Thermoregulatory responses of the immature rat following repeated postnatal exposures to 2,450-MHz microwaves

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Abstract
This study was designed to determine the changes that occur in the thermoregulatory ability of the immature rat repeatedly exposed to low‐level microwave radiation. Beginning at 6–7 days of age, previously untreated rats were exposed to 2,450‐MHz continuous‐wave microwaves at a power density of 5 mW/cm2 for 10 days (4 h/day). Microwave and sham (control) exposures were conducted at ambient temperatures (Ta) which represent different levels of cold stress for the immature rat (ie, “exposure” Ta = 20 and 30 °C). Physiological tests were conducted at 5–6 and 16–17 days of age, in the absence of microwaves, to determine pre‐ and postexposure responses, respectively. Measurements of metabolic rate, colonic temperature, and tail skin temperature were made at “test” Ta = 25.0, 30.0, 32.5, and 35.0 °C. Mean growth rates were lower for rats exposed to Ta = 20°C than for those exposed to Ta = 30°C, but microwave exposure exerted no effect at either exposure Ta. Metabolic rates and body temperatures of all exposure groups were similar to values for untreated animals at test Ta of 32.5 °C and 35.0 °C. Colonic temperatures of rats repeatedly exposed to sham or microwave conditions at exposure Ta = 20 °C or to sham conditions at exposure Ta = 30 °C were approximately 1°C below the level for untreated animals at test Ta of 25.0 °C and 30.0 °C. However, when the exposure Ta was warmer, rats exhibited a higher colonic temperature at these cold test Ta, indicating that the effectiveness of low‐level microwave treatment to alter thermoregulatory responses depends on the magnitude of the cold stress.