Characterization of nuclear 3,5,3'-triiodothyronine receptors in the developing rat lung: effects of hypo- and hyperthyroidism.

In this study, we measured the changes in binding characteristics of T3 to its nuclear receptor in lungs of rats from 2 days of age to adulthood. In all ages studied, we found a single class of binding sites with a mean Ka of 1.16 +/- 0.05 x 10(10) M-1. The specificity of the nuclear receptor, as judged by the relative affinities of thyroid hormone analogs, is in general agreement with data previously reported for lung and other organs. A significant decrease in the maximum binding capacity of the lung T3 receptor occurs between 30 and 40 days of age: 0 to 30 days, 0.208 +/- 0.005 pmole T3/mg protein; 40 days to adulthood, 0.111 +/- 0.007 pmole T3/mg protein. Our experiments showed that this reduction is not due to differential recovery of nuclei or efficiency of extraction at various ages. The relative saturation of the nuclear receptor increases from 26% at 2 days of age to 48% in the adult animal. Thyroid status has no significant effect on the nuclear T3 binding capacity of developing rat lung up to 11 days of age. However, hyperthyroidism substantially increased the binding capacity in lungs of adult animals (0.157 +/- 0.009 versus 0.108 +/- 0.006 pmole T3/mg protein), without a corresponding reduction in hypothyroidism (0.116 +/- 0.007 pmole T3/mg protein). These findings suggest that thyroid hormones play a yet unspecified important role in lung physiology during the first month of life in the rat.