Abstract
The expression of c- erbAα and -β encoded thyroid hormone receptors (TR) was investigated in rat placenta between 16 and 21 days of gestation (dg), and in fetal liver and brain at 16dg, using semi-quantitative RT-PCR and nuclear 3,5,3′-triiodothyronine (T3) binding. TRα1, TRβ1, c-erbAα 2 and c- erbAα 3 mRNA abundance was unchanged in placenta between 16 and 21dg, as was the dissociation constant (Kd) of T3binding. The maximal T3binding capacity (Bmax) in placenta doubled over this period, suggesting placental TR binding activity is post-transcriptionally regulated. Transcript abundance in tissues at 16dg can be summarized: TRα1, placenta=fetal liver<fetal brain; TRβ1, placenta=fetal liver>fetal brain; c- erbAα 2 and α3, placenta=fetal liver<fetal brain; TRβ2; none detected. T3binding in fetal liver and brain exhibited equivalent Kdand Bmax, the Kdbeing less than 50 per cent of that in placenta, though Bmaxwas unchanged. The higher Kdin placenta may reflect tissue-specific patterns of TR modification. In conclusion, rat placenta expresses significant levels of c-erbAα and -β transcripts and protein, providing a possible mechanism of action for T3of maternal and fetal origin in this tissue.
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