The expression of c-erbA alpha and -beta encoded thyroid hormone receptors
(TR) mas investigated in rat placenta between 16 and 21 days of gestation (
dg), and in fetal liver and brain at 16 dg, using semi-quantitative RT-PCR
and nuclear 3,5,3'-triiodothyronine. (T-3) binding. TR alpha1, TR beta1, c-
erbA alpha2 and c-erbA alpha3 mRNA abundance was unchanged in placenta betw
een 16 and 21 dg, as was the dissociation constant (K-d) of T-3 binding. Th
e maximal T-3 binding capacity (B-max) in placenta doubled over this period
, suggesting placental TR binding activity is post-transcriptionally regula
ted. Transcript abundance in tissues at 16 dg can be summarized: TR alpha1,
placenta=fetal liver<fetal brain; TR<beta>1, placenta=fetal: liver>fetal b
rain; c-erbA alpha2 and alpha3, placenta=fetal liver<fetal blain; TR<beta>2
; none detected. T-3 binding in fetal liver and brain exhibited equivalent
K-d and B-max the K-d being less than 50 per cent of that in placenta, thou
gh B-max was unchanged. The higher K-d in placenta may reflect tissue-speci
fic patterns of TR modification. In conclusion, rat placenta expresses sign
ificant levels of c-erbA alpha and -beta transcripts and protein, providing
a possible mechanism of action for T-3 of maternal and fetal origin in thi
s tissue. (C) 2001 Harcourt Publishers Ltd.