Thermogenesis in thyroidectomized, protein-malnourished rats

To determine the requirement of thyroid hormones for the expression of diet-induced thermogenesis (DIT) in the protein-malnourished rat, groups of male weanling Sprague-Dawley rats were thyroidectomized (Tx) or left surgically intact, and fed isoenergetic diets containing normal (22%, control) or decreased (8%, protein malnourished, PM) casein for 8 weeks postweaning. Half the Tx rats of either group received thyroxine (T4) replacement. Weight gains were least in PM-Tx rats, intermediate in control-Tx, PM and PM-Tx + T4, and greatest in control and control-Tx + T4 groups. Interscapular brown adipose tissue (IBAT) weight and brown adipocyte diameter were similar in control and PM rats and were greater in both Tx groups. Brown adipocyte diameter was normal but IBAT greater in both Tx + T4 groups. Both resting (ROC) and minimal (MOC) oxygen consumptions were increased in PM rats after 7 weeks, but the differences between ROC and MOC were similar in both dietary groups. MOC and ROC were decreased following Tx and became normalized in both Tx groups with T4. The effects of norepinephrine (100 micrograms/kg body weight, s.c.) on MOC were markedly reduced in both Tx groups and were normal after T4 replacement. These observations indicate that the increased thermogenesis observed in the protein-deficient rat is due to a thyroid-dependent increase in MOC, and that the thyroid hormones are required for the sympathetic-dependent component of DIT and total thermogenesis in the protein-malnourished rat model.