range of positive responses would have been observed. No consistent pattern is found across biologic systems or exposure conditions.

Transient Changes in Cells Exposed In Vitro to Electric and Magnetic Fields

The data showing that magnetic fields can induce transient changes in cell expression are significant. Those data fit into three categories: changes in the signal-transduction pathway including changes in concentrations of ODC, changes in gene expression, and changes in intracellular calcium levels.

Signal Transduction

Signal refers to molecular systems, both at the cell membrane and inside the cell, in which signals from the environment and from other cells are received, and which regulate intracellular processes, such as metabolic activities, gene expression, differentiation, and cell proliferation in response to the signals received. Signal-transduction processes present an interesting possible mechanism for electric and magnetic fields to influence cell function. In particular, membrane signal-transduction processes have been an area of intense focus. One reason for the interest is that the cell membrane presents a substantial barrier to electric fields, especially in the range of field strengths and frequencies present in the ambient environment. Attenuation of electric fields by the plasma membrane of mammalian cells has been estimated at 103-105 between the external plasma membrane surface and the interior of the cell (Polk 1992b). For all intents and purposes, no significant penetration of information-containing electric signal across the cell membrane can be postulated for the 60-Hz ambient fields encountered in ordinary household exposures (Polk 1992a,b). Because membrane-mediated signal transduction by hormones and other signaling agents involves the transmission of signals across the plasma membrane without requiring that the signal itself penetrate the membrane, low-frequency electric or magnetic fields have been postulated to act on intracellular processes by influencing only the initial extracellular steps of signal transduction (Adey 1992a). A number of studies have been interpreted by the investigators to indicate that weak electric or magnetic fields can produce changes in membrane signal-transduction pathways. Numerous reviews of low-frequency, low-energy electric-and magnetic-field interactions with biologic systems, including cells, animals, and humans, have been conducted (Adey 1992a,b; Cardossi et al. 1992; Cleary 1993; Liburdy 1992a; Luben 1991, 1993; Tenforde 1991, 1992; Walleczek 1992).

Although many types of signals can be found in biologic systems, the mechanisms for transmitting the information contained in those signals across the cell membrane are relatively few. In all known signal-transduction systems, a signal interacts with a cellular protein (a receptor or voltage-sensitive ion channel) and

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