after administration of paraquat or diquat, rats were lightly anesthetized with ether and bled by decapitation. Brain, heart, lung, liver, kidney and spleen were removed and fixed in 10% formalin. Paraffin sections were then stained with hematoxylin-eosin. Results Au topsy findings Par-aqua t At 2 hours petechiae
appeared in the visible subserosa of the lung, at 24 hours they covered a wider area and at death the organ was congested, presenting a color like that of the liver. Histologically, at 2 hours the lung showed congestion, localized hemorrhage and edema of the alveolar wall and slight round cell infiltration at the alveolar septa; at 24 hours, the damage further included alveolar edema and round cell infiltration around the bronchi and vessels; and at death secondary pneumonia had developed. The liver was congested at all times, the kidney weight increased at death, but other organs showed no significant abnormalities. Diquat At 2 hours the stomach was expanded and the ileac serosa was light
green due to contents but no histologic findings were obtained. At 24 hours the intestines were distended, especially at the cecum; the lung was congested and hemorrhaged though less severely than in the paraquat group. At death, the intestines were distended and the mucosa slightly hemorrhaged. The liver was less markedly congested than in the paraquat group. All other organs were only slightly congested and exhibited no significant histological difference from the paraquat group. Biochemical
analysis
The results of the determinations are shown in Figs. 1 and 2. Paraqua t At 2 hours the highest concentrations
were in kidney and lung. At 24 hours concentrations increased in all organs, but particularly in the lung, heart and kidney. In lung the concentration was approximately three times higher than the value at 2 hours. At 48 hours concentrations were decreased, but were still higher than at 2 hours (p < 0.05). The paraquat concentrations in lung and kidney at all times were higher than those in other organs.
J
24
12
I 48hrs
48hrs
brain
‘2
4ahri
liver
1
24
0:
paraquat,
mean
4 S.D.
I 48
hrs
??:
diquat
‘2
?A
4
hrr
Fig. 1. Tissue concentration of paraquat and diquat in rats 2, 24 and 48 hours after LD50 administration. The values for 48 hours are mean only.
For further observation, the LD5c/2 of the poison was given to other rats. On the 2nd day the concentrations in lung and kidney were higher than in other organs but thereafter no difference was seen. Diquat At 2 hours the concentrations in lung and heart were higher than in other organs. With the exception of kidney, at 24 hours concentrations decreased, particularly in brain and heart, but there was no significant change by 48 hours. Diquat concentration in kidney tended to increase. After LD5c/2 administration, a low concentration was detected in all organs, but there was no difference in concentration among them, except that the value in the kidney was a little higher on the 2nd day.
Discussion The most typical feature found in patients who ingest paraquat is pulmonary damage. In the present study, in keeping with the findings of others, we found severe congestion, edema, slight cell infiltration and hemor-
169
heart
i
.:
days
0:
paraquat,
.:
'day
1
days
diquat
Fig. 2. Tissue concentration administration.
of paraquat and diquat in rats 2, 5 and 9 days after LD5o/2
rhage in the lung, but no fibrosis. The absence of pulmonary fibrosis accords with the acute course of poisoning. In contrast, with diquat, the lungs showed only moderate congestion and hemorrhage. However, dilatation of the intestines was seen at 24 hours and diarrhea occurred in all animals. Histologically, however, changes severe enough to cause death following diquat were difficult to find. Crabtree et al. [ 151 assumed that in diquat-poisoned rats diarrhea causes dehydration and hemoconcentration resulting in death. We found paraquat concentrations to be higher in the lungs and kidneys than in the other organs examined at 2,24 and 48 hours after dosing, but on 5th and 9th days there was no difference between the organs. Following diquat, the concentration in the lung and heart was highest at 2 hours, but no damage or histological changes occurred in heart. The accumulation of paraquat in the lungs has also been reported by others [16 - 211. Our findings suggest that diquat is more rapidly excreted than paraquat, and that the selective toxicity of paraquat to the lungs is probably related to its accumulation and/or retention in the lungs.
References 1 D. J. L. Carson, The increasing use of paraquat as a suicidal agent. Forensic Sci., 7 (1976) 151 - 160. 2 D. M. Conning, K. Fletcher and A. A. B. Swan, Paraquat and related bipyridyls. Brit. Med. Bull., 25 (1969) 245 - 249.
