Ontogeny of glucocorticoid receptors in rat liver

Specific receptors for glucocorticoids are present in liver cytosols of rat fetuses at least as early as the 18th day of gestation. The concentration of the receptor begins to decline after the 20th day reaching undetectable levels shortly before and after parturition. The receptor can be detected again 1 to 2 hours after birth, and its levels increase markedly to higher than adult values between the 2nd and 5th day. The reason for the failure to detect specific hormone binding near parturition appears to be due to occupation of binding sites by endogenous steroids rather than the absence of the receptor. This is indicated by the demonstration of both cytoplasmic and nuclear receptor sites in liver slices of newborn rats incubated with labeled dexamethasone at 37 degrees. The cytoplasmic receptors of fetal and adult liver differ in their relative affinity for cortisol and corticosterone. The fetal receptors have a higher affinity for corticosterone than cortisol while the reverse is true for the adult receptors. These observations suggest either the existence of dissimilar receptors in fetal and adult liver or the presence of more than one type of receptor sites. It is therefore possible that subtle differences in the nature of hepatic receptors may be partly responsible for the maturation-dependent qualitative differences in tissue responsiveness to glucocorticoids.