Rat pancreatic nuclear thyroid hormone receptor: Characterization and postnatal development

Abstract

To document the presence of 3,5,3′-triiodothyronine nuclear receptor in the rat pancreas, the solubilized pancreatic nuclear fraction was characterized for its binding to 3,5,3′-l-triiodothyronine. Binding was found to be reversible, saturable, and specific. Scatchard analysis revealed a single class of 3,5,3′-l-triiodothyronine receptors in the pancreata of adult rats. The dissociation constant was 0.29 ± 0.02 × 10−9 M, and maximal binding capacity was 242 ± 10 fmol of T3 bound per milligram of non-histone protein at 30 °C (n = 20). Both parameters are comparable with those of adult rat liver nuclei. The degree of receptor occupancy was 30%–40% of the total sites in the adult rat pancreas. Developmentally, 3,5,3′-l-triiodothyronine binding was not detectable at birth and very low in pups < 5 days old. It reached a peak at the third to fourth weeks. The degree of receptor occupancy changed with development from a low (19.2% ± 2.0%) in 5–10-day-old pups to a high (50.5% ± 3.0%) in 11–20-day-old pups, then returned to a moderate level (36.9% ± 3.0%). The dissociation constants remained unchanged at various postnatal ages. Our data suggest that the rat pancreas is a target tissue for thyroid hormone and that thyroxine plays a role in pancreatic development.