Abstract
ABSTRACT The pinocytic step in thyroid hormone secretion was studied using double isotope technique in rats. TSH administration caused an increase in the radioactivity of the 15 000 × g pellet (L). The kinetics of the thyroglobulin pool (Tg) involved were studied. Tg was labelled 5 and 1 days before the day of the experiment with 125I and 131I respectively. Groups of animals were fed with either Forramez (400 μg 127I/kg) or Labina (200 μg 127I/kg). Both tracers increased after TSH injection in the last group, while only 125I was increased in the Forramez-fed animals. Sucrose density gradient showed that the radioactive material involved in this process is Tg. The per cent distribution of radioactivity for iodo-amino acids was estimated for both isotopes by ascending paper chromatography. The ratio 131I/125I was calculated for T4 and T3 in the L and S fractions. An increase in this ratio was found in the L fraction after TSH administration. These results suggest that storage of Tg takes place in an "oinion like" structure, the outer layers corresponding to the more recently synthetized molecules, being more available to pseudopods during the pinocytic process. This hypothesis fits with the present results.
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