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
tetraiodothyroacetic acid (Tetrac) and rT3 in anterior pituitary cells
with those of T4 and T3. Cells were isolated from adult male Wistar rats
and cultured for 3 days in medium with 10% fetal calf serum. Uptake was
measured at 37 C in medium with 0.1% BSA for [125I]Tetrac (200,000 cpm;
240 pM) and [125I]T4 (100,000 cpm; 175 pM) or with 0.5% BSA for [125I]rT3
(100,000 cpm; 250 pM) and [125I]T3 (50,000 cpm; 50 pM). The free fraction
of Tetrac was 1% that of T4 (in medium with 0.1 and with 0.5% BSA), and
the free fraction of rT3 was half that of T3. Uptake of the four tracers
increased sharply up to 1 h of incubation and then leveled off. Expressed
as femtomoles per pM free hormone, uptake at equilibrium was 1.16 +/- 0.16
(n = 6) for Tetrac, 0.15 +/- 0.01 (n = 6) for T4, 0.023 +/- 0.003 (n = 6)
for rT3, and 0.21 +/- 0.02 (n = 6) for T3. Cell-associated radioactivity
after incubation for 24 h with [125I]Tetrac was represented for 15% by
[125I]Triac; after incubation with [125I]T4 for 15-20% by [125I]T3, after
incubation with [125I]rT3 for 6% by [125I]3,3'-T2, while [125I]T3 was
still for 98% [125I]T3. Exposure of cells for 2 h to 100 nM TRH stimulated
TSH release by 90-135%. Tetrac was effective in reducing this response at
a free concentration of 0.05 pM, but rT3 was effective only at a free
concentration of 16 nM. A free Tetrac concentration of 5 pM was equally
effective as 50 pM free T4 in reducing the TSH response to TRH. In human
serum, Tetrac was exclusively bound to T4-binding prealbumin. The free
Tetrac fraction was 0.001% in control subjects and rose 2- to 12-fold in
patients with nonthyroidal illness. As uptake of [125I]Tetrac in the
pituitary was higher than that of T4 and T3, and it was more potent than
T4 in reducing TSH release, Tetrac may be of potential significance for
the regulation of TSH secretion in vivo