170 D. C. Flenley and H. Matthew, Paraquat poisoning. Quart. J. Med., 42 3 N. J. Cooke, (1973) 683 - 692. 4 N. Kodagoda, R. P. Jayewardene and D. Attygalle, Poisoning with paraquat. Forensic sci., 2 (1973) 107 - 111. 5 S. Ranachandran, C. N. A. Rajapakse and M. U. F. Perera, Further observations on paraquat poisoning in man. Forensic Sci., 4 (1974) 257 - 266. and J. McEuoy, Diquat poisoning. Postgrad. Med. J., 45 (1969) 6 D. G. Oreopoulos 635 - 637. 7 H. Schonborn, H. P. Schuster and F. K. K&sling, Klinik und Morphologie der akuten peroralen Diquatintoxikation (Reglone). Arch. Toxikol., 27 (1971) 204 - 216. 8 W. M. Thurlbeck and S. M. Thurlbeck, Pulmonary effects of paraquat poisoning. Chest, 69 (1976) 276 - 280. 9 S. M. Wasan and T. F. McElligott, An electron microscopic study of experimentally induced interstitial pulmonary fibrosis. Amer. Rev. Resp. Dis., 105 (1972) 276 - 282. 10 P. Smith, D. Heath and J. M. Kay, The pathogenesis and structure of paraquat induced pulmonary fibrosis in rats. J. Puthol., 114 (1974) 57 - 67. 11 D. G. Clark, T. F. McElligott and E. W. Hurst, The toxicity of paraquat. &-it. J. Ind. Med., 23 (1966) 126 - 132. 12 D. G. Clark and E. W. Hurst, The toxicity of diquat. Lb-it. J. Ind. Med., 27 (1970) 51 - 55. 13 S. Tsunenari, H. Muto, S. Inoue, S. Sasaki, H. Sugita and M. Kanda, Forensic toxicolJup. J. Leg. Med., 29 (1975) 88 - 102 (in ogical studies of herbicide Gramoxone. Japanese). 14 J. W. Daniel and J. C. Gage, Absorption and excretion of diquat and paraquat in rats. Bit. J. Ind. Med., 23 (1966) 133 - 136. 15 H. C. Crabtree, E. A. Lock and M. S. Rose, Effects of diquat on the gastrointestinal tract of rats. Toxicol. Appl. Pharmacol., 41 (1977) 585 - 595. 16 C. W. Sharp, A Ottolenghi and H. S. Posner, Correlation of paraquat toxicity with tissue concentration and weight loss of the rat. Toxicol. Appl. Pharmacol., 22 (1972) 241 - 251. 17 K. F. Ilett, B. Stripp, R. H. Menard, W. D. Reid and J. R. Gillette, Studies on the mechanism of the lung toxicity of paraquat: Comparison of tissue distribution and some biochemical parameters in rats and rabbits. Toxicol. Appl. Pharmacol., 28 (1974) 216 - 226. J. W. Daniel and S. Longshaw, The tissue distribution of the bi18 M. H. Litchfield, pyridylium herbicides diquat and paraquat in rats and mice. Toxicology, 1 (1973) 155 - 165. 19 R. E. Murray and J. E. Gibson, Paraquat disposition in rats, guinea pigs and monkeys. Toxicol. Appl. Pharmacol., 27 (1974) 283 - 291. 20 M. S. Rose, L. L. Smith and I. Wyatt, Evidence for energy-dependent accumulation of paraquat into rat lung. Nature, 252 (1974) 314 - 315. 21 J. S. Bus, M. M. Precae, S. Z. Gagen, H. S. Posner, B. C. El&on, E. W. Sharp and J. E. Gibson, Fetal toxicity and distribution of paraquat and diquat in mice and rats. Toxicol. Appl. Pharmacol., 33 (1975) 450 - 460.
