RNA synthesizing activities and glucocorticoid binding capacities in thymus cells during rat development

This investigation was designed to study specific glucocorticoid binding to cytoplasmic fraction and nuclei of thymus cells during rat development and to find out whether these data can be correlated to changes of RNA synthesizing activity of nuclei and cytoplasm. The dexamethasone binding capacity of cytoplasm rose rapidly in rats weighing up to 125 g and decreased significantly in animals weighing more than 160 g. Hormone binding to nuclei revealed similar but less pronounced changes. RNA synthesizing activity of nuclei measured by [3H]uridine incorporation in vitro decreased from 57 +/- 4.6 dpm/microng DNA in young to 23 +/- 2.2 dpm/microng DNA in adult rats. RNA synthesizing activity of the cytoplasmic fraction fell 21.6% and that of purified polymerase IIIB fell 27.8% during development. Inhibition of RNA synthesis by dexamethasone applied in vivo and in vitro showed age-dependent differences. The RNA synthesizing capacity of nuclei was inhibited up to 39% in animals weighing 130 g and only 9% in aging rats. Similar changes were observed by incubation of intact thymocytes with and without hormone. The observed inhibitory effect of dexamethasone on RNA synthesis is well correlated to changes of cytoplasmic hormone receptor capacities during development.