Chanoine J P, Alex S, Fang S L, Stone S, Leonard J L, Körhle J, Braverman L E
Department of Medicine, University of Massachusetts Medical School, Worcester 01655.
Endocrinology. 1992 Feb;130(2):933-8. doi: 10.1210/endo.130.2.1733735.
T4 is bound to transthyretin (TTR; 75%) and albumin (Alb; 25%) in rat serum and only to TTR in cerebrospinal fluid (CSF). In addition to the liver, TTR is synthesized in large amounts in the choroid plexus and then secreted into the CSF, suggesting that serum T4 could be transported to the CSF and brain via the choroid plexus. We determined whether serum T4 bound to TTR is transported into the choroid plexus and CSF. N-Bromoacetyl-L-[125I]T4, a derivative of T4 that binds covalently to TTR, was used as the affinity label for the T4-binding site on TTR. Rats were injected with [125I]T4, acetyl-[125I]T4 covalently bound to human TTR ([125I]T4Ac.human hTTR), or acetyl-[125I]T4 covalently bound to human Alb ([125I]T4Ac.hAlb). The quantities of [125I]T4Ac.hTTR and [125I]T4Ac.hAlb present in the choroid plexus, CSF, and brain 90 min later were barely detectable. In contrast, [125I]T4 injected as the unbound form accumulated in the choroid plexus and CSF to levels 6-11 times higher than with [125I]T4Ac.hTTR (P less than 0.005). We then used a synthetic flavonoid (EMD) that competitively inhibits binding of T4 to serum TTR and transiently increases serum free T4 to determine the role of choroid plexus TTR and CSF TTR in the transport of T4 from serum to brain. Rats were given 110 microCi [125I]T4 15 min after the injection of vehicle, a low (0.3 mumol/100 g BW) or high dose of EMD (2.0 mumol/100 g BW). Rats were killed 60 min later. In serum, the percentage of [125I]T4 bound to TTR decreased and free T4 increased similarly in the low and high dose EMD-treated rats. In contrast, the percentage of [125I]T4 bound to TTR in choroid plexus and, subsequently, CSF was significantly decreased in rats given the high dose of EMD, but was not affected by the low dose of EMD, suggesting that in high doses, EMD crossed from serum to choroid plexus and CSF and occupied TTR-binding sites for T4. There was a significant decrease (P less than 0.05) in the percentage of injected [125I]T4 in the high dose vs. the low dose EMD-treated rats in total choroid plexus (61%), 1 ml CSF (94%), and 1 g cerebral cortex (46%) and cerebellum (46%).(ABSTRACT TRUNCATED AT 400 WORDS)
在大鼠血清中，甲状腺素（T4）与甲状腺素转运蛋白（TTR；75%）和白蛋白（Alb；25%）结合，而在脑脊液（CSF）中仅与TTR结合。除肝脏外，脉络丛中也大量合成TTR，然后分泌到脑脊液中，这表明血清T4可能通过脉络丛转运至脑脊液和脑。我们研究了与TTR结合的血清T4是否能转运至脉络丛和脑脊液。N-溴乙酰-L-[125I]T4是T4的一种衍生物，可与TTR共价结合，用作TTR上T4结合位点的亲和标记物。给大鼠注射[125I]T4、与人TTR共价结合的乙酰-[125I]T4（[125I]T4Ac.hTTR）或与人Alb共价结合的乙酰-[125I]T4（[125I]T4Ac.hAlb）。90分钟后，脉络丛、脑脊液和脑中的[125I]T4Ac.hTTR和[125I]T4Ac.hAlb含量几乎检测不到。相反，以未结合形式注射的[125I]T4在脉络丛和脑脊液中的积累水平比[125I]T4Ac.hTTR高6 - 11倍（P小于0.005）。然后，我们使用一种合成类黄酮（EMD），它能竞争性抑制T4与血清TTR的结合并短暂增加血清游离T4，以确定脉络丛TTR和脑脊液TTR在T4从血清转运至脑过程中的作用。给大鼠注射溶剂、低剂量（0.3 μmol/100 g体重）或高剂量（2.0 μmol/100 g体重）的EMD 15分钟后，注射110微居里的[125I]T4。60分钟后处死大鼠。在血清中，低剂量和高剂量EMD处理的大鼠中，与TTR结合的[125I]T4百分比下降，游离T4增加。相反，高剂量EMD处理的大鼠脉络丛及随后脑脊液中与TTR结合的[125I]T4百分比显著下降，但低剂量EMD未产生影响，这表明高剂量时EMD从血清进入脉络丛和脑脊液并占据T4的TTR结合位点。高剂量与低剂量EMD处理的大鼠相比，在总脉络丛（61%）、1毫升脑脊液（94%）、1克大脑皮质（46%）和小脑（46%）中，注射的[125I]T4百分比有显著下降（P小于0.05）。
