Abstract
To investigate the uptake of triiodothyronine sulfate (T 3S) and its effect on thyrotropin-releasing hormone (TRH)-induced thyrotropin (TSH) secretion, anterior pituitary cells were isolated from euthyroid rats and cultured for 3 days in medium containing 10% fetal calf serum. Incubation was performed at 37°C in medium containing 0.5% bovine serum albumin (BSA). Exposure of the pituitary cells to TRH (0.1 μmol/L) for 2 hours stimulated TSH secretion by 176%. This effect was reduced by approximately 45% after a 2-hour preincubation with T 3 (0.001 to 1 μmol/L). A significant inhibitory effect of T 3S on TRH-induced TSH release was only observed at a concentration of 1 μmol/L. The uptake of [ 125I]T 3 after 1 hour of incubation was reduced by 40% ± 4% ( P < .001) by simultaneous addition of 10 nmol/L unlabeled T 3, whereas 1 μmol/L T 3S was required to obtain a reduction of the [ 125I]T 3 uptake by 34% ± 2% ( P < .001). The amount of T 3 present in the unlabeled T 3S preparation was 0.25% as determined by radioimmunoassay. When pituitary cells were incubated for 1 hour with [ 125I]T 3S or [ 125I]T 3 both 50,000 cpm/0.25 mL), the uptake of [ 125I]T 3zS expressed as a percentage of the dose was 0.04% ± 0.02% (mean ± SE, n = 4), whereas that of [ 125I]T 3 amounted to 3.0% ± 0.4% (n = 4). In contrast, when hepatocytes were incubated for 1 hour with [ 125I]T 3S, the uptake amounted to 5.1% ± 0.8% (n = 9), whereas that of [ 125I]T 3 was 22.1% ± 1.7% (n = 9). Furthermore, [ 125I]T 3S was as rapidly deiodinated (iodide production, 14.9% ± 2.6%; n = 9) as [ 125I]T 3 (12.1% ± 0.8%, n = 9) by hepatocytes. It is concluded that (1) T 3S is poorly taken up by pituitary cells, and (2) the suppressive effect of high concentrations of T 3S on TRH-induced TSH secretion and on [ 125I]T 3 uptake can be explained by slight contamination with T 3. Thus, it appears that T 3S has only a minor biological effect, if any, on the pituitary.
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