T3 plus high doses of beta-blockers: effects on energy intake, body composition, bat and heart in rats.

The effects of a combined treatment with supraphysiological doses of the thyroid hormone T3 (15 micrograms/kg BW/day, s.c.) and high doses of a predominant beta 1-blocker (atenolol, 12.5 and 25 mg/kg BW, 3X/day, s.c.) or a non-specific beta-blocker (propranolol, 5 mg/kg BW s.c. and 33 mg/kg BW p.o., each 3X/day) on energy intake, body composition and the heart were studied in overfed rats with an increased body fat content. The goal of the study was to investigate whether the above treatment constitutes a therapy for obesity in that T3 causes weight and fat loss and the beta-blockers prevent the unwanted T3-effects on the heart (tachycardia and increased heart weight). T3 did not increase energy intake above the level seen in overfed animals. It caused loss of body weight due to loss of fat but not protein, an increase in interscapular brown adipose tissue (IBAT) weight and fat, tachycardia and an increase in heart weight. Atenolol and propranolol blocked T3-induced tachycardia. With the exception of the highest propranolol dose which abolished the T3-induced increase in IBAT fat content, the beta-blockers did not modify the other T3 effects. Thus, in spite of the weight and fat loss and the lack of significant protein loss and tachycardia observed under T3/high dose beta-blockers treatment, the T3-induced increase in heart weight makes this treatment unsuitable as a therapy for obesity.