Thyroxine inner ring monodeiodinating activity in fetal tissues of the rat

TS Huang, IJ Chopra, R Boado, DH Solomon… - Pediatric …, 1988 - nature.com
TS Huang, IJ Chopra, R Boado, DH Solomon, GN Chua Teco
Pediatric research, 1988nature.com
We studied thyroxine (T 4) inner ring monodeiodinating activity (5-MA) in various tissues of
fetal, maternal, and adult male rats. Tissue homogenates were incubated with 0.26 μM T 4 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 (rT 3) generated was
measured by radio-immunoassay of ethanol extracts of incubation mixture and the result
was corrected for rT 3 degradation during incubation. Compared to maternal tissues, T 4 to …
Abstract
We studied thyroxine (T 4) inner ring monodeiodinating activity (5-MA) in various tissues of fetal, maternal, and adult male rats. Tissue homogenates were incubated with 0.26 μM T 4 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 (rT 3) generated was measured by radio-immunoassay of ethanol extracts of incubation mixture and the result was corrected for rT 3 degradation during incubation. Compared to maternal tissues, T 4 to rT 3 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 rT 3/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 T 4 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 rT 3 during incubation. Relative to mother, T 4 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 T 4 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 T 4 5-MA is exceedingly active in several fetal tissues, most notably in skeletal muscle followed by intestine and cerebral cortex.
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