Effects of TSH on iodination in rat thyroid follicles studied by autoradiography

The possible functional relation between TSH-stimulated exocytosis and TSH-stimulated iodination in the thyroid gland was studied using quantitative EM autoradiography and cytochemistry. The study was performed in rats pretreated with thyroxine for 2 days. TSH, giving i.v. 10 min before sacrifice to thyroxine-treated rats, increased organification of 125I by about 50%. TSH decreased the number of peroxidase-positive vesicles in the apical cytoplasm and increased the width of the peroxidase reaction at the apical plasma membrane, suggesting a redistribution of peroxidase. EM autoradiography after labelling with [3H]leucine showed that TSH caused a rapid redistribution by exocytosis of newly synthesized protein to the follicle lumen. The protein deposited in the lumen remained to a large extent in the microvillus region. 10 min after injection of 125I, newly iodinated protein was distributed in a gradient in the lumen periphery. TSH, given 5 min before 125I, caused a significant increase in the labelling of the colloid in the microvillus region, indicating a selective incorporation of iodine into newly synthesized molecules deposited in this region by stimulated exocytosis. Our results confirm and extend earlier observations on a functional link between exocytosis and iodination. Redistribution of peroxidase as well as newly synthesized protein to the site of iodination might be of importance.