Triiodothyronine nuclear receptors in liver, brain and lung of neonatal rats. Effect of hypothyroidism and thyroid replacement therapy.

In this study we have examined the effect of neonatal hypothyroidism and triiodothyronine (T3) replacement therapy on the maximal binding capacity (MBC) and the affinity of T3 receptors prepared from liver, brain and lungs. Rats were radiothyroidectomized at birth and administered T3 or its placebo starting at 8 days of age. The thyroid state in normal, hypothyroid and hypothyroid T3-treated rats was assessed by serum determination of thyroxine, T3 and by the measurement of the hepatic alpha-glycerophosphate dehydrogenase. The animals were sacrificed at 8, 16 or 24 days of age and the T3 binding was estimated in isolated nuclei and in salt nuclear extracts. The MBC of the T3 receptors was higher in the hypothyroid rats at all ages when it was determined in isolated nuclei, but not when was measured in nuclear extracts. At 8 days, the MBC had risen twofold or more in the receptors from brain (1.0 +/- 0.37 vs. 0.4 +/- 0.1 ng T3/mg DNA in controls) and from lungs (0.6 +/- 0.25 vs. 0.3 +/- 0.15 ng T3/mg DNA in controls), but was only slightly elevated in the hepatic receptor (0.62 +/- 0.08 vs. 0.48 +/- 0.15 ng T3/mg DNA in controls). At 16-24 days, the highest value of MBC was observed in the hepatic receptor (0.83 +/- 0.04 vs. 0.41 +/- 0.1 ng T3/mg DNA in controls) followed by the brain receptor (0.65 +/- 0.03 vs. 0.35 +/- 0.02 ng T3/mg DNA in controls), and that of lung (0.39 +/- 0.07 vs. 0.20 +/- 0.03 ng T3/mg DNA in controls).(ABSTRACT TRUNCATED AT 250 WORDS)