Thyroidal regulation of rat pancreatic nuclear triiodothyronine receptor during postnatal development

We have shown previously that the rat pancreas contains nuclear T3 receptors which exhibit a characteristic maturation pattern during development. To investigate whether these receptors are subjected to autologous regulation by thyroid hormones, the effect of T4 on the binding capacity (Bmax), dissociation constant (Kd), and receptor occupancy were followed in intact rat pups at various ages. Hyperthyroidism (by daily injection of T4 0.1 micrograms/g body wt to intact pups starting 4 days before death at 5, 10, 15, and 20 days of age) increased while hypothyroidism (by propylthiouracil feeding) decreased the total T3 binding capacity during preweaning ages (mean maximal binding capacities as estimated by Scatchard analysis, at 30 C for 14-20 days old eu-, hyper-, and hypothyroid pups: 186, 229, and 129 fmol/mg non-histone protein (NHP). The thyroid conditions also affected the percentage of T3 receptor occupancy but not the affinity of binding (as measured by Kd). Concomitantly, these conditions also caused corresponding changes in pancreatic weights, DNA and protein contents, and the concentrations of amylase, trypsinogen, and lipase. The postnatal developmental retardation induced by 6-n-propyl-2-thiouracil treatment was reversed by T4 replacement. The results suggest that rat pancreatic T3 nuclear receptors during postnatal ages are modulated by T4, and such modulation apparently in turn affects the development of the exocrine enzymes.