A small temperature rise may contribute towards the apparent induction by microwaves of heat-shock gene expression in the nematodeCaenorhabditis Elegans

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Abstract
We have previously reported that low intensity microwave exposure (0.75–1.0 GHz CW at 0.5 W; SAR 4–40 mW/kg) can induce an apparently non-thermal heat-shock response in Caenorhabditis elegans worms carrying hsp16-1::reporter genes. Using matched copper TEM cells for both sham and exposed groups, we can detect only modest reporter induction in the latter exposed group (15–20% after 2.5 h at 26 °C, rising to ∼50% after 20 h). Traceable calibration of our copper TEM cell by the National Physical Laboratory (NPL) reveals significant power loss within the cell (8.5% at 1.0 GHz), accompanied by slight heating of exposed samples (∼0.3 °C at 1.0 W). Thus, exposed samples are in fact slightly warmer (by ≤0.2 °C at 0.5 W) than sham controls. Following NPL recommendations, our TEM cell design was modified with the aim of reducing both power loss and consequent heating. In the modified silver-plated cell, power loss is only 1.5% at 1.0 GHz, and sample warming is reduced to ∼0.15 °C at 1.0 W (i.e., ≤0.1 °C at 0.5 W). Under sham:sham conditions, there is no difference in reporter expression between the modified silver-plated TEM cell and an unmodified copper cell. However, worms exposed to microwaves (1.0 GHz and 0.5 W) in the silver-plated cell also show no detectable induction of reporter expression relative to sham controls in the copper cell. Thus, the 20% “microwave induction” observed using two copper cells may be caused by a small temperature difference between sham and exposed conditions. In worms incubated for 2.5 h at 26.0, 26.2, and 27.0 °C with no microwave field, there is a consistent and significant increase in reporter expression between 26.0 and 26.2 °C (by ∼20% in each of the six independent runs), but paradoxically expression levels at 27.0 °C are similar to those seen at 26.0 °C. This surprising result is in line with other evidence pointing towards complex regulation of hsp16-1 gene expression across the sub-heat-shock range of 25–27.5 °C in C. elegans. We conclude that our original interpretation of a non-thermal effect of microwaves cannot be sustained; at least part of the explanation appears to be thermal. Bioelectromagnetics 27:88–97, 2006.