Dynamics of brown fat thermogenesis in week-old rats: evidence of relative stability during moderate cold exposure.

Neonates of many mammalian species, including humans, depend primarily on thermogenesis by brown adipose tissue as a defense against cold challenge. Although the steady-state thermogenic responses of brown adipose tissue to various air temperatures are well known, the dynamic responses have received relatively little attention. In this article, we examine the relative stability of brown adipose tissue thermogenesis during brief perturbations of air temperature. Specifically, week-old rats were allowed to settle at one of two levels of cold exposure. These two levels were defined on the basis of previous work as being moderate (30.5 degrees C) or extreme (23 degrees C). After pups had settled at these temperatures, they were exposed to positive or negative air temperature perturbations of approximately 3.7 degrees C. Pups experiencing perturbations from the moderate air temperature, unlike those exposed to the extreme air temperature, exhibited organized thermogenic responses that allowed them to return quickly to their preperturbation conditions. These data suggest that brown adipose tissue thermogenesis is more stably controlled than has previously been suspected.