Study type: Medical/biological study (experimental study)

A small temperature rise may contribute towards the apparent induction by microwaves of heat-shock gene expression in the nematode Caenorhabditis Elegans med./bio.

By: Dawe AS, Smith B, Thomas DW, Greedy S, Vasic N, Gregory A, Loader B, de Pomerai DI
Published in: Bioelectromagnetics 2006; 27 (2): 88-97
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Aim of study (acc. to editor)

To study the effects of microwave exposure on heat shock (HSP) responses in transgenic nematodes (Caenorhabditis elegans). The aim of the investigation was to replicate data of previous studies (see publication 4466, publication 4467, and publication 6611).

Background/further details

Worms were incubated at 15, 26, 26.2, and 27°C.

Endpoint

Exposure

Exposure Parameters
Exposure 1: 1 GHz
Modulation type: CW
Exposure duration: continuous for 2.5 hours
Exposure 2: 1 GHz
Modulation type: CW
Exposure duration: nearly continuous for up to 20 hours (see add. information)

Exposure 1

Main characteristics
Frequency 1 GHz
Type
Charakteristic
  • guided field
Exposure duration continuous for 2.5 hours
Modulation
Modulation type CW
Exposure setup
Exposure source
Chamber The TEM cell used here was identical to that described in [Daniells et al., 1998], except that it was constructed of copper rather than aluminium. It was 34 cm long and of a square cross-section, tapering from a maximum of 24 x 24 cm at the center to 1.5 x 1.5 cm at the ends. The inner septum (waveguide) was central and 27/32 of the total width, giving a 50 Ω impedance, which matched the load and cables. Power was limited by the matched load to 500 mW (27 dBm).
Setup Identical loaded 24-well multiwell plates (containing 1.0 ml of K medium per well) were placed in two copper TEM cells located on the same shelf of a large incubator set at 26.0 ± 0.1 °C, one for RF and the other one for sham exposure.
Sham exposure A sham exposure was conducted.
Additional info In later experiments, an unmodified copper TEM cell was used for shams, while the TEM cell used for exposures was modified with the aim of reducing both power loss and consequent heating, including removal of internal polystyrene foam from beneath the septum, replacement of BNC by APC 3.5 connectors, and silver plating the copper surfaces of the cell.
Parameters
Measurand Value Type Method Mass Remarks
power 0.5 W - - - -
Measurement and calculation details

The S parameters of the TEM cell were measured using a calibrated network analyzer both with and without the presence of a loaded 24-well multiwell plate, containing 1.0 ml of K medium per well. Temperature rises corresponding to 1.0 W and 10 W power inputs (into a 50 Ω load) were measured using a temperature probe dipping into the liquid K medium in a loaded 24-well plate, as above. Following modifications to the TEM cell, the frequency response and temperature measurements were repeated. In the modified silver-plated cell, power loss was only 1.5% at 1.0 GHz, and sample warming was reduced to ±0.15 °C at 1.0 W (i.e., ±0.1 °C at 0.5 W).

Exposure 2

Main characteristics
Frequency 1 GHz
Type
Charakteristic
  • guided field
Exposure duration nearly continuous for up to 20 hours (see add. information)
Modulation
Modulation type CW
Exposure setup
Exposure source
Sham exposure A sham exposure was conducted.
Additional info Sham and microwave-exposed PC161 worms (as for Part A) were kept for 20 h in black non-fluorescent multiwell plates, which were removed at 2 or 4 h intervals (for <5 min per plate) to allow determination of GFP fluorescence levels.
Parameters
Measurand Value Type Method Mass Remarks
power 0.5 W - - - -

Reference articles

Exposed system:

Methods Endpoint/measurement parameters/methodology

Investigated system:
Time of investigation:
  • after exposure

Main outcome of study (acc. to author)

Using copper TEM cells for both exposed and sham exposed groups, only modest reporter gene induction in the exposed group was detected (15-20% after 2.5 h at 26°C, rising to approximately 50% after 20 h). Calibration of the copper TEM cell by the National Physical Laboratory (Teddington, UK) revealed significant power loss (8.5% at 1.0 GHz) within the cell, accompanied by slight heating of exposed samples (approximately 0.3°C at 1 W). Thus, exposed samples were slightly warmer (by less/equal 0.2°C at 0.5 W) than sham-exposed controls.
The TEM cell design was modified with the aim of reducing both power loss and consequent heating. In the modified silver-plated TEM cell, power loss was only 1.5 % at 1.0 GHz, and sample warming was reduced to approximately 0.15°C at 1.0 W (i.e., less/equal 0.1°C at 0.5 W).
Under sham exposure/sham exposure conditions, there was no difference in reporter gene expression between the modified silver TEM cell and an unmodified copper TEM cell. However, worms exposed to microwaves (1.0 GHz, 0.5 W) in the silver TEM cell also showed no detectable induction of reporter gene expression relative to sham exposed controls in the copper TEM cell. Thus, the 20 % "microwave induction" revealed using two copper TEM cells may be caused by a small temperature difference between sham exposure and exposure conditions.
The authors conclude that previous interpretation of a non-thermal effect of microwaves cannot be sustained.

Study character:

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