Thyrotropin-induced acceleration of calcium efflux from mouse thyroid: evidence for inhibition by excess iodide

From the molecular characterization of iodide transporters to the prevention of radioactive iodide exposure

In the event of a nuclear reactor accident, the major public health risk will likely result from the release and dispersion of volatile radio-iodines. Upon body exposure and food ingestion, these radio-iodines are concentrated in the thyroid, resulting in substantial thyroidal irradiation and accordingly causing thyroid cancers. Stable potassium iodide (KI) effectively blocks thyroid iodine uptake and is thus used in iodide prophylaxis for reactor accidents. The efficiency of KI is directly related to the physiological inhibition of the thyroid function in the presence of high plasma iodide concentrations. This regulation is called the Wolff-Chaikoff effect. However, to be fully effective, KI should be administered shortly before or immediately after radioiodine exposure. If KI is provided only several hours after exposure, it will elicit the opposite effect e.g. lead to an increase in the thyroid irradiation dose. To date, clear evaluation of the benefit and the potential toxicity of KI administration remain difficult, and additional data are needed. We outline in this review the molecular characterization of KI-induced regulation of the thyroid function. Significant advances in the knowledge of the iodide transport mechanisms and thyroid physiology have been made. Recently developed molecular tools should help clarify iodide metabolism and the Wolff-Chaikoff effect. The major goals are clarifying the factors which increase thyroid cancer risk after a reactor accident and improving the KI administration protocol. These will ultimately lead to the development of novel strategies to decrease thyroid irradiation after radio-iodine exposure.

Regulation of calcium fluxes in the thyroid

Calcium (Ca2+) exchanges were studied in dog thyroid slices incubated in vitro. With 45Ca2+-prelabeled slices, carbamylcholine 10(-7)-10(-5) M (Cchol) induced an important transitory spike efflux, inhibited by procaine and atropine while the stimulated efflux obtained with high concentrations of TSH (10 mU/ml) was progressive and sustained over time. The effects observed with both agents did not require extracellular Ca2+ and were insensitive to verapamil 10(-6)-10(-4) M. Neither dibutyryl (Bu2)-cAMP, nor any agent raising intracellular cAMP (prostaglandin E2, choleratoxin, inhibitors of phosphodiesterases with low concentrations of TSH) were able to reproduce the action of TSH 10 mU/ml, forskolin 10(-5) M being the only exception. Replacement of sodium by choline (+ atropine) in the incubation medium decreased the basal efflux and inhibited the TSH effect. Ouabain 10(-3) M also abolished the TSH-induced Ca2+ efflux, while having no influence on carbamylcholine action. TSH 10 mU/ml and 1 mU/ml, Bu2-cAMP 10(-3) M, choleratoxin and prostaglandin E2 with inhibitors of phosphodiesterase decreased the total 45Ca2+ uptake of the slices, while no effect of Cchol could be detected on this parameter. The results obtained suggest that (1) Cchol and TSH stimulate 45Ca2+ efflux from dog thyroid slices with different kinetics, by mobilization of intracellular Ca2+ stores; (2) this effect of TSH is not mediated by cAMP; (3) independently TSH at low concentrations (1 mU/ml), through cAMP, decreased 45Ca2+ uptake; this suggests that increased 45Ca2+ efflux and decreased uptake result from different mechanisms, as has been described for iodide exchange in FRTL-5 cells.

In vitro release of triiodothyronine and thyroxine from thyroid glands of the domestic fowl (Gallus domesticus)

Basal and thyrotrophin (TSH)-stimulated release of iodothyronines (triiodothyronine, T3, and thyroxine, T4) from intact chicken thyroid glands was determined in vitro. In the absence of TSH, T3 and T4 were released in measurable amounts in the incubation media. The release of both iodothyronines was directly related to the media TSH concentrations and incubation period. Lineweaver-Burke analysis revealed that the Vmax for T3 was 99.4 pg/gland, with an apparent Km of 17.8 mU TSH, and that the Vmax for T4 was 323.35 ng/gland, with an apparent Km of 51.5 mU TSH, demonstrating that T4 is the major iodothyronine released by avian thyroid glands. The basal release of T4 was suppressed by the addition of a calcium chelator (ethyleneglycol-bis-(beta-aminoethylether)-N,N,N', N'-tetraacetic acid; EGTA), a calcium antagonist (cobalt chloride, CoCl2), or prostaglandin E1 (PGE1) to the incubation media. Basal T4 released was increased in the presence of a calcium agonist (lanthanum chloride, LaCl3), a calcium ionophore (A23187), dibutyryl cyclic adenosine 3'3'-monophosphate (dbcAMP), isobutylmethylxanthine (IBMX), indomethacin, magnesium chloride (MgCl2), and potassium iodide (KI). Thyrotrophin-stimulated T4 release was reduced by CoCl2, PGE1, and indomethacin but enhanced by LaCl3, MgCl2, and KI. These results demonstrate that it is possible to measure the release of thyroid hormones in an in vitro system in the chicken. Basal and stimulated iodothyronine release from the chicken thyroid gland appears to be mediated by calcium- and cAMP-dependent mechanisms.