Physical Mechanisms for Biological Effects of Low Field Intensity ELF Magnetic Fields

Biological effects of high intensity ELF electric fields, such as muscle stimulation and - at still higher intensities - cell damage by electroporation and heating, are reasonably well (although not completely) understood. However power frequency effects of magnetically induced electric fields of less than 10 -3 V/m or alternating magnetic fields of less than about 100 μT cannot be explained very well at the present time. The biological processes involved, notably signal transduction at the cell membrane and subsequent biochemical processes involving “second messengers” are beginning to be identified (Adcy, 1990; Luben, 1993). These processes involve vast amplification of the incident “signal” with metabolically supplied energy. The physical mechanisms of conversion from an electric or magnetic field to biochemical process are however unknown at the present time. As a consequence we do not know which aspect of the applied or environment field is important. Properties such as polarization, duration and/or intermittency of the signal, frequency and harmonic content may turn out to be as important as amplitude. The biological, chemical and mechanical condition of the organism (e.g. stage in cell cycle, stimulation by a mitogen or cell density) will also influence the effectiveness of the applied field. A key question, which is still only resolved for a few experimentally observed effects, is whether specific biological results arc caused directly by the magnetic field or by the electric field that is induced in tissue by a time varying magnetic field. Particularly for fields larger than about 100 μT direct “magne-tochemical” effects have been suggested and the presence of magnetite particles in some tissues may also play a biological role. Experimental findings suggest that effects of induced electric fields are particularly likely when their magnitudes at the tissue or cell level are relatively “large”, of the order of 10-3 V/m or larger, (McLeod et al., 1993b; Liburdy, 1992; Lyle et al., 1988), and when multiple cells are connected by gap junctions. Induced electric fields may also affect the motion of “counterions” on the cell surface and possibly the structural fluctuations of nucleic acid molecules. While most proteins have a small net electrostatic charge, nucleic acids are polyelectrolytes with large net charge and are surrounded by counterions (McCammon and Harvey, 1987). However in view of the inefficiency of electric field induction by an ELF magnetic field, illustrated by equation (25). The mechanisms for “direct” interaction of time varying magnetic fields with biological processes need serious consideration. This is particularly necessary when the applied ELF magnetic fields that produce unambiguous biological effects are of such amplitude and orientation (in relation to the culture medium or animal) as to produce very small induced electric fields. Experimental results have suggested that the relative magnitude and direction of a static magnetic field (such as that of the earth) can determine the biological effectiveness of a simultaneously present alternating field in some biological systems under controlled laboratory conditions.