Nuclear receptors for thyroxine in rat liver during postnatal development in relation to DNA-dependent RNA polymerase...

Specific binding of 125I-thyroxine on liver nuclear extract of rat fetuses aged 15 and 18 days, of young rats aged 1, 2, 7, 12-14 and 18 days and of adult rats was quantified. The highest value representing the capacity of specific 125I-thyroxine binding was noted in rats aged 7 days. A marked diminution of specific 125I-thyroxine binding was demonstrated in rats aged 12-14 days. Practically the same values of Ka in liver nuclear extract were found in fetuses aged 18 days (Ka = 4.44 X 10(9) 1 mol-1) and in a group of young animals aged 7 days (Ka = 4.17 X 10(9) 1 mol-1). In nuclear extract of the animals aged 12-14 days and in adult rats diminished values (Ka = 2.72 X 10(9) 1 mol-1 and Ka = 1.18 X 10(9) 1 mol-1, respectively) were noted in comparison with the above-mentioned groups of rats. A positive correlation between the binding of 125I-thyroxine and Mn2+-activated DNA-dependent RNA polymerase activity in liver nuclei of rats aged 4 and 14 days and adult rats was found (r = 0.700). These results support the suggestion that thyroxine in the perinatal period of life plays an important role in the induction of mRNA.