Requirement of Insulin Growth Factor I plus Hydrocortisone for the Regeneration of Thyrotropin (TSH)-Dependent Mechanism of I-Efflux and Ca2+Mobilization in FRTL-5 Cells during TSH Depletion*

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Abstract
FRTL-5 rat thyroid cells grown in culture medium supplemented with serum and 6H (TSH, insulin, hydrocortisone, transferrin, glycylhistidyllysine, and somatostatin) showed a significant increase in TSH-dependent cAMP accumulation and I- efflux after prolonged incubation (5 to 10 days) of the cells in culture medium containing 5H (6H-TSH) or serum. The induction of the cAMP response was at least partly reproduced when both serum and 5H were omitted from the medium. However the I- efflux response was completely abolished under such conditions and regenerated when serum or 5H was present. The serum or 5H effect on I- efflux response was mimicked by 2H (insulin + hydrocortisone). Insulin was replaced by 1/1000 less insulin-like growth factor-I than insulin. TSH-dependent Ca2+ mobilization of the cells was similarly affected by the presence of serum or 2H. However, the I- efflux and Ca2+ responses to an agonist other than TSH (extracellular ATP) were not substantially influenced by serum and/or 2H as well as TSH in the medium. The results indicate that serum or insulin-like growth factor-I plus hydrocortisone are required rather specifically for the regeneration of the TSH-receptor mechanism coupled with I- efflux and/or Ca2+ mobilization mechanism.

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