TOXICOLOGY
AND APPLIED PHARMACOLOGY32,527-533(1975)
Interaction of Dietary Methylmercury and Selenium Accumulation and Retention of These Substances Rat Organs
on in
GEN OHI, SUSIJMU NISHIGAKI, HIRONOBU SEKI, YUKIHIRO TAMURA, TOSHIO MAKI, HIROKO MAEDA, SETSUKO OCHIAI, HIROSHI YAMADA, YASUHIRO SHIMAMURA, AND HIROSHI YAGYU Department
of Environmental
Receired
Medicine, Public Health,
October
2,1974;
Tokyo Tokyo
Metropolitan 160, Japan
accepted
December
Research
Laboratory
oj
21,1974
Interaction of Dietary Methylmercury and Seleniumon Accumulation and Retention of TheseSubstancesin Rat Organs.OHI, G., NISHIGAKI, S., SEKI, H., TAMURA,~., MURA,
Y.,
AND
YAGYU,
MAKI,T., MAED& H., OCHIAI, S., YAMADA, H., SHIMAH. (1975). Toxicol. Appl. Pharmacol. 32,521-533.
Four groups of male Wistar rats were fed the following regimenfor 40 days: (I) 20 ppm methylmercury chloride (MMC); (2) 20 ppm MMC + 3 ppm sodiumselenite;(3) 3 ppm sodiumselenite;(4) basaldiet. The basal diet which contained0.4 ppm “organic selenium”originating mainly from fish mealand wheatwasresumedon day 41. Protective effect of seleniteover toxicity of methylmercury was observed in terms of both growth rate and morbidity. Concentrations of total mercury, methylmercury and seleniumwere determinedon Days 0,20,41,47,54, and 61 in the brain, liver, kidney, and blood. It wasnoted that methylmercury increasedaccumulationof selenium in all the organsanalyzed while seleniumretention varied according to the type of seleniumand the organs. Modification by selenite, despite its protective effect, remainedequivocal in regard to the organ accumulation of mercury and its retention therein.
Prior to the establishment of the etiology of Minamata disease, an unusually high accumulation of selenium was noted in the organs of cats, crows, and people who died from this disease as well as in the fish and oysters caught in Minamata Bay (Ueda, 1960). Within the same year Uzioka (1960) found both selenium and mercury at remarkably high concentrations in the organs of cats and people in the Minamata area and suggested this had a casual association. Recently, Koeman et al. (1973) found in the analysis of trace elements in the liver of seals and dolphins a strong correlation between the accumulation of both selenium and that of mercury, and several workers have demonstrated that selenium protects against both inorganic and organic mercury poisoning in experimental animals (Parizek and Ostadalova, 1967; Ganther et al., 1972; Ganther and Sunde, 1974; Stoewsand et al., 1974; Potter and Matrone, 1974). Our study was conducted with the purpose of seeing if any mutual influence existed between selenium and methylmercury with regard to their organ accumulation, Copyright 0 1975 by Academic Press, Inc. All rights of reproduction in any from reserved. Printed in Great Rritain
527
528
OHI ET AL.
