Estimated heat contribution of brown fat in arousing ground squirrels (Citellus lateralis)

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Abstract
To evaluate the thermal contribution of brown fat relative to that of the intact ground squirrel during arousal from hibernation, 2 approximations of the tissue oxygen consumption in vivo were derived as a function of arousal time. Equations for one estimate were obtained from the recorded brown fat "warming" curves by assuming the metabolism in vitro reflects that in vivo and the brown fat temperature increases linearly with the QO2 [oxygen consumption quotient] druing most of the arousal period. Brown fat thermogenesis was also approximated as the sum of the calories needed for the tissue temperature change observed over a given time interval plus heat lost to the blood. Comparing the calculated tissue O2 consumption with the measured total, the estimated brown fat contribution ranged from 6-15% initially to 5-8% as the squirrels approached normothermic temperatures. In view of the assumptions involved in these derivations, it was concluded that the calculated average of 7-11% of the total energy cost represents minimum estimates of brown fat thermogenesis in support of arousal at low ambient temperatures.