Cytoplasmic glucocorticoid receptors in the developing small intestine of the rabbit fetus

Glucocorticoid binding and alkaline phosphatase activity in the small intestine of the fetal rabbit were studied to investigate the relationship of glucocorticoid receptors and the development of the tissue. In the cytosol fraction, the binding of (3H)dexamethasone involves a macromolecule with high affinity (Kd = nM) for the hormone and a limited number of binding sites (saturable at a hormone concentration of 10 nM). That the binding reaction involves a protein and sulfhydryl groups was demonstrated by the absence of binding of the steroid in the presence of Pronase and sulfhydryl blocking reagents. In sucrose density gradients, the complexes have sedimentation coefficients of about 4S and 7S at low ionic strength, but only 4S at high ionic strength (0.4m KCl). The binding protein is thermolabile, and is stabilized by complexing with the hormone. The ability of different steroids to compete with (3H)dexamethasone for the binding sites correlates well with their glucocorticoid potency. During development of the fetal rabbit small intestine, the total number of glucocorticoid-binding sites in the cytosol increases in parallel with the increases in the tissue weight until term. However, the concentration of the binding sites (pmol/mg cytosol protein) is maximum at day 25 of gestation, followed by a decrease to the adult level within a few days after birth. Alkaline phosphatase activity is first detectable on day 25 of gestation and increases rapidly thereafter. These observations suggest that there may be a temporal relatioship between the development of the fetal rabbit small intestine, as reflected in the alkaline phosphatase and the levels of glucocorticoid receptors in the cytosol.