UBC Theses and Dissertations
Corticosteroid effects on fetal metabolism and studies on steroid-receptor complexes in placenta of mice Wong, Ming Dak
Teratogenic and lethal effects have been reported from treatment of pregnant animals with both synthetic and natural corticosteroids, and were also observed in this investigation. Injection of dexamethasone into pregnant mice on gestational day 18 resulted in decreasing net transfer of labeled glucose from mother to fetus with increasing steroid dosage. Inhibition was also demonstrable when injections were made into the fetus in utero. It was concluded that the deleterious effects of dexamethasone on the survival of mouse fetuses might be attributable to this action. All steroid systems studied to date appear to involve a similar sequence of interactions that precede the different biochemical effects that are observed in target tissues. Binding of the steroid to a specific cytoplasmic macromolecule and translocation of this complex to the nucleus are required in the mechanism of steroid action. The placenta, being the site of maternal-fetal transmission of nutrients, was therefore examined for the presence of intracellular steroid receptors. Using Sephadex chromatography, the in vitro binding of radioactive steroids to components in mouse placental nuclei and cytoplasm was investigated. Specificity was indicated in competition studies using excess unlabeled competing steroids. This specificity was confirmed since only the active glucocorticoids formed complexes which demonstrated the ability to translocate to the nucleus. The binding properties of the cytoplasmic steroid-receptor interaction were also studied. From the time course of binding the complex was shown to possess more stability at 0°C than at 37°C, and the distribution of receptors in the cytosol appeared to be homogeneous. The cytoplasmic complex showed lability when heat denaturation and proteolytic digestion were investigated, but did not appear to be affected by nucleases or the sulfhydryl reagents. Kinetic analysis of the binding revealed the presence of high affinity specific binding sites with a dissociation constant of 17.5 nM and a receptor site concentration of 0.26 pmoles/mg protein. Using sucrose density gradient centrifugation, the molecular weight of the cytoplasmic corticosterone-receptor complex was estimated to be approximately 55•800. This investigation has revealed the existence of glucocorticoid receptors in a target tissue in which the regulation of glucose availability to the fetus may be mediated by corticosteroids. In view of the critical role that glucose plays in the nutritional status of the fetus; and the preeminent influence that glucocorticoids have on glucose uptake in other target tissues, it is then very plausible that the corticosteroids have a regulatory function in fetal growth and viability.
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