retention, and toxicity. Selenium in this casewas either added as sodium selenite or contained as “organic selenium” in the diet. METHODS One hundred and fifty male Wistar rats weighing 121 + 3 g were divided into four groups and were fed ad libitum the following regimen for 40 days and thereafter basal diet (Nippon Formula Compound Feed Co.) was resumed: Group A-20 ppm methylmercury; Group B-20 ppm methylmercury + 3ppm selenium; Group C-3 ppm selenium; Group D-Control. Methylmercury was added to the diet as methylmercury chloride in acetone and selenium was administered as sodium selenite dissolved in tap water. Selenium levels in the constituent of the basal diet are shown in Table 1. TABLE 1 COMPOSITION OF BASALDIET
Wheat Corn Soybeanmeal(45 % protein) Fish meal Wheat bran Rice bran Kaoliang Alfalfa CaC03 NaCl I Yeast Vitamine complex
%
Selevel (ppm)
27.4 18.0 13.0 10.5 9.6 9.0 4.0 3.0
0.88 0.05 0.43 1.43 1.28 0.13 0.12 0.73 0.01
5.5
100.0
0.01 0.02 0.04 0.41
On Days 0,20,41,47,54, and 61 five animals from each group (total samplesize for Day 0 was 5) were sacrificed and the brain, liver, kidney, and blood were frozen immediately after dissectionand storedat-25°C until the time of analysisfor total mercury, methylmercury, and selenium. The animals were weighedtwice a week and mortality and morbidity (roughened hair, neurological signsmanifested asparalysis or crossingof hind legs)were recorded daily. Specimenswere homogenized singly or in pools so that the homogenated samples would weigh 0.2-2.0 g according to the levels of analyzed substances.Extraction of methylmercury followed essentially the method described by West66 (1968); gas chromatography with electron capture detector was used for analysis. The recoveries for methylmercury were 80% with a 5% coefficient of variation. Total mercury was analyzed by flamelessatomic absorption spectrophotometry of vaporized mercury after digesting the specimen in sulfuric-nitric acid with pentaoxyvanadium (Deitz et al., 1973). Average recoveries were calculated as 98 % with a 3 % coefficient of variation. For seleniumdetermination the specimen was digested in nitric-perchloric acid and a
METHYLMERCURY
AND
SELENIUM
INTERACTION
529
selenium-diaminonaphthalene complex was measured by fluorometry (Watkinson, 1966). Recovery was 95 % with a 2 % coefficient of variation. Statistical analysis were performed by Students t test. RESULTS
Figure 1 shows the acute toxicity of selenite manifested as the initial sag in growth rate of groups B and C; the toxic effect of MMC became progressively but more gradually evident. By the fourth week a protective effect of selenite in terms of growth rate became clear (p < 0.001); by week 6 36 % (9 out of 25) of group A animals developed either paralysis or crossing of hind legs while none of the group B showed these signs. Following the resumption of the basal diet, the weight of animals poisoned by methylmercury and/or selenite turned from a downward to an upward trend. It is notable that
FIG. 1. Effect of sodium selenite and/or methylmercury on growth rate. The growth rate is expressed as weight/weight at start.
there was a considerable decrease in growth rate in the animals fed selenite after the termination of the treated diet and water. Two animals in group A succumbed during the experimental period (on Days 54 and 61) while none died in the other groups. Methylmercury concentrations in the brains of those two animals were higher than in the surviving ones (20,22 ppm). Figure 2 shows that methylmercury and sodium selenite caused enlargement of the kidney and liver, respectively, on Day 41, but the simultaneous administration of these substances nullified the enlargement (p < 0.05). Accelerated accumulation of selenium in the animals which were fed selenite together with MMC was most clearly seen in the kidneys [e.g., on Day 411 the selenium concentration of group B animals showed an g-fold rise compared to that of the animals fed selenite alone (group C)], but it was also apparent in all the analyzed organs (Fig. 3). On the other hand administration of selenite alone in tap water did not significantly raise the selenium concentration in the kidneys (group C) and it quickly fell to the control value when selenite feeding was terminated. A notable phenomenon was the marked elevation of selenium in the kidneys of group
530
OHI
ET
AL.
A animals which were fed methylmercury but no added selenite. Analysis of the basal diet and its component shows that selenium mainly originates from fish meal, wheat, and bran (Table 1) presumably in the form of organic selenium. As far as the retention pattern is concerned, after the steep rise of selenium concentration in the kidney noted on Day 41, the animals in group B showed a rapid decrease in the wake of the end of selenite feeding in contrast to group A which show continued elevation of selenium content in the kidneys.
% 6-1
LIVER
BRAIN
KIDNEY
8 -ii 5i .g 3 42 8 \ 3+ .$ s 2c % 6 l-
0
20
44754Q
0
20
4l475m
--Group -Group -*Grwp --.-- Group
A:Hg 6:Hg’Se C : Se D : Control
0
847546l
20
Days
FIG. 2. Effect of selenite and/or methylmercury on organ weight. The organ weight is corrected for bodyweight.
