Norepinephrine and Thyroid-Stimulating Hormone Induce Inositol Phosphate Accumulation in FRTL-5 Cells

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Abstract
3H-Labeled inositol phosphate accumulation is observed when prelabeled FRTL-5 cells (a rat thyroid cell line) are exposed to norepinephrine (NE) or TSH. The presence of inositol trisphosphate among the products implicates a phosphodiesterase-catalyzed hydrolysis of phosphatidylinositol 4,5-bisphosphate. The response to NE is much greater than that to TSH. This may be explained by the ability of cAMP to inhibit inositol phosphate accumulation in these cells. The stimulation by NE is inhibited by .alpha.1-adrenergic receptor antagonists and is markedly potentiated in medium of reduced Ca2+ concentration. After chronic withdrawal of TSH from the growth medium, the magnitude of the response to NE is considerably reduced; however, there is no substantial shift in the dose-response curve. This reflects the dependency of .alpha.1-adrenergic receptor expression on TSH in the FRTL-5 cell. In contrast, the characteristics of inositol phosphate accumulation induced by acute treatment with TSH are similar in cells maintained in the presence or absence of a low concentration of this hormone, and correlate well with the iodide efflux and iodination of thyroglobulin observed in response to TSH. These results support the hypothesis that TSH may mediate certain of its physiological effects through cAMP-independent mechanisms, such as phospholipid/Ca2+ and C-kinase pathways.