In general terms, two possible scenarios have been addressed experimentally: (1) the interaction of electric fields (either ambient or induced in the medium adjacent to the cell by oscillating magnetic fields) with ions or charged molecules at the surface of the membrane; and (2) the interaction of magnetic fields with atoms, ions, or molecules in the membrane or within the cytosol or nucleus of the cell. Either of these possible interaction mechanisms is postulated to modulate a step (or steps) in some signal-transduction event, leading to further changes in the function of the cell. The focus of some experiments was on the observable effects of exposure to electric and magnetic fields alone (i.e., attempt to show the existence of receptors for such fields). The focus of other work was on possible interactions between electric or magnetic fields and the existing signal-transduction systems for other ambient signals, such as hormones or neurotransmitters (i.e., to look for electric-or magnetic-field effects on receptors for other agents).

A number of laboratories have examined the effects of electric and magnetic fields on bone and connective tissue cells, including studies of signal transduction as well as other regulatory and differentiation processes. Those studies are summarized in Chapter 4 of this report in the section on bone healing and will not be repeated here. It is important to emphasize, however, that most of the studies on bone and connective tissue have used field strengths much higher than those encountered in either residential exposures or most occupational exposures. The lowest magnetic-field strength that has been shown to have reproducible effects on connective-tissue cells is approximately 100 µT (1 G), and most of the studies relating to clinical effectiveness of magnetic fields have been at strengths of 500-2,000 µT (5-20 G) (Brighton and McCluskey 1986).

Other examples have been presented of interactions between magnetic fields and already-recognized signal-transduction pathways. For example, Walleczek and Liburdy (1990) observed that 60-Hz magnetic fields caused increased 45Ca influx during concanavalin-A (Con-A)-induced signal transduction in lymphocytes. A 60-min exposure of rat thymic lymphocytes to a 22-mT (220-G) magnetic field (induced electric field = 1.0 mV/cm) at 37°C was performed in the presence or absence of Con-A. Nonactivated cells (no mitogen) were unresponsive to the magnetic field; 45Ca influx was not altered. When Con-A was present, the magnetic field led to an increase in 45Ca influx of 50-200%. In these studies, as in those of Luben's group on bone cells (Luben et al. 1982; Luben 1991, 1994), the effects of magnetic-field exposure were prevalent mainly at low concentrations of the signaling molecule, suggesting that power-frequency magnetic-field exposures could cause changes in the affinity of the receptor for the ligand or in the effectiveness of the transduction process at low field strengths, but could not produce a change in the maximal responsiveness of the cells to the signal. Liburdy et al. (1993a) also recently reported that cell-surface antibody binding to human lymphocytes could be altered by a 60-Hz 200-G magnetic field. T lymphocytes were reported to exhibit an approximate doubling of anti-CD3 antibody released

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