Cytogenetic damage related to low levels of methyl mercury contamination in the Brazilian Amazon

The mercury rejected in the water system, from mining operations and lixiviation of soils after deforestation, is considered to be the main contributors to the contamination of the ecosystem in the Amazon Basin. The objectives of the present study were to examine cytogenetic functions in peripheral lymphocytes within a population living on the banks of the Tapajós River with respect to methylmercury (MeHg) contamination, using hair mercury as a biological indicator of exposure. Our investigation shows a clear relation between methylmercury contamination and cytogenetic damage in lymphocytes at levels well below 50 micrograms/gram, the level at which initial clinical signs and symptoms of mercury poisoning occur. The first apparent biological effect with increasing MeHg hair level was the impairment of lymphocyte proliferation measured as mitotic index (MI). The relation between mercury concentration in hair and MI suggests that this parameter, an indicator of changes in lymphocytes and their ability to respond to culture conditions, may be an early marker of cytotoxicity and genotoxicity in humans and should be taken into account in the preliminary evaluation of the risks to populations exposed in vivo. This is the first report showing clear cytotoxic effects of long-term exposure to MeHg. Although the results strongly suggest that, under the conditions examined here, MeHg is both a spindle poison and a clastogen, the biological significance of these observations are as yet unknown. A long-term follow-up of these subjects should be undertaken.

Methylmercury intoxication and histochemical demonstration of NADPH-diaphorase activity in the striate cortex of adult cats

The effects of methylmercury (MeHg) on histochemical demonstration of the NADPH-diaphorase (NADPH-d) activity in the striate cortex were studied in 4 adult cats. Two animals were used as control. The contaminated animals received 50 ml milk containing 0.42 µg MeHg and 100 g fish containing 0.03 µg MeHg daily for 2 months. The level of MeHg in area 17 of intoxicated animals was 3.2 µg/g wet weight brain tissue. Two cats were perfused 24 h after the last dose (group 1) and the other animals were perfused 6 months later (group 2). After microtomy, sections were processed for NADPHd histochemistry procedures using the malic enzyme method. Dendritic branch counts were performed from camera lucida drawings for control and intoxicated animals (N = 80). Average, standard deviation and Student t-test were calculated for each data group. The concentrations of mercury (Hg) in milk, fish and brain tissue were measured by acid digestion of samples, followed by reduction of total Hg in the digested sample to metallic Hg using stannous chloride followed by atomic fluorescence analysis. Only group 2 revealed a reduction of the neuropil enzyme activity and morphometric analysis showed a reduction in dendritic field area and in the number of distal dendrite branches of the NADPHd neurons in the white matter (P<0.05). These results suggest that NADPHd neurons in the white matter are more vulnerable to the long-term effects of MeHg than NADPHd neurons in the gray matter.

Effects of methylmercury exposure during the second stage of rapid postnatal brain growth on negative geotaxis and on delta-aminolevulinate dehydratase of suckling rats

In the present study, we examined the effects of exposure to methylmercury (0, 2.3, 4.6, 6.9 and 9.2 mg/kg, daily for 5 consecutive days, sc) during the second stage of rapid postnatal brain development (8 to 12 days of age) on the sulfhydryl-containing enzyme delta-aminolevulinate dehydratase (ALA-D, E.C. 4.2.1.24) from brain, liver and kidney and on motor performance (latency to complete a negative geotaxis response) of rats. ALA-D specific activity of 13-day old rats of both sexes (7-12 per group) was reduced significantly in rats treated with 6.9 mg/kg and 9.2 mg/kg in brain (about 40 per cent , P < 0.05) and in liver (about 25 per cent , P < 0.05). Renal ALA-D specific activity was not affected by methylmercury treatment. The in vitro IC50 for inhibition of brain, liver and renal ALA-D was 79.3, 81.8 and 39.1 microM, respectively. The latency to complete the negative geotaxis response of 12-day old rats was increased by 6.9 (7.9 +/- 0.7 s, mean +/- SEM) and 9.2 mg/kg methylmercury (7.8 +/- 0.5 s) when compared with control rats (5.8 +/- 0.3 s), suggesting an impairment in motor performance of exposed rats. These results demonstrate that exposure to relatively high doses of methylmercury during the second stage of brain development causes a significant reduction in brain and hepatic ALA-D. The absence of inhibition of ALA-D by lower doses may be related to the relatively low in vitro sensitivity of the enzyme to methylmercury. The possible involvement of ALA-D inhibition on the neurotoxicity of methylmercury deserves additional investigation