Einfluss einer achtwöchigen 50-Hz-Magnetfeld-Exposition auf DNA-Reparatur und mitochondriale DNA-Synthese von Sammelrohr-Epithelzellen der Niere

Abstract

1) Currently no generally accepted scientific model on the long-term effects of weak electromagnetic fields exists. Therefore, there is a considerable demand for a better analysis of the impact of electromagnetic fields on living organisms. To do this, a mouse model was used to elucidate the question whether a nuclear exposure to a 50 Hz magnetic field over a period of eight weeks causes DNA-damage and influences mitochondrial mtDNS-synthesis.2) Altogether, 81 male, seven months old mice were exposed over a period of eight weeks to a magnetic field (MF) of 50 Hz with a magnetic flux density of 0,1 and 1 mT, or they were sham-exposed. Following the exposure to the magnetic field, 3H-thymidine (3H-TdR) was injected. Depending on the subgroup, this was either done five minutes, 24 hours or seven days after the end of MF exposure. This way, a) the nuclear (n) DNA repair synthetic rate (so-called unscheduled DNA-synthesis, UDS) for measuring MF-induced nDNA damage as well as b) the mtDNA synthetic rate as an indicator of metabolic cell activity was examined cell-type specifically in situ. After considering the background labelling, normalization to nuclear DNA content as well as determination of the cytoplasmatic grain densities the measured data of different cell types were directly comparable. 115 minutes after the injection of 3H-TdR the mice were narcotized and subsequently sacrifized through perfusion fixation. Autoradiograms were prepared from paraffin sections of the kidneys and afterwards microscopically evaluated with the help of the computer program „Discus“. For that, 100 successive kidney collecting tubule cells and their adjacent cytoplasm in medulla and cortex were analyzed in view of grain counts and the nuclear as well as cytoplasmatic profile areas. Finally, cumulative frequency distributions of the grain counts were drawn. On the basis of these graphs, mice that were suspected to be affected by artifacts were taken out of the assessment. 3) Univariance analysis showed a statistically significant result only for nDNA-repair of epithelial cells of the cortical collecting tubule regarding the applied MF dose. However, this could not be substantiated when considering a Bonferroni correction. This means that one has to expect nDNA damage in these cell types after exposure to a magnetic field. As there is no indication for MF induced nDNA damage after 24 hours and seven days after the end of MF exposure, one can assume that existing damage is repaired within a short period of time (< 24 hours). 4) Regarding mtDNA synthesis measurements and the corresponding metabolic cell activity, only the univariance analysis generated significant results for the variables “dose” and “time of 3H-TdR-injection” of cortical collecting tubule cells. These results could not be confirmed with post-hoc t-tests. A linear regression analysis including the data of Freuding (2004), which were generated using the same method for a dose of 1,5 mT, showed a dose-dependant significant raise of the metabolic cell activity for the range between 0,1 and 1,5 mT five minutes after the end of MF exposure. This coincides with Freuding’s (2004) data that showed 5 minutes after ending the magnetic exposure a significant difference between the mice that were exposed to a 1,5 mT magnetic field and the controls. No evidence for a longer lasting effect could be found. 5) The range of the obtained data within the test groups was remarkably higher than it was in the previous analysis of Freuding (2004) after a MF exposure of 1,5 mT. This might contribute to the fact that no significant correlations could be found