Intracellular potentials of brown fat cells in lightly anesthetized 
cold-acclimated rats were measured in vivo. The effects of adrenergic agonists 
and antagonists on these potentials were examined in an attempt to relate the 
electrical activity of the cells to the adrenergic-induced stimulation of brown 
fat thermogenesis.Norepinephrine, the physiological mediator of brown fat heat 
production, significantly depolarized the membrane of these cells in vivo. This 
was effected either upon norepinephrine administration (3-100 mug/kg body wt) or 
excitation of the transsected nerve trunk to the interscapular fat pad and 
appreciably inhibited (55%) by doses of propranolol (1 mg/kg) sufficient to 
abolish the temperature increase of the tissue. Since theophylline (325 mum/kg) 
did not depolarize the cells, although it stimulated thermogenesis in the 
tissue, the depolarizing effect of norepinephrine is interpreted as being at 
least partially associated with biochemical events terminating in the activation 
of adenylate cyclase. However, the norepinephrine-induced electrical changes and 
the ensuing increase in brown fat thermogenesis appear to be causally 
independent and experimentally separable. On the other hand, our data do not 
preclude the speculation that the membrane phenomenon, if accompanied by 
increased Na(+), may serve partially to regulate the metabolic rate of brown fat 
during long-term physiological stimulation (e.g., cold stress) by increasing the 
rate of ATP utilization via the Na(+)/K(+) pump.