Reciprocal relation between serum thyrotropin levels and intrapituitary 3,5,3'L-triiodothyronine generating activity from thyroxine in perinatal rats.
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Thyroxine (T<SUB>4</SUB>) 5'-monodeiodinating activity in rat pituitary was studied, using paper chromatography. Rat anterior pituitary homogenates were incubated with <SUP>125</SUP>I-T<SUB>4 </SUB>, dithiothreitol and unlabeled L-T<SUB>4 </SUB>at 37°C. The incubation mixture was extracted by ethanol and the extracts were subjected to descending paper chromatography. The conversion ratio of T<SUB>4 </SUB>to 3, 5, 3'L-triodothyronine (T<SUB>3</SUB>) was calculated from the radioactivity of T<SUB>3</SUB>spot to total radioactivity on the paper strip. The T<SUB>3</SUB>amount generated was estimated from the T<SUB>4 </SUB>concentration in the incubation mixture and the conversion ratio.<BR>The T<SUB>3</SUB>generating activity from T<SUB>4 </SUB>increased as the amount of tissue increased. It was temperature-and pH-dependent, and thiol sensitive. These results suggest the enzymatic nature of T<SUB>4 </SUB>5'-monodeiodinating activity of the anterior pituitary. A kinetic study revealed low Km for T<SUB>4 </SUB> (7.9±1.6nM, Mean±SE), with Vmax of 68.0±12.7 fmoles T<SUB>3</SUB>/mg Protein/min.<BR>T<SUB>4 </SUB>monodeiodinating activity was consistently, though minimally, detected in fetal rat pituitaries and increased after birth, reaching the maximum at 22days. It declined thereafter to the young adult level. Serum TSH levels were markedly elevated in fetus. They decreased after birth, reaching the nadir at 22days, and then increased to young adult levels. Serum T<SUB>4 </SUB>and T<SUB>3</SUB>levels were markedly diminished in fetus and gradually increased after birth, reaching the young adult levels at 17 days. Thus, a reciprocal relationship was observed between intrapituitary T<SUB>3</SUB>generating activity from T<SUB>4 </SUB>and serum TSH levels in developing rats. It suggests that the conversion of T<SUB>4 </SUB>to T<SUB>3</SUB>in the pituitary plays a role in regulating TSH secretion.
- 一般社団法人 日本内分泌学会の論文
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- Reciprocal relation between serum thyrotropin levels and intrapituitary 3,5,3'L-triiodothyronine generating activity from thyroxine in perinatal rats.