DIVERGENT DEIODINATION OF THYROID-HORMONES IN THE SEPARATED PARTS OF THE FETAL AND MATERNAL PLACENTA IN PIGS

Previous work from this laboratory has shown that the thyroid gland of the fetal pig begins to function at about day 46-47 (0.40-0.415 fract ion of gestational age). Sera from fetuses contain lower thyroxine (T- 4), 3,3',5-triiodothyronine (T-3) and 3,3',5'-triiodothyronine (rT(3)) concentrations than maternal sera, except for about 2 weeks before te rm. The fetal T-4 metabolism is dominated by the 5'-monodeiodinating a ctivity (5'-MD). In the present study we measured the iodothyronines c ontent, and the outer (5'-MD) and inner (5-MD) monodeiodinases activit y, in homogenates of the placenta. The pig placenta, which is of the e pitheliochorial type, was separated into the fetal and the maternal pa rt. The concentrations of T-4, T-3 and rT(3) were lower, and the deiod inating activity of 5'-MD and 5-MD higher, in the fetal than in the ma ternal placenta. The fetal placenta not only deiodinated more actively T-4 to T-3 and T-4 to rT(3), but degraded T-3 to 3,3'-diiodothyronine (3,3'-T-2) more actively than rT(3) to 3,3'-T-2 Such divergent deiodi nating activity of T-4 to T-3, T-3 to 3,3'-T-2 and rT(3) to 3,3'-T-2 m ight favor establishing a relatively high and constant rT(3) concentra tions in fetal and maternal placentas, and a lower T-3 in the fetal pl acenta. The inner ring deiodinating activity (excluding a day before p arturition) was always more active in the fetal placenta, while the ou ter ring deiodinations varied in this respect, depending on the gestat ion stage. These results support the hypothesis that in the fetal pig, enzymatic deiodination of thyroid hormones forms a barrier which redu ces transplacental passage of the hormones and that the fetal part of the placenta is the primary factor in the mechanism regulating the hor monal transfer. In spite of the presence of the barrier, there is an a dequate maternal supply of thyroid hormones to the fetus in early gest ation, which suggests that the enzymatic mechanism is influenced in so me way by the thyroid status of the fetus.