Characterization of specific thyroid hormone receptors in bone

Area of Science:

• Endocrinology
• Bone Biology
• Molecular Pharmacology

Background:

• Thyroid hormones (TH) influence bone turnover and calcium release.
• Specific nuclear receptors for TH mediate their biological effects.
• The cardiotonic agent milrinone shares structural similarities with thyroxine (T4) and affects bone resorption.

Purpose of the Study:

• To characterize specific nuclear receptors for tri-iodothyronine (T3) in neonatal mouse calvaria.
• To investigate the binding affinity and kinetics of T3 and thyroxine (T4) to these receptors.
• To determine if milrinone mediates its effects on bone turnover via the TH receptor.

Main Methods:

• Isolation and characterization of specific nuclear receptors for [125I]tri-iodothyronine (T3) in neonatal mouse calvaria.
• Binding assays to determine maximal binding, dissociation kinetics, and IC50 values for T3.
• Competition assays using unlabeled T3, thyroxine (T4), diiodotyrosine, and milrinone.

Main Results:

• Maximal specific [125I]T3 binding to isolated nuclei occurred within 60 minutes at 22°C.
• [125I]T3 binding was rapidly displaced by unlabeled T3 with a half-life of 36 minutes and an IC50 of approximately 10(-8) M.
• Thyroxine (T4) showed 10-fold lower affinity than T3, while milrinone and diiodotyrosine exhibited weak competition for the T3 receptor.

Conclusions:

• Neonatal mouse calvaria possess specific nuclear receptors for tri-iodothyronine (T3).
• The binding characteristics are consistent with known thyroid hormone receptor properties.
• Milrinone's effects on bone turnover and collagen synthesis in vitro are likely not mediated through the thyroid hormone receptor.