PPm BRAIN
15-1
LIVER
KIDNEY
BLOOD
Group A : Hg -GroupB:?q+Se --Group C Se -.*-. Group D : Control
IOi: z..I 2
0
20
44754610
M
4l475461
0
20
$4754610
20
L147546l Day5
FIG. 3. Selenium concentrations in several organs and blood. Each point is the mean value expressed on a wet basis. SD is indicated by the vertical bars.
The accumulation pattern of selenium in the brain and blood (Fig. 3) was similar to the one seen in the kidney and liver as far as peak selenium concentration of group B is concerned. However, in contrast to the finding noted in the liver and kidney, the selenium concentration of group B remained persistently elevated following the termination of treatment. Even on Day 61, selenium concentrations in the brain of group B animals were significantly higher (p < 0.001) than in the animals from the other groups.
METHYLMERCURY
AND
SELENIUM
531
INTERACTION
Both total mercury and methylmercury concentrations, as expected, were distinctly elevated in the groups A and B (Figs. 4 and 5). In spite of the protective effect of selenite over toxicity of simultaneously fed MMC, there was no marked difference in the mercury concentration curves between groups A and B. Rather, methylmercury in the brain, liver, and kidney of group B tended to reach and retain higher concentrations than group A.
0
m
445451
0
20
4475461
0
m
wo5m
0.
m
WL7Y.61 Darj
FIG. 4. Methyl mercury concentrations. Points represent mean values on a wet basis with the vertical bars representing SD. For the determination of low concentrations of mercury all samples were pooled.
BRAIN -Croq, -troy, ---Group ------Group
A B C 0
LIVER
KIDNEY
BLOOD
: Hg : Hg. se : Se : Cmtro,
FIG. 5. Total mercury concentrations. Points represent mean values on a wet basis with the vertical bars representing SD. For the determination of low concentrations of mercury all samples were pooled.
DISCUSSION
When sodium selenite is administered to rats in tap water at the level of 3 ppm, the initial response is reduction in the diet and water consumption and subsequent weight loss (Hadjimarkos, 1966), however, when this substance is fed together with a high level of methylmercury, several workers have demonstrated that it delays and mitigates the expression of methylmercury intoxication (Ganther et al., 1972; Ganther and Sunde,
532
OH1
ET AL.
1974; Stoewsand et al., 1974; Potter and Matrone, 1974). Our observations are in accord with these reports. Studying the protective effect of selenium on acute methylmercury poisoning, Iwata et al. (1973) observed that administration of sodium selenite accelerated the accumulation of methylmercury in the brain, but shortened its retention and this led them to propose that the protective effect of selenite was due to the enhanced elimination of methylmercury from the vital organs. We failed to see this pattern in our study; accumulation of methylmercury in the brain, liver, kidney, and blood attained more or less similar concentrations with or without simultaneous administration of sodium selenite and the retention of methylmercury in the group fed both selenite and MMC, when measured I, 2, or 3 wk after termination of treatment, was not at all shortened. Our data thus support the observation that the concentration of methylmercury in the organs is not correlated with the expression of methylmercury poisoning (Stoewsand et al., 1974). The finding that an &fold accumulation of selenium occurred in the kidney when selenite was fed with methylmercury may shed some light on the remarkably high concentration of selenium noted in the sea shells, fish, cats, crows, and people in the Minamata area when one considers the historical fact that another substance which was permitted in excessive amounts at that time to those fauna was methylmercury (Ueda, 1960; Uzioka, 1960). Ueda (1960) found that selenium concentrations in the liver of eight cats suffering from Minamata disease ranged from 5.2 to 38.1 ppm (mean 16.4) while those of ten asymptomatic cats obtained in a village 10 miles north of Minamata showed a range of from 0.3 to 67.8 ppm (mean 38.9). Uzioka (1960) also gave an example of an apparently healthy cat in the same area which had in the liver selenium and mercury concentrations of 86.9 and 301 ppm, respectively. These interesting observations as well as our present study suggest that these toxic substances can counterbalance the toxicity of each other at amazingly high levels if the conditions such as the rate of intake and the ratio of these substances are adequate. Our study shows that the group of animals which were fed methylmercury alone continued to retain high concentrations of selenium while the group fed both selenite and methylmercury showed a quick decrease from the initial high concentration after termination of their administration. This phenomenon may be explained if one assumes that there are two different simultaneously operating processes; the accumulation of selenium, regardless of its source or chemical form, is greatly facilitated by the presence of methylmercury but only organic selenium originating from the fish meal and bran as well as wheat (Smith, 1938) in the diet is persistently retained in the tissues while inorganic selenium is rather readily excreted. This abrupt release of selenite which is highly toxic might also account for the dip in the growth rate observed among the animals fed selenite. Steinwall and Olsson (1969) reported that the blood-brain barrier was impaired in methylmercury poisoning and that it caused plasma exudation and disturbed bloodbrain exchange of nutrients. If selenite which does not normally accumulate in the brain (Smith, 1938) was transferred to the brain by plasma exudation in our study, the persistent elevation of selenium concentrations in the brain along with those in the blood suggest a persistence of blood-brain barrier damage even after the improvement of general physical conditions has started.
