Increase in liver nuclear tri-iodothyronine receptors associated with increased deoxyribonucleic acid by long-term administration of tri-iodothyronine in thyroidectomized rats.

The effect of long-term administration of small amounts of tri-iodothyronine was examined on the nuclear tri-iodothyronine receptors in rat liver. The maximal binding capacity (C(max.)) and association constant (K(a)) of the receptors were determined in thyroidectomized rats given vehicle alone (group A), 2mug of tri-iodothyronine/100g body wt. (group B) or 7mug of tri-iodothyronine/100g body wt. (group C) for 2 weeks. Scatchard analyses with correction for the amount of endogenous tri-iodothyronine revealed that C(max.) values per g of liver were increased to 1.5 and 2.7 times the control value in groups B and C respectively. Since concentrations of DNA per g of liver were significantly increased in the two groups of hormone-treated rats, C(max.) values per mg of DNA were nearly the same in group B, but still increased significantly in group C compared with group A. K(a) values remained unchanged in all three groups of animals. Mitochondrial alpha-glycerophosphate dehydrogenase activity was 9.6 and 28.7 times as high in groups B and C, respectively, as in group A. Concentrations of endogenous tri-iodothyronine bound to non-histone protein were substantially increased in groups B and C, although concentrations of serum tri-iodothyronine remained rather low. The results obtained indicate that the long-term administration of tri-iodothyronine in small doses induces an increase in the nuclear receptors associated with increased DNA with and without accompanying a relative increase in the receptor concentration in thyroidectomized rats. Also the hormonal effect is closely related to the total number of the nuclear receptors and the concentrations of nuclear tri-iodothyronine bound to the receptors rather than the serum tri-iodothyronine concentrations.