Thyroxine inner ring monodeiodinating activity in fetal tissues of the rat
Journal
Pediatric Research
Journal Volume
23
Journal Issue
2
Pages
196-199
Date Issued
1988
Author(s)
Abstract
We studied thyroxine (T4) inner ring monodeiodinating activity (5-MA) in various tissues of fetal, maternal, and adult male rats. Tissue homogenates were incubated with 0.26 μM T4 in 0.1 M phosphate buffer (pH 7.4) containing 10 mM EDTA and 400 mM dithiothreitol (final volume 0.7 ml) for 10 min at 37° C; the 3,3′,5′-triiodothyronine (rT3) generated was measured by radioimmunoassay of ethanol extracts of incubation mixture and the result was corrected for rT3 degradation during incubation. Compared to maternal tissues, T4 to rT3 5-MA in the 14-day-old fetus was increased about 70 times in skeletal muscle (mean ± SEM, velocity, 5.4 ± 0.9 versus 0.08 ± 0.01, pmol rT3/h/mg protein); ~8 times in intestine (0.72 ± 0.17 versus 0.09 ± 0.03); and ~4 times in cerebral cortex (19 ± 0.5 versus 4.5 ± 0.9), while it was similar in skin (3.2 ± 0.48 versus 2.6 ± 0.52). Hepatic T4 5-MA approximated 1.1 ± 0.63 in the 14-day-old fetus; it could not be measured reliably in maternal or 19-day fetal tissue because of extensive (>90%) degradation of rT3 during incubation. Relative to mother, T4 5-MA in 19-day fetal tissues was increased ~30-fold in intestine, ~ 20-fold in skeletal muscle, and ~6-fold in cerebral cortex while it was similar in skin. The T4 5-MA in maternal rat tissues did not differ significantly from corresponding values in adult male rat, except skin, where it was lower in the mother rat (2.6 ± 0.52 versus 4.6 ± 0.61, p < 0.05). In summary, relative to adult tissues T4 5-MA is exceedingly active in several fetal tissues, most notably in skeletal muscle followed by intestine and cerebral cortex. © 1988 International Pediatric Research Foundation, Inc.
Type
journal article