Sympathetic activity in thyroid-treated Zucker rats.

Treatment of lean and obese 3- to 4-mo-old male Zucker rats with 0.03% thyroid powder (TP) for up to 41 days produced a decreased weight gain and carcass lipid content in obese but not lean rats, without reduction in food intake. Improved thermogenesis in TP-treated obese rats was evidenced by increased basal rectal temperatures and improved cold tolerance, although TP did not alter the lower stress-evoked levels of plasma norepinephrine (NE) or epinephrine of the obese rats. Levels of NE and/or NE turnover were lower in hearts, aortas, pancreata, interscapular brown adipose, and epididymal white adipose pads of obese compared with lean rats. Both NE levels and turnover were increased in the hearts and aortas of TP-treated obese rats, suggesting increased sympathetic activity in these organs. Basal but not stress-evoked hyperglycemia, hyperinsulinemia, and hyperglycerolemia in the obese rats were partly or totally corrected by TP treatment as was defective in vivo utilization of fatty acids. Therefore 0.03% TP administration had a selective effect on the obese Zucker rat compared with the lean and led to decreased weight gain associated with improved thermogenesis. These effects appeared to be related in part to increased sympathetic function in some organs and improved utilization of fatty acids.