Graduate Thesis Or Dissertation
 

Pathology, tissue residues and viral-induced mortality in mice exposed to lead and cadmium

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  • Lead (Pb) and cadmium (Cd) are naturally occurring heavy metals which have been redistributed to a large extent by man in the environment. These metals have certain common toxic sites of action on biological systems of mammals. Simultaneous exposure to Pb and Cd is highly probable due to common sources of exposure in the environment. The purpose of this study was to observe the effects of simultaneous, chronic exposure to Pb and Cd on pathology, tissue residues and susceptibility to viral challenge in mice. Mice were exposed, via the drinking water, to either Pb or Cd or both metals simultaneously at various doses for ten weeks and then challenged with encephalomyocarditis virus (EMCV). Mortality was enhanced in all groups of mice exposed to Pb and suppressed in groups of mice treated with Cd. Mice simultaneously coexposed to Pb and Cd had mortality rates intermediate to those observed after exposure to the individual metals. It is proposed that Pb suppresses immunity in mice by inhibition of basic metabolic functions deoendent on sulfhydrylcontaining enzymes. The effect of this action results in a decreased ability of cells to produce antibodies, interferon and anti-viral proteins. It is postulated that Cd enhances the immune response in mice by inhibition of zinc-dependent enzymes such as peptidases and RNA polymerase. This results in increased stability and persistance of antibodies, interferon and anti-viral proteins in addition to decreased ability of the virus to replicate. Coexposure to Pb and Cd did not appear to alter histopathologic lesions observed by light microscopy that were produced by exposure to Pb or Cd only. Pb and Cd residues in tissues of mice exposed to these metals, as compared to residues in tissues of mice which received Pb or Cd only, indicate that interaction occurs in regard to metabolism, storage and excretion of these elements. Coexposure to Pb caused increased Cd accumulation at low doses. The effectwasreversed at high doses and Cd tissue residues are reduced. Pb tissue residues, except in kidneys, were generally lower in mice coexposed to Cd. Kidney Pb levels increased in these mice. It is significant that chronic coexposure to Pb and Cd at subclinical doses alters certain effects produced by exposure to only one of the metals. Simultaneous exposure to low levels of Pb and Cd better simulate natural conditions of exposure in the environment.
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