METHYLMERCURY AND SELENIUM INTERACTION
533
ACKNOWLEDGMENT We are deeply indebted to Dr. N. Imura for the most enlightening critical discussions and are also grateful to Messrs. Yunome, Honda, and Seto for their useful technical assistance. REFERENCES DEITZ, F. D., SELL, J. L., AND BRISTOL,D. (1973). Rapid sensitive method for determination of mercury in a variety of biological samples. J. AOAC 56, 378-382. GANTHER, H. E., GOUDIE, C., SUNDE, M. L., KOPECKY, M. J., WAGNER, R., SANG-HWANG OH, AND HOEKESTRA,W. G. (1972). Selenium: relation to decreased toxicity of methylmercury added to diets containing tuna. Science 75, 1122-l 124. GANTHER, H. E. AND SUNDE, M. L. (1974). Effect of tuna fish and selenium on the toxicity of methylmercury; a progress report. J. Food Sci. 39, 1-5. HADJIMARKOS, D. M. (1966). Effect of selenium on food and water intake in the rat. Experientia 15, 117-l 18. IWATA, H., OKAMOTO, H. AND OHSAWA, Y. (1973). Effect of seleniumon methylmercury poisoning.ResearchCommun.Pathol. Pharmacol.5, 673-680. KOEMAN,J. H., PEETERS, W. H. M., KOUDSTAAL-HOL, C. H. M., TJIOE,P. S., ANDDEGOEIJ, J. J. M. (1973).Mercury-seleniumcorrelations in marine mammals.Nature (London)245, 385-386. PARIZEK,J. ANDOSTADALOVA, I. (1967).The protective effect of smallamountsof selenitein sublimateintoxication. Experimentia23, 142-143. POTTER, S. ANDMATRONE,G. (1974).Effect of seleniteon the toxicity of dietary methylmercury and mercuric chloride in the rat. J. Nutr. 104, 638-647. SMITH,M. I. (1938).Studieson the fate of seleniumin the organism.U.S. Pub. He&z Rep.53, 119991216 STEINWALL,0. ANDOLSSON, Y. (1969). Impairement of the blood-brain barrier in mercury poisoning.Acta Neurol. Stand. 45, 351-361. STOEWSAND, G. S., BACHE,C. A., AND LISK, D. J. (1974). Dietary seleniumprotection of methylmercury intoxication of Japanesequails.Bull. Enoiron.Contam.Toxicol. 11,152-l 56. UEDA,K. (1960).Experimental studieson seleniumpoisoning.J. KumamotoMed. Sot. 34 (Suppl. I), 141-155. UZIOKA,T. (1960).Analytical studieson methylmercury in the animal organsand foodstuff. J. KumamotoMed. Sot. 34 (Suppl. 2), 383-399. WATKINSON,J. H. (1966).Fluorometric determinationof seleniumin biological material with 2,3-diaminonaphthalene.Anal. Chem.38, 92-97. WEST%, G. (1968). Determination of methylmercury salts in various kinds of biological material. Acta Chm.Stand. 22, 2277-2280.
