Norepinephrine-induced depolarization of brown fat cells.

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.