CHRONIC ADMINISTRATION OF BETA-ADRENERGIC AGONISTS CAN MIMIC THE STIMULATIVE EFFECT OF COLD-EXPOSURE ON PROTEIN-SYNTHESIS IN RAT BROWN ADIPOSE-TISSUE

When mammals are exposed to a cold environment for a long time, the ca pacity of nonshivering thermogenesis by brown adipose tissue (BAT) inc reases in association with the stimulation of synthesis of some specif ic proteins and tissue hyperplasia, which are totally dependent on sym pathetic innervation to this tissue. To clarify the roles of the adren ergic mechanisms for the cold-induced protein synthesis and hyperplasi a in BAT, in this study, the effects of chronic treatment with adrener gic agonists using an osmotic mini-pump were examined in rats. Continu ous administration of noradrenaline or isoproterenol (beta-agonist) fo r 10 days resulted in increased synthesis of the mitochondrial uncoupl ing protein and an isoform of glucose transporter (GLUT4), and tissue hyperplasia, in the same way as after cold exposure of the same durati on. Phenylephrine (alpha-agonist) administration did not have any sign ificant effect. Surgical sympathetic denervation completely abolished the effects of cold exposure, whereas it did not influence those of ad renergic agonists at all. These results indicate that the stimulative effects of cold exposure on protein synthesis and hyperplasia of BAT a re attributable solely to the beta-adrenergic action of noradrenaline secreted from the sympathetic nerves in this tissue,