EFFECTS OF INORGANIC IODIDE, EPIDERMAL GROWTH-FACTOR AND PHORBOL ESTER ON HORMONE SYNTHESIS BY PORCINE THYROID-FOLLICLES CULTURED IN SUSPENSION

Porcine thyroid follicles cultured in suspension for 96 h synthesized and secreted thyroid hormones in the presence of thyrotropin (TSH). The secretion of newly synthesized hormones was assessed by determining the contents of thyroxine (T4) and triiodothyronine (T3) in the media and by paperchromatographic analysis of I-125-labelled hormones in the media where the follicles were cultured in the presence and absence of inhibitors of hormone synthesis. The hormone synthesis and secretion was modified by exogenously added NaI (0.1 - 100-mu-M). The maximal response was obtained at 1-mu-M. Thyroid peroxidase (TPO) activity in the cultured follicles with TSH for 96 h was dose-dependently inhibited by NaI. One hundred-mu-M of NaI completely inhibited TSH-induced TPO activity. Moreover, both epidermal growth factor (EGF:10(-9) and 10(-8) M) and phorbol 12-myristate 13-acetate (PMA: 10(-8) and 10(-7) M) inhibited de novo hormone synthesis. An induction of TPO activity by TSH was also inhibited by either agent. These data provide direct evidences that the thyroid hormone synthesis is regulated by NaI by as well as TSH at least in part via regulation af TPO activity and also that both EGF and PMA are inhibitory on thyroid hormone formation.