Forensic Science International, 14 (1979) 165 - 170 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
TISSUE DISTRIBUTION OF PARAQUAT ADMINISTRATION IN RATS
AND DIQUAT AFTER ORAL
EMIKO KURISAKI and HIROKO SAT0 Department
of Legal Medicine, Fukushima
(Received November 3,1978;
Medical College, Fukushima
in revised form February 27,1979;
(Japan)
accepted May 1,1979)
Summary The differential distribution of paraquat and diquat in liver, kidney, lung, brain and heart has been studied after oral administration of the LD~,J or LD50/2 to rats. Paraquat concentrations were highest in all organs at 24 hours; at this time concentrations in kidney and lung were 2 to 3 times higher than at 2 hours. In contrast, with the exception of kidney, tissue diquat concentrations were highest at 2 hours. There was severe lung damage at 24 hours after paraquat; diquat did not produce severe lung lesions, but caused intestinal distention and diarrhea. These findings suggest that the difference in the toxicity of paraquat and diquat is related to their difference in tissue distribution and excretion.
Introduction Paraquat (l,l’-dimethyl-4,4’-bipyridylium) and diquat (l,l’-ethylene4,4’-bipyridylium) are nonselective contact herbicides available under the commercial names of Gramoxone and Reglox. These compounds are widely used throughout the world, and accidental and suicidal poisoning has increased proportional to usage [ 11. These two bipyridylium compounds show differences in toxicity [2 71. For example, paraquat causes severe pulmonary lesions [ 8 - lo] , whereas diquat does not have a marked pulmonary toxicity. To study further the qualitative difference in the toxicity of the two compounds, we have determined their tissue distributions after oral administration in rats. Materials and methods Animals
and materials
Male Wistar rats (180 - 225 g) were used; they were allowed free access
to food and water. Rats were given either the LD50 (150 mg paraquat cation per kg or 231 mg diquat cation per kg) or LDs0/2, as determined by Clark et al. [ 11 - 121 ; a dose-volume of 0.2 ml/kg was used.
166
For biochemical analysis, thirteen rats were each given the LD5c of paraquat and another thirteen the LD5e of diquat. For each group, five animals were sacrificed at 2 hours, five at 24 hours, three at 48 hours. The LD5s/2 was given to groups containing five animals, of which two were sacrificed at 2 days, two at 5 days and one at 9 days. For histological study, nine rats were given the LD5c of paraquat and another nine rats the LD5a of diquat. For each group, two animals were sacrificed at 2 hours and two at 24 hours; five died between 3 and 5 days after paraquat and another five died on the 3rd or 4th day after diquat. Reagents
Gramoxone (containing 24% paraquat, Nippon Noyaku Co. Ltd.) and Reglox (containing 30% diquat, Nippon Noyaku Co. Ltd.) were used. The formulated solutions were appropriately diluted with distilled water. Determination
of paraquat
and diquat
Animals were lightly anesthetized with ether, and exsanguinated by cutting the inferior vena cava. Brain, heart, lungs, liver and kidneys were removed and stored in a freezer. Paraquat
[13]
Frozen organ samples were rapidly thawed, homogenized in 1 N HCl, centrifuged at 3000 rpm for 15 min and the supernatant retained. The precipitate was resuspended in 1 N HCl and recentrifuged. The combined supernatants were subjected to ion-exchange resin column chromatography. A column (1 cm i.d.) was prepared from Dowex 5OW-X8 (H+) cation-exchange resin and packed 5 cm high. The column was washed with water and the 1 N HCl supernatant passed through the column. The column was then washed successively with 25 ml portions of water, 1 N HCl and water. Subsequently, the column was eluted with 5 N HCl, the eluate evaporated to dryness on the water-bath, the residue dissolved in 4 ml of water and used for paraquat determination. 0.375 ml of 1 N NaOH and 0.75 ml of freshly prepared 10% sodium dithionite were added to 3 ml of the eluate, and the absorbance measured spectrophotometrically at 630 nm. The standard curve was constructed on the basis of an accurately prepared paraquat solution. Recovery of paraquat added to blood was 90%. Diquat
[14]
Samples for diquat determinations were prepared as for paraquat except that 5% trichloroacetic acid was used in place of 1 N HCl. The column was the same as that used for paraquat. The column was washed with water, then with 6 M sodium chloride to convert the resin to the sodium form, and then with water. After passing the trichloroacetic acid supematant through the column, the column was washed with 25 ml portions of water, 0.6 M sodium chloride, and water and then eluted with 20 ml of 5 M ammonium chloride. 0.8 ml of freshly prepared 1% sodium dithionite in 5% aqueous sodium
