Thresholds for electromagnetic field-induced hypoxia protection: evidence for a primary electric field effect

We have recently reported that weak electromagnetic (EM) field exposure of chick embryos induces a response that can be used to protect against subseq uent hypoxic insult. This work is continued here with an exposure response study using 20-min exposure to 60 Hz magnetic fields over a range of 2-10 m uT. Once again, the biomarker used was induction of hypoxia protection. A s igmoidal response curve was found, with exposures to magnetic field strengt hs greater than or equal to 4 muT inducing maximum hypoxia protection (68% survival). We also attempted to determine whether the magnetic or induced e lectric component of the EM field was responsible for the observed protecti on. This was accomplished by making measurements with two different orienta tions of the magnetic fields (perpendicular and parallel to the major axis of the egg). Owing to the configuration of the embryo in the egg, the induc ed electric field at the embryo was lower when the magnetic field was paral lel to the major axis even though the magnetic field strength was the same for each orientation. Exposure of the embryos to the parallel orientation r esulted in a reduced protective response. An exposure-response curve genera ted for this orientation of the field also showed a more "drawn-out" appear ance, consistent with the observed distribution of embryo positions within the egg. Our results suggest, that the induced electric, not the applied ma gnetic field, plays a primary role in the protective effect observed in thi s chick embryo model. (C) 2000 Elsevier Science S.A. All rights reserved.