INSULIN-LIKE GROWTH-FACTOR-I MESSENGER-RIBONUCLEIC-ACID EXPRESSION INPORCINE THYROID-FOLLICLES IS REGULATED BY THYROTROPIN AND IODINE

Insulin-like growth factor I (IGF-I) has been shown to be released fro m thyrocytes in vitro. We investigated IGF-I mRNA expression during tr eatment with thyrotropin (TSH), forskolin and potassium iodide (KI) in intact porcine thyroid follicles ex vivo. Porcine thyroid follicles w ere prepared by collagenase digestion and cultured in the presence of TSK, forskolin or EI. After different incubation times, mRNA was isola ted and examined by Not-them hybridization with a porcine IGF-I cDNA p robe of 405 bp in length. In untreated follicles no IGF-I mRNA was fou nd, whereas in follicles stimulated with TSH an IGF-I mRNA of 7.0 kb w as detected after 24h, which persisted for another 24 h. Forskolin tre atment mimicked the TSK effect, indicating that IGF-I mRNA expression may be stimulated by the adenylate cyclase pathway. Preincubation of t he porcine follicles with KI decreased dose dependently the TSH-induce d IGF-I mRNA expression, with complete inhibition at 10 mu mol/l KI. T hese results suggest that TSK acts via the cAMP pathway to enhance EGF -I mRNA expression, which then may lead to an autocrine IGF-I stimulat ion. The IGF-ImRNA expression is under negative control of iodide.