from cells compared with that released from the sham-treated cells (Liburdy 1992b). Again, the changes induced by power-frequency magnetic fields were most significant at low concentrations of antibody—a result consistent with a change in receptor affinity but not in the total number of receptors expressed on the cells. Those results and the anti-CD3 antibody results mentioned above suggest that the binding of ligands to their receptors would be fruitful to investigate in other cell systems. It should also be pointed out that none of the studies described have been independently replicated, and this type of replication is a pressing research need.

Blank's group has reported effects of very-low-frequency electric fields on the Na, K-ATPase ion pump in membranes (Blank 1992; Blank and Soo 1992). Electric fields at 30-300 Hz were applied for 15 min to membrane preparations at current densities of 0.05-50 mA/cm2 (0.001-1 mV/cm); the response was complex, with either increases or decreases in enzyme activity, depending on the level of Na and K ions in the medium. Field inhibition of ATPase activity occurred when the enzyme was in a medium containing optimal concentrations of activating cations, and field stimulation occurred when the enzyme activity was reduced by using ouabain or by lowering temperature. Blank estimated the threshold for effects at electric-field strengths of approximately 5 µV/cm (5 × 10-4 V/m) across the membrane, and this threshold was associated with a current density of 8 mA/cm2. This threshold value, although low by comparison with ambient electric fields in air near power lines, is much higher than those believed to be induced by environmental exposures to electric fields. The results can be interpreted in terms of the electric field inducing changes in the binding of substrate ions (Na+ and K+) to the ion pump at high and low concentrations of the ligands, similarly to the studies of Liburdy and Luben described above. Blank's work has not been replicated by other investigators, but neither have failures to replicate been reported.

ODC activity is modulated by membrane-mediated signaling events, and its activation is associated with the activity of mitogens and tumor-promoting agents of various types during carcinogenesis. Byus et al. (1987) reported that three cell lines—human lymphoma cells (CEM), mouse myeloma cells (P3), and rat hepatoma (Reuber H35) cells—exhibited increases of 50-300% in ODC activity when exposed to sinusoidal 60-Hz electric fields at 10 mV/cm. Increases in ODC were detected as low as 0.1 mV/cm in Reuber H35 cells. For comparison, phorbol ester at doses associated with tumor promotion produced activation of ODC levels by more than 1000%. The investigators interpreted these results as indicative of an electric-field effect on the cell membrane, resulting in a signal-transduction effect on ODC activation by mechanisms not directly investigated in these or subsequent studies. These findings have been used as basis for a hypothesis that electric fields might act as a copromoter with tumor-promoting agents, producing more activation of ODC and more growth promotion of carcinogen-induced cells in the presence of low electric-field strengths than in the absence of electric

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