AND APPLIED PHARMACOLOGY32,527-533(1975)
Interaction of Dietary Methylmercury and Selenium Accumulation and Retention of These Substances Rat Organs
on in
GEN OHI, SUSIJMU NISHIGAKI, HIRONOBU SEKI, YUKIHIRO TAMURA, TOSHIO MAKI, HIROKO MAEDA, SETSUKO OCHIAI, HIROSHI YAMADA, YASUHIRO SHIMAMURA, AND HIROSHI YAGYU Department
of Environmental
Receired
Medicine, Public Health,
October
2,1974;
Tokyo Tokyo
Metropolitan 160, Japan
accepted
December
Research
Laboratory
oj
21,1974
Interaction of Dietary Methylmercury and Seleniumon Accumulation and Retention of TheseSubstancesin Rat Organs.OHI, G., NISHIGAKI, S., SEKI, H., TAMURA,~., MURA,
Y.,
AND
YAGYU,
MAKI,T., MAED& H., OCHIAI, S., YAMADA, H., SHIMAH. (1975). Toxicol. Appl. Pharmacol. 32,521-533.
Four groups of male Wistar rats were fed the following regimenfor 40 days: (I) 20 ppm methylmercury chloride (MMC); (2) 20 ppm MMC + 3 ppm sodiumselenite;(3) 3 ppm sodiumselenite;(4) basaldiet. The basal diet which contained0.4 ppm “organic selenium”originating mainly from fish mealand wheatwasresumedon day 41. Protective effect of seleniteover toxicity of methylmercury was observed in terms of both growth rate and morbidity. Concentrations of total mercury, methylmercury and seleniumwere determinedon Days 0,20,41,47,54, and 61 in the brain, liver, kidney, and blood. It wasnoted that methylmercury increasedaccumulationof selenium in all the organsanalyzed while seleniumretention varied according to the type of seleniumand the organs. Modification by selenite, despite its protective effect, remainedequivocal in regard to the organ accumulation of mercury and its retention therein.
Prior to the establishment of the etiology of Minamata disease, an unusually high accumulation of selenium was noted in the organs of cats, crows, and people who died from this disease as well as in the fish and oysters caught in Minamata Bay (Ueda, 1960). Within the same year Uzioka (1960) found both selenium and mercury at remarkably high concentrations in the organs of cats and people in the Minamata area and suggested this had a casual association. Recently, Koeman et al. (1973) found in the analysis of trace elements in the liver of seals and dolphins a strong correlation between the accumulation of both selenium and that of mercury, and several workers have demonstrated that selenium protects against both inorganic and organic mercury poisoning in experimental animals (Parizek and Ostadalova, 1967; Ganther et al., 1972; Ganther and Sunde, 1974; Stoewsand et al., 1974; Potter and Matrone, 1974). Our study was conducted with the purpose of seeing if any mutual influence existed between selenium and methylmercury with regard to their organ accumulation, Copyright 0 1975 by Academic Press, Inc. All rights of reproduction in any from reserved. Printed in Great Rritain
527
528
OHI ET AL.
