Polychlorinated biphenyls in anuran frogs: Occurrence in native Sierra Nevada hylids and biochemical markers of exposure in ranids

To test the hypothesis that certain chlorinated organic pesticides and contaminants have the potential to bioaccumulate in native Sierra Nevada amphibians, we surveyed Pacific treefrog (Hyla regilla) tadpoles collected from numerous populations throughout the Sierra Nevada range for tissue levels of polychlorinated biphenyls (PCBs) and the cotton insecticide toxaphene. Several factors were found to influence the population distribution of these contaminants, including elevation, distance from California's Central Valley, latitude, and site drainage basin aspect. The profile of PCB homolog distributions and concentrations among the sampled populations suggests that the primary sources of PCB inputs to the Sierra Nevada mountain range lie to the west of the mountain range, in California proper.; To investigate the biological and environmental significance of polychlorinated biphenyl body burdens in amphibians, we exposed adult northern leopard frogs (Rana pipiens) to a commercial PCB mixture (Aroclor 1260), and assessed the effect of this PCB exposure combined with cold temperature exposure on several biomarkers of immune function and carbohydrate metabolism. Aroclor 1260 was found to interact with cold exposure by diminishing levels of hepatic glucose transporter proteins, which facilitate diffusion of glucose through the hepatocyte plasma membrane and thereby enable appropriate carbohydrate storage and utilization. Aroclor 1260 also acted synergistically with cold exposure in reducing levels of serum complement, which is a critical component of humoral immunity that provides protection from fungal, bacterial, and viral pathogens. In an attempt to understand the significance of PCB exposure on carbohydrate metabolism and define some of the molecular signaling events associated with PCB and environmental stress exposure in Ranid frogs, subadult and adult California red-legged frogs (Rana draytonii) were exposed to a mixture of non-ortho-coplanar PCBs and subsequently subjected to either cold or dehydration stress. Using a novel polymerase chain reaction (PCR)-based assay, dramatic changes in expression of nuclear transcription factors c/EBPα and c/EBPδ were found in several tissues. In vitro hepatic and muscle glucose uptake capacity was also affected by PCB and cold or dehydration exposure. Taken as a whole, these findings help fill some knowledge gaps relating to the biological effects of chlorinated organic contaminants in amphibians.