Effects of thyroxine on rat brown fat and muscle thermogenesis in the cold

Improving efficacy of the adjustable gastric band: studies of the use of adjuvant approaches in a rodent model

BACKGROUND

The laparoscopic adjustable gastric band (AGB) has been effective in reducing excess weight by approximately 50% for at least 16 years. However, as with all weight loss approaches, reduction in weight resulting from bariatric surgery is associated with a compensatory reduction in energy expenditure, which may confound and limit weight loss. Adjuvant therapies that reduce food intake and increase energy expenditure may be used to improve weight loss outcomes by ameliorating, or even reversing, this reduction in energy expenditure.

METHODS

Rats were either fitted with an AGB or were sham operated and received one of 2 adjunctive pharmacologic treatments, (1) thyroxine or (2) bupropion/naltrexone (Contrave), at a range of doses and matched with vehicle controls (n = 6-8/group) over a 4-week period of combined treatments. Metabolic parameters including food intake, weight, fat mass, and energy expenditure in brown adipose tissue (BAT), whole body calorimetry, and physical activity were assessed.

RESULTS

Inflation of the AGB caused a reduction in weight gain that was further enhanced by cotreatment with either thyroxine or Contrave (P<.05). Thyroxine completely ameliorated the reduction in AGB-induced BAT thermogenesis and significantly improved weight loss, particularly in fat mass. Contrave also augmented the loss of weight and fat mass associated with the AGB and increased BAT thermogenesis in banded rats even at doses below that required to change food intake.

CONCLUSION

Adjuvant therapies can improve the efficacy of the AGB, at least in part by negating the compensatory reduction in energy expenditure, but also via a combined effect on food intake.

Modulation of liver mitochondrial NOS is implicated in thyroid-dependent regulation of O(2) uptake

Changes in O(2) uptake at different thyroid status have been explained on the basis of the modulation of mitochondrial enzymes and membrane biophysical properties. Regarding the nitric oxide (NO) effects, we tested whether liver mitochondrial nitric oxide synthase (mtNOS) participates in the modulation of O(2) uptake in thyroid disorders. Wistar rats were inoculated with 400 microCi (131)I (hypothyroid group), 20 microg thyroxine (T(4))/100 g body wt administered daily for 2 wk (hyperthyroid group) or vehicle (control). Basal metabolic rate, mitochondrial function, and mtNOS activity were analyzed. Systemic and liver mitochondrial O(2) uptake and cytochrome oxidase activity were lower in hypothyroid rats with respect to controls; mitochondrial parameters were further decreased by L-arginine (-42 and -34%, P < 0.05), consistent with 5- to 10-fold increases in matrix NO concentration. Accordingly, mtNOS expression (75%) and activity (260%) were selectively increased in hypothyroidism and reverted by hormone replacement without changes in other nitric oxide isoforms. Moreover, mtNOS activity correlated with serum 3,5,3'-triiodothyronine (T(3)) and O(2) uptake. Increased mtNOS activity was also observed in skeletal muscle mitochondria from hypothyroid rats. Therefore, we suggest that modulation of mtNOS is a substantial part of thyroid effects on mitochondrial O(2) uptake.

Neuropeptide Y suppressed the T3-induced rise in brown fat mitochondrial respiration in hypothyroid rats

It has been postulated that a cold-induced decrease in hypothalamic neuropeptide Y (NPY) might disinhibit the sympathetic outflow that activates brown adipose tissue (BAT) thermogenesis. The present work has assessed the interscapular BAT NPY and norepinephrine (NE) concentrations 7 days after surgical excision of the sympathetic nerves (Sx) of each BAT lobe in rats exposed to 4 degrees C or 24 degrees C during 24 h. In addition, the effects of NPY treatment on BAT oxygen (O2) consumption in normal and T3-treated hypothyroid rats was determined. Sprague-Dawley rats weighing 220-240 g were used. BAT was removed and homogenized in 0.1 M HCl. BAT NPY content at 24 degrees C was 109 +/- 19 pg/mg protein. Sx decreased this value significantly (P < 0.001) and cold increased it by two-fold (P < 0.001). Neither cold nor Sx altered hypothalamic NPY. BAT O2 consumption was depressed by NPY treatment (P < 0.001) and increased by NE (P < 0.001). In euthyroid Sx rats, NPY decreased O2 consumption (P < 0.001). NPY failed to alter O2 consumption in hypothyroid rats and sharply decreased it in T3-treated rats. The results show that NPY blocked the effects of T3 on BAT O2 consumption, and that cold-induced activation of the thermogenic process did not produce measurable changes in hypothalamic NPY.