AbstractThyroid hormones stimulate bone turnoverin vivoand increase Ca release from bonein vitro.To investigate further the effects of thyroid hormones in bone, we have characterized specific nuclear receptors for [125]triiodothyronine (T3) in neonatal mouse calvaria. Maximal specific binding of [125I]T3to isolated nuclei occurred within 60 min at 22°C. [125I]T3binding was completely and rapidly displaced by the addition of 10−6Munlabeled T3; the dissociation appears to be first order witht1/2= 36 min. The IC50for competition by unlabeled T3was approximately 10−8M.The relative affinity of thyroxine (T4) for the receptor was ˜10 × lower than T3, consistent with its lower biological potency in most target tissues for thyroid hormones. Only weak competition was observed with diiodotyrosine at concentrations up to 10−4M.We have previously shown that the cardiotonic agent milrinone stimulates bone resorptionin vitrowith characteristics similar to those of T4. Structural homology between milrinone and T4was recently reported. Milrinone, like diiodotyrosine, was only a weak competitor for binding at concentrations up to 10−4M.Milrinone inhibited collagen synthesis in the calvaria. The results suggest that the effects of milrinone on bone turnover in calvariain vitroare probably not mediated through a thyroid hormon