The calcium ionophore A23187 (0·1–1 μmol/l) inhibited membrane electrical polarization, uptake of 125I, fluid transport and TSH-stimulated release of radioiodine from the organic pool in follicular cultures of porcine thyroid cells. At higher concentrations (1–30 μmol/l), A23187 promoted release of radioiodine from the organic pool. Stimulation of release of radioiodine from the organic pool by veratridine (a sodium channel agonist, 0·4–1 mmol/l) and A23187 was dependent on the calcium concentration of the medium, while TSH action was independent. Incubation in medium of very low calcium concentration (0·0177 mmol/l) resulted in enhanced release from the organic pool, which was inhibited by TSH (256 μU/ml), A23187 (25 μmol/l) or veratridine (0·5 mmol/l). These data therefore do not support the hypothesis that calcium acts as a mediator of the secretomotor action of TSH, but suggest the possibility of a TSH-induced increase in intracellular calcium as a regulatory negative-feedback mechanism.
J. Endocr. (1988) 116, 373–380
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