Intrafollicular re-cycling of organic iodine in the hypophysectomized rat as observed by a long-term kinetic study

Abstract

Abstract. A long-term kinetic study was started 8 days after hypophysectomy in rats previously adapted to receive 5 μg iodide daily, and maintained on the same diet after surgery. At each time, the renewed fraction for thyroglobulin (Tg) and lysosomes in the gland, and hormones in the plasma was calculated by the method of isotopic equlibrium. All the pools studied are renewed very slowly, but with different kinetics. Between time zero and the 25th day of the experiment, the renewed fraction in plasma hormones always exceeds that of Tg. Thus, in hypophysectomized rats, Tg pool is composed of at least two compartments with different turn-over rates. Tg molecules of both compartments are endocytosed, but that of the faster compartment participate in secretion in a preferential way. Peeling of the data for the 60-day period of experiment confirms these results. In addition, peeling demonstrates a 15-day delay in the turn-over of the second compartment. Such a delay was not found in the turn-over of this compartment in lysosomes. Thus, we may again postulate, in hypophysectomized rats, a recycling of iodine in organic form from the lysosomes back to the luminal colloid, as previously proposed for control rats.