EFFECTS OF TRIIODOTHYRONINE, TRIIODOTHYROACETIC ACID, IOPANOIC ACID AND IODIDE ON THE THYROTROPIN-RELEASING HORMONE-INDUCED THYROTROPIN RELEASE FROM SUPERFUSED RAT PITUITARY FRAGMENTS

The effects of triiodothyronine, triiodothyroacetic acid, iopanoic acid and potassium iodide were investigated on basal and stimulated thyrotropin release in an in vitro experimental model. Rat pituitary fragments were superfused by Medium-199 with or without T3 (10(-7) mol/l), triiodothyroacetic acid (10(-8)-10(-6) mol/l), iopanoic acid (10(-7)-10(-5) mol/l) or potassium iodide (10(-7)-10(-4) mol/l). This was followed by a 6-min pulse of thyrotropin-releasing hormone (10(-8) mol/l). TSH was measured in 3-min fractions. The TRH-induced TSH release from the pituitary fragments was inhibited by T3 (10(-7) mol/l), by triiodothyroacetic acid (10(-7)-10(-6) mol/l), and by high concentrations of iodide (10(-4) or 10(-5) mol/l). Iopanoic acid had no significant effect at the concentrations tested. It is assumed that in vitro, and at similar concentrations, the inhibitory effect of triiodothyroacetic acid on the TRH-induced TSH secretion is comparable to that of T3, whereas iopanoic acid may have no direct detectable effect. In contrast, a direct inhibitory effect of inorganic iodide, at least in pharmacological concentrations in vivo, cannot be excluded.