Thyroxine inner ring monodeiodinating activity in fetal tissues of the rat

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 microM 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 degrees 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); approximately 8 times in intestine (0.72 +/- 0.17 versus 0.09 +/- 0.03);and approximately 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 (greater than 90%) degradation of rT3 during incubation. Relative to mother, T4 5-MA in 19-day fetal tissues was increased approximately 30-fold intestine, approximately 20-fold in skeletal muscle, and approximately 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 less than 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.