revised final - Agency for Toxic Substances and Disease Registry ...

MERCURY 261
2. HEALTH EFFECTS
administered to adult cats (0.003, 0.0084, 0.020, 0.046, 0.074, or 0.176 mg Hg/kg/day, 7 days a week for
2 years), either as pure methylmercuric chloride in corn oil added to a diet containing uncontaminated fish or
as methylmercury-contaminated fish. In the two highest dose groups (0.074 and 0.176 mg Hg/kg body
weight at 100 weeks of exposure), no significant differences were seen in total mercury concentrations in the
blood between groups receiving the dose as methylmercuric chloride or as contaminated fish at the same dose
level. In addition, monthly blood levels were comparable for all dose groups. No significant differences were
seen at 100 weeks in total mercury concentrations in nervous tissue or other tissues (renal cortex, renal
medulla, liver, spleen, adrenal, bladder, atria, ventricle, ovary, testis, muscle) between the two highest dose
groups receiving the dose as methylmercuric chloride or as contaminated fish at the same dose level.
Regarding the effect of selenium on methylmercury toxicity, most studies have shown that the simultaneous
administration of mercury and selenium in equimolar doses to animals has resulted in decreased toxicity of
both elements for acute- and chronic-duration exposures. This effect has been observed with inorganic and
organic mercury and with either inorganic or organic selenium compounds, although inorganic forms of
selenium appear to be more effective than organic forms (Chang 1983; Skerfving 1978). Selenium protects
against the acute nephrotoxicity of the mercuric ion and methylmercuric ion in rats (Ganther et al. 1972,
1980; Hansen 1988; Magos et al. 1987; Parizek and Ostadolva 1967) and possibly against acute neurotoxicity
of methylmercuric ion in rats (Ohi et al. 1980).
Somewhat paradoxically, the protective effect of selenium has been associated with a higher whole-
body retention of mercury (Hansen 1988; Magos et al. 1987). In a study of selenium excretion in workers
exposed to low levels of metallic mercury vapor in a chloralkali plant, Ellingsen et al. (1995) found
that even in a low-to-moderate occupational exposure, mercury may reduce the urinary selenium
concentration in humans in a manner that is not yet fully known. Evidence from human autopsy tissues
suggests that distribution of mercury throughout the body may be influenced by the presence of selenium
(Suzuki et al. 1993). In this study, however, the level of selenium was found to negatively correlate with the
level of mercury in some tissues including the cerebrum, spleen, and kidney cortex. Suzuki et al. (1993) also
report that hair selenium values negatively correlated with total organ mercury and inorganic mercury
levels. The association between concentrations of inorganic mercury and selenium in both the occipital lobe
and the thalamus of the brains of methylmercury-exposed female monkeys was reported by Bjorkman et al.
(1995). These authors observed a tendency to a "hockey stick-shaped" relationship between concentrations of
selenium and inorganic mercury in the thalamus of monkeys with ongoing exposure to methylmercury, and