retention, and toxicity. Selenium in this casewas either added as sodium selenite or contained as “organic selenium” in the diet. METHODS One hundred and fifty male Wistar rats weighing 121 + 3 g were divided into four groups and were fed ad libitum the following regimen for 40 days and thereafter basal diet (Nippon Formula Compound Feed Co.) was resumed: Group A-20 ppm methylmercury; Group B-20 ppm methylmercury + 3ppm selenium; Group C-3 ppm selenium; Group D-Control. Methylmercury was added to the diet as methylmercury chloride in acetone and selenium was administered as sodium selenite dissolved in tap water. Selenium levels in the constituent of the basal diet are shown in Table 1. TABLE 1 COMPOSITION OF BASALDIET
Wheat Corn Soybeanmeal(45 % protein) Fish meal Wheat bran Rice bran Kaoliang Alfalfa CaC03 NaCl I Yeast Vitamine complex
%
Selevel (ppm)
27.4 18.0 13.0 10.5 9.6 9.0 4.0 3.0
0.88 0.05 0.43 1.43 1.28 0.13 0.12 0.73 0.01
5.5
100.0
0.01 0.02 0.04 0.41
On Days 0,20,41,47,54, and 61 five animals from each group (total samplesize for Day 0 was 5) were sacrificed and the brain, liver, kidney, and blood were frozen immediately after dissectionand storedat-25°C until the time of analysisfor total mercury, methylmercury, and selenium. The animals were weighedtwice a week and mortality and morbidity (roughened hair, neurological signsmanifested asparalysis or crossingof hind legs)were recorded daily. Specimenswere homogenized singly or in pools so that the homogenated samples would weigh 0.2-2.0 g according to the levels of analyzed substances.Extraction of methylmercury followed essentially the method described by West66 (1968); gas chromatography with electron capture detector was used for analysis. The recoveries for methylmercury were 80% with a 5% coefficient of variation. Total mercury was analyzed by flamelessatomic absorption spectrophotometry of vaporized mercury after digesting the specimen in sulfuric-nitric acid with pentaoxyvanadium (Deitz et al., 1973). Average recoveries were calculated as 98 % with a 3 % coefficient of variation. For seleniumdetermination the specimen was digested in nitric-perchloric acid and a
METHYLMERCURY
AND
SELENIUM
INTERACTION
529
selenium-diaminonaphthalene complex was measured by fluorometry (Watkinson, 1966). Recovery was 95 % with a 2 % coefficient of variation. Statistical analysis were performed by Students t test. RESULTS
Figure 1 shows the acute toxicity of selenite manifested as the initial sag in growth rate of groups B and C; the toxic effect of MMC became progressively but more gradually evident. By the fourth week a protective effect of selenite in terms of growth rate became clear (p < 0.001); by week 6 36 % (9 out of 25) of group A animals developed either paralysis or crossing of hind legs while none of the group B showed these signs. Following the resumption of the basal diet, the weight of animals poisoned by methylmercury and/or selenite turned from a downward to an upward trend. It is notable that
FIG. 1. Effect of sodium selenite and/or methylmercury on growth rate. The growth rate is expressed as weight/weight at start.
there was a considerable decrease in growth rate in the animals fed selenite after the termination of the treated diet and water. Two animals in group A succumbed during the experimental period (on Days 54 and 61) while none died in the other groups. Methylmercury concentrations in the brains of those two animals were higher than in the surviving ones (20,22 ppm). Figure 2 shows that methylmercury and sodium selenite caused enlargement of the kidney and liver, respectively, on Day 41, but the simultaneous administration of these substances nullified the enlargement (p < 0.05). Accelerated accumulation of selenium in the animals which were fed selenite together with MMC was most clearly seen in the kidneys [e.g., on Day 411 the selenium concentration of group B animals showed an g-fold rise compared to that of the animals fed selenite alone (group C)], but it was also apparent in all the analyzed organs (Fig. 3). On the other hand administration of selenite alone in tap water did not significantly raise the selenium concentration in the kidneys (group C) and it quickly fell to the control value when selenite feeding was terminated. A notable phenomenon was the marked elevation of selenium in the kidneys of group
530
OHI
ET
AL.
A animals which were fed methylmercury but no added selenite. Analysis of the basal diet and its component shows that selenium mainly originates from fish meal, wheat, and bran (Table 1) presumably in the form of organic selenium. As far as the retention pattern is concerned, after the steep rise of selenium concentration in the kidney noted on Day 41, the animals in group B showed a rapid decrease in the wake of the end of selenite feeding in contrast to group A which show continued elevation of selenium content in the kidneys.
% 6-1
LIVER
BRAIN
KIDNEY
8 -ii 5i .g 3 42 8 \ 3+ .$ s 2c % 6 l-
0
20
44754Q
0
20
4l475m
--Group -Group -*Grwp --.-- Group
A:Hg 6:Hg’Se C : Se D : Control
0
847546l
20
Days
FIG. 2. Effect of selenite and/or methylmercury on organ weight. The organ weight is corrected for bodyweight.
