Brown fat nuclear triiodothyronine receptors in rats

Brown adipose tissue (BAT) thermogenesis is important in the adaptation of body energy expenditure to cold exposure, fasting, and overfeeding and in the pathogenesis of obesity. Thyroid hormones are required for the normal functioning of BAT. The mechanism of their effect on BAT, however, has not yet been elucidated. Since most thyroid hormone effects are mediated via the binding of T3 to nuclear receptors (NT3R) the present studies were performed to investigate whether BAT contained NT3R and whether these NT3R were affected by cold exposure, fasting, overfeeding, or hypo- or hyperthyroidism. Rat BAT was found to contain NT3R with a maximum binding capacity (MBC) of 0.28 ng T3/mg DNA and a dissociation constant (Kd) of 3.2 X 10(-10) M. These parameters were unaffected by any of the experimental conditions studied. A major alteration of BAT NT3R MBC or Kd is thus not a causal factor in the changes in BAT thermogenesis induced by the above experimental conditions. In contrast, 3-wk overfeeding increased (+48%) and fasting decreased (-29%) the MBC of hepatic NT3R. Overfeeding increased serum T3 (+110%), while fasting decreased it (-37%).