CORTICOSTEROID-BINDING GLOBULIN (CBG) PRODUCTION BY HEPATIC AND EXTRAHEPATIC SITES IN THE OVINE FETUS - EFFECTS OF CBG ON GLUCOCORTICOID NEGATIVE FEEDBACK ON PITUITARY-CELLS IN-VITRO

Plasma cortisol levels increase in fetal sheep during late gestation a nd this is associated with an increase in plasma corticosteroid-bindin g globulin (CBG) concentrations. However, the relative tissue sources of plasma CBG, the ontogeny of its biosynthesis and glycoform composit ion have not been established in the ovine fetus. Therefore we examine d whether changes in plasma corticosteroid binding capacity (CBC) in f etal sheep during late gestation were associated with different patter ns of glycosylation and reflected changes in tissue CBG expression. Si nce free cortisol is considered the bioactive fraction, we measured ch anges in the percent and absolute free cortisol in fetal plasma during late gestation. In order to examine whether CBG alters cortisol negat ive feedback at the level of the fetal pituitary, we also examined the effect of exogenous CBG in mediating the glucocorticoid-induced suppr ession of basal and corticotrophin-releasing hormone (CRH)-stimulated ACTH release from fetal pituitary cells in culture. The mean free cort isol concentration in plasma was not different between days 15 and 20 prior to parturition, and between 5 and 10 days prepartum, although it did rise between these times. Plasma CBC in chronically catheterized fetuses rose from 23.3 +/- 4.6 ng/ml at day 115 to 86.5 +/- 20.8 ng/ml at term and then decreased rapidly after birth. Between day 125 and d ay 140 of pregnancy approximately 10% of fetal plasma CBG was retarded by Concanavalin-A chromatography. This proportion increased at birth and attained adult values >70% by one month of age. By Northern blotti ng the relative levels of CBG mRNA in the fetal liver did not change b etween days 100 and 125, then increased significantly at day 140, but declined at term and in newborn lambs. CBG mRNA was undetectable in to tal RNA from lung, kidney, hypothalamus and placentomes, but was prese nt in the fetal pituitary at days 125 and 140. Reverse transcription-P CR was used to confirm the presence of CBG mRNA in pituitary tissue fr om term fetuses. In cultures of term fetal pituitary cells, added CBG attenuated the cortisol- but not the dexamethasone-mediated suppressio n of basal and CRH-stimulated ACTH release. We conclude that in fetal sheep there is an increase in the corticosteroid binding capacity of p lasma during late pregnancy which regulates, in part, free cortisol le vels in the circulation. The liver is the major site of CBG biosynthes is in the fetus and at least until day 140 of gestation the rise in pl asma CBC is associated with an increase in hepatic CBG mRNA levels. Th e fetal pituitary was also established as a site of CBG production. Ou tput of ACTH by cultured pituitary cells was inhibited by cortisol and this effect was diminished in the presence of added CBG. This study s upports a role for systemic CBG in modulating the availability of cort isol to the fetal pituitary and suggests an additional way of modifyin g feedback effects of cortisol at the pituitary through its own produc tion of CBG.