Effects of mercury compounds on the spontaneous and potassium-evoked release of [3H]dopamine from mouse striatal slices
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The effects of mercury compounds on the spontaneous and potassium-evoked release of [3H]dopamine from mouse striatal slices have been examined. All mercury compounds examined produced concentration-dependent increases in the spontaneous release of [3H]dopamine, with an order of potency of methylmercury greater than mercuric (Hg2+) mercury greater than p-choloromercuribenzene sulfonic acid. Methylmercury had no effect on the 25 mM potassium evoked release of [3H]dopamine in the presence of 1.3 mM calcium. However, in calcium-free conditions, methylmercury significantly increased the potassium-evoked release of [3H]dopamine. Mercuric mercury significantly reduced the 25 mM potassium evoked release of [3H]dopamine in the presence of 1.3 mM calcium, and this response was not reversible with brief washing of the tissue. In calcium-free conditions, mercuric mercury significantly elevated the evoked release of [3H]dopamine, similar to the result obtained with methylmercury. It is suggested that mercury compounds alter dopaminergic synaptic function, possibly by disrupting calcium homeostasis or calcium-dependent processes, and that methylmercury and mercuric mercury can have differential effects to alter dopaminergic neurotransmission.Keywords:
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Studies are reported on the methylation and accumulation of mercury in fish populations of the pond. Results showed that the majority of the mercury in the fish muscles was in the methyl mercury form. Very low concentrations of mercury were found in the pond sediments. Physiological methylation of the mercury could not be demonstrated by in vivo or in vitro techniques. Organic ligand methylation and reduction of the mercury was shown to be a possible mechanism to account for both the mercury distribution in the pond and the concentrations of methylmercury in the fish. Recommendations are made for remedial treatments to eliminate the mercury problem. (HLW)
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the 1950's and the ornithological di saster in Sweden in the 1960's, the health hazard imposed by environmental mercury pollution has received international atten tion. It seems that the form of mercury discharged into the environment is unim portant, because any form of mercury may be converted to methylmercury by micro organisms. Numerous studies have been directed toward delineating the sources of mercury contamination, its movement in the environment, and the route through which mercury reaches human beings. The general physical-chemical properties of mercury have been studied since the days of alchemy; however, a paucity of data exists concerning mercury's dynamic properties. These include the mechanisms by which mercury is released, mobilized, and subsequently transformed into methyl mercury in the water environment and the external environmental factors affecting these transformations. Several studies have been conducted in volving the biomethylation occurring in sediments and the subsequent fish uptake of the methylmercury produced. Unfortu nately, the majority of these studies were conducted without proper consideration of environmental factors such as temperature, degree of organic pollution, chloride ion concentration, mercury concentration, and pH. Furthermore, no known experiments have varied these factors in a single study. It is obvious that the methylation of mer cury contained in sediments needs quantifi cation in terms of different environmental conditions, and those factors affecting methylmercury uptake by fish require eluci dation. Objectives The objectives of these studies were (a) to examine the dynamic properties of in organic mercury and methylmercury, (b) to determine the uptake rates of methyl mercury biosynthesized in sediment by fish and the effects on the uptake rates of varying environmental conditions, and (c) to observe the effects of varying environ mental parameters on the overall methyla tion activities of microorganisms. In addi tion, an estimate of the retention time of mercury in contaminated sediments was to be ascertained. Methodology
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This chapter contains sections titled: Introduction Global mercury cycle Processes of mercury cycles in lakes The methylmercury cycle in aquatic systems Environmental change/methylmercury concentration in fish Mercury concentrations in aquatic systems Objectives for mercury monitoring Conclusions
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THE HIGH TOXICITY OF MERCURY COM POUNDS has long been known,1-2 but their hazardousness as pollutants of aquatic environments became a matter of concern only after the deaths from poisoning at Minamata Bay, Japan.3'4 The cause was traced to the presence of effluents contain ing methylmercuric chloride, and Kurland 3 suggested the possibility of microbial trans formation of mercury to methylmercury. The experimental support for this hypoth esis was furnished by Jensen and Jernelov,5 who showed that inorganic mercury was converted to monomethyland dimethyl mercury in aquarium sediments. The microbial methylation of mercury is the most hazardous pathway through which mercury contributes towards en vironmental pollution because methyl mercury concentrates progressively in aquatic organisms from minute concentra tions in water.6 Once in living organisms, it circulates in the blood unchanged, with a very slow rate of elimination.7 More over, methylmercury is able to penetrate the blood-brain barrier resulting in severe damage to nerve cells even at very low concentrations.8 Earlier work suggests that approximately 95 percent of mercury found in fish is methylmercury,9 although recent work indicates that in some cases, the levels may be considerably lower. Thus, fish, as an important dietary source, form the major route of methylmercury transfer to higher trophic levels. Figure 1 schematically illustrates the biotransforma tion of mercury in sediments as described by Jernelov.10 The environmental levels of mercury and methylmercury in a variety of fish, and also the accumulation of mercury by fish in aquaria, have been the subject of con siderable study. Yet, there is no published data on the relative methylating capacities of different mercury-contaminated sedi ments, although work has been in progress in Sweden for some time.10
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