Preferential saturation of brain 3,5,3'-triiodothyronine receptor during development in fetal lambs

The concentrations of T4 and T3 were measured in brain, liver, and lung of fetal lambs at 100 days gestational age. The highest concentrations of T4 were found in lung (26.8 ng/g). Brain T4 (8.8 ng/g) was only 30% of lung T4. In contrast, higher concentrations of T3 were found in brain (1.8 ng/g) than in lung (0.39 ng/g) or liver (0.36 ng/g). Nuclear T3 was 16% of the total T3 in brain and 44% of that in lung. The degree of saturation of the nuclear T3 receptor was estimated from the concentrations of nuclear T3 and nuclear receptor. Receptor saturation was low in liver and lung (10%) and high in brain (74%). Receptor occupancy was also measured directly in vitro by comparing the binding of [125I]T3 in nuclear extracts at 0 and 20 C. This method is based on the different rates of dissociation of the T3-receptor complex at these temperatures (0.045 and 0.618 h-1, respectively). Therefore, [125I]T3 was bound mainly to unoccupied sites at 0 C, whereas at 20 C it bound to unoccupied sites plus a fraction (70%) of endogenously occupied sites. There was no difference in binding at the two temperatures using lung extract, reflecting a very low occupancy. Data from brain suggested 61% receptor saturation. Total and free T3 were measured in plasma and in lung and brain cytosols, and the figures were compared to the intranuclear free T3 calculated by the law of mass action, from the affinity and saturation of receptor. In lung, the concentrations of cytosolic (5.4 +/- 1.9 pM) and nuclear (8.6 pM) free T3 were similar to that of plasma T3 (3.7 +/- 0.99 pM). In contrast, brain cytosolic (14.9 +/- 1.2 pM) and nuclear (203 pM) free T3 revealed the presence of free T3 gradients from cytosol to plasma (4-fold) and from nucleus to cytosol (13.6-fold). The data suggest that the sheep brain is a major target of thyroid hormone action at the end of the neuroblast proliferation period. Mechanisms are locally present in the brain at this stage of development to ensure a high saturation of the nuclear T3 receptor.