Energy balance, diet-induced thermogenesis and brown adipose tissue in lean and obese (fa/fa) Zucker rats after adrenalectomy.

Five-week-old male, lean (+/?) and genetically obese (fa/fa) Zucker rats received either bilateral adrenalectomy or sham operations, and energy balance was measured over the subsequent 21 days. Body weight and energy intake were similar for intact lean and obese rats, but the latter group showed a marked increased in body energy gain and energetic efficiency, and reduced energy expenditure. Adrenalectomy did not significantly influence energy balance in lean rats but caused decreases in food intake and body weight gain in obese rats, and restored their energetic efficiency and body energy gain to the level of lean animals. The lower thermic response to a single meal (40 kJ) in intact obese rats was restored to normal by adrenalectomy. Brown adipose tissue (BAT) mass was larger in sham-operated obese rats, but tissue protein concentration, mitochondrial yield and mitochondrial GDP binding were all markedly reduced in obese rats. BAT mass, composition and GDP binding were almost identical in adrenalectomized obese and all lean rats. These findings demonstrate that the reduced fat deposition in Zucker rats after adrenalectomy is mainly due to the large decrease in the efficiency of energy utilization associated with a restoration of brown fat activity.