Multiple power‐density windows and their possible origin

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Abstract
We have previously reported that in vitro exposure of chick forebrain tissue to 50‐MHz radiofrequency (RF) electromagnetic radiation, amplitude modulated (AM) at 16 Hz, would enhance the efflux of calcium ions within only two power‐density ranges: one from 1.44 to 1.67 mW/cm2, and the other including 3.64 mW/cm2. No effect on efflux occurred at 0.37, 0.72, 2.17, and 4.32 mW/cm2. We confirmed and extended these results by testing at another set of power densities, which included the range of the previous study. Forebrain tissue from 1–7‐day‐old chickens was labeled in vitro with radioactive calcium ions (30 min, at 37 °C), rinsed, placed in a physiological salt solution, and then exposed for 20 min to 50‐MHz radiation, AM at 16 Hz, in a transverse electric and magnetic field (TEM) cell maintained at 37 °C. The solution was then assayed for radioactive calcium activity. A power‐density series was tested. An enhanced efflux of calcium ions was found at 1.75, 3.85, 5.57, 6.82, 7.65, 7.77, and 8.82 mW/cm2; no change was observed at 0.75, 2.30, 4.50, 5.85, 7.08, 8.19, 8.66, 10.6, and 14.7 mW/cm2. Power density is converted to specific absorption rate (SAR) by 0.36 mW/kg per mW/cm2. Even the highest SAR tested (0.005 W/kg) is much too low to result in generalized heating of the sample and thus to be the underlying cause of the enhanced response. A hypothetical mechanism is proposed involving dynamic systems that may account for the power‐density dependency as well as for part of the frequency dependency observed with both modulated RF radiation and extremely‐low‐frequency (ELF) fields.