Krysin E, Brzezińska-Slebodzińska E, Slebodziński A B
Department of Developmental and Experimental Endocrinology, Polish Academy of Sciences, Poznań, Poland.
J Endocrinol. 1997 Nov;155(2):295-303. doi: 10.1677/joe.0.1550295.
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 fraction of gestational age). Sera from fetuses contain lower thyroxine (T4), 3,3',5-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) concentrations than maternal sera, except for about 2 weeks before term. The fetal T4 metabolism is dominated by the 5'-monodeiodinating activity (5'-MD). In the present study we measured the iodothyronines content, and the outer (5'-MD) and inner (5-MD) monodeiodinases activity, in homogenates of the placenta. The pig placenta, which is of the epitheliochorial type, was separated into the fetal and the maternal part. The concentrations of T4, T3 and rT3 were lower, and the deiodinating activity of 5'-MD and 5-MD higher, in the fetal than in the maternal placenta. The fetal placenta not only deiodinated more actively T4 to T3 and T4 to rT3, but degraded T3 to 3,3'-diiodothyronine (3,3'-T2) more actively than rT3 to 3,3'-T2. Such divergent deiodinating activity of T4 to T3, T3 to 3,3'-T2 and rT3 to 3,3'-T2 might favor establishing a relatively high and constant rT3 concentrations in fetal and maternal placentas, and a lower T3 in the fetal placenta. The inner ring deiodinating activity (excluding a day before parturition) was always more active in the fetal placenta, while the outer ring deiodinations varied in this respect, depending on the gestation stage. These results support the hypothesis that in the fetal pig, enzymatic deiodination of thyroid hormones forms a barrier which reduces transplacental passage of the hormones and that the fetal part of the placenta is the primary factor in the mechanism regulating the hormonal transfer. In spite of the presence of the barrier, there is an adequate maternal supply of thyroid hormones to the fetus in early gestation, which suggests that the enzymatic mechanism is influenced in some way by the thyroid status of the fetus.
本实验室之前的研究表明，胎猪的甲状腺在妊娠约46 - 47天（占妊娠期的0.40 - 0.415）开始发挥功能。除了足月前约2周外，胎儿血清中的甲状腺素（T4）、3,3',5 - 三碘甲状腺原氨酸（T3）和3,3',5' - 三碘甲状腺原氨酸（反T3，rT3）浓度低于母体血清。胎儿的T4代谢主要由5' - 单脱碘酶活性（5' - MD）主导。在本研究中，我们测量了胎盘匀浆中的碘甲状腺原氨酸含量以及外环（5' - MD）和内环（5 - MD）单脱碘酶活性。猪的胎盘属于上皮绒毛膜型，被分离为胎儿部分和母体部分。胎儿胎盘部分的T4、T3和rT3浓度较低，5' - MD和5 - MD的脱碘活性高于母体胎盘部分。胎儿胎盘不仅将T4更活跃地脱碘为T3以及将T4脱碘为rT3，而且将T3降解为3,3' - 二碘甲状腺原氨酸（3,3' - T2）比将rT3降解为3,3' - T2更活跃。T4到T3、T3到3,3' - T2以及rT3到3,3' - T2这种不同的脱碘活性可能有助于在胎儿和母体胎盘中建立相对较高且恒定的反T3浓度，以及在胎儿胎盘中建立较低的T3浓度。内环脱碘活性（分娩前一天除外）在胎儿胎盘部分总是更活跃，而外环脱碘在这方面则因妊娠阶段而异。这些结果支持了这样的假设，即在胎猪中，甲状腺激素的酶促脱碘形成了一道屏障，减少了激素的跨胎盘转运，并且胎盘的胎儿部分是调节激素转运机制中的主要因素。尽管存在这道屏障，但在妊娠早期母体仍能为胎儿提供充足的甲状腺激素，这表明酶促机制在某种程度上受胎儿甲状腺状态的影响。
