UPTAKE OF 3,3',5,5'-TETRAIODOTHYROACETIC ACID AND 3,3',5'-TRIIODOTHYRONINE IN CULTURED RAT ANTERIOR-PITUITARY-CELLS AND THEIR EFFECTS ON THYROTROPIN SECRETION

We compared the uptake, metabolism, and biological effects of tetraiod othyroacetic acid (Tetrac) and rT(3) in anterior pituitary cells with those of T-4 and T-3. Cells were isolated from adult male Wistar rats and cultured for 3 days in medium with 10% fetal calf serum. Uptake wa s measured at 37 C in medium with 0.1% BSA for [I-125]Tetrac (200,000 cpm; 240 pM) and [I-125]T-4 (100,000 cpm; 175 pM) or with 0.5% BSA for [I-125]rT(3) (100,000 cpm; 250 pM) and [I-125]T-3 (50,000 cpm; 50 pna ). The free fraction of Tetrac was 1% that ofT(4) (in medium with 0.1 and with 0.5% BSA), and the free fraction of rT(3) was half that of T- 3. Uptake of the four tracers increased sharply up to 1 h of incubatio n and then leveled off. Expressed as femtomoles per pM free hormone, u ptake at equilibrium was 1.16 +/- 0.16 (n = 6) for Tetrac, 0.15 +/- 0. 01 (n = 6) for T-4, 0.023 +/- 0.003 (n = 6) for rT(3), and 0.21 +/- 0. 02 (n = 6) for T-3. Cell-associated radioactivity after incubation for 24 h with [I-125]Tetrac was represented for 15% by [I-125]Triac; afte r incubation with [I-125]T-4, for 15-20% by [I-125]T-3, after incubati on with [I-125]rT(3) for 6% by [I-125]3,3'-T-2, while [I-125]T-3 was s till for 98% [I-125]T-3. Exposure of cells for 2 h to 100 nM TRH stimu lated TSH release by 90-135%. Tetrac was effective in reducing this re sponse at a free concentration of 0.05 pM, but rT(3) was effective onl y at a free concentration of 16 nM. A free Tetrac concentration of 5 p M was equally effective as 50 pM free T-4 in reducing the TSH response to TRH. In human serum, Tetrac was exclusively bound to T-4-binding p realbumin. The free Tetrac fraction was 0.001% in control subjects and rose 2- to 12-fold in patients with nonthyroidal illness. As uptake o f [I-125]Tetrac in the pituitary was higher than that of T-4 and T-3, and it was more potent than T-4 in reducing TSH release, Tetrac may be of potential significance for the regulation of TSH secretion in vivo .