Two independent laboratories have demonstrated that electromagnetic radiation at specific frequencies can cause a change in the efflux of calcium ions from brain tissue in vitro. In a local geomagnetic field (LGF) at a density of 38 microTesla (microT), 15- and 45-Hz electromagnetic signals (40 Vp-p/m in air) have been shown to induce a change in the efflux of calcium ions from the exposed tissues, whereas 1- and 30-Hz signals do not. We now show that the effective 15-Hz signal can be rendered ineffective when the LGF is reduced to 19 microT with Helmholtz coils. In addition, the ineffective 30-Hz signal becomes effective when the LGF is changed to +/- 25.3 microT or to +/- 76 microT. These results demonstrate that the net intensity of the LGF is an important variable. The results appear to describe a resonance-like relationship in which the frequency of the electromagnetic field that can induce a change in efflux is proportional to a product of LGF density and an index, 2n + 1, where n = 0,1. These phenomenological findings may provide a basis for evaluating the apparent lack of reproducibility of biological effects caused by low-intensity extremely-low-frequency (ELF) electromagnetic signals. In future investigations of this phenomenon, the LGF vector should be explicitly described. If the underlying mechanism involves a general property of tissue, then research conducted in the ambient electromagnetic environment (50/60 Hz) may be subjected to unnoticed and uncontrolled influences, depending on the density of the LGF.

Bawin and her coworkers have reported changes in binding of calcium after exposure of avian brain tissue to nonionizing electromagnetic radiation. Because calcium is intimately involved in the electrical activity of the brain, their results reveal a heretofore unrecognized potential for nonionizing radiofrequency radiation to affect biological function. We have verified and extended their findings. The forebrains of newly hatched chickens, separated at the midline to provide treatment-control pairs, were labeled in vitro with radioactive calcium. Samples of tissue were exposed for 20 minutes in a Crawford irradiation chamber to 147-MHz radiation, which was amplitude modulated sinusoidally at selected frequencies between 3 and 30 Hz. Power densities of incident radiation ranged between 0.5 and 2 mW cm -2. Compared with nonirradiated samples, a statistically significant increase in efflux ofcalcium ions (P < 0.01) was observed in irradiated samples at a modulation frequency of 16 Hz and at a power density of 0.75 mW cm -2. Our data confirm the existence of the frequency "window" reported by Bawin et al., as well as a narrow power-density "window" within which efflux of calcium ions is enhanced. 0048-6604/79/1112-S014501.00 93 94 BLACKMAN ET AL.

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A companion paper describes a predictive ion parametric resonance (IPR) model of magnetic field interactions with biological systems based on a selective relation between the ratio of the flux density of the static magnetic field to the AC magnetic field and the charge-to-mass ratio of ions of biological relevance. Previous studies demonstrated that nerve growth factor (NGF)-stimulated neurite outgrowth (NO) in PC-12 cells can be inhibited by exposure to magnetic fields as a function of either magnetic field flux density or AC magnetic field frequency. The present work examines whether the PC-12 cell response to magnetic fields is consistent with the quasi-periodic, resonance-based predictions of the IPR model. We tested changes in each of the experimentally controllable variables [flux densities of the parallel components of the AC magnetic field (Bac) and the static magnetic field (Bdc) and the frequency of the AC magnetic field] over a range of exposure conditions sufficient to determine whether the IPR model is applicable. A multiple-coil exposure system independently controlled each of these critical quantities. The perpendicular static magnetic field was controlled to less than 2 mG for all tests. The first set of tests examined the NO response in cells exposed to 45 Hz Bac from 77 to 468 mG(rms) at a Bdc of 366 mG. Next, we examined an off-resonance condition using 20 mG Bdc with a 45 Hz AC field across a range of Bac between 7.9 and 21 mG(rms). Finally, we changed the AC frequency to 25 Hz, with a corresponding change in Bdc to 203 mG (to tune for the same set of ions as in the first test) and a Bac range from 78 to 181 mG(rms). In all cases the observed responses were consistent with predictions of the IPR model. These experimental results are the first to support in detail the validity of the fundamental relationships embodied in the IPR model.