A Study of the Cytoplasmic Receptors for Glucocorticoids in Intestine of Pre- and Postweanling Rats

Glucocorticoids cause both enzymic and morphologic changes in the rat intestine during the time of weaning. To obtain information regarding the mechanism of these actions, we examined the cytoplasmic fraction of intestines from 18-day-old rats for the presence of specific glucocorticoid-binding proteins which are characteristics of target tissues. Incubation of slices of intestine with [3H]dexamethasone in a physiological medium at 2 degrees showed the presence of a cytoplasmic binding macromolecule with high specificity for steroids having glucocorticoid activity. The binding reaction was saturable (concentration of binding sites equals 0.24 pmol per mg of protein) and of high affinity (dissociation constant equals 9.3 nM). Binding was reversible on addition of nonlabeled dexamethasone (t 1/2 equals 5.2 hours), indicating that the usual assay procedure measured both corticosterone-filled and unoccupied binding sites. Sucrose density gradient centrifugation showed that the receptor-dexamethasone complex from intestinal cytosol sedimented at the same rate as that from liver (8.2 S). The receptor-dexamethasone complex was stable at 2 degrees for at least 24 hours in intestinal slices, but in isolated cytosol fractions there was considerable loss of binding even in the presence of high concentrations of [3H]dexamethasone. Furthermore, mixing experiments showed that the presence of cytosol from intestinal mucosa (but not from the muscle layers) caused a dissociation of dexamethasone from receptors of liver cytosol. This suggested the presence of some interfering factor in isolated mucosal cytosol and meant that quantitative studies had to be confined to intact slices. Although the reasons for the instability of steroid-receptor complexes in the presence of isolated intestinal cytosol are not understood, the instability is believed to be associated with homogenization and, therefore, is believed to have no physiological significance. Finally, the ontogenesis of cytoplasmic glucocorticoid receptors in intestinal slices was examined and the pattern compared with that in liver and lung. Receptor activity was present in intestine from late fetal life through adulthood, but concentrations were significantly higher during the first two postnatal weeks than at all other times. By contrast, receptor activity detected in cytosol prepared from rat lung was high around the time of birth, while that in liver rose steadily during the first postnatal week and remained at high levels. Thus specific receptors for glucocorticoids are present in the rat intestine during periods of both responsiveness and unresponsiveness. This suggests that although corticosteroids exert their effects through the cytoplasmic receptors, this early event in glucocorticoid action may not be a controlling step for changes in responsiveness during development.