The sensitivity of cells and tissues to exogenous fields: effects of target system initial state.

The effect of the initial biochemical or metabolic state of a cell membrane target pathway on its sensitivity to exogenous electromagnetic (EMF) fields is considered. It is shown that the resting or initial transmembrane voltage can affect the frequency response of the membrane pathway and substantially alter the signal to thermal noise threshold (SNR) of the target. EMF sensitivity is examined using a model which describes the response to applied fields of both single cells and cells in gap junction contact via a distributed parameter electrical circuit analog, wherein a voltage-dependent membrane impedance, relating to the initial biochemical state of the target cell(s), is considered. Application of the Hodgkin-Huxley K(+)-conduction pathway membrane to this model results, at a given transmembrane voltage, in a preferential array response to applied field frequencies in the 1-100 Hz range, centered at approximately 16 Hz for 1-10 mm array lengths. Extension of the model to consider the voltage dependence of the Hodgkin-Huxley K+ pathway results in a significant modulation of array frequency response with changing membrane resting potential. The result is EMF sensitivity (SNR) depends upon the initial state of the target tissue, providing a possible explanation of why, e.g., repairing, rather than resting, bone exhibits a physiologically relevant response to certain weak EMF signals.