Fetal rat thyroid gland in organ culture

Abstract

Using the organ culture technique, the action of TSH and of insulin on 131I metabolism and on amino acid 14C incorporation into protein was studied in fetal rat thyroid glands. By addition of TSH to the culture medium, nonfunctioning fetal thyroid could be made to initiate iodine metabolism. In functioning fetal thyroid, TSH greatly improved 131I utilization which is reflected primarily in its increased incorporation into thyroxine. Under similar conditions, it is found that TSH failed to influence the incorporation of 14C-leucine into protein and that puromycin, at a concentration that inhibited protein formation, failed to abolish the TSH-induced stimulation of 131I incorporation into protein-bound iodotyrosines and thyroxine. It seems, therefore, that TSH stimulation of thyroid hormone production is not dependent on increased synthesis of protein. We made use of perchlorate and tapazole to explore the mechanism of action of TSH. It is suggested that the effect of TSH on organ culture of embryonic thyroid glands is not dependent only on the stimulation of iodide concentrating ability of the explants since TSH accelerates the conversion of MIT to DIT and the formation of thyroid hormones. Insulin also has a pronounced effect on embryonic thyroid glands explanted into organ culture. Its presence as the only hormone in the culture medium enhanced significantly the incorporation of 131I into protein-bound iodotyrosines and thyroxine. When insulin was added to the TSH-containing medium, 131I utilization exceeded that observed in the presence of TSH alone; the formation of DIT and T 4 pas proportionately more affected than was that of MIT. The 14C of 14C-labeled amino acids was readily incorporated into thyroidal protein, and in each case the presence of insulin in the medium enhanced the incorporation. Addition of insulin to the culture medium stimulated also the incorporation of 14C of adenine-8- 14C into ribonucleic acid by the fetal glands. It is conceivable that insulin enhances protein formation in embryonic thyroid explants solely or partly via its influence on the metabolism of ribonucleic acid.