Active transport of iodide in isolated porcine thyroid cells. Application to an in vitro bioassay of thyrotropin

Abstract

Porcine thyroid cells were cultured for 2 days with or without dibutyryl cyclic AMP or thyrotropin (TSH). Then they were isolated post-culture by a gentle treatment with a calcium chelating agent. Some characteristics of the iodide transport system were studied in these thyroid cell suspensions. Iodide influx is a saturable, temperature- and energy-dependent phenomenon. It is blocked by ouabaïn, N-ethylmaleimide, dinitrophenol, cardiotoxin, low Na + concentration and harmaline. Only 3% of the intracellular iodide is trichloracetic-acid-insoluble at equilibrium. The apparent Michaelis constant ( K m) of the transport system is 30 μM. For monolayer cells, the decrease of C/M ratio, increase of apparent K m, and decrease of V max between day 0 (freshly isolated cells) and day 6, indicate a loss of iodide-trapping ability up to passive diffusion. To the contrary, high values of C/M and normal K m (30 μM) are observed in TSH follicles from reassociated cells. At iodide equilibrium, thryotropin, prostaglandins E 1 and E 2 and long-acting thyroid stimulator (LATS), induce a fast release of iodide. This release is dose-dependent in the first 5 min. It has been used to develop a bioassay of TSH and a fast detection of LATS.