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DIEGO BELLABARBA, SUZANNE BÉDARD, SUZANNE FORTIER, JEAN-GUY LEHOUX, 3,5,3′-Triiodothyronine Nuclear Receptor in Chick Embryo. Properties and Ontogeny of Brain and Lung Receptors, Endocrinology, Volume 112, Issue 1, 1 January 1983, Pages 353–359, https://doi.org/10.1210/endo-112-1-353
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Nuclear T3 binding sites were detected in the whole brain and lung of chick embryos. Gel chromatography on Sephadex G-100 of the solubilized receptors showed an identical elution profile (peak 1: T3 nonspecifically bound; peak 2: T3 bound to the receptor; peak 3: free T3). The sedimentation coefficient on glycerol density gradient was 4.0S for both receptors, and the estimated mol wt is 60,000.
Scatchard analysis performed with purified nuclei and the solubilized receptors revealed a single class of binding sites. The values of the association constants (Ka) and the maximal binding capacities (MBC) at 17 days of embryogenesis were as follows (mean ± SD): brain: purified nuclei, Ka = 0.64 ± 0.1 × 109 M-1, MBC ± 1.44 fmol/μg DNA; solubilized receptor, Ka = 1.16 ±0.1 × 109 M-1, MBC = 1.95 ± 0.08 fmol/μg DNA; lung: purified nuclei, Ka = 0.77 ± 0.1 ± 109 M-1 MBC = 2.72 fmol/μg DNA; solubilized receptor, Ka = 2.02 × 109 M-1, MBC = 1.86 ± 0.1 fmol/μg DNA.
Serial studies performed at different days of embryogenesis demonstrated a rapid and significant increase of the MBC of the brain receptor between 9 and 12 days, followed by a sharp decline, particularly after hatching. The MBC of the lung receptor decreased slightly from day 12 to day 17 but fell markedly from 17 to 19 days and continued to diminish rapidly in the neonatal period. The Ka values of the two receptors did not change significantly during embryogenesis and after hatching.
In competition studies 3,3′,5-triiodothyroacetic acid, L-T3, D-T3, and L-T4 were the more effective inhibitors of [125I]T3 binding. At 9 days of embryogenesis, L-T4 was half as potent as L-T3, but at 19 days its affinity had diminished, and it was 5 times less potent than T3.
These results indicated the presence of putative T3 receptors in the nuclei of brain and lung of chick embryo. The variations in the ontogeny of these receptors among the various organs suggest that local factors may intervene in their development.
