Energy Metabolism of the White Rat After Acclimation to Warm and Cold Environments

The heat production (indirect) of 30°C acclimated (I) and 6°C acclimated (II) Sprague-Dawley adult male rats was determined with an open-circuit metabolism apparatus and with a low-lag, closed-circuit apparatus over the temperature range −36° to 30°C. Heat production of II was linearly related to temperature between −36° and 20°C and was always higher than that of I. At the lower temperatures heat production of I fell rapidly with time of exposure to the test temperature, and showed a maximal initial value (obtained between the 3rd and 20th minute of exposure) which was independent of temperature between −35° and −25°C and fully elicited at the start of the measurements. Heat balance of I and II measured at −15°C indicated that body cooling in I was a resultant of the failure to produce enough heat to balance the heat loss. These results indicate that the metabolic response of white rats to cold exposure occurs within 3 minutes of the beginning of exposure and that I reach a maximum metabolic rate at temperatures at which II can still show an increase on further lowering of the environmental temperature. Submitted on September 14, 1956