Inner-ring deiodination of 3,5,3'-triiodothyronine in the in situ perfused guinea pig placenta.

Abstract

IntroductionBroken cell preparations of rat and human placentas contain an inner (tyrosyl)-ring iodothyronine deiodinase enzyme with greatest activity when the substrate is 3,5,3'-triiodothyronine (T3). This report describes the deiodination of T3 in the intact placenta and the effect of sodium iopanoate (IA) and propylthiouracil (PTU) on T3 deiodination. Under nembutal anesthesia, the placenta of60-65-d-old pregnant guinea pigs was surgically exposed, a single umbilical artery and the umbilical vein were cannulated, and the fetus was removed. In a temperature-controlled chamber (37°C), the fetal side of the placenta was perfused through the umbilical artery at a rate of 1 ml/min with 3% bovine serum albumin Krebs-Henseleit buffer containing 0.14 nM outer ring labeled 1'"IJT3. Placenta effluent fractions were collected at timed intervals from the umbilical vein canulla throughout a 120-min perfusion period. The contents of the perfusion buffer and the various effluent fractions were analyzed for their iodothyronine content by high pressure liquid chromatography. In five experiments, the percent composition of '25I-labeled iodothyronines in the perfusion buffer and placenta effluent was 95.3±1.0 (mean±SE) and 70.2±2.1 for T3 (P < 0.01), 2.5±0.7 and 20.1±1.8 for 3,3'-T2 (P < 0.01), and 0 and 8.2±0.9 for 3'-Ti. The placenta receives a large percentage of the fetal cardiac output (1), and, as we and others have reported (2-7), contains enzymes that deiodinate thyroid hormones. These observations support the possibility that the placenta is an important site for fetal thyroid hormone metabolism. Deiodination of the iodothyronines can occur in the phenolic (5' or outer) or in the tyrosyl (5 or inner) rings. These deiodinative pathways play a major role in the metabolism of thyroxine (T4),' generating the metabolically active iodothyronine, 3,5,3'-triiodothyronine (T3), and the metabolically inactive iodothyronine, 3,3',5'-triiodothyronine (rT3). Deiodination is also the principal pathway for the metabolism of T3, which results in the formation of the inactive diiodothyronines, 3,3'-T2 and 3,5-T2, as well as the inactive monoiodothyronines, 3'-T, and 3-T1. Thus, placental deiodination is potentially an important mechanism for the modulation of fetal thyroid hormone action. While detailed studies of deiodination have been performed in placenta homogenates and subcellular fractions (6,8), there is little information on iodothyronine deiodination in the intact placenta (9).In this study, we discovered the metabolic fate of T3 when perfused through the fetal side ofthe guinea pig placenta in situ. Since previous studies employing placenta homogenates have shown an inhibitory effect ofpropylthiouracil (PTU) and sodium iopanoate (IA) on T4 deiodination (8), the effect of these drugs on T3 metabolism in the intact placenta was evaluated.

Maternal compound W serial measurements for the management of fetal hypothyroidsm

“…Since limited amounts of iodothyronines cross the placenta (33)(34)(35)(36), administration of T4 or T3 to the mother is of limited value for therapy of fetal hypothyroidism and carries the risk of worsening of maternal hyperthyroidism. The amount of maternal hormone reaching the fetus in cases of severe fetal hypothyroidism appears insufficient to prevent the fetal pathology (36)(37)(38).…”

American Thyroid Association Guide to Investigating Thyroid Hormone Economy and Action in Rodent and Cell Models

“…Whereas placental D2 and D3 activities can be measured in cell dispersions or tissue sonicates (179)(180)(181)(182), in situ preparations can be used to study the placental D3 pathway (183,184). In an anesthetized pregnant guinea pig, the placenta is surgically exposed and a single umbilical artery and the umbilical vein cannulated, while the fetus is removed.…”

“…Placenta effluent fractions are collected at timed intervals (up to 2 hours) from the umbilical vein cannula. The contents of the perfusion buffer and the various effluent fractions are then separated and analyzed by HPLC for their iodothyronine content (183). In this setting outer-ring deiodination of T 4 or rT 3 is minimal (184) possibly because placental outer-ring deiodination is greatest in the zone immediately adjacent to the uterine wall (185), distant from the fetal side of the organ.…”

Insights Into the Role of Deiodinases from Studies of Genetically Modified Animals

“…Other deiodinase inhibitors that have been used in vivo, such as the radiographic contrast agents iopanoic acid and sodium ipodate, inhibit the function of all three deiodinases (8,9). However, their use in some circumstances has been informative, such as in defining an important role for 5Ј-deiodination in the feedback regulation of thyrotropin (TSH; 10) and during tadpole metamorphosis (11), and in exploring the consequences of D3 expression in placental function (12,13). Finally, selenium deficiency has been used as an interesting and informative model system in rodents to examine the effects of inhibiting the D1 in specific tissues such as the liver and kidney (14).…”