167
tetraborate was added to 4 ml of the eluate, and the absorbance measured at 380 nm. The standard curve was constructed on the basis of an accurately prepared diquat solution. Recovery of diquat was 81%. Histological
study
Two and 24 hours after administration of paraquat or diquat, rats were lightly anesthetized with ether and bled by decapitation. Brain, heart, lung, liver, kidney and spleen were removed and fixed in 10% formalin. Paraffin sections were then stained with hematoxylin-eosin. Results Au topsy findings Par-aqua t At 2 hours petechiae
appeared in the visible subserosa of the lung, at 24 hours they covered a wider area and at death the organ was congested, presenting a color like that of the liver. Histologically, at 2 hours the lung showed congestion, localized hemorrhage and edema of the alveolar wall and slight round cell infiltration at the alveolar septa; at 24 hours, the damage further included alveolar edema and round cell infiltration around the bronchi and vessels; and at death secondary pneumonia had developed. The liver was congested at all times, the kidney weight increased at death, but other organs showed no significant abnormalities. Diquat At 2 hours the stomach was expanded and the ileac serosa was light
green due to contents but no histologic findings were obtained. At 24 hours the intestines were distended, especially at the cecum; the lung was congested and hemorrhaged though less severely than in the paraquat group. At death, the intestines were distended and the mucosa slightly hemorrhaged. The liver was less markedly congested than in the paraquat group. All other organs were only slightly congested and exhibited no significant histological difference from the paraquat group. Biochemical
analysis
The results of the determinations are shown in Figs. 1 and 2. Paraqua t At 2 hours the highest concentrations
were in kidney and lung. At 24 hours concentrations increased in all organs, but particularly in the lung, heart and kidney. In lung the concentration was approximately three times higher than the value at 2 hours. At 48 hours concentrations were decreased, but were still higher than at 2 hours (p < 0.05). The paraquat concentrations in lung and kidney at all times were higher than those in other organs.
J
24
12
I 48hrs
48hrs
brain
‘2
4ahri
liver
1
24
0:
paraquat,
mean
4 S.D.
I 48
hrs
??:
diquat
‘2
?A
4
hrr
Fig. 1. Tissue concentration of paraquat and diquat in rats 2, 24 and 48 hours after LD50 administration. The values for 48 hours are mean only.
For further observation, the LD5c/2 of the poison was given to other rats. On the 2nd day the concentrations in lung and kidney were higher than in other organs but thereafter no difference was seen. Diquat At 2 hours the concentrations in lung and heart were higher than in other organs. With the exception of kidney, at 24 hours concentrations decreased, particularly in brain and heart, but there was no significant change by 48 hours. Diquat concentration in kidney tended to increase. After LD5c/2 administration, a low concentration was detected in all organs, but there was no difference in concentration among them, except that the value in the kidney was a little higher on the 2nd day.
Discussion The most typical feature found in patients who ingest paraquat is pulmonary damage. In the present study, in keeping with the findings of others, we found severe congestion, edema, slight cell infiltration and hemor-
169
heart
i
.:
days
0:
paraquat,
.:
'day
1
days
diquat
Fig. 2. Tissue concentration administration.
of paraquat and diquat in rats 2, 5 and 9 days after LD5o/2
rhage in the lung, but no fibrosis. The absence of pulmonary fibrosis accords with the acute course of poisoning. In contrast, with diquat, the lungs showed only moderate congestion and hemorrhage. However, dilatation of the intestines was seen at 24 hours and diarrhea occurred in all animals. Histologically, however, changes severe enough to cause death following diquat were difficult to find. Crabtree et al. [ 151 assumed that in diquat-poisoned rats diarrhea causes dehydration and hemoconcentration resulting in death. We found paraquat concentrations to be higher in the lungs and kidneys than in the other organs examined at 2,24 and 48 hours after dosing, but on 5th and 9th days there was no difference between the organs. Following diquat, the concentration in the lung and heart was highest at 2 hours, but no damage or histological changes occurred in heart. The accumulation of paraquat in the lungs has also been reported by others [16 - 211. Our findings suggest that diquat is more rapidly excreted than paraquat, and that the selective toxicity of paraquat to the lungs is probably related to its accumulation and/or retention in the lungs.
