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.