Atrazine mediated mechanisms

Changes in plasma corticosterone and thyroxine concentrations were reported in salamander larvae Ambystoma tigrinum) exposed to atrazine at concentrations of 75 and 250 g/L (29). The authors did not suggest that this was an atrazine-specific mechanism but rather that it was a conq>ensatory response to stress from an environmental contaminant. In addition, since die development of the laryngeal dilator muscle is dependent on both androgen and thyroid hormone, the fact that effects were not observed in male X, laevis in all but one of the studies conducted with atrazine supports the conclusion that atrazine did not disrupt thyroid hormone status in these frogs (17,19,27), Overall, there is no evidence that atrazine affects thyroid-mediated processes in amphibians. 

Atrazine was not carcinogenic to mice or Fischer rats after oral administration in the diet. An increase incidence of mammary tumors has been found in female Sprague-Dawley females treated similarly. The lARC has determined that the mammary tumors associated with atrazine exposure involve a mechanism that is non-DNA-reactive and hormonally mediated. They further stated that this mechanism is not relevant to humans. The lARC concluded that there was sufficient evidence for the carcinogenicity of atrazine in experimental animals and inadequate evidence of carcinogenicity in humans. ... 

It is well known that plants had developed mechanisms and strategies to get rid of harmful substances, such as herbicides. In com, benzoxazinones are involved in detoxification of the herbicides atrazine and simazine. Their hydroxylation is mediated by direct nucleophilic attack of the benzoxazinone oxygen atom [178]. 

Understanding of mechanisms of action of endocrine disrupters reduces uncertainties and may offer useful biomarkers for further study. These biomarkers can be usefrd in risk assessment and for confirming responses under field conditions. Thusfar, laboratory responses are inconsistent and, where some of fiiese potential biomarkers have been measured in the field, no trends have been apparent. The lines of evidence thus does not support a causal link between atrazine exposures and possible endocrine-mediated effects in anqihibians. The lack of consistent biomarkers makes it inqiossible to conduct a risk assessment as no hazard has been identified. 

If and when consistent and robust responses to atrazine are identified in frogs, the mechanisms by which these effects are mediated should be elucidated. This will better allow the application of biomarkers to assess the significance of responses in the field situations and increase the precision and accuracy of extrapolation of effects to other an hibians and assessment of their relevance at the level of the population. 

Comprehensive reviews by Smith, Cripps and Roberts, and Hassall have covered many of the main classes of herbicides such as the ureas (e.g., diuron, linuron, chlorbromuron, chloroxuron, and chlortoluron), triazines (e.g., atrazine, simazine, trietazine, and prometryn), phenoxyalkanoates (e.g., 2,4-D, MCPA, and mecoprop), phenylcarbamates (e.g., propham and barban), benzoic acids (e.g., dichlobenil, dicamba, and chloramben), and dinitroanilines (e.g., trifluralin, nitralin, and isopropalin). Smith classified the major types of reactions into 12 general classes, 9 of which are biologically mediated and 3 of which can occur as a result of either biochemical or chemical mechanisms. Hill has also devised a scheme of 11 general classes. 

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