To study the thermal response of interscapular brown fat (IBF) to norepinephrine (NE), urethan-anesthetized rats (1.2 g/kg ip) maintained at 28-30 degrees C received a constant venous infusion of NE (0-2 x 10(4) pmol/min) over a period of 60 min. IBF temperatures (T(IBF)) were recorded with a small thermistor fixed under the IBF pad. Data were plotted against time and expressed as maximal variation (Deltat degrees C). Saline-injected rats showed a decrease in T(IBF) of approximately 0.6 degrees C. NE infusion increased T(IBF) by a maximum of approximately 3.0 degrees C at a dose of 10(4) pmol x min(-1) x 100 g body wt(-1). Surgically thyroidectomized (Tx) rats kept on 0.05% methimazole showed a flat response to NE. Treatment with thyroxine (T(4), 0.8 microg x 100 g(-1) x day(-1)) for 2-15 days normalized mitochondrial UCP1 (Western blotting) and IBF thermal response to NE, whereas iopanoic acid (5 mg x 100 g body wt(-1) x day(-1)) blocked the effects of T(4). Treatment with 3,5, 3'-triiodothyronine (T(3), 0.6 microg x 100 g body wt(-1) x day(-1)) for up to 15 days did not normalize UCP1 levels. However, these animals showed a normal IBF thermal response to NE. Cold exposure for 5 days or feeding a cafeteria diet for 20 days increased UCP1 levels by approximately 3.5-fold. Nevertheless, the IBF thermal response was only greater than that of controls when maximal doses of NE (2 x 10(4) pmol/min and higher) were used.
Conclusions: 1) hypothyroidism is associated with a blunted IBF thermal response to NE; 2) two- to fourfold changes in mitochondrial UCP1 concentration are not necessarily translated into heat production during NE infusion.