Nuclear 3,5,3′-Triiodothyronine Receptors in Rabbit Lung: Characterization and Developmental Changes*

Abstract

Studies both in vivo and in vitro suggest that thyroid hormones participate in the hormonal regulation of fetal lung maturation. The nuclear T₃ receptors identified in the lung tissue of several species are one possible site for the regulation of hormone action. In this study we characterized the T₃-binding properties of lung nuclei from fetal and adult rabbits and examined the developmental pattern of T₃ receptor concentration and occupancy. The developmental profile of nuclear binding capacity was multiphasic. The concentration of sites increased from 0.267 ± 0.021 fmol T₃ bound/μg DNA at 21 days of gestation to 0.384 ± 0.011 fmol/μg DNA at 28 days of gestation and then decreased to 0.273 ± 0.018 fmol/μg DNA by term. Within 2-3 weeks after birth, the concentration rose to 0.321 ± 0.016 fmol/μg DNA before decreasing to the adult concentration (0.238 ± 0.013 fmol/ μg DNA), which was 62% of the maximum value found at 28 days of gestation. Maximal T₃ binding to lung nuclei was achieved after incubation of fetal nuclei for 90 min at 37 C and adult nuclei for 4 h at 25 C. The half-times of T₃ dissociation from fetal nuclei were 28 ± 3 min (n = 4) at 37 C, 2.5 h at 30 C, and 24–36 h at 2 C; the dissociation rates for adult nuclei were similar. The relative order of potency of T₃ analogs for both fetal and adult nuclei was T₃ proprionate > 3,3′,5-triiodothyroacetic acid > L-T₃ > DT₃ > L-T₄ > 3,5-diethyl-3′-isopropyl-D,L-thyronine > rT₃ > 3,5- dimethyl-3′-isopropyl-L-thyronine. The release of receptors from nuclei incubated under optimal conditions was similar for fetal and adult nuclei (9.8 ± 0.4% and 10.2 ± 1.0%, respectively). Receptor release was independent of gestational age and T₃ and Ca²⁺ concentrations, but was dependent on incubation temperature and time. Nuclear receptors from adult, but not fetal, lung were inactivated during incubation at 37 C and were protected by the presence of a saturating T₃ concentration, suggesting the greater stability of occupied than unoccupied receptors. We also describe a procedure for estimating the occupancy of fetal receptors by endogenous thyroid hormone. This assay is based on the lower T₃ binding to nuclei at 2 C tha- at 37 C, and was validated by altering the nuclear T₃ content both in vitro and in vivo. Occupancy of receptors increased from approximately 11% to 23% of the total binding capacity between 21 and 28 days of gestation. Thus, the nuclear T₃ receptors in rabbit lungs undergo changes in concentration, stability, and occupancy during preand postnatal life. These findings support a possible direct action of both endogenous and exogenous thyroid hormones in fetal lung maturation.