References 1 D. J. L. Carson, The increasing use of paraquat as a suicidal agent. Forensic Sci., 7 (1976) 151 - 160. 2 D. M. Conning, K. Fletcher and A. A. B. Swan, Paraquat and related bipyridyls. Brit. Med. Bull., 25 (1969) 245 - 249.
170 D. C. Flenley and H. Matthew, Paraquat poisoning. Quart. J. Med., 42 3 N. J. Cooke, (1973) 683 - 692. 4 N. Kodagoda, R. P. Jayewardene and D. Attygalle, Poisoning with paraquat. Forensic sci., 2 (1973) 107 - 111. 5 S. Ranachandran, C. N. A. Rajapakse and M. U. F. Perera, Further observations on paraquat poisoning in man. Forensic Sci., 4 (1974) 257 - 266. and J. McEuoy, Diquat poisoning. Postgrad. Med. J., 45 (1969) 6 D. G. Oreopoulos 635 - 637. 7 H. Schonborn, H. P. Schuster and F. K. K&sling, Klinik und Morphologie der akuten peroralen Diquatintoxikation (Reglone). Arch. Toxikol., 27 (1971) 204 - 216. 8 W. M. Thurlbeck and S. M. Thurlbeck, Pulmonary effects of paraquat poisoning. Chest, 69 (1976) 276 - 280. 9 S. M. Wasan and T. F. McElligott, An electron microscopic study of experimentally induced interstitial pulmonary fibrosis. Amer. Rev. Resp. Dis., 105 (1972) 276 - 282. 10 P. Smith, D. Heath and J. M. Kay, The pathogenesis and structure of paraquat induced pulmonary fibrosis in rats. J. Puthol., 114 (1974) 57 - 67. 11 D. G. Clark, T. F. McElligott and E. W. Hurst, The toxicity of paraquat. &-it. J. Ind. Med., 23 (1966) 126 - 132. 12 D. G. Clark and E. W. Hurst, The toxicity of diquat. Lb-it. J. Ind. Med., 27 (1970) 51 - 55. 13 S. Tsunenari, H. Muto, S. Inoue, S. Sasaki, H. Sugita and M. Kanda, Forensic toxicolJup. J. Leg. Med., 29 (1975) 88 - 102 (in ogical studies of herbicide Gramoxone. Japanese). 14 J. W. Daniel and J. C. Gage, Absorption and excretion of diquat and paraquat in rats. Bit. J. Ind. Med., 23 (1966) 133 - 136. 15 H. C. Crabtree, E. A. Lock and M. S. Rose, Effects of diquat on the gastrointestinal tract of rats. Toxicol. Appl. Pharmacol., 41 (1977) 585 - 595. 16 C. W. Sharp, A Ottolenghi and H. S. Posner, Correlation of paraquat toxicity with tissue concentration and weight loss of the rat. Toxicol. Appl. Pharmacol., 22 (1972) 241 - 251. 17 K. F. Ilett, B. Stripp, R. H. Menard, W. D. Reid and J. R. Gillette, Studies on the mechanism of the lung toxicity of paraquat: Comparison of tissue distribution and some biochemical parameters in rats and rabbits. Toxicol. Appl. Pharmacol., 28 (1974) 216 - 226. J. W. Daniel and S. Longshaw, The tissue distribution of the bi18 M. H. Litchfield, pyridylium herbicides diquat and paraquat in rats and mice. Toxicology, 1 (1973) 155 - 165. 19 R. E. Murray and J. E. Gibson, Paraquat disposition in rats, guinea pigs and monkeys. Toxicol. Appl. Pharmacol., 27 (1974) 283 - 291. 20 M. S. Rose, L. L. Smith and I. Wyatt, Evidence for energy-dependent accumulation of paraquat into rat lung. Nature, 252 (1974) 314 - 315. 21 J. S. Bus, M. M. Precae, S. Z. Gagen, H. S. Posner, B. C. El&on, E. W. Sharp and J. E. Gibson, Fetal toxicity and distribution of paraquat and diquat in mice and rats. Toxicol. Appl. Pharmacol., 33 (1975) 450 - 460.