Exposure of DMBA-treated female rats in a 50-Hz, 50 microTesla magnetic field: effects on mammary tumor growth, melatonin levels, and T lymphocyte activation.

There is growing public concern about the possible health risks, particularly increased cancer risks of exposure to magnetic fields (MF) associated with power distribution systems. Recently, we have started a series of animal studies to investigate this issue, using the DMBA (7,12-dimethylbenz[a]anthracene) model of breast cancer in female rats. In the present study, female rats were chronically exposed to a 50-Hz, 50 microTesla (microT) MF with or without DMBA treatment. Because alterations in circulating levels of the pineal hormone melatonin and impaired immune system functions have been involved in breast cancer growth, and both melatonin and immune system are thought to be targets for MF-effects, serum melatonin and the proliferative capacity of splenic lymphocytes were determined in MF-exposed and sham-exposed rats. For this purpose, 216 female Sprague-Dawley rats were divided into four groups. Two of the groups (with 99 animals each) received oral applications of DMBA and were either sham-exposed or exposed in a 50-Hz, 50 microT MF for 24 h/day 7 days/week for a period of 91 days. The other two groups (9 animals each) were either sham-exposed or MF-exposed without DMBA treatment. The exposure chambers and all other environmental factors were identical for MF-exposed and sham-exposed animals. The DMBA-treated animals were palpated once weekly to assess the development of mammary tumors. At the end of the three-month period of MF exposure, the number and size of mammary tumors was determined by autopsy. In controls, DMBA induced tumors in approximately 55% of the animals within the 3 month period of sham-exposure. Already 8 weeks after DMBA application, the MF-exposed group exhibited significantly more tumors than sham-exposed animals. At time of autopsy, significantly more MF-exposed DMBA-treated rats exhibited macroscopically visible mammary tumors than DMBA-treated controls, thus indicating that MF exposure enhances the development and growth of cancers in this model. Comparison of the data from 50 microT with recent data from other flux densities indicated that long-term MF exposure of DMBA-treated rats increases the incidence of palpable and/or macroscopically visible mammary tumors in a highly dose-related fashion. Determination of nocturnal serum melatonin after 9 and 12 weeks of exposure at 50 microT did not yield significant differences between MF-exposed rats and sham-exposed controls, whereas a marked suppression of T cell proliferative capacity was seen in MF exposed rats. The data add further evidence to the hypothesis that hormone-dependent tissues such as breast might be particularly sensitive to MF-effects and indicate that immune system depression is involved in the increased breast cancer growth observed in MF exposed rats.