Biosynthesis of hepatic corticosteroid-binding globulin: ontogeny and effect of thyroid hormone

The present study reports on the ontogeny and the effect of thyroid hormones on liver corticosteroid-binding globulin (CBG) biosynthesis, in relation to plasma CBG binding capacity in the rat. We show that mRNACBG contents were high in liver of 18-day-old fetuses and decreased with age to reach almost undetectable levels by postnatal day 1. Interestingly, at the latter time point plasma CBG concentration remained elevated and disappeared thereafter from the circulation with a half-life of about 3 days. The messenger was localized in parenchymatous cells and not in hematopoietic ones, although the latter constitute the major cell population in fetal liver. It is not until after 10 days of age that mRNACBG and plasma CBG levels increased in concert, with a sex-difference being noticed by postnatal day 30. Treatment of rats with 3,3',5-triiodo-L-thyronine (T3), but not reverse T3 (rT3) (the predominant form present in fetal serum), enhanced CBG biosynthesis. These findings show that liver mRNACBG contents undergo dramatic changes during ontogeny and suggest the existence of a differential regulation of the messenger in fetal and neonatal rats.