Association of the changes in cytosolic Ca2+ and iodide efflux induced by thyrotropin and by the stimulation of alpha 1-adrenergic receptors in cultured rat thyroid cells.
Abstract Thyrotropin causes a time- and concentration-dependent increase in cytosolic Ca2+ in FRTL-5 rat thyroid cells as measured by Quin2 fluorescence; the half-maximal response occurs in response to 1 X 10(-7) M thyrotropin. The effect of added thyrotropin is the same whether cells have been previously and chronically exposed to thyrotropin or whether they have been thyrotropin "starved" for several days. The thyrotropin effect on cytosolic Ca2+ has no relationship to intracellular cAMP levels with respect to dose and time course. Norepinephrine (1 X 10(-7) M) also causes increases in cytosolic Ca2+ in FRTL-5 thyroid cells. With the use of a variety of adrenergic inhibitors, norepinephrine was found to exert its effect via an alpha 1-adrenergic receptor. The exposure of FRTL-5 cells to physiological thyrotropin concentrations enhances the effect on cytosolic Ca2+ level induced by norepinephrine in vitro; the shape of the dose-response curve indicates a cooperative effect of the thyrotropin and norepinephrine. The increase in cytosolic Ca2+ seems to be derived from an intracellular pool rather than from the extracellular space. It is not prevented by nifedipine, a blocker of Ca2+ channels; it is present in cells exposed to ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid; and it is not associated with increased Ca2+ uptake into the cell. the thyrotropin- and norepinephrine-induced increase in cytosolic Ca2+ parallels the efflux of iodide and the organification of thyroglobulin in a dose-dependent manner
