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Distinction between Heating Rate and Total Heat Absorption in the Microwave-Exposed Mouse

This investigation assesses the ability of the heat-dissipating system of the mouse to respond to equivalent heat loads (e.g., J/g) administered at varying intensities (e.g., J/g/s or W/kg). Use of a microwave exposure system provided a means to administer exact amounts of energy at varying rates in awake, free-moving mice. Male AJ mice were administered average heat loads of 12-13 J/g at rates of 19, 68, or 194 W/kg by exposure to 2,450 MHz microwave radiation while evaporative heat loss (EHL) was continuously monitored with dew-point hygrometry. The sensitivity of the EHL response was calculated as the heat dissipated by evaporation in response to microwave exposure divided by the heat absorbed. This ratio yields a dimensionless number that is related to the sensitivity of the EHL response and, in this study, is defined as the open loop gain (OLG) of EHL (i.e., motor response/error signal). When mice were exposed at an ambient temperature of 32.5 C to a specific absorption rate (SAR) of 19 W/kg, the OLG averaged 0.17; thus 17% of the absorbed heat was dissipated by EHL and the remaining 83% of the energy was passively dissipated. Absorbing the same amount of heat but at an SAR of 68 W/kg resulted in an OLG of 0.42 and at 194 W/kg, OLG equaled 0.50. Since the OLG parameter is normalized to total heat absorption, these results demonstrate that the sensitivity of the mouse to a microwave heat load is highly dependent on the rate of energy absorption and less dependent on the total exchange of energy. The response time to microwave radiation (time required for EHL to increase to 63% above baseline) decreased as a power function with increasing SAR. These data contradict the notion that mammals have a fine control over whole-body heat exchange (i.e., thermoregulatory control) and instead support the hypothesis that the heat-dissipating system is highly influenced by the rate of temperature increase (i.e., temperature regulatory control).