PPm BRAIN
15-1
LIVER
KIDNEY
BLOOD
Group A : Hg -GroupB:?q+Se --Group C Se -.*-. Group D : Control
IOi: z..I 2
0
20
44754610
M
4l475461
0
20
$4754610
20
L147546l Day5
FIG. 3. Selenium concentrations in several organs and blood. Each point is the mean value expressed on a wet basis. SD is indicated by the vertical bars.
The accumulation pattern of selenium in the brain and blood (Fig. 3) was similar to the one seen in the kidney and liver as far as peak selenium concentration of group B is concerned. However, in contrast to the finding noted in the liver and kidney, the selenium concentration of group B remained persistently elevated following the termination of treatment. Even on Day 61, selenium concentrations in the brain of group B animals were significantly higher (p < 0.001) than in the animals from the other groups.
METHYLMERCURY
AND
SELENIUM
531
INTERACTION
Both total mercury and methylmercury concentrations, as expected, were distinctly elevated in the groups A and B (Figs. 4 and 5). In spite of the protective effect of selenite over toxicity of simultaneously fed MMC, there was no marked difference in the mercury concentration curves between groups A and B. Rather, methylmercury in the brain, liver, and kidney of group B tended to reach and retain higher concentrations than group A.
0
m
445451
0
20
4475461
0
m
wo5m
0.
m
WL7Y.61 Darj
FIG. 4. Methyl mercury concentrations. Points represent mean values on a wet basis with the vertical bars representing SD. For the determination of low concentrations of mercury all samples were pooled.
BRAIN -Croq, -troy, ---Group ------Group
A B C 0
LIVER
KIDNEY
BLOOD
: Hg : Hg. se : Se : Cmtro,
FIG. 5. Total mercury concentrations. Points represent mean values on a wet basis with the vertical bars representing SD. For the determination of low concentrations of mercury all samples were pooled.
DISCUSSION
When sodium selenite is administered to rats in tap water at the level of 3 ppm, the initial response is reduction in the diet and water consumption and subsequent weight loss (Hadjimarkos, 1966), however, when this substance is fed together with a high level of methylmercury, several workers have demonstrated that it delays and mitigates the expression of methylmercury intoxication (Ganther et al., 1972; Ganther and Sunde,
532
OH1
ET AL.
1974; Stoewsand et al., 1974; Potter and Matrone, 1974). Our observations are in accord with these reports. Studying the protective effect of selenium on acute methylmercury poisoning, Iwata et al. (1973) observed that administration of sodium selenite accelerated the accumulation of methylmercury in the brain, but shortened its retention and this led them to propose that the protective effect of selenite was due to the enhanced elimination of methylmercury from the vital organs. We failed to see this pattern in our study; accumulation of methylmercury in the brain, liver, kidney, and blood attained more or less similar concentrations with or without simultaneous administration of sodium selenite and the retention of methylmercury in the group fed both selenite and MMC, when measured I, 2, or 3 wk after termination of treatment, was not at all shortened. Our data thus support the observation that the concentration of methylmercury in the organs is not correlated with the expression of methylmercury poisoning (Stoewsand et al., 1974). The finding that an &fold accumulation of selenium occurred in the kidney when selenite was fed with methylmercury may shed some light on the remarkably high concentration of selenium noted in the sea shells, fish, cats, crows, and people in the Minamata area when one considers the historical fact that another substance which was permitted in excessive amounts at that time to those fauna was methylmercury (Ueda, 1960; Uzioka, 1960). Ueda (1960) found that selenium concentrations in the liver of eight cats suffering from Minamata disease ranged from 5.2 to 38.1 ppm (mean 16.4) while those of ten asymptomatic cats obtained in a village 10 miles north of Minamata showed a range of from 0.3 to 67.8 ppm (mean 38.9). Uzioka (1960) also gave an example of an apparently healthy cat in the same area which had in the liver selenium and mercury concentrations of 86.9 and 301 ppm, respectively. These interesting observations as well as our present study suggest that these toxic substances can counterbalance the toxicity of each other at amazingly high levels if the conditions such as the rate of intake and the ratio of these substances are adequate. Our study shows that the group of animals which were fed methylmercury alone continued to retain high concentrations of selenium while the group fed both selenite and methylmercury showed a quick decrease from the initial high concentration after termination of their administration. This phenomenon may be explained if one assumes that there are two different simultaneously operating processes; the accumulation of selenium, regardless of its source or chemical form, is greatly facilitated by the presence of methylmercury but only organic selenium originating from the fish meal and bran as well as wheat (Smith, 1938) in the diet is persistently retained in the tissues while inorganic selenium is rather readily excreted. This abrupt release of selenite which is highly toxic might also account for the dip in the growth rate observed among the animals fed selenite. Steinwall and Olsson (1969) reported that the blood-brain barrier was impaired in methylmercury poisoning and that it caused plasma exudation and disturbed bloodbrain exchange of nutrients. If selenite which does not normally accumulate in the brain (Smith, 1938) was transferred to the brain by plasma exudation in our study, the persistent elevation of selenium concentrations in the brain along with those in the blood suggest a persistence of blood-brain barrier damage even after the improvement of general physical conditions has started.
