REGULATION OF PORCINE SKELETAL-MUSCLE NUCLEAR 3,5,3'-TRI-IODOTHYRONINE RECEPTOR-BINDING CAPACITY BY THYROID-HORMONES - MODIFICATION BY ENERGY-BALANCE

Thyroid hormones have been implicated in the regulation of nuclear 3,5 ,3'-tri-iodothyronine (T-3) receptor binding capacity (B-max) but, des pite numerous in vivo and in vitro studies, there is considerable cont roversy regarding their exact role. Since changes in thyroid status al ter energy balance and hence may influence T-3 receptor numbers, the e ffects of chronic hypothyroidism and T-4 treatment have been studied i n young pigs under conditions of controlled energy intake. Four groups of animals comprising a hypothyroid, a euthyroid and a hyperthyroid g roup, all on the same level of food intake, and a hyperthyroid group o n twice the amount of food were used. After 3 weeks on the treatment r egimes, both the hypothyroid animals on the same level of food intake and the hyperthyroid animals on twice the amount of food had significa ntly increased B-max values (97% and 137% higher respectively) compare d with euthyroid controls. However, there was no difference between co ntrols and the hyperthyroid animals on the same level of food intake. In a second study, the effects of short-term treatment of euthyroid an imals with T-3 was investigated. Results showed that in two groups of controls that received intravenous saline, those on a higher food inta ke had higher B-max values (76% increase). Intravenous T-3 administrat ion to animals on a low food intake did not change the receptor number s. In none of the studies was there any change in the dissociation con stant of the receptors as a result of different treatments. It is sugg ested that, at least in postnatal life, thyroid hormones per se have n o significant effect on nuclear T-3 receptor numbers in skeletal muscl e. Instead, changes in B-max in response to thyroid status may be seco ndary to alterations in energy balance induced by these hormones.