Quantitative Studies on the Iodinated Components Secreted by the Rat Thyroid Gland as Determined by<i>in Situ</i>Perfusion

Abstract

A technique is described for perfusing the rat thyroid gland in situ and analyzing the effluent from the thyroid vein. This procedure allows determination of radioactive components secreted from prelabeled thyroid glands by avoiding contamination from recirculating radioactive materials. Filtration of blood with Dacron or glass wool before itsi use in perfusion greatly reduced hemorrhage into the follicles and the consequent appearance of thyroglobulin in the thyroid effluent. Doublelabeling with 125I given 24 hr before and 131I given 10 min before perfusion permitted analysis of materials labeled with iodine entering the thyroid gland at these different time intervals in the same animal. TSH added to the perfusing blood at a concentration of 12.5 mU/ml markedly increased the secretion of iodothyronines and of iodide and the T3 + T4/I− ratio in rats labeled 24 hr previously with 125I. TSH also induced an increase in iodothyronine secretion in rats acutely labeled with 131I. The T3/T4 ratio of both 125I and 131I-labeled iodothyronines appearing in the thyroid effluent was always slightly higher than the ratio of these materials found in the thyroid gland itself, both before and after perfusion. TSH did not cause any dramatic shift in the T3/T4 ratio either in the thyroid gland or in the thyroid effluent in the perfused animals. Addition of 2 × 10−4m ClO4− to the TSH-containing perfusate markedly increased the secretion of both 131I- and 125I-labeled inorganic iodide and inhibited the usual increase of the T3 + T4/I− ratio but did not prevent the increased secretion of T3 and T4 in response to TSH. This suggests that the labeled iodide appearing in the thyroid effluent originates chiefly from deiodination of iodotyrosines during hydrolysis of thyroglobulin in the process of secretion. Most of this liberated iodide is apparently normally reorganified before it can leave the thyroid gland, but it is readily discharged by CIO4−. (Endocrinology81: 946, 1967)