METHYLMERCURY AND SELENIUM INTERACTION
533
ACKNOWLEDGMENT We are deeply indebted to Dr. N. Imura for the most enlightening critical discussions and are also grateful to Messrs. Yunome, Honda, and Seto for their useful technical assistance. REFERENCES DEITZ, F. D., SELL, J. L., AND BRISTOL,D. (1973). Rapid sensitive method for determination of mercury in a variety of biological samples. J. AOAC 56, 378-382. GANTHER, H. E., GOUDIE, C., SUNDE, M. L., KOPECKY, M. J., WAGNER, R., SANG-HWANG OH, AND HOEKESTRA,W. G. (1972). Selenium: relation to decreased toxicity of methylmercury added to diets containing tuna. Science 75, 1122-l 124. GANTHER, H. E. AND SUNDE, M. L. (1974). Effect of tuna fish and selenium on the toxicity of methylmercury; a progress report. J. Food Sci. 39, 1-5. HADJIMARKOS, D. M. (1966). Effect of selenium on food and water intake in the rat. Experientia 15, 117-l 18. IWATA, H., OKAMOTO, H. AND OHSAWA, Y. (1973). Effect of seleniumon methylmercury poisoning.ResearchCommun.Pathol. Pharmacol.5, 673-680. KOEMAN,J. H., PEETERS, W. H. M., KOUDSTAAL-HOL, C. H. M., TJIOE,P. S., ANDDEGOEIJ, J. J. M. (1973).Mercury-seleniumcorrelations in marine mammals.Nature (London)245, 385-386. PARIZEK,J. ANDOSTADALOVA, I. (1967).The protective effect of smallamountsof selenitein sublimateintoxication. Experimentia23, 142-143. POTTER, S. ANDMATRONE,G. (1974).Effect of seleniteon the toxicity of dietary methylmercury and mercuric chloride in the rat. J. Nutr. 104, 638-647. SMITH,M. I. (1938).Studieson the fate of seleniumin the organism.U.S. Pub. He&z Rep.53, 119991216 STEINWALL,0. ANDOLSSON, Y. (1969). Impairement of the blood-brain barrier in mercury poisoning.Acta Neurol. Stand. 45, 351-361. STOEWSAND, G. S., BACHE,C. A., AND LISK, D. J. (1974). Dietary seleniumprotection of methylmercury intoxication of Japanesequails.Bull. Enoiron.Contam.Toxicol. 11,152-l 56. UEDA,K. (1960).Experimental studieson seleniumpoisoning.J. KumamotoMed. Sot. 34 (Suppl. I), 141-155. UZIOKA,T. (1960).Analytical studieson methylmercury in the animal organsand foodstuff. J. KumamotoMed. Sot. 34 (Suppl. 2), 383-399. WATKINSON,J. H. (1966).Fluorometric determinationof seleniumin biological material with 2,3-diaminonaphthalene.Anal. Chem.38, 92-97. WEST%, G. (1968). Determination of methylmercury salts in various kinds of biological material. Acta Chm.Stand. 